Coping with groundwater pollution in high-nitrate leaching areas: The efficacy of denitrification

The Ningxia Yellow River irrigation area, characterized by an arid climate and high leaching of NO3--N, exhibits complex and unique groundwater nitrate (NO3--N) pollution, with denitrification serving as the principal mechanism for NO3--N removal. The characteristics of N leaching from paddy fields and NO3--N removal by groundwater denitrification were investigated through a two-year field observation. The leaching losses of total nitrogen (TN) and NO3--N accounted for 10.81-27.34% and 7.59-12.74%, respectively, of the N input. The linear relationship between NO3--N leaching and N input indicated that the fertilizer-induced emission factor (EF) of NO3--N leaching in direct dry seeding and seedling-raising and transplanting paddy fields was 8.2% (2021, R2 = 0.992) and 6.7% (2022, R2 = 0.994), respectively. The study highlighted that the quadratic relationship between the NO3--N leaching loss and N input (R2 = 0.999) significantly outperformed the linear relationship. Groundwater denitrification capacity was characterized by monitoring the concentrations of dinitrogen (N2) and nitrous oxide (N2O). The results revealed substantial seasonal fluctuations in excess N2 and N2O concentrations in groundwater, particularly following fertilization and irrigation events. The removal efficiency of NO3--N via groundwater denitrification ranged from 42.70% to 74.38%, varying with depth. Groundwater denitrification capacity appeared to be linked to dissolved organic carbon (DOC) concentration, redox conditions, fertilization, irrigation, and soil texture. The anthropogenic-alluvial soil with limited water retention accelerated the leaching of NO3--N into groundwater during irrigation. This process enhances the groundwater recharge capacity and alters the redox conditions of groundwater, consequently impacting groundwater denitrification activity. The DOC concentration emerged as the primary constraint on the groundwater denitrification capacity in this region. Hence, increasing carbon source concentration and enhancing soil water retention capacity are vital for improving the groundwater denitrification capacity and NO3--N removal efficiency. This study provides practical insights for managing groundwater NO3--N pollution in agricultural areas, optimizing fertilization strategies and improving groundwater quality.

Effects of warming on rice production and metabolism process associated with greenhouse gas emissions

Evaluating the impact of global warming on rice production and greenhouse gas (GHG) emissions is critical for ensuring food security and mitigating the consequences of climate change. Nonetheless, the impacts of warming on crop production, GHG emissions, and microbial mechanisms in the single-cropping rice systems remain unclear. Here, a two-year field experiment was conducted to explore the effects of warming (increased by 2.7-3.0 °C on average) in the rice growing season on crop production and functional microorganisms associated with GHG emissions. Results showed that warming resulted in significant reduction (p < 0.01) in the aboveground biomass and grain yield as well as in grain weight, the number of spikelets per panicle, and the seed-setting rate. However, it caused a significant increase (p < 0.01) in the number of panicles by 15.6 % and 34.9 %, respectively. Furthermore, warming significantly increased (p < 0.01) seasonal methane (CH4) emissions but reduced nitrous oxide (N2O) emissions, particularly in 2022.The relative abundance of genes associated with CH4 metabolism and nitrogen metabolism was increased by 40.7 % and 32.7 %, respectively, in response to warming. Moreover, warming had a positive impact on the abundance of genes related to CH4 production and oxidation processes but did not affect the denitrification processes associated with N2O production. These results showed that warming decreased rice yield and biomass in the single cropping rice system but increased CH4 emissions and global warming potential. Taken together, to address the increasing food demand of a growing population and mitigate the impacts of global warming, it is imperative to duce GHG emissions and enhance crop yields.

Exploring dissolved N2O characteristics and unearthing indirect N2O emission factors in the shallow groundwater of paddy and upland fields

Indirect emissions of nitrous oxide (N2O) stemming from nitrogen (N) leaching in agricultural fields constitute a significant contributor to atmospheric N2O. Groundwater nitrate (NO3--N) pollution is severe in the Ningxia Yellow River Irrigation Area (NYRIA), coupled with high NO3--N leaching, exacerbates the risk of indirect N2O emissions from groundwater. Over two years of field observations, this study investigated the characteristics and interannual variations of dissolved N2O (dN2O) concentrations and indirect N2O emission factors (EF5g) in shallow groundwater. The research focused on three typical farmlands in the NYRIA, each subjected to six levels of N fertilizer application. The mean dN2O concentrations in the groundwater of paddy, corn and vegetable fields were 5.17, 8.40 and 16.35 μg N·L-1, respectively. Notably, the dN2O concentrations in the shallow groundwater of upland fields exceeded those in paddy fields, with maximum levels in vegetable fields nearly an order of magnitude higher. Elevated N application significantly increased dN2O concentrations across various farmlands, showing statistically significant variation. However, differences in EF5g-A and EF5g-B within the same farmland were negligible. Denitrification was the primary process contributing to N2O production in groundwater, with nitrification also played a crucial role in upland fields. Factors such as NO3--N, NH4+-N, dissolved oxygen (DO), and pH critically influenced N2O production. EF5g-B, which considers the NO3--N consumption during denitrification processes in groundwater, was deemed more appropriate than EF5g-A for assessing the indirect N2O emission in the NYRIA. The EF5g of agricultural fields exhibited minimal sensitivity to N input but was significantly affected by other factors, such as the planting pattern. The study revealed the rationality of adopting EF5g-B in assessing indirect N2O emissions, providing valuable insights for N management strategies in regions with high NO3--N leaching. Minimizing N fertilizer application while ensuring crop yield, especially in upland fields, is beneficial for reducing N2O emissions.

Resupply, diffusion, and bioavailability of Hg in paddy soil-water environment with flood-drain-reflood and straw amendment

Mercury (Hg) poses a significant risk in paddy fields, particularly when it is converted to methylmercury (MeHg) and accumulates in rice. However, the bioavailability and resupply kinetics of Hg in the paddy soil-water environment are not well understood. In this study, the diffusive gradients in thin films (DGT) and the 'DGT-induced fluxes in sediments' model (DIFS) were first adopted to investigate the Hg resupply kinetics, diffusion fluxes and bioavailability in a paddy environment subjected to flood-drain-reflood treatment and straw amendment. Our results shown that although the straw amendment limited the bioavailability of Hg (38.2%-47.9% lower than control) in porewater by decreasing its resupply capacity, especially with smaller straw particles, the net production of MeHg in paddy fields was significantly increased after straw amendment (73.5%-77.9% higher than control). The results of microbial sequencing indicate that enhanced methylators (e.g., family Geobacter) and non-Hg methylators (e.g., Methanosarcinaceae) played a crucial role in MeHg production following straw amendment. Moreover, Hg-containing paddy soils generally tend to release Hg into the overlying water, while drain-reflood treatment changes the direction of Hg diffusion fluxes in the paddy soil-water interface. The drainage-reflooded treatment decreases the Hg reactive and resupply capacity of the paddy soil, thereby hindering the release of Hg from soil into overlying water during the early stages of reflooding. Overall, this study provides novel insights into the behavior of Hg in paddy soil-water surface microlayers.

Microbiological mechanism for "production while remediating" in Cd-contaminated paddy fields: A field experiment

Security utilization measures (SUMs) for "production while remediating" in moderate and mild Cd-polluted paddy fields had been widely used. To investigate how SUMs drove rhizosphere soil microbial communities and reduced soil Cd bioavailability, a field experiment was conducted using soil biochemical analysis and 16S rRNA high-throughput sequencing. Results showed that SUMs improved rice yield by increasing the number of effective panicles and filled grains, while also inhibiting soil acidification and enhancing disease resistance by improving soil enzyme activities. SUMs also reduced the accumulation of harmful Cd in rice grains and transformed it into FeMn oxidized Cd, organic-bound Cd, and residual Cd in rhizosphere soil. This was partly due to the higher degree of soil DOM aromatization, which helped complex the Cd with DOM. Additionally, the study also found that microbial activity was the primary source of soil DOM, and that SUMs increased the diversity of soil microbes and recruited many beneficial microbes (Arthrobacter, Candidatus_Solibacter, Bryobacter, Bradyrhizobium, and Flavisolibacter) associated with organic matter decomposition, plant growth promotion, and pathogen inhibition. Besides, special taxa (Bradyyrhizobium and Thermodesulfovibrio) involved in sulfate/sulfur ion generation and nitrate/nitrite reduction pathway were observably enriched, which effectively reduced the soil Cd bioavailability through adsorption and co-precipitation. Therefore, SUMs not only changed the soil physicochemical properties (e.g., pH), but also drove rhizosphere microbes to participate in the chemical species transformation of soil Cd, thus reducing Cd accumulation in rice grains.

Effects of abrupt and gradual increase of atmospheric CO2 concentration on methanotrophs in paddy fields

The simulation of abrupt atmospheric CO₂ increase is a common way to examine the response of soil methanotrophs to future climate change. However, atmosphere is undergoing a gradual CO₂ increase, and it is unknown whether the previously reported response of methanotrophs to abrupt CO₂ increase can well represent their response to the gradual increase. To improve the understanding of the effect of elevated CO₂ (eCO₂) on methanotrophs in paddy ecosystems, the methane oxidation potential and communities of methanotrophs were examined via open top chambers under the three following CO₂ treatments: an ambient CO₂ concentration (AC); an abrupt CO₂ increase by 200 ppm above AC (AI); a gradual CO₂ increase by 40 ppm each year until 200 ppm above AC (GI). Relative to AC treatment, AI and GI treatments significantly (p < 0.05) increased the methane oxidation rate by 43.8% and 36.7%, respectively, during rice growth period. Furthermore, the abundance of pmoA genes was significantly (p < 0.05) increased by 62.4% and 32.5%, respectively, under AI and GI treatments. However, there were no significant variations in oxidation rate or gene abundance between the two eCO₂ treatments. In addition, no obvious change of overall community composition of methanotrophs was observed among treatments, while the proportions of Methylosarcina and Methylocystis significantly (p < 0.05) changed. Taken together, our results indicate similar response of methanotrophs to abrupt and gradual CO₂ increase, although the magnitude of response under gradual increase was smaller and the abrupt increase may somewhat overestimate the response.

Nitrogen migration and transformation in a saline-alkali paddy ecosystem with application of different nitrogen fertilizers

With the increasing transformation of saline-alkali land into paddy, the nitrogen (N) loss in saline-alkali paddy fields becomes an urgent agricultural-environmental problem. However, N migration and transformation following the application of different N fertilizers in saline-alkali paddy fields remains unclear. In this study, four types of N fertilizers were tested to explore the N migration and transformation among water-soil-gas-plant media in saline-alkali paddy ecosystems. Based on the structural equation models, N fertilizer types can change the effects of electrical conductivity (EC), pH, and ammonia-N (NH₄⁺-N) of surface water and/or soil on ammonia (NH₃) volatilization and nitrous oxide (N₂O) emission. Compared with urea (U), the application of urea with urease-nitrification inhibitors (UI) can reduce the potential risk of NH₄⁺-N and nitrate-N (NO₃^--N) loss via runoff, and significantly (p < 0.05) reduce the N₂O emission. However, the expected effectiveness of UI on NH₃ volatilization control and total N (TN) uptake capacity of rice was not achieved. For organic-inorganic compound fertilizer (OCF) and carbon-based slow-release fertilizer (CSF), the average TN concentrations in surface water at panicle initiation fertilizer (PIF) stage were reduced by 45.97% and 38.63%, respectively, and the TN contents in aboveground crops were increased by 15.62% and 23.91%. The cumulative N₂O emissions by the end of the entire rice-growing season were also decreased by 103.62% and 36.69%, respectively. Overall, both OCF and CSF are beneficial for controlling N₂O emission and the potential risks of N loss via runoff caused by surface water discharge, and improving the TN uptake capacity of rice in saline-alkali paddy fields.

A GIS-based method for modeling methane emissions from paddy fields by fusing multiple sources of data

Quantification of regional methane (CH₄) gas emission in the paddy fields is critical under climate warming. Mechanism models generally require numerous parameters while empirical models are too coarse. Based on the mechanism and structure of the widely used model CH4MOD, a GIS-based Regional CH₄ Emission Calculation (GRMC) method was put forward by introducing multiple sources of remote sensing images, including MOD09A1, MOD11A2, MOD15A2H as well as local water management standards. The stress of soil moisture condition (f(water)) on CH₄ emissions was quantified by calculating the redox potential (Eh) from days after flooding or falling dry. The f(water)-t curve was calculated under different exogenous organic matter addition. Combining the f(water)-t curve with local water management standards, the seasonal variation of f(water) was obtained. It was proven that f(water) was effective in reflecting the regulation role of soil moisture condition. The GRMC was tested at four Eddy Covariance (EC) sites: Nanchang (NC) in China, Twitchell (TWT) in the USA, Castellaro (CAS) in Italy and Cheorwon (CRK) in Korea and has been proven to well track the seasonal dynamics of CH₄ emissions with R² ranges of 0.738-0.848, RMSE ranges of 31.94-149.22 mg C/m²d and MBE ranges of -66.42- -14.79 mg C/m²d. The parameters obtained in Nanchang (NC) site in China were then applied to the Ganfu Plain Irrigation System (GFPIS), a typical rice planting area of China, to analyse the spatial-temporal variations of CH₄ emissions. The total CH₄ emissions of late rice in the GFPIS from 2001 to 2013 was in the range of 14.47-20.48 (10³ t CH₄-C). Ts caused spatial variation of CH₄ production capacity, resulting in the spatial variability of CH4 emissions. Overall, the GRMC is effective in obtaining CH₄ emissions from rice fields on a regional scale.

Characteristics of soil N2O emission and N2O-producing microbial communities in paddy fields under elevated CO2 concentrations

The effects of elevated carbon dioxide (CO₂) concentration (e[CO₂]) on nitrous oxide (N₂O) emissions from paddy fields and the microbial processes involved in N₂O emissions have recently received much attention. Ammonia-oxidizing microorganisms and denitrifying bacteria dominate the production of N₂O in paddy soils. To better understand the dynamics of N₂O production under e[CO₂], a field experiment was conducted after five years of CO₂ fumigation based on three treatments: CK (ambient atmospheric CO₂), T1 (CK + increase of 40 ppm per year until 200 ppm), and T2 (CK + 200 ppm). N₂O fluxes, soil physicochemical properties, and N₂O production potential were quantified during the rice-growth period. The functional gene abundance and community composition of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were analyzed using quantitative polymerase chain reaction (qPCR) and those of ammonia-denitrifying bacteria (nirS- and nirK-type) were analyzed using Illumina MiSeq sequencing. N₂O emissions decreased by 173% and 41% under the two e[CO₂] treatments during grain filling and milk ripening, respectively (P < 0.05). N₂O emissions increased by 279% and 172% in the T2 treatment compared with T1 during the tillering and milk-ripening stages, respectively (P < 0.05). Furthermore, the N₂O production potential was significantly higher in the CK treatment than in T1 and T2 during the elongation stage. The N₂O production potential and abundance of AOA amoA genes in T1 treatment were significantly lower than those in CK treatment during the high N₂O emission phase caused by mid-season drainage (P < 0.05). Although nirK- and nirS-type denitrifying bacteria community structure and diversity did not respond significantly (P > 0.05) to e[CO₂], the abundance of nirK-type denitrifying bacteria significantly affected the N₂O flux (P < 0.05). Linear regression analysis showed that the N₂O production potential, AOA amoA gene abundance, and nirK gene abundance explained 47.2% of the variation in N₂O emissions. In addition, soil nitrogen (N) significantly affected the nirK- and nirS-type denitrifier communities. Overall, our results revealed that e[CO₂] suppressed N₂O emissions, which was closely associated with the abundance of AOA amoA and nirK genes (P < 0.05).

Using HYDRUS-2D model to simulate the water flow and nitrogen transport in a paddy field with traditional flooded irrigation

In recent years, agricultural non-point source pollution (ANPSP) has become increasingly prominent, and nitrogen plays an important role in ANPSP. Therefore, we carried out traditional flooded irrigation (TFI) experiments in the paddy field, and applied HYDRUS-2D model to simulate the nitrogen transport in this study. Three observation points A1, A2, and A3 were arranged on the diagonal of the paddy field. We observed ponding water depth on soil surface and nitrogen concentrations in ponding water and soil water at 0.1 m, 0.2 m, and 0.3 m below soil surface. HYDRUS-2D model was proved to be effective in simulating the ponding water depth with root mean squared error (RMSE) = 0.717 cm and Nash-Sutcliffe coefficient (NSE) = 0.805 for the simulated and measured ponding water depth. The simulated and measured NH₄⁺-N concentrations at different depths below soil surface at point A1 basically had the same trend, and the simulated NH₄⁺-N concentrations in ponding water had better agreement with the measured data with RMSE = 1.323 mg/L, and NSE = 0.958. The measured NH₄⁺-N concentrations at depths of 0.1 m, 0.2 m, and 0.3 m below soil surface at point A2 were larger than the simulated values, but they had the same trend on the whole. The simulated NH₄⁺-N concentrations at different depths below soils' surface at point A3 did not fit well with the measured values. The overall trend of the simulated and measured NO₃^--N concentrations in ponding water on soil surface at point A1 was consistent, but the peak values of the simulated NO₃^--N concentrations were larger than the measured ones. The simulated and measured NO₃^--N concentrations at different depths below soil surface at points A2 and A3 did not agree well although they had the same trend, which became worse with the increase of soil depth. This indicated that the HYDRUS-2D model was effective in simulating water flow and nitrogen transport in TFI paddy fields. Sensitivity analysis suggested different simulated nitrogen concentrations in different water depths at different time were sensitive to different model parameters.

Death in the paddy field: Carcass decomposition and associated arthropods in subunits of a rice field landscape

Although rice production landscapes are often the scene of homicides, vertebrate decomposition and associated arthropods have never been described from rice paddies. Tropical rice landscapes are typically composed of irrigated/non-irrigated rice fields, fallow land (i.e., uncultivated fields), and low earthen levees (bunds) used as access pathways. The specific microclimatic and environmental conditions associated with each subunit of rice field landscapes are likely to impact carcass decomposition as well as the arthropod species associated with carrion. Here, we tested the hypothesis that the combined effects of constraints on arthropod colonization and survival, scarcity of necromass, limited habitat architecture, and recurrent disturbance limit the ability of carrion-related arthropods to colonize corpses and large carcasses in tropical rice-fields. Our results from monitoring pig carcasses in Philippine rice fields indicated that vertebrate decomposition in irrigated fields was slower and incomplete when compared to non-irrigated fields and bunds. Carcasses were colonized by a small complex of carrion-related arthropods that differed in composition and relative species abundance between dry bunds and relatively humid rice paddies. Fire ants (Solenopsis germinata) were observed frequently on carcasses exposed on bunds, rarely in non-irrigated fields, and almost never in irrigated fields. The presence of fire ants was associated with reduced blow fly (Chrysomya megacephala) abundance. Taken together, this indicates that the arthropod fauna associated with carcasses in tropical rice fields is relatively simple in contrast to the generally high arthropod diversity reported for other ecosystems at tropical latitudes. The limited richness of the community also means that an understanding of the development of one abundant calliphorid blow fly, C. megacephala, may be sufficient to investigate deaths and homicides under conditions similar to the ones described in this study.

Seasonal variations of toxic metal(loid)s in groundwater collected from an intensive agricultural area in northwestern Turkey and associated health risk assessment

İpsala district located in the northwest of Turkey is an intensive agricultural area, where paddy cultivation has been carried out for more than 50 years. The main source for drinking water in the area is groundwater. Since large amounts of agrochemicals are applied to the paddy fields, groundwater in the study area can be contaminated with toxic metal (loid)s (TMs). In this study, levels of eight TMs in the drinking water samples taken from the district and its 22 villages in the dry and wet seasons were measured and compared with drinking water quality guidelines. In addition, non-carcinogenic and carcinogenic health risks, and pollution status of TMs were assessed. The mean values of Cd, Ni, Cu, Zn, Mn, Pb, As and Cr in both seasons were below the drinking water limits. High clay content and low infiltration rate of the soils in the study area may have caused low TM concentrations. The TMs levels were higher in the wet season due to high rainfall intensity. Metal pollution indices indicated that groundwater quality is suitable for potable uses. All hazard quotient and hazard index results for children and adults in both seasons were lower than the acceptable risk level of 1. Carcinogenic risk results of As and Cr in both seasons were within or below the acceptable risk range. These findings revealed that the TMs in the drinking water would not pose health risks to the local residents.

Controlled release urea improves rice production and reduces environmental pollution: a research based on meta-analysis and machine learning

To reveal the comprehensive impacts of controlled release urea (CRU) on rice production, nitrogen (N) loss, and greenhouse gas (GHG) emissions, a research based on global meta-analysis and machine learning (ML) was conducted. The results revealed that the CRU application instead of conventional fertilizer can increase rice yield, N use efficiency (NUE), and net benefits by 5.24%, 20.18%, and 9.30%, respectively, under the same amount of N. Furthermore, the emission of N₂O and CH₄, global warming potential (GWP), the loss of N leaching, and NH₃ volatilization were respectively reduced by 25.64%, 18.33%, 21.10%, 14.90%, and 35.88%. The enhancing effects of CRU on rice yield and NUE were greater when the nitrogen application rate was 150 kg N ha^-1. Nevertheless, the reducing effects of CRU on GHG emission reduction, nitrogen leaching, and NH₃ volatilization was greater at high nitrogen application rate (≥150 kg ha^-1). Mitigating effects of CRU on N₂O and CH₄ emission were significant when soil pH ≥ 6, while CRU posed a measurable effect on reducing nitrogen leaching and NH₃ volatilization in paddy fields with soil organic carbon lower than 15 g kg^-1 and pH lower than 6. Based on the data collected from meta-analysis, the results of ML demonstrated that it was feasible to use soil data and N application rate to predict N losses in rice fields under CRU. The performance of random forest is better than multilayer perceptron regression in predicting N losses from paddy fields. Thus, it is necessary to promote the application of CRU in paddy fields, especially in coarse soil, in which scenario the environmental pollution would be decreased and the rice yields, NUE, and net benefits would be increased. Meanwhile, machine learning models can be used to predict N losses in CRU paddy fields.

Impact of paddy fields on water quality of Gala Lake (Turkey): An important migratory bird stopover habitat

Gala Lake is an internationally important lake due to its location on one of the world's most important bird migration routes. For this reason, water quality of the lake is of great concern. However, the lake surrounded by paddy fields may face eutrophication and toxic metal contamination due to excessive use of fertilizers and pesticides. In this study, impact of paddy fields on water quality of the Gala Lake was investigated. The concentrations of metal(loid)s and physico-chemical parameters in surface water samples taken from the lake were measured and compared with water quality guidelines. Also, human health risks and contamination status of metal(loid)s were assessed. The mean NO₂, SRP and BOD concentrations in the lake exceeded the permissible levels for both salmonid and cyprinid waters. The mean BOD value indicated contaminated water quality in the lake, while mean COD and SRP values indicated lightly contaminated water quality. The mean As, Cr and Pb values in the winter exceeded the drinking water limits set by WHO and EC, while the mean Cr and Zn values exceeded the limit values for the protection of freshwater aquatic organisms set by USEPA. Similarly, heavy metal pollution index and the degree of contamination values in the winter indicated that the lake water is moderately polluted. Health risk assessment results revealed that As and Cr in the lake water via ingestion exposure pathway may pose both non-carcinogenic and carcinogenic health risks to the residents. The results of this study indicated that paddy fields are a major source of nutrients, organic matter and toxic metal(loid)s to the Gala Lake. To improve the water quality of the lake, we suggest that excessive use of fertilizers and pesticides should be controlled to reduce metal(loid) and nutrient loads from the paddy fields.

The input-output balance of microplastics derived from coated fertilizer in paddy fields and the timing of their discharge during the irrigation season

Polymer-based microcapsules derived from coated fertilizers are not recovered after use. Therefore, they are a source of microplastics to the agricultural lands and coastal areas of Japan. In this study, we investigated the input-output balance of microcapsules in three paddy fields and the timing of microcapsule discharge from the fields with the aim of developing effective techniques to reduce microcapsule discharge. Microcapsules were discharged from the paddy fields primarily during puddling, when the weir plate was overflowed, and when surface drainage was implemented. About 50% of the total discharge during the irrigation period occurred during puddling, which is a process for leveling paddy fields. Therefore, contamination of the surrounding environment by microcapsules from paddy fields can be effectively reduced by preventing the release of microplastics from paddy fields during puddling. We also showed that the total microcapsule discharge cannot be controlled solely by irrigation water management, such as by adjusting the height of the weir plate. We found that about 0.067-0.076% of the total number of microcapsules accumulated in the soil of the paddy fields was discharged during the irrigation season in 2020. Furthermore, 70% of the microcapsules discharged from one field in 2020 had resided in the soil for at least two years. The use as fertilizer coatings of biodegradable polymers that would degrade completely in the soil within a few years could therefore substantially reduce the amount of microplastics released into the ocean from agricultural fields, and their development is thus urgently needed.

The role of coated fertilizer used in paddy fields as a source of microplastics in the marine environment

Fixed-point and multi-point field survey was conducted to determine the actual contamination of the Japanese coast by microcapsules derived from coated fertilizers. The results indicated that the amount of microcapsules increased during the irrigation season (9423-74,869 pieces/m²) and decreased in the non-irrigation season (77-1471 pieces/m²). Microcapsules accounted for 90% of all microplastics sampled during the irrigation season. The lack of correlation between the area of agricultural land in the watershed and the microcapsule density on the coast suggested that the density of microcapsules on the coast was not only dependent on the total load to the ocean, but also on topographic and meteorological conditions. More than 90% of the microcapsules collected from the shoreline were made of polyethylene. Scanning electron microscopic observation of the surface of the microcapsules revealed that portions of the microcapsules were removed as secondary microplastics in the process of migration to the ocean.

Decadal shifts in soil pH and organic matter differ between land uses in contrasting regions in China

Soil organic matter (SOM) and pH are critical soil properties strongly linked to carbon storage, nutrient cycling and crop productivity. Land use is known to have a dominant impact on these key soil properties, but we often lack the ability to examine temporal trajectories across extensive spatial scales. Large-scale monitoring programmes provide the data to evaluate these longer-term changes, and under different climatic conditions. This study used data from Chinese soil surveys to examine changes in soil pH and SOM across different land uses (dry farmland, paddy fields, grassland, woodland, unused land), with surface soil (0-20 cm) collected in the periods 1985-90 (Survey 1; 890 samples) and 2006-10 (Survey 2; 5005 samples) from two contrasting areas. In the southern part of China the mean pH of paddy soils fell sharply over the two decades between surveys - from pH 5.81 to 5.19 (p < 0.001), while dry farmlands in the northern sampling area fell slightly (from pH 8.15 to 7.82; p < 0.001). The mean SOM content of dry farmland soil rose in both areas and the mean SOM of paddy fields in the southern area also rose (all p < 0.001). Woodland soil pH in the south showed an increase from 4.71 to 5.29 (p < 0.001) but no significant difference was measured in the woodlands of the northern area, although the trend increased. The SOM content of woodland top soils rose in the northern (p = 0.003) and southern (p < 0.001) study areas. The implications and potential causes of these changes over the two decade timespan between surveys are discussed and suggestions made as to how large scale soil sampling campaigns can be designed to monitor for changes and potential controlling factors.

Adaptation of soil fungi to heavy metal contamination in paddy fields-a case study in eastern China

Soil fungi have been widely studied, but the effects of heavy metal contamination at various levels as well as the abundance and diversity of heavy metal tolerant fungi in the contaminated paddy soils are still unknown. The purpose of this study is to analyze the adaptability of fungi at different levels of heavy metal contamination to identify species that have strong adaptability to heavy metals. In this research, the technology of high-throughput sequencing was applied to study fungal communities in severe level (SL), moderate level (ML), light level (LL), and clean level (CL) for soil samples polluted by heavy metal, as well as to analyze the relations between environmental variables and fungal communities. The spearman analysis showed that 6 dominant fungal phyla and 18 dominant fungal genera were significantly correlated with these environmental variables. The α-diversity indexes of the soil fungal community from SL, ML, and CL were, mostly, drastically higher than the LL samples (p < 0.05). Meanwhile, Ascomycota, the main fungal phylum, was spotted to yield a strong tolerance towards heavy metals, especially in ML. The most dominant genera of tolerant fungi in this area, which are Aspergillus, Penicillium, and Fusarium, could absorb and transport the heavy metals with the help of nutrients under certain heavy metal contamination levels. Therefore, this study indicated that some fungi, which have strong biodegradability on heavy metals, can reduce toxicity of heavy metals and create a proper soil environment to grow food crops. Graphical abstract.

Effect of long term fertilization management strategies on methane emissions and rice yield

Optimum fertilization is an efficient method to maintain rice yield and reduce N-losses. It is essential though to evaluate methane emissions from paddy fields, to further understand its impact on greenhouse gas budget. Therefore, a field experiment was conducted to investigate the effect of long-term optimum fertilization on CH₄ emissions and rice yield. We collected data in the 7th and 8th year from a field experiment initiated in 2010. Four optimum fertilization strategies, reduced N-fertilizer and zero-P treatment (RNP, 200 kg N/ha), sulfur-coated urea combined with uncoated urea treatment (SCU, 200 kg N/ha), organic fertilizer combined chemical fertilizer treatment (OCN, 200 kg N/ha), organic fertilizer treatment (OF, 200 kg N/ha); and two controls, the farmers' N management (FN, 270 kg N/ha) and zero-N treatment (N0), were employed. The results showed the rice yields achieved for the optimum fertilization treatments (RNP, SCU, OCN, and OF) were similar with those for the FN. No significant differences in CH₄ emissions among all treatments. Cumulative seasonal CH₄ emissions were negatively correlated with grain yield (P < 0.05). In the RNP and SCU treatments, soil available K, mcrA gene and available P were the key variables affecting CH₄ emissions; soil available K, available P and SOC contents were the key emissions factors for OCN and OF treatments. The SCU achieved the highest rice yield and lowest CH₄ emission intensity among optimum fertilization treatments. These results suggest that long-term application of sulfur-coated urea combined with uncoated urea can maintain rice yield and reduce methane emissions from rice paddies.

Developing a management-oriented simulation model of pesticide emissions for use in the life cycle assessment of paddy rice cultivation

Management of the negative environmental impact of pesticides used in paddy fields requires the quantification of the emissions reaching relevant environmental compartments and the determination of the factors that influence such emissions. The present study is the first to develop a simulation model for estimating the fractions emitted to the air (f_air), surface water (f_sw), and the fractions that leach vertically (f_vl) of pesticides applied in paddy fields for life cycle assessments (LCA). The emission quantification methods are based on the properties of the active ingredients of the pesticides and management aspects such as formulations (granular or liquid) and the water-holding period following application. The emission fractions of 37 pesticides used commonly in Japan were estimated using the presented model. A sensitivity analysis was conducted by adjusting parameters applied in the model to reveal their influence on the emission fractions. The parameters influencing f_air included the formulation applied, while the water-holding period and waterproofing of the levee influenced f_sw regardless of the formulation adopted, suggesting that local management could effectively reduce emissions to surface water. In addition, soil organic carbon content influenced f_vl greatly, suggesting considerable regional variation in the emission factor. The developed model is expected to greatly improve the realism of impact assessment of pesticide in LCAs for paddy rice cultivation, considering it fills a gap in the fate model used in LCAs to estimate pesticide emissions to air, surface water, and soil in paddy fields.

Year-to-year climate variability affects methane emission from paddy fields under irrigated conditions

A field experiment in Yangtze River delta of China was conducted to investigate the effects of four treatments (conventional chemical fertilizers (CF), chemical fertilizers incorporated with wheat straw (CF-WS), slow-release urea fertilizer incorporated with wheat straw (SCF-WS), and no fertilizers (Non-F)) on methane emission, rice grain yield, and straw biomass in three (2013, 2014, and 2015) consecutive growing seasons. Similar air temperature and precipitation were recorded in 2014 and 2015 normal seasons; however, the 2013 season with a 2.1-2.3 °C higher mean air temperature and a 61-64% lower precipitation than the normal seasons was considered as a warm-and-dry season.No significant differences in seasonal total methane emissions across all treatments were observed in 2014 and 2015 seasons. Seasonal total methane emissions were 58-294% higher in 2013 season compared to 2014 and 2015 seasons, which suggests that higher ambient temperatures increase methane emissions from paddy fields. The enhanced methane emission was seen mainly during the mid-to-late growth stages after mid-season drainage. CF-WS and SCF-WS significantly increased methane emissions before mid-season drainage relative to CF, and thereby enhanced methane emissions in both normal and warm-and-dry seasons in a seasonal scale.While rice grain yield in each treatment in 2013 season was close to or lower than that in 2014 and 2015 seasons, straw biomass in CF, CF-WS, and SCF-WS in 2013 season were significantly higher than those in 2014 and 2015 seasons, by both 29%, 33-41%, and 15-16%, respectively. Enhancements in methane emissions at higher air temperatures were significantly correlated to its greater straw biomass. Furthermore, combination of slow-release urea fertilizer and straw by ploughing can significantly increase above-ground biomass yields and nitrogen use efficiency in both normal and warm-and-dry seasons.

Comparing ammonia volatilization between conventional and slow-release nitrogen fertilizers in paddy fields in the Taihu Lake region

Pollution arising from ammonia volatilization in paddy fields could be reduced by using slow-release nitrogen fertilizers. In recent years, slow-release nitrogen fertilizers have been commonly used to replace conventional nitrogen fertilizers in the Taihu Lake region to reduce ammonia volatilization and improve nitrogen-use efficiency. To compare ammonia volatilization losses and examine the effects of different factors (N rates, types, field water NH₄⁺, pH, and rainfall) between conventional nitrogen fertilizer and slow-release nitrogen fertilizer, paddy field experiments were conducted using conventional urea and sulfur-coated urea (SCU) fertilizers. The results indicated that ammonia volatilization flux positively increased with N application rate following an exponent function and depended on field water NH₄⁺ concentration and pH. The ammonia volatilization under SCU treatment was 37.95-70.48 kg/hm², accounting for 40.66-52.86% of the fertilizer application rate. Compared with the same N input, the ammonia volatilization loss rate was 11.53-25.33% lower under the SCU treatment. Besides, SCU produced an unfavorable environment for ammonia volatilization, with a 1.15-2.61% decrease in pH and a 40.83-43.58% decrease in field water NH₄⁺ concentration.

Geographical variation in arsenic, cadmium, and lead of soils and rice in the major rice producing regions of China

Rapid industrialization and urbanization have accelerated the contamination of paddy soils with potentially toxic elements (PTEs). However, the status and the key factors responsible for the geographical variation in PTE concentrations in rice remain poorly understood. Here, a total of 113 pairs of soil and rice plant samples were collected from 19 provinces in four major rice producing areas of China to assess the geographical variation in total arsenic (As), cadmium (Cd) and lead (Pb) concentrations in the soil-rice system. Average total concentrations of As, Cd and Pb were 11.8, 0.45 and 25.7 mg kg^-1, respectively, in the soils and 0.089, 0.087 and 0.036 mg kg^-1 in the polished rice. The national maximum allowable concentrations of total soil As and Cd were exceeded in 6.19 and 33.6% of soils and that of Cd was exceeded in 7.96% of polished rice and no polished rice exceed the Pb limit. The As, Cd and Pb concentrations of rice were significantly and positively correlated (p < 0.05) with their corresponding soil available concentrations rather than with their soil total concentrations. Due to the combined effects of local rice varieties, cultivation of varieties with high Cd translocation factors and high Cd availability in acid soils, the highest rice Cd risk occurred in south China. The Cd concentrations in polished rice exceeded the maximum allowable by 4.0 and 15.8% in uncontaminated and contaminated soils, respectively. Results from 113 fixed samples may represent the actual current As, Cd and Pb status of rice in the main rice production areas nationally as they were very consistent with 574 random samples. In view of the high Cd contamination risk in acid soils of south China, countermeasures are needed to minimize Cd accumulation in rice crops in this region.

Effects of biochar addition on the NEE and soil organic carbon content of paddy fields under water-saving irrigation

The addition of biochar has been reported as a strategy for improving soil fertility, crop productivity, and carbon sequestration. However, information regarding the effects of biochar on the carbon cycle in paddy fields under water-saving irrigation remains limited. Thus, a field experiment was conducted to investigate the effects of biochar addition on the net ecosystem exchange (NEE) of CO₂ and soil organic carbon (SOC) content of paddy fields under water-saving irrigation in the Taihu Lake region of China. Four treatments were applied: controlled irrigation (CI) without biochar addition as the control (CA), CI with biochar addition at a rate of 20 t·ha^-1 (CB), CI with biochar addition at a rate of 40 t·ha^-1 (CC), and flooding irrigation (FI) with biochar addition at a rate of 40 t·ha^-1 (FC). Biochar addition increased rice yield and irrigation water use efficiency (IWUE) by 24.0-36.3 and 33.4-42.5%, respectively, compared with the control. In addition, biochar addition increased the NEE of CI paddy fields. The average NEE of paddy fields under CB and CC was 2.41 and 30.6% higher than that under CA, respectively. Thus, the increasing effect of biochar addition at a rate of 40 t·ha^-1 was considerably better than those of the other treatments. Apart from biochar addition, irrigation mode was also identified as an influencing factor. CI management increased the NEE of paddy fields by 17.6% compared with FI management. Compared with CA, CB increased total net CO₂ absorption by 10.0%, whereas CC decreased total net CO₂ absorption by 13.8%. Biochar addition also increased SOC, dissolved organic carbon, and microbial biomass carbon contents. Therefore, the joint regulation of biochar addition and water-saving irrigation is a good technique for maintaining rice yield, increasing IWUE, and promoting soil fertility. Furthermore, when amended at the rate of 20 t·ha^-1, biochar addition will be a good strategy for sequestering carbon in paddy fields.

Geographical pattern of methanogenesis in paddy and wetland soils across eastern China

Large variation of CH₄ emissions from paddy and wetland ecosystems exists across different geographical locations in China. To obtain mechanistic understanding of this variation, we investigated the dynamics of methanogenesis over the course of glucose degradation in fourteen paddy field soils and five wetland soils collected from different regions of China. The results revealed that the maximal rate (2-3 mM per day) and the total amount (25-30 mM) of CH₄ produced were similar across soil samples. The lag phase of methanogenesis, however, differed substantially with the shortest lag phase of 4 days in a paddy soil from north China and the longest of 32 days in a soil from south China, and this difference reflected a general geographical trend among all soils tested. Nitrate was reduced completely within 4 days in all soils. The reduction of Fe(III) and sulfate was completed after 21 days and 29 days, respectively. The depletion time of Fe(III) and sulfate were positively correlated with the lag phase of methanogenesis. Competition for common substrates between methanogens and iron and sulfate reducers, however, does not explain this coincidence because a slow production of CH₄ was detected at the very beginning. It appears that the geographical variations in methanogenesis and the reduction of ferric iron and sulfate are related to the variation in soil pH but not to temperature, soil organic C and nutrient conditions in paddy and wetland soils across eastern China.

Were the sharp declines of dragonfly populations in the 1990s in Japan caused by fipronil and imidacloprid? An analysis of Hill's causality for the case of Sympetrum frequens

Neonicotinoids and fipronil are the most widely used insecticides in the world. Previous studies showed that these compounds have high toxicity to a wide taxonomic range of non-target invertebrates. In rice cultivation, they are frequently used for nursery-box treatment of rice seedlings. The use of fipronil and neonicotinoid imidacloprid is suspected to be the main cause of population declines of red dragonflies, in particular Sympetrum frequens, because they have high lethal toxicity to dragonfly nymphs and the timing of the insecticides' introduction in Japan (i.e., the late 1990s) overlapped with the sharp population declines. However, a causal link between application of these insecticides and population declines of the dragonflies remains unclear. Therefore, we estimated the amount of the insecticides applied for nursery-box treatment of rice seedlings and analyzed currently available information to evaluate the causality between fipronil and imidacloprid usage and population decline of S. frequens using Hill's causality criteria. Based on our scoring of Hill's nine criteria, the strongest lines of evidence were strength, plausibility, and coherence, whereas the weakest were temporality and biological gradient. We conclude that the use of these insecticides, particularly fipronil, was a major cause of the declines of S. frequens in Japan in the 1990s, with a high degree of certainty. The existing information and our analyses, however, do not allow us to exclude the possibility that some agronomic practices (e.g., midsummer drainage or crop rotation) that can severely limit the survival of aquatic nymphs also played a role in the dragonfly's decline.

Differences in the digestive enzyme activity, intestinal mucosa and microbial community in loach cultivated in two separate environments

BACKGROUND

Fish culture in rice paddies can contribute to increasing yields of rice and surplus fish products. Environmental impacts and food-safety issues have become important topics in aquaculture, and organic foods currently were paid attention by researchers and industry practitioners. But the mechanism of differences in quality of Loach (Paramisgurnus dabryanus) reared in rice fields and ponds remains largely uncharacterized. In this study,digestive enzyme activity, intestinal mucosa cells and the gut microbial community of loach were determined under the two separate cultivation modes.

RESULTS

The levels of intestinal digestive enzyme activity of fish reared in the paddy-cultivated mode (PACM) were higher (P < 0.05) than those in the pond-cultivated mode (POCM). It was extremely significant (P < 0.01) for the activity of lipase in the liver, foregut and midgut, and for the activities of amylase and trypsin in the hindgut. Acid mucous cells in the loach foregut in PACM were fewer than in POCM (P < 0.01). In summer, the abundance of the Firmicutes, Lactobacillus spp., Aeromonas hydrophila, Enterobacteriaceae and Streptococcus spp. in loach intestinal mucosa in PACM was higher than in POCM. In fall, the abundance of total bacteria, the Bacteroidetes, Bifidobacterium and Enterobacteriaceae in the intestinal mucosa in PACM was likewise higher than in POCM. These differences were significant (P < 0.05 or P < 0.01) between loach in the two separate culture modes for all microorganisms except for A. hydrophila and Streptococcus spp. In addition, quantitative PCR assays showed that some microorganisms presented consistently similar abundances in the gut as in the culture water.

CONCLUSIONS

These results showed some enzymatic activities involved in digestion in liver and intestine of loach in PACM were higher than those in POCM, as using digestive enzyme analysis and histological observation of intestinal sections. These findings suggest most of the microorganisms examined in the gut mucosa of loach in the two culture modes significantly differed in abundance between summer and fall. However, some pathogenic bacteria in the gut, particularly A. hydrophila, presented lower abundance in PACM in fall, yet did not differ in abundance between loach in the two cultivation modes.

Organic fertilizer application increases the soil respiration and net ecosystem carbon dioxide absorption of paddy fields under water-saving irrigation

Quantifying carbon sequestration in paddy soil is necessary to understand the effect of agricultural practices on carbon cycles. The objective of this study was to assess the effect of organic fertilizer addition (MF) on the soil respiration and net ecosystem carbon dioxide (CO₂) absorption of paddy fields under water-saving irrigation (CI) in the Taihu Lake Region of China during the 2014 and 2015 rice-growing seasons. Compared with the traditional fertilizer and water management (FC), the joint regulation of CI and MF (CM) significantly increased the rice yields and irrigation water use efficiencies of paddy fields by 4.02~5.08 and 83.54~109.97% (p < 0.05). The effects of organic fertilizer addition on soil respiration and net ecosystem CO₂ absorption rates showed inter-annual differences. CM paddy fields showed a higher soil respiration and net CO₂ absorption rates during some periods of the rice growth stage in the first year and during most periods of the rice growth stage in the second year. These fields also had significantly higher total CO₂ emission through soil respiration (total R_soil) and total net CO₂ absorption compared with FC paddy fields (p < 0.05). The total R_soil and net ecosystem CO₂ absorption of CM paddy fields were 67.39~91.55 and 129.41~113.75 mol m^-2, which were 27.66~135.52 and 12.96~31.66% higher than those of FC paddy fields. The interaction between water and fertilizer management had significant effects on total net ecosystem CO₂ absorption. The frequent alternate wet-dry cycles of CI paddy fields increased the soil respiration and reduced the net CO₂ absorption. Organic fertilizer promoted the soil respiration of paddy soil but also increased its net CO₂ absorption and organic carbon content. Therefore, the joint regulation of water-saving irrigation and organic fertilizer is an effective measure for maintaining yield, increasing irrigation water use efficiency, mitigating CO₂ emission, and promoting paddy soil fertility.

Molecular ecological perspective of methanogenic archaeal community in rice agroecosystem

Methane leads to global warming owing to its warming potential higher than carbon dioxide (CO₂). Rice fields represent the major source of methane (CH₄) emission as the recent estimates range from 34 to 112 Tg CH₄ per year. Biogenic methane is produced by anaerobic methanogenic archaea. Advances in high-throughput sequencing technologies and isolation methodologies enabled investigators to decipher methanogens to be unexpectedly diverse in phylogeny and ecology. Exploring the link between biogeochemical methane cycling and methanogen community dynamics can, therefore, provide a more effective mechanistic understanding of CH₄ emission from rice fields. In this review, we summarize the current knowledge on the diversity and activity of methanogens, factors controlling their ecology, possible interactions between rice plants and methanogens, and their potential involvement in the source relationship of greenhouse gas emissions from rice fields.

Organic carbon and nitrogen availability determine bacterial community composition in paddy fields of the Indo-Gangetic plain

Soil quality is an important factor and maintained by inhabited microorganisms. Soil physicochemical characteristics determine indigenous microbial population and rice provides food security to major population of the world. Therefore, this study aimed to assess the impact of physicochemical variables on bacterial community composition and diversity in conventional paddy fields which could reflect a real picture of the bacterial communities operating in the paddy agro-ecosystem. To fulfill the objective; soil physicochemical characterization, bacterial community composition and diversity analysis was carried out using culture-independent PCR-DGGE method from twenty soils distributed across eight districts. Bacterial communities were grouped into three clusters based on UPGMA cluster analysis of DGGE banding pattern. The linkage of measured physicochemical variables with bacterial community composition was analyzed by canonical correspondence analysis (CCA). CCA ordination biplot results were similar to UPGMA cluster analysis. High levels of species-environment correlations (0.989 and 0.959) were observed and the largest proportion of species data variability was explained by total organic carbon (TOC), available nitrogen, total nitrogen and pH. Thus, results suggest that TOC and nitrogen are key regulators of bacterial community composition in the conventional paddy fields. Further, high diversity indices and evenness values demonstrated heterogeneity and co-abundance of the bacterial communities.

Vertical distribution of denitrification end-products in paddy soils

Knowledge of denitrification process and its end-product at various depths of paddy soil is very important for our understanding of its role in mitigating reactive N and indirect nitrous oxide (N₂O) emission. In this study, the end-products of denitrification were determined at four depths in a long-term field lysimeter experiment in southeast China over a rice season. Three treatments were included: (1) chemical fertilizer (NPK); (2) NPK plus pig manure (NPKM); and (3) NPK plus straw (NPKS). The concentration of dissolved N₂O increased with soil depth across all treatments and the highest concentration of excess dinitrogen (N₂) was observed at 0.2m depth, as was the highest dissolved organic carbon (DOC) content. Denitrification reduced the amount of nitrate by 48-54% in the paddy soil profile, especially at 0.2m depth (68-88%), whereas the lower reduction of NO₃^- (17-44%) in the subsoil (at 0.6 and 0.8m depth) was accompanied by a higher concentration of NO₃^-. Our results demonstrated that DOC was the major limiting factor of denitrification in the subsoil. The application of pig manure markedly increased the amount of DOC in the surface soil, resulting in a high rate of denitrification, whereas the addition of straw had no effect on denitrification. The indirect emission factors for N₂O (EF5-g, 0.001-0.006) were comparable with the default value (0.0025) reported by the Intergovernmental Panel on Climate Change. The low N₂O production was probably caused by the complete reduction of N₂O to N₂, as reflected by the lower N₂O/(N₂O+N₂) ratios in the paddy soil profile. Although the surface soil was identified as a hotspot for denitrification, a considerable amount of excess N₂ was observed in the subsoil for all three treatments. We therefore conclude that the loss of N through denitrification may be significantly underestimated if only the surface soil is considered.

Reducing nitrogen runoff from paddy fields with arbuscular mycorrhizal fungi under different fertilizer regimes

Nitrogen (N) runoff from paddy fields serves as one of the main sources of water pollution. Our aim was to reduce N runoff from paddy fields by fertilizer management and inoculation with arbuscular mycorrhizal fungi (AMF). In northeast China, Shuangcheng city in Heilongjiang province, a field experiment was conducted, using rice provided with 0%, 20%, 40%, 60%, 80%, and 100% of the local norm of fertilization (including N, phosphorus and potassium), with or without inoculation with Glomus mosseae. The volume, concentrations of total N (TN), dissolved N (DN) and particulate N (PN) of runoff water were measured. We found that the local norm of fertilization led to 18.9kg/ha of N runoff during rice growing season, with DN accounting for 60%-70%. We also found that reduction in fertilization by 20% cut down TN runoff by 8.2% while AMF inoculation decreased N runoff at each fertilizer level and this effect was inhibited by high fertilization. The combination of inoculation with AMF and 80% of the local norm of fertilization was observed to reduce N runoff by 27.2%. Conclusively, we suggested that the contribution of AMF inoculation combined with decreasing fertilization should get more attention to slow down water eutrophication by reducing N runoff from paddy fields.

Influence of green waste compost on azimsulfuron dissipation and soil functions under oxic and anoxic conditions

Concerns have been raised over the sustainability of intensive rice cultivation, where the use of chemical fertilizers and pesticides has been associated with numerous environmental problems. The objective of this study was to test the effect of the herbicide azimsulfuron on important soil functions as affected by amendment with a byproduct of the olive oil industry. Soil was collected from a Mediterranean rice field. Part of it was amended with alperujo compost (AC). Amended and unamended soils were incubated for 43days in presence or not of azimsulfuron, under anoxic-flooded (AF) and oxic-unflooded (OU) conditions. We monitored the dissipation of the herbicide azimsulfuron, C mineralization, soil microbial biomass (SMB) and dissolved organic carbon (DOC) content and its nature. Under AF conditions, the application of compost produced an increase in the dissipation of the herbicide (up to 12.4%). It was related with the higher DOC content, 4 times higher than under OU conditions. Though increases in carbon turnover (under AF and OU conditions) and reduction of SMBC after herbicide application (only under AF conditions) were observed, the differences were not statistically significant. The application of this organic amendment is presented as an efficient management strategy to increase C turnover in agricultural soils and reduce some of the negative effects derived from the application of azimsulfuron under flooded conditions.

Biologically derived fertilizer: A multifaceted bio-tool in methane mitigation

Methane emissions are affected by agricultural practices. Agriculture has increased in scale and intensity because of greater food, feed and energy demands. The application of chemical fertilizers in agriculture, particularly in paddy fields, has contributed to increased atmospheric methane emissions. Using organic fertilizers may improve crop yields and the methane sink potential within agricultural systems, which may be further improved when combined with beneficial microbes (i.e. biofertilizers) that improve the activity of methane oxidizing bacteria such as methanotrophs. Biofertilizers may be an effective tool for agriculture that is environmentally beneficial compared to conventional inorganic fertilizers. This review highlights and discusses the interplay between ammonia and methane oxidizing bacteria, the potential interactions of microbial communities with microbially-enriched organic amendments and the possible role of these biofertilizers in augmenting the methane sink potential of soils. It is suggested that biofertilizer applications should not only be investigated in terms of sustainable agriculture productivity and environmental management, but also in terms of their effects on methanogen and methanotroph populations.

Can arbuscular mycorrhiza and fertilizer management reduce phosphorus runoff from paddy fields?

Our study sought to assess how much phosphorus (P) runoff from paddy fields could be cut down by fertilizer management and inoculation with arbuscular mycorrhizal fungi. A field experiment was conducted in Lalin River basin, in the northeast China: six nitrogen-phosphorus-potassium fertilizer levels were provided (0, 20%, 40%, 60%, 80%, and 100% of the recommended fertilizer supply), with or without inoculation with Glomus mosseae. The volume and concentrations of particle P (PP) and dissolved P (DP) were measured for each runoff during the rice growing season. It was found that the seasonal P runoff, including DP and PP, under the local fertilization was 3.7 kg/ha, with PP, rather than DP, being the main form of P in runoff water. Additionally, the seasonal P runoff dropped only by 8.9% when fertilization decreased by 20%; rice yields decreased with declining fertilization. We also found that inoculation increased rice yields and decreased P runoff at each fertilizer level and these effects were lower under higher fertilization. Conclusively, while rice yields were guaranteed arbuscular mycorrhizal inoculation and fertilizer management would play a key role in reducing P runoff from paddy fields.