Straw incorporation into microplastic-contaminated soil can reduce greenhouse gas emissions by enhancing soil enzyme activities and microbial community structure

Research on soil bacterial community assembly and function under different straw returning practices in arid and semi-arid agricultural ecosystems over multiple years

Introduction

Straw return has gained attention for its potential to improve soil quality and crop yields, particularly in semi-arid regions like the Tumu Chuan Plain Irrigation Area. Soil bacteria play a crucial role in regulating soil biological processes, and understanding how straw return affects bacterial populations can guide better agricultural management practices.

Methods

We investigated the impact of continuous straw return on soil bacterial communities using 16S rRNA gene sequencing. Four treatments were applied: Farmers' shallow rotation (CK), straw incorporated with deep tillage (DPR), straw incorporated with subsoiling (SSR), and no-tillage mulching straw return (NTR). Bacterial community structure, metabolic pathways, and assembly mechanisms were analyzed using Bugbase and PICRUSt2 for phenotypic and metabolic pathway predictions.

Results

The study found that straw return practices significantly altered the relative abundance and life history strategies of bacterial phyla, mainly influenced by soil organic matter (SOM) and enzyme activity. The K-strategist to r-strategist ratio was highest in CK (2.06) and lowest in SSR (1.89). DPR and NTR treatments significantly changed bacterial community structure compared to CK (p < 0.05), resembling SSR. Predictions showed that DPR and NTR enhanced carbohydrate and amino acid metabolism and promoted more stable bacterial networks, with homogenous selection and drift effects. Bacterial aggregation in all treatments was driven by random processes, with varying aggregation levels: CK (20%), DPR (38.6%), SSR (16.5%), and NTR (30.7%).

Discussion

The study demonstrates that continuous straw return practices significantly impact soil bacterial communities. DPR and NTR notably improved microbial diversity, bacterial cooperation, and ecosystem stability. These findings provide valuable insights for sustainable agricultural practices in semi-arid regions, enhancing soil microbial ecology and soil health through strategic straw return.

Response of microbial structure characteristics and enzyme activity to different altitude

Studying the influence of altitude on soil microorganisms and enzyme activity is crucial for protecting land environmental changes in karst areas. The study focused on the soil at different altitudes in the Luganyou area, Baise City. The Multidimensional statistical analysis encompassed assessments of soil nutrient properties, soil microorganisms, and soil enzymes across these altitude gradients. The results indicate that enzyme activity in the 0-15 cm soil layer surpasses that in the 15-30 cm soil layer. Catalase, urease, and soil cellulase exhibit consistent trends on both shady and sunny slopes, increasing with altitude. Alpha diversity analysis shows that the abundance and diversity of bacteria and fungi on the shaded side exhibit the following trend of low altitude > high altitude. In contrast, on the sunny side, bacterial richness displayed a decreasing-increasing pattern with rising altitude, while the diversity trend mirrored that of the shady side. Overall, fungal richness on the sunny side exhibited a slight decrease, whereas diversity increased with altitude. Across shady slopes, overall soil nutrient levels tend to increase with altitude. The comprehensive ecological stoichiometry analysis of the soil indicated an elevated C/N ratio, while the N/P and C/P ratios were relatively low on both shady and sunny slopes. These findings suggest that the study area as a whole is constrained by N availability. Correlation and redundancy analyses revealed that organic matter is the primary factor influencing enzyme activity in shaded slopes, while organic matter and available phosphorus are the key factors affecting enzyme activity in sunny slopes. The key nutrient factors affecting microorganisms include total nitrogen content, organic matter content, as well as enzyme factors such as invertase, catalase, and acid phosphatase. In summary, the study revealed a negative correlation between microbial diversity on shady slopes and altitude in the low altitude areas of southwest China (160-380 m). Additionally, bacterial richness on sunny slopes displayed a "low-high-low" trend, underscoring the significance of organic matter (R 2 = 0.683) and available phosphorus (p < 0.05) as pivotal drivers of microbial communities. These findings aim to offer a theoretical framework for guiding crop cultivation, land management, and enhancement strategies in southwestern China.

Response of soil heavy metal forms and bioavailability to the application of microplastics across five years in different soil types

Microplastics (MPs) potentially alter physicochemical and transformation of heavy metals (HMs) in soils, which may depend on the specific characteristics of soil types. However, the dynamical and long-term mechanisms remain to be elucidated. A five-year incubation experiment was conducted to evaluate the influence of MPs on the chemical speciation of Pb, Ni, Cu, Cr, Cd, and As in the meadow, tidal, cinnamon, saline-alkali, and brown soils. From the first year to the fifth year, the clay value of the meadow, tidal, cinnamon, and saline-alkali soils was increased by 31.35 %, 9.63 %, 30.12 %, and 33.12 %, respectively; the pH values of the cinnamon and saline-alkali soils were increased by 15.02 % and 15.86 %, respectively. Besides, speciation distribution results suggested that the application of MPs reduced the liable available (LB) form (F2-dissolved and F3-ion exchangeable) of HMs and increased the potentially available (PB) form (F5-minerals and F6-organic-bound fraction) of HMs in all soils. Compared with other forms, F2 HMs fraction was the most responsive to MPs. Furthermore, the average bioconcentration factor (BCF) of Cr and Pb decreased by 73.75 % and 70.41 % in soils, respectively. Interestingly, soil type showed more impact on the form of HMs, which was associated with the different physicochemical parameters of soils, while application time displayed more impact on the bioavailability of HMs. Moreover, our results suggested that soils with higher clay content and pH values (such as cinnamon and saline-alkali soils) may mitigate the bioavailability of HMs more effectively in the presence of MPs, while soils with lower clay content may be more vulnerable to HMs contamination over time. This work highlights the importance of long-term monitoring of the impact of MPs on HMs dynamics for effective mitigation of soil contamination risks. Our study provides valuable guidance for soil remediation strategies and environmental quality management across different soil types.

PBAT biodegradable microplastics enhanced organic matter decomposition capacity and CO2 emission in soils with and without straw residue

Recent studies show that biodegradable microplastics (BMPs) could increase soil CO2 emission, but whether altered carbon emission results from modified soil organic matter (SOM) decomposition remains underexplored. In this study, the effect and mechanisms of BMPs on CO2 emission from soil were investigated, using poly(butylene adipate-co-terephthalate) (PBAT, the main component of agricultural film) as an example. Considering that straw returning is a common agronomic measure which may interact with microplastics through affecting microbial activity, both soils with and without wheat straw were included. After 120 d, 1 % (w/w) PBAT BMPs ificantly increased cumulative CO2 emission by 1605.6 and 1827.7 mg C kg-1 in soils without and with straw, respectively. Cracks occurred on the surface of microplastics, indicating that CO2 was partly originated from plastic degradation. Soil dissolved organic matter (DOM) content, carbon degradation gene abundance (such as abfA, xylA and manB for hemicellulose, mnp, glx and lig for lignin, and chiA for chitin) and enzyme activities increased, which significantly positively correlated with CO2 emission rate (p < 0.05), suggesting that PBAT enhanced carbon emission by stimulating the decomposition of SOM (and possibly the newly added straw) via co-metabolism and nitrogen mining. This is supported by DOM molecular composition analysis which also demonstrated stimulated turnover of carbohydrates, amino sugars and lignin following PBAT addition. The findings highlight the potential of BMPs to affect SOM stability and carbon emission.

Microplastic effects on carbon cycling in terrestrial soil ecosystems: Storage, formation, mineralization, and microbial mechanisms

Soil is the largest environmental reservoir of microplastics (MPs) on the earth. Incremental accumulation of MPs in the soil can cause significant changes in soil physicochemical and microbial traits, which may in turn interfere with soil biogeochemical processes such as carbon cycling. With published research regarding MPs impacts on soil carbon cycling growing rapidly, a systematic review summarizing the current knowledge and highlighting future research needs is warranted. As carbon-rich polymers, MPs can contribute to soil organic carbon (SOC) storage via degradation and leaching. MPs can also affect the humification of dissolved organic matters (DOM), consequently influencing the stability of SOC. Exposure to MPs can cause substantial impacts on the growth performance, litter decomposition, and root secretion of terrestrial plants as well as soil microbial carbon turnover, inducing changes in the formation of SOC. The presence of MPs has contrasting effects on the emissions of both CO2 and CH4 from the soil. The diverse effects of MPs on soil carbon metabolism could be partly attributed to the varying changes in soil microbial community structure, functional gene expression, and enzyme activity under MPs exposure. Further research is still highly needed to clarify the pathways of MPs impacts on soil carbon cycling and the driving biological and physicochemical factors behind these processes.

Biochar for sustainable agriculture: Improved soil carbon storage and reduced emissions on cropland

Climate change, driven by excessive greenhouse gas (GHG) emissions from agricultural land, poses a serious threat to ecological security. It is now understood that significant differences exist in the responses of soil GHG emissions and soil carbon (C) sequestration to the application of different C-based materials (i.e., straw, organic manure (OM), and biochar). Therefore, elucidating the mechanisms by which differences in the properties of these materials affect soil GHG emissions is essential to comprehensively investigate the mechanisms through which variations in material properties influence soil GHG emissions. Herein, we conducted a field experiment to evaluate the responses of soil GHG emissions to cropland application of different C-based materials and employed molecular modeling calculations to explore the mechanisms by which differences in the properties of these materials affect soil GHG emissions. The results showed that biochar demonstrated superior resistance to biochemical decomposition and soil GHG adsorption capacity, leading to a significant reduction in soil GHG emissions due to its excellent physicochemical properties. The active surface properties of straw and OM enhanced their interaction with decomposing enzymes and accelerated their biochemical decomposition. Wheat-maize rotation with biochar application reduced CO2 emissions by 1089.8 kg CO2eq ha-1 and increased soil organic carbon by 141.8% compared to the control after one year. Collectively, these results contribute to the optimization of cropland application strategies for crop residues to balance soil C sequestration and soil GHG emissions, and to ensure sustainable agriculture and ecological security.

Effects of Trichoderma harzianum on Growth and Rhizosphere Microbial Community of Continuous Cropping Lagenaria siceraria

This study analyzed the effects of Trichoderma harzianum on the growth of continuous cropping Lagenaria siceraria and the physical and chemical properties of rhizosphere soil and microbial community structure, using Illumina Miseq (PE300) high-throughput sequencing technology along with physiological and biochemical detection. The results indicated that after applying T. harzianum, the growth of L. siceraria was significantly promoted, with increases in plant height, fresh weight, and dry weight of 21.42%, 24.5%, and 4.5%, respectively. The pH of the rhizosphere soil decreased from 7.78 to 7.51, while the electrical conductivity, the available phosphorus, the available potassium, and the total nitrogen were markedly higher compared to the control group and increased by 13.95%, 22.54%, 21.37%, and 16.41%, respectively. The activities of catalase and sucrase in the rhizosphere increased by 18.33% and 61.47%, and the content of soil organic carbon (SOC) increased by 27.39%, which indicated that T. harzianum could enhance soil enzyme activity and promotes the transformation of organic matter. The relative abundance of beneficial bacteria such as Pseudomonas increased, while the relative abundance of harmful fungi such as Fusarium and Podosphaera decreased significantly.

Soil nutrients and enzyme activities based on millet continuous cropping obstacles

In order to evaluate the effects of continuous cropping of millet on soil nutrients and soil enzyme activities, the present study was based on four treatments of 2 years of continuous cropping (T1), 3 years of continuous cropping (T2), 4 years of continuous cropping (T3) and rotational cropping (CK), based on 4 years of no fertilizer positioning experiments, and the soil nutrients, soil enzyme activities and millets yields were determined, respectively. The results showed that with the increase of continuous cropping years, the millet yield decreased and was significantly lower than that of rotating with legume crops, and compared with CK, the yields of T1, T2 and T3 treatments were reduced by 8.92%, 13.73% and 37.60%, respectively; the soil nitrogen and phosphorus contents were reduced, the quick-acting potassium content did not change obviously, and the soil pH was increased; Soil urease, alkaline phosphatase, sucrase and catalase activities generally showed a decreasing trend and the decrease was more significant with the increase in the number of years of continuous cropping. Therefore, in order to maintain the soil fertility and increase the millet yield, it is necessary to practice crop rotation and stubble reversal between millets and leguminous crops such as kidney beans, and to apply certain fertilizers.

Microplastic pollution promotes soil respiration: A global-scale meta-analysis

Microplastic (MP) pollution likely affects global soil carbon (C) dynamics, yet it remains uncertain how and to what extent MP influences soil respiration. Here, we report on a global meta-analysis to determine the effects of MP pollution on the soil microbiome and CO2 emission. We found that MP pollution significantly increased the contents of soil organic C (SOC) (21%) and dissolved organic C (DOC) (12%), the activity of fluorescein diacetate hydrolase (FDAse) (10%), and microbial biomass (17%), but led to a decrease in microbial diversity (3%). In particular, increases in soil C components and microbial biomass further promote CO2 emission (25%) from soil, but with a much higher effect of MPs on these emissions than on soil C components and microbial biomass. The effect could be attributed to the opposite effects of MPs on microbial biomass vs. diversity, as soil MP accumulation recruited some functionally important bacteria and provided additional C substrates for specific heterotrophic microorganisms, while inhibiting the growth of autotrophic taxa (e.g., Chloroflexi, Cyanobacteria). This study reveals that MP pollution can increase soil CO2 emission by causing shifts in the soil microbiome. These results underscore the potential importance of plastic pollution for terrestrial C fluxes, and thus climate feedbacks.

Ecological effect of microplastics on soil microbe-driven carbon circulation and greenhouse gas emission: A review

Soil organic carbon (SOC) pool, the largest part of terrestrial ecosystem, controls global terrestrial carbon balance and consequently presented carbon cycle-climate feedback in climate projections. Microplastics, (MPs, <5 mm) as common pollutants in soil ecosystems, have an obvious impact on soil-borne carbon circulation by affecting soil microbial processes, which play a central role in regulating SOC conversion. In this review, we initially presented the sources, properties and ecological risks of MPs in soil ecosystem, and then the differentiated effects of MPs on the component of SOC, including dissolved organic carbon, soil microbial biomass carbon and easily oxidized organic carbon varying with the types and concentrations of MPs, the soil types, etc. As research turns into a broader perspective, greenhouse gas emissions dominated by the mineralization of SOC coming into view since it can be significantly affected by MPs and is closely associated with soil microbial respiration. The pathways of MPs impacting soil microbes-driven carbon conversion include changing microbial community structure and composition, the functional enzyme's activity and the abundance and expression of functional genes. However, numerous uncertainties still exist regarding the microbial mechanisms in the deeper biochemical process. More comprehensive studies are necessary to explore the affected footprint and provide guidance for finding the evaluation criterion of MPs affecting climate change.

Response of soil erosion resistance to straw incorporation amount in the black soil region of Northeast China

Straw incorporation has been considered as an effective environmental management application to improve soil erosion resistance (SER) and organic carbon sequestration. SER is useful to evaluate soil erosion subjected to concentrated flow. Nevertheless, few studies have been performed to examine how SER varied with the amount of straw incorporation on sloping croplands in high latitude and cool regions. In the current study, the fixed bed scouring tests were conducted in a large hydraulic flume using undisturbed soil samples taken from Hebei small watershed in the black soil region of Northeast China. The response of SER to different straw incorporation amounts (0, 1.125, 2.25, 4.5, 6.75, 9.0 and 13.5 t ha-1) was quantified after three months of straw decomposition. The major influencing factors and the corresponding mechanisms were determined. The findings demonstrated that rill erodibility firstly decreased exponentially with straw incorporation amount (R2 = 0.93), while it slightly increased when straw incorporation amount was more than 9.0 t ha-1. Critical shear stress firstly increased logarithmically (R2 = 0.90) and then slightly decreased when the amount exceeded 9.0 t ha-1. Compared to the treatment of 0 t ha-1, rill erodibility reduced by 17.0%-92.8% and critical shear stress increased by 59.6%-127.2% across different treatments of straw incorporation. Rill erodibility had significant and negative correlations with soil organic matter content, aggregate stability, cohesion, root mass density, straw mass density and straw decomposition amount. The key mechanisms for promoting SER were derived by the direct and indirect effects of straw incorporation and its decomposition on soil physicochemical properties and crop roots. The amount of 9.0 t ha-1 was recommended as the optimum amount of straw incorporation in croplands in Northeast China. These findings are useful to understand how soil erosion resistance responds to the amount of straw incorporation and make rational environmental management policy for semi-humid and cool regions.