Hydrochar more effectively mitigated nitrous oxide emissions than pyrochar from a coastal soil of the Yellow River Delta, China

Rare Taxa as Key Drivers of Soil Multi-Nutrient Cycling Under Different Crop Types

Soil microorganisms are crucial for nutrient cycling, with abundant and rare taxa playing distinct roles. However, the mechanisms by which soil microbes influence nutrient cycling under different crop types remain unclear. In this study, we investigated the network structure, diversity, and microbial composition of croplands in the Yellow River Delta, focusing on four primary crops: soybean, maize, cotton, and sorghum. The findings revealed that the co-occurring network structure of sorghum planting-soils exhibited greater complexity than other crop types. Bacterial alpha diversity in cotton-planting soil is the highest and susceptible to environmental variations. The diversity of both rare and abundant taxa responds differently to soil nutrients depending on the crop type. While abundant taxa play a crucial role in soil multi-nutrient cycling, rare taxa are key drivers of variations in nutrient cycling expression. The diversity of rare taxa showed a strong correlation with critical nutrients. Structural equation modeling revealed that the alpha diversity of rare bacterial and fungal taxa significantly influenced the soil multi-nutrient cycling index (MNC). Specifically, higher Shannon indices of rare bacterial taxa were associated with lower MNC, while the opposite was true for soil fungi. Soil organic carbon and soil total nitrogen are the key factors influencing alpha diversity in rare bacterial and fungal taxa. Moreover, this study provides new insights into the role of rare soil microbial diversity in the nutrient cycling of agricultural ecosystems.

Variation in microbial communities and network ecological clusters driven by soil organic carbon in an inshore saline soil amended with hydrochar in Yellow River Delta, China

Char materials (e.g., hydrochar) can enhance carbon sequestration, improve soil quality and modulate soil microbial communities to recuperate soil health. However, little is known about the soil organic carbon (SOC) content, as well as the microbial communities and co-occurrence networks in response to hydrochar amendment in an inshore saline soil. Here, the effect of Sesbania cannabina (a halophyte) straw derived hydrochar (SHC) amendment on SOC and labile organic carbon (LOC) fractions and the potential associations among SOC content change, soil C-cycling enzyme activities and microbial communities were illustrated using a pot experiment. SHC effectively improved the contents of SOC and LOC, particularly particulate organic carbon (POC), and stimulated the activities of C-cycling enzymes. Furthermore, SHC induced shift in microbial community compositions and co-occurrence networks, result in decrease in relative abundance of Actinobacteriota and its corresponding ecological cluster, which may favor SOC accumulation. Functional annotation of prokaryotic taxa (FAPROTAX) analysis also revealed a decrease in microbial ecological function related to carbon degradation. These findings provided a deeper insight about the hydrochar-induced SOC enhancement and suggested an efficient approach to improve C sequestration and improve soil health in the coastal salt-affected soil.

Modified biochar affects CO2 and N2O emissions from coastal saline soil by altering soil pH and elemental stoichiometry

The application of biochar in degraded farmland improves soil productivity while achieving the recycling of agricultural waste. The collapse of the physical structure of coastal saline soils will greatly reduce the carbon sequestration potential of biochar. Phosphorus- and magnesium-modified biochar greatly improve the stability of biochar, which endows them with the potential to greatly improve the organic carbon pool of coastal saline soil. However, changes in the properties of modified biochar increase the uncertainty of microbial driven CO2 and N2O release by affecting soil chemistry properties. In this study, through laboratory soil microcosmic experiment, we investigated the effects of magnesium-modified biochar (BCMg) and phosphorus-modified biochar (BCP) on CO2 and N2O releases from coastal saline soils, and further uncovered their potential mechanisms. Compared with unapplied biochar (CK) and unmodified biochar (BC) treatment, BCMg reduced both the releases of CO2 and N2O, and BCP decreased N2O release but enhanced CO2 release. pH is the medium through which BCMg affects the release of CO2 and N2O. Specifically, BCMg increased soil pH above 8.5, which reduced the metabolic activity of the microbial community, and the abundance of bacteria directly or indirectly involved in N2O production, thereby decreasing the releases of CO2 and N2O. The amendment of BCP changed soil elemental stoichiometry causing microbial N-limitation. Increasing CO2 release and decreasing N2O release were strategies for microorganisms to cope with N-limitation. These findings suggested that BCMg is superior to BCP in mitigating greenhouse gas emissions, providing a basis for the application of modified biochar to improve the carbon pool and reduce greenhouse gas emissions of coastal saline soil.

Cattle manure hydrochar posed a higher efficiency in elevating tomato productivity and decreasing greenhouse gas emissions than plant straw hydrochar in a coastal soil

Rehabilitation of degraded soil health using high-performance and sustainable measures are urgently required for restoring soil primary productivity and mitigating greenhouse gas (GHG) emission of coastal ecosystems. However, the effect of livestock manure derived hydrochar on GHG emission and plant productivity in the coastal salt-affected soils, one of blue carbon (C) ecosystems, was poorly understood. Therefore, a cattle manure hydrochar (CHC) produced at 220 °C was prepared to explore its effects and mechanisms on CH4 and N2O emissions and tomato growth and fruit quality in a coastal soil in comparison with corresponding hydrochars derived from plant straws, i.e., sesbania straw hydrochars (SHC) and reed straw hydrochars (RHC) using a 63-day soil column experiment. The results showed that CHC posed a greater efficiency in reducing the global warming potential (GWP, 54.6 % (36.7 g/m2) vs. 45.5-45.6 % (22.2-30.6 g/m2)) than those of RHC and SHC. For the plant growth, three hydrochars at 3 % (w/w) significantly increased dry biomass of tomato shoot and fruit by 12.4-49.5 % and 48.6-165 %, respectively. Moreover, CHC showed the highest promotion effect on shoot and fruit dry biomass of tomato, followed by SHC ≈ RHC. Application of SHC, CHC and RHC significantly elevated the tomato sweetness compared with CK, with the order of CHC (54.4 %) > RHC (35.6 %) > SHC (22.1 %). Structural equation models revealed that CHC-depressed denitrification and methanogen mainly contributed to decreased GHG emissions. Increased soil phosphorus availability due to labile phosphorus supply from CHC dominantly accounted for elevated tomato growth and fruit production. Comparably, SHC-altered soil properties (e.g., decreased pH and increased total carbon content) determined variations of GHG emission and tomato growth. The findings provide the high-performance strategies to enhance soil primary productivity and mitigate GHG emissions in the blue C ecosystems.

Corn straw biochar addition elevated phosphorus availability in a coastal salt-affected soil under the conditions of different halophyte litter input and moisture contents

Improving salt-affected soil health using different strategies is of great significance for Sustainable Development Goals. The effects of biochar as a sustainable carbon negative soil amendment on phosphorous (P) pools in the degraded salt-affected soils of the of coastal wetlands (as one of the primary blue carbon ecosystems) with halophyte litter input under different water conditions (the two intrinsic characteristics of coastal wetlands) are poorly understood. Thus, a corn straw derived biochar (CBC) was added into a coastal salt-affected soil collected from the Yellow River Delta to investigate its effect on P fractions and availability under the input of three different local halophyte litters (i.e., Suaeda salsa, Imperata cylindrica and Phragmites australis) and under the unflooded and flooded water conditions. The results showed that the individual input of Suaeda salsa increased soil P availability by 28.2-40.9 %, but Imperata cylindrica and Phragmites australis had little effect on P availability. CBC individual amendment more efficiently enhanced P availability in the unflooded soil than the flooded soil. However, the co-amendment of CBC with litters showed little synergistic effect on P availability. CBC sharply increased the proportion of Ca-bound labile P fraction, but moderately lifted the proportion of Al/Fe-bound mediumly labile P fraction. CBC-enhanced P availability and altered inorganic P fractions were mainly resulted from the provision of labile inherent P by biochar, improved soil properties (i.e., increased CEC), and altered bacterial community composition (i.e., elevated abundance of P-solubilizing and phosphate-accumulating bacteria). These findings give new insights into understanding P biogeochemical cycling in the coastal salt-affected soils amended with biochars, and will be helpful to develop biochar-based technologies for enhancing P pools and improving soil health of the blue carbon ecosystems.

Wheat straw hydrochar induced negative priming effect on carbon decomposition in a coastal soil

The mechanisms underlying hydrochar-regulated soil organic carbon (SOC) decomposition in the coastal salt-affected soils were first investigated. Straw-derived hydrochar (SHC)-induced C-transformation bacterial modulation and soil aggregation enhancement primarily accounted for negative priming effects. Modification of soil properties (e.g., decreased pH and increased C/N ratios) by straw-derived pyrochar (SPC) was responsible for decreased SOC decomposition.

Nitrogen loss, nitrogen functional genes, and humification as affected by hydrochar addition during chicken manure composting

This study investigated the effect of hydrochar addition on nitrogen (N) transformation, N functional genes, and humification during chicken manure composting. The addition of 10 % hydrochar reduced cumulative ammonia (NH₃) and nitrous oxide emissions by 55.24 % and 45.30 %, respectively, and N losses by 32.07 %. Further, it increased the relative abundance of amoA while decreasing that of nirK, nirS, and nosZ in compost. Hydrochar reduces NH₃ emissions during composting owing to its acid-carbon properties that lower the pH of the composting pile and promote ammonia oxidation. Moreover, hydrochar addition enhances the humification of the composting pile and significantly increases the content of humic substances. Moreover, after hydrochar addition, the germination index of the compost product reached >80 % 10 days earlier. The results demonstrate that hydrochar is a suitable composting additive for reducing N loss and shortening the composting time.

Biochar improved the composting quality of seaweeds and cow manure mixture and altered the microbial community

The beneficial effects of biochar addition during composting have been proved for many feedstocks, like manures and crop straws. However, the effect of biochar on the quality of composting product with seaweed as the feedstock and the bacterial response has not been investigated. In this study, the wheat straw biochar addition on the quality of the composting product and the bacterial response was explored at the rate of 0-10%. The results showed that biochar addition at the optimal rate (5%, w/w) could increase the germination index and the ratio of the optical density of humic acid at 460 nm to that at 660 nm (E4/E6) of the composting product, which indicated the decreased biotoxicity and enhanced compost maturity. The significant increase of the nitrate nitrogen (NO₃ ^--N) content of the composting product proved the improvement of N cycling during composting process with biochar addition. The bacterial community of composting product was shifted and the relative abundance of some beneficial taxa (e.g., Muricauda and Woeseia) was significantly increased with biochar addition. Furthermore, the relative abundance of some bacterial genes related to amino acid metabolism and carbohydrate metabolism was also increased with biochar addition. The results of our study provided the positive effect of biochar addition on the composting of seaweed and could help to produce high quality seaweed fertilizer by composting with biochar addition.