Residue Amendment Research Articles

Effects of biochar and biogas residue amendments on N2O emission, enzyme activities and functional genes related with nitrification and denitrification during rice straw composting

This study was carried out to determine the response characteristics of N2O emission, enzyme activities, and functional gene abundances involved in nitrification/denitirification process with biochar and biogas residue amendments during rice straw composting. The results revealed that N2O release mainly occurred during the second fermentation phase. Biogas residue amendment promoted N2O emission, while biochar addition decreased its emission by 33.6%. The nirK gene was abundant through composting process. Biogas residues increased the abundance of denitrification genes, resulting in further release of N2O. Biochar enhanced nosZ gene abundance and accelerated the reduction of N2O. Nitrate reductase (NR), nitrite reductase (NiR), N2O reductase (N2OR), and ammonia monooxygenase (AMO) activities were greatly stimulated by biochar or biogas residue rather than their combined addition. Pearson regression analysis indicated that N2O emission negatively correlated with ammonium and positively correlated with nosZ, nirK, 18S rDNA, total nitrogen, and nitrate (P < 0.05).

Absence of a home-field advantage within a short-rotation arable cropping system

AimsThe home-field advantage (HFA) hypothesis predicts faster decomposition of plant residues in home soil compared to soils with different plants (away), and has been demonstrated in forest and grassland ecosystems. It remains unclear if this legacy effect applies to crop residue decomposition in arable crop rotations. Such knowledge could improve our understanding of decomposition dynamics in arable soils and may allow optimisation of crop residue amendments in arable systems by cleverly combining crop-residue rotations with crop rotations to increase the amount of residue-derived C persisting in soil.MethodsWe tested the HFA hypothesis in a reciprocal transplant experiment with mesh bags containing wheat and oilseed rape residues in soils at three stages of a short-rotation cropping system. Subsets of mesh bags were retrieved monthly for six months to determine residue decomposition rates, concomitantly measuring soil available N, microbial community structure (phospholipid fatty acid analysis), and microbial activity (Tea Bag Index protocol) to assess how plants may influence litter decomposition rates via alterations to soil biochemical properties and microbial communities.ResultsThe residues decomposed at similar rates at all rotational stages. Thorough data investigation using several statistical approaches revealed no HFA within the crop rotation. Soil microbial community structures were similar at all rotational stages.ConclusionsWe attribute the absence of an HFA to the shortness of the rotation and soil disturbance involved in intensive agricultural practices. It is therefore unlikely that appreciable benefits could be obtained in short conventionally managed arable rotations by introducing a crop-residue rotation.

Soil microbial metabolism on carbon and nitrogen transformation links the crop-residue contribution to soil organic carbon

The beneficial effect of crop residue amendment on soil organic carbon (SOC) stock and stability depends on the functional response of soil microbial communities. Here we synchronized microbial metagenomic analysis, nuclear magnetic resonance and plant-15N labeling technologies to gain understanding of how microbial metabolic processes affect SOC accumulation in responses to differences in N supply from residues. Residue amendment brought increases in the assemblage of genes involved in C-degradation profiles from labile to recalcitrant C compounds as well as N mineralization. The N mineralization genes were correlated with the C and N accumulation in the particulate and mineral-associated C pools, and plant-derived aliphatic forms of SOC. Thus, the combined C and N metabolic potential of the microbial community transforms residue into persistent organic compounds, thereby increasing C and N sequestration in stable SOC pools. This study emphasizes potential microbially mediated mechanisms by which residue N affects C sequestration in soils.

Nutrient Management Drives the Direction and Magnitude of Nitrous Oxide Flux in Crop Residue-Returned Soil Under Different Soil Moisture

Crop residues as key organic carbon inputs have the potential for soil carbon sequestration. However, previous studies have shown an inconsistent effect of residue return on the direction and magnitude of soil nitrous oxide (N2O) emission. We used a laboratory-based soil incubation study to test the response of N2O emission to crop residue type, soil moisture, and how nutrient management modulates these responses. In this study, we incorporated crop residues with different qualities (wheat, rice, soybean, and maize) at two soil moisture contents {80% field capacity (FC) and 60% FC} and under seven nutrient levels: N0P0K0 (no nutrients), N0PK, N100PK, N150PK, N100PK + manure@ 5 Mg ha−1, N100PK + biochar@ 5 Mg ha−1, and N150PK + biochar@ 5 Mg ha−1. The results demonstrated significant (p &amp;lt; 0.01) differences in the magnitude of N2O emissions among treatments. However, only the interaction effect of residue × nutrient and nutrient × moisture was significant (p &amp;lt; 0.05). N100PK and N150PK at 80% FC mitigated N2O emission by approximately 20% in wheat residue-amended soil (cf. control soil without residue). In contrast, maize residue amendment (cf. control soil) increased N2O emission by 130% under N0P0K0 and 80% FC. Residue effects were negatively correlated with the C:N ratio, and a strong positive correlation (p &amp;lt; 0.01) was obtained between N2O emission and CO2 respiration, labile carbon, mineral N, and residue total nitrogen (TN). When no nutrients were added, N2O emission was higher in residue returned soil. However, cumulative fluxes of N2O decreased by 6–17% when maize and wheat residues (cf. control soil) were applied with nutrients. Negative fluxes of N2O indicating consumption were observed in every treatment after 57 days of incubation and were most pronounced in control soil without residue and nutrients. Decreasing the soil moisture from 80% FC to 60% FC, the N2O consumption rate increased by 6.6 times across residue types and nutrient management. The regression analysis and structural equation modeling (SEM) results showed that residue TN, soil CO2 emission, NO3-N, and labile SOC were the key predictor variables and could explain 82% variability in the soil N2O emission in the Vertisols of Central India. The results suggested that nutrient addition (NPK) could alter the magnitude and direction of soil N2O flux by residue type and soil moisture by influencing the underlying soil microbial processes of the C and N cycle in the Vertisol of subtropical India.

Different contributions of ammonia oxidizing archaea and bacteria to nitrification in soils amended with equivalent amounts of either ammonium-N or organic-N

Although considerable knowledge exists on the impact of crop residues on soil N availability, information about their impact on nitrification specifically is lacking. The unique physiologies of ammonia oxidizing archaea (AOA) suggests their activities might respond differently to crop residue amendment compared to ammonia oxidizing bacteria (AOB). An incubation study was conducted on two widely distributed Oregon agricultural silt loam soils of the Walla Walla (WW) and Woodburn (CC) series, amended with a fertilizer rate of 70 μg N g−1 soil in the form of (1) NH4+-N, (2) clover stems (C:N ~ 25), or (3) clover leaves (C:N ~ 13). NH4+ accumulation in the presence of acetylene, and the relative amounts of AOA and AOB activities, in the presence of the selective AOB inhibitor, 1-octyne, were examined over a 28-31d incubation. Nitrifying activity in the crop residue-amended WW soil matched the rate of ammonification and was predominately octyne-resistant suggesting that nitrifying activity was dominated by AOA. In CC soil, the relative contributions of AOA and AOB to emergent activities was influenced by the nature of the residue and in a soil specific manner. In leaf amended CC soil, AOB contributed the majority of initial nitrifying activity over 0-7d, with the contribution of AOA activity increasing post 7d to represent 100% of the total rate. In stem amended CC soil, emergence of nitrifying and ammonifying activities were delayed, but after 7d incubation AOA activity emerged at a rate greater than the accompanying rate of ammonification. In NH4+-N amended soils, AOB activity emerged quickly at rates greater than when N was supplied by residues. The response of AOA activity to NH4+-N was soil specific being immediate and at a greater rate than leaf residue supported activity in WW soil, whereas in CC soil, AOA activity was temporally delayed (as with residue treatment) and emerged at a rate similar to that supported by leaf residue. From a practical perspective, the findings of this study show that an application of 70 μg N g−1 soil as crop residues can prevent AOB driven nitrifying activity from reaching its potential compared to an inorganic NH4+-N application, and also suggest the potential of AOA activity to be modified by residues in a soil specific manner.

Lowering the water solubility of phosphorus fertilisers impacts leaching, plant growth and residual soil phosphorus

Context Leaching of phosphorus (P) from water soluble agricultural fertilisers applied to sandy soil can adversely impact downstream water quality. Less soluble fertilisers may reduce P leaching and increase production. However, repeated application of low water soluble P (LWSP) fertiliser with high citrate soluble P (CSP) has potential to increase P leaching and P accumulation. Methods We examined the effect of LWSP fertilisers (single superphosphate, SSP; fertilisers low in water soluble P, WSP) on leaching losses in soil with low P retention/low P status; with/without bauxite residue amendment and with/without plants. Key results On low P retentive soils with a low P status, P leaching was reduced by reducing the WSP of fertiliser. Plants and soil amendment reduced P leaching further, but P loss remained proportional to WSP applied. Under field conditions, a subset of LWSP fertilisers greatly increased pasture dry matter production and increased soil test P values. Soil test P increases were positively correlated to fertiliser citrate soluble P content. Soils already containing at least 80% of the required plant available P did not require a WSP fertiliser to provide early season P. Conclusions LWSP fertilisers and soil amendment have potential to reduce P leaching in low P retentive soils. Fertiliser formulations targeting reduced WSP and high CSP require attention. Implications LWSP fertilisers may increase soil P residues reducing future P needs; however, ongoing soil testing is required for assessment of soil P accumulation, requirements and leaching potential.

Amendment of crop residue in different forms shifted micro-pore system structure and potential functionality of macroaggregates while changed their mass proportion and carbon storage of paddy topsoil

Soil organic carbon (SOC) sequestration in croplands was well known through physical protection of particulae organic carbon (POC) in macroaggregates, particularly with crop residue amendment. How different forms of crop residue could affect development of macroaggregates and their micro-pore system remained unclear. In a field experiment, a rice paddy was amended respectively with fresh (CS), manured (CM) and pyrolyzed (CB) maize residue at 10 t ha−1 in carbon equivalent, in comparison to no residue amendment (CK). In 3 years following amendment, undisturbed topsoil (0–15 cm) cores were collected under the treatments respectively. Following size fractionation of water-stable aggregates and SOC pool analysis, macroaggregate (MAC) samples were characterized with SR-µCT tomography. Compared to CK, bulk SOC storage increased by 13.9 % and 31.3 %, POC storage by 32.6 % and 63.3 % while MAC mass proportion by 7.4% and 24.5%, under CM and CB, respectively. Coincidently, total micro-porosity of MAC was increased by ca 30 % but porosity ratio of connected pores to isolated pores increased almost 1-fold though fractal dimension (FD) increased slightly, both under CM and CB. However, indices of pore throats were seen higher under CB than other treatments though micro-pore networking promoted under both CM and CB. In addition, micro-porosity and pore structure as well as POC were all observed similar between fresh residue and no residue treatments. Thus, crop residue amendment in different forms shifted micro-pore structure and potential functionality of MAC while changed their mass proportion and carbon storage of the paddy topsoil. Overall, crop residue recycled as biochar to paddy could ensure SOC sequestration through enhanced physical protection of POC in MAC, which could further potentially contribute to soil function improvement. How the short-term changes in macroaggregation and the pore system could contribute to the soil fertility and functioning deserve further studies.

RETRACTED: Activities of functional enzymes involved in C, N, and P conversion and their stoichiometry during agricultural waste composting with biochar and biogas residue amendments

This experiment was carried out to explore the effects of biochar, biogas residue and their combination amendment on enzyme activities and their stoichiometry during agricultural waste composting. A comprehensive analysis of the variation in, and stoichiometric correlations between, β-glucosidase (BG), N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and alkaline phosphatase (AKP) were determined. The results showed that biochar, biogas residue, and their combined addition significantly increased those enzyme activities. The potential C:P and N:P acquisition activities represented by ln(BG): ln(AKP) and ln(LAP + NAG): ln(AKP), were significantly decreased with biogas residue addition. BG, NAG and LAP were significantly negatively correlated with temperature, organic matter and water-soluble carbon. Redundancy analysis also showed that moisture and water-soluble carbon were significantly related to the variations of enzyme activities. Biochar and biogas residue changed the characteristics of the composting substrate, thus affecting the activity and stoichiometry of functional enzymes involved in C, N and P cycling.

SURFACE RESIDUES: EFFECTS ON SOIL MOISTURE AND TEMPERATURE1

ABSTRACT This study aimed to determine how crop residue placement and composition would affect soil water content and temperature during the dry season in the central region of Espírito Santo state, Brazil. A 19-week field study was conducted from April to August 2017. A 2 x 4 factorial study with four replications was implemented using a randomized complete block design. Factors were soil management [conventional tillage (CT) and no soil disturbance (ND)] and residue amendment [maize (Zea mays L.), sunn hemp (Crotalaria juncea L.), a maize-sunn hemp mixture, and a no amendment control]. Soil water content and temperature were measured weekly at predetermined soil depth intervals. Soil water content was higher in ND plots amended with surface residues than under all other treatments in the 0 to 0.05 m depth range. All residue amendments in this range were equally effective in conserving soil water. Surface residues reduced soil temperature by up to 8.4 °C relative to the control in ND plots. Incorporating residue amendments by CT cancelled all temperature-moderating benefits provided by surface residues. These results indicate that surface residues from cereals, legumes, or cereal/legume mixtures are equally effective in conserving soil water and moderating soil temperature during the dry season. Additional research is needed to determine how improved soil environmental conditions, generated by surface residues, would affect nutrient acquisition and crop performance.

Allelopathy effects of invasive alien Ageratina adenophora on native shrub species of chir pine forest in the central Himalaya, India

ABSTRACT Ageratina adenophora Spreng. is one of the most troublesome invasive species that has invaded various forest ecosystems in the central Himalaya. The chir pine (Pinus roxburghii Sarg.) forests in the region facing a high risk of invasion and loss understory diversity of native species. The present study comprehends the allelopathy effect of the invasive species A. adenophora residues bioassay (extract) and soil amendment against two co-occurring native shrubs viz Berberis asiatica Roxb. Ex DC. and Rubus ellipticus Sm. in a laboratory and natural forest conditions. Results revealed that a higher concentration (4% and 8%) of A. adenophora leaf, root and shoot extract have significantly inhibited (23% to 76%) and low concentration (1%) did not inhibit the seed germination, root and shoot length of B. asiatica and R. ellipticus. Furthermore, 8% (w/w) A. adenophora residue (leaf, root and shoot) amendments with the natural soil also showed significant inhibition effects (13% to 37%) on seedling growth and biomass of both the test species. Weekly A. adenophora leaf residue amendment showed higher inhibition in shoot elongation than root and shoot amendment during the experiment period. The study demonstrated that A. adenophora extract and residue amendment have strong allelopathic effects on woody shrub species and indicating allelopathy played important role in the successful establishment and rapid spread of A. adenophora in the understory of chir pine forests.

Can aged biochar offset soil greenhouse gas emissions from crop residue amendments in saline and non-saline soils under laboratory conditions?

Applying biochar in association with crop residues might optimize costs and effectiveness in the reclamation of saline soils. Here, we explored the potential effects of biochar in association with crop residue amendments on soil greenhouse gas (GHG) emissions, and microbial communities. Previously, we found that soil N2O emission significantly increased with increasing salinity levels followed by cotton straw addition. In the present study, microcosm experiments were performed to investigate the interaction of salinity (0 and 1.2% salt) with the aging of biochar following soil amendments over an incubation period of 80 days. The results indicated that N2O emissions were approximately 5–10 times higher in saline soils than in non-saline soils, and the cumulative N2O emissions following two straw amendments treatment were the highest of all the treatments. Salinity increased the contribution of nitrification to soil N2O emissions stimulated by the cotton straw amendments, and aged biochar performed better in decreasing soil N2O emissions in saline soils than in non-saline soils. In addition, aged biochar increased soil C mineralization and CO2 emissions under saline conditions. Soil CO2 and N2O emissions were affected by both soil abiotic and biotic factors under non-saline and saline conditions. Moreover, much more specific but fewer microbial groups survived and utilized crop residues under saline than non-saline conditions, and aged biochar decreased salt stress in soil microorganisms. These findings indicated that aged biochar and crop residues together would be an optimal way to address soil C storage and mitigate N2O emissions under saline conditions.

Plant-microbe Interactions in Rhizospheric soil Treated with Different Quality Organic Residues

The objective of this research was to study plant-microbe interactions in rhizospheric soil treated with different quality organic residues in short-term incubation of the soil and subsequent planting of maize crop. The treatments combinations were, 1) untreated soil (control; CT), 2) groundnut stover (GN) with high nitrogen (N) but low lignin (L) and polyphenol (PP), 3) tamarind leaf and petiole litter (TM) with medium N, L and PP, 4) rice straw (RS) with low N, L and PP but high cellulose, 5) GN+TM, 6) GN+RS and 7) TM+RS. Single and mixed residue additions of GN and TM, both high and intermediate quality, resulted in higher soil microbial properties and nutrients than the application of RS as a low quality. Accordingly, the application of the former group increased microbial abundances (i.e., bacteria, archaea, and fungi), elevated the enzymes related to the decomposition of organic residue (i.e., invertase, protease, phenoloxidase and peroxidase activity), and enhanced soil nutrients and plant growth. The results indicated that the chemical compositions (N, L, and PP) of the organic residues amendment are key factors regulating soil microbial abundance and enzyme activity both in after incorporation and after planting. Moreover, bacterial and archaeal abundance, and microbial activities including soil respiration, invertase, protease, and peroxidase activity in the soil after planting higher than those in the soil before planting.

Structural and hydraulic responses of humid tropical soils to lime and organic residue amendments

In humid tropical environments, where soils are characteristically acidic and low in organic matter, lime and organic residues have been used to improve soil quality. A systematic consideration of their interaction is, therefore, crucial for land-based ecosystem management. A 28-day incubation pot study was carried out to investigate the main and interactive effects of lime and organic residue type (corn stover and vermicompost) on aggregate stability under rapid wetting (WSAr), saturated hydraulic conductivity (Ksat), and soil water repellency (SWR) on three acidic soils with contrasting clay content from Trinidad: Cunupia (Aquic Hapludalfs), Sangre Grande (Fluvaquentic Endoaquepts), and Talparo (Aquertic Eutrudepts). Organic residue had a significant (P ≤ 0.001) increasing effect on WSAr and Ksat for all three soils, this being highest for corn stover and lowest for no residue. Lime and organic residue interactive effects were only significant (P ≤ 0.05) for WSAr in the Cunupia soil, where lime significantly reduced WSAr in the vermicompost and no residue, but not in the corn stover treatment. Soil water repellency increased with clay content and was highest in the lime–corn stover treatment of the Talparo soil. Overall, our results suggest that applying crop residue with lime may help minimise the short-term deleterious effects of lime on the structural and hydraulic properties of humid tropical soils. Nonetheless, future experiments with a wider range of soils and organic residues need to be carried out for a longer term to validate our results.

Incorporation of maize crop residue maintains soybean yield through the stimulation of nitrogen fixation rather than residue-derived nitrogen in Mollisols

Crop residue amendment to soil is recommended as an effective management practice to return nutrients, especially in the maize-soybean rotation system where large amounts of maize residues are produced. Quantifying the utilisation of maize-residue N by the subsequent soybean crop is essential for optimising the N fertilisation strategy for sustainable production. However, whether and how maize residue amendment alters N acquisition in soybean plants are largely unknown. It was hypothesised that maize residue would supply N and enhance N2 fixation to meet the N requirements of subsequent soybeans. Three treatments, namely: 1) chemical fertiliser (55.2, 35.2 and 22.4 kg ha−1 of N, P and K, respectively), 2) maize residue (8 t ha−1), and 3) non-fertiliser were applied in a maize-soybean rotation system in a Mollisol soil. It was demonstrated that soybean seed yield in the maize-residue treatment was the same as that in the chemical fertiliser treatment, with 2.9 vs. 3.2 t ha−1 in 2014, 2.7 vs. 2.6 t ha−1 in 2016, and 3.0 vs. 3.1 t ha−1 in 2018. A follow-up pot experiment using 15N-labelled residue indicated that the residue-derived N accounted for 0.5 % of the total N in soybean seeds and the proportion of symbiotically fixed N reached 82 %. The amount of fixed N during the pod-filling period in the residue treatment was 0.66 g plant-1, which was 49 % and 41 % higher than those in the chemical fertiliser and non-fertiliser treatments, respectively. The stimulation of N2 fixation was associated with an increase in fixed N per nodule and the enrichment of diazotrophs in the rhizosphere of soybean. With maize residue amendment, the increased N2-fixing capability of nodules during the reproductive period, rather than residue-derived N, fulfilled the N demand for maintaining seed yield of soybean. In the maize-soybean rotation system, maize residue amendment would facilitate the N2 fixation to partly substitute for N fertiliser for soybean production in Mollisols.

Differential long-term fertilization alters residue-derived labile organic carbon fractions and microbial community during straw residue decomposition

Straw residue amendment and fertilization are the key global management strategies for achieving more sustainable agriculture. However, the temporal changes in labile soil organic carbon (SOC) fractions and microbial community (MB) in response to differential long-term fertilization during straw residue decomposition remain unclear. We collected topsoil samples (0–20 cm; Mollisols) from three fertilizer management strategies (CK, no fertilization control, IF, inorganic fertilizer, and IFM, inorganic fertilizer plus manure) in a long-term field experiment. Subsequently, we conducted an in-situ micro-plot incubation experiment with and without 13C-labeled maize straw residue (δ13C = 246.9‰). We found that the straw-residue C in soil was mainly retained as particulate organic carbon (POC). The residue-derived POC was significantly increased, by 3, 5, and 20 times, whereas the residue-derived dissolved organic carbon (DOC) was significantly decreased by 71 %, 57 %, and 95 % in CK, IF, and IFM treatments, respectively, with straw addition (abbreviated as CKS, IFS, and IFMS respectively) during straw residue decomposition. The content of residue-derived microbial biomass carbon (MBC) was higher at 40.6 mg kg−1 (IFMS) and 33.0 mg kg−1 (IFS) compared to 27.0 mg kg−1 in the unfertilized (CKS) treatment at the end of the incubation period (day 150). The number of edges of the bacterial network was decreased by 16 %, 53 %, and 73 % in the treatments of CKS, IFS, and IFMS, respectively, compared with the corresponding fertilizer treatments without straw application. While the number of edges of fungal network also decreased by 57 % in CKS treatment, those in IFS and IFMS treatments increased by 160 % and 310 %, respectively. This indicated that straw residue addition decreases the bacterial microbial network complexity in all treatments, but it increases fungal network complexity in IFS and IFMS treatments. The highest microbial activities of the bacterial and fungal keystone taxa were observed on the 1 st day in the IFS treatment and on the 150th day in the CKS treatment. However, the highest microbial activities of bacterial keystone taxa were observed on the 60th day, and the highest microbial activities of fungal keystone taxa were detected on the 150th day in the IFMS treatment. The observed temporal changes in the microbial community suggested that independent of agricultural fertilizer management, straw residue-derived POC and DOC promoted fungal C processing, whereas for bacterial C, this was facilitated only by straw residue-derived MBC in these Mollisols. Highlighting straw residue incorporation helps to sustain microbial diversity and associated carbon processing in agricultural soils.

Effect of Biochar and Straw Application on Nitrous Oxide and Methane Emissions from Eutric Regosols with Different pH in Sichuan Basin: A Mesocosm Study

Adoption of crop residue amendments has been increasingly recommended as an effective management practice for mitigating greenhouse gas emissions while enhancing soil fertility, thereby increasing crop production. However, the effect of biochar and straw on nitrous oxide (N2O) and methane (CH4) emissions in soils of differing pH remains poorly understood. Three treatments (control (i.e., no amendment), maize straw, and biochar derived from maize straw) were therefore established separately in soils with different pH levels, classified as follows: acidic, neutral, and alkaline. N2O and CH4 were investigated using a static chamber–gas chromatography system during 57 days of a mesocosm study. The results showed that cumulative N2O emissions were significantly higher in acidic soils than in other experimental soils, with the values ranging from 7.48 to 11.3 kg N ha−1, while CH4 fluxes ranged from 0.060 to 0.089 kg C ha−1, with inconclusive results. However, a weak negative correlation was observed between log N2O and log NO3-N in acidic soil with either biochar or straw, while the same parameters with CH4 showed a moderate negative correlation, suggesting a likelihood that these amendments could mitigate GHGs as a result of the NO3-N increase in acidic soils. It is also possible, given the alkaline nature of the biochar, that incorporation had a significant buffer effect on soil acidity, effectively increasing soil pH by &gt;0.5 pH units. Our findings suggest that for the rates of application for biochar and straw used in this study, the magnitude of reductions in the emissions of N2O and CH4 are dependent in part on initial soil pH.

Linking rhizospheric diazotrophs to the stimulation of soybean N2 fixation in a Mollisol amended with maize straw

Crop residue amendment is likely to stimulate symbiotic N2 fixation, and clarifying its effect on N2-fixing bacteria, i.e., diazotrophs in the rhizosphere of legume crops, is important for sustainable N management in legume-cereal cropping systems. Therefore, this study aimed to reveal the diazotrophic community composition in the rhizosphere of soybean in response to maize residue amendment. Being designed with treatments of maize residue, chemical fertilizer, and non-fertilizer applications, this study deployed the 15N-labeling technology combined with high-throughput sequencing of the nifH gene as a molecular marker for diazotrophs to quantify the symbiotically-fixed N2 in soybean plants and link symbiotically fixed N2 to the diazotrophic community diversity in the rhizosphere. Residue amendment increased the abundance of diazotrophs and fundamentally altered the composition of its community in the rhizosphere. It increased the relative abundances of Bradyrhizobium and Azohydromonas compared to the chemical fertilizer treatment. The copy number of nifH in the rhizosphere was associated with dissolved organic carbon and N2 fixation. Residue-induced increase in dissolved organic carbon may provide sufficient carbon sources for diazotroph enrichment and thus enhance nodulation. The maize residue amendment may enrich N2 fixers to facilitate nodulation and subsequent N2 fixation of soybean, highlighting the eco-functional importance of diazotrophs fixing extra N into the rotation system.

Amendment of straw biochar increased molecular diversity and enhanced preservation of plant derived organic matter in extracted fractions of a rice paddy

While biochar enhanced carbon sequestration and stability of soil organic matter (SOM), changes in organic molecular composition in biochar-amended soils had been poorly addressed. In this study, molecular composition changes of a paddy topsoil 2 years following amendments at 10 t ha−1 OC equivalent with untreated (CS), manured (CM) and charred (CB) maize straw were compared to no amendment (CK). Topsoil SOM was sequentially extracted with ultrapure water (UWE), solvent (TSE), base hydrolysis (BHY) and CuO oxidation (CUO) and molecular compounds in these extracted fractions were detected quantitatively by GC/MS. Compared to CK, SOC content was increased respectively by 12% under CS and CM, and by 36% under CB. Fraction abundance both of UWE and CUO was increased but that of TSE unchanged under CS and CM, while that of BHY unchanged under CS but increased by > 60% under CM and CB, respectively. Under CB relative to CS and CM, abundance of TSE and BHY fractions was greatly increased but that of UWE and CUO unchanged. Specifically, abundances of water-soluble monosaccharides, low molecular weight organic acids and lignin-derived phenols, especially cinnamyl-based monomers, were all enhanced under CS, but abundances of n-alkanols, fatty acids in free lipids and di-acids and hydroxyl fatty acids in bound lipids were decreased under both CS and CM. In contrast, CB amendment increased abundances of n-alkanols, alkanes and sterols in free lipids while enhanced molecular and functional group diversity of UWE and TSE fractions. Overall, short-term crop residue amendment altered the abundance and molecular diversity of OM mainly associated with short-lived UWE and labile TSE fractions and biochar enhanced preservation of plant derived molecules mainly in lipids. Thus, returning crop residue as biochar could be a sustainable approach to enhance not only SOM pool but molecular diversity also in agricultural soils.

Mixing of biochar, vinegar and mushroom residues regulates soil microbial community and increases cucumber yield under continuous cropping regime

Continuous cropping is a common practice in protected agriculture, although it results in a significant decline in soil quality and crop yield. Considering the large yearly production of vinegar and mushroom residues in China, the current study aimed to investigate the effects of different organic residue amendments on soil quality in the protected cultivation of cucumber under continuous cropping regime. Accordingly, a pot experiment was conducted with the following treatments: soil without amendment (CK), soil amendment with commercial conditioner (C), biochar (B), vinegar (V) and mushroom (M) residue (VM, V:M, 3:1), and the mixtures of vinegar, mushroom residue, and biochar (VMB, V:M:B, 3:1:4). The results showed that the soil pH, soil water content (SWC), and organic carbon (OC) content significantly increased after soil amendments. Vinegar and mushroom residues improved the content of soil N, P, and K. In contrast, biochar substantially decreased the total phenol content and electrical conductivity. Moreover, microbial diversity significantly increased with VMB amendment, and structural equation modeling (SEM) indicated that the microbial Chao1 index was strongly linked to cucumber yield. Furthermore, the yield of cucumber had a strong correlation with microbial diversity compared with soil physico-chemical properties. Chloroflexi, Actinobacteria and Gemmatimonadetes were significantly enriched by different amendments. Our results suggest that soil amendments can directly regulate the soil microbial community or induce compositional shifts of microbes by changing soil physico-chemical characteristics under continuous cropping regime. The coamendment of soil with vinegar and mushroom residues plus biochar is an effective method to reduce the negative effects of continuous cropping in protected cultivation. The study provides convincing evidence that the application of organic amendments and management of residues are potential sustainable strategies to increase agricultural productivity in protected horticulture.

Microbial metabolic efficiency and community stability in high and low fertility soils following wheat residue addition

Soil microbial metabolic efficiency and microbial community stability following the amendment of plant residue to soils are of great importance to the improvement of soil carbon storage and soil fertility. However, heterogeneity of microbial metabolic efficiency and community stability in soils with different fertility defined based on the crop yield, as well as the underlying mechanisms still remains elusive. Here, soils with high and low fertility (HF and LF) were incubated with 13C-labeled wheat residue and analyzed periodically for microbial metabolic quotient and functional bacterial populations using DNA-stable isotope probing technique combined with high-throughput sequencing. Results revealed that soil organic matter (SOM) decomposers following wheat residue amendment were suppressed in HF but stimulated in LF, leading to a higher microbial metabolic efficiency and lower priming effect in HF. This difference in SOM decomposers' responses could be due to that microbes in nutrient- limited LF has to mine recalcitrant SOM for nutrient requirement to support the utilization of wheat residue, the ample nutrients in HF, however, render the microbes to directly utilize wheat residue. Both the resistance (disturbance stability) and resilience (temporal stability) of bacterial community were higher in HF than in LF following disturbance of wheat residue addition. Higher abundance and lower composition variation of wheat residue decomposers in HF than in LF might result in the higher stability of microbial community in HF. The results suggest that plant residue amendment to fertile soils is likely more effective for soil carbon accumulation and soil fertility buildup than to infertile soils, due to higher microbial metabolic efficiency and higher microbial community stability.