Our increasing dependence on a small number of agricultural crops, such as corn, is leading to reductions in agricultural biodiversity. Reductions in the number of crops in rotation or the replacement of rotations by monocultures are responsible for this loss of biodiversity. The belowground implications of simplifying agricultural plant communities remain unresolved; however, agroecosystem sustainability will be severely compromised if reductions in biodiversity reduce soil C and N concentrations, alter microbial communities, and degrade soil ecosystem functions as reported in natural communities. We conducted a meta-analysis of 122 studies to examine crop rotation effects on total soil C and N concentrations, and the faster cycling microbial biomass C and N pools that play key roles in soil nutrient cycling and physical processes such as aggregate formation. We specifically examined how rotation crop type and management practices influence C and N dynamics in different climates and soil types. We found that adding one or more crops in rotation to a monoculture increased total soil C by 3.6% and total N by 5.3%, but when rotations included a cover crop (i.e., crops that are not harvested but produced to enrich the soil and capture inorganic N), total C increased by 8.5% and total N 12.8%. Rotations substantially increased the soil microbial biomass C (20.7%) and N (26.1%) pools, and these overwhelming effects on microbial biomass were not moderated by crop type or management practices. Crop rotations, especially those that include cover crops, sustain soil quality and productivity by enhancing soil C, N, and microbial biomass, making them a cornerstone for sustainable agroecosystems.

Does agricultural crop diversity enhance soil microbial biomass and organic matter dynamics? A meta-analysis

The impact of animal manure application on soil organic carbon (SOC) stock changes is of interest for both agronomic and environmental purposes. There is a specific need to quantify SOC change for use in national greenhouse gas (GHG) emission inventories. We quantified the response of SOC stocks to manure application from a large worldwide pool of individual studies and determined the impact of explanatory factors such as climate, soil properties, land use and manure characteristics. Our study is based on a meta-analysis of 42 research articles totaling 49 sites and 130 observations in the world. A dominant effect of cumulative manure-C input on SOC response was observed as this factor explained at least 53% of the variability in SOC stock differences compared to mineral fertilized or unfertilized reference treatments. However, the effects of other determining factors were not evident from our data set. From the linear regression relating cumulative C inputs and SOC stock difference, a global manure-C retention coefficient of 12% ± 4 (95% Confidence Interval, CI) could be estimated for an average study duration of 18 years. Following an approach comparable to the Intergovernmental Panel on Climate Change, we estimated a relative SOC change factor of 1.26 ± 0.14 (95% CI) which was also related to cumulative manure-C input. Our results offer some scope for the refinement of manure retention coefficients used in crop management guidelines and for the improvement of SOC change factors for national GHG inventories by taking into account manure-C input. Finally, this study emphasizes the need to further document the long-term impact of manure characteristics such as animal species, especially pig and poultry, and manure management systems, in particular liquid vs. solid storage.

Animal manure application and soil organic carbon stocks: a meta-analysis

Biochar as a carbon-rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta-analysis procedures to examine responses of soil carbon dioxide (CO2) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO2 fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO2 fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N-fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land-use types. Responses of soil CO2 fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO2 fluxes and MBC content, respectively. Both soil CO2 fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO2 fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO2 flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land-use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/gcbb.12265

Response of soil carbon dioxide fluxes, soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

Controls over soil microbial biomass responses to carbon amendments in agricultural systems: A meta-analysis

Changes in total organic C (CT), water-soluble organic C (CWS), microbial biomass C (CMB), C mineralization, particulate organic C (CP), labile organic C (CL), C management index (CMI), and C storage in surface Hapli-Ustic Cambisol (0–20 cm) under straw incorporation after both 2- and 10-year durations were investigated in a maize (Zea mays L.) field experiment in northeast China, in order to examine the effectiveness of these active C fractions and CMI as early indicators for total C change. The treatments included straw removal (0%S), 50% of straw incorporation (50%S), and 100% of straw incorporation (100%S). Under the straw incorporation, CT concentration and C storage did not significantly change under 2-year duration, while were significantly increased under 10-year duration. However, CMB, total C mineralization (CTM), CP, and CL, and CMI were significantly increased under the straw incorporation even after only 2-year duration, and the responses were more significant after 10-year duration. There were positive correlations between all these C indicators with each other. Our findings demonstrate that the measured active C fractions (except for CWS) and CMI can provide an early indication of change in total soil organic C induced by straw incorporation.

Soil organic carbon active fractions as early indicators for total carbon change under straw incorporation

Little is known about effects of high nitrogen (N) fertilization on soil chemical properties, enzyme and microbial activities involved in N and C transformation. Soil EC, pH, mineral N, three enzyme activities, nitrification capacity, and microbial functional diversity were evaluated under conventional N rate (270 or 300 kg N ha−1 season−1, N4), 80% (N3), 60% (N2), 40% (N1) and 0% (N0) of N4 during a 2-year cucumber/tomato rotation in polytunnel greenhouse lands in Eastern China. Soil EC, NH4+-N and NO3−-N were significantly increased, whilst pH, activities of dehydrogenase, urease and neutral phosphatase, nitrification capacity, and microbial functional diversity decreased significantly with N application rate. Microbial diversity indices deducted from the C-source utilization pattern were significantly lower under the highest N4 rate in the same vegetable season, but varied with seasons among N rates. Both principal component analysis and C substrate utilization patterns displayed significant separation of soil microbial communities between the higher N4 or N3 and the lower N0, N1 and N2 rates. Our results indicated that a 40% reduction of the conventional N fertilization rate was practical to reduce excess N input while maintaining the sustainability of such greenhouse-based intensive vegetable systems in China’s Yangtze River Delta.

Higher rates of nitrogen fertilization decrease soil enzyme activities, microbial functional diversity and nitrification capacity in a Chinese polytunnel greenhouse vegetable land

As labile organic pools, soluble organic matter and soil microbial biomass are sensitive to changes in soil management and therefore good indicators of soil quality. Effects of a 17-year long-term fertilization on soil microbial biomass C (SMBC) and N (SMBN), soluble organic C, and soluble organic N during the maize growing season were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwest China. The fertilization treatments included no fertilizer (CK), inorganic N, P, and K fertilizer (NPK), cattle manure plus NPK fertilizer (MNPK), and straw plus NPK fertilizer (SNPK). Our results showed that C storage in the 0–20 cm soil layer was 28% to 81% higher in the fertilized treatments compared to the unfertilized treatment. In the 0–10 cm soil layer, SMBC and SMBN in the three fertilized treatments were higher than in the unfertilized treatment on all sampling dates, while microbial biomass C and N in the 0−10 cm soil layers were the highest at grain filling. In the same soil layer, soil-soluble organic C generally decreased in the order MNPK > SNPK > NPK > CK, while soluble organic N was the highest in the MNPK followed by the SNPK treatment. There was no significant difference in soluble organic N in the NPK and CK treatments throughout most of the maize growing season. Changes in soluble organic N occurred along the growing season and were more significant than those for soluble organic C. Soluble organic N was the highest at grain filling and the lowest at harvest. Overall, our results indicated that microbial biomass and soluble organic N in the surface soil were generally the highest at grain filling when maize growth was most vigorous. Significant positive relationships were found between soluble organic C and SMBC and between soluble organic N and SMBN.

Effects of 17-year fertilization on soil microbial biomass C and N and soluble organic C and N in loessial soil during maize growth

Due to the ever-increasing worldwide plantation of sweet leaf Stevia rebaudiana (Bertoni), how to efficiently and effectively utilize the huge amounts of leaf residues that contain abundant nutrients after sweetener extraction becomes an eminent issue. One option is to return these residues into soil, as organic manure in the fresh or composted form, in order to both sustain soil fertility and avoid potential environmental pollution. In a field experiment, we studied if the Stevia leaf residue returning affected both plant and soil microbial growths as well as the possible change of soil microbial community composition. In doing so, four treatments were employed: (1) no chemical fertilization and no Stevia residue returning (no-fertilization control); (2) chemical N, P, and K fertilization (NPK); (3) fresh Stevia residue plus NPK (FS + NPK); and (4) composted Stevia residue plus NPK (CS + NPK). Responses of plant and soil microbial communities to Stevia residue input after 1-year fertilizations were investigated by multiple approaches, including soil enzyme assay, real-time quantitative polymerase chain reaction, and PCR–denaturing gradient gel electrophoresis. Our results showed that compared to the sole NPK and no-fertilization control, returning Stevia residues to soil stimulated the enzyme activities of dehydrogenase, invertase, and urease, except neutral phosphomonoesterase; thereby, both the Stevia leaf biomass and sweet glycoside of rebaudioside A were increased. The soil microbial community abundance was increased by the returning of Stevia residues, and their composition was shifted, evidenced by an increase of relative abundance of some genotypic groups, such as Bacillales. Further molecular analysis of Bacillus confirmed that this guild composition was positively influenced by Stevia residue returning, especially for Bacillaceae. Our results demonstrated an effective use of Stevia residues as organic manure for promoting Stevia yield and quality through stimulating soil microbial growth and enzyme activities.

Soil microbial mechanisms of Stevia rebaudiana (Bertoni) residue returning increasing crop yield and quality

The nitrogen difference method (NDM) for quantifying N2 fixation, based on the same amount of soil N exploited by N2-fixing and non-N2-fixing plant, may not be suitable to plants with different root traits. We tested the reliability of NDM in legume-based intercropping systems by two field experiments in Northwest China. In experiment 1, faba bean (Vicia faba), pea (Pisum sativum), and soybean (Glycine max) grew solely or intercropped with maize (Zea mays) with two N application rates (0, 225 kg ha−1). The biomass of faba bean, pea, and maize was significantly increased, whereas that of soybean was decreased when intercropped than solely grown. Aggressivity analyses demonstrate greater N competition ability of faba bean and pea, but not soybean, than maize. An improved NDM (INDM) could mitigate these effects: $$ {N_{{\text{fix - int}}}} = \left[ {{N_{{\text{leg - int}}}} + \frac{{1 - x}}{x}{N_{{\text{ref - int}}}} - \frac{{{N_{{\text{ref - sole}}}}}}{x}} \right] + \left[ {{\text{soil}}{N_{{\text{leg - int}}}} + \frac{{1 - x}}{x}{\text{soil}}{N_{{\text{ref - int}}}} - \frac{{{\text{soil}}{N_{{\text{ref - sole}}}}}}{x}} \right] $$
 , where x and 1 − x are planting area of legume and non-legume in the intercropping system. Compared to traditional NDM (TNDM, $$ {N_{{\text{fix - int}}}} = \left[ {{N_{{\text{leg - int}}}} - {N_{{\text{ref - sole}}}}} \right] + \left[ {{\text{soil}}{N_{{\text{leg - int}}}} - {\text{soil}}{N_{{\text{ref - sole}}}}} \right] $$
 ), %N
 dfa (N derived from air) by INDM was decreased by 54.3% and 39.8% for faba bean, 44.7% and 5.0% for pea, but increased by 113.5% and 191.0% for soybean at the two N application rates, indicating different %N
 dfa quantifications between the two methods. In experiment 2, %N
 dfa of sole or intercropped faba bean was quantified by TNDM, INDM, and 15N natural abundance method (NA). The %N
 dfa only by INDM correlated significantly with that from NA. Both interspecific root interactions and N loss affect %N
 dfa estimation. Our results suggested that INDM could be more suitable than TNDM for quantifying %N
 dfa of a N2-fixing plant in intercropping systems.

Xinhua He

Papers

Higher rates of nitrogen fertilization decrease soil enzyme activities, microbial functional diversity and nitrification capacity in a Chinese polytunnel greenhouse vegetable land

Effects of 17-year fertilization on soil microbial biomass C and N and soluble organic C and N in loessial soil during maize growth

Soil microbial mechanisms of Stevia rebaudiana (Bertoni) residue returning increasing crop yield and quality

An improved nitrogen difference method for estimating biological nitrogen fixation in legume-based intercropping systems