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Nitrogen (N) fertilization is the major contributor to nitrous oxide (N2O) emissions from agricultural soil, especially in post‐harvest seasons. This study was carried out to investigate whether ryegrass serving as cover crop affects soil N2O emissions and denitrifier community size. A microcosm experiment was conducted with soil planted with perennial ryegrass (Lolium perenne L.) and bare soil, each with four levels of N fertilizer (0, 5, 10 and 20 g N m−2; applied as calcium ammonium nitrate). The closed‐chamber approach was used to measure soil N2O fluxes. Real‐time PCR was used to estimate the biomass of bacteria and fungi and the abundance of genes involved in denitrification in soil. The results showed that the presence of ryegrass decreased the nitrate content in soil. Cumulative N2O emissions of soil with grass were lower than in bare soil at 5 and 10 g N m−2. Fertilization levels did not affect the abundance of soil bacteria and fungi. Soil with grass showed greater abundances of bacteria and fungi, as well as microorganisms carrying narG, napA, nirK, nirS and nosZ clade I genes. It is concluded that ryegrass serving as a cover crop holds the potential to mitigate soil N2O emissions in soils with moderate or high NO3− concentrations. This highlights the importance of cover crops for the reduction of N2O emissions from soil, particularly following N fertilization. Future research should explore the full potential of ryegrass to reduce soil N2O emissions under field conditions as well as in different soils. Highlights: This study was to investigate whether ryegrass serving as cover crop affects soil N2O emissions and denitrifier community size;Plant reduced soil N substrates on one side, but their root exudates stimulated denitrification on the other side;N2O emissions were lower in soil with grass than bare soil at medium fertilizer levels, and growing grass stimulated the proliferation of almost all the denitrifying bacteria except nosZ clade II;Ryegrass serving as a cover crop holds the potential to mitigate soil N2O emissions. [ABSTRACT FROM AUTHOR]

Plant roots interact with rhizosphere microorganisms to accelerate soil organic matter (SOM) mineralization for nutrient acquisition. Root‐mediated changes in SOM mineralization largely depend on root‐derived carbon (root‐C) input and soil nutrient status. Hence, intraspecific competition over plant development and spatiotemporal variability in the root‐C input and nutrients uptake may modify SOM mineralization. To investigate the effect of intraspecific competition on SOM mineralization at three growth stages (heading, flowering, and ripening), we grew maize (C4 plant) under three planting densities on a C3 soil and determined in situ soil C‐ and N‐mineralization by 13C‐natural abundance and 15N‐pool dilution approaches. From heading to ripening, soil C‐ and N‐mineralization rates exhibit similar unimodal trends and were tightly coupled. The C‐to‐N‐mineralization ratio (0.6 to 2.6) increased with N availability, indicating that an increase in N‐mineralization with N depletion was driven by microorganisms mining N‐rich SOM. With the intraspecific competition, plants increased specific root lengths as an efficient strategy to compete for resources. Root morphologic traits rather than root biomass per se were positively related to C‐ and N‐mineralization. Overall, plant phenology and intraspecific competition controlled the intensity and mechanisms of soil C‐ and N‐ mineralization by the adaptation of root traits and nutrient mining. Soil C‐ and N‐mineralization rates are tightly coupled throughout maize growth stages and dependent on soil N availability. Plant phenology and intraspecific competition govern the intensity and mechanisms of the C‐ and N‐mineralization by the adaptation of root traits and nutrient mining. [ABSTRACT FROM AUTHOR]

Quantification of legume nitrogen (N) rhizodeposition (N derived from roots) and its fate in agroecosystems is crucial for managing soil fertility, land productivity, and agriculture sustainability. In contrast to forage legumes, the N rhizodeposition by grain legumes is nearly unknown. Therefore, N rhizodeposition of four grain legumes and its transfer to subsequent wheat crops was quantified using the 15N stem labeling method under field conditions. The N rhizodeposition of the grain legumes: peanut, soybean, mungbean, and adzuki bean amounted to 25, 51, 20, and 63 kg N ha−1, respectively. N rhizodeposition was not affected by fertilization, and it was 53–257% more accumulated in topsoil (0–20 cm) than that in subsoil (20–40 cm). However, N rhizodeposition per unit of root biomass in subsoil was 3.5‐times as much as that in topsoil (p < 0.05), indicating the importance of legumes for soil fertility and exploration in subsoil. Remarkably, subsequent wheat utilized 13–85% of legume N rhizodeposition, which contributed to 4–20% of total wheat N uptake. Combining the present data with the literature review, the average N rhizodeposition of legumes (both grain and forage legumes) is 83 kg N ha−1 (n = 75), and one‐fourth of which was utilized by subsequent cereals. Increasing root biomass by 1 g increases rhizodeposition by 53 mg N. In conclusion, legume N rhizodeposition is crucial for the sustainability of legume‐based crop rotations resulting in soil N build‐up and is an important N source for subsequent crops. [ABSTRACT FROM AUTHOR]

The responses of soil denitrifiers to inorganic and organic mixed fertilization in paddies have not been well evaluated. The abundance and diversity of nirS- and nirK-type denitrifiers in paddies after 30-year fertilization were estimated using quantitative PCR and Illumina MiSeq sequencing. The nirS gene abundance was two orders of magnitude higher than the nirK gene and both were reduced by fertilization. The high manure addition (NPK + 60% OM) decreased nirK gene abundance by 32.7% compared to inorganic fertilization alone (NPK) and decreased its community Chao1 index by 30.2% compared to unfertilized control (CK). Fertilization significantly changed the proportion of dominant (mean proportion > 1%) operational taxonomic units (OTUs) of both denitrifiers. Fertilization increased the proportion of OTUs belonged to cluster III (48.7–97.3%) of the nirS-type denitrifiers compared to CK. NPK + 60% OM increased that belonged to cluster II (48.0%) compared to the low manure addition (NPK + 30% OM). Total nitrogen and organic carbon correlated significantly with the abundance, diversity and community structure of the nirK-type denitrifiers, while they solely correlated with the nirS-type denitrifier community structure. Our results demonstrate that abundance and diversity of the nirK-type denitrifiers are more, while the community structure is less, sensitive to inorganic and organic mixed fertilization than the nirS-type in paddy soils. [ABSTRACT FROM AUTHOR]

Climate warming and anthropogenic nitrogen (N) loads are two major global change components interactively affecting carbon cycling. However, the effects of N forms and amounts on temperature sensitivity (Q10) of soil organic matter (SOM) mineralization remain incomplete. With this goal, soil was sampled after 23 years of mineral and (or) organic N fertilization, and then incubated for one year at 10, 20, and 30°C. For the first time, we compared four approaches (Equal time, Equal C, 1‐C pool, and 2‐C pool model) to evaluate the Q10 of SOM mineralization. All approaches showed that the Q10 decreased by more than one third with N fertilization compared to unfertilized control at low temperatures. The '1‐C pool model' was not adequate for Q10 estimation with various C availability. The Q10 estimated by '2‐C pool model' was strongly depended on incubation duration. The 'Equal C' approach was more powerful for separating SOM pools and it revealed the decreased Q10 of the recalcitrant pool at high N rates. The impact of N fertilization on Q10 was more evident at high N than at low N. Notably, the Q10 decreased more by mineral N compared to organic fertilizers (~60% vs. ~40% decreased in Q10) at 10–20oC. The added benefit of N fertilization in protecting SOM under climate warming was demonstrated by decreased Q10. Such one‐third reduction of temperature sensitivity by N fertilization is large enough to be considered in predictions of global SOM stocks under warming and anthropogenic N loads. [ABSTRACT FROM AUTHOR]

The photosynthate carbon (C) released in the rhizosphere plays a crucial role in C sequestration, microbial activities and nutrient availability in soil. Nitrogen (N) fertilization modifies the allocation and dynamics of photosynthates in paddy rice systems, but these effects depend on plant growth stages. Rice (Oryza sativa L.) plants were pulse labelled with 13CO2 at the tillering, elongation, heading and grain‐filling stages with 0 and 225 kg N ha−1 fertilizer. The plants and soil were sampled shortly after each pulse labelling and at harvest. Relative 13C (as % of assimilated C) in the roots and rhizosphere soil was largest at the early growth stage (tillering) and subsequently decreased. At harvest, 68% of the rhizodeposited C remained in bulk soil without N fertilizer, which corresponded to 6.2% of the net assimilated 13C. The absolute amount of net belowground C input (root + rhizodeposition) by rice was 268 and 468 kg C ha−1 under 0 and 225 kg N ha−1 fertilizer, of which rhizodeposition accounted for 60 and 40%, respectively. We concluded that N fertilization raised the belowground C input by rice mainly by increasing root biomass rather than by rhizodeposition. Highlights: Rice photosynthesis‐derived carbon (C) was quantified in soil by multiple pulse labelling with 13CO2Young rice plants allocated more assimilates into the soil compared to mature plantsNitrogen deficiency led to greater C retention in bulk soil than in the rhizosphereNitrogen fertilization increased the net belowground C input mainly with larger root biomass [ABSTRACT FROM AUTHOR]

Abstract: The stability and turnover of soil organic matter (SOM) are a very important but poorly understood part of carbon (C) cycling. Conversion of C3 grassland to the C4 energy crop Miscanthus provides an ideal opportunity to quantify medium‐term SOM dynamics without disturbance (e.g., plowing), due to the natural shift in the δ13C signature of soil C. For the first time, we used a repeated 13C natural abundance approach to measure C turnover in a loamy Gleyic Cambisol after 9 and 21 years of Miscanthus cultivation. This is the longest C3–C4 vegetation change study on C turnover in soil under energy crops. SOM stocks under Miscanthus and reference grassland were similar down to 1 m depth. However, both increased between 9 and 21 years from 105 to 140 mg C ha−1 (P < 0.05), indicating nonsteady state of SOM. This calls for caution when estimating SOM turnover based on a single sampling. The mean residence time (MRT) of old C (>9 years) increased with depth from 19 years (0–10 cm) to 30–152 years (10–50 cm), and remained stable below 50 cm. From 41 literature observations, the average SOM increase after conversion from cropland or grassland to Miscanthus was 6.4 and 0.4 mg C ha−1, respectively. The MRT of total C in topsoil under Miscanthus remained stable at ~60 years, independent of plantation age, corroborating the idea that C dynamics are dominated by recycling processes rather than by C stabilization. In conclusion, growing Miscanthus on C‐poor arable soils caused immediate C sequestration because of higher C input and decreased SOM decomposition. However, after replacing grasslands with Miscanthus, SOM stocks remained stable and the MRT of old C3‐C increased strongly with depth. [ABSTRACT FROM AUTHOR]

Organic management enhances the formation of distinct and stable soil microbial communities, however, its influence on the temporal recovery of microbiome and multifunctionality of sterilized soil remains poorly understood. We used amplicon sequencing and metagenomic sequencing to investigate the effects of microbial communities in long‐term organic and conventional managed soils on restoring soil microbiome and functionality. We calculated multifunctionality of soils at days 30 and 90 of recolonization using the averaging approach. Results showed that organic management (O) significantly increased alpha diversity, niche width and network complexity of microbial community compared to conventional management (C). The alpha diversity, niche width and network complexity of microbial community in soils with organic soil suspension were significantly increased compared to conventional management at days 30 and 90 of recolonization. Soil multifunctionality of sterilized organic managed soil inoculated with organic soil suspension (OO) was 14.6% to 70.6% higher than that of the rest treatments. Macrogenomic analysis revealed that O significantly enriched functional pathways of ABC transporters, carbon metabolism, biosynthesis of amino acids, two‐component and nitrogen metabolism as well as most of the functional genes for carbon degradation, carbon fixation, nitrogen cycling and phosphorus cycles compared to C. These functional pathways and genes were also significantly enriched in soils with organic soil suspension at day 30 and 90 of recolonization. Furthermore, alpha diversity, niche width, network complexity, functional pathways and functional genes of microbiome correlated positively with soil multifunctionality. Synthesis and applications. Our results emphasize the importance of organic management induced changes in diversity, network complexity, and functionality of microbial communities for promoting recovery to soil microbial and functional losses, providing the theoretical basis for sustainable impact of organic management in agronomic production on soil microbiome and function. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Global warming and increasing anthropogenic nitrogen (N) loads are two major worldwide issues that arose with industrialization, and both factors permanently impact carbon (C) cycling. For the first time, we compared four independent approaches to evaluate the temperature sensitivity (Q10) of soil organic matter (SOM) mineralization: Equal time, Equal C, 1-C pool model and 2-C pool model. With this goal, soil was sampled from a wheat-maize double cropping system, after 23-year of no N (control), low N (organic and mineral N), high organic N (manuring), or high mineral N fertilization. Four Q10 estimation approaches were compared based on the CO2 released from long-term N fertilized soil at 10, 20 and 30 ºC during one-year incubation. All four approaches clearly showed large Q10 decrease for all SOM pools by N fertilization compared to unfertilized control (Q10 = 0.8-2.3 vs. 2.7-3.7) at low temperature. N fertilization decreased the activation energy (from 85 to 56 kJ mol-1) and led to lower Q10 according to Arrhenius formalism. The lower Q10 under long-term N fertilization resulted also in the decreased C:N ratio, accelerated C turnover and increased microbial C use efficiency. All Q10 estimation approaches captured the decomposition of different SOM pools (from labile to recalcitrant) at various time scales. The '1-C pool model' is not adequate for Q10 estimation because it ignores the response of various C pools to soil warming. The '2-C pool model' fit data very well but the fitted decomposition rates strongly depend on incubation duration. The 'Equal C' approach was more dynamic in separating various SOM to provide a mechanistic understanding of soil C sequestration and cycling under increasing temperature. However, 'Equal C' approach was largely C quality dependent and disregards the incubation duration. The advantages and shortcomings of the four Q10 estimation approaches were intensively discussed. Long-term incubation and continuous measurement of SOM mineralization is always needed to separate various C pools, especially very recalcitrant C. N fertilization lead to added benefit in protecting SOM under warming by decreasing Q10 of SOM. Such reduction of temperature sensitivity caused by N fertilization is large enough to be considered in predictions of the magnitude of global SOM stocks under warming and increasing anthropogenic N loads. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) represent an important route for plant carbon (C) input to soil, and regulate below-ground organic matter (SOM) storage. However, the C sequestration depends on plant C input and rhizosphere priming effect (RPE), and both processes affected by AMF colonization, which makes the C balance under AMF remains largely unknown. A mycorrhizal wild type progenitor (MYC) and its mycorrhiza defective mutant of tomato (reduced mycorrhizal colonization: rmc) were used to control the formation of AMF symbiosis. The plants were labelled with 15N mineral fertilizer and continuously 13CO2. AMF symbiosis decreased relative C incorporation (% of total assimilated C) into roots, but increased the net rhizodeposition and C incorporated into soil. N fertilization decreased relative C incorporation into roots, rhizosphere, and bulk soil. However, the absolute amount of rhizodeposition remaining in soil was not changed by fertilization. A positive RPE was observed for both MYC and rmc plants, which ranging from 16-71% and 25-101% of native SOM decomposition, respectively. Remarkably, the positive RPE induced by AMF was decreased with plant age, which may associate with the increased nutrients competition between AMF and free-living decomposers in rhizosphere. The RPE and extracellular enzyme activities decreased with N fertilization compared with unfertilized soil at 8 and 12 weeks after transplanting, suggesting N fertilization decreased microbial N demand through SOM mining. Sixteen weeks after transplanting, the amount of rhizodeposits remained in rhizosphere and bulk soil by MYC was 0.02-0.04, 1.67-1.75 mg C kg-1, which accounted for 0.7-1.1% and 42-46% of belowground plant C, respectively. The rhizodeposition remaining in SOM for rmc was three times lower than that for MYC plant. We conclude that AMF symbiosis and N fertilization facilitates C sequestration in soil not only by higher plant C input, but also by reducing native SOM decomposition. [ABSTRACT FROM AUTHOR]

Emissions of greenhouse gases (GHGs), such as carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) have great impact on global warming and atmospheric chemistry. Biochar addition is a potential option for reducing GHGs emissions through carbon (C) sequestration and N 2 O mitigation. However, the influences of biochar on C and nitrogen (N) transformations in soil are still unclear, resulting in a poor understanding of the mechanisms of N 2 O mitigation effects of biochar. Here we carried out two soil incubation experiments to investigate the influence of two common biochars addition (corn cob and olive pulp) with ammonium sulfate on CO 2 and N 2 O emissions from two contrasting soil types (acidic sandy and alkaline clay soil). Furthermore, four extracellular enzymes activities that related to C and N cycling, i.e. cellobiohydrolase, chitinase, xylanase and β-glucosidase, were analyzed to gain insights into the underlying mechanisms of biochar’s effects on CO 2 and N 2 O evolutions. Contrasting effects of two biochars on CO 2 and N 2 O emissions were observed in the two different soils. The corn biochar addition had no significant effect on CO 2 and N 2 O emissions in the alkaline clay soil, but significantly decreased CO 2 emissions by 11.8% and N 2 O emissions by 26.9% in the acidic sandy soil compared to N-fertilizer only treatment. In contrast, olive biochar addition showed no significant effect on CO 2 emissions but decreased N 2 O emissions by 34.3% in the alkaline clay soil, while in the acidic sandy soil addition of olive biochar triggered about a twofold higher maximum CO 2 emission rate and decreased N 2 O emissions by 68.4%. Up to 50–130% higher specific CO 2 emissions (per unit of C-related enzyme activity: cellobiohydrolase, chitinases and β-glucosidase) were observed after addition of olive biochar compared to corn biochar addition in the acidic sandy soil. We concluded that biochar’s effects on N 2 O and CO 2 emissions are more pronounced in acidic soils. Alkaline biochar’s N 2 O mitigation potential in acidic soils seems to be dependent on soil NO 3 − content as drastically higher N 2 O emissions were measured in early phase of the experiment (where soil NO 3 − was high) and significantly lower N 2 O fluxes were obtained in later phases (with lower soil NO 3 − content). [ABSTRACT FROM AUTHOR]

Straw management strategies are highly important for maximizing the benefits of straw incorporation, which should aim to increase crop production while improving soil fertility. Ammoniated straw incorporation may be one of the potential candidates for achieving these goals. However, the effects of ammoniated straw incorporation on wheat yield, yield stability and soil properties as well as their potential relationships remain poorly understood. Based on an ongoing long-term field experiment commenced in 2011 on the Chinese Loess Plateau, we investigated the responses of soil properties, wheat yield and yield stability of winter wheat (Triticum aestivum L.) to ammoniated and conventional straw incorporation during 2017–2020. The three treatments were: (i) no straw (Control), (ii) conventional straw incorporation (CSI), and (iii) ammoniated straw incorporation (ASI). We found that the ASI treatment on average significantly increased wheat yield by 10.1% and yield stability by 19.5% compared to the CSI treatment, and significantly increased wheat yield by 26.9% and yield stability by 38.7% compared to the Control treatment. Changes in wheat yield and yield stability were positively related to ASI-induced increases in soil water storage. When compared to the Control and CSI treatments, the ASI treatment on average significantly increased soil organic carbon (SOC) content by 17.2% and 14.2% and total nitrogen (TN) content by 27.3% and 18.3% in 0–10 cm depth, and it significantly increased SOC content by 19.2% and 12.4% and TN content by 27.8% and 19.4% in 10–20 cm depth, respectively. There were positive relationships between changes in wheat yield and SOC and TN content. These results demonstrate that it is feasible to achieve higher wheat yield and yield stability while increasing SOC and TN content by optimizing straw management practices in semi-arid areas. [Display omitted] • We compared ammoniated and conventional straw incorporation (ASI and CSI) for their performances in wheat cropping. • ASI treatment increased grain yield, yield stability, SOC and TN compared to CSI and Control treatments. • Increases in grain yield and yield stability are positively correlated with changes in soil water storage. • There were positive relationships between changes in grain yield and SOC and TN. [ABSTRACT FROM AUTHOR]