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Manure application is effective in promoting soil carbon sequestration, but its impact on N2O emission is not well understood. A field experiment was conducted in a maize‐cultivated black soil in Northeast China with six treatments: inorganic fertilizer (NPK), 75% inorganic fertilizer N plus 25% pig (PM1) or chicken (CM1) manure N, 50% inorganic fertilizer N plus 50% pig (PM2) or chicken (CM2) manure N, and no N fertilizer (CK). Annual N2O emission significantly increased from 0.34 kg N ha−1 for CK to 0.86 kg N ha−1 for NPK and further to 1.65, 1.02, 1.17, and 0.93 kg N ha−1 for PM1, CM1, PM2, and CM2, respectively. A 15N tracing study showed that 71–79% of total N2O was related to nitrification at 30–70% water‐filled pore space (WFPS), and heterotrophic nitrification contributed 49% and 25% to total N2O at 30% and 70% WFPS, respectively. In an incubation, N2O emission was only stimulated when nitrate and glucose were applied together at 60% WFPS, indicating that denitrification was carbon limited. PM had a stronger effect on denitrification than CM due to higher decomposability, and the lower N2O emission at higher manure application rate was associated with decreased mineral N supply. After compiling a worldwide database and establishing an empirical model that related N2O emissions (kg N ha−1) to precipitation (Pr, m) and fertilizer N application rate (Nr, kg N ha−1) (N2O = 1.533Pr + 0.0238PrNr), annual N2O emission from global‐cultivated black soil applied with inorganic fertilizer N was estimated as 347 Gg N. Our results suggested that N2O emission from cultivated black soils in China was low primarily due to low precipitation and labile organic carbon availability, and would be stimulated by manure application; thus, increased N2O emission should be taken into consideration as applying manure increases soil organic carbon sequestration.

26Extreme precipitation is predicted to be more frequent and intense accompanying global 27 warming, and may have profound impacts on soil respiration (Rs) and its components, i.e., 28 autotrophic (Ra) and heterotrophic (Rh) respiration. However, how natural extreme rainfall or 29 snowfall events affect these fluxes are still lacking, especially under nitrogen (N) fertilization. 30In this study, extreme rainfall and snowfall events occurred during a 3-year field experiment,

Partial nitrate nutrition was testified to improve rice (Oryza sativa L.) growth. However, how partial nitrate nutrition is related to root growth in rice cultivars with different N-use efficiency is still unclear. Two rice cultivars, Nanguang (high N-use efficiency) and Elio (low N-use efficiency), were grown on six ratios of NH 4 + /NO 3 -in the solution. The response of root growth to partial nitrate nutrition was investigated and N status and auxin concentration were recorded in order to elucidate the mechanisms by which the optimal ratio of NH 4 + /NO 3 -that controls the architecture of the root system. The length of adventitious and lateral roots was stimulated only in cv. Nanguang by partial nitrate nutrition. Nitratestimulated root length in Nanguang resulted mainly from root initiation rather than root elongation. Root biomass was similar between two rice cultivars under NH 4 + supplied, while higher root biomass was observed in cv. Nanguang than in cv. Elio under 25% nitrate supplied for 10 weeks. Significant IAA increase was recorded in cv. Nanguang under 25% nitrate presence in the nutrition compared to under sole NH 4 + solution. The presence of nitrate increased root initiation in the rice with high N-use efficiency, which led to faster biomass accumulation and higher N-use efficiency at later growth stages.

Understanding the change in chemical composition of crop residues during their decomposition is crucial to elucidate the mechanisms underlying the effects of residue retention on soil carbon (C) sequestration and nutrient cycling. Here a field experiment was carried out to investigate the decomposition process of maize, soybean, and wheat residues in a cultivated Mollisol in northeast China over a year. Using a litterbag method, we monitored the dynamics of residue mass loss, concentration of C and nitrogen (N), and C/N ratio, and evaluated the decomposition rates of residues and C functional groups. Chemical compositions of the crop residues were determined by solid-state 13 C-nuclear magnetic resonance spectroscopy, and the cellulose crystallinity, lignin concentration, and syringyl to guaiacyl (S/G) ratio of lignin were estimated. After one year of incubation in field conditions, mass loss was 57% for soybean residues, significantly greater than 52% for maize and 45% for wheat. The decomposition rate of residues significantly decreased from 0.223-0.379 month −1 during the first month to 0.054-0.076 month −1 over the whole period. The proportion of decomposed C ranged at 57-63% among different residues, and had a positive relationship with the mass loss of O-alkyl C, di-O-alkyl C, and carbonyl C. The decomposition rates of these three C functional groups were greater in soybean than those in maize and wheat residues, which was also the case for lignin S/G ratio, possibly accounting for the higher decomposition extent of soybean residues. As decomposition progressed, the C chemistry of maize, soybean, and wheat residues exhibited an increasing divergence, which was mainly related to relative decreases in O-alkyl C and di-O-alkyl C contents, and relative increases in 3 phenolic C and aromatic C contents in the residues. Net N release was observed for all residues after one-year decomposition, and was significantly related to the mass loss of alkyl C, O-alkyl C, and aromatic C. Overall, our study provides insight into chemical changes of crop residues over the degradation processes in the field, and highlights the significant effects of organic C chemistry on residue decomposition.

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