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Background: The respiratory quotient, that is, the CO2–O2‐exchange ratio during aerobic heterotrophic soil respiration, (RQAHR) is one of the key variables of the barometric process separation (BaPS) technique. For reliable application, the BaPS method requires the adjustment of RQAHR to soil‐specific substrate conditions. Aim: The objective of the present study was to determine RQAHR in five arable soils varying between pH 4 and 8 and carbon/nitrogen (C/N) ratios between 8.4 and 11.9 using the BaPS–15N method. Further, we aimed to elucidate whether RQAHR can be estimated from basic soil properties. Results: Determined RQAHRs were soil specific and ranged between 0.74 and 1.53. RQAHR particularly affected the calculation of gross nitrification rates but not soil respiration rates. Based on a multiple linear regression analysis, soil pH and the C/N ratio of the soil organic matter pool were identified to explain well (R2 = 0.984) the measured RQAHRs. Applying the thus derived RQMLR to the calculation procedure led to good agreement between gross nitrification rates derived by the 15N pool dilution technique. Conclusions: Future studies are needed to test whether this empirical relationship between RQAHR, pH, and C/N ratio is generalizable. If this would be the case, the BaPS method could be applied without the use of a stable isotope and thus, being less resource demanding than the 15N pool dilution technique. [ABSTRACT FROM AUTHOR]

Excessive amounts of nitrate have accumulated in many soils on the North China Plain due to the large amounts of chemical N fertilizer used combined with low carbon inputs. The present study investigated the promotion of soil nitrate transformation to soil organic N by different carbon amendments. A laboratory incubation experiment using 15N tracer (K15NO3) was employed to elucidate the proportion of soil organic N derived from accumulated soil nitrate following amendment with glucose (G) or maize straw (S) at controlled soil temperature and moisture content. During the 56-d incubation period we determined the dynamics and isotopic abundance of mineral N (NO3- and NH4+) and soil organic N and greenhouse gas (N2O and CO2) emissions. Carbon amendment markedly stimulated transformation of nitrate to soil organic N. Mineralization of native soil organic N was also enhanced because soil organic N at the end of the incubation period was not significantly different from that in the control without added C. Glucose had a greater effect on nitrate-N immobilization (25.0 mg kg-1) than maize straw (9.4 mg kg-1) and glucose addition also led to highest greenhouse gas emissions. With glucose and straw amendment total N2O-N emissions were 1.01 and 0.03 mg kg-1, respectively. Similarly, glucose amendment lowered the concentration and abundance of NO3--N and increased that of NH4+-N more than maize straw. In agricultural practice straw amendment may be an effective strategy to deplete the excess NO3--N pool and restrict leaching, to promote nitrate-N immobilization and to decrease greenhouse gas emissions.

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N2O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N2O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N2O fluxes in an arable soil, combined with in situ high‐frequency measurements of soil matrix oxygen (O2) and N2O concentrations, in situ 15N labeling, and N2O 15N site preference (SP). The in situ O2 concentration and further microcosm visualized spatiotemporal distribution of O2 both suggested that O2 dynamics were the proximal determining factor to matrix N2O concentration and fluxes due to quick O2 depletion after N fertilization. Further SP analysis and in situ 15N labeling experiment revealed that the main source for N2O emissions was bacterial denitrification during the hot‐wet summer with lower soil O2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold‐dry winter with higher soil O2 concentration. The robust positive correlation between O2 concentration and SP values underpinned that the O2 dynamics were the key factor to differentiate the composite processes of N2O production in in situ structured soil. Our findings deciphered the complexity of N2O production processes in real field conditions, and suggest that O2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N2O production processes in undisturbed structure soils. Our results help to develop targeted N2O mitigation measures and to improve process models for constraining global N2O budget. [ABSTRACT FROM AUTHOR]