Early season Nsub2/subO emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile

摘要

Soil moisture strongly affects the balance between nitrification, denitrification and Nsub2/subO reduction and therefore the nitrogen (N) efficiency and N losses in agricultural systems. In rice systems, there is a need to improve alternative water management practices, which are designed to save water and reduce methane emissions but may increase Nsub2/subO and decrease nitrogen use efficiency. In a field experiment with three water management treatments, we measured Nsub2/subO isotope ratios of emitted and pore air Nsub2/subO ( δsup15/supN , δsup18/supO and site preference, SP) over the course of 6?weeks in the early rice growing season. Isotope ratio measurements were coupled with simultaneous measurements of pore water NO 3 - , NH 4 + , dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths. We then used the relationship between SP × δsup18/supO - Nsub2/subO and SP × δsup15/supN - Nsub2/subO in simple two end-member mixing models to evaluate the contribution of nitrification, denitrification and fungal denitrification to total Nsub2/subO emissions and to estimate Nsub2/subO reduction rates. Nsub2/subO emissions were higher in a dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to water-seeded + alternate wetting and drying (WS-AWD) and water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD treatment the highest emissions were associated with a high contribution from denitrification and a decrease in Nsub2/subO reduction, while in the WS treatments, the highest emissions occurred when contributions from denitrificationitrifier denitrification and nitrification/fungal denitrification were more equal. Modeled denitrification rates appeared to be tightly linked to nitrification and NO 3 - availability in all treatments; thus, water management affected the rate of denitrification and Nsub2/subO reduction by controlling the substrate availability for each process ( NO 3 - and Nsub2/subO ), likely through changes in mineralization and nitrification rates. Our model estimates of mean Nsub2/subO reduction rates match well those observed in sup15/supN fertilizer labeling studies in rice systems and show promise for the use of dual isotope ratio mixing models to estimate Nsub2/sub losses.