Early season N 2 O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile
2018
Abstract. Soil moisture strongly affects the balance between nitrification, denitrification
and N 2 O 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 N 2 O and decrease nitrogen
use efficiency. In a field experiment with three water management treatments,
we measured N 2 O
isotope ratios of emitted and pore air N 2 O
( δ 15 N , δ 18 O 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 × δ 18 O - N 2 O and
SP × δ 15 N - N 2 O in simple two end-member
mixing models to evaluate the contribution of nitrification, denitrification
and fungal denitrification to total N 2 O emissions and to estimate
N 2 O reduction rates. N 2 O 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 N 2 O reduction, while in the WS
treatments, the highest emissions occurred when contributions from
denitrification/nitrifier 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
N 2 O reduction by controlling the substrate availability for each
process ( NO 3 - and N 2 O ), likely through changes in
mineralization and nitrification rates. Our model estimates of mean
N 2 O reduction rates match well those observed in 15 N
fertilizer labeling studies in rice systems and show promise for the use of
dual isotope ratio mixing models to estimate N 2 losses.
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