The emission of greenhouse gases (GHG) from wetland soils depends on C availability, particularly from soil and dissolved organic carbon (DOC) sources. In addition, plant residues inputs supply an additional C source influencing DOC levels. We examined GHG emissions in degraded wetlands undergoing restoration with rice cultivation. To identify sources of soil and plant residue C, we used C-13 labeled rice straw in lab and field studies. Our experimental design included soils with C contents from 2 to 15%. Rice straw addition resulted in a 3-fold increase in DOC from SOC, defined as the priming effect (PE), with the largest amount in the 15% C soil. The release of DOC from the PE was associated with the reduction of Fe3+ to Fe2+. In rice residue amended soils, SOC was the dominant source of DOC, however the primed DOC did not result in a proportional increase in CH4 production. The contribution of SOC to CH4 production increased from 30 to 50% with residue addition as SOC content increased. The rice residue induced priming of CH4 from SOC was positively correlated to the degree of residue decomposition, while the residue priming efficiency on CH4 production was negated by other electronic accepters. In a field study, CH4 and N2O emissions were monitored in an N rate trial experiment with rice plants pulse labeled with 13CO2. Both CH4 and N2O emission were unaffected by N rates up to 160 kg ha-1. However increasing N rates increased plant derived DOC. Across N treatments, plant derived C contributed 27% of emitted CH4, and up to 9.1% of DOC. N2O consumption was observed across all N rates. In a separate study, N2O consumption was related to SOC with the highest consumption rates in the highest C soils. The highest consumption rates amounted to 8.2 tons of CO2 equivalents ha-1 y-1, 32% of the average CO2 emissions from disturbed drained peat soils. The highest N2O consumption rates occurred during soil drainage associated with rice cultivation. Overall, N fertilization decreased CH4 emissions particularly in the low C soils. Annual budgets of N2O emissions were not affected by N fertilizer and in all cases regardless of soil C content acted as a sink for this important GHG. The capacity of rewetted wetlands to act as an atmospheric N2O sink is significant and can be made a stronger sink by manipulating water depth. The use of rice cultivation to restore degraded peat soils generally emitted less GHG than business as usual crops growing under drain conditions.

Influence of soil carbon and priming effects on greenhouse gases emissions in degraded wetlands restored with rice cultivation

255th National Meeting (2018)

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