Quantifying the Contribution of Plants and Soils to CH4 Fluxes and Net Seasonal N2o Emissions in an Agricultural Wetland.

Rice cultivation has been implemented in the Sacramento-San Joaquin Delta (California, U.S.) in an effort to mitigate greenhouse gas emissions, soil loss, and subsidence. Here we examined the effects of nitrogen fertilization on carbon biogeochemistry and greenhouse gas (GHG) emissions from rice fields at Twitchell Island, CA, in 2012. Given that N is often a limiting factor for rice production and N fertilizers are significant sources of N₂O, we hypothesized that lower nitrogen rates would reduce the emissions of nitrous oxide (N₂O) and methane (CH₄), due to reductions in total N availability and plant labile carbon input to soils. We applied N at rates of 0, 80, and 160 kg N ha^-1 as urea, and quantified GHGs emissions with the static chamber method. Additionally, we determined the contributions of rice plants to soil C pools and subsequent CH₄ emissions using a ¹³C pulse-labeling experiment carried out at two growing stages of rice plants under different field conditions. We found no nitrogen effect on total above and below ground biomass production, but rice grain yields declined significantly at the highest (160 kg N ha^-1) rate of nitrogen addition. This indicates that rice plants were able to obtain enough nitrogen from the existing soil organic pools and upwelling of subsurface water. The ¹³C labeling experiment showed no effect of N treatment on the contribution of rice plants to dissolved (pore water) and emitted CH₄, however, the higher nitrogen application rates of 160 resulted in more plant derived dissolved inorganic carbon (DIC) than the 80 N rate. Plant derived carbon contributed from 0 to 27% of CH₄ emissions, from 0 to 74% of dissolved CH₄, and from 13 to 78% towards DIC. These ranges probably reflect shifts in growing stages and changes in soil water levels. Contradicting our expectations, emissions of N₂O and CH₄ were not affected by nitrogen fertilization rates. While negative N₂O fluxes were observed, N₂O proved to be a weak source with an average of 0.06 μg N₂O-N m^-2 day^-1. Methane emissions were consistently positive with average emissions of 74 mg CH₄-C m^-2 day^-1.