Quantifying The Contribution Of Rice Plants To Carbon Pools and Efflux: An Analysis Of Nitrogen Fertilization Effects.

Rice fields have been implemented in the Sacramento-San Joaquin Delta (California, U.S.) in an effort to address the long-term issue of soil subsidence. Here we examined the effects of nitrogen fertilization on carbon biogeochemistry and greenhouse gas (GHG) emissions from rice fields. Previous studies indicated no significant increase in rice grain yields under nitrogen fertilization, suggesting an opportunity to use N management to influence GHGs. We hypothesized lower nitrogen rates would decrease nitrous oxide (N₂O) and methane (CH₄) emissions, 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, and quantified GHG with the static chamber method. This was in combination with a ¹³C pulse-labeling experiment to determine the contribution of rice plants to total CH₄ emissions and pore water carbon pools, as rice plants provide labile carbon to soil microorganisms, which may influence soil carbon dynamics and GHG production and emission. Rice grain yields declined at the highest rate of nitrogen fertilization; however, there was no nitrogen effect on total above and below ground biomass. It was likely that rice plants were able to obtain required nitrogen through other means, such as mineralization of existing organic pools or upwelling of subsurface water.

                The ¹³C labeling showed no effect of N treatment on contribution of rice plants to emitted CH₄, dissolved CH₄, or dissolved inorganic carbon (DIC). Carbon derived from rice plants contributed to up to 20%  of the total CH₄ emissions, 24% of dissolved CH₄, and 37% of DIC. Overall, CH₄ emissions did not appear to be influenced by N fertilization rates.