Influence of Variable Soil Carbon on CH4 and N2O Emissions from Rice Fields.

Reclamation of the Sacramento-San Joaquin Delta’s peatlands provided fertile soils for agriculture, but resulted in the oxidation of peat soils, extensive greenhouse gas (GHG) emissions, and subsidence.  Rice cultivation has been proposed as a regional solution to mitigate past agricultural impacts.  Multiple factors are capable of influencing GHGs emissions.  In the present study, we investigate the influence of nutrient management and soil carbon content on CH₄ and N₂O emissions from highly organic peat soils under rice cultivation.  The experiment was conducted in rice fields at Twitchell Island, California and consisted of 4 soil carbon contents (5%, 10%, 20%, and 25%) in combination with 2 nitrogen fertilization rates (0 and 80 kg N ha^-1 as urea).  We determined CH₄ and N₂O emissions with a typical closed chamber method during the growing season of 2013.  In general, N fertilization did not affect CH₄ and N₂O emissions regardless of soil C content, while both CH₄ and N₂O emissions varied considerably across the soil C gradient.  CH₄ emissions were generally higher from the 5% C field than others.  CH₄ emissions were maximal during the drainage event, averaging 48.3, 17.2, 5.2, and 11.4 mg CH₄ hr^-1 m^-2 for 5%, 10%, 20%, and 25% C fields, respectively.  N₂O emissions were low during the flooding period and negative emissions were frequently observed in all fields, especially in the 25% C field, suggesting the consumption of N₂O was greater than soils with lower carbon.  High N₂O emissions were observed in all fields right after the drainage, except for the 25% C field where the emission was largely negative.  Our results suggested that, in addition to carbon quantity, soil carbon quality maybe also important in controlling the GHGs emissions.  Furthermore, N management many not be as effective as water management to mitigate GHGs emissions from the rice fields with highly organic, but subsiding soils.