Nitrogen Cycling and Rice Crop Management: Implications for Subsidence of Delta Peat Soils.

Since being drained for agriculture in the mid-1800s, continuous farming of peat soils in the Sacramento-San Joaquin Delta has led to severe soil subsidence up to >6 meters in places, primarily due to microbial oxidation of the exposed peat. Rice production has been proposed as an alternative to aerobic cropping systems in the Delta to protect the soil from further oxidation and loss; however, there remains a need to assess the impact of rice cultivation on actual rates of subsidence. A nitrogen (N) fertilizer trial on Twitchell Island in 2011 revealed no significant positive rice grain yield response to N (p<0.05) and high yields in all plots. We hypothesize that peat oxidation resulted in sufficient N mineralization to sustain crop production, indicating that soil loss is continuing. We conducted a series of field experiments including water, fertilizer, and ¹⁵N-labelled crop residue manipulations using crop N uptake as a proxy to estimate subsidence. This presentation describes N cycling and estimated subsidence in Twitchell Island rice fields based on these experiments.

Results from the 2012 experiments suggest the majority of plant-available N is still being supplied by the soil when no fertilizer is applied, and estimated subsidence rates at Twitchell Island based the N budgeting approach were 3.1 (0.3) mm/year and 4.7 (0.6) mm/year in the two fields studied. These results raise questions about the degree to which rice systems can be used to halt subsidence on Sacramento-San Joaquin Delta peat soils.