Factors influencing the decomposition of organic soils of the California delta

Since the subsidence rate in the peat and muck lands of the California Delta is approximately double that reported in other areas, and since biological decomposition is a major cause of subsidence, reasons for the high decomposition rate were investigated. Estimates of decomposition in field soils based on ash analyses failed to account for all the subsidence which has occurred, suggesting that considerable soil has been lost by means other than oeroblc decomposition of the leyers above the ground water level. Other possibilities are wind erosion and anaerobic decomposition below the water table. Appreciable decomposition rates measured in peat incubated under conditions of limited oxygen supply suggest that anaerobic decomposition contributes to subsidence in the field. Alternate wetting and drying were found to accelerate the rate of decomposition. Immediately after each wetting a high Tate was observed, which declined gradually. Mixture of peat and muck soils with mineral soils was found to increase, rather than retard, loss of carbon from the organic soils. This was partly due to the higher pH of the mineral soils in some instances, but a similar effect was observed when no pH change occurred. Addition of plant residues to peat soils tends to counteract the effects of subsidence, even though normal amounts of residues are insufficient to maintain the soil elevation. Evidence is presented, which indicates that subsidence rates in the Delta may be decreasing, and may eventually attain values comparable with those reported elsewhere. The experiments suggest that as a means of conserving peat and muck soils infrequent cultivation, high water table, and maximum return of crop residues to the soil are desirable.