DIVISION S-10—WETLAND SOILS Organic Matter Oxidation Potential Determination in a Periodically Flooded Histosol under Sugarcane

trol both soil oxidation and P losses include maintaining water tables as high as possible without jeopardizing Histosols of the Everglades Agricultural Area (EAA) are subsiding crop yields and growing crops that are tolerant to high primarily from aerobic microbial oxidation. An experiment was conwater table levels (Anderson and Flaig, 1995). ducted in a Histosol to evaluate methods [14C-benzoate oxidation (BO), soil respiration (CO2 evolution; RESP), and microbial biomass There are few methods that can be used to monitor carbon (MBC)] of determining short-term organic matter (OM) oxisoil loss under replicated field experiments (Lal, 1998). dation potential under alternate flood and drain management. SugarSoil subsidence in the EAA was measured approxicane (interspecific hybrids of Saccharum species) was grown in field mately every 5 yr from the early 1900s until 1978 by lysimeters containing Pahokee muck soil (euic, hyperthermic Lithic surveying marker posts driven to bedrock (Shih et al., Haplosaprist). Soils were flooded for 7 d followed by drainage to 1997). Marker posts cannot be placed in sugarcane fields three depths (16, 33, and 50 cm) for 14 d. A continuously drained because they would easily be damaged by heavy equipcontrol treatment was also included. Thus, soils under four flood and ment that must pass through the fields during planting, drain cycles were sampled during the drain period. Increased water fertilizing, cultivating, and harvesting. Short-term methods table levels after flooding reduced BO, tended to increase MBC, and to monitor OM degradation potential under cropped conhad little effect on RESP. Consequently, BO was not correlated with either MBC or RESP, which suggests that microbes responsible for ditions are necessary to evaluate the effects of crop and OM oxidation are a small portion of the total microbial population. water table management practices in the EAA to reduce Alternate flooding and draining increased BO compared with the or prevent soil loss and mineralization of P. Three methdrain control. Soil moisture (SM) and soluble organic carbon (SOC) ods have potential for use. The first method applies a were both negatively correlated with BO, but not with either MBC recalcitrant 14C-labeled substrate to the soil to measure the or RESP. Soil moisture and SOC may provide alternate indicators oxidative activity of microorganisms. The second method for OM oxidation potential. Since BO responded according to soil measures soil CO2 evolution to determine microbial resOM oxidation on the basis of field measurements previously reported, piration, and the third method estimates soil MBC to BO was the best of the three methods for predicting short-term effects indicate changes in microbial populations. of water table management on soil OM oxidation potential. The 14C-labeled organic compound method was used by Tate and Terry (1980) who reported that oxidation of aromatic ring compounds (14C-labeled salicylate) in H of the EAA are subsiding at a rate of aerobic Histosols was correlated to moisture content of 1.4 cm yr 1 (Shih et al., 1998). Most of the subsithe soil. Under flooded conditions for 25 d, measuring dence is due to soil loss resulting from aerobic microbial catabolism from aromatic ring compounds (salicylate) activity (Tate, 1980a). Currently, continual net oxidative was better than measuring catabolism from simple carlosses from these organic soils have a negative impact bohydrates (glucose) and amino acids (mixture), beon farmers in the EAA because there are only about cause the aromatic ring compound was more resistant 0.3 to 1 m of soil left on top of limestone bedrock to decomposition than the other chemicals (Tate, 1979a). (Ingebritsen et al., 1999). In another study, extrapolation of oxidation rates from An additional problem for producers is mineraliza14C-labeled acetate and succinate applied to soil taken tion of P from soil OM. Phosphorus release from OM from a drained field showed close agreement with actual oxidation has been estimated at 72 kg ha 1 yr 1 (Diaz long-term rates for soil loss calculated from field marker et al., 1993). Since federal and state governments have posts (Tate, 1979b). Disadvantages for using the 14C targeted P for reductions in the waterways, producers method are the high cost of purchasing and disposing must reduce P levels in drainage waters by 25%, pay a of labeled compounds, and compliance with federal and land use privilege tax of $61.50 ha 1 yr 1, and follow best state regulations regarding use of radioactive substances. management practices to reduce P levels in waterways Carbon dioxide evolution by soil microorganisms is (Izuno et al., 1999). Best management practices to cona direct measure of soil C loss and has been used to measure subsidence potential from fallow organic soil D.R. Morris and B. Glaz, USDA–ARS, Sugarcane Field Station, ( 85% OM) in the laboratory (Knipling et al., 1970; 12990 U.S. Hwy. 441, Canal Point, FL 33438; S.H. Daroub, Everglades and Volk, 1973). However, when plant roots are present, Research and Education Center, Univ. of Florida, 3200 E. Palm Beach Road, Belle Glade, FL 33430. Received 13 Nov. 2002. *Corresponding author ([email protected]). Abbreviations: BO, 14C-benzoate oxidation; EAA, Everglades Agricultural Area; MBC, microbial biomass carbon; OM, organic matter; Published in Soil Sci. Soc. Am. J. 68:994–1001 (2004).  Soil Science Society of America RESP, soil respiration (CO2 evolution); SM, soil moisture; SOC, soluble organic carbon; T7, temperature at the 7-cm depth. 677 S. Segoe Rd., Madison, WI 53711 USA