Nitrogen Cycling in Seasonal Wetlands in Subtropical Cattle Pastures.

Understanding the effects of agricultural land use on wetland N cycling is critical in areas such as south-central Florida, where widespread agricultural activities intersect with extensive wetland systems. We examined annual net N mineralization (Nmin) (buried-core method) and denitrification rates (acetylene-block method) in 24 small seasonal wetlands on a cattle ranch in this region, 12 each in intensively managed improved pastures (IP) and less intensively managed semi-native pastures (SNP). Wetlands in IP had less detritus, higher N concentrations, lower C/N ratios (0-15 cm), and higher microbial biomass N than did wetlands in SNP Cumulative annual net Nmin was lower in IP wetlands (98 ± 17 kg N ha-1) than in SNP wetlands (133 ± 18kg N ha-1). Nitrification was much lower in IP than in SNP wetlands and dominated net Nmin during the dry season (December-June), but was negligible during the flooded period (July-October). Cumulative annual denitrification was lower in IP wetlands (17.7 ± 3.4 kg N ha-1) than in SNP wetlands (34.7 ± 6.3 kg N ha-1). Soil N cycling rates correlated with NO3- and NH4+ concentrations, which correlated with soil C content. Our results show that the more intensive management of improved pastures was associated with declines in wetland soil C content and lower rates of nitrification and denitrification. [ABSTRACT FROM AUTHOR]