Influence of Nitrate and Phosphorus Loading on Denitrifying Enzyme Activity in Everglades Wetland Soils

Abstract

There has been recent concern about the impact of increased nutrient loading on the northern Everglades ecosystem. We investigated the spatial and temporal distribution of denitrifying enzyme activity (DEA) along a P‐enrichment gradient in the Water Conservation Area 2A (WCA‐2A) and determined the effects of added P and NO−3 on DEA. The DEA in soil and detritus layers was measured under anaerobic conditions four times during 2 yr, using the acetylene blockage technique. The DEA ranged from 0.004 to 7.75 mg N2O–N kg−1 h−1 Highest rates of DEA were found in the detritus and surface (0–10 cm) soils, and rates decreased exponentially with increasing distance from the surface‐water inflow point, where nutrients are loaded to the wetland. Nitrate was found to be limiting, while the addition of P had no effect on the distribution of DEA in these soils. There was a seasonal effect on DEA, with higher activity observed during the summer when temperatures and hydraulic and nutrient loading were highest. Soils from outside the impacted zone demonstrated denitrifying potentials, within 10 h when spiked with inflow concentrations of NO−3, similar to DEA of soils from within the impacted zone. This suggests that soils from outside the impacted zone can increase denitrification rates when exposed to higher NO−3 concentrations in a relatively short time. Agricultural drainage water discharge, and consequent NO−3 loading, has created a zone of elevated DEA proximal to the S‐10C surface‐water inflow point in WCA‐2A.