Denitrification, organic matter decomposition, and N mineralization in organic and mineral soils of two riparian ecosystems

Denitrification (reduction of soluble NO{dollar}sb3sp-{dollar} to gaseous N{dollar}sb2{dollar}) within riparian areas is a pivotal process that helps reduce N loading into streams from nonpoint sources. This study was conducted to evaluate the functionality of riparian ecosystems with soil and vegetation characteristics not previously studied. Because denitrification rates are closely related to the availability of N and C released from decomposing soil organic matter, soil respiration and N mineralization rates were also measured. Denitrification measurements were conducted in two ecosystem types ((a) Atlantic white cedar swamps (Chamaecyparis thyoides) with organic soil, (b) mixed hardwood forests with mineral soil) using several indirect methods (acetylene inhibition, denitrifying enzyme activity (DEA), ecosystem mass balance, and decrease in groundwater nitrate concentration) and a direct method measuring actual N{dollar}sb2{dollar} flux, previously used only for submerged sediments. A model was developed to estimate the diffusive flux of trapped atmospheric nitrogen from soil porespace, so that N{dollar}sb2{dollar} flux due to denitrification could be measured. Over four seasons, average seasonal denitrification rates based on direct N{dollar}sb2{dollar} flux ranged from 0 to 235 {dollar}mu{dollar}mol N/m{dollar}sp2{dollar}/hr for organic soils which were 1-2 orders of magnitude greater than results by acetylene inhibition (0.2 to 3.0 {dollar}mu{dollar}mol N/m{dollar}sp2{dollar}/hr). In the mineral soils, N{dollar}sb2{dollar} flux rates (30 to 130 {dollar}mu{dollar}mol N/m{dollar}sp2{dollar}/hr) were also 1-2 orders of magnitude greater than results by acetylene inhibition (0.1-10 {dollar}mu{dollar}mol N/m{dollar}sp2{dollar}/hr) and were 1 to 5 times greater than DEA rates (2-60 {dollar}mu{dollar}mol N/m{dollar}sp2{dollar}/hr). Over an annual cycle in one forested site, the direct N{dollar}sb2{dollar} flux rate measurement (6 kg N/ha/yr) was comparable to the denitrification estimate based on ecosystem mass balance (7 kg N/ha/yr) and was within a factor of two of the denitrification estimate based on the decrease in groundwater NO{dollar}sb3{dollar} concentration. CO{dollar}sb2{dollar} production rates were not significantly different between soil types in a given season, but rates were greater in summer than fall or spring. The average net N mineralization rates for organic soil in summer and spring (177 and 53 mg NH{dollar}sb4{dollar}-N/m{dollar}sp2{dollar}/day, respectively) were more than double that of mineral soils (77 and {dollar}-{dollar}53 mg NH{dollar}sb4{dollar}-N/m{dollar}sp2{dollar}/day). There was no correlation between denitrification rates and decomposition rates or N mineralization rates.