Simultaneous Measurements of Dissolved Organic Carbon and Soil Respiration Reveal Reduced Soil Carbon Loss Under Nitrogen Addition in a Montane Forest

Abstract

AbstractA major uncertainty in the estimation of soils acting as net carbon (C) sinks or sources stem from the effects of anthropogenic nitrogen (N) input on the balance between plant C input and soil C loss. In contrast to the generally observed increasing pattern of plant C input, the response of soil C loss to increased N deposition remains elusive, largely due to its large temporal variation. Here simultaneous measurements of two major soil C loss pathways, including dissolved organic carbon (DOC) leaching and soil respiration, were conducted for 5 and 3 yr, respectively, to assess the effects of N addition on soil C loss in an N‐limited montane forest. The effects were seasonal, depth and N level dependent and the two pathways responded asynchronously to N addition. Significant decreases in DOC concentrations and fluxes in leachates from the organic layer were observed during autumn/winter under a high N addition rate (40 kg N/ha/yr). No significant impact of N addition on DOC concentrations or fluxes was observed for leachates from the mineral soil horizon. Biodegradability was low for DOC from both soil layers and was not consistently influenced by N addition. Soil respiration was significantly decreased under high N addition. Annual soil C loss (estimated by summing DOC leaching from the mineral horizon and soil respiration) showed that N addition reduced soil C loss consistently over years, implying that the forest soil is likely a C sink under excess N deposition, which should be confirmed with longer term monitoring.