Export of dissolved organic carbon and nitrate from grassland in winter using high temporal resolution, in situ UV sensing

Abstract

Co-deployment of two reagentless UV sensors for high temporal resolution (15min) real time determination of wintertime DOC and nitrate-N export from a grassland lysimeter plot (North Wyke, Devon, UK) is reported. They showed rapid, transient but high impact perturbations of DOC (5.3–23mg CL−1) and nitrate–N export after storm/snow melt which discontinuous sampling would not have observed. During a winter freeze/thaw cycle, DOC export (1.25kg Cha−1d−1) was significantly higher than typical UK catchment values (maximum 0.25kg Chad−1) and historical North Wyke data (0.7kg Cha−1d−1). DOC concentrations were inversely correlated with the key DOC physico-chemical drivers of pH (January r=−0.65), and conductivity (January r=−0.64). Nitrate–N export (0.8–1.5mgNL−1) was strongly correlated with DOC export (r≥0.8). The DOC:NO3–N molar ratios showed that soil microbial N assimilation was not C limited and therefore high N accrual was not promoted in the River Taw, which is classified as a nitrate vulnerable zone (NVZ). The sensor was shown to be an effective sentinel device for identifying critical periods when rapid ecosystem N accumulation could be triggered by a shift in resource stoichiometry. It is therefore a useful tool to help evaluate land management strategies and impacts from climate change and intensive agriculture.