Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO3-) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35-50 kg N ha(-1) y(-1) as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5-16.9 kg N ha(-1) y(-1)). DON leaching increased 35-162% following 2.5 years of experimental input of 50-150 kg N ha(-1) y(-1). The fertilizer-driven increase of DON leaching comprised 4-17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC: DON ratios declined in all three forests. Our data showed that DON accounted for 23-38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO3- leaching in all three forests. We hypothesize that abiotic incorporation of excess NO3- (through chemically reactive NO2-) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China.