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

Abstract

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 longterm 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.