Revealing the Drivers of Unexpected High Nitrogen Levels in Tropical Rainforest Rivers in Winter

Elevated river nitrogen (N) concentrations have contributed to eutrophication and subsequent ecological damage worldwide. However, current research has mostly focused on nitrate (NO 3 − ) in rivers, with relatively few studies on the sources of different N forms, especially in tropical rainforest rivers. This study combined multiple natural abundance isotopes (δ 15 N/δ 18 O-NO 3 − , δ 15 N-NH 4 + , δ 15 N-PON, and δD/δ 18 O-H 2 O) and 15 N-pairing incubations to demonstrate the processes underlying the unexpectedly high N concentrations in the rivers draining the National Park of Hainan Tropical Rainforest (the largest rainforest in China) in winter. The natural abundance isotopes revealed that NO 3 − in the rivers was largely from forest soils. NH 4 + -N in the rivers was from precipitation (30.3%), sewage (19.3%), forest soil (27.4%), and agriculture (23.1%), and the PON were largely terrestrial plants and, to a lesser extent, phytoplankton. 15 N-pairing experiments revealed that nitrification in the soils was associated with the soil organic carbon and NH 4 + -N contents. The strong nitrification, together with the high NO 3 − -N contents and moisture, contributed to the substantial denitrification. Meanwhile, we found that the soil-to-river transports of NO 3 − and NH 4 + were largely regulated by soil N production rates. Increases in the soil N contents, either anthropogenically or naturally, can directly increase the river NO 3 − -N and NH 4 + -N loadings. These results are essential for advancing our understanding of elevated river N concentrations in rainforests and other minimally impacted or pristine regions.