Nitrogen sink in a small forested watershed of subtropical China

Global nitrogen (N) emission and deposition have been increased rapidly due to massive mobilization of N which may have longreachingimpacts on ecosystems. Many agricultural and forest ecosystems have been identified as secondary N sources. In the presentstudy, the input-output budget of inorganic N in a small forested watershed of subtropical China was investigated. Inorganic N wetdeposition and discharge by stream water were monitored from March, 2007 to February, 2009. The concentrations and fluxes ofinorganic N in wet precipitation and stream water and net retention of N were calculated. Global N input by dry deposition andbiological fixation and N output by denitrification for forested watersheds elsewhere were reported as references to evaluate whetherthe studied forested watershed is a source or a sink for N. The results show that the inorganic N output by the stream water is mainlycaused by NO3??-N even though the input is dominated by NH4+-N. The mean flux of inorganic N input by wet precipitation andoutput by stream water is 1.672 and 0.537 g N/(m2 yr), respectively, which indicates that most of inorganic N input is retained in theforested watershed. Net retention of inorganic N reaches 1.135 g N/(m2 yr) considering wet precipitation as the main input and streamwater as the main output. If N input by dry deposition and biological fixation and output by denitrification are taken into account, thissubtropical forested watershed currently acts as a considerable sink for N, with a net sink ranging from 1.309 to 1.913 g N/(m2 yr)which may enhance carbon sequestration of the terrestrial ecosystem.