Nitrogen addition reduces dissolved organic carbon leaching in a montane forest

Dissolved organic carbon (DOC) plays a significant role in the forest soil carbon cycle and can be regulated by nitrogen (N) addition. However, the regulatory direction, mechanism and seasonal pattern of DOC under N addition are less clear. Here, in a montane evergreen forest located at the eastern edge of the Tibetan Plateau, 2 levels of N were applied over 2 years to determine the effects of N addition on DOC release from organic (O layer) and mineral soil. Frequent sampling revealed that high levels of N addition could decrease the concentration of DOC and the flux from the O layer but not from mineral soil and that moderate N addition had no effect on DOC leaching from either the O or mineral layer. The effect of N addition on DOC leaching from the O layer was seasonally dependent, showing a significant reduction in DOC leaching during autumn/winter but no changes during summer and spring. This seasonally different response of DOC to N addition affected the seasonal pattern of DOC leaching. Soil and leachate pH were not influenced by N addition in the short term, indicating that there was not enough difference in DOC retention by mineral soil to significantly affect DOC leaching under N addition. In contrast, N addition-derived reduction in DOC leaching was likely to be due to suppressed fresh litterfall-derived DOC production during autumn/winter; this speculation was supported by lower values of O layer water-extractable organic carbon and microbial biomass carbon as well as lower saccharase and cellulose activities found with high N addition. These results suggested that the processes in control of DOC leaching and their responses to N addition were different for O and mineral soil and that short-term N addition could decrease O-layer DOC leaching, which is likely associated with decreased DOC production rather than greater DOC retention.