The Stimulation Effect of Nitrogen‐Addition on Soil Nitrogen Leaching Weakens Over Time in an Alpine Forest

Abstract Elevated anthropogenic nitrogen (N) input to headwater forests over the years increases soil N leaching and is expected to result in negative consequences like eutrophication of downstream regions. However, the long‐term response of N leaching to elevated N input in alpine forests remains elusive. Here, we explored the long‐term effects of the addition of N, as 15 N‐labeled ammonium nitrate ( 15 NH 4 15 NO 3 ), on N leaching in a three‐N‐level experiment via a frequent sampling of soil leachates from two soil layers for eight years. The results showed that N addition increased the total dissolved N (TDN) concentrations and fluxes in leachates from the organic layer only for the high N addition (4 g N/m 2 /yr) treatment. Regardless of N treatment, dissolved organic N was the main leaching form of N. The effect of N addition on N leaching attenuated over time and it was partly attributed to the gradual increase in N retention by the aggrading forest. Overall, our data illustrate that N leaching showed declined trends over time due to the increased plants uptake, suggesting the important role of aggrading alpine forests in regulation of N leaching under elevated N deposition.