Rainfall partitioning and associated nitrate and sulfate fluxes along a slope gradient in a subtropical broadleaved forest

Abstract

Forest canopies create a high spatial variability of both canopy interception (Ei) and acid deposition. However, spatial characteristics of these hydrochemical fluxes along a slope gradient in forest watersheds remain unclear. Here we monitored bulk precipitation (BP), throughfall (TF), stemflow (SF), and associated nitrate (NO3−) and sulfate (SO42−) in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved forest, Southeast China. Results showed that annual rates of TF, SF and Ei to BP in UP were 85.2%, 2.6%, and 12.2%, respectively. Corresponding values in LP were 88.6%, 2.2%, and 9.2%. There were no significant differences among these rainfall partitioning components between UP and LP. Additionally, annual volume weighted mean concentrations of NO3−-N in TF and SF were significantly lower than in BP, whereas no significant differences were found for SO42−-S among BP, TF and SF. Unlike annual TF NO3−-N fluxes were similar in UP and LP, annual TF SO42−-S fluxes significantly increased along the UP to LP. Net TF + SF (NTF) fluxes were negative for NO3−-N and SO42−-S at the two slope plots, illustrating retained/uptake in the canopy. Dormant season NTF S fluxes were close to zero, indicating that dry deposition does not appear to play a major role, whereas exchange with foliage surfaces should be the dominant factor controlling the spatial patterns of TF N and S fluxes in the watershed. This study can contribute to the evaluation of watershed-scale water and acidifying input into the soil during canopy passage, and further consequent effects on forest ecosystems.