Soil Nitrogen and Sulfur Leaching in a Subtropical Forest at a Transition State under Decreasing Atmospheric Deposition

Anthropogenic emissions of nitrogen- (N) and sulfur (S)-containing pollutants have declined across China in recent years. However, the responses of N and S depositions and dynamics in soil remain unclear in subtropical forests. In this study, the wet and throughfall depositions of dissolved inorganic N (DIN) and SO42− were continuously monitored in a mildly polluted subtropical forest in Southeast China in 2017 and 2018. Moreover, these solutes in soil water along the soil profile were monitored in 2018. Throughfall deposition of DIN and S decreased by 59% and 53% in recent 3 years, respectively, which can be majorly attributed to the decreases in wet depositions of NO3− and SO42−. Meanwhile, NH4+ deposition remained relatively stable at this site. Even though N deposition in 2018 was below the N saturation threshold for subtropical forests, significant N leaching still occurred. Excess export of N occurred in the upper soil layer (0–15 cm), reaching 6.86 ± 1.54 kg N/ha/yr, while the deeper soil (15–30 cm) was net sink of N as 8.29 ± 1.71 kg N/ha/yr. Similarly, S was excessively exported from the upper soil with net flux of 14.7 ± 3.15 kg S/ha/yr, while up to 6.37 ± 3.18 kg S/ha/yr of S was retained in the deeper soil. The significant N and S leaching under declined depositions suggested that this site possibly underwent a transition state, recovering from historically high acid deposition. Furthermore, the rainfall intensity remarkably regulated leaching and retention of SO42− and DIN at this site. The impacts of climate changes on N and S dynamics require further long-term monitoring in subtropical forests.</jats:p

Prominent role of sulfate reduction in robust sulfur retention in subtropical soil

Abstract Sulfur budgets in catchments indicated that about 80% of the deposited sulfur was retained in the subtropical soil, it alleviates the historical acidification caused by elevated deposition. The strong sulfur retention was attributed to the reversible sulfate adsorption in previous studies. Here we report that sulfate reduction is a prominent yet thus far overlooked mechanism for sulfur retention, based upon the comprehensive evidence of soil sulfur storage and multi-isotope within entire soil profile along a hydrological continuum in a typical subtropical catchment of China. Using a dual isotopic mass balance model, we determined that annual flux of reduction accounted for approximately 38% of sulfur retention, which was close to the proportion of reduced species in soil. Consequently, the release of sulfur legacy would be less serious with the decreasing sulfur deposition in China, compared to the projections only considering adsorption.</jats:p