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

Piaopiao Ke,Qian Yu,Lei Duan,Gaoyue Si,Yao Luo,Zhenglin Cheng

doi:10.3390/f12121798

Abstract

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.

Highlights

Elevated nitrogen (N) and sulfur (S) depositions have been widely reported to cause acidification and eutrophication in terrestrial ecosystems, influencing nutrient balance, vegetation growth, and biodiversity in Europe [1,2] and North America [3] in 1970s, and in East Asia in around a decade later [4]
Following dramatic economic development since the 1970s, the anthropogenic emissions of these air pollutants rapidly increased in China [4]
The concomitant high loads of N and S to forests have caused significant soil acidification, especially in South China, where the forest soils are susceptible to acid deposition [6,7]

Summary

Introduction

Elevated nitrogen (N) and sulfur (S) depositions have been widely reported to cause acidification and eutrophication in terrestrial ecosystems, influencing nutrient balance, vegetation growth, and biodiversity in Europe [1,2] and North America [3] in 1970s, and in East Asia in around a decade later [4]. Enhanced atmospheric S and N depositions resulted primarily from increased anthropogenic emissions of sulfur dioxides (SO2 ), nitrogen oxides (NOx ), and ammonia (NH3 ) [4]. Following dramatic economic development since the 1970s, the anthropogenic emissions of these air pollutants rapidly increased in China [4]. The concomitant high loads of N and S to forests have caused significant soil acidification, especially in South China, where the forest soils are susceptible to acid deposition [6,7]

Methods

Results

Conclusion