Critical loads of headwater streams in China using SSWC model modified by comprehensive F-factor.

In order to evaluate the potential risk of surface water acidification in regions with historically-elevated acid deposition and to measure the recovery of such ecosystems after policy changes, critical loads and their exceedances were estimated for 349 headwater streams across China using a modified SSWC model. Such a model considered the acid-neutralizing capacity derived from high base cation deposition and the robust retention of sulfate and nitrate. Results indicated that China's streams had higher critical loads (averaged at 4.7 keq·ha-1·yr-1) and were less sensitive to acid deposition as compared to Europe and North America. The proportion of surveyed streams with acid deposition exceeded critical load decreased from 40.4% in 2005 to 29.5% in 2018, indicating a significant decrease in risk of surface water acidification, and thus a benefit from the emission abatement in recent years. Nonetheless, a relatively high risk of acidification still existed in southeast China with lower critical loads and most critical load exceedances. More efforts should be put into implementing emission control policies in the future.

High hydrogen sulfide emissions from subtropical forest soils based on field measurements in south China.

The present estimate of global hydrogen sulfide (H2S) emission from natural sources has large uncertainty mainly due to the lack of valid field data, particularly for subtropical forest soil in China with elevated atmospheric S deposition. In this study, the field observation of H2S fluxes over subtropical forest soil was conducted for the first time in south China to measure the magnitude of emissions of H2S, and evaluate its contribution to the large S sink, using the Dynamic Flux Chamber (DFC) method. Daily variations of H2S fluxes showed an increasing emission with the increasing air temperature in the morning, a peak at the middle of the day, and a decreasing emission thereafter, then approximated to zero at night. The H2S flux had positive values in all seasons, with the highest in summer, followed by spring, and relatively lower values in fall and winter. The H2S flux measurements showed relatively large emission with annual average value of 0.028 g S m-2 yr-1, possibly due to the elevated sulfate concentration in the soil solution by S deposition, the hot and humid climate, as well as the lower soil pH in subtropical China. Thus, not only tropical, subtropical soils need to be included as significant H2S sources to accurately portray the global H2S budget. Although the soil in subtropical forests acted as a strong source for H2S to atmosphere, H2S emission from soil had limited contribution (about 0.2%) to the large S sink in this forest catchment.

Nitrogen deposition increases N2O emission from an N-saturated subtropical forest in southwest China.

Nitrous oxide (N2O) is a major greenhouse gas, with elevated emission being reported from subtropical forests that receive high nitrogen (N) deposition. After 10 years of monthly addition of ammonium nitrate (NH4NO3) or sodium nitrate (NaNO3) to a Mason pine forest at Tieshanping, near Chongqing city in Southwest China, the simulated N deposition was stopped in October 2014. The results of soil N2O emissions monitoring in different seasons during the nitrogen application period showed that nitrogen addition significantly increased soil N2O emission. In general, the N2O emission fluxes were positively correlated to nitrate (NO3-) concentrations in soil solution, supporting the important role of denitrification in N2O production, which was also modified by environmental factors such as soil temperature and moisture. After stopping the application of nitrogen, the soil N2O emissions from the treatment plots were no longer significantly higher than those from the reference plots, implying that a decrease in nitrogen deposition in the future would cause a decrease in N2O emission. Although the major forms of N deposition, NH4+ and NO3-, had not shown significantly different effects on soil N2O emission, the reduction in NH4+ deposition may decrease the NO3- concentrations in soil solution faster than the reduction in NO3- deposition, and thus be more effective in reducing N2O emission from N-saturated forest soil in the future.

Threshold and multiple indicators for nitrogen saturation in subtropical forests.

The influence of nitrogen (N) deposition on forest ecosystems largely depend on the N status. Developing threshold and practical indicators for N saturation in subtropical forests, with extremely high N deposition, would both enhance forest management and the assessments of global N balance and carbon (C) sequestration. Here, we quantified the N mass balance and assessed current N status at a number of subtropical forest sites in South China, using both N content, C/N ratio, and 15N natural abundance (δ15N) as potential indicators of N saturation. Among the studied sites, N deposition ranged from 13.8 to 113 kg N ha-1 yr-1 in throughfall, and was dominated by ammonium (NH4+). The threshold for N leaching in subtropical forest was first found to be 26-36 kg N ha-1 yr-1, which was 160% higher than in temperate forest (based on prescribed minimum). This indicates that critical parameter inputs in global models of the impact of N deposition are in need of revision, based on specific ecosystem characteristics. We found a critical C/N ratio of 20 for the O/A horizon as indicator of N saturation. Foliar N content and δ15N were positively correlated with N deposition and were well suited to indicate regional N status. The δ15N enrichment factor (Ɛfoli/So2, δ15Nfoliage - δ15NSoil2) was between -10‰ and -1‰, and had similar trend to those obtained from other regions with increasing N deposition. These suggest that the enrichment factor could be used to investigate the influence of N deposition in forest ecosystems, regardless of spatial heterogeneity in δ15N of N input, soil N availability and geomorphology.