Trends of two decadal precipitation chemistry in a subtropical rainforest in East Asia

Long-term monitoring of precipitation chemistry provides a great opportunity to examine the evolution of air pollutant emissions and effectiveness of air pollution control measures. We evaluated the characteristics and trends of precipitation chemistry at both annual and seasonal scales based on the records of 1994-2013 at Fushan Experimental Forest (FEF) of northeastern Taiwan. The results showed that 77% of the weekly precipitation had pH<5.0. The two-decadal average annual pH was 4.62, without a significant inter-annual trend, possibly due to the concurrent declines of both acidic pollutants and base cations. There was a significant positive relationship between [SO₄^2-+NO₃^-] and [Ca²⁺+NH₄⁺] indicating that their deposition was likely dominated by NH₄NO₃, (NH₄)₂SO₄, Ca(NO₃)₂, and CaSO₄. There was a significant negative relationship between precipitation pH and the difference between [SO₄^2-+NO₃^-] and [Ca²⁺+NH₄⁺], not just [SO₄^2-+NO₃^-], suggesting that precipitation acidity was not solely determined by acidic pollutants but by the balance between acidic pollutants and base cations. We also found temporal decreases of Ca²⁺ and NH₄⁺ concentrations in precipitation which contributed to the low acid neutralization capacity of precipitation. Annual deposition of NO₃^- and SO₄^2- was 23 and 55kgha^-1yr^-1, which is much higher than most forest sites in the industrialized countries suggesting that acid deposition is still a major environmental issue in Taiwan. Annual deposition of NH₄⁺, Ca²⁺ and NO₃^- showed significant decreasing trends during the 20-year period, which was mostly due to the decreases in the summer deposition associated with air pollution mitigation strategies. Winter deposition showed no decreasing patterns for the same period. The high contribution to annual acid deposition from autumn-winter and spring rains (50%) associated with northeast monsoon implies that long-range transport of anthropogenic emissions from East Asia played a key role on acid depositions at FEF and possibly many areas in the region. Therefore, intergovernmental cooperation is urgently needed to effectively mitigate the threat of acid deposition in East Asia.

Modeling the terrestrial N processes in a small mountain catchment through INCA-N: A case study in Taiwan

Riverine dissolved inorganic nitrogen (DIN) is an important indicator of trophic status of aquatic ecosystems. High riverine DIN export in Taiwan, ~3800kg-Nkm^-2yr^-1, which is ~18 times higher than the global average, urges the need of thorough understanding of N cycling processes. We applied INCA-N (Integrated Nitrogen Catchment Model) to simulate riverine DIN export and infer terrestrial N processes using weekly rainwater and streamwater samples collected at the Fushan Experimental Forest (FEF) of northern Taiwan. Results showed that the modeled discharge and nitrate export are in good agreement with observations, suggesting the validity of our application. Based on our modeling, the three main N removal processes, in the order of descending importance, were plant uptake, riverine N transport and denitrification at FEF. The high plant uptake rate, 4920kg-Nkm^-2yr^-1, should have led to accumulation of large biomass but biomass at FEF was relatively small compared to other tropical forests, likely due to periodic typhoon disruptions. The low nitrate concentration but high DIN export highlights the importance of hydrological control over DIN export, particularly during typhoons. The denitrification rate, 750kg-Nkm^-2yr^-1, at FEF was also low compared to other tropical forest ecosystems, likely resulting from quick water drainage through the coarse-loamy top soils. The high DIN export to atmospheric deposition ratio, 0.45, suggests that FEF may be in advanced stages of N excess. This simulation provides useful insights for establishing monitoring programs and improves our understanding N cycling in subtropical watersheds.