Simulated effects of climate change and acid deposition on soil chemical conditions in a Masson Pine forest of SW China

In SW China, acid deposition has been associated with forest damage such as defoliation and mortality due to serious soil acidification. These effects may be exacerbated by ongoing climate change. Understanding the integrated effects of climate change and acid deposition on soil chemistry of forest ecosystems is the key to alleviate forest damage and recover forest health. In particular, the long-term development of integrated effects is unclear and, thus, prevents setting up cost-effective controls of atmospheric deposition for improved forest-health management. We employed the Nutrient Cycling Model (NuCM) to predict the changes of soil chemistry in a Masson pine (Pinus massoniana) forest at Tie-Shan-Ping (nearby Chongqing, the biggest city in SW China) under two scenarios of climate change and five scenarios of atmospheric deposition. Field-monitoring data were used to calibrate and validate the NuCM model. It is shown that the maintaining of current acid deposition both in its quantity and composition would not enhance soil acidification although it would take further 20 years to reach a steady state with higher SO42- and Ca2+ concentrations in soil solution. This simulated trend is in contradiction to the decrease of soil pH observed in field during last several decades. The possible reason of this may be the obviously elevated deposition of Ca2+, which may help to raise the pH of soil solution but occurred only in recent several years following the rapid increase of local construction industry. However, this enhanced Ca2+ input may not be maintained for long time. A decreased S input would lead to some positive effects on soil chemistry such as the increase of soil base saturation. A high N deposition has implicated the forest ecosystem to be saturated with N, and increased N input will further aggravate soil acidification and nutrient imbalances. The future climate change projected by IPCC, i.e., the increase of temperature and rainfall may slightly enhance the negative effect of increased N input. The integrated effect of climate change and N-deposition increase may counteract the positive effects of decreased S input to a considerable degree. This supports the need for a rigorous implementation of new technology to decrease the emission of both S and N.