Bulk deposition of base cationic nutrients in China's forests: Annual rates and spatial characteristics
Base cations, such as potassium (K+), calcium (Ca2+) and magnesium (Mg2+), are essential nutrients for plant growth and their atmospheric inputs can buffer the effect of acid deposition by nitrogen (N) and sulphur (S) compounds. However, the spatial variation in atmospheric deposition of these base cationic nutrients is less understood compared with N and S deposition. By synthesizing bulk deposition data for K+, Ca2+ and Mg2+, we assessed their annual rates and spatial characteristics at 34 forested sites across China. Our synthesis showed relatively high levels of bulk deposition of base cationic nutrients in China's forests, being an order of magnitude higher than in the USA and Europe. On average, K+, Ca2+ and Mg2+ accounted for 13%, 72% and 15% of the bulk deposition of base cationic nutrients, respectively. Surprisingly, base cation deposition was lower at sites near semi-arid regions compared with sites in eastern and southern China, which were far from semi-arid regions. Moreover, elevated base cation deposition was associated with urban hotspots, exhibiting a significant power-law increase with closer distance to the nearest large cities. We estimated that on average base cationic nutrients neutralized a significant proportion (76%) of the potential acid load due to acid deposition. Our findings suggest that in China there is considerable anthropogenic alteration of the regional cycling of base cationic nutrients, which plays an important role in counteracting the risk of soil acidification and base cation depletion in forest ecosystems, especially in the southern regions.