Calcium change of transported soils from 1995 to 2010 in acid rain areas in China

Acid rain, a significant environmental threat, negatively impacts soil health and ecosystems. It can leach calcium (Ca) from soils, affecting plant growth and other ecological processes. China has experienced widespread acid rain, particularly in the southern regions. Two national geochemical mapping projects: Environmental Geochemical Monitoring Networks (EGMON) and China Geochemical Baselines (CGB), were conducted to establish baselines for soil Ca. This study compares these baselines to quantify Ca changes in China's acid rain areas from 1995 to 2010 and identify key controlling factors. Our analysis revealed a significant decrease in Ca content in transported soils across these regions during this period. Notably, the median Ca concentration in the top soil (TS) decreased from 0.47 % to 0.36 %, and in the deep soil (DS) from 0.39 % to 0.31 %. Areas experiencing Ca depletion contents were roughly twice as those with increases. Estimated net Ca change rates were −0.07 Pg/a for TS and 0.04 Pg/a for DS, indicating a net Ca loss of 1.25 Pg from the study region's top 125 cm of soil. This translates to a loss rate of 0.083 Pg/a or 0.026 g Ca²⁺ kg⁻¹ soil per year. Calcium contents in transported soils were influenced by the interplay of acid rain, rainfall, and parent rocks. Acid rain could dissolve Ca from carbonate-rich rocks, enriching nearby rivers with Ca²⁺. Upon deposition, clay minerals in the soils could adsorb Ca²⁺, potentially increasing soil Ca content. However, intense acid rain (pH <5.0) could leach Ca²⁺ from the soil and outweigh the positive influence of parent carbonate-rich rocks, leading to Ca depletion. Additionally, in regions with sufficient rainfall (>1600 mm/a), combined with the leaching effect of acid rain, Ca content could decrease due to overall mobilization and transport out of the soil.