Evidence that acidification-induced declines in plant diversity and productivity are mediated by changes in below-ground communities and soil properties in a semi-arid steppe

1. Anthropogenic acid deposition-induced soil acidification is one of the major threats to biodiversity, ecosystem functioning and services. Few studies, however, have explored in detail how above-ground changes in plant species richness and productivity resulting from soil acidification are mediated by effects on below-ground biota and soil properties. 2. To increase our understanding of this linkage, we collected data on below- and above-ground communities and soil properties in a 3-year field experiment with seven levels of acid addition rate to build-up broad intensities of soil acidification in the semi-arid Inner Mongolian grassland. 3. Acid addition directly elevated concentrations of soil Al3+ ions, decreased the base cations Ca2+, Mg2+ and Na+, and increased soil moisture and available phosphorus. Acid addition also appears to have altered the soil microbial community via changes in H+ and Al3+ ions and altered the nematode community via changes in H+ ions and soil moisture. 4. The observed changes in soil N availability (i.e. net N mineralization, NO3--N and NH4+-N could be explained by mediating changes in the H+ and Al3+ ions, microbial community (i.e. community structure, bacteria and fungi/bacteria as indicated by phospholipid fatty acids analysis) and the nematode community (i.e. total abundance, taxa richness and maturity index). 5. Declines in plant species richness and productivity were greater at high intensities of soil acidification in the second sampling year than in the first sampling year. The changes in plant community observed were mostly explained by soil nutrient pathways (e.g. N availability or base mineral cations), which were in turn regulated by the soil microbial or nematode communities as well as by the direct effects of the increase in H+ or Al3+ ions. 6. Synthesis. Our results suggest that the below-ground microbial and nematode communities are more sensitive to soil acidification than the plant communities are, and further that soil acidification-induced changes in plants are mediated by changes in below-ground communities and soil nutrients. These findings improve our understanding of the links between below- and above-ground communities in the Inner Mongolia grassland, especially in the context of anthropogenic acid enrichment.