Soil acid cations induced reduction in soil respiration under nitrogen enrichment and soil acidification

Atmospheric nitrogen (N) deposition and soil acidification both can largely change soil microbial activity and root growth with a consequent impact on soil respiration (R_s). However, it remains unclear which one, N enrichment or soil acidification, plays more important role in impacting soil respiration. We conducted a manipulative experiment to simulate N enrichment (10gm^-2yr^-1 NH₄NO₃) and soil acidity (0.552molH⁺m^-2yr^-1 sulfuric acid) and compared their effects on R_s and its components in a subtropical forest. The results showed that soil pH was reduced by 0.4 similarly under N addition or acid addition after 3years' treatment. Acid addition decreased autotrophic respiration (R_a) by 22-35% and heterotrophic respiration (R_h) by 22-23%, resulting in a reduction of R_s by 22-26% in the two years. N addition reduced R_a, R_h, R_s less than acid addition did. The reductions of R_s and its components were attributed to increase of soil acid cations and reduction of cellulose degrading enzymes activity. N addition and soil acidification significantly enhanced fungal to bacterial ratio. All the cellulose degrading enzymes were reduced more by soil acidity (43-50%) than N addition (30-39%). The principal component scores of degrading enzymes activity showed significantly positive relationships with R_h. Structural equation model showed that soil acidification played more important role than N enrichment in changing R_s and its components. We therefore suggest that soil acidification is an important mechanism underlying soil respiration changes, and should be incorporated into biogeochemical models to improve the prediction of ecosystem C cycling in the future scenarios of anthropogenic N deposition and acid enrichment.