Afforestation mitigates soil nitrogen limitation by enhancing mineralization and lowering denitrification in central China

Abstract Afforestation has profoundly altered soil nitrogen (N) transformation, particularly the key processes governing soil organic N and inorganic N dynamics which determine soil N availability. However, the divergences in soil N transformation and the main influencing factors following different afforestation types have not been systematically clarified. Here, we examined soil net N mineralization rate (NMR), net nitrification rate (NNR) and potential denitrification rate (PDR) at 144 paired sampling sites by comparing different afforestation types (initial land use types and tree species) in central China. Afforestation significantly enhanced soil NMR, but lowered NNR and PDR, respectively, compared to non‐afforested lands. Soil NMR and PDR responses to afforestation from cropland were more sensitive than those from shrubland. Notably, broadleaf afforestation (i.e. Quercus variabilis ) had a stronger effect on altering NMR compared to coniferous afforestation (i.e. Platycladus orientalis ) from cropland. These variations in soil NMR and NNR responses were primarily attributed to soil properties (e.g. soil organic nitrogen [SON], soil organic carbon [SOC], C: N ratio) across both non‐afforested and afforested lands, whereas the main influencing factor for PDR shifted from microbial traits in non‐afforested lands to soil properties in afforested lands. Although higher SON contents promoted soil NMR in afforested lands, significantly higher microbial biomass N in the afforested lands compared to non‐afforested lands suggested intensified microbial inorganic N assimilation, which consequently constrained NNR and PDR relative to non‐afforested lands, further avoiding N loss and N limitation. Our findings emphasized that afforestation types divergently influenced soil net N transformation rates with overall impacts on alleviating possible soil N constraints, thereby providing a theoretical basis for the regulation of soil N cycling under future afforestation and forest management. Read the free Plain Language Summary for this article on the Journal blog.