Land use overrides climatic controls on soil organic nitrogen transformations: Contrasting responsiveness between forest and cropland ecosystems

1. Soil organic nitrogen (SON) transformation is critical for global nutrient cycling and ecosystem productivity, yet how its responsiveness to climate change differs across diverse land use types remains poorly resolved.

2. We measured gross protein depolymerization (GPD), microbial N growth, gross N mineralization (GNM) and microbial N use efficiency (NUE) in paired forest and cropland soils along a broad climatic gradient in subtropical China to quantify differential climate associations and identify governing biogeochemical controls.

3. Forest soils exhibited substantially higher GPD (82%), microbial growth (132%) and NUE (26%) compared to adjacent croplands, while GNM rates were similar between land uses. Across the observed spatial climate gradient, SON transformations in forests showed strong positive co-variation with climate: GPD, Ngrowth and GNM increased with mean annual temperature (MAT) and mean annual precipitation (MAP) (slopes for MAT = 0.59, 0.84, 0.49; for MAP = 0.55, 0.62, 0.23), whereas NUE declined with both MAT and MAP (slopes = −0.68 and −0.63, respectively). In contrast, cropland SON processes were largely insensitive to MAT and MAP except that Ngrowth and NUE increased modestly with MAT. Mechanistic analyses indicated contrasting regulatory pathways: in forests, climatic effects were transmitted mainly through mineral–enzyme interactions (e.g. iron/aluminium oxides modulating protease activity) and resource stoichiometry (e.g. dissolved organic carbon:available phosphorus ratio), with GPD tightly coupled to Ngrowth and GNM and acting as a rate-limiting step. In croplands, temperature effects were largely indirect, operating through base cation to iron/aluminium-oxide ratios, resource availability (free amino acids, carbon:N ratio), and microbial functional gene abundances, yielding a decoupling of depolymerization from downstream processes.

4. These results show that land use strongly modulates the climate-associated responsiveness of SON transformations: forest soils are more vulnerable to climate-driven changes in N cycling than intensively managed croplands. Our findings have implications for land-use-specific management and for improving predictions of N dynamics under global change.