Latitudinal patterns of soil nitrogen density across soil profiles and their driving factors in the arid valleys of southwest China

Purpose Identifying patterns and drivers of soil total N (TN) and its fractions along latitudinal gradients provides a comprehensive understanding of the response of soil N availability to environmental changes. Methods In the present study, we collected soil samples at depths of 0–10 cm, 10–20 cm, 20–30 cm, and 30–50 cm from 98 natural shrublands along a latitudinal gradient (23.2° N to 32.3° N) in the arid valleys of southwest China. We investigated the soil TN density (STND) and inorganic N densities (SIND)—which is the sum of nitrate–N [NO3D] and ammonium-N [NH4D] densities—and their environmental driving factors. Results Latitudinal patterns of STND followed an inverse unimodal distribution, whereas those of NH4D followed a unimodal distribution, regardless of the soil layer. SIND in the 0–10 cm layer followed a unimodal distribution with increasing latitude, while NO3D had an inverse unimodal distribution with increasing latitude in the 10–20 cm, 20–30 cm, and 30–50 cm layers. Across the four soil layers, variations in STND with latitude were largely explained by soil organic carbon content. Soil sorption capacity and vegetation composition including shrub cover [SC] and herb cover [HC] strongly influenced the SIND. Variations in NH4D and NO3D with latitude were jointly driven by the mean annual temperature, SC, HC, dissolved organic carbon, clay content, and pH. Although soil properties determined the latitudinal patterns of soil N density in all the four soil layers tested, the relative contributions of climatic and vegetation factors increased with soil depth. Conclusion Our study elucidated the latitudinal patterns in STND and its fractions, and our findings highlight the potential impacts of climatic and vegetation factors on subsoil N pools and dynamics.