Land-use change affects stocks and stoichiometric ratios of soil carbon, nitrogen, and phosphorus in a typical agro-pastoral region of northwest China

Abstract

The impacts of land-use change on dynamics of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the subsoil (> 30 cm) are poorly understood. This study aims to investigate whether the effects of land-use change on stocks and stoichiometric ratios (RCN, RCP, and RNP) of SOC, TN, and TP can be different between topsoil (0–30 cm) and subsoil (30–60 cm) in the Ili River Valley, northwest China. Soil samples (0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm) were collected from a pasture (PT), a 27-year-old cropland (CL) converted from PT, and a 13-year-old poplar (Populus tomentosa Carr.) plantation (PP) converted from CL. SOC, TN, and TP concentrations and soil bulk density were determined to calculate stocks and stoichiometric ratios (molar ratios) of SOC, TN, and TP. Conversion from PT to CL led to substantial losses in SOC, TN, and TP pools in both topsoil and subsoil, and the reduction rates in subsoil (13.8–24.7%) were higher than those in topsoil (8.5–17.3%), indicating that C, N, and P pools in subsoil could also be depleted by cultivation. Similar to topsoil, significant increases in SOC, TN, and TP stocks were detected after afforestation on CL in subsoil, although the increase rates (31.2–56.2%) were lower than those in topsoil (47.8–69.1%). Soil pH and electrical conductivity (EC), which generally increased after conversion from PT to CL while decreased after CL afforestation, showed significant negative correlations with SOC, TN, and TP, suggesting that cultivation might lead to soil degradation, whereas afforestation contributed to soil restoration in this area. Significant changes in C:N:P ratios in topsoil were only detected for RNP after conversion from CL to PP. By contrast, land-use change significantly altered both RCN and RNP in the subsoil, demonstrating that the impacts of land-use change on RCN and RNP were different between topsoil and subsoil. The significant relationship between soil EC and RNP suggested that RNP might be a useful indicator of soil salinization. Stocks of SOC, TN, and TP as well as RCN and RNP in subsoil showed different responses to land-use change compared to those in topsoil in this typical agro-pastoral region. Therefore, it is suggested that the effects of land-use change on dynamics of SOC, TN, and TP in subsoil should also be evaluated to better understand the role of land-use change in global biogeochemical cycles.