Soil Development and Land Use Decouples Soil C:N:P Stoichiometry: A Case Study on Yangtze River Alluvial Fans

Abstract

Coupled soil carbon (C), nitrogen (N), and phosphorus (P) cycles significantly influence biogeochemical processes and ecosystem services. Soil development, as an ecosystem driver, displays different driving mechanisms for nutrients originating from various processes. However, little is known about how changes in soil development simultaneously affect C:N:P stoichiometry across multiple land uses, over long timespans. Variations in soil C:N:P ratios with changes of soil development and land use were investigated along a soil chronosequence comprising c. 0, 60, 100, 280, 1000, and 1500 years. Soil development induced nutrient decoupling, as it facilitated C and N accumulation from biological processes, but restricted P supply controlled by geochemical processes. The soil’s ability to sequester C and N reached a stable state after 1000-years, while P declined continuously from 60 years. Soil C/P ratio (R CP ) and N/P ratio (R NP ) increased with soil development, suggesting that C and N increase potentially promotes P uptake by plants and ultimately intensifies P-limitation. Soil C/N ratio (R CN ) declined in the first 60 years and then reached a threshold of 10:1 . Our analysis of land use impacts on nutrient stoichiometry indicated another decoupling of C, N and P. Paddies commonly had higher organic carbon (OC) and total nitrogen (TN) but lower adsorption capacity of total phosphorous (TP), compared to adjacent drylands, resulting in significant R CP and R NP differences between land uses. At the initial stage, natural vegetation could accelerate soil development and regulate nutrient structure. Additionally, soil development leads soil properties to evolve in a direction conducive to storing more nutrients, which includes decreasing pH (alkaline weakening), electrical conductivity (EC), bulk density (BD), and sand content, while increasing clay content. According to a Random Forest model, soil OC, TN and TP were mainly affected by sand content, available N (AN) and available P (AP), while clay particles could best predict R CP and R NP variations. Our study suggests that old soils require higher nutrient inputs to maintain their productivity and ecological balance, and that any fertilization strategy should be developed based on C and N deficiencies in dryland, and lack of P in paddies.

Keywords: Soil chronosequence, soil ecological stoichiometry, Land use, nutrient decoupling, edaphic properties