Subsoil Holds Over 60% of Nutrient Stocks in Yangtze Plain-Hilly Transition Zone

A soil survey in Jiayu County, in the plain-hilly transition zone of the middle and lower Yangtze River, has found that more than 60% of the region's carbon, nitrogen, and phosphorus inventory is stored below the top 20 centimetres of soil. The study, published in Environmental Geochemistry and Health on 23 October 2026, combined 422 topsoil samples with soil-profile data extending to a depth of 1.20 metres across five soil types. The authors, Song Hu, Shuo Xiong, Jie Luo, Kaili Xu, and Quan Li, argue that surveys which stop at the surface layer can substantially underestimate the true regional nutrient stocks in these landscapes.

The plain-hilly transition zone of the middle and lower Yangtze has undergone marked land-use change, altering soil nutrient cycling, yet the contribution of deeper subsoil layers to regional nutrient budgets has remained insufficiently understood. By quantifying carbon, nitrogen, and phosphorus stocks and their stoichiometric characteristics down to 1.20 metres, the study provides depth-resolved estimates for the county. The finding that the 0.20 to 1.20 metre subsoil layers hold the majority of the regional nutrient inventory is a measured result of this sampling design, not a model prediction, and it applies to the five soil types studied in Jiayu County.

Nutrient patterns differed clearly among soil types, reflecting the combined effects of land use, parent material, and geomorphic setting. Paddy soil carried high carbon and nitrogen stocks, alluvial soil was enriched in phosphorus, and red soil represented a low-nutrient background pool. These contrasts mean that a single management prescription for the whole transition zone would ignore the distinct nutrient profiles of its constituent soils.

The authors suggest differentiated approaches: phosphorus control in paddy and alluvial soils and carbon enhancement in red soil, which they state may support nutrient balance and sustainable agricultural management in plain-hilly transition zones. When converted to molar units, the average topsoil carbon-to-nitrogen-to-phosphorus ratio was approximately 37.4:3.5:1. The researchers interpret this as indicating stoichiometric imbalance rather than direct evidence of phosphorus saturation, an important distinction for how the results are applied.

Available phosphorus and soil pH emerged as the key factors associated with the spatial heterogeneity of nutrient distributions. The study notes that these are associations with spatial patterns; the abstract does not establish causal effects of pH or available phosphorus on nutrient cycling. The central technical message is that depth matters.

Because subsoil layers store more than 60% of the regional nutrient inventory, surface-only sampling would miss most of the carbon, nitrogen, and phosphorus held in these landscapes, and any regional nutrient budget built from topsoil data alone would be biased low. The study's scope is a single county case study of five soil types, so the specific stock values and stoichiometric ratios should be read as local measurements for Jiayu County rather than general figures for the entire Yangtze plain-hilly transition zone. Even so, the depth-resolved sampling strategy shows how quantifying nutrient inventories in transition landscapes where land-use change is reshaping soil nutrient cycling can reveal the substantial, previously undercounted role of subsoil layers.

Subject of Research: Distribution of soil carbon, nitrogen, and phosphorus stocks across five soil types in the plain-hilly transition zone of the middle-lower Yangtze River

Article Title: Distribution characteristics of soil carbon, nitrogen, and phosphorus stocks in the plain-hilly transition zone of the middle-lower Yangtze River: a case study of five soil types in Jiayu County

Article References: Hu, S., Xiong, S., Luo, J., Xu, K., & Li, Q. (2026). Distribution characteristics of soil carbon, nitrogen, and phosphorus stocks in the plain-hilly transition zone of the middle-lower Yangtze River: a case study of five soil types in Jiayu County. Environmental Geochemistry and Health, 48(16), Article 657. https://doi.org/10.1007/s10653-026-03564-3

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03564-3

Keywords: soil carbon, nitrogen, phosphorus, nutrient stocks, soil stoichiometry, subsoil, Yangtze River, paddy soil, alluvial soil, red soil, land use, Jiayu County