Stoichiometry of carbon, nitrogen, and phosphorus in soil: Effects of agricultural land use and climate at a continental scale

Agricultural land use profoundly alters soil carbon (C), nitrogen (N), and phosphorus (P) contents, impacting crop productivity, environmental quality, and soil C sequestration. However, how the soil C:N:P stoichiometry is affected by upland and paddy cropping over broad geographical scale remains largely unknown. The objective of this study is to examine the soil C:N:P stoichiometry in woodland (as control), agricultural upland and paddy from four climate zones (tropics, subtropics, warm temperate, and mid-temperate) across eastern China. 720 surface soil samples were collected from 240 sites with adjacent woodland, upland, and paddy at a depth of 0–15 cm. Total C, N, and P contents and their ratios were determined. Among climate zones, C and N contents and C:N ratios decreased in the order of mid-temperate > tropics > subtropics > warm temperate, whereas C:P and N:P ratios followed the order of subtropics > mid-temperate and tropics > warm-temperate. Compared to woodland, upland agriculture decreased the C content by 4.0–6.4 g kg−1, but increased P content by 0.2–0.5 g kg−1, resulting in the decreases of C:N, C:P, and N:P ratios by 1.4–5.1, 3.4–91.5, and 1.6–4.1, respectively. Hence, uplands are relatively limited by C and N but enriched with P, particularly in warm temperate zone. By contrast, the C, N, and P contents in paddy soils were all increased compared to woodland soils, but larger N and P increase leads to the decreases in C:N and C:P ratios. The higher P content, and consequently lower C:N:P ratios in both agricultural soils are the consequences of intensive fertilization. As a whole, the direction of soil C, N, and P contents and their stoichiometric ratios in response to agricultural use was similar in the four climate zones: P increased, but C:N:P ratios decreased. The more intensive agricultural practices and stronger biological and geochemical processes with higher temperature and precipitation in warmer and wetter zones lead to more pronounced differences of soil C, N, and P contents and their stoichiometric ratios among land-use types than those in cooler and drier climates.