Soil nutrient stoichiometry impacts on soil organic carbon stocks in long-term phosphorus fertilisation experiments

Managing agroecosystems to enhance soil organic carbon (SOC) storage is important for mitigating climate change. However, the transformation of SOC is intimately connected to nutrient cycling, particularly nitrogen (N) and phosphorus (P). While P constraints on plant growth are known, their effects on carbon (C) and N cycling remain uncertain. The study uses several long-term experiments (LTEs) to determine the importance of N-P interactions and the optimal C:N:P stoichiometry for long-term SOC stocks in managed agricultural systems. The aim was to determine the influence of multi-decadal P fertilisation on SOC stocks and stoichiometric interactions of C, N and P in agricultural soils (up to 50 cm) across different soil textural classes and land uses. For this, the soils were sampled at three depths 0–10, 10–30 and 30–50 cm from six LTEs in Europe (three grasslands and three arables) to determine soil physico-chemical properties. The results showed comparable SOC stocks in contrasting P treatments across land uses. In grasslands, SOC stocks at 0–50 cm depth ranged from 9.7 to 40.6 t C ha−1 while in arable sites, they were between 11.0 and 48.3 t C ha−1. The SOC stocks did not vary significantly across P treatments indicating that long-term P fertilisation did not affect C storage. Grassland sites had higher SOC stocks in the 0–10 cm, while at arable sites they were higher at 10–30 cm depths. The maximum predicted SOC stock of 30.9 t C ha−1 was with SOC/TN (total nitrogen) ratio of 10.1 and SOC/TP (total phosphorus) ratio of 32.6 in grassland sites, while these ratios were 10.9 and 29.4, respectively, in arable sites, where the predicted maximum SOC stock was 33.3 t C ha−1. Overall, the study shows that the long-term phosphorus fertilisation of grassland and arable soils did not affect SOC stocks at the studied LTEs.