Impacts of long term P availability on Soil Organic Carbon stocks and cycling – insights from the EJP ICONICA project

The management of agro-ecosystems enhancing soil and subsoil organic carbon (SOC) sequestration could be a potential strategy to mitigate climate change. However, soil quality and organic matter (SOM) transformation is highly regulated by the cycle and interactions with other nutrients such as carbon (C), nitrogen (N) and phosphorus (P). In agricultural management, the addition of P fertilizers and its effect on C and N cycling (CNP stoichiometry) is still largely unknown. Therefore, the ICONICA project intends to provide context-based information on soil nutrients (C & N) and their stocks and cycling in P fertilizer scenarios. To determine the influence that P fertilization has on SOC stabilizations and greenhouse gas (GHG) emissions in agricultural systems, ICONICA employed a unique set of long-term P fertilizer experiments (LTEs) across Europe, including a range of P treatments to establish relationship between long-term P availability and interaction with C and N. In this project soils up to 50 cm depth were collected from six LTEs distributed across Europe. The LTEs feature different soil type/textural classes and land uses. The objective was to quantify C stocks and provide initial reference value of soil C storage for soil depths and link that to the soil stoichiometric ratios (C:N:P) in different management systems. Overall, in grassland sites the average SOC stocks ranged from 54.76±6.65 (tC/ha) in The Netherlands, followed by 43.07±5.71 (tC/ha) in Ireland. The average SOC stocks in arable sites in Sweden were 21.22±1.19 (tC/ha) and the least SOC stocks (18.60±2.35 (tC/ha) occurred in arable site in Denmark. Grassland sites showed a higher SOC stock within the topsoil (0-10 cm) while arable sites contributes most of the C in the subsoil especially at 10-30 cm. The mean soil C/N ratio was higher in the topsoil (0-10 cm) in the arable with respect to the grasslands and ranged from 13.74 to 10.28 with similar trend at soil depth gradients. The variations in C flow and in stocks through the sampled profile of different land uses such as grassland to arable is highly complex and driven by multiple factors indicating the need for further assessment. Nevertheless, the observations indicated the indispensable nature of long-term field experiments to quantify the optimum C:N:P stoichiometry that may enable efficient SOC sequestration as opposed to production in the managed agricultural systems.