Articles | Volume 13, issue 4
Research article
24 Feb 2016
Research article |  | 24 Feb 2016

Phosphorus fertilisation under nitrogen limitation can deplete soil carbon stocks: evidence from Swedish meta-replicated long-term field experiments

Christopher Poeplau, Martin A. Bolinder, Holger Kirchmann, and Thomas Kätterer

Abstract. Increasing soil organic carbon (SOC) in agricultural soils can mitigate atmospheric CO2 concentration and also contribute to increased soil fertility and ecosystem resilience. The role of major nutrients in SOC dynamics is complex, due to simultaneous effects on net primary productivity (NPP) that influence crop residue carbon inputs and in the rate of heterotrophic respiration (carbon outputs). This study investigated the effect on SOC stocks of three different levels of phosphorus and potassium (PK) fertilisation rates in the absence of nitrogen fertilisation and of three different levels of nitrogen fertiliser in the absence of PK fertiliser. This was done by analysing data from 10 meta-replicated Swedish long-term field experiments (> 45 years). With N fertilisation, SOC stocks followed yield increases. However, for all PK levels, we found average SOC losses ranging from −0.04 ± 0.09 Mg ha−1 yr−1 (ns) for the lowest to −0.09 ± 0.07 Mg ha−1 yr−1 (p =  0.008) for the highest application rate, while crop yields as a proxy for carbon input increased significantly with PK fertilisation by 1, 10 and 15 %. We conclude that SOC dynamics are mainly output-driven in the PK-fertilised regime but mostly input-driven in the N-fertilised regime, due to the much more pronounced response of NPP to N than to PK fertilisation. It has been established that P rather than K is the element affecting ecosystem carbon fluxes, where P fertilisation has been shown to (i) stimulate heterotrophic respiration, (ii) reduce the abundance of arbuscular mycorrhizal fungi and (iii) decrease the crop root : shoot ratio, leading to higher root-derived carbon input. The higher export of N in the PK-fertilised plots in this study could (iv) have led to increased N mining and thus mineralisation of organic matter. More integrated experiments are needed to gain a better understanding of the relative importance of each of the above-mentioned mechanisms leading to SOC losses after P addition.

Short summary
Nutrients determine the balance between inputs and outputs to and from the soil and thus exert a strong impact on the total soil organic carbon stock. However, for phosphorus, this impact has not been comprehensively addressed. Here we show in 10 different long-term experiments that phosphorus fertilisation can significantly deplete soil carbon stocks, despite a positive impact on plant growth and thus carbon inputs. Thus, soil carbon decay is most likely stimulated even more strongly.
Final-revised paper