Gross Nitrogen Transformations in Long‐Term Phosphorus Fertilisation Experiments Across Three European Agroecosystems

ABSTRACT

The effects of long‐term phosphorus (P) fertilisation in agroecosystems on nitrogen (N) cycling are still elusive, in terms of P‐induced increase or reduction of N availability for plants and microorganisms, thereby affecting plant growth and also N ₂ O emissions. Therefore, we performed detailed ¹⁵ N tracing analyses in soils from three different European long‐term P fertilisation experiments (LTE) in the laboratory and compared high‐P application treatments to the corresponding non‐P‐fertilised controls. In addition, carbon (C) fluxes (CO ₂ , CH ₄ ) were examined. The soils were sourced from Johnstown Castle, Ireland (JC, grassland), Jyndevad, Denmark (JY, arable soil), and Lanna Skara, Sweden (LS, arable soil) and were treated with two different combinations of ¹⁵ N‐labelled NH ₄ NO ₃ and ¹³ C‐labelled dried maize leaves ( ¹⁵ NH ₄ NO ₃  +  ¹³ C; NH ₄ ¹⁵ NO ₃  +  ¹³ C). Gross N transformation rates were quantified by using the Ntrace _Gas analysis tool. We found more pronounced effects of long‐term P fertilisation on N and C transformations in soils from the annually P‐fertilised LTEs (JC and JY) than in soils fertilised with P every 6 years (LS). In all soils, N immobilisation exceeded N mineralisation. The annually P‐fertilised soils showed increased mineralisation and immobilisation turnover, and across all LTEs, P fertilisation led to slightly elevated nitrification rates. Autotrophic and heterotrophic nitrification pathways varied among the LTEs. P fertilisation did not alter N ₂ O emissions, but raised CO ₂ emissions in all LTEs. Only the annually P‐fertilised soils showed higher microbial biomass C concentration and CH ₄ uptake. These results highlight the importance of comparative studies to obtain an integrative perspective of long‐term P fertilisation affecting N and C transformations in soils.