Long-Term Phosphorus Fertilisation: Effects on Nitrogen and Carbon Cycle Dynamics and Greenhouse Gas Fluxes in European Agricultural Soils

Phosphorus (P) is a crucial nutrient for plant growth, its limitation reduces plant and microbial biomass, affecting soil organic carbon (SOC) sequestration. Changes in soil P content may influence microbial composition, shaping pathways in the carbon (C) and nitrogen (N) cycles and impacting greenhouse gas emissions. In this lab incubation experiment we investigate the impact of different P fertilisation levels in three European long-term experiments (LTE) on N and C transformation processes and greenhouse gas fluxes in agricultural soils using stable isotope techniques (¹⁵N and ¹³C). The study is part of the EJP SOIL project “ICONICA” (Impact of long-term P additions on C sequestration and N cycling in agricultural soils).

The soil samples derived from Johnstown Castle, JC (grassland soil, Ireland), Lanna Skara, LS (arable soil, Sweden) and Jyndevad, JY (arable soil, Denmark). Two P levels were examined from each LTE: low P (0 kg P/ha and year) and high P additions (different P application rates among LTEs). The soils were mixed with ¹³C- and ¹³C¹⁵N- labelled maize biomass, respectively, and received ammonium nitrate (NH₄NO₃) in the ¹³C treatment as ¹⁵NH₄NO₃ and NH₄¹⁵NO₃, respectively, and unlabelled NH₄NO₃ in the ¹³C¹⁵N treatment. Soil and gas samples were taken 0, 1, 3, 7 and 10 days after addition of NH₄NO₃ and were analysed for ⁽¹⁵⁾NH₄⁺-N, ⁽¹⁵⁾NO₃^--N, organic ⁽¹⁵⁾N, organic ⁽¹³⁾C contents as well as for nitrous oxide (⁽¹⁵⁾N₂O), carbon dioxide (⁽¹³⁾CO₂), and methane (CH₄) fluxes.

Preliminary findings display clear differences among the three LTEs as well as the two P levels. Regarding the impact of P fertilisation history: JC soil exhibited elevated CO₂ emissions at high P compared to low P level. Significantly, high P levels showed higher CH₄ uptake rates in JC and JY soils compared to the respective low P levels. JY had the highest N₂O emissions, while JC had the lowest. JC had higher NH₄-N values than LS. The highest NO₃-N values were measured in JC, and the lowest in JY. In JC, higher NO₃-N values were measured in high P compared to low P.

The results so far underscore the complex interactions within the carbon-nitrogen-phosphorus cycles under varying P inputs. Further analyses and interpretations are in progress.