Impact of Long-Term Phosphorus Fertilization on Nitrogen and Carbon Cycle Dynamics, Greenhouse Gas Fluxes and Plant Growth in an Irish Grassland Soil

Phosphorus is an essential element for all organisms. A sub-optimal supply of agricultural soils may influence yields, soil quality and health, carbon sequestration and nutrient turnover processes. Changes in soil P levels can influence microbial communities, altering key pathways in the nitrogen (N) and carbon (C) cycles and affecting greenhouse gas emissions. In this laboratory incubation experiment, we examine the effects of three long-term P fertilization levels in an Irish grassland soil on N and C transformation processes, associated greenhouse gas fluxes and plant growth using stable isotope techniques (¹⁵N and ¹³C). This research 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 were obtained from the low, medium and high P fertilizer treatments of the long-term P-fertilization experiment on grassland at Johnstown Castle, Ireland. The soil was sieved, filled into plant pots and sown with three maize seeds per pot. All pots received 5 days after sowing the same amount of glycine and ammonium nitrate (NH₄NO₃), only differing in the isotopic labels of N and C, respectively: The ¹³C-labelled glycine was applied together with NH₄¹⁵NO₃, the ¹⁵N-labelled glycine together with the unlabelled NH₄NO₃, and the unlabelled glycine together with ¹⁵NH₄NO₃. Plant, soil and gas samples were taken 0, 1, 3, 7 and 10 days after label application by glycine-NH₄NO₃ addition 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.

We aim to deepen our understanding of the complex relationships between the nitrogen-carbon-phosphorus cycles and their impacts on plant growth under varying levels of phosphorus application. The development of the Ntrace analysis tool into the CNtrace analysis tool is expected to enhance insights into these interactions and transformation processes. Key hypotheses include increased plant biomass, elevated CO₂ emissions and reduced N₂O emissions at high P-levels. Further analyses and interpretations are ongoing.