Soil phosphorus pools with addition of fertiliser phosphorus in a long-term grazing experiment

Abstract

Published Online 8 November 2019Grasslands are a globally important use of land for food and fibre production, which often require the addition of phosphorus (P) fertiliser to maximise plant production. However, a large proportion of the added P can accumulate in pools of poorly available inorganic and organic P in the surface soil layer under grasslands. The aim of this study was to identify the chemical nature of the organic P in soils from a long-term fertiliser by grazing permanent pasture experiment that have received varying additions of phosphatic fertiliser (cumulative P input of 27, 169, 311, 513, 745 and 1035 kg P ha⁻¹) over a period of 37 years. The design of the experiment uniquely provides insight into the response of soil organic P to the addition of fertiliser P on the decadal scale. On average, 46% of the added fertiliser P was recovered as total P in the 0–100 mm soil layer after 37 years of phosphate addition. The content of both inorganic and organic forms of soil P increased with the addition of fertiliser P. The accumulation of organic P increased linearly up to a cumulative P input of 745 kg P ha⁻¹ and plateaued thereafter. The majority of organic P in all treatments was detected as a broad signal in the phosphomonoester region of solution ³¹P nuclear magnetic resonance (NMR) spectra; this also accounted for 79% of the accumulated organic P in fertilised soil. Our results indicate that accumulation of P in the organic portion as complex forms eventually reaches a new equilibrium where no net accumulation would be expected with further addition of phosphate.Timothy I. McLaren, Ronald J. Smernik, Michael J. McLaughlin, Therese M. McBeath, Malcolm R. McCaskill, Fiona A. Robertson, Richard J. Simpso