Simulating long-term nutrient dynamics to better assess soil fertility in subtropical cropping

Soil chemical fertility has steadily declined in tropical and subtropical agriculture. Assessing effect of phosphorus (P), nitrogen (N), and carbon (C) on crop productivity is complex because climate often dictates crop nutrient response, causing suboptimal yield and fertiliser inefficiency. The Agricultural Productions Systems sIMulator (APSIM) model accounts for C x N x climate interactions, but modelling P is constrained by a dearth of suitable data. We simulated P, N, and C dynamics at a 35-year long-term field trial, where a range of N (0, 40, 80, 120 kg ha-1 ) and P (0, 10, 20 kg ha-1 ) fertiliser rates were consistently applied. We parameterised the model by assuming correspondence between conceptual soil P pools and Hedley fractionation pools, and with quantified P adsorption isotherms, measured organic N, C, and charcoal content to estimate organic matter decay coefficients, pool sizes, and C:N ratios. APSIM accounted for variation in mean N export (94 %), crop yield (88 %), and P export (62 %) across the 12 treatments, and reproduced interannual variation in N × P effects for crop yield and N export. APSIM also identified depletion or accumulation of soil P, N, and C in most treatments. P fractionation and isotherm measurements are labour intensive but worthwhile and future efforts should work to consolidate a database for different soil types. Using this P modelling approach will enable better assessment of climate variability on soil fertility and crop productivity, and guide management practices to deliver better fertiliser efficiency and maintain soil organic C