Simulating long-term phosphorus, nitrogen, and carbon dynamics to advance nutrient assessment in dryland cropping

Soil chemical fertility has steadily declined in tropical and subtropical agriculture with depleted stocks of phosphorus (P), nitrogen (N), and carbon (C). Assessing the dynamics of these elements and their interactions on crop productivity in dryland cropping are complex because climate often dictates crop nutrient response. This results in under- or over- fertilising crops, suboptimal crop yield, and fertiliser inefficiency. The Agricultural Productions Systems sIMulator (APSIM) model accounts for C x N x climate interactions, but simulation of P dynamics is constrained by a dearth of suitable data. To address this problem, we used a novel approach to simulate P, N, and C dynamics at a 35-year long-term field trial, where a broad 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 soil P model with quantified adsorption isotherms and by assuming correspondence between conceptual soil P pools and Hedley fractionation pools. Soil N and C dynamics were parameterised with 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, where crop response was strongly mediated by N supply and water availability. APSIM also identified the long-term 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. Better informed P modelling will provide insights into the effects of climate variability on soil fertility and crop productivity, and guide management practices to deliver better fertiliser efficiency and maintain soil organic C.