Nutrient Addition to a Subtropical Rangeland: Effects on Animal Productivity, Trophic Efficiency, and Temporal Stability

A comprehensive understanding of how native grasslands (rangelands) respond to chronic nutrient addition requires considering interactions with climate variability, water availability in particular, and with stocking rate management. We assessed animal production responses to phosphorus plus nitrogen fertilization sustained over a decade on a highly diverse subtropical grassland. Between 2002 and 2011, paddocks (2 ha) stocked with Hereford heifers received 15 kg P + 50 kg N/ha in April/May, plus 50 kg N/ha in August/September, or were left as unfertilized controls (n = 2). Monthly, independent per-paddock, put-and-take adjustment of stocking rates kept sward height within the 6–12 cm range across all paddocks and years. Herbage production, stocking rate, and liveweight gain per animal (LWG_A) and per hectare (LWG_H) were measured over the 10 consecutive one-year cycles. Fertilization increased LWG_H (+53%) because it increased both herbage production (+32%) and liveweight production per unit produced herbage (trophic efficiency +29%). Such higher trophic efficiency was not a direct effect but a consequence of the increased grazing intensity brought about by the 48% higher stocking rate required to maintain target sward heights in fertilized paddocks, especially in winter and spring. Fertilization increased LWG_H substantially more in years with higher summer precipitation. Considering that nutrients were added in autumn–winter, greater responses in wetter summers suggest active excreta-driven nutrient recycling between the sequential growth of cool- and warm-season species. Nutrient addition did not decrease temporal stability of herbage production, LWG_A or LWG_H. A stocking rate management that precluded fertilized paddocks from becoming taller than unfertilized controls, hence mitigating competition for light, could be the reason underlying this unexpected result. By disentangling intrinsic effects of fertilizers from grazing regime-mediated effects, this study helps understand how extensive animal production (agroeco) systems reliant on highly diverse rangelands respond to progressive nutrient enrichment.