Loss of P Fertilizer Effectiveness in Raising Soil P Availability in a Grazed Grassland Enriched With CO2 for 24 Years

ABSTRACT Phosphorus (P) is a finite resource and an essential macronutrient for plant growth. The importance of low soil P availability in constraining plant biomass responses to elevated CO2 (eCO2) is increasingly recognized. P fertilization could alleviate these constraints, but biogeochemical feedbacks under eCO2 may diminish the effectiveness of P fertilizer in raising soil P availability. Here, we present data from a botanically diverse grazed pasture enriched with CO2 (+84–111 ppm) and supplied with P fertilizer (1.5 g P m−2 year−1) for approximately 24 years, showing (1) a sustained 27% reduction in topsoil Olsen P under eCO2 prior to annual fertilizer application, and (2) an approximate halving of the short‐term (approximately 4 months) effectiveness of P fertilizer in raising Olsen P by 1 unit under eCO2. Similar results occurred with the Bray‐1 soil P test. These effects soon disappeared after CO2 enrichment stopped. Accumulation of moderately labile organic P in the eCO2 topsoil shortly after fertilization indicated rapid biological immobilization of newly applied P occurring under eCO2. Alternative P loss mechanisms under eCO2, including inorganic P depletion due to increased pasture growth, increased P offtake versus return through the plant→animal→dung pathway, or P movement down the soil profile, were not supported by the available evidence. Despite this, pasture P concentration and uptake were similar under eCO2 and ambient CO2, and the biomass of the P‐sensitive legume Trifolium repens was often greater under eCO2. Thus, either the fertilizer regime was sufficient to maintain a non‐limiting pasture P status, or integrated plant–soil biological adjustments under eCO2 compensated for reduced P availability. If compensatory mechanisms play a greater role in supporting crop P nutrition under eCO2 but are neglected by routine soil P availability tests focused on inorganic P, overapplication of P fertilizers will occur as CO2 levels continue to rise.