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

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 CO (eCO) is increasingly recognized. P fertilization could alleviate these constraints, but biogeochemical feedbacks under eCO may diminish the effectiveness of P fertilizer in raising soil P availability. Here, we present data from a botanically diverse grazed pasture enriched with CO (+84-111 ppm) and supplied with P fertilizer (1.5 g P m year) for approximately 24 years, showing (1) a sustained 27% reduction in topsoil Olsen P under eCO 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 eCO. Similar results occurred with the Bray-1 soil P test. These effects soon disappeared after CO enrichment stopped. Accumulation of moderately labile organic P in the eCO topsoil shortly after fertilization indicated rapid biological immobilization of newly applied P occurring under eCO. Alternative P loss mechanisms under eCO, 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 eCO and ambient CO, and the biomass of the P-sensitive legume Trifolium repens was often greater under eCO. Thus, either the fertilizer regime was sufficient to maintain a non-limiting pasture P status, or integrated plant-soil biological adjustments under eCO compensated for reduced P availability. If compensatory mechanisms play a greater role in supporting crop P nutrition under eCO but are neglected by routine soil P availability tests focused on inorganic P, overapplication of P fertilizers will occur as CO levels continue to rise.