Loss of P Fertilizer Effectiveness in Raising Soil P Availability in a Grazed Grassland Enriched With CO 2 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 CO 2 (eCO 2 ) is increasingly recognized. P fertilization could alleviate these constraints, but biogeochemical feedbacks under eCO 2 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 2 (+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 eCO 2 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 2 . Similar results occurred with the Bray‐1 soil P test. These effects soon disappeared after CO 2 enrichment stopped. Accumulation of moderately labile organic P in the eCO 2 topsoil shortly after fertilization indicated rapid biological immobilization of newly applied P occurring under eCO 2 . Alternative P loss mechanisms under eCO 2 , 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 2 and ambient CO 2 , and the biomass of the P‐sensitive legume Trifolium repens was often greater under eCO 2 . Thus, either the fertilizer regime was sufficient to maintain a non‐limiting pasture P status, or integrated plant–soil biological adjustments under eCO 2 compensated for reduced P availability. If compensatory mechanisms play a greater role in supporting crop P nutrition under eCO 2 but are neglected by routine soil P availability tests focused on inorganic P, overapplication of P fertilizers will occur as CO 2 levels continue to rise.

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