Climatic Effects on Phosphorus Fractions of Native and Cultivated North American Grassland Soils

The climatic regime influences the turnover of P in soils, resulting in shifts in P fractions of different bonding forms. Here, we aimed at (i) identifying possible changes in P fractions as related to mean annual temperature (MAT) and mean annual precipitation (MAP), as well as (ii) elucidating how these patterns change under long-term agriculture. We analyzed different P fractions after sequential extraction according to the Hedley procedure from the top 10 cm of 18 native grassland sites and adjacent long-term agricultural fields along a temperature and precipitation transect from central Saskatoon to south Texas. The analyses were performed on bulk soils and clay fractions. The results showed that total P (Ptot) concentrations decreased with increasing MAT but were not clearly related to MAP. The contributions of total inorganic P (Pitot) and total organic P (Potot) to Ptot did neither change with MAT nor with MAP. The proportions of individual soil fractions were shifted from the labile and easily extractable P bonding forms to the more stable, residual P fractions with increasing annual temperature. Arable cropping induced loss of organic P (Po). The relationships to the climatic factors were sustained for the arable soil, but better reflected in the clay fractions than in the bulk soil. We conclude that there is a gross pattern of soil P distribution related to the climate, likely reflecting the increased biological processing as well as and progressing weathering and neoformation of secondary mineral phases as annual temperatures rise