Stratification of Phosphorus Forms from Long-Term Conservation Tillage and Poultry Litter Application

Abstract

Phosphorus stratification leaves high P concentrations at the soil surface, which are vulnerable to loss in runoff. Understanding P forms at the soil surface may help control P loss, but little information is available on how P forms stratify in soils. We used 31P nuclear magnetic resonance (NMR) spectroscopy and chemical analysis to characterize P forms and the elements controlling P cycling in Cecil soil (a fine, kaolinitic, thermic Typic Kanhapludult) that became P stratified after 11 yr of cropping management conducted under combinations of tillage (conventional and no-till, CT and NT, respectively) and inorganic chemical (CF) and poultry litter (PL) fertilization. Samples from three soil depths (0–2.5, 2.5–5, and 5–15 cm) were sequentially extracted in NaOH–ethylenediaminetetraacetic acid (EDTA) and HCl before 31P-NMR spectroscopy. The NaOH–EDTA extraction recovered 63 to 83% of the total P in a range of P forms, and HCl recovered 2 to 16% of the total P, mainly orthophosphate. Phosphorus forms and concentrations were evenly distributed in soil layers only in the CT treatment with CF. Stratification of orthophosphate and phytate occurred in NT samples fertilized with PL and CF, with the highest concentrations in the top 2.5 cm. Increased orthophosphate and phytate concentrations were detected at the soil surface for CT–PL samples, indicating that PL is not as well mixed by tillage as CF. These results suggest that P fertilizers should be placed below the soil surface under NT to avoid stratification and thus reduce potential P loss in surface runoff.