Abstract
Seasonal fluctuations in bioavailable soil P can influence soil test results and associated assessment of off‐site transport risk. Our objective was to evaluate changes in soil P speciation and availability with time following applications of grain fed cattle (Bos taurus) manure or monoammonium phosphate (MAP). Beef cattle manure or MAP was applied at a targeted rate of 200 kg P ha−1 on a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustolls) in 2005 and 2006 and planted to grain sorghum [Sorghum bicolor (L.) Moench]. Soil samples (0–0.15 m) were collected before and throughout the growing season and analyzed for Mehlich‐3 phosphorus (Me3P), 1:10 water‐extractable phosphorus (WEP10), water extractable cations, pH, and fractions of bioactive soil phosphorus (TBIOP), which comprised 1:100 water extractable P, ethylenediamine‐N,N,N′,N′‐tetraacetate (EDTA)‐exchangeable inorganic phosphorus (EEPi) and the EDTA‐exchangeable phosphohydrolase‐labile phosphorus (EPHP). Levels of soil Me3P, WEP10, and all fractions of TBIOP in MAP‐amended plots fluctuated significantly (p < 0.05) during both seasons. Except for Me3P, manure amended plots also exhibited significant (p < 0.05) seasonal variations in soil extractable P and a delayed release of P that extended well into the growing season. In contrast, fluctuations in extractable soil P in unamended plots were not significant except EPHP. In water extracts, a significant (p < 0.05) dependence of solution P on pH and Ca suggested that precipitation‐dissolution reactions contributed to observed seasonal fluctuations in P. Fluctuations in total bioactive soil P were two to four times greater than aboveground biomass P highlighting the importance of accounting for seasonal dynamics in assessing offsite P transport risks.
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