Manure and Mineral Fertilizer Effects on Seasonal Dynamics of Bioactive Soil Phosphorus Fractions
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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 Pull-man 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. 2011 by the American Society of Agronomy.
