Soil Phosphatase Activity and Foliar Uptake of Phosphorus As Influenced by Nitrogen Fertilization Regime in a Grazed Pasture System.

Efficiency of P utilization from P-enriched soils may be enhanced by management practices that facilitate phytoextraction of P, and increasing activity of soil phosphatase may be an effective method for increasing P bioavailability. In October 2010, 6 0.28-ha plots characterized by high soil-test P were overseeded with triticale (Triticum secale) and crimson clover (Trifolium incarnatum) into a tall fescue (Lolium arundinacea)/bermudagrass (Cynodon dactylon) sod and assigned randomly to 1 of 3 treatments (n = 2): 100% of N recommendation for tall fescue in a split-application, 50% of N recommendation, and 0% of N recommendation. In February 2011, 6 cattle (339 ± 11 kg) were randomly assigned to plots (1 animal/plot) for grazing until May; soil (0 – 20 cm) and forage samples were taken from each pasture prior to grazing by cattle. In June, plots were overseeded with cowpea (Vigna unguiculata) and maintained on the same N-fertilizer regimes, based on N recommendation for bermudagrass. In August, 6 cattle (361 ± 23 kg) were randomly assigned to plots (1 animal/plot) for grazing until September, at which time another set of soil and forage samples was taken. There were no differences among treatments for acid (P > 0.417) or base phosphatase (P > 0.225). Cool-season acid and base phosphatase activities were lower (0.09 and 0.13 μg pNP/g/h, respectively) than warm-season (0.22 and 0.24 μg pNP/g/h, respectively) (P = 0.034 and P = 0.045, respectively). Forage DM availability (P = 0.002) and uptake of P (P = 0.01) were greater for cool- than warm-season forage (4,441 vs. 2,311 kg/ha and 8.44 vs. 5.30 kg P/ha, respectively), but were not different among treatments.  Results indicate that growing season and grazing of seasonally adapted forage species greatly affected soil phosphatase activity, but these were not affected by manipulation of N fertilization regime.