Effects of Nitrogen Fertilization on Phosphorus Uptake and Movement Among Forage, Soil and Water in a Year-Round Pasture System
Dillard, Sandra Leanne
Type of Degreedissertation
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A 3-year study was conducted to investigate manipulation of nitrogen (N) fertilization regime and incorporation of seasonally adapted annual forages for re-establishing phosphorus (P) equilibrium and sustainability in a year-round grazed pasture system characterized by high background soil-test P. In the fall of 2009, 2010 and 2011, six 0.28-ha plots were overseeded with triticale (Triticum secale) and crimson clover (Trifolium incarnatum) into a tall fescue (Lolium arundinacea)/bermudagrass (Cynodon dactylon) sod and randomly assigned to 1 of 3 N fertilizer treatments (n = 2): 100% of N recommendation in a split application, 50% in a single application, and 0% of N recommendation for triticale. Cattle were placed into plots the following spring for grazing until May. In the summer, plots were overseeded with cowpea (Vigna unguiculata), fertilized at the same rates by reference to N recommendations for bermudagrass, and grazed by cattle until September. There were no effects of N fertilization rate on acid or alkaline soil phosphatase activity, soil pH, soil electrical conductivity, or soil concentrations of water-soluble P or N. Concentration of extractable soil-P was decreased in plots receiving 50% of the recommendation for N. However, increasing N fertilization to 100% of recommendation in a split application resulted in no further reduction in soil-test P. Forage mass, foliar P and N concentrations and forage P mass were not affected by N fertilization rates at the plant-community level;, but responses were observed within individual forage species. Forage characteristics were affected more by growing season and forage management than by fertilization, similar to effects observed for soils, suggesting that foliar P mass could be increased with minimal N inputs, use of seasonally adapted annual forages and incorporation of grazing cattle. Runoff water from plots contained greater concentrations of extractable P, PO4-P and NH4-N in unfertilized than fertilized paddocks, possibly due to lower pasture productivity. Neither N fertilization regime nor grazing season affected intake or fecal excretion of water-soluble P by grazing cattle, but total P excretion was greater with greater N fertilization in the spring grazing period. Cattle P requirements for growth were met by grazed forage, and sufficient P was returned to pasture to meet forage requirements, with no N fertilization. There was no effect of N fertilization on rates of decomposition or P disappearance from fecal pats, or on P concentration in soil beneath decomposing fecal pats; however, N fertilization increased N removal from and increased soil N concentrations beneath fecal pats. Results are interpreted to mean that, in order to effectively decrease nutrient concentrations in water runoff and to increase forage P mass and soil quality in a year-round grazed grass-legume pasture, N fertilization at no more than 50% of the recommended rate for the seasonally-adapted grass species within the stand was sufficient. Also, intake and fecal returns of P and foliar uptake of P were causally associated with N status of the P-enriched pasture system, but rate and extent of assimilation of P returns into the soil profile from degradation of fecal material were not.