Phosphorus loss in runoff from grasslands related to soil test phosphorus and poultry litter application

Abstract

Phosphorus in runoff from agricultural fields to surface waters has been identified as an important contributor to eutrophication. The objective of this research were to determine the relationship between total P (TP), dissolved reactive P (DRP), and bioavailable P (BAP) in runoff from a benchmark soil (Cecil series) and Mehlich III, DI water, and Fe2O3 extractable soil P, and the effects of application rate and initial runoff timing on the long-term loss of P in runoff from surface applied poultry litter There was significant correlation between all forms of P in runoff and all soil P test methods used. The highest correlation between STP method and DRP in runoff occurred with water extractable soil P (R 2 = 0.68) Poultry (Gallus gallus domesticus) litter application rates of 2, 7, and 13 Mg ha -1 were used along with rainfall scenarios included; 1) sufficient rainfall to produce 30 min. of runoff immediately after litter application (R1); 2) no rainfall for 30 days after manure application, then sufficient rainfall to produce 30 min. of runoff (R2); and 3) small rainfall events every 7 d (5 min. at 75 mm hr -1 ) for 30 days and then sufficient rainfall to produce 30 min. of runoff (R3). P loss was the greatest from the high application rate (13 Mg ha -1 ) and from the immediate runoff (R1) plots. Non-linear regression procedures were used to develop equations relating to P loss over time to litter application. The resulting equations produced fairly accurate prediction of P concentration (0.68 to 0.91 R 2 ) under the conditions in this study. These equations were effective at predicting DRP concentration in runoff (r = 0.8). The prediction seemed to be most accurate for runoff events that occurred shortly after litter application. However, predictions became less accurate for events that occurred long after application due to fluctuations in observed DRP concentration. These fluctuations may be explained by changes in the pool of soluble P. Fluctuations in the soluble P pool way be related to variable source areas within the field and lysis of microbial cells caused by rapid wetting during large rainfall events.