Jeq50402 1101..1109

Phosphorus is an essential plant nutrient and critical to agricultural production, but it is also a problem when excessive amounts enter surface waters. Summer rotational grazing and winter feeding beef pasture systems at two fertility levels (56 and 28 kg available P ha) were studied to evaluate the P losses from these systems via surface runoff and subsurface flow using eight small (0.3–1.1 ha), instrumented watersheds and spring developments. Runoff events from a 14-yr period (1974–1988) were evaluated to determine the relationships between event size in mm, total dissolved reactive phosphorous (TDRP) concentration, and TDRP transport. Most of the TDRP transported was via surface runoff. There were strong correlations (r 5 0.45–0.66) between TDRP transport and event size for all watersheds, but no significant (P 5 0.05) correlations between TDRP concentration and event size. Flow-weighted average TDRP concentrations from the pasture watersheds for the 14-yr period ranged from 0.64 to 1.85 mg L with a few individual event concentrations as high as 85.7 mg L. The highest concentrations were in events that occurred soon after P fertilizer application. Average seasonal flow-weighted TDRP concentrations for subsurface flow were ,0.05 mg L. Applying P fertilizer to pastures in response to soil tests should keep TDRP concentrations in subsurface flow at environmentally acceptable levels. Management to reduce runoff and avoidance of P fertilizer application when runoff producing rainfall is anticipated in the next few days will help reduce the surface losses of P. PHOSPHORUS is one of the essential nutrients for all living things, but excessive amounts in surface waters can cause excessive growth of aquatic biota. Such accelerated eutrophication can limit water use for drinking, recreation, and industry in water bodies near the source of the excess P as well as at great distances from the P sources. Agricultural systems have been clearly linked with P movement into surface water bodies (Cassell et al., 1998; Correll, 1998; Sharpley et al., 1994). Various aspects of Pmovement and its impacts have been studied extensively. Considerable literature on the impacts of agricultural Pon eutrophication has been reviewed (Daniel et al., 1998; Correll, 1998) including P loss in agricultural drainage (Sims et al., 1998). Much of the research on the impacts of P management in the United States has dealt with non-forage crops. Phosphorus from pasture systems has received considerable attention outside the United States, especially in Australia, New Zealand, and the United Kingdom. New Zealand research on P in pasture runoff was reviewed by Gillingham and Thorrold (2000). There is an increasing amount of research on direct fertilizer effects on P runoff from agricultural systems that were mainly pastoral (reviews by Nash and Halliwell, 1999; Hart et al., 2004). Research on P movement in surface runoff from pasture systems at various scales was reviewed by Dougherty et al. (2004). One of the difficulties in addressing P movement to surface water is that the critical P concentration above which eutrophication may occur is an order of magnitude smaller than the P concentrations in the soil solution critical for plant growth (Daniel et al., 1998). Additionally various water quality standards for P have been established for receiving waters. To control eutrophication total phosphorus (TP) should not exceed 0.05 mg L in streams entering lakes and reservoirs, nor 0.025 mg L within lakes and reservoirs (Daniel et al., 1998). For lowland streams in New Zealand, the Australian and New Zealand Environment and Conservation Council (2000) set limits of 0.01 and 0.033 mg P L for dissolved reactive phosphorus (DRP) and TP, respectively. Primary algal production requires a P concentration of .0.02 mg L (Hayes and Green, 1984). Dissolved reactive P may cause eutrophication in lowland rivers around 0.01 mg L (Meybeck, 1982; Australian and New Zealand Environment and Conservation Council, 2000). In a laboratory study of 11 species of freshwater algae, Grover (1989) found that PO4–P levels required to maintain equilibrium algal growth ranged from 0.003 to 0.8 mg L. Research of Sawyer (1947) and Vollenweider and Kerekes (1980) suggested that critical P concentrations above which accelerated eutrophication may occur are 0.01 and 0.02 mg L for dissolved P and TP, respectively. Correll (1998) stated that TP may be more meaningful in evaluating eutrophication enhancement. In the 1970s eutrophication and TP concentrations increased in the Chesapeake Bay while dissolved P concentrations remained relatively unchanged. Although acceptable concentrations of TP may vary with the situation, for most surface bodies of water TP concentrations of 0.1 mg L are unacceptably high and concentrations of 0.02 mg L are often a problem (Correll, 1998). Using rainfall simulators on grassed plots to relate dissolved P in surface runoff to soil test P, several studies in the United States (Pote et al., 1999a, 1999b; Schroeder et al., 2004; Torbert et al., 2002) reported dissolved P concentrations ranging up to nearly 2.0 mg L. Based on this type of information, Vadas et al. (2005) concluded that soil test P can be a good predictor for dissolved P in surface runoff from non-calcareous soils. Pierson et al. (2001) recommended caution, however, when using such relationships if poultry litter was the P source and soil test P was determined shortly after a litter application. They also reported that dissolved P concentrations in USDA-ARS, P.O. Box 488, Coshocton, OH 43812. Received 18 Oct. 2005. *Corresponding author ([email protected]). Published in J. Environ. Qual. 35:1101–1109 (2006). Technical Reports: Surface Water Quality doi:10.2134/jeq2005.0402 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: TDRP, total dissolved reactive phosphorus; TP, total phosphorus; WS, watershed. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 1101 Published online May 31, 2006