Eutrophication of freshwater bodies is frequently attributed to elevated phosphorus (P) concentrations in surface runoff from P-enriched agricultural soils. In the Mid-Atlantic region of the USA, eutrophication of the Chesapeake Bay and its tributaries is a first-order environmental issue. Agriculture is implicated as a cause of eutrophication via the high concentration of poultry feeding operations in the Delmarva region and the numerous dairy cattle farms within the northern and western tributary basins of the Chesapeake Bay. Livestock manures from these operations long have been used as sources of crop nutrients. Unfortunately, applying manures to fields based upon crop nitrogen requirements has resulted in oversupplying crop phosphorus requirements by three to four times. Over the years these applications have resulted in soil phosphorus concentrations that have reached levels considered to be excessive for many fields in the region. This has generated concern about soil phosphorus leaching potential and was this project's first objective. The second objective of this research has focused upon the effect that grain and forage cropping systems common to the region have upon soils with elevated soil phosphorus concentrations.
Research plots were established at four locations in Maryland USA beginning in 1994. Three of the sites have Atlantic Coastal Plain soil types and the fourth has a soil type representative of the Piedmont region. Five distinct initial soil P concentrations (treatments) were established at each location using four annual applications (1994-1997) of five different rates [0 (control), 100, 200, 300, and 400 kg total P ha-1 y-1] of either poultry manure, dairy manure or commercial fertilizer. To measure soil phosphorus leaching potential, beginning in 1998 each plot from the sites with the Atlantic Coastal Plains soils was sampled to a depth of 150 cm in seven depth increments. The soil samples were analyzed for soil test phosphorus (STP), water-extractable soil phosphorus (WSP), and degree of phosphorus saturation (DPS). The DPS of the 0 to 15 cm depths confirmed that at the 100 kg P ha--1 yr--1 application rate, all sites had exceeded the threshold for P saturation (30%). At depths greater than 30 cm, DPS was typically below the 30% saturation threshold. The DPS change points ranged from 25 to 34% for the 0 to 90 cm depths. This research concluded that the risk of P leaching through the matrix of the Atlantic Coastal Plain soils studied was not high; however, P leaching via macropore bypass may contribute to P loss from these soils.
Beginning in 2001, these plots have been used to evaluate the effects that forage and grain cropping systems have upon P-enriched soils. Base line soil P concentrations for this study were established by soil samples collected in March 2001. At each of the four locations, one of three forage systems (Forage I = cereal rye (Secale cereale L.) silage and corn (Zea mays L.) silage annually; Forage II = alfalfa (Medicago sativa L.); Forage III = annual ryegrass (Lolium multiflorum Lam.) and corn silage annually) and the grain system [corn, either wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) and soybean (Glycine max Merr.) rotation] have been maintained using production practices to attain optimum yield since 2001. No phosphorus fertilizer has been applied to any of these plots since 1997. Variables measured are soil phosphorus concentration (soil samples collected each March) and crop phosphorus uptake and removal (a function of crop yield and crop phosphorus concentration). Through 2004 (2005 data will be included) and across all P loading treatments at all locations, the three forage systems have had approximately twice the crop removal of P compared to the grain system. However, changes in soil P concentration for the two cropping systems have not reflected those differences in crop P removal. Few significant reductions in soil P concentration have been observed for either cropping system. The few reductions that have occurred have been location specific and at the greatest soil P loading treatments. Additionally, no significant reductions in soil P concentration have occurred for the control treatment for either cropping system. It is estimated it will take years and possibly decades before soil P concentrations for the P-enriched treatments for either cropping system will return to levels considered optimum. Considerable variability in crop P concentrations have been observed among species at locations and among years produced. One consistent outcome, however, has been an increase in crop P concentrations as soil P loading treatments increased. This indicates that higher consumption of P occurs in agronomic crops produced on P-enriched soils. Additionally, these crop P concentrations have been greater than those reported in the National Resource Council (NRC) standards.