Dairy Manure Influence on Soil and Sediment Composition: Implications for Phosphorus Retention

Manure-derived P can jeopardize surface water quality in regions dominated by sandy soils, due to low P retention capacity of these soils. Surface (Ap) horizons from dairy-intensive areas in the Lake Okeechobee Basin were recently found to release P readily, despite abundant Ca and high pH. The purpose of this study was to determine the inorganic components hat influence or reflect the stability of P in these horizons and in a related stream sediment that released much less P than soil material, despite comparable total P. Coarse fragments (>2 ram), sand, silt, and clay were separated by sieving and centrifugation. Whole soil material and coarse fragments were examined using a dissecting microscope. Crystalline and noncrystalline components were identified using a combination of optical microscopy, x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, electron microprobe analysis, thermogravimetry, density separation, and selective dissolution techniques. The >2-pro fraction of Ap horizons was dominated by quartz, but appreciable biogenic opal was present in the silt. Clay from these horizons was mainly noncrystalline Si (opal A), which persisted after Ca and P were removed via selective dissolution. The clay-fraction Si had high CEC, abundant adsorbed water, amorphous morphology, and low affinity for P. It was probably a degradation product of opaline forage phytoliths, since dried manure contained Si bodies similar in morphology to those found in silt and clay. Lack of Ca-P minerals suggests that manure components (i.e., organic acids, Mg, Si, etc.) inhibited crystallization of stable Ca-P, thereby maintaining high P solubility. The stream sediment contained a calcium phosphate mineral resembling poorly crystalline apatite, and a ferrous phosphate mineral (vivianite). The §ux of P from dairy to aquatic systems in regions of dominantly sandy soils could be markedly reduced if the barrier to the crystallization of Ca-P could be eliminated. D MANURE can appreciably elevate levels of P and other components in soils (Dantzman et al., 1983). The fate of added P is dependent on the soil’s potential for P assimilation into stable (i.e., immobile) forms. Many soils effectively retain P due to the presence of mineral components (metal oxides, secondary phyllosilicates, etc.) with high surface affinity for orthophosphate (Sample et al., 1980). However, movement of from dairy to aquatic systems does occur under certain conditions, and has been linked to eutrophication of surface water (Federico, 1977; Federico et al., 1981; Webber, 1981; Allen et al., 1982). This movement may be related to erosion (Sharpley and Smith, 1983) or subsurface transport (Burgoa, 1984; Flaig et al., 1986; Burgoa, 1989; Mansell et al., 1991). Subsurface transport of P can be significant in sandy soils due to low surface area or a paucity of P-retaining components (Neller et al., 1951; Yuan and Lucas, 1982; Rhue and Hensel, 1983; Allen, 1987; Rhue and Everett, 1987; Soil and Water Science Dep., P.O. Box 110510, Univ. of Florida, Institute of Food and Agric. Science, Gainesville, FL 32611. Contribution of the Florida Agric. Exp. Stn. Journal Set. no. R-03469. Received 25 Oct. 1993. *Corresponding author. Published in J. Environ. Qual. 23:1071-1081 (1994). Scinto and Reddy, 1990; Burgoa, 1991). Lateral transport of P from sandy, manure-affected surface horizons to streams is exacerbated by high water tables (Allen, 1987; Mansell et al., 1991). Stability of fertilizer and manure-derived forms of P is a relevant environmental consideration for deep sandy soils, given the potential for subsurface transport. The forms, stabilities, and transformations of applied P have been studied for a variety of soil materials and amendments (Bell and Black, 1970a,b; Fixen and Ludwick, 1982; Sharpley et al., 1984; O’Connor et al., 1986; Pierzynski et al., 1990). We recently found that high levels of P could be leached from surface (Ap) horizons of four sandy Florida soils heavily loaded with dairy manure despite high pH and abundant Ca2÷ in solid and solution phases (Wang et al., 1994). Solutions were supersaturated with respect to apatite, and even to some of the more soluble metastable forms of Ca-P. These soils were Aquods (Soil Survey Staff, 1975), which are dominated by stripped quartz sand in A and E horizons. These horizons have minimal P retention capacity (Burgoa, 1989). Thus, manure appeared to provide the reactive (nonquartz) components that dictated P stability terms of both sorption and precipitation. The P in dairy manure is mainly in inorganic form (Morse et al., 1992). This study was conducted to determine the composition of samples used by Wang et al. (1994), including four dairy-impacted Ap horizons and a stream sediment influenced by dairy-barn flushing. The sediment was included because it inexplicably released far less P upon leaching than the soil samples, despite comparable total P (Wang et al., 1994). We believe the results help to explain the labile nature of P in such Ap horizons, and have implications for water quality in regions where dairies are located on deep sandy soils. MATERIALS AND METHODS Sample Collection and Preparation Samples of surface soil horizons (Ap; 0-20 cm) from four representative dairy-intensive areas (Sites I-4; fenced areas near milking barns with heavy manure loading) in Okeechobee Basin, and of a stream sediment (sampled at approximately 0-20 and 20-40 cm) from Site 2 were used in the study (Fig. I). The intensive-area Ap horizons were heavily affected by manure, and had little or no vegetative cover. The dominant soils for all sites were Aquods. The P forms by horizon were previously characterized for Sites I, 2, and 4 by Nair and Graetz (1991), who found (in concurrence with Wang et al., 1994) that the P in intensive-area Ap horizons was mainly associated with Ca. Samples had relatively high sand (810890 g kg-~) and organic C (44-72 g kg-I) contents (Table I). Abbreviations: CEC, cation exchange capacity; DCB, dithionite-citratebicarbonate; XRD, x-ray diffraction; TG, thermogravimetry; SEM, scanning electron microscopy; EM, electron microprobe.