Proc. 57th Southern Pasture and Forage Crop Improvement Conference, Athens, GA April 23-25, 2002

The Salem Road Study: Restoration of Degraded Land with Pasture
-Water Quality of Overland Flow

D.H. Franklin, J.A.Stuedemann, A.J. Franzluebbers, J.L. Steiner, and M.L. Cabrera

USDA-Agriculture Research Service, Watkinsville, GA
University of Georgia, Athens, GA

Abstract

Identifying agricultural land management systems that are productive and protect water quality is important for the sustenance of the community as a whole. Historical loss of top soil in overland flow has degraded the soil base in the Southern Piedmont. Grassland restoration through grazed systems may be a productive means with which to sustain small family farms, green space and improve water quality. However, nutrient losses in overland flow from grazed land may reduce productivity and abase aquatic systems. This work has been carried out to evaluate nutrient losses in runoff from pastures with variable fertilizer management strategies. Twenty small in-field runoff collectors were installed at six Salem Road Study paddocks fertilized with either mineral fertilizer or broiler litter. Runoff samples from ten runoff events from May 1998 to February 2001 were collected and analyzed for concentrations of dissolved reactive phosphorus. Pastures fertilized with broiler litter and mineral fertilizer had runoff losses of DRP ranging from 0.5 to 6.0 mg P/L and were not found to be different. These fertilization strategies did not significantly impact nutrient losses in overland flow during this period of lower-than-normal precipitation. Further research is needed to better understand land application of manure in these systems under normal and higher-than-normal precipitation.

Introduction

Phosphorus (P) loss from agricultural land is an economic loss for the producer and may elevate nutrient concentrations in nearby streams. In the Southern Piedmont, animal agriculture dominates and cattle/poultry enterprises are a common practice. Cattlemen graze and manage their lands in a variety of ways across the watershed and across the individual farm. Pastures and hayfields are commonly fertilized with inorganic fertilizer and/or broiler litter. The issue of N and P losses in surface runoff from pastures and haylands, and potential eutrophication of receiving waters has been described (Sharpley, 1995, Moore et al., 1995). Loss of P in runoff occurs in both dissolved and particulate forms (Sharpley et al., 1992). Currently, the scientific and advisory community has not come to consensus on acceptable levels of P in edge-of-field runoff or in surface waters. Part of the problem lies in the scarcity of measurement at the field, farm, and watershed levels (Daniel et al., 1995), and in the potentially inherent temporal and spatial variability of N and P found in soil and surface waters.

In the state of Georgia, a P-index was developed to assess the risk of P loss from agricultural fields. The Georgia P-index has three components: Source, Transport and Management. Source refers to the main sources of P in the field, namely P present in the soil (which is measured by soil test P) and P added as fertilizer (which is measured by the rate of application). Transport takes into account the main mechanisms of P movement in the field, namely soluble P in surface runoff, particulate P in surface runoff, and soluble P in leachate. Management considers those management practices that can reduce the loss of P from a field, such as method of P application, timing of the application, and use of vegetated buffers. These practices could prove to be a useful strategy to increase nutrient utilization on site, limit losses and improve productivity.

Methods and Materials

The climate of the Georgia Piedmont is temperate, humid and rainfall is on average 50 inches (1250 mm) per year. On average, there are about 20 runoff events/year, with 74% of them occurring from October through March (Tyson-Pierson, 2000). The Salem Road Study has 18 grazed, 0.7-ha paddocks with three fertilization treatments which are replicated three times. See J. A. Stuedemann’s paper ( this proceedings) for in depth details of the paddocks and long-term study. This work will only consider two fertilization strategies (mineral and manure) for the low grazing pressure paddocks (Figure 1). In Year 1999 fertilizer timing strategy changed and fertilization with broiler litter increased from two times a year to three times a year to accommodate optimum fescue production (Table 1). An increase in the number of applications resulted in a difference in total amount applied per year (6.6 Mg ha^-1 yr^-1 in years 1994 to 1998 and 10.0 Mg ha^-1 yr^-1 in years 1999 to 2003). Soil samples were collected and Mehlich-I extractable soil P of the 0-6-cm and 0-15-cm depths was determined each year from early 1994 to early 1999 (Franzluebbers et al., 2002). Linear regressions of extractable soil P during this time were used to predict extractable soil P at the beginning of the growing seasons of 1999 and 2000.

Runoff was collected with small in-field runoff collectors (SIRC, Franklin et al., 1999). Three to four SIRC were placed at the edge of each paddock (May 1998) to collect runoff. Runoff collectors were inspected and cleaned at least twice monthly. The morning following a rainfall event, sites were inspected for runoff and if runoff was present, samples were collected. Runoff samples were taken from SIRC collection tanks and collection tanks were cleaned and rinsed with deionized water. Runoff samples were placed in dark, iced coolers upon exiting each field and sent to an analytical laboratory upon completion of sampling. In the laboratory, runoff samples were filtered through 0.45-Fm Cellulose Nitrate (CNA) and analyzed colorimetrically using the molybdate blue method (Murphy and Riley, 1962).

Statistical Analysis

Univariate analysis (SAS Inst. Inc., 1994) was executed to summarize descriptive statistics and to determine likelihood of a normal distribution for runoff data. When data within groups (“treatments”) are not normally distributed or when they do not have equal variances, nonparametric methods of analysis are advised (Helsel and Hirsch, 1992; Conover, 1980). Two pathways of nonparametric multi-factor analysis are possible (Helsel and Hirsch, 1992). One approach is to compute the Kruskal-Wallis test and the other is a rank transformation followed by ANOVA on the computed ranks. The Kruskal Wallis test makes multiple comparisons on ranks and determines if a group differs from all others. Rank transformation/ANOVA makes multiple comparisons on ranks and determines differences among groups. Rank transformation/ANOVA was used in this study.

Table 1. Fertilization strategies for years 1994 to 2003 at the Salem Road Grazing Land Study, Farmington, GA.

Fertilization Strategy	Years 1994 to 1998		Years 1999 to 2003
	Source	Amount	Source	Amount
Mineral	Inorganic (N-P2O5-K2O)	225-36-63 ( kg ha-1 yr-1)	Inorganic (N-P2O5-K2O)	270-45-90 ( kg ha-1 yr-1)
Manure	Broiler Litter (N-P2O5-K2O)	6.6 Mg ha-1 yr-1 194-283-201 ( kg ha-1 yr-1)	Broiler Litter (N-P2O5-K2O)	10.0 Mg ha-1 yr-1 270-394-280 ( kg ha-1 yr-1)
Timing	All N sources applied 2 times per year		All N sources applied 3 times per year

 

 

 

Results and Discussion

During three years, we collected overland flow (runoff) from the Salem Road Grazing Land Study 9 times. Year 2000 had twice as many runoff events as Year 1999 and Year 1998 had no events. Runoff concentrations of dissolved reactive phosphorus ranged from 0.5to 6.0 mg P L^-1 for paddocks fertilized with either mineral fertilizer or broiler litter. No significant difference in DRP concentration in runoff between paddocks fertilized with mineral fertilizer or broiler litter was indicated across both years and there were too few events to analyze each year separately.

An increase of 6 % average annual DRP concentration in runoff from 1999 to 2000 was measured for paddocks fertilized with mineral fertilizer (Figure 2), whereas paddocks fertilized with broiler litter had a 171% increase in average annual DRP concentration in runoff. These increases observed from 1999 to 2000 may have been caused in part by the lower annual runoff volume observed in 1999 and in part by the change in fertilization strategy in 1999 (from two times a year to three times a year).

A 4% increase between years was measured for soil test phosphorus (STP) from paddocks fertilized with mineral fertilizer for depth 0 to 6 cm, whereas a 12% increase was measured in STP in mineral paddocks for depth 0 to 15 cm. This suggests that the mineral fertilizer may have moved below 6 cm. Paddocks fertilized with broiler litter had similar, but, measurable changes in STP at both depths. The shallower sampling depth ( 0 to 6 cm) had a 13% change and the standard sampling depth (0 to 15 cm) had a 14% change. These results indicate that most of the broiler litter P was still on the surface.

The standard sampling depth protocol for pastures in Georgia is four to six inches or 10 to 15 cm which would be where most of the root mass is present. Because P is considered to be a fairly immobile nutrient in the soil one would expect a greater change in the shallower STP. This was not the case; a possible explanation for this is the lack of precipitation in 1998 and 1999 thus removing the mechanism by which the surface applied broiler litter P enters the soil. Soil test P was 172% and 174% higher in the shallower sampling depth for years 1999 and 2000, respectively. These results indicate that there is a large amount of phosphorus at or above the soil surface. When a runoff event occurs, some of the soluble P present is desorbed from broiler litter on the soil and grass surface and entrained in the runoff. This mechanism could also explain why runoff P concentration in the broiler litter treatment increased by 171% while STP in the 0-6 cm increased by only 13%. The larger increase observed in runoff P concentration than in STP suggests that some of the broiler litter P was on the surface, exposed to solubilization by rainfall and surface runoff. These results clearly indicate that STP alone is not a clear indicator of the potential for P loss in runoff. In addition to STP, rate and timing of fertilizer P applications as well as risk of runoff are needed to compute the Georgia P Index.

Summary

Over the past three years the Southeast has experienced a significant drought. Under these conditions, there was little evidence measured on this study to declare a difference in DRP concentration in runoff between paddocks fertilized with mineral fertilizer or broiler litter. These results can not be extrapolated to normal or wet conditions within in the region. However, we can surmise that soil test phosphorus alone is not an adequate criterion to assess the risk of P loss from grasslands.

The Salem Road Study is a long-term research project. Additional runoff collectors will be placed in paddocks currently without collectors. In 2002 small in-field runoff collectors will be placed in mineral and broiler paddocks with high grazing pressure. This may better simulate on-farm practices and should further our understanding of edge-of field nutrient losses for pastures with intensive grazing.

References

Conover, W.J. 1980. Practical nonparametric statistics. John Wiley & Sons, New York, NY.

Daniel, T.C., D.R. Edwards, and D.J. Nichols. 1995. Edge-of-field losses of surface-applied animal manure. p.89-98. In K. Steele (ed.) Animal Waste and the Land Water Interface. Lewis Publishers, New York, NY.

Franzluebbers, A.J., J.A. Steudemann, and S.R. Wilkinson. 2002. Bermudagrass Management in the Southern Piedmont USA. II. Soil Phosphorus. SSSAJ 66:291-298

Helsel, D.R., and R.M. Hirsch. 1992. Statistical methods in water resources. Elsevier, New York, NY.

Moore, P.A. Jr., T.C. Daniel, A.N. Sharpley, and C.W. Wood. 1995. Poultry manure management: environmentally sound options. J. Soil and Water Conserv. 50:321-327.

Murphy, J., and J.P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27:31-36.

Sharpley, A.N., S.J. Smith, O.R. Jones, W.A. Berg, and G.A. Coleman. 1992. The transport of bioavailable phosphorus in agricultural runoff. J. Environ. Qual. 21:30-35.

Sharpley, A.N. 1995. Dependence of runoff phosphorus on extractable soil phosphorus. J. Environ. Qual. 24:920-926.

Tyson-Pierson, S.K. 2000. Surface water quality in grasslands fertilized with broiler litter. Dissertation, Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA.