The potential of forage and bioenergy crops as alternatives to remediate P in P-enriched soils

Dr. Maria L. Silveira and Joao Vendramini
UF/IFAS Range Cattle Research & Education Center

Phosphorus (P) is an essential nutrient in modern agriculture. Phosphorus fertilization can increase crop yields and resistance to pest and diseases. Profitable crop production depends on many factors, including a sound P management program; however, increased public perception of the role of agriculture as a nutrient source to the environment has prompted a growing interest in obtaining information about alternative to minimize the potential impacts of P pollution. 

Long-term history of continual P application of fertilizer and manure has increased soil test P levels in several regions in the U.S., to a point where P can become an environmental concern. In spite of many soils in the U.S. be P deficient and require annual application of P to sustain profitable crop production, improper management of P in certain limited areas has caused soil P accumulation and subsequent water quality problems. 

Numerous strategies have been examined to reduce P mobility in P-enriched soils such as the use of chemical amendments (i.e. aluminum- and iron-based amendments). However, important questions relative to the long-term stability and availability of immobilized P by soil amendments remains unknown. Moreover, soil amendments do not prevent accumulation of P in the soil; thus sustainable P management strategies should be focused on reducing soil P concentration and subsequent risks associated with P transport. On the other hand, the use of forage crops has been suggested as a means by which significant amounts of P can be removed from the soil. Because of the relatively high dry matter yields, persistence, and extended growing period, perennial warm-season grasses represent a viable alternative to remediate P in P-enriched soils. 

A 3-yr study was conducted at the Range Cattle REC in Ona to identify potential crop species adapted to Florida’s environmental conditions that could be used for both remediation of soil P and as forage or a cellulosic fuel-stock for production of renewable energy. Fifteen forage species were selected based on dry matter yields, adaptability to Florida conditions, and the potential to be used as a forage and bioenergy crops (Table 1).

The results from this study indicated that dry matter yields and crop P uptake varied considerably among species. Averaged across the 3-yr, dry matter yields ranged from 9,680 lb/A per year for Pensacola bahiagrass to 38,720 lb/A per year for elephantgrass. Switchgrass showed poor persistence and plots were not harvested after the second year because of excessive weed infestation. The two cultivars of bahiagrass (Pensacola and Argentine) did not exhibit significant differences in dry matter yield; neither did the two cultivars of stargrass (Florico and Ona stargrass) or the four cultivars of bermudagrass. Elephantgrass and sugarcane had significantly greater dry matter yields (38,720 lb/A per year for elephantgrass and 36,960 lb/A per year for sugarcane) than the other species. Elephantgrass exhibited the greatest P-removal potential (122 lb P/A per year) compared with other forage species (average = 57 lb P/A per year). Soil test P concentrations decreased by as much as 27% over the 3-yr study in response to crop P uptake. 

Despite the ability of forage crops to remove significant amount of P from the soil, this approach may require long time (years to decades) to reduce soil P concentrations to desirable levels. In addition, the use of crops to remove excess P represents a costly alternative because of the relatively high nitrogen fertilization levels required to maintain maximum crop production and P uptake potential. Thus, the two fold use of selected forage crops for both phytoremediation of P and as a renewable energy source may represent a potentially feasible alternative. Although elephantgrass appears to be the most appropriate species to phytoremediate P-impacted soils, one concern with the growth of elephantgrass is the large amount of nitrogen required to maintain adequate dry matter yields. Therefore, long-term studies evaluating the impacts of nitrogen fertilization on elephantgrass yields, P removal potential, environmental impacts and net energy balance are critically needed.