Perennial grasses are a critical component of mineral nutrient cycling in biological systems. Nutrient cycling is essential to maintaining a flow of mineral nutrients between available forms and unavailable sequestered forms. When plants extract mineral nutrients from the soil these nutrients become part of the plant biomass. When the plant senesces, these minerals are eventually released to the soil through microbial action. There are various loss pathways that minerals can enter which remove them from plant availability (Follett and Wilkinson, 1995). These differ depending on the mineral nutrient in question and environmental conditions.
Perennial grasses such as tall fescue are better able to capture available nutrients than are annual crop species for a number of reasons. The fibrous root system and long active growing season of tall fescue allow for extended access to available soil nutrients, resulting in better nutrient uptake efficiency than for many annual crops. This is especially true for N, which can be lost from the rooting zone in runoff, leached as nitrate, volatilized as ammonia, or lost as N2 or N2O gas through denitrification from waterlogged soils (Follett and Wilkinson, 1995). Nitrogen recovery from fertilizer applications has been studied extensively in tall fescue and other forage grasses, inasmuch as N is the primary fertilizer mineral limiting production.
Evaluations comparing economically optimum N rates for production of cool-season grasses in the U.S. Northeast determined that tall fescue required 32 kg N Mg-1 forage produced (Hall et al., 2003). More importantly, when N was applied at economically optimum N rates, NO3--N in the soil did not increase, indicating that the forage species were making effective use of applied N. Nitrogen not recovered in harvested forage could have leached below the sampling depth, but a large amount of N could also be sequestered in the extensive fibrous root system of tall fescue. Regardless, the stability of soil NO3- levels indicated the ecological service provided by the forage species tested.
In studies conducted in the Appalachian region of the United States, tall fescue and switchgrass both recovered around 30% of the N applied under a variety of soil conditions (Staley et al., 1991). Only on shallow soils at moderate fertilizer N rates did switchgrass recover 63% more of the applied N. Various factors could contribute to this difference, but the most likely explanation is that the greater water use efficiency of warm-season grasses than cool-season grasses is manifested more in shallow soils. Any factor that limits growth of grasses will also limit N uptake. This is true for shallow soils with low water-holding capacity and also situations with low solar irradiance, such as agroforestry alley cropping (Burner and MacKown, 2005).
Human and animal wastes also have been used as pasture fertilizer sources. One of the issues related to land application of wastes is the availability of N from these sources. The ability of forage grasses to make use of this N source is important for protection of the environment and optimization of N cycling. Cogger et al. (2001) reported that tall fescue could recover 62% of the inorganic N provided through municipal sewage sludge applications over a 5-yr period. In addition, the grass was able to maintain soil NO3--N levels below 25 kg ha-1. Production systems other than pasture also may enhance nutrient cycling from grass systems. Perennial grasses grown for biomass energy potentially can be processed in a manner that will allow almost complete recovery of N contained in the harvested biomass (Anex et al., 2007). Coupling this type of system with the utilization of wastes as a N source could certainly enhance ecosystem nutrient cycling.
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