A review of soil erosion potential associated with biomass crops
Introduction
An assessment of the probable role of biomass energy in the future global energy situation by the Intergovernmental Panel on Climate Change (IPCC) suggests that if steps are not taken to slow the release of greenhouse gasses into the atmosphere, global air temperatures can be expected to rise 0.3°C per decade and sea level to rise by 6 cm per decade.[1] The release of CO2 from utilization of biomass energy would not contribute to net global CO2 levels since the gasses released during its utilization were only recently removed from the atmosphere. With biomass values for energy production of $30(U.S.) Mg−1 or less, it seems likely that production of biomass crops would be competitive only on marginal cropland, such as that now used for pasture and hay production, or idled under government programs. Much of this land would be considered to be highly erodible. In assessing the potential of biomass production, it is therefore essential to consider its impact on soil erosion.
The U.S. has ca 200 million ha of forest land, 140 million ha of cropland, 47 million ha of marginal cropland/pasture and 32 million ha in federal programs.[2] Robertson and Shapouri[3] estimated that as much as 60 million ha could be converted to biomass production in the U.S, most of it marginal land.
Before environmentally sensitive areas are converted to biomass energy production, the sustainability of such systems must be assessed. The designation of land as marginal frequently results from high erodibility or past cropping history involving significant erosion losses. In a national workshop on biomass and global warming, the lack of information on the sustainability of biomass production on marginal land was cited as a major constraint to the assessment of the role of biomass.[4]
Soil erosion has contributed to the loss of 430 million ha of cropland worldwide, 30% of the world cropland base.[5] Soil loss negatively affects characteristics associated with crop productivity including water holding capacity, soil nutrients, soil density, soil organic matter and others.[6] In addition, sediments removed during water erosion collect in rivers, lakes and reservoirs resulting in future economic losses. The loss of soil water holding capacity is a primary effect contributing to productivity loss.[7] Slightly eroded soil held 14% more water in the top 1 m than severely eroded soil and, when plant-extractable water fell to 55–60% of total water holding capacity under moisture stress conditions, corn on slightly eroded soil had significantly higher evapotranspiration levels.[7]
Soil formation is a very slow process. It is estimated that 100 yr are required for the formation of 2.5 cm of topsoil. In the U.S., the rate of soil loss may exceed the suggested maximum sustainable rate of ca 11 Mg ha−1 yr−1 on 39 million ha of cropland.[8] Estimates for Illinois indicate that soil erosion loss is excessive on 42% of the state's 10 million ha cropland base.[9]
Erosion reduces the long-term productivity of soils. The actual effect varies considerably due to differences in topsoil depth, subsoil composition and depth, the crop being produced and other variables.10, 11 Water erosion rates increase with increasing slope.[12] The long-term productivity loss due to erosion of a Minnesota soil was calculated to be 5%, but was greater on soils with >6% slope.[13] In other cases the impact of soil erosion on productivity may be small. On Piedmont soils (Typic Hapludults) of North Carolina, Daniels et al.[11] measured yields of corn, sorghum [Sorghum bicolor (L.) Moench] wheat (Triticum aestivum L. em Thell), barley (Hordeum vulgare L.) and soybean (Glycine max (L.) Merr.) on slightly and severely eroded soils for 34 field years of data and found that average yield reductions averaged only 0.1 Mg ha−1 across all crops. Annual crop value loss to erosion averaged $4.44(U.S.) ha−1.
Susceptibility to erosion is among the variables considered in classification of cropland in the U.S. Only ca 3% of U.S. land is suitable for continuous cropping with minimal erosion concern. About 40% of the remaining land has row crop potential but with limitations to continuous cropping, most often erosion hazard. Conservation Compliance components of current U.S. farm policy will require reduced tillage practices on 22% of Class IIe land and all of Class IIIe and IVe land in Kentucky.[14] Of the 10 million ha total land area in Kentucky, ca 5.5 million ha are potential cropland (Classes I–VI).[15] However, >90% of this cropland is below Class I, with limitations to crop suitability or cropping patterns. Erosion is the primary concern on ca 75% of land in classes II–VI. Wetness and drought are the primary limitations on the remaining area.
The susceptibility of a particular site to water erosion is estimated using the universal soil loss equation (USLE).[16] Using the equation, soil loss is estimated as:where R is the Rainfall erosion index factor, K is the soil erodibility factor, L is the slope length factor, S is the slope steepness factor, C is the cover and management factor and P is the cropping practice factor.
Within the U.S., values for R for estimating raindrop and runoff impacts range from 100–200 in the North Central region to values between 200 and 400 or more in the Southeast. Soil cover by canopy or mulch determines the C factor and greatly impacts estimates of soil loss. With no mulch, values for C decline from 1 with no canopy cover to only 0.15 with 100% canopy cover.[16] Plant cover intercepts falling raindrops and reduces the force with which they impact the surface.[17]
Section snippets
Herbaceous biomass
Herbaceous biomass crop production could be a land use option on a large part of the 214 million ha of land in the U.S. classified as cropland or pasture land, depending upon production costs, crop value, and other factors.[18]
Keeney and DeLuca[19] assessed options for biomass energy production in the midwestern U.S. and concluded that ethanol from corn (Zea mays L.) grain would not be a primary source of energy in the U.S. due to the high inputs of energy required in its production. Cellulosic
Riparian or other stabilization plantings
Trees have been planted for the purpose of reducing water erosion by creating a living buffer area adjacent to water courses to catch sediment from eroding land to reduce runoff by increasing water infiltration, and to physically stabilize stream banks with their roots. Van Kraayenoord[43] outlines New Zealand recommendations for anti-erosion plantings of poplars (Populus spp. ). Poplar plantations are recommended on slopes subject to slippage, especially the sides of actively expanding
Summary
In assessing the use of land resources for biomass energy cropping, potential effects of such systems on soil erosion and sustainability of soil productivity must be addressed. With proper management herbaceous perennial species, especially grasses, provide year-round protection and minimal soil erosion. Growth of these species frequently improves soil productivity by increasing soil organic matter, improving soil structure, and by increasing soil water and nutrient-holding capacity. Tillage
Acknowledgements
Contribution of the Department of Agronomy, University of Kentucky, Lexington, KY 40546-0091 (Kentucky Agricultural Experimental Station Article No. 95-06-173) and the Prairie Farm Rehabilitation Administration Shelterbelt Centre, Agriculture and Agri-Food Canada, Indian Head, Saskatchewan, Canada S0G 2K0.
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