Crop Residue Removal Impacts on Soil Productivity and Environmental Quality

Crop residues are a potential source of renewable feedstocks for cellulosic ethanol production because of their high cellulose content and easy availability. Indiscriminate removal as biofuel may, however, have adverse impacts on soil, environment, and crop production. This article reviews available information on the impacts of crop residue removal on soil properties, crop yields, and soil erosion across a wide range of soils and ecosystems. It explicitly synthesizes data on the independent impacts of crop residue removal on soil and environment rather than on the interrelated tillage-crop-residue management impacts. Published literature shows that residue removal adversely impacts near-surface soil physical, chemical, and biological properties. Unmulched soils are prone to particle detachment, surface sealing, crusting, and compaction. Residue removal reduces input of organic binding agents essential to formation and stability of aggregates. It also closes open-ended biochannels by raindrop impacts and reduces water infiltration, saturated/unsaturated hydraulic conductivity, and air permeability, and thereby increases runoff/soil erosion and transport of non-point source pollutants (e.g., sediment and chemicals). Residue removal accelerates evaporation, increases diurnal fluctuations in soil temperature, and reduces input of organic matter needed to improve the soils' ability to retain water. It reduces macro- (e.g., K, P, N, Ca, and Mg) and micronutrient (e.g., Fe, Mn, B, Zn, and S) pools in the soil by removing nutrient-rich residue materials and by inducing losses of soil organic matter (SOM)-enriched sediments in runoff. Residue removal drastically reduces earthworm population and microbial carbon (C) and nitrogen (N) biomass. It adversely affects agronomic production by altering the dynamics of soil water and temperature regimes. The short-term (<10 yr) data show nevertheless that residue removal may not always degrade soil physical properties and decrease crop yields in the short term depending on the soil type, topography, and fluctuations in annual weather conditions. Sloping and erosion-prone soils are more rapidly and adversely affected by residue removal than those on flat terrains with heavy texture and poorly drained conditions. Sloping terrains are not only highly susceptible to water and wind erosion but also to tillage erosion. In these soils, therefore, a fraction of the total crop residue produced may be available for biofuel production and other expanded uses. Standard guidelines on when, where, and how much residues to remove need to be, however, established. Modeling rates of residue removal are presently based on the needs of soil cover to control erosion without consideration to maintaining SOM and nutrient pools, enhancing soil physical, chemical, and biological quality, and sustaining crop production. Threshold levels of residue removal must be assessed for principal soil types based on the needs to maintain or enhance soil productivity and improve environmental quality. For those soils in which some residues are removed, best management practices (e.g., cover crops, diverse crop rotations, and manure application) must be adopted to minimize adverse impacts of residue removal. Because indiscriminate harvesting of crop residues for biofuel may deteriorate soil properties, reduce crop yields, and degrade the environment, there exists an urgent research need for developing alternative sustainable renewable energy feedstocks (e.g., warm season grasses and short-rotation woody crops).