Soil quality

Harvesting of corn stover (plant residues) for cellulosic ethanol production must be balanced with the requirement for returning plant residues to agricultural fields to maintain soil structure, fertility, crop protection, and other ecosystem services. High rates of corn stover removal can be associated with decreased soil organic matter (SOM) quantity and quality and increased highly erodible soil aggregate fractions. Limited data are available on the impact of stover harvesting on soil microbial communities which are critical because of their fundamental relationships with C and N cycles, soil fertility, crop protection, and stresses that might be imposed by climate change. Using fatty acid and DNA analyses, we evaluated relative changes in soil fungal and bacterial densities and fungal-to-bacterial (F:B) ratios in response to corn stover removal under no-till, rain-fed management. These studies were performed at four different US locations with contrasting soil-climatic conditions. At one location, residue removal significantly decreased F:B ratios. At this location, cover cropping significantly increased F:B ratios at the highest level of residue removal and thus may be an important practice to minimize changes in soil microbial communities where corn stover is harvested. We also found that in these no-till systems, the 0- to 5-cm depth interval is most likely to experience changes, and detectable effects of stover removal on soil microbial community structure will depend on the duration of stover removal, sampling time, soil type, and annual weather patterns. No-till practices may have limited the rate of change in soil properties associated with stover removal compared to more extensive changes reported at a limited number of tilled sites. Documenting changes in soil microbial communities with stover removal under differing soil-climatic and management conditions will guide threshold levels of stover removal and identify practices (e.g., no-till, cover cropping) that may mitigate undesirable changes in soil properties.

Biochar, a co-product of thermochemical conversion of lignocellulosic materials into advanced biofuels, may be used as a soil amendment to enhance the sustainability of biomass harvesting. We investigated the impact of biochar amendments (0, 5, 10, and 20 g-biochar kg− 1 soil) on the quality of a Clarion soil (Mesic Typic Hapludolls), collected (0–15 cm) in Boone County, Iowa. Repacked soil columns were incubated for 500 days at 25 °C and 80% relative humidity. On week 12, 5 g of dried and ground swine manure was incorporated into the upper 3 cm of soil for half of the columns. Once each week, all columns were leached with 200 mL of 0.001 M CaCl2. Soil bulk density increased with time for all columns and was significantly lower for biochar amended soils relative to the un-amended soils. The biochar amended soils retained more water at gravity drained equilibrium (up to 15%), had greater water retention at − 1 and −5 bars soil water matric potential, (13 and 10% greater, respectively), larger specific surface areas (up to 18%), higher cation exchange capacities (up to 20%), and pH values (up to 1 pH unit) relative to the un-amended controls. No effect of biochar on saturated hydraulic conductivity was detected. The biochar amendments significantly increased total N (up to 7%), organic C (up to 69%), and Mehlich III extractable P, K, Mg and Ca but had no effect on Mehlich III extractable S, Cu, and Zn. The results indicate that biochar amendments have the potential to substantially improve the quality and fertility status of Midwestern agricultural soils.

Harvesting crop residues for bioenergy or bio-product production may decrease soil organic matter (SOM) content, resulting in the degradation of soil physical properties and ultimately soil productivity. Using the least limiting water range (LLWR) to evaluate improvement or degradation of soil physical properties in response to SOM changes has generally been hampered by the extensive amount of data needed to parameterize limiting factor models for crop production. Our objective was to evaluate five pedotransfer functions to determine their effectiveness in predicting soil water holding capacity in response to different SOM levels. Similarly, two other pedotransfer functions were evaluated to determine the effects of SOM on cone index values. Predictions of field capacity and wilting point water content as well as the cone index–water content–bulk density relationship of soil strength using the pedotransfer functions were compared with field data from two tillage experiments near Akron, CO that had a range of SOM concentrations. Equations previously developed by da Silva and Kay gave the best estimates of LLWR for the pedotransfer functions we evaluated. These equations were then used to illustrate LLWR changes in response to different soil and crop management practices on a Duroc loam near Sidney, NE. The results showed that tillage and, possibly, soil erosion decreased the LLWR as tillage intensity increased. Therefore, we recommend that crop residue removal rates be limited to rates that maintain or increase SOM content to ensure soil physical conditions are not degraded.