Residue, tillage, and nitrogen effects on soils

Harvesting corn (Zea mays L.) residue for use as a biofuel feedstock may alter important chemical and physical properties of soils. Coupled with the common management practices of nitrogen fertilizer application and tillage, this practice could lower soil organic carbon (SOC), as well as increase erodible sediments and soil compaction. A study was initiated in the 2006 growing season that included combinations of three levels of residue removal (none, partial, or full removal), two tillage treatments (no-till or chisel plow), and four nitrogen application rates (67 to 268 kg ha-1). These experiments were established in continuous corn (CC) at four Illinois sites, with treatments remaining in the same plots each year. After 7 growing seasons, plots with no residue removal at Monmouth had 37.3 g SOC kg-1 in the top 10 cm, while plots with full residue removal had 34.7 g SOC kg-1. At Perry and DeKalb, no-till plots with no residue removal had higher levels of SOC than no-till plots with residue removed. Perry no-till plots had 23.3 g SOC kg-1 with no residue removed and 19.8 g SOC kg-1 with all residue removed; DeKalb no-till plots had 45.7 g SOC kg-1 with no residue removed and 38.1 g SOC kg-1 with all residue removed. In the first 10 cm, no-till soils at Perry and Monmouth had significantly more SOC than tilled soils (22.6 vs. 20.5 g SOC kg-1 at Perry; 36.7 vs. 35.1 g SOC kg-1 at Monmouth). Tillage also had a significant effect on bulk density (ρb) at all four sites, with tilled soils having a lower mean ρb at 0-10 cm than no-till soils by 0.09 – 0.13 g cm-3. Penetration resistance (PR) was similarly affected, with tilled soils having a lower PR than no-till soils by 301 – 610 kPa. Soil pH decreased in the top 10 cm with higher rates of N application at all four sites. Perry soils showed the largest decrease in pH; soils with 67 kg N ha-1 applied had a pH of 5.49, while soils with 268 kg N ha-1 applied had a pH of 4.72. Water aggregate stability (WAS) was only significantly impacted at Urbana, where no-till plots had 84% WAS while tilled plots had 81% WAS. Compared to soils with 134 kg N ha-1 applied, soils with 268 kg N ha-1 applied the previous year had higher ammonium (NH4+) levels at two sites and higher nitrate (NO3-) levels at three sites. The practices of residue removal and tillage may decrease SOC; soils under residue removal and tillage had lower SOC than residue cover and no-till at several sites. However, from this study’s results, the magnitude of the differences in SOC stocks is relatively small and most likely has not materially impacted the productivity of the soil. Future study will clarify whether the relative differences in SOC between residue and tillage management become more significant over time