Soil Carbon and Microfauna Changes as Influenced by Combinations of Bio-covers and Cropping Sequences of Glyphosate Tolerant Corn, Soybean, and Cotton

The objective of this research was to compare the effects of combinations of a wide range of cropping sequences and soil bio-covers on soil organic carbon (SOC) and Heterodera glycines under no-tillage. The experiment used a split-block design with four replications at the Milan (RECM) and Middle TN (MTREC) Research & Education Centers. The whole-block treatment was cropping sequences of corn (Zea mays), soybean (Glycine max), and cotton (Gossypium hirsutum). The split-block was bio-covers of winter wheat (Triticum aestivum), hairy vetch (Vicia villosa), poultry litter, and fallow. Soil samples were taken over four years of experimentation. A novel technique was developed to measure SOC called near infrared reflectance spectroscopy (NIR). Partial least squares regression was used to predict SOC as measured by NIR. Results were compared to assess the reliability of NIR determination of organic carbon. NIR analysis correlated well (r greater than 0.9) with SOC as determined by combustion. NIR was therefore considered sufficiently accurate for quantifying soil organic carbon.

Overall, both sites showed a small but consistent loss in carbon over all treatments. The average losses of SOC at the 0-5 cm depth were 1.06 and 1.40 Mg ha^-1 at RECM and MTREC, respectively. In the subsurface, mean SOC loss at RECM was 1.43 Mg ha^-1. Conversely, the subsurface SOC at MTREC was 3.14 Mg ha^-1 higher than preexperiment levels. Crop sequence had a significant effect on change in SOC (P ≤ 0.05) in both surface and subsurface regions at RECM. Sequences with two or more years of cotton tended to lose significantly more (P ≤ 0.05) SOC than those with two or more years of soybean. The poultry litter bio-cover lost less surface SOC (0.58 Mg ha^-1) than those under vetch (1.33 Mg ha^-1) or fallow (1.8 Mg ha^-1).

Soybean cyst nematode (SCN) eggs were found in sequences containing no soybean throughout the experiment, indicating survival of over seven years without soybean. Sequences with two or more years of soybean had significantly higher (P ≤ 0.01) SCN egg density than others, with continuous soybean having the greatest egg population density. One year of rotation with a non-host crop typically reduced SCN egg population density by more than 50%.