Impact of winter cover crops on soil properties under soybean cropping systems

Agricultural systems in Uruguay exhibit a high frequency of soybean in crop rotations. This could adversely affect soil organic carbon (SOC) storage in the long term. The introduction of winter annual cover crops (CC) in these systems may improve SOC balance as well as reduce the impact of rainfall on soil erosion rates. The objective of this study was to evaluate the impact of the inclusion of CC on SOC storage and soil physical properties under continuous soybean (Glycine max [L.] Merr) cropping systems. A field experiment was established in 2003 in a vertic Argiudoll. A split plot design was used with CC and non CC as whole plot arranged in four randomized complete blocks. Cover crops were: oats (Avena sativa L)., annual ryegrass (Lolium multiflorum L.). Two termination dates comprised split plots: 60 or 30 days before soybean planting. Mean annual forage production over eight years was higher (P ≤ 0.05) in oats compared to annual ryegrass for both termination dates. The inclusion of CC increased SOC concentration at the 0-7.5 cm depth, and at the 7.5-15 cm depth in the annual ryegrass treatment. Oats and annual ryegrass had 60 and 80 % more particulate organic carbon (POC) concentrations at the 0-7.5 cm, respectively than no cover crop treatment. No significant differences among treatments were observed for mineral associated organic C-MAOM at any depth. Nitrogen mineralization potential under ryegrass had a three-fold increase compared to the no cover crop treatment. Both CC treatments showed a higher (P ≤ 0.05) percentage of macropores compared to the no cover crop treatment. In most of the years soybean grain yield was not affected by cover crops treatments. However, late termination dates often reduced soybean yields. The inclusion of CC in high frequency soybean cropping systems may enhance soil C storage, and improve related soil physical properties.