Cutting management and alfalfa stand age effects on organically grown corn grain yield and soil N availability

Alfalfa is recommended as a rotational crop in corn production, due to its ability to contribute to soil nitrogen (N) and carbon (C) stocks through atmospheric N₂ fixation and above- and belowground biomass production. However, there is little information on how alfalfa management practices affect contributions to soil and subsequent corn crop yields, and research has not been targeted to organic systems. A study was conducted to determine the effects of alfalfa stand age, cutting frequency and biomass removal on soil C and N status and corn yields at three organically managed Minnesota locations. In one experiment, five cutting treatments were applied in nine environments: two, three and four cuts with biomass removal; three cuts with biomass remaining in place; and a no-cut control. In the other experiment, corn was planted following 1-, 2-, 3- or 4-year-old alfalfa stands and a no-alfalfa control. Yield was measured in the subsequent corn crop. In the cutting experiment, the two- and three-cut treatments with biomass removal reduced soil mineral N by 12.6 and 11.5%, respectively, compared with the control. Potentially mineralizable N (PMN) was not generally affected by cutting treatments. The three-cut no-removal increased potentially mineralizable C by 17% compared with the other treatments, but lowered soil total C in two environments, suggesting a priming effect in which addition of alfalfa biomass stimulated microbial mineralization of native soil C. Although both yields and soil mineral N tended to be higher in treatments where biomass remained in place, this advantage was small and inconsistent, indicating that farmers need not forgo hay harvest to obtain the rotational benefits of an alfalfa stand. The lack of overall correlation between corn grain yields and mineral and potentially mineralizable N suggests that alfalfa N contribution was not the driver of the yield increase in the no-removal treatments. Alfalfa stand age had inconsistent effects on fall-incorporated N and soil N and C parameters. Beyond the first year, increased alfalfa stand age did not increase soil mineral N or PMN. However, corn yield increased following older stands. Yields were 29, 77 and 90% higher following first-, second- and third-year alfalfa stands than the no-alfalfa control, respectively. This indicates that alfalfa may benefit succeeding corn through mechanisms other than N contribution, potentially including P solubilization and weed suppression. These effects have been less studied than N credits, but are of high value in organic cropping systems.