The environmental impact of agriculture on the landscape is driven by management decisions that include frequency of disturbance (tillage), external inputs (fertilizer, pesticides, herbicides), and cropping system intensity and diversity. Reducing negative impacts of one management choice often results in increasing negative impacts of another. For example, no-tillage soybean production reduces system erosion potential but relies on increased herbicide application to suppress weeds. Organic systems reduce inputs of herbicides and pesticides but rely on increased frequency of tillage to control weeds. Recent results suggest organic no-tillage systems are feasible when a cover crop is employed as a weed suppressing mulch and the following grain crop is no-till planted directly through the mulch. There is a need for more in-depth understanding of how cover crop surface mulch management impacts soil surface physical conditions, biogeochemistry and soybean interactions with above and below ground organisms.
Rye mulch levels were manipulated in the field by first cutting the Rye stand and either removing or supplementing pre-designated micro-plots to achieve a gradient of mulch levels which corresponded to 0, 2700, 5500, 8250 & 11000 kg/ha. Soybeans were no-till drilled at a seeding rate of 150,000 seed/ha. At 10 weeks soybeans (R5 growth stage) and soils were sampled from each micro-plot and analyzed for carbon, nitrogen & elemental composition using near infrared reflectance spectroscopy (NIR). Natural abundance of 15N was measured in soybean tissue and compared to a non-leguminous reference plant (pigweed) to calculate the proportion of plant nitrogen derived from biological nitrogen fixation. Weed/crop competition was measured weekly. Ambient light levels were measured above and below the mulch layer weekly and soil moisture and temperature was measured hourly for the entirety of the experiment.
Results/Conclusions
Mulch levels impacted both above and belowground system responses such as weed competition, soil and plant biochemistry and soybean interaction with symbiotic nitrogen fixing organisms. We observed a positive relationship between mulch levels, soil moisture & soil and plant potassium. Inverse relationships were observed between increasing mulch levels and light infiltration, soil Fe, soil and plant Mg & Na. We observed a negative effect of mulch levels on the proportion of plant nitrogen derived from biologically fixed sources. Based on weed/plant competition data we suggest that the observed differences in biological nitrogen fixation is a direct effect of mulch level on soil surface physical and biogeochemical conditions rather than an indirect effect of the mulches suppressive effects on weed resource competition.
