A Mechanistic Approach to Weed Management in a Cover Crop Mulched System

Two, 4-site year studies were conducted in North Carolina to evaluate the effects of maize and soybean planting timings after roll-crimping a cover crop mulch, on soil moisture, crop stand, weed pressure and corn and soybean yield. Two, 2-site year studies were conducted in NC and Baltimore, MD to determine the nutritive weed suppressive mechanisms along with decomposition dynamics of cover crop mulches. Two cover crop mixtures were compared; winter pea (P), and AU Early Cover hairy vetch a (V) were both mixed with Wrens Abruzzi rye. Delayed corn planting after roll-kill did not enhance the soil water content; however, at Kinston (2012), the soil water content was 23% greater in the hairy vetch (V) when compared to winter pea (P) and no-till without cover crop mulch treatments. The corn planting date across all 4 site-years did not affect weed biomass; however, weed coverage at Salisbury (2012) was affected by planting date. Cover crop treatments yielded equivalent to their weed-free no-till without cover crop mulch counter parts in 2011; however, despite high biomass production from pea and vetch, in-row weeds did escape control in the CC-H (weedy) treatments, at 3 of the 4 site-years when compared to the CC+H (weed-free) treatments. During the soybean experiment, planting timing included roll-kill/planting and roll-kill/delayed planting where soybean planting occurred either on the same day or approximately two-weeks later, respectively. Soybean row spacing included 76, 38, and 17 cm and all treatments included a weedy check and weed-free treatment. The high rye biomass levels (10,000 kg ha-1) resulted in excellent weed control. Weed coverage was highest in the 76 cm row-space treatment when compared to both the 17 cm and 38 cm row spacing in two of the four site-years. Row spacing also influenced soybean yield; the 38 cm spacing out-yielded the 17 cm and the 76 cm spacing by 3.2% and 12.7%, respectively. Soil volumetric water content was higher in the mulch treatments compared to the no rye checks. Delaying soybean planting lowered soil water evaporation. The increased soil VWC in the rolled-rye treatment did not translate into increased soybean yield. The rolled-rye treatment exhibited significant (p < 0:01) increases in soil volumetric water content (VWC) when compared to the no-rye treatment at Goldsboro, Kinston, and Salisbury (2012) with 27.4%, 13.4%, and 29.7%, respectively. Fungal translocation of N from the soil to the surface mulch has been demonstrated in laboratory experiments, but this mechanism has not been documented under field conditions. Translocation of N into surface mulches may play an important role in weed suppression. To assess the presence of fungal mediated N immobilization, the isotope 15N as (NH 4)2SO4 (99.7 at.) was injected below the soil surface in situ at a rate of 1 mg 15N kg -1 soil into (4.4 cm depth) buried steel frames (0.58 m2). N movement was monitored throughout the season. Treatments received, or did not, receive fungicide every 2-weeks after soybean planting. The cereal rye mulches reduced in N content at both BARC and KINS, at 15 and 50 kg N ha-1. An increase of 15N atom% access of 10--15% in the cereal rye surface mulch tissue was detected. Only one sampling date showed significant impact of the fungicide treatment. The increase in 15N atom% excess in the cereal rye mulch supports laboratory observations that soil inorganic N is fungal translocates into surface mulches. At six weeks after soybean planting (averaged over locations), the non-fungicide treatment was 36% greater in 15N concentrations, providing evidence of microbial competition via fungal translocation as a mechanism. Soil extractable inorganic N was reduced by as much as 60% from the initial low of 10 kg N ha-1, where both the soybeans and cereal rye residues sequestered and immobilized considerable amounts of N. These findings support the nutritive weed suppressive mechanism of high biomass cereal rye cover crop mulches during the critical weed free period of soybeans, thereby contributing to the weed suppression of the system.