Annual Weeds Suppression and Oat Forage Yield Responses to Crop Density Management in an Oat-Cultivated Grassland: A Case Study in Eastern China

Abstract

Although weeds can be inhibited by high planting densities, canopy shading, elemental balance and soil microbial recruitment are not yet adequately considered when measuring competitive effects on weed control. The effects of oat (Avena sativa) planting density (60 to 600 plants m−2) on the biomass and shoot element balance of oat and weeds were evaluated in a field experiment. The shift in the microbial community of the dominant weed species was examined in a pot experiment by growing the weed alone and in competition with 360 oat plants m−2 (recommended planting density) under greenhouse conditions. Increasing oat planting density beyond 360 plants m−2 did not improve oat forage yield or weed suppression. Compared to 60 plants m−2, the biomass of broadleaf and grass weeds decreased by 1122% and 111%, respectively, at a density of 360 plants m−2, while oat forage biomass increased by 60% and leaf area index by 24%. The improved canopy properties suppressed competing weeds through increased shading. Typically, the C:N and C:P ratios of shoots of Echinochloa crus-galli and Digitaria sanguinalis were higher than those of Portulaca oleracea and Chenopodium album. At high planting densities, E. crus-galli and D. sanguinalis exhibited high P contents and low N:P ratios, suggesting a limited supply of N nutrients for growth. Soil bacterial community assay showed that the composition of microbial communities of the two grass weeds were shaped by the presence of oat competition, which also considerably depleted several important functional microbes associated with nutrient cycling in the weeds’ rhizosphere. These results highlight that increased crop density significantly improves the crop competitive advantage over weeds through increased shading, reduced elemental balance, and beneficial microorganisms of weeds, thereby reducing the need for herbicides or physical weed control in oat cropping system.