Natural grassfields developed on sandy soils from Northern Uruguay are formed by communities of forage species, composed mainly of seasonal growing grasses, with low biomass production during the fall/winter period. The objective of this study was to evaluate the effect of herbicides, applied on native grassfield to established winter forage species, on soil properties and root biomass. This experiment began in 1994 and was established as a complete block design, with three replications. In the main plots, to establish winter-growing the forage-species black oat (Avena strigosa L.), triticale (X Triticosecale Wittmack), and ryegrass (Lolium multiflorum L.) on native grassfield, herbicides were applied (paraquat 0,60gha-1a.i., glyphosate 0,36gha-1a.i., glyphosate 1,44gha-1a.i.) and a test without herbicide was used for comparison. The main plots were divided in 1995 forming a split-plot design, where each plot received the same treatments on a half plot. The split-plots were divided in split-split-plot design in 1996, where each split-plot received the same treatments on a half split-plot. Soil samples to measure root biomass, bulk density, soil organic carbon, exchangeable bases and aluminum, and soil pH were taken in three separate samplings using a 7.65cm diameter by 40cm long metallic cylinder. The extracted soil monoliths were stratified down to 30cm to the following depths: 0-5, 5-10, 10-15, 15-20 and 20-30cm. Root biomass was higher where no herbicide was applied, as compared to herbicide treatments only at 0-5cm depth as well as for paraquat application, as compared to glyphosate. The continuous herbicide application in 1995 and 1996 produced progressive root biomass reduction. There was a high positive correlation between root biomass and soil organic carbon (SOC) and the latter reduced 13% due to the higher rate of chemical control (1.44gha-1a.i. of glyphosate applied in 1994, 95 and 96). There was no effect of tested treatments on exchangeable bases, but Al3+ was affected. SOC changes were closely related to changes in soil structure, increasing bulk density and decreasing aggregate stability. The system with high rate of herbicide use, developed to obtain high winter forage biomass production and suppression of native grasses, induced great changes on plant community presenting, as a consequence, less root biomass and SOC with negative effect on soil acidity and aggregation.
roots; soil structure; soil organic carbon; pasture management; exchangeable aluminum