Root exudation of glyphosate by Brachiaria decumbens and its effects on eucalypt plants and microbial soil respiration

摘要

This study aimed to evaluate root exudation of the herbicide glyphosate by Brachiaria decumbens and its effects on eucalypt cultivated in soil and in nutritive solution; and to quantify microbial respiration in soil under different managements. One Eucalyptus grandis and four Brachiaria decumbens seedlings were planted in pots with lids with five perforations holding 8.0 L of the nutritive solution. A randomized block design in six replications was used, each pot being considered an experimental plot. The eucalypt and brachiaria plants were interplanted in a hydroponic solution for 30 days. Fifteen days after the transplant, the brachiaria plants were pruned to stimulate tillering. After this period, glyphosate treatments of 0, 720, 1440, 2160, and 2880 g a.e. ha-1 were applied to the brachiaria plants. In the soil experiment, E. grandis seedlings were planted in 72 10-liter pots, half containing sandy soil and half clayey soil. The experiment was set up in a randomized block design with six replications, in a 2 x 6 factorial scheme (two soil types and six management combinations). Following the eucalypt seedlings, five Brachiaria decumbens seedlings per pot were planted in 48 pots (24 of each soil), and interplanted with a eucalypt seedling. The remaining eucalypt pots were cultivated in monoculture. The tested treatments were: 1- interplanted eucalypt and brachiaria (control); 2- Eucalypt without brachiaria + 1440 g a.e. ha-1 of glyphosate applied in the soil; 3- interplanted eucalypt and brachiaria cut after spray with 1440 g ha-1 glyphosate; 4, 5 and 6- interplanted eucalypt and sprayed brachiaria with 720, 1440, and 2880 g a.e ha-1 glyphosate, respectively. In treatments 4, 5, and 6 the eucalypt plants were protected from contact with the herbicide applied to the brachiaria plants. In treatment 2, glyphosate was applied directly to soil. The eucalypt plants in treatment 3 were treated with the brachiaria plant cut shoots, seven days after the latter had been sprayed with 1440 g ha-1 glyphosate. All the tested rates controlled over 95% of the grass species in both assays, and no toxicity symptoms were verified in the eucalypt plants. The microbial activity was greater in the sandy soil, mainly with the increase of the glyphosate rates applied to the brachiaria plants.