Potential allelopathic effect of Brachiaria decumbens root exudates on neotropical tree seedlings

The reforestation of degraded areas in the neotropics is hampered by the invasion of Brachiaria spp. The aggressiveness of these grasses may be related to the release of allelochemicals in the soil, and the sensitivity of tree seedlings of distinct successional classes to exudates of Brachiaria roots has not been addressed. Here, we evaluated the effects of exudates from Brachiaria decumbens roots on the growth, physiology and nitrogen metabolism of seedlings of six tree species native to Brazilian Atlantic Forest: Trema micrantha, Heliocarpus popayanensis and Cecropia pachystachya (pioneers), Cariniana estrellensis, Eugenia brasiliensis and Guarea kunthiana (non-pioneers). The seedlings were cultivated in fertile soil without (control) and with B. decumbens root exudates (AL treatment). Total leaf area and root and shoot dry weights of seedlings of pioneer species were drastically reduced by AL, whereas non-pioneers were less negatively affected. The gas exchange parameters and chlorophyll content of seedlings of both successional groups were altered by AL. AL decreased endogenous nitrate contents in the seedlings, in accordance with the inhibition of soil nitrification by this treatment. As consequence, AL reduced the endogenous protein contents of pioneer seedlings. Overall, these results demonstrate the negative allelopathic potential of exudates from B. decumbens roots on neotropical tree seedlings. Pioneer species, which are highly dependent on nitrate, were more negatively affected by the grass exudates, suggesting an indirect allelopathic effect mediated by the inhibition of soil nitrification, which may hamper the restoration of degraded areas. However, direct allelopathic mechanisms cannot be ruled out.