Phytotoxicity of decomposing below-ground residues of Ageratum conyzoides: nature and dynamics of release of phytotoxins

Abstract

We investigated the dynamics of phytotoxicity and the quantitative changes in phenolics in decomposing root residues of Ageratum conyzoides over a 60-day period. A set of four treatments, viz. residues alone (R), residues mixed into soils (R + S, R + 3S), and soil alone (S), were maintained and the quantitative changes in phytotoxicity were monitored on 1, 5, 10, 15, 20, 25, 30, 45, and 60 day after decomposition (DAD). The phytotoxicity during the decomposition process was evaluated in a laboratory bioassay against radish (Raphanus sativus). The phytotoxicity of R, R + S, and R + 3S treatments increased during initial period of decomposition (up to 20-DAD), and declined afterwards (i.e., at >20-DAD). In general, the phytotoxicity was in the order R > R + S > R + 3S treatments. It was paralleled by a similar trend of changes in the amounts of water-soluble phenolics that increased up to 20-DAD and thereafter declined. The amount of phenolics was in the order R > R + S > R + 3S. At 1-DAD, the amount of water soluble phenolics in R, R + S, and R + 3S treatments was 765.3, 594.5, and 251.3 μg/ml, respectively. It enhanced to 1,266.76, 845.5, and 416.0 μg/ml at 20-DAD. However, at 60-DAD, the amounts of phenolics in R, R + S, and R + 3S treatments was 149.93, 142.6, and 100.0 μg/ml, respectively. The study concludes that the phytotoxicity of below-ground residues of A. conyzoides changes during decomposition and was reduced upon the addition of soil to the residues.