INFLUENCE OF HERBICIDE APPLICATIONS ON THE DECOMPOSITION, MICROBIAL BIOMASS, AND MICROBIAL ACTIVITY OF PASTURE SHOOT AND ROOT LITTER

Pure swards of each of four pasture species (Lolium perenne L., Trifol ium repens L., Senecio jacobaea L., and Carduus nutans L.) were establ ished in glasshouse conditions and subjected to one of three treatment s: spraying with 2,4-D/picloram mix; spraying with glyphosate; or unsp rayed. After the sprayed swards died, all above-ground and below-groun d tissue was harvested, air-dried, and placed in nylon mesh litter-bag s which were positioned in the field. Decomposition, microbial basal r espiration, and substrate-induced respiration (proportionally related to the glucose-responsive microbial biomass) of this litter was then m onitored over 338 days. Both herbicide treatments inhibited decomposit ion of T. repens and L. perenne shoot tissue and C. nutans root tissue , but stimulated that of C. nutans shoot tissue, indicating that herbi cides may influence decomposition of different species in different wa ys; the possible reasons for this are discussed. However, the rapid de composition of most of the tissues considered in this study suggest th at herbicides are unlikely to exert substantial long-term effects on p lant litter persistence. Microbial basal respiration and substrate-ind uced respiration of most of the litter types considered were initially very strongly enhanced by both herbicide treatments; however, this ef fect was highly transitory for all tissue types except one, and for so me of the tissue types a strong inhibition of these microbial variable s in the herbicide treatments followed. It therefore appears that micr obial build-up on litter from herbicide-killed plants (and the subsequ ent decline) occurs earlier than that from unsprayed plants, probably because herbicide-induced plant damage increases the availability of r eadily utilisable microbial substrates. The retardation of leaf litter decomposition in herbicide treatments was often associated with reduc ed microbial activity and biomass, indicating strong linkages between soil-associated microflora and decomposition processes. This study als o indicates that newly developed approaches for simultaneously assessi ng decomposition and the microbial biomass of leaf litter have conside rable potential for investigating impacts of ecological factors on pla nt litter-microbial interactions.