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., Trifolium repens L., Senecio jacobaea L., and Carduus nutans L.) were established in glasshouse conditions and subjected to one of three treatments: spraying with 2,4‐D/picloram mix; spraying with glyphosate; or unsprayed. After the sprayed swards died, all above‐ground and below‐ground tissue was harvested, air‐dried, and placed in nylon mesh litter‐bags which were positioned in the field. Decomposition, microbial basal respiration, and substrate‐induced respiration (proportionally related to the glucose‐responsive microbial biomass) of this litter was then monitored over 338 days. Both herbicide treatments inhibited decomposition of T. repens and L. perenne shoot tissue and C. nutans root tissue, but stimulated that of C. nutans shoot tissue, indicating that herbicides may influence decomposition of different species in different ways; the possible reasons for this are discussed. However, the rapid decomposition of most of the tissues considered in this study suggest that herbicides are unlikely to exert substantial long‐term effects on plant litter persistence. Microbial basal respiration and substrate‐induced respiration of most of the litter types considered were initially very strongly enhanced by both herbicide treatments; however, this effect was highly transitory for all tissue types except one, and for some of the tissue types a strong inhibition of these microbial variables in the herbicide treatments followed. It therefore appears that microbial build‐up on litter from herbicide‐killed plants (and the subsequent decline) occurs earlier than that from unsprayed plants, probably because herbicide‐induced plant damage increases the availability of readily utilisable microbial substrates. The retardation of leaf litter decomposition in herbicide treatments was often associated with reduced microbial activity and biomass, indicating strong linkages between soil‐associated microflora and decomposition processes. This study also indicates that newly developed approaches for simultaneously assessing decomposition and the microbial biomass of leaf litter have considerable potential for investigating impacts of ecological factors on plant litter‐microbial interactions. Keywords: basal respiration2,4‐Dplant litterdecompositionglyphosateherbicidemicrobial biomass