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Abstract

Grazing or mowing is central to the management of grasslands and may alter mineralization of soil organic matter (SOM) and soil carbon (C) stocks. Some studies have shown grazing-induced reductions in total soil respiration suggesting decreases in SOM mineralization. However, it has also been suggested that grazing may increase SOM mineralization, based on observations of increased soluble C, microbial biomass and mineral nitrogen (N) in soil after clipping. No studies to date have directly measured SOM mineralization to determine the effects of grazing on SOM mineralization and the underlying mechanisms. We examined the effect of clipping on soil-derived CO2 efflux (Rs) for six gramineae and one leguminous species typical of temperate grasslands. Continuous 13C labeling of monocultures coupled with a new method of directly measuring Rs and the rhizosphere priming effect (RPE i.e. Rs from planted soils minus respiration from bare soils) in perennial herbaceous plants was used. For a model species, Lolium perenne, the clipping effects on aboveground biomass production, mineral N and soluble C in soil, microbial biomass and microbial community composition were also quantified. We found that clipping decreased the RPE and Rs (SOM mineralization) within 48 hours for all the studied species. For Lolium perenne, this reduced SOM mineralization persisted for one month after clipping. Moreover, clipping reduced the production of aboveground biomass and the total N assimilated by the plants. These changes increased N availability in soil and induced shifts in the soil microbial community structure favoring gram positive bacteria (i16:0) over saprophytic fungi (18:2u6). The strong correlation of fungi (18:2u6) with Rs across treatments suggests that saprophytic fungi play a key role in SOM mineralization. In conclusion, our study shows that plant clipping decelerates SOM mineralization and induces shifts in microbial community structure, most likely as an indirect effect of clipping on plant N uptake.