The impact of herbivores on nitrogen mineralization rate: consequences forsalt-marsh succession

Soil net N-mineralization rate was measured along a successional gradient i n salt-marsh sites that were grazed by vertebrate herbivores, and in 5-year -old exclosures from which the animals were excluded. Mineralization rate w as significantly higher at ungrazed than at grazed sites. In the absence of grazing, mineralization rate increased over the course of succession, wher eas it remained relatively low when sites were grazed. The largest differen ces in mineralization rate between grazed and ungrazed sites were found at late successional stages where grazing pressure was lowest. The amount of p lant litter was significantly lower at grazed sites. In addition, the amoun t of litter and potential litter (non-woody, live shoots) was linearly rela ted to net N-mineralization rate. This implies that herbivores reduced mine ralization rate by preventing litter accumulation. Bulk density was higher at grazed salt-marsh sites than at ungrazed sites. This factor may also hav e contributed to the differences in net N-mineralization rate between graze d and ungrazed sites.