Cattle grazing drives nitrogen and carbon cycling in a temperate salt marsh

We examined the impact of long-term cattle grazing on soil processes and microbial activity in a temperate salt marsh. Soil conditions, microbial biomass and respiration, mineralization and denitrification rates were measured in upper salt marsh that had been ungrazed or cattle grazed for several decades. Increased microbial biomass and soil respiration were observed in grazed marsh, most likely stimulated by enhanced rates of root turnover and root exudation. We found a significant positive effect of grazing on potential N mineralization rates measured in the laboratory, but this difference did not translate to in situ net mineralization measured monthly from May to September. Rates of denitrification were lowest in the grazed marsh and appeared to be limited by nitrate availability, possibly due to more anoxic conditions and lower rates of nitrification. The major effect of grazing on N cycling therefore appeared to be in limiting losses of N through denitrification, which may lead to enhanced nutrient availability to saltmarsh plants, but a reduced ability of the marsh to act as a buffer for land-derived nutrients to adjacent coastal areas. Additionally, we investigated if grazing influences the rates of turnover of labile and refractory C in saltmarsh soils by adding 14C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of 14CO2. Grazing had little effect on the rates of mineralization of 14C used as a respiratory substrate, but a larger proportion of 14C was partitioned into microbial biomass and immobilized in long- and medium-term storage pools in the grazed treatment. Grazing slowed down the turnover of the microbial biomass, which resulted in longer turnover times for both leaf litter and root exudates. Grazing may therefore affect the longevity of C in the soil and alter C storage and utilization pathways in the microbial community.