The impact of sheep grazing on net nitrogen mineralization rate in two temperate salt marshes

Nitrogen mineralization rate was studied in grazing trials with three diffe rent stocking rates (0, 3, 10 sheep ha(-1)) in two man-made salt marshes, v iz. a Puccinellia maritima-dominated low salt marsh and a high salt marsh d ominated by Festuca rubra. Mineralization rates were derived from the amoun ts of mineral N which accumulated in situ during six-week incubation period s in tubes containing undisturbed soil cores from the upper 10 cm soil laye r. The annual rates of net N mineralization were significantly higher in th e better drained, high salt marsh (71 - 81 kg ha(-1) yr(-1)) than in the lo w salt marsh (39-49 kg ha(-1) yr(-1)). High amounts of belowground litter a ccumulated in the low salt marsh due to frequent water logging. Both IN min eralization and nitrification rate were negatively correlated with soil wat er content. In the Puccinellia maritima salt marsh, grazing had neither an effect on N mineralization rates during any of the incubation periods nor o n annual mineralization rates. In the Festuca rubra salt marsh, N mineraliz ation rates increased earlier during spring at the intensively grazed site than at the moderately grazed and the ungrazed site. N mineralization and n itrification rates were significantly higher at the ungrazed site: than at the intensively grazed site during the period of peak net IN mineralization from the end of April until mid-June. Although sheep grazing affected the seasonal pattern of N mineralization in the high marsh, grazing did not aff ect the annual rate of net N mineralization.