The Impact Of Sheep Grazing On Net Nitrogen Mineralization Rate In Two Temperate Salt Marshes

Abstract: Nitrogen mineralization rate was studied in grazing trials with three different stocking rates (0, 3, 10 sheep ha￢ﾀﾐ1) in two man￢ﾀﾐmade salt marshes, viz. a Puccinellia maritima￢ﾀﾐdominated low salt marsh and a high salt marsh dominated by Festuca rubra. Mineralization rates were derived from the amounts of mineral N which accumulated in situ during six￢ﾀﾐweek incubation periods in tubes containing undisturbed soil cores from the upper 10 cm soil layer. The annual rates of net N mineralization were significantly higher in the better drained, high salt marsh (71 ￢ﾀﾐ 81 kg ha￢ﾀﾐ1 yr￢ﾀﾐ1) than in the low salt marsh (39 ￢ﾀﾐ 49 kg ha￢ﾀﾐ1 yr￢ﾀﾐ1). High amounts of belowground litter accumulated in the low salt marsh due to frequent water logging. Both N mineralization and nitrification rate were negatively correlated with soil water content. In the Puccinellia maritima salt marsh, grazing had neither an effect on N mineralization rates during any of the incubation periods nor on annual mineralization rates. In the Festuca rubra salt marsh, N mineralization rates increased earlier during spring at the intensively grazed site than at the moderately grazed and the ungrazed site. N mineralization and nitrification rates were significantly higher at the ungrazed site than at the intensively grazed site during the period of peak net N mineralization from the end of April until mid￢ﾀﾐJune. Although sheep grazing affected the seasonal pattern of N mineralization in the high marsh, grazing did not affect the annual rate of net N mineralization.