Nitrogen cycling in grazed pastures at elevated CO 2 : N returns by ruminants

Abstract

In pastures grazed by large herbivores, nutrients cycle both through litter and animal excreta. We compared nitrogen (N) returns from sheep grazing a temperate pasture exposed to ambient or elevated CO 2 (475 μmol mol −1 ) in a FACE (Free Air CO 2 Enrichment) experiment established in the spring of 1997. In the spring of 2000 and 2001, we measured the chemical composition of the diet, sheep faeces and of individual plant species before grazing to characterize feed intake and to compare the intake of N to the N produced in faeces. In both years under elevated CO 2 , leaves of the individual species exhibited lower N concentrations and higher water‐soluble carbohydrate (WSC) concentrations. There was a significantly greater proportion of legume in the diet at elevated CO 2 but, together with the changes in chemical composition of individual species, this resulted in diets that had similar N but higher WSC and digestibility for both ambient and elevated CO 2 . We found that a greater proportion of dietary N was partitioned to urine at elevated CO 2 , probably because of the higher proportion of legume N in the diet, with possible differences in protein quality. A potentially significant consequence of this change in partitioning is greater N loss through volatilization at higher CO 2 levels.

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