Extensive Estimate of Nitrogen Budget and Critical Loads at the Scale of a Semi-Arid Tropical Grazed Site (Dahra, Senegal)
Résumé
Based on rare and original in situ measurements together with published data, we estimate a complete N budget for a semi-arid Sahelian grazed grassland ecosystem (or open tree savanna) located in Dahra (Senegal) for the years 2014 and 2017. Nitrogen (N) budgets in Africa have been estimated primarily for croplands, and very little data are available for grassland ecosystems. N inputs include biological N fixation, dry and wet N atmospheric deposition, and N input from livestock manure. N outputs include NO and N 2 O emissions from soils, NO and NH 3 emissions from domestic fires and biomass burning, NH 3 volatilization from manure, N ingestion from livestock grazing, N uptake by trees and N soil leaching. Upper and lower bounds were estimated, depending on the method used to quantify the contribution of livestock to the N budget. N inputs ranged between 11.7 ± 0.5 and 34.4 ± 0.5 kg N ha -1 yr -1 and N outputs ranged between 16.8 ± 1.5 and 46.1 ± 1.5 kg N ha -1 yr -1 on average for both years. N depletion (inputs minus outputs) was consequently estimated to be between -5.1 ± 2.0 and -11.1 ± 2.0 kg N ha -1 yr -1 . The budget is dominated by the impact of livestock through grazing (62% of the outputs), NH 3 volatilization (15% of the outputs), manure (68% of the inputs) and atmospheric deposition (19% of the inputs). The nitrogen critical load (estimated by the Steady State Mass Balance approach) ranged from 16.7 ± 0.8 to 47.5 ± 1.7 kg N ha -1 yr -1 , showing that the grazed grassland of Dahra was not yet threatened by an excess of nitrogen. The main uncertainties are related to the assessment of the critical load and to the quantification of parameters linked to livestock.
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