New Paper Alert: “Nitrogen budget and critical load determination at a Sahelian grazed grassland site”

Claire Delon (Laboratoire d’Aérologie, CNRS, Université de Toulouse) and co-authors recently published their paper, Nitrogen budget and critical load determination at a Sahelian grazed grassland site, in the journal Nutrient Cycling in Agroecosystems.

The paper’s co-authors (see full list below) include five participants of the INSA project (Corinne Galy-Lacaux, Ousmane Ndiaye, Dominique Serça, Eric Mougin et Yélognissè Frédi Agbohessou), and benefited from work completed during an INSA funded secondment of Yélognissè Frédi Agbohessou to the Laboratoire d’Aérologie in 2021.

The paper includes new, rare, in situ measurements, and, for the first time, estimates a complete nitrogen (N) budget for a semi-arid Sahelian grazed grassland located in Dahra (Senegal). The authors find that the budget is dominated by the impact of livestock, both through grazing (outputs) and manure (inputs) (Figure 1), emphasising the important role livestock plays in maintaining the ecosystem equilibrium.

Figure 1. Schematic representation and % for each contribution of the N cycle at Dahra. Green colour is for inputs, orange colour for outputs. Small arrows = contribution ≤ 10%, medium arrows = contribution between 11 and 50%, large arrows = contribution ≥ 50%. Red numbers give the range in kg N ha−1 yr−1 for each input and output.

By calculating the nitrogen use efficiency and nitrogen critical load, the authors conclude that the Dahra open tree savanna site is not yet threatened by an excess of N deposition, but that the N budget is unbalanced and favours N depletion. Authors emphasize the need to acquire new data to improve N budget and critical load evaluations.

The next step? Authors envisage new field experiments (NitroAfrica project, 2023-2026) to assess the impact of N deposition on the soil-plant-atmosphere continuum, and the collection of new data on the N budget at the Dahra site in the Sahel, as well as at two other sites in Côte d’Ivoire representative of wet and mesic savannas.  Separate on-going research, performed within the framework of the EU DESiRA CASSECS program (https://www.cassecs.org/en/projet/descriptif-du-projet), is looking at improving knowledge about the impact of livestock on N soil stocks (in particular their evolution).

Reference : Claire Delon, Corinne Galy‑Lacaux, Blandine Barret, Ousmane Ndiaye, Dominique Serça, Frédéric Guérin, Eric Gardrat, Eric Mougin, Yélognissè F. Agbohessou and Anne Probst. Nitrogen budget and critical load determination at a Sahelian grazed grassland site. Nutrient Cycling in Agroecosystems 2022. The online version is available at: https:// doi.org/ 10. 1007/ s10705- 022- 10220-6.

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