Modelling the effect of soil moisture and organic matter degradation on biogenic NO emissions from soils in Sahel rangeland (Mali)

Abstract

Abstract. This work is an attempt to provide seasonal variation of biogenic NO emission fluxes in a Sahelian rangeland in Mali (Agoufou, 15.34 • N, 1.48 • W) for years 2004, 2005, 2006, 2007 and 2008. Indeed, NO is one of the most important precursors for tropospheric ozone, and previous studies have shown that arid areas potentially display significant NO emissions (due to both biotic and abiotic processes). Previous campaigns in the Sahel suggest that the contribution of this region in emitting NO is no longer considered as negligible. However, very few data are available in this region, therefore this study focuses on model development. The link between NO production in the soil and NO release to the atmosphere is investigated in this modelling study, by taking into account vegetation litter production and degradation, microbial processes in the soil, emission fluxes, and environmental variables influencing these processes, using a coupled vegetation-litter decompositionemission model. This model includes the Sahelian Transpiration Evaporation and Productivity (STEP) model for the simulation of herbaceous, tree leaf and faecal masses, the GENDEC model (GENeral DEComposition) for the simulation of the buried litter decomposition and microbial dynamics, and the NO emission model (NOFlux) for the simulation of the NO release to the atmosphere. Physical parameters (soil moisture and temperature, wind speed, sand percentage) which affect substrate diffusion and oxygen supply in the soil and influence the microbial activity, and biogeochemical parameters (pH and fertilization rate related to N content) are necessary to simulate the NO flux. The reliability of the simulated parameters is checked, in order to assess the robustness of the simulated NO flux. Simulated yearly average of NO flux ranges from 2.09 to 3.04 ng(N) m −2 s −1 (0.66 to 0.96 kg(N) ha −1 yr −1 ), and wet season average ranges from 3.36 to 5.48 ng(N) m −2 s −1 (1.06 to 1.73 kg(N) ha −1 yr −1 ). These results are of the same order as previous measurements made in several sites where the vegetation and the soil are comparable to the ones in Agoufou. This coupled vegetationlitter decomposition-emission model could be generalized at the scale of the Sahel region, and provide information where few data are available.

Effets de la pâture sur la dynamique de la végétation herbacée au Sahel (Gourma, Mali) : une approche par modélisation

“…Le climat joue un rôle important dans la dégradation saisonnière de la masse herbacée au Sahel (Delon et al, 2015). Dans cette région où il fait chaud et parfois humide, l'humidité du sol, mais aussi l'humidité de l'air, participent à la décomposition de la litière au sol et à la minéralisation de la litière enfouie, en particulier sous l'action des termites.…”

The contribution of soil biogenic NO and HONO emissions from a managed hyperarid ecosystem to the regional NO<sub><i>x</i></sub> emissions during growing season

“…The considerable uncertainty about the range of soil biogenic NO emissions stems from widely differing estimates of the NO emissions. Based on field measurements worldwide Davidson and Kingerlee (1997) estimated the global NO soil source strength to be 21 Tg a −1 (with an error margin of 4 to 10 Tg a −1 , 40 % of the total), while the fourth IPCC estimate is 8.9 Tg a −1 (Denman et al, 2007), which is higher than the third IPCC estimate of 5.6 Tg a −1 (IPCC, 2001). Moreover, the uncertainties in the NO emission data from semiarid, arid, and hyperarid regions are very large (mainly due to a very small number of measurements being available).…”

“…Ozone is usually generated in polluted, industrialized regions, where ambient levels of NO x are high. Moreover, the intensification of agriculture is associated with increased NO emissions, which also causes an increase in available nitrogen in the atmosphere (Denman et al, 2007). NO x in the troposphere originates mostly as NO, which reaches a photo-stationary equilibrium with NO 2 within a few minutes.…”

“…The current evolution of anthropogenic as well as biogenic NO x sources triggers a potential increase in global tropospheric O 3 concentrations. According to recent estimates (Kasibhatla et al, 1993;Davidson and Kingerlee, 1997;Denman et al, 2007;Feig et al, 2008), anthropogenic sources amount to 45-67 % of the mid-2000s total global NO x emissions (42-47 Tg a −1 , in terms of mass of N), and other globally important sources are soil biogenic NO emissions (10-40 %), biomass burning (13-29 %), and lightning (5-16 %). The considerable uncertainty about the range of soil biogenic NO emissions stems from widely differing estimates of the NO emissions.…”

The contribution of soil biogenic NO emissions from a managed hyper-arid ecosystem to the regional NO<sub>2</sub> emissions during growing season

“…However, today's knowledge of soil biogenic NO emission rates from managed arid soils is still rather limited (c.f. Behrendt et al, 2014;Mamtimin et al, 2015;Delon et al, 2015). The laboratory incubation method is usually dynamic chamber system which are consists gas dilution, thermostat valve, thermostat cabinet, and analysers (Behrendt et al, 2014).…”