Comparaison des émissions de N2O de deux sites africains aux gestions et climats contrastés

Sites and field campaigns description 

Dahra rangeland station

The Dahra field station has been in operation since 2002 and is managed by the University of Copenhagen. It is part of the Centre de Recherches Zootechniques (CRZ) and is located in Senegal (Ferlo) in West Africa (15°24’10’’N, 15°25’56’’W, elevation 40m). This site is a semiarid savanna used as a grazed rangeland. The soils are mainly sandy with low N and C content and neutral pH (Table 5.1). The region is influenced by the West African Monsoon (cool wet south-westerly wind) during the rainy season and the Harmattan (hot dry north-easterly wind) during the dry season. The rainy season occurs between July and October. Annual average rainfall is 416 mm calculated from historical data collected between 1951 and 2013 (Table 5.1) and was 356 mm in 2013 and 412 mm in 2017. Tree cover is around 3% of the surface with the Chapitre 5. Comparaison des émissions de N2O de deux sites africains aux gestions et climats contrastés 124 most abundant species being the Balanites aegyptiaca, Accacia tortilis and Acacia Senegal (Tagesson et al., 2015a). The herbaceous vegetation is dominated by annual C4 grasses (e.g. Dactyloctenium aegyptum, Aristida adscensionis, Cenchrus biflorus and Eragrostis tremula) (Tagesson et al., 2015a). Livestock is mainly composed of cattle, sheep and goats. Grazing is permanent and occurs year-round (Assouma et al., 2017). The site was previously described in Tagesson et al. (2015b) and Delon et al. (2017, 2019). The 2017 field campaign was part of the French national program called Cycle de l’Azote entre la Surface et l’Atmosphère en afriQUE (CASAQUE – Nitrogen cycle between surface and atmosphere in Africa), whereas results of the 2013 field campaigns were already published in Delon et al. (2017). Three field campaigns were carried out to quantify N2O emissions. The campaigns lasted eight days in July 2013 (11 – 18 July 2013), ten days in November 2013 (29 October – 7 November) and eight days in September 2017 (21 – 27 September 2017). 

Mbita cropland region

The Mbita region is composed of eight plots with different vegetation, soil characteristics and fertilisation input. Five out of these eight plots are experimental plots located within the International Centre of Insect Physiology and Ecology (ICIPE) close to the Victoria Lake in Kenya and will be referred as ICIPE from here onwards. Each of the five plots has a size of ~ 12×12 m2 and receives 100 kg N ha-1 yr-1 of either di-ammonium phosphate or calcium ammonium nitrate fertiliser during the two rainy seasons. Crops are regularly watered by overhead irrigation sprinklers. These plots are annually cultivated with maize (2 plots referred to as Maize 1 and Maize 2), sorghum (1 plot referred to as Sorghum), napier grass (1 plot referred to as Napier Grass) and permanent grassland (1 plot referred to as Grassland). The grassland plot has been a permanent non-cultivated grassland since 2010. The three other plots are cultivated fields located around the city of Kisumu: one with maize in Fort Ternan (referred to as Maize Fort Ternan, with no fertilization), a 20 years tea plantation in Kaptumo (referred to as Tea Field Kaptumo) with NPK fertilization of 50 kgN ha-1 yr-1 and a sugar cane plot in Kisumu (referred to as Sugar Cane Kisumu) with no fertilization (Fig. 5.1). These crops are representative of the main country cultivated crops since they represent 54 % of the total cultivated areas (Oseko et al., 2015). Two field campaigns were carried out to quantify daily N2O emissions. The first one lasted nine days in January 2018 (17 – 25 January 2018) during the dry season and the second one ten days in November 2018 (29 October – 07 November 2018) during the rainy season for the purpose of the CASAQUE program. The measurement plots in the Mbita region were located in a region of mixed crops and grassland. All sites in the Mbita region are characterized by an equatorial climate with two distinct rainy seasons throughout the year: one long rainy season from March to May and a shorter one from November to December (five months in total), often referred as long and short rain seasons. The average rainfall and temperatures are 1100 mm and 24 °C in ICIPE from 1986 to 2018 (Table 5.1). In 2018, ICIPE experienced a rainfall of 1070 mm and an average temperature of 20 °C. All four locations (ICIPE, Fort Ternan, Kaptumo and Kisumu) have clay soils with high N and C content (Table 5.1). Soils in ICIPE, Fort Ternan and Kisumu have a relative neutral pH whereas the soil in Kaptumo has a low (acid) pH (Table 5.1). Chapitre 5. Comparaison des émissions de N2O de deux sites africains aux gestions et climats contrastés 126 Fig. 5.1. Plots location in Dahra (Senegal) and around and in the Mbita region (Kenya). The borders define with dark and blue lines referred to African countries and Kenyan districts’ boundaries, respectively.

Field Data

Hydro-meteorological data

In Dahra, the available hydro-meteorological variables used for this study are rainfall (mm), air and soil temperature (°C) and soil water content (SWC) (%) at 0.05, 0.10 and 0.30 m depth. These variables were collected every 30 s and averaged and saved every 15 min (or sum for rainfall). All details on sensors and measured variables are given in Tagesson et al. (2015b) and Chapitre 5. Comparaison des émissions de N2O de deux sites africains aux gestions et climats contrastés 127 Delon et al. (2019). Due to technical issues in 2017, rainfall was measured by a manual rain gauge with direct reading. This rain gauge was installed in the frame of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) network (https://indaaf.obs-mip.fr/). The gauge is set up one meter above the soil surface according to the World Meteorological Organization (WMO) recommendations. It is graded from 0 to 150 mm, with a minimum resolution of 0.25 mm. Reading of the manual gauge was done every day at 8:00 AM by a local partner. In the Mbita region, soil temperatures were not measured during the field campaigns due to probe unavailability. SWC was measured manually with a thetaprobe ML3 (Delta-T Devices Ltd, 1% precision) at an hourly time step during the day. Rainfall was measured using the same protocol as in Dahra during 2017, with a manual rain gauge read daily by a local collaborator. For the purpose of the study, WFPS was calculated as follows and used in the modelling process of N2O emissions at the Dahra site: 𝑊𝐹𝑃𝑆 = 𝑆𝑊𝐶 ∗ 𝐷𝑎 𝐷𝑎− 𝐷𝑏 with 𝐷𝑎 = the soil actual density (Da = 2.6 g cm-3 ) and 𝐷𝑏 = the soil bulk density, calculated from the dry weight of soil samples collected in situ (see section 2.2.3 chapter 5 for details) with a cylinder of known volume (290 cm3 ) and dried at 40°C for 24h. Db = dry soil mass/total volume was 1.50 ± 0.06 g cm-3 for Sahelian soils. The use of WFPS values instead of SWC is useful to include the concept of soil porosity together with soil humidity as it is a way to indicate how soil pores are saturated or not, and to compare more objectively denitrification thresholds between ecosystems. For both regions, all hydro-meteorological variables were averaged to daily values for the purpose of this study.