Can best agricultural practices improve crop productivity and at the same time enhance SCS? Does a saturation threshold of soil carbon content exist ? What is the role of the root system in soil carbon sequestration ? What is the impact of agricultural practices on the GHG balance and on negative emission through soil carbon sequestration ?
1.1: Assessing practices increasing nutrient and water use efficiency at the field scale
Main Objectif :To study selected agricultural practices that are potentially able to increase crop productivity and carbon inputs to the soil, through enhanced nutrient and water use efficiency (NUE and WUE).
Specific Objectives :To assess the productivity of targeted cropping systems
To assess how NUE and WUE can be modified to increase primary crop productivity
To predict when and where more C and N are stored in the soil and how this C and N interact with native SOMs
Materials and Methods : Existing on-station and on-farm experiments in France (Diams, Restinclières, Vézénobres), Senegal (Niakhar), Zimbabwe (CIMMYT long-term trials) and Kenya (Soil fertility management Long term trials) will be used to assess the impact of different soil and crop management options on yield, NUE and WUE and SOC and nutrients (mainly N and P) dynamics.
1.2: Analyzing soil carbon and its stability under various cropping systems
Main Objectif : To assess the C inputs efficiency to increase SCS in different pedoclimatic conditions and practices
Specific Objectives :
To quantify the SOC storage potential and SOC saturation deficit under cropped soils depending on pedoclimatic conditions
To determine the labile and resistant forms of SOC under different practices under pedoclimatic conditions
Materials and Methods : The SCS potential will be estimated through the “saturation deficit” concept, based on the amount of carbon present in the fine (clay + silt) soil fraction (Feller and Beare 1997, Hassink 1997). Particulate organic matter fractions and short-term mineralizable C will be used to assess nutrients cycling and SOC vulnerability to environmental changes (Balesdent et al. 1998).
Other indicators to characterize the different SOC pools that will be used are: the permanganate-oxidizable carbon (POXC) (Hurisso et al. 2016) and the Rock-Eval pyrolysis (Saenger et al. 2015, Sebag et al. 2016).
Soil samples collected in various crops and not cultivated controls in task 1.4 and 3.1 will be selected to have relevant results to test these various indicators of the soil carbon dynamics.
1.3: Analyzing the link between plant productivity increase and SOC changes through root dynamics
Main Objectif : To evaluate the potential contribution of roots on SCS in cropping and agroforestry systems
Specific Objectives : To assess the contribution of roots to total organic inputs into the soil through root biomass production, mortality and turnover;
To measure the rate of root decomposition and of the root-derived C stabilization at different soil depths compared with the topsoil depending on the edaphic conditions
To assess the fate of root origin C depending on edaphic conditions and plant species
Materials and Methods : Making use of rather unique experimental sites/facilities to probe and monitor root biomass, root production and estimate root turnover in situ under field conditions in France and Senegal. The activities of roots and of soil microbial communities will be linked to soil C stocks and fluxes measured along the soil profiles under various crops (task 1.4). Coarse and fine root density distribution will be assessed by destructive sampling. Minirhizotrons equipped with automatic scanners and root litter bags will be used to assess root turnover. The biochemical composition of soil organic matter will be determined by Rock-Eval pyrolysis. Destructive sampling and controlled conditions will be used in order to measure the priming effect and catabolic activity of soil microbial communities along the whole soil profile.
Main Objectif : To establish the baseline of soil C balance as function of land use in all sites
Specific Objectives :
To provide data on C stocks and fluxesTo assess GHG fluxes at the soil-plant-atmosphere interface (be limited to Senegal and France sites)
Materials and Methods : Sampling will be designed to cover the heterogeneity of the land-uses, soil types, and agricultural practices after landscape stratification using remote sensing images (see also WP3.1). SOC (0-30 cm depth) will be assessed based on calibration models built by IR spectroscopy against dry combustion analysis (CHN). Bulk density measurements will be part of soil inventories for each situation.
The improved version of the full GHG emission protocol proposed by (Assouma 2016) will be implemented on the main landscape units of each site. This protocol includes:
soil surface measurements with automatic and multiplexed (Senegal; DIAMs/FR) chambers of CO2, and N2O fluxes during one full year in France and Senegal
CO2 and H2O emissions measured at the scale of one ecosystem (Senegal) by eddy-covariance. Three flux-towers are available for CO2 (net ecosystem C balance, gross photosynthesis, ecosystem respiration) and H2O (evapo-transpiration) and settled on (i) the overall agro-silvo-pastoral system; (ii) pure millet; (iii) pure groundnut
When not measured in some of the sites, emission factors proposed in the literature will be used
In all sites, total C accumulation in natural biomass will be evaluated with in situ surveys and specific allometric equations available in the literature for the main species encountered in the region
