Christophe Jourdan

Background and Aims: Increasing C storage in cultivated soils requires a better understanding of C dynamics, particularly at depth, where root litter decomposition dynamics is expected to be slower than in ploughed layers. Methods: We assessed the effect of barley root diameter on root decomposition in situ using a non-invasive method at different depths. Temporal decreases in root diameter and length were measured using images acquired by optical scanners buried at depths of 20, 50 and 90 cm from seeding and for 1.5 years. A parallel root litterbag experiment was performed to measure root mass loss. Results: Root decomposition was observed on the scanned images before the flowering stage, with up to 85 % of the maximum root volume achieved being lost at harvest. Thinner roots (<0.3 mm) decomposed more slowly than thicker roots. Root length decreased faster at 20 cm than at 50 cm, but soil depth had no significant impact on the dynamics of root-diameter decrease. Conclusions: Optical scanner-based image analysis complements litterbags by enabling individual root tracking and in situ decomposition assessment without root manipulation. This method offers the opportunity to measure root decomposition at various soil depths over long periods, and could improve the estimation of root-derived soil C inputs.

Background and aims: Root traits of tree species are of great importance for selection of trees in dry zone agroforestry systems. Climate models predict increased frequency and severity of drought in the Sahel but our understanding on the strategies of root growth of tree species under drought is limited. To increase our knowledge of species differences in rooting patterns we investigated root growth of nine dryland tree species in West Africa during one year when exposed to drought stress. Methods: Seedlings were planted in 2 m tubes inserted into the soil and subjected to irrigation and drought (no irrigation during the dry season). After 4, 8 and 12 months, we assessed root growth and root biomass, supplemented with assessments of water potential, leaf phenology, and above-ground growth. Results: Tree species showed significant differences in rooting profile. Drought induced deeper root growth, especially in deciduous species. Adansonia digitata, Sterculia setigera and Anacardium occidentale had shallow roots when irrigated. Acacia senegal, Acacia nilotica and Faidherbia albida had deeper root growth and showed clear differences in leaf water potential between treatments while A. digitata and S. setigera appeared to be water conserving under drought. Conclusions: Our results link root growth with phenology, physiology and above-ground growth and provide an attempt to group dryzone tree species in functional types based on their root growth.

Soil hydraulic parameters (SHPs) required as inputs for numerical models are scarce in Sahelian regions. Instead, they are estimated using pedotransfer functions (PTFs), but their ability to simulate soil water dynamics has not been evaluated yet. Therefore, this study aims to parametrise SHPs with seven different PTFs and inverse modelling to gain insight into their ability to simulate water fluxes in Senegal’s Groundnut basin. We used four years of field measurements of soil moisture (SWC) and actual evapotranspiration (ET_a) under pearl-millet and groundnut crop rotation for model evaluation. Inverse modelling for SWC (RMSE ≤ 0.015 cm³ cm⁻³) and ET_a (RMSE ≤ 0.62 mm d⁻¹) yielded the best model performance values compared to PTFs (0.024-0.175 cm³ cm⁻³ and 0.68-0.96 mm d⁻¹). Where field measurements are lacking for inverse estimation, three of the seven tested PTFs yielded good modelling results and could be used as a parsimonious approach for cultivated Sahelian soils.