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Abstract: Hydraulic redistribution is the process of passive water movement from deeper moist soil to shallower dry soil layers using plant roots as conduits. Results from this study indicate that this phenomenon exists among two shrub species (Guiera senegalensis and Piliostigma reticulatum) that co-exist with annual food crops in Sahelian agro-ecosystems. Real-time measurements were conducted for soil water content, soil water potential and microclimate variables notably; air temperature, relative humidity, wind speed, precipitation and solar irradiance. Additionally, sap flow measurements were conducted in shrub roots using the thermal dissipation technique on intact and coppiced shrubs. Monthly predawn leaf water potential was measured using a portable pressure chamber. Soil water potential (Ψ s) at the 20 cm depth declined significantly during the dry season with diel changes in Ψ s of −0.6 to −1.1 MPa. These variations were attributed to passive water release from shrub roots resulting in overnight rewetting of drier upper soil layers. Sap flow measurements on tap and lateral shrub roots indicated daily reversals in the direction of flow. During the peak of the dry season, both positive (toward shrub) and negative (toward soil) flows were observed in lateral shrub roots with sap flow in the lateral roots frequently negative at night and rapidly becoming positive soon after sunrise. The negative sap flow at night in superficial lateral roots and the periodic positive flow in the descending tap roots were indicative of hydraulic redistribution. Hydraulic redistribution may be an important mechanism for drought stress avoidance while maintaining plant physiological functions in both shrubs and neighboring annuals in water-limited environments. [Copyright &y& Elsevier]

Summary: The interactions of the tree component with soils in the parkland systems of Africa have been investigated but little research has been done on unrecognized native woody shrubs that coexist with crops in farmers’ fields, particularly in the Sahel. The two most important species are Guiera senegalensis and Piliostigma reticulatum, which are coppiced and burned prior to crop planting in Senegal and throughout the Sahel. The litter inputs and year-round presence of the shrub rhizosphere should have implications for soil quality and crop productivity, but the interactions of these shrubs with microbial communities and biogeochemical processes are uninvestigated. Hence, the objective was to determine the influence of shrub rhizospheric soil and residue type on microbial community composition and activity during decomposition as a first step towards developing a non-thermal residue management system. Two experiments, one for each shrub species, had a 2×3 factorial design with two soil treatments (0–10cm depth beneath and outside the influence of the shrub) and three residue amendments (leaf, stem+leaf and control). The amended soil microcosms were incubated for 105 days in the lab and destructively sampled at 0, 7, 14, 45, 75, or 105 days. At each sampling, the soil microbial communities were profiled by phospholipids fatty acid (PLFA) analysis and analyzed for the activity of two C hydrolyzing enzymes (β-glucosidase and cellulase). Interestingly, the shrub canopy effect on the microbial communities was stronger than the residue type. The fungal biomarkers were more closely correlated than other microbial groups to residue chemistry whereas, Gram-positive bacterial and fungal markers (18:2ω6c, 18:1ω9c) were highly correlated with enzyme activities. In contrast, the actinomycete marker (10Me 16:0) was poorly correlated with enzyme activities. We conclude that shrub rhizospheres in Sahaliean agroecosystems are distinct components in controlling microbial community composition, promoting fungi, microbial diversity, and litter decomposition. [Copyright &y& Elsevier]

Abstract: Allometric equations and community biomass stocks are presented for Guiera senegalensis J.F. Gmel (Gs) and Piliostigma reticulatum (DC.) Hochst (Pr) – two native shrub species in the Sahel. These shrubs are of interest because they dominate semi-arid sub-Sahalien Africa but have been largely overlooked as a key biomass component and regulator of ecosystem composition and function in this landscape. In Year 1, best predictors of aboveground biomass were height and number of stems (Gs) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R 2 =0.90 for Gs and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58tonha−1 (mean=2.38tonha−1) in the Gs sites and from 0.33 to 7.38tonha−1 (mean=3.71tonha−1) in the Pr communities. Both species exhibited unusually large root:shoot ratios (4.5:1 for Gs and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for Gs and Pr. [Copyright &y& Elsevier]

Estimating regional carbon (C) stocks and understanding their dynamics is crucial, both from the perspective of sustainable landscape management and global change feedback. This study combines remote sensing techniques and a coupled GIS-CENTURY model to estimate regional biomass C stocks and SOC dynamics for Guiera senegalensis shrub communities in Senegal''s Peanut Basin. A statistical model relating field-measured shrub aboveground biomass C at training plots to satellite image-derived shrub abundances was developed and used to estimate regional biomass C across a major part of the Basin. Regional SOC dynamics were modeled by coupling the CENTURY model and GIS databases. Significant correlation (r =0.73; p =0.05) was observed between aboveground biomass C and satellite image-derived shrub abundance at the training plots. Aboveground biomass C stocks ranged from 0.01 to 0.45Mgha−1 with an approximate total of 247,000MgC for the 3060km2 study area. CENTURY model predictions indicate that C sequestration in these systems is contingent on long-term effectiveness of non-thermal management of shrub residue and that the actual rates depend strongly on soil type and scenarios of future land management. Compared with the traditional “pruning-burned” management practice, returning prunings for 50 years would increase soil C sequestration by 200–350% without fertilization, and increase soil C sequestration by 270–483% under a low (35kgha−1 Nyr−1; 20kgha−1 Pyr−1) fertilization regime, depending on soil type and climate conditions. These results indicate that altered land management could contribute to transforming these degraded semiarid agroecosystems from a source to a sink for atmospheric CO2. [Copyright &y& Elsevier]