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Nitrate in the unsaturated zone between the soil surface and the water table was studied in agroforestry Parklands in north western Senegal by examination of samples obtained by hand auger. Depending on location, water tables existed at depths between 10 and 35m below ground. Previous studies of groundwater in this region had found that large concentrations of nitrate were unconnected with anthropogenic activity. The objective of this study was to determine whether nitrogen fixing vegetation had a role in groundwater nitrate accumulation and whether roots of trees were located deeply enough to access the nitrate. Accordingly, sample profiles were augered close to stems of nitrogen fixing trees, non-nitrogen fixing trees and also in adjacent areas that were unaffected by tree presence. These adjacent areas were typically open pasture or cultivated fields. Tree fine roots were quantified in the samples and examined for the presence of mycorrhizas. Similarly, sand/soil samples were examined and tested for the presence of nitrogen fixing rhizobia that were capable of forming functional nodules on appropriate host plants. Concentrations of nitrate were greatest in soils beneath nitrogen fixing trees and nitrate was more plentiful in profiles augered beneath nitrogen fixing crops than it was elsewhere suggesting that N-fixation was the source of the nitrate. The concentrrations of nitrate that were found in the unsaturated zone were greatly in excess of the WHO recommended limit for nitrate in drinking water. High NO3-N/Cl ratios confirm insitu production of nitrate, and indicate that this is a natural baseline occurrence related to N-fixation. The nitrate is moving down the profile and impacts the groundwater unless it can be intercepted by plant roots. NO3-N amounts in solution in the soil profile varied between 75 and 1000kg ha−1 beneath trees and between 120 and 400kg ha−1 in areas outwith tree crowns. Although these quantities of N occupy the lower end of the range of N values obtained in north American deserts, they comprise a considerable dryland resource where amounts of organic fertilizer are limited and where cost prohibits the use of commercial fertilizers. Roots of both nitrogen fixing and non-nitrogen fixing trees were deep enough to access the nitrate but the small amounts of available water at intermediate depths suggest that large scale uptake of nitrate will only be possible in the wetter zones located close to the water table. Shallow roots tended to be more heavily colonized by mycorrhizas than deeper roots but mycorrhizas were recovered from roots located 22m below ground. Tree roots and rhizobia had similar patterns of distribution. They were commonest close to the soil surface, less frequent at intermediate depths and tended to increase in frequency close to the water table.

Survival, biomass production, nutrient use and fertility of soil were examined in experimental stands of 10-year-old N-fixing indigenous Acacia nilotica (L.) Del. (eight provenances), A. senegal (L.) Willd. (one provenance), Acacia tortilis (Forsskal) Hayne (six provenances), Acacia raddiana syn A. tortilis (Forssk.) Hayne ssp. raddiana (Savi) Brenan (three provenances) and exotic N-fixing A. holosericea A. Cunn. ex G. Don (one provenance), A. cowleana Tate (one provenance), Acacia aneura F. Muell. ex Benth. (twelve provenances), Prosopis juliflora (Sw.) DC. (one naturalised provenance), Prosopis cineraria (L.) Druce (five provenances) and P. chilensis (Molina) Stuntz (two provenances) growing on a coastal site in semi-arid Senegal. Similar data were obtained for the non-N-fixing fast-growing exotic species Azadirachta indica Adr. Juss. and Eucalyptus camaldulensis Dehnh. of the same age that were growing on adjacent plots. P. juliflora produced more biomass than the other species and showed the greatest potential for use as a multipurpose tree. A. aneura showed the greatest potential for fodder production and despite its small stature, it ranked second for biomass among the N-fixing species. Of the indigenous species, A. nilotica was the most efficient user of non-renewable elements in biomass production. A. raddiana seemed profligate in its consumption of both N and P in wood production. E. camaldulensis was efficient in its use of N in wood but its wood contained large concentrations of P and K. A. aneura, P. juliflora and A. nilotica had the smallest concentrations of P in wood. A. raddiana and A. indica contained large concentrations of K in their wood in contrast to A. aneura and A. nilotica which were the most efficient users of K in wood production. There were few significant differences in fertility in soils beneath the differing tree species. Nevertheless, plots containing P. juliflora and P. cineraria tended to be the most fertile. Soil beneath A. indica was more fertile than soil beneath some N-fixing species. The extent of removal of plant nutrients in biomass suggests that without inputs of nutrient elements, harvest of wood and foliage is unlikely to be sustainable at the measured rate of production for more than two tree rotations of 10 years duration.

Intact soil cores were sampled from around three leguminous tree species (Acacia nilotica, Acacia tortilis and Prosopis juliflora) in 10-year-old alley-cropping plots at Thienaba, Senegal. Cores were removed from two depths (0–25 and 25–50 cm) and at two distances from the trunk (1 and 5 m). Duplicate soil cores were taken for assessment of root concentration (cm/100 cm3 soil), mycorrhizal infection (% of infected root length) and spore concentration in the soils. In order to determine the mycorrhizal inoculum potential (MIP) of the soils, a mycorrhizal bioassay of the soil cores was conducted in the greenhouse using millet seedlings. For all plots, seedlings grown in soils from the surface layer (0–25 cm) were larger and formed higher levels of infection than those grown in soils from 25 to 50 cm depth. Mycorrhizal infection of the seedlings was greatest in soil from the A. tortilis plots and, unlike the other tree species, also the greatest in soil collected near the tree. Positive relationships were found between the growth and infection of the bioassay seedlings and the root and spore concentrations in the field soils. Seedling growth and infection may also have been related to higher levels of carbon in the field soils. The results indicated that root and spore concentrations in the field soils were good indicators of MIP, but that levels of root infection were not. The results also indicated the potential benefit to crop yield of maintaining high levels of mycorrhizal propagules in alley-cropping soils and a possible role of the trees in maintaining these sources of inoculum.

Roots and soil were sampled from around four leguminous tree species in 10-year-old plots at Bandia, Senegal. Assessments of root concentration (cm per 100 cm3 soil) and mycorrhizal colonisation (% of root length) were made and related to the abundance of spores in the soil and the above ground growth (stem diameter at 30 cm height) of the trees. Root concentrations in Acacia nilotica and Acacia tortilis plots were greater than those found in Prosopis juliflora and Acacia aneura plots at all three depths examined (0–10, 10–25, 25–50 cm). Root concentration decreased with soil depth in all plots and was greatest nearest the tree in the Acacia nilotica and Acacia tortilis plots. Mycorrhizal colonisation was highest on Prosopis juliflora roots and lowest on Acacia tortilis roots. Colonisation was not affected by distance from the tree and decreased with depth only in the Acacia aneura plot. Numbers of spores recovered from soils were generally low (27 per 100 g dry wt. soil) and were concentrated in the upper 10 cm of soil. Spores were most numerous in the Acacia aneura plot and least numerous in the Prosopis juliflora plot. Positive relationships were found between spore numbers and root concentration in the Prosopis juliflora plot and between spore numbers and mycorrhizal colonisation in the Acacia aneura plot. Of the four tree species examined, root and mycorrhizal distributions of Prosopis juliflora and Acacia nilotica showed most promise for use in agroforestry systems.