Soil aggregation and associated nutrient content along a distance-from-tree gradient in a low input cropping system. Comparing the effect for two tree species: Mangifera indica (mango, Anacardiaceae) vs. Faidherbia albida (faidherbia, Fabaceae)

Trees can play a major role in improving soil quality and thus have the potential to enhance sustainability of agroecosystems. Although this has been well established, knowledge on the effect of individual trees on aggregate stability and associated-nutrient content remains scarce. This study aims to quantify the spatial extent and magnitude of influence of single-standing Mangifera indica (mango, Anacardiaceae) and Faidherbia albida (faidherbia, Fabaceae) trees on soil aggregate stability and aggregate-associated nutrient content (total carbon, nitrogen and phosphorus) in a lowinput maize cropping system. Mango and faidherbia differ fundamentally in their properties (e.g., phenology and N-fixing capacity) providing an interesting study system. We hypothesize that 1) the extent of tree influence on soil will be proportional to the tree crown, 2) soil aggregation and aggregate-associated nutrient content will decrease with distance from tree for both species due to greater organic matter inputs from leaves and roots closer to the tree, 3) the magnitude of tree influence will be greater for faidherbia relative to mango due to its N-fixing property. To test these hypotheses, soil samples, collected along four transects, laid out around each single tree and taken at four different distances (1, 4, 10 and 15 m) from the tree trunk, were fractionated and total carbon, nitrogen and phosphorus content within the different fractions was determined. Our results show that both species had an impact on soil fertility, but only the spatial influence of mango was proportional to its crown. We could identify no change in the measured indicators with distance around faidherbia suggesting the possibility that we were likely still in its impact zone. These results support the hypothesis that faidherbia has an extensive positive impact on the surrounding soil, and that mango crown also improves subcanopy soil fertility via a better aggregation and an increase in associated-nutrient content.