Nitrogen and phosphorus uptake from isotope‐labeled fertilizers by sorghum and soil microorganisms

On nutrient‐depleted Lixisols from Burkina Faso, nutrient acquisition by crops and soil microbes mainly relies on the limited amounts of mineral and organic fertilizers applied by small‐scale farmers. The objective of this study was to determine simultaneously the uptake of nitrogen (N) and phosphorus (P) contained in organic and mineral fertilizers by sorghum [ Sorghum bicolor (L.) Moench ] and soil microbial biomass. Double 15 N and 33 P direct and indirect labeling techniques were applied in a pot experiment to determine the contributions of different fertilizers to sorghum N and P uptake during 52 d of growth. In parallel, soil respiration, available, and microbial N and P were tracked in an incubation experiment. Sorghum derived 83–90% of P from fertilizers. Nitrogen from cattle manure was poorly available, contributing only 20% of the N taken up by sorghum. Water‐soluble mineral fertilizers increased soil N and P availability, resulting in the highest total N and P uptake by sorghum from fertilizers and soil among all treatments. The application of cowpea [ Vigna unguiculata (L.) Walp.] residues induced microbial N and P immobilization, reducing sorghum N and P uptake to the level of the non‐fertilized treatment. The use of double 15 N and 33 P labeling elucidated the impact of fertilizers on soil nutrient pools. The low plant N/P ratio suggested N limitation for sorghum in the manure treatment. Cowpea residues were inefficient for sorghum nutrition, but they increased soil microbial nutrient pools. This study gives insights on the potential effects of legume residues used as green manure to build soil fertility.