Mineralization and N-use efficiency of tree legume prunings from fertilizer tree systems and low quality crop residues in Malawi

Abstract

There is substantial evidence that fertilizer tree systems are capable of maintaining increased and sustainable crop production on low fertility soils in southern Africa, thus reducing the required amount of chemical fertilizer. However, crop yield increase in soils amended by fertilizer tree systems can only be optimized if nutrient release by the organic materials and nutrient demand by the crop are in synchrony. The decomposition and N release patterns of high quality tree prunings (gliricidia and sesbania) and crop residues (pigeon pea leaves and roots, and maize stover) were studied to understand the N use efficiency of fertilizer tree systems. The treatment were (1) quality pruning residues from gliricidia (Gs) and sesbania (Ss), (2) three medium quality residue levels including pigeon pea leaves (Pea-L), pigeon pea leaves + roots (Pea-LR) and pigeon pea roots (Pea-R), and (3) two rates of maize stover (Stover-1 and Stover-2) as low quality residues, and control (no crop residues, no tree prunings). The treatment combinations were laid out as a randomized complete blocks design. Mixtures of tree prunings with 2.5 t ha-1 maize stover increased maize N uptake and grain yield whereas 5 t ha-1 maize stover reduced maize N uptake and grain yield during the wetter season. Mixtures of Pea-R, Stover-1 or Stover-2 with tree prunings depressed yields during the drier season. Stover-2 had the highest N fraction immobilized N, respectively 15 and 35% N during the wetter and drier conditions. We
conclude that (1) mixing of high quality tree prunings with crop residues may enhance the decomposition of low quality crop residues but there is no special interaction, and (2) remineralization of N immobilized early in the season by the low quality organic materials is stimulated by well
distributed rainfall.