Improved fallows with leguminous trees have been developed in Southern Africa as a viable alternative to inorganic fertilizers but the changes in soil properties that are responsible for crop productivity improvement and implications of mixing litter and fresh leaves from the same tree species on soil fertility are not fully understood. Our objectives were to quantify (1) some changes in soil properties that are responsible for crop production improvement under improved fallow systems; (2) the N mineralization patterns of mixtures of litter and fresh leaves from the same tree species. The treatments used in the study were 2-year planted Sesbania sesban (sesbania) and Cajanus cajan (cajanus) and controls of natural fallow, continuous fertilized and unfertilized maize. At fallow clearing sesbania contributed 56 kg N ha1 through litter and fresh leaves. Sesbania (fresh leaves + litter) showed high N mineralization after 10 weeks compared to the mixture of cajanus fresh leaves with litter. Maize yields were significantly correlated with preseason NO3-N and total inorganic-N content of the top 20-cm soil layer. Soil penetrometer resistance at 4 weeks after planting was lowest in the sesbania land-use system (2.2 Mpa), whereas the highest percentage of water-stable aggregates at fallow clearing and crop harvest was in sesbania (83%) and cajanus (77%), respectively. The improved soil conditions and N contribution of sesbania and cajanus fallows to the subsequent maize crop was evidenced by increased maize yields of between 170–200% over maize without fertilizer.
Tag: mineralization
Mineralization and N-use efficiency of tree legume prunings from fertilizer tree systems and low quality crop residues in Malawi
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.
Influence of vegetation on soil organic nitrogen composition and mineralization in a subtropical wetland
Studies have investigated the role of litter type on soil organic nitrogen (SON) mineralization and the resulting chemical composition in aerobic soils; however, the role of anaerobic conditions have been limitedly explored. We determined the SON chemical composition and N mineralization rate with depth of two different vegetation types (emergent aquatic vegetation, EAV and submerged aquatic vegetation, SAV) and prevailing flooded soil conditions in a subtropical treatment wetland. The treatment wetland provided similar water quality and climate conditions across the vegetation types. Three soil depths were sampled, the surficial floc, recently accreted soil, and soil present before constructing the treatment wetland. The surficial floc material created from emergent plants exhibited a mean total carbon (TC) and total N (TN) of 364 g C kg−1 and 27 g N kg−1 vs 185 g C kg−1and 12 g N kg−1 for submerged plants. Despite these differences, there were no significant differences in the proportion of N that was soil organic nitrogen (SON). Mean extractable ammonium was lower (95 mg N kg−1) in SAV systems and may be due to increased pH (~9) during the day compared to EAV systems (207 mg N kg−1). For the chemical composition, the N-associated aromatics comprised 4.8% of the SON in SAV areas compared with just 1.6% for EAV, which suggests an increased SON recalcitrance in SAV created floc. The difference in EAV and SAV chemical composition may be important for the long-term storage of SON and future studies should focus on additional environmental factors that favor the formation of stable recalcitrant SON.