Integrating legumes to improve N cycling on smallholder farms in sub-humid Zimbabwe: resource quality, biophysical and environmental limitations

Abstract

The release of mineral-N in soil from plant residues is regulated bytheir `quality' or chemical composition. Legume materials used byfarmers in southern Africa are often in the form of litter with Nconcentration < 2%. We investigated the decomposition of Sesbaniasesban and Acacia angustissima litter in the field using litterbags, andN mineralization of a range of legume materials using a leaching tubeincubation method in the laboratory. The mass loss of the litter couldbe described using a modified exponential decay model: Y = (Y-0 -Q)e(-kt) + Q. The relative decomposition constants for Sesbania andAcacia litter were 0.053 and 0.039 d(-1), respectively. The % Nmineralized from fresh Sesbania prunings was 55% compared with only27% for the Sesbania litter after 120 days of incubation under leachingconditions. During the same period, fresh prunings of Acacia releasedonly 12% of the added N while Acacia litter released 9%. Despite thelarge differences in N concentration between Acacia prunings and itslitter, the total mineralized N was similar, as mineralization fromprunings was depressed by the highly active polyphenols. While N supplymay be poor, these slow decomposing litter materials are potentiallyuseful for maintaining soil organic matter in smallholder farms. In twofield experiments with contrasting soil texture, Sesbania, Acacia andCajanus produced large amounts of biomass ( > 5 Mg ha(-1)) and improvedN cycling significantly ( > 150 kg N ha(-1)) on the clay loam soil, butadapted poorly on the sandier soil. There was a rapid N accumulation inthe topsoil at the beginning of the rains in plots where large amountsof Sesbania or Acacia biomass had been incorporated. Despite the widedifferences in resource quality between these two, there was virtuallyno difference in N availability in the field as this was, among otherfactors, confounded by the quantity of N added. A substantial amount ofthe nitrate was leached to greater than 0.4 m depth within a three-weekperiod. Also, the incidence of pests in the first season, and drought inthe second season resulted in poor nitrogen use efficiency. Ourmeasurements of gaseous N losses in the field confirmed that N2Oemissions were < 0.5 kg N ha(-1). As we had measurements of all major Nflows, we were able to construct overall N budgets for the improvedfallow - maize rotation systems. These budgets indicated that, in anormal rainfall season with no major pest problems, reducing nitrateleaching would be the single largest challenge to increased N recoveryof added organic N in the light textured soils.