T. P. Tauro

Nitrogen fixation and biomass productivity of indigenous legumes for fertility restoration of abandoned soils in smallholder farming systems

Abstract Most legume-based soil fertility technologies often fail to make the desired impact on nutrient-depleted soils partly due to low N2-fixation rates and poor biomass productivity. A study was conducted in the 2004/05 and 2005/06 rainfall seasons to evaluate biomass productivity and N2-fixation of indigenous legumes on nutrient-depleted fields under low (450- 650 mm yr−1) to high (> 800 mm yr−1) rainfall areas of Zimbabwe. Legume species, mostly of Crotalaria, Indigofera and Tephrosia genera, were sown in mixtures on disturbed soil. Indigenous legume fallows (indifallows) produced 2–5 t biomass ha−1 under low and 5–15 t biomass ha−1 under high rainfall. They significantly (P<0.05) out-yielded natural fallows by 84% and a sunnhemp (Crotalaria juncea) green manure fallow by 32% over a growth period of six months. Cumulatively, indifallows produced more biomass (~29 t ha−1) than natural fallows (~ 8 t ha−1) over two seasons. Indigenous legumes derived 61–90% of their N from the atmosphere with amounts fixed ranging from 2–57 kg N ha−1 under medium rainfall conditions to 1–173 kg N ha−1 under high rainfall. Application of P increased indifallow biomass productivity by 15% under low rainfall, and N2-fixation by 32% and 18% under low and high rainfall, respectively. These results demonstrated that indigenous legumes generate sufficiently high biomass and fix nitrogen on nutrient-depleted fields where most conventional green manure and grain legume cultivars often fail to establish.

Comparative short-term effects of sewage sludge and its biochar on soil properties, maize growth and uptake of nutrients on a tropical clay soil in Zimbabwe

Abstract Soil application of biochar from sewage could potentially enhance carbon sequestration and close urban nutrient balances. In sub-Saharan Africa, comparative studies investigating plant growth effect and nutrients uptake on tropical soils amended with sewage sludge and its biochar are very limited. A pot experiment was conducted to investigate the effects of sewage sludge and its biochar on soil chemical properties, maize nutrient and heavy metal uptake, growth and biomass partitioning on a tropical clayey soil. The study compared three organic amendments; sewage sludge (SS), sludge biochar (SB) and their combination (SS+SB) to the unamended control and inorganic fertilizers. Organic amendments were applied at a rate of 15 t ha −1 for SS and SB, and 7.5 t ha −1 each for SS and SB. Maize growth, biomass production and nutrient uptake were significantly improved in biochar and sewage sludge amendments compared to the unamended control. Comparable results were observed with F, SS and SS+SB on maize growth at 49 d of sowing. Maize growth for SB, SS, SS+SB and F increased by 42, 53, 47, and 49%, respectively compared to the unamended control. Total biomass for SB, SS, SS+SB, and F increased by 270, 428, 329, and 429%, respectively compared with the unamended control. Biochar amendments reduced Pb, Cu and Zn uptakes by about 22% compared with sludge alone treatment in maize plants. However, there is need for future research based on the current pot experiment to determine whether the same results can be produced under field conditions.

Indigenous legumes biomass quality and influence on C and N mineralization under indigenous legume fallow systems

Non-cultivated N2-fixing indigenous legumes can be harnessed to enhance soil fertility replenishment of smallholder farms. Understanding N release patterns of biomass generated by such legumes is key in managing N availability to crops. Nitrogen and C mineralization patterns of indigenous legume species, mainly ofTephrosia andCrotalaria genera, and of soils sampled at termination of 1- and 2-year indigenous legume fallows (indifallows)were investigated in leaching tube incubations under laboratory conditions. With the exception ofTephrosia longipes Meisn (2.4%) andCrotalaria cylindrostachys Welw.ex Baker (1.8%), all indigenous legumes had >2.5% N. Total polyphenols and lignin were <4% and 15%, respectively, for all species.Crotalaria pallida (L.) andEriosema ellipticum Welw.ex Baker mineralized >50% of the added N in the first 30 days of incubation. Similar to mixed plant biomass from natural weed fallow,C. Cylindrostachys immobilized N during the 155-day incubation period. Indifallow fallow biomass reached peak N mineralization 55 days after most legumes had leveled off. Carbon release by legume species closely followedN release patterns,with mostCrotalaria species releasing >500 mg CO2-C kg−1 soil. Soils sampled at termination of fallows reached peak N mineralization in the first 21 days of incubation, with indifallows mineralizing significantly (P<0.05) more N than natural fallows. Application of mineral P fertilizer to indifallows and natural fallows increased C and N mineralization relative to control treatments. It was concluded that (i) indigenous legumes generate biomass of high quality within a single growing season, (ii) the slow N release of biomass generated under indifallow systems suggests that such fallows can potentially be manipulated to enhance N availability to crops, and (iii) N and C mineralization of organic materials in sandy soils is likely controlled by availability of P to the soil microbial pool.