J. Nyamangara

Cumulative effects of sewage sludge and effluent mixture application on soil properties of a sandy soil under a mixture of star and kikuyu grasses in Zimbabwe

Abstract Although sewage effluent and sludge provides nutrients for plant growth, its continual use over extended periods can result in the accumulation of heavy metals in soils and in grass to levels that are detrimental to the food chain. This study was carried in 2001 out at Firle farm, owned by the Municipality of Harare, to assess heavy metal loading on a sandy soil and uptake of the metals by pasture grass consisting of a mixture of Cynodon nlemfuensis (star grass) and Pennisetum clandestinum Chiov (kikuyu grass) following sewage effluent and sludge application for 29 years. Firle Farm receives treated effluent and sludge emanating from domestic and industrial sources. Soil and grass samples were taken from the study area, consisting of 3 ha of non-irrigated area (control) and 1.3 ha of irrigated area. Both the soil and grass samples were tested for Cu, Zn, Ni and Pb using atomic absorption spectrophotometry. Sewage sludge addition resulted in high levels of soil pollution, especially in the 20 cm horizon, in the irrigated area when compared to the control. Grasses took up moderate levels of Cu and Zn, and limited levels of Pb. Nickel was not detectable in grasses despite high levels in the irrigated soil. Copper uptake was several times higher than the suggested potentially toxic level of 12 mg/kg [Soil Science Society of America, Micronutrients in agriculture, second ed., Wisconsin, USA, 1991]. Lead uptake averaged 1.0 mg/kg, which was below 10 mg/kg the suggested limit for agronomic crops [E.M. Seaker, Zinc, copper, cadmium and lead in minespoil, water and plants from reclaimed mine land amended with sewage sludge, 1991]. Cu and Zn showed relatively higher mobility down the soil profile than Ni and Pb. Even then, the concentrations in the lower soil layers were very small, suggesting that the metals were unlikely to contaminate groundwater. There was no direct correlation between metal levels in soils and grasses. It was postulated that it is the bio-available metal fraction in the soil that is correlated to plant uptake. The grasses appeared healthy even though they contained moderately high levels of Zn and Cu. This raises the possibility of beef animals grazing on “healthy” looking grass that has very high concentrations of heavy metals. The fact that the total metal concentrations in the experimental soil were very high but did not cause any toxicity symptoms to the grass suggested that the limit soil concentration do not necessarily imply toxicity to all plants. However, limit concentrations are set not only for plant growth, but also for the protection of soil microorganisms and the latter are more sensitive to heavy metal pollution.

EFFECT OF CONSERVATION AGRICULTURE ON MAIZE YIELD IN THE SEMI-ARID AREAS OF ZIMBABWE

SUMMARY Globally, a range of agronomic factors have been reported to have an impact on the performance of conservation agriculture (CA) and often determine its performance in relation to conventional agriculture (CONV). To assess this performance in Zimbabwe, 48 CA experiments were conducted by the International Crops Research Institute for the Semi-Arid Tropics in the semi-arid areas of southern Zimbabwe from 2004 to 2010, to calculate the weighted mean difference (WMD) through meta-analytical methods. The two CA practices, planting basins (Basins) and ripper tillage (Ripper), were compared with CONV. It was hypothesised that CA results improved yield compared with CONV and that the effect of CA practices on yield is affected by soil type, rainfall amount and distribution and selected management practices, which included rates of inorganic fertilisers and manures and mulching. Basins were superior to CONV in 59% of the experiments and the overall effect was significant (p < 0.001). The effect of Ripper was non-significant. The hypothesis that CA practices result in improved maize grain yield over CONV was accepted for Basins. The WMD for experiments conducted on sandy soils was 0.365 t ha −1 for Basins and 0.184 t ha −1 for Ripper, and in both cases was significant (p < 0.05). For clay soils, only the WMD for Basins was significant. A higher rainfall regime (500–830 mm) resulted in a lower WMD for Basins (0.095 t ha −1 ) and Ripper (0.105 t ha −1 ) compared with 0.151 t ha −1 for Basins and 0.110 t ha −1 for Ripper under lower rainfall (320–500 mm). The overall effect of Basins under the higher rainfall regime was not significant. There was better yield performance for Basins when the rainfall was well distributed; the reverse was noted for the Ripper. The application of 10–30 kg ha −1 of N (micro-dose range) resulted in a higher WMD for Basins than zero N application. Without N application, the WMD of Basins was not significant. For zero manure application in Basins, the WMD was 0.043 t ha−1 compared with 0.159 t ha−1 when manure was applied. The application of mulch depressed the WMD in Basins by 44% and Ripper by 89%. The hypothesis that yield performance under CA is influenced by soil type, rainfall amount and distribution, inorganic fertiliser and manure application was accepted.

Opportunities for optimization of in-field water harvesting to cope with changing climate in semi-arid smallholder farming areas of Zimbabwe

Climate change has resulted in increased vulnerability of smallholder farmers in marginal areas of Zimbabwe where there is limited capacity to adapt to changing climate. One approach that has been used to adapt to changing climate is in-field water harvesting for improved crop yields in the semi- arid regions of Zimbabwe. This review analyses the history of soil and water conservation in Zimbabwe, efforts of improving water harvesting in the post independence era, farmer driven innovations, water harvesting technologies from other regions, and future directions of water harvesting in semi arid marginal areas. From this review it was observed that the blanket recommendations that were made on the early conservation method were not suitable for marginal areas as they resulted in increased losses of the much needed water. In the late 1960 and 70s’, soil and water conservation efforts was a victim of the political environment and this resulted in poor uptake. Most of the water harvesting innovations which were promoted in the 1990s’ and some farmer driven innovations improved crop yields in marginal areas but were poorly taken up by farmers because they are labour intensive as the structures should be made annually. To address the challenges of labour shortages, the use of permanent in-field water harvesting technologies are an option. There is also need to identify ways for promoting water harvesting techniques that have been proven to work and to explore farmer-led knowledge sharing platforms for scaling up proven technologies.

Nitrous oxide emissions from wetland soil amended with inorganic and organic fertilizers

Agricultural soils are a primary source of anthropogenic trace gas emissions, and the subtropics contribute greatly, particularly since 51% of world soils are in these climate zones. A field experiment was carried out in an ephemeral wetland in central Zimbabwe in order to determine the effect of cattle manure (1.36% N) and mineral N fertilizer (ammonium nitrate, 34.5% N) application on N2O fluxes from soil. Combined applications of 0 kg N fertilizer + 0 Mg cattle manure ha−1 (control), 100 kg N fertilizer + 15 Mg manure ha−1 and 200 kg N fertilizer + 30 Mg manure ha−1 constituted the three treatments arranged in a randomized complete block design with four replications. Tomato and rape crops were grown in rotation over a period of two seasons. Emissions of N2O were sampled using the static chamber technique. Increasing N fertilizer and manure application rates from low to high rates increased the N2O fluxes by 37–106%. When low and high rates were applied to the tomato and rape crops, 0.51%, 0.40%, and 0.93%, 0.64% of applied N was lost as N2O, respectively. This implies that rape production has a greater N2O emitting potential than the production of tomatoes in wetlands.

Effectiveness of cattle manure and nitrogen fertilizer application on the agronomic and economic performance of maize

The effects of N fertilizer and aerobically composted cattle manure, applied separately or in combination, on maize (Zea mays L.) grain yields and net benefits were determined over three seasons. A field experiment was established on a moderately leached sandy loam soil (Typic Kandiustalf). Manure was a poor source of N. In the first year, increase in grain yield was much higher when manure (12.51 ha−1 and 37.51 ha−1) was combined with the 60 kg N ha−1 mineral N rate (40% and 25.1%, respectively), and a relatively smaller further increase of 17.5% was recorded for the 37.5 t ha−1 rate while there was a decrease of 3.7% for the 12.5 t ha−1 rate, when mineral N rate was doubled to 120 kg N ha−1. In the third season increase in grain yield was also much higher when manure (12.51 ha−1 and 37.51 ha−1) was combined with the 60 kg N ha−1 mineral N rate (66.2% and 16%, respectively) and relatively smaller further increases were recorded when the mineral N rate was doubled to 120 kg N ha−1 (21.4% and 15.1 %, respectively). Net benefit indications are that residual effects of cattle manure last for at least three seasons and thus farmers could apply up to 40 t ha−1 in the first season and benefit from its residual fertility in subsequent seasons. It was concluded that smallholder farmers in Zimbabwe and other countries of Sub-Saharan Africa could positively exploit the combined application of manure and N fertilizer to increase maize yield and net benefits.

Effect of cotton–cowpea intercropping on C and N mineralisation patterns of residue mixtures and soil

Carbon and nitrogen mineralisation potential of mixed cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) crop residues produced under intercropping, as well as a reddish-brown soil classified by FAO as Ferralic Cambisol previously under intercrops, were studied over a 10-week incubation period under controlled conditions (25°C and moisture content of 70% field capacity, 125 mm) in the laboratory. Treatments consisted of cotton residues (100 : 0), cowpea residues (0 : 100), and cotton–cowpea residues (50 : 50, 70 : 30, and 30 : 70). These ratios were based on yields obtained in different cotton–cowpea intercrop treatments from a field study. Cowpea residues (0 : 100) released the highest amount of mineral N of 36.4 mg/kg soil, and cotton residues (100 : 0) least, 19.2 mg/kg soil, while the other mixtures were in between. All treatments except for cowpea residues (0 : 100) and the 30 : 70 mixture showed immobilisation of soil N during the first 2 weeks of incubation. The trend for C mineralisation was similar to that of N, and cowpea residues (0 : 100) released the highest amount, 492 mg C/kg soil, while cotton residues (100 : 0) recorded the least, 315 mg C/kg soil. The C mineralisation patterns of cowpea residues (0 : 100) and 30 : 70 treatments were exponential and were well described by the equation: where CE is exponentially mineralisable C fraction, k is the rate constant, and t is time in days. The mineralisation patterns for other treatments were sigmoidal and were well described by the equation: where CS is sigmoidally mineralisable C fraction; t 0 is time in days required for complete mineralisation of CS , while k is rate constant. The amount of N released from soil previously under cotton–cowpea intercrops and sole crops was approximately one-third of the amount released when the residues were incorporated. The highest amount of N released (12.2 mg/kg soil) was from soil previously under sole cowpea, while soil from the 1 : 1 cotton–cowpea intercrop released 9.9 mg/kg soil and soil from sole cotton released 5.9 mg/kg soil. There was no significant effect (P > 0.05) of previous crop on C mineralisation patterns of the soil. Mixtures slow down N losses and increase nutrient use efficiency of legume residues, especially in the short-term. When cotton is grown as a sole crop, starter N to offset negative effects of initial N-immobilisation at the start of season is required. A better understanding of controlling parameters of decomposition can make it possible to predict C and N mineralisation patterns in mixtures. Reduced C mineralisation in cotton–cowpea mixtures may result in more C sequestration and, hence, SOM build-up and improved sustainability in the long term in intercropping systems.

Approaches to reinforce crop productivity under rain-fed conditions in sub-humid environments in Sub-Saharan Africa

Smallholder farming in much of Sub-Saharan Africa is rain-fed and thus exposed to rainfall variability. Among the climate variables, rainfall is projected to decline and have an overriding effect on crop productivity. With little opportunity for supplementary irrigation for the majority of farmers, a plausible strategy to maintain crop production under water-limited conditions includes balanced nutrient management for enhancing efficiency of use of limited soil water. Co-application of judicious rates of organic and mineral nutrient resources, particularly including the use of phosphorus (P) on P-limited soils, will facilitate development of an extensive crop rooting system for efficient exploration and capture of soil water, especially at a depth >0.8 m. This chapter explores case studies across Eastern and Southern Africa where various soil water conservation and nutrient management approaches have been used to gain ‘extra miles’ with limited available soil water. Firstly, an approach is described that varies nitrogen (N) fertilizer application across growing seasons, by adjusting N application rates to match current season rainfall trends. The approach offers opportunities for farmers to increase crop productivity to >6 t ha−1 in high agro-potential areas, compared to a ceiling of 4.5 t ha−1 for the fixed fertilization model, while minimizing economic losses due to investments in N fertilizer during drought years. Secondly, we deal with the subject of fertilization across nutrient gradients, where a poor agronomic N use efficiency of 35 kg grain kg−1 of N applied when soil organic carbon >0.5 %. Thirdly, the conservation agriculture (CA)-nutrient management nexus is examined, where maize yields in farmers’ fields with CA alone were barely 0.5 t ha−1 compared to an average of 2.5 t ha−1 for CA combined with fertilizers. Fourthly, a novel system that involves intercropping two legumes with contrasting phenology for enhanced cropping system functioning is described. Finally, an approach that can be used for co-learning with farmers on soil fertility management principles for risk management is presented. The data lead to the conclusion that the ‘doubled-up’ legumes system results in reduced fertilizer requirements for cereal crops grown in sequence, which benefits yield stability over time. Variable use of N fertilizer according to season quality and more tailored targeting of nutrients are vital for profitable investments in fertilizers in Africa. The Africa RISING project in Eastern and Southern Africa is currently harnessing some of these principles as vehicles for intensification of smallholder farming systems.

The effects of catena positions on greenhouse gas emissions along a seasonal wetland (dambo) transect in tropical Zimbabwe

Wetlands are major natural sources of greenhouse gases (GHGs). In central and southern Africa, one of the most extensive wetlands are dambos (seasonal wetlands) which occupy 20–25% of land area. However, there are very little data on GHG methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) emissions from dambos, and this study presents the first estimates from dambos in Zimbabwe. The objective was to evaluate the effects of catena positions; upland, dambo mid-slope and dambo bottom, on GHG emissions along an undisturbed dambo transect. Methane emissions were −0.3, 29.5 and −1.3 mg m−2 hr−1, N2O emission were 40.1, 3.9 and 5.5 µg m2 hr−1, while CO2 emissions were 2648.9, 896.2 and 590.1 mg m−2 hr−1 for upland, mid-slope and bottom catena, respectively. Our results showed that uplands were important sources of N2O and CO2, and a sink for CH4, while the dambo mid-slope position was a major source of CH4, but a weak source of CO2 and N2O. Dambo bottom catena was weak source GHGs. Overall, dambos were major sources of CH4 and weak sources of N2O and CO2.We concluded that, depending on catenal position, dambos can be major or minor sources of GHGs.

Effect of soil fertility management strategies and resource-endowment on spatial soil fertility gradients, plant nutrient uptake and maize growth at two smallholder areas, north-western Zimbabwe

Abstract We investigate the effects of smallholder farmer resource—endowment and soil nutrient management strategies on plant nutrient uptake and growth across soil fertility gradients under semi-arid conditions. Soil fertility gradients as influenced by farmers’ resource availability may affect the response of crops to fertilizer addition and therefore productivity. The study was conducted in Njelele and Nemangwe smallholder areas (450-800 mm per annum, unimodal) in north-western Zimbabwe. Soil and maize cobleaf samples were collected from fields of farmers (varying resource endowment) located near to (homefields) and far away (outfields) from the farmers’ homesteads during the 2005–6 season. The samples were analysed for selected soil fertility indicators and soil samples were further used to test maize growth response to various nutrient applications under greenhouse conditions. Soil fertility (organic C, total N and available P) significantly (P<0.05) decreased from resource-endowed to resource-constrained farmers, and from outfields to homefields, but the latter differences were only significant for available P. Besides resource-endowment and field type, response to nutrient applications also depended on soil texture. In sandy soil, both N and P were limiting to MAIZE growth in outfields while N was most limiting in the homefield. It was concluded that resource-endowment and nutrient resource management strategies employed by farmers result in soil fertility gradients which affect response of crops to fertilisation and therefore need to be factored in the development of fertiliser recommendations.

Optimizing dambo (seasonal wetland) cultivation for climate change adaptation and sustainable crop production in the smallholder farming areas of Zimbabwe

Most smallholder farming areas of Zimbabwe have low soil fertility and low rainfall, which has continually decreased over the past years. In recent years, most of the smallholder farming areas have experienced perennial droughts, poor rainfall distribution and crop failures and these have been attributed to climate change and variability. Cultivation of dambos, which are seasonal wetlands, presents a climate change and variability adaptation option for smallholder farmers. This synthesis analyses the role of dambo cultivation in climate change and variability adaptation and discusses future directions for sustainable dambo utilization. The data on current dambo farming practices were collected from literature, surveys and field observations. The results showed that farmers grow crops in dambos as an adaptive strategy to climate change and variability and have largely abandoned upland fields where yields are <1 t ha−1 in preference of dambos where yields average 2–3 t ha−1. Dambo cultivation offers a buffer against crop failures and has resulted in improved household food security. We conclude that dambo cultivation is a potentially beneficial farmer-driven climate change and variability adaptation strategy. However, if not properly designed and managed, dambo cultivation may result in their degradation hence there is need for further research to evaluate options for sustainable dambo utilization as intensification of dambo agriculture is important for food security.