A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions

Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact

Groundwater recharge in basement aquifers in subhumid drylands of sub-Saharan Africa

Characterising groundwater recharge is fundamental for sustainable groundwater management. This study focuses on assessing recharge in drylands using four experimental plots under different land-use practices in crystalline basement aquifers in three southern African countries (Chitedze in Malawi, Kabeleka and Liempe in Zambia, and Domboshawa in Zimbabwe). Several methods, including water-table fluctuation (WTF), chloride mass balance (CMB), water stable isotopes (δ18O and δ2H) and dissolved gases, were used to quantify annual recharge rates, recharge sources and groundwater residence times. This informed the development of a conceptual model of groundwater recharge in unpumped basement aquifers. Using WTF, across all sites/years, the range of annual median recharge was found to be in the range of 2.8–14.1% rainfall. Recharge was observed for most years across all sites and was controlled by hydrogeological settings, rainfall totals and antecedent conditions, i.e. the groundwater level at the end of the preceding dry season. Based on groundwater level observations and water stable isotope analysis, for sites where there has been extensive use of conservation agriculture (in time and space), there is some evidence of earlier and greater recharge compared to conventional agriculture at paired sites. Additionally, there is evidence of high lateral connectivity in shallow, permeable layers and high local connectivity in the aquifers which facilitate discharge to surface drainage. This leads to a lower proportion of modern recharge at these unpumped sites (typically <10%) compared to other studies using comparable methods in pumped boreholes, which highlights the importance of groundwater capture due to pumping

Nitrogen effect on zinc biofortification of maize and cowpea in Zimbabwean smallholder farms

Agronomic biofortification of crops with zinc (Zn) can be enhanced under increased nitrogen (N) supply. Here, the effects of N fertilizer on grain Zn concentration of maize (Zea mays L.) and cowpea (Vigna unguiculata L.) were determined at two contrasting sites in Zimbabwe over two seasons. All treatments received soil and foliar zinc‐sulphate fertilizer. Seven N treatments, with three N rates (0, 45, and 90 kg ha−1 for maize; 0, 15, and 30 kg ha−1 for cowpea), two N forms (mineral and organic), and combinations thereof were used for each crop in a randomized complete block design (n = 4). Maize grain Zn concentrations increased from 27.2 to 39.3 mg kg−1 across sites. At 45 kg N ha−1, mineral N fertilizer increased maize grain Zn concentration more than organic N from cattle manure or a combination of mineral and organic N fertilizers. At 90 kg N ha−1, the three N fertilizer application strategies had similar effects on maize grain Zn concentration. Co‐application of N and Zn fertilizer was more effective at increasing Zn concentration in maize grain than Zn fertilizer alone. Increases in cowpea grain Zn concentration were less consistent, although grain Zn concentration increased from 39.8 to 52.7 mg kg−1 under optimal co‐applications of N and Zn. Future cost/benefit analyses of agronomic biofortification need to include information on benefits of agro‐fortified grain, complex farmer management decisions (including cost and access to both N and Zn fertilizers), as well as understanding of the spatial and site‐specific variation in fertilizer responses

Fitting and comparing water retention curves for soils under contrasting experimental treatment: Examples from conservation agriculture trials in southern Africa

Conservation Agriculture (CA) is proposed as a ‘climate-smart’ intervention for resilient crop production in dryland areas affected by climate change. Evidence is needed for how these practices affect fundamental properties of the soil. The soil water retention curve (SWRC) is a physical attribute of the soil which provides information on its porous structure and physical quality. It is also critical for modelling processes in the soil such as water movement, water availability for plants and infiltration into the soil during rainfall events. In this paper we estimate parameters of the van Genuchten model of the SWRC from experiments on CA interventions in southern Africa, using a linear mixed modelling framework. The method we use, stochastic approximation maximization, allows for maximum likelihood estimation of the parameters without use of linearizing approximation. We show how sequential fitting of model parameters, with marginal false discovery rate control, allows us to make robust inferences about differences in the SWRC between soils under contrasting experimental management. We also show how the method allows us to draw samples from distribution of SWRC parameters, reflecting the uncertainty which arises from variation within the management treatments. Indices of soil physical quality may be computed from the parameter estimates to compare treatments, and by computing them from the samples, the uncertainty in these indices can also be assessed. We use the estimated model parameters to simulate infiltration of water into the soils under different management during a rainfall event. Again, by using the samples from the joint distributions of the parameters the effects of uncertainty in these parameters as propagated through the model can be computed. We applied these methods to soils collected from experimental plots under CA and conventional tillage (CV) at sites in Zimbabwe, Zambia and Malawi. We observed differences in the SWRC for the CA and CV plots at the Zambian site where a physically vulnerable soil showed greater macroporosity under CA than CV. In contrast, a sandy and organic-poor soil at the site in Zimbabwe showed somewhat greater macroporosity under cultivation rather than CA management. There was no detectable treatment effect of the management system on the SWRC for the soils at the site in Malawi