Browsing by Subject "agroecology, Plant Production Sciences"

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  • Deng, Biar (Helsingin yliopisto, 2020)
    Rainfed agriculture is a vital land use practice for food security and economic development in most of drylands, but particularly in sub-Saharan Africa (SSA). However, it is becoming an increasingly uncertain and inefficient practice in SSA because of climate change and extremes (i.e. low and erratic rainfall, high temperatures, floods, and drought occurrence), and low soil fertility and water supply. For example, yields of sorghum, which is the main staple food crop in South Sudan, are dwindling under rainfed cultivation in its main production areas in the north of the country due to the previously mentioned factors. Nevertheless, soil amendment materials, such as biochars, along with integration of sorghum production into agroforestry systems, which can improve soil fertility and water storage capacity, could assist in improving the crop yields. In this dissertation, the effects of Acacia seyal-based agroforestry and addition of biochar on soil water retention and supply and on sorghum yields were examined. The research focused on 1) the potential of using biochar as a soil amendment combined with A. seyal-based agroforestry in a field experiment, 2) the effect of biochar on alleviating water stress on sorghum yield in greenhouse conditions, and 3) simulation of the potential effect of biochar amendments on improving sorghum biomass and grain yield, especially as indicated by differences in yield between wet and dry years. The two-year agroforestry field experiment (Paper I) was carried out at Magara Village north of Renk in South Sudan, during the growing seasons of 2011 and 2012. The split block experiment included three A. seyal tree density treatments: no trees; scattered trees (100 trees ha-1) and dense trees (400 trees ha-1) and two biochar amendment treatments (0 t ha-1 and 10 t ha-1). The soil consisted of silty loam underlain by clay, and the biochar source was A. seyal trees. A soil analysis showed that agroforestry resulted in lower soil pH, N, and total and exchangeable Ca2+ contents and higher C/N ratios compared to sole sorghum cultivation. The application of biochar significantly increased the soil C and exchangeable K+ contents as well as the pedotransfer-derived field capacity and plant available water contents, but significantly decreased the content of exchangeable Ca2+ and cation exchange capacity. The inclusion of A. seyal trees significantly decreased the sorghum grain yields, and the effect of biochar on grain yield compared to sole sorghum cultivation without amendment was not significant. The Land Equivalent Ratio (LER, the sum of the fractions of the intercropped yields divided by the sole-crop yields) value was 0.3 for dense A. seyal intercropping combined with biochar in both 2011 and 2012 and with scattered A. seyal intercropping in 2011, but it was twofold greater (0.6) in 2012 with biochar amendment. The greenhouse experiment (Paper II) was carried out at the Viikki Campus, Helsinki, Finland, during May–December 2011. The main factor was drought stress with three levels of soil moisture content: 60% of field capacity (well-watered), 40% (medium drought) and 20% (severe drought). The same type A. seyal biochar, in the same amounts as applied in the field experiment (0 t ha-1 and 10 t ha-1), was used. Drought stress had a significant effect on sorghum gas exchange but not on sorghum stomatal traits. The stomatal conductance and photosynthesis and transpiration rates were all significantly reduced under severe drought compared to values found in plants that were under medium drought or well-watered. The photosynthetic water use efficiency (WUE) increased with the level of drought stress. Drought stress significantly reduced the sorghum biomass and grain yields compared to those observed in well-watered plants. Biochar addition did not have a significant effect on any of sorghum stomatal traits, gas exchange or grain yield. The biochar/sorghum simulation study (Paper III) was carried out using the water-driven crop-growth model AquaCrop (version 6.1). The model was parameterized for the field experiment site and soil conditions. Soil fertility stress parameters were adjusted so that simulated biomass and grain yield values best matched the levels recorded in the field experiment. Climate data for 2011 and 2012, both wet years, and for 1990, an extremely dry year, were extracted from the Climate Forecast System Reanalysis (CFRS) online dataset. The effects of biochar were simulated using the changes in soil hydraulic properties (increases in field capacity, available water capacity and saturated conductivity) reported in a published meta-analysis study. Generally, the simulated biochar amendments having the greatest effect on soil hydraulic properties increased the water content of the rooting depth in all three years, but an increase in sorghum production was only discernible for 1990. The results from paper I showed that sorghum yields are lowered when the crop is grown in agroforestry systems. As sorghum is not tolerant of shade, the reduction in sorghum production with increasing tree density was probably due to canopy cover and shading. This effect thus overrode any benefit of having the trees in the cultivation system. The results from paper II indicated that biochar has no significant effect on alleviating drought stress on sorghum production and grain yields, while the results from paper III showed biochar, while improving soil hydraulic properties, only resulted in increased sorghum biomass production and grain yield in very dry years. Overall, the results from this study showed that the propounded benefits of agroforestry and biochar need further study and critical assessment, particularly in semi-arid environments where the water supply through rainfall is low and erratic but the water demand is constantly high. The results may well vary with different crops and agroforestry systems, as well as with different soil types and the type and dose of biochar. Furthermore, the results may only become apparent with sufficient time and therefore long-term studies are needed.