Water use efficiency of maize under selected soil and water conservation practices and climate change scenarios in Ruzizi Plain Arenosols, Eastern Democratic Republic of Congo

Abstract

The Ruzizi plain in the eastern Democratic Republic of Congo (DRC), where maize production is almost entirely rainfed, experiences high intra- and inter-annual rainfall variability, an imbalance between cumulative rainfall and evapotranspiration and climate change effects that affect soil water deficit, potentially leading to reduced water use efficiency of maize. Therefore, it is very important to identify soil and water conservation (SWC) practices that improve soil water balance (SWB) and water use efficiency (WUE). However, concerns can be raised about the current and future effectiveness of these SWC practices as climate change adaptation options, as their effectiveness varies over time and is site-specific. This study evaluated the effectiveness of selected SWC practices (Tied ridges, Zai pits and conventional tillage as controls) in improving SWB and WUE under current and future climatic conditions. The effectiveness of tied ridges and Zai pits compared to conventional tillage as a control was tested on three slopes (0-2, 2-8 and 8-15%) using a factorial design over three growing seasons for the current situation. Each practice was replicated three times on each slope. For the future climate condition, the AgMIP Regional Integrated Assessment (RIA) approach was used to simulate WUE within the Cropping System Model-Crop Environment Resource Synthesis (CSM-CERES-Maize) of the Decision Support System for Agrotechnology Transfer (DSSAT) model. The model was calibrated using experimental data from nine cropping seasons (2011-2018) and 100 farms. The sensitivity of the model was assessed as a function of temperature, water and SWC practices for the same environments. Initial conditions of crop management practices were used as input data for CSM-CERES maize. Current climate data were extracted from AgMERRA datasets corrected with local data for 1980 to 2021. Future climate projections (2023-2099) were obtained after downscaling the data from the 29 GCMs of CMIP5 and subsetting to five GCMs based on climate regimes. The results of this research show that under the current climatic conditions, tied ridges and Zai pits significantly improved the cumulative soil water balance by 148.7 and 21.1% respectively compared to conventional tillage. However, the efficiency of these SWC practices decreased significantly with slope (p<0.05). The effectiveness of tied ridges and, to some extent, Zai pits in storing soil water was observed when rainfall exceeded 10 mm per day. WUE was higher for tied ridges (5.34 kgha-1mm-1) than for Zai pits (3.52 kgha-1mm-1) and conventional tillage (3.78 kg ha-1mm-1). WUE decreased significantly with slope gradient (p<0.01). The decrease in stored soil water (SWS) during the silking and maturing stages significantly reduced WUE. The results of the future climate as predicted by different GCMs indicated that the Ruzizi Plain would experience an increase in mean annual temperatures for RCP 4.5 and 8.5, with the highest increase (3.05°C) under hot/dry conditions for RCP8.5 and the lowest (1.04°C) under cool/dry conditions for RCP 4.5. All models selected for five climate regimes for 2022-2099 showed no change in rainfall trends for RCP 4.5 (p>0.05). In both RCP scenarios, maize WUE will decrease from 3% to 26.2% compared to baseline, with a CV of 16.3-28.3%. It should be noted that despite the reduction in effectiveness under future climate conditions, tied ridges as an adaptation practice will increase maize yield, WUE and soil water balance by 38.2 and 50.2% respectively compared to conventional tillage. Tied ridges offer an opportunity to improve SWB and WUE, reduce potential losses due to climate change, and sustain rainfed agriculture in the Ruzizi plain of eastern DRC. Therefore, tied ridges can be used to improve crop productivity and environmental conservation as component of integrated soil water and nutrient management strategies.