CLIMATE CHANGE ANALYSIS, IMPACT ASSESSMENT, AND NITROGEN OPTIMIZATION USING CERES-SORGHUM MODEL FOR SORGHUM [Sorghum bicolor (L.) Moench] PRODUCTION IN THE SEMI- ARID RIFT VALLEY AREAS OF ETHIOPIA

Abstract

Climate variability and change are currently significant global issues. Certainly, understanding the observed and projected climatic change, can serve as an indispensable tool in devising effective strategic frameworks to enhancing the socioeconomic welfare of a significant cohort of agricultural practitioners. The objective of the study was to explore past and projected climate variability and trend, assessing and quantifying the loss of nitrogen as result of climate change and its impact on sorghum yield as well as to determine optimum nitrogen fertilizer for sorghum using CERES-Sorghum model in semi-arid Rift Valley areas of Ethiopia. To achieve this objective the study mainly focused on the four components; in understand past climate variability and trends, projected climate change and extremes trends, projected climate change impact on nitrogen loss and its impact on sorghum yield and nitrogen optimization using CERES-sorghum under observed climate condition. The primary approach followed was collecting long term (1981-2021) daily rainfall and air temperature for eleven stations from EMI. Three and two stations were, respectively, from NRVE and ERVE while six of the stations were from the CRV. The variability of rainfall and temperature over the past four decades were assessed using mean, CV, and SAI. Trend analysis was performed following Mann–Kendall test and Sen’s slope estimator. The start and end of the growing season, length of the growing period, and dry spells were analyzed for the stations. The second approach was that weather data for 11 stations were generated using AgMIP-5 technique for RCP 4.5 and 8.5 and for the period of 2050s and 2080s.MAKESENS was employed for the detection of trend of extreme indices. Instat v3.37 was used for the analysis of start, end, and length of growth season. The third approach was to quantify the loss of nitrogen from the sorghum treated with level of nitrogen with three time of deliver and its impact on sorghum yield and the final approach nitrogen fertilizer optimization using CERES-sorghum model in observed climate condition for the sorghum at Melkassa, Mieso and Kobo areas. The months of July, August, and September were the primary contributors to the overall annual rainfall at all the sites studied. Conversely, December, January, November, and February exhibited the lowest levels of rainfall across the locations. In comparison to the longest rainy period, the short rainy period contributed less rainfall to the annual amount and was also associated with high inter-seasonal variability. The xix range of minimum and maximum air temperatures experienced during the short rainy season varied between 12.24-19.4 oC and 27.7- 36̊ oC, whereas during main rainy season ranged from 11.9 -20.1oC and 24.5-33.4 oC respectively in the studied regions. The trend of rainfall in months of March and April showed a decline in range of 0.075 -1.78 mmyr-1,0.06-0.97mmyr-1 and 0.11 - 0.84mmyr-1in the CRV, ERVE, and NRVE, respectively. The non-significantly (P ≤0.05) increasing amounts of rainfall in the months of June in range s of 0.14-1.17mmyr-1and September month to increase in range of 0.05-0.81mmyr-1. where June as the start and September as end of the season, respectively, in the studied regions. Rainfall decreases from 0.31–2.76 mmyr-1and increases to a 0.19–5.12 mmyr-1 in the short and main rainy seasons respectively except decrease in rainfall in the main rainy season in Dhera, Matahara, and Melka Werer in CRV. The maximum and minimum air temperatures in the month of March to May show increasing trends, while from June to September there were mixed trends in the studied sites. The start of the season throughout the main growing period varies between 177 and 196 DOY. Conversely, the end of the season spans from 274 to 285. Similarly, the length of growing period in the studied regions spans from 78 to 143 days, and the number of rainy days during the main growing season ranges from 67 to 110 days. The projected annual minimum air temperatures in CRV, ERVE and NRVE could increase by 1.9 ̊ C (2050s) and 2.6 ̊ C (2080s), 1.8 ̊ C (2050s) and 2.5 ̊ C (2080s) and 1.88 ̊ C (2050s) and 2.69 ̊ C (2080s) respectively under RCP 4.5. However, at the same location, it is projected to increase in both time frames (2050s and 2080s) in all studied sites under RCP 8.5. The annual maximum air temperature, projected under RCP 4.5 in the CRV, ERV, and NRVE will increase by 1.59 ̊ C (2050s) and 2.18°C(2080s),1.42 ̊ C (2050s) and 2.08 ̊ C(2080s) and 1.46 ̊ C (2050s) and 2.09 ̊ C(2080s) respectively. However, at same regions, under the RCP 8.5 it rise in both periods (2050s and 2080s). This will be convoy with increase of the hot and cold extremes’ indices in regions. The change of annual rainfall decrease insignificantly by 0.6-5.5% and 0.32% and increase 0.85– 12.3% and 22.3% in half of the stations located in CRV and ERVE, whereas, in NRVE will increase 6.1-14.6% under RCP 4.5 in all stations in 2050s.Though the annual rainfall under RCP 4.5 in 2080s will decline in range of 0.67–10.1%,3.12–4.5% and 0.9-4.6% at CRV, ERVE and NRVE respectively. In most of the location in CRV and ERVE stations growing season rainfall decline from 1.45% to 53.8%.and 0.8 to 8.8%. Whereas, in NRVE will increase in 9.2 to 19% under RCP 4.5 in period of 2050. The CERES-Sorghum model coupled in DSSAT -CSM calibrated using five years (2010–2016) and evaluated with six years (2017–2022) experimental xx data of days to anthesis and physiological maturity, grain yield, LAI, and biomass for Melkassa, Mieso and Kobo. The projected SNR is expected to increase from 1 to 36.7% in the 2050s and 1 to 43.3% in the 2080s under RCP 4.5. Under RCP 8.5, the SNR is expected to increase in range of 1% to 53.2% in the 2050s and in the 2080 in range of 2.4 to 63.9% . Under RCP4.5, the projected N2O emission will rise in the range of 0.39% to 13.5% in the 2050s and in 2080s and decline at Kobo by 2.8% (2050) and 16.2% (2080). Under RCP 8.5, the projected N2O emission will rise in Melkassa and Mieso but it will decline at Kobo site in the 2050s and 2080s. Under RCP 8.5 in both time frames, the N-leaching is projected to increase at N application to sorghum.In general, the lowest SNR, N2O emission and N-leaching in both time frames and scenarios is projected to be the N fertilizer application rate of 46 N kg ha–1.The projected decrease in sorghum grain yield is in the range of 6.8 to 11.9% in the 2050s and by 9.45 to17,8% in the 2080s under RCP 4.5. However, under RCP 8.5, projected decrease in sorghum grain yield is in the range of 7.9 to 17.7% in the 2050s and in the range of 17.3 to 24.3% in the 2080. Therefore, to reduce the risk that may arise because of climate change and its impact on the environment, sorghum producers should integrate cultivation of the crop with soil moisture conservation and nutrient and water use efficient genotypesAt the site under study, nitrogen fertilizer had a significant impact on the mean simulated sorghum grain yield, biomass yield, harvest index, and NUE of the CERES sorghum model. Consequently, the EONR rates at the Mieso in ERVE, Kobo in NRVE, and Melkassa in CRV were 29, 34, and 30 N kg N ha–1. These rates of nitrogen fertilizer the most cost-effective and environmentally optimal recommendations because of the package availability and the negligible difference between EONR and 46 kg N ha– 1 at three application times: a quarter at planting, half in the mid of the growing season, and a quarter at the booting stage