Modeling potential impact of climate change on sorghum and cowpea performance under different tillage and cropping systems in semi-arid areas of Kenya

Abstract

Field trials were carried out at Kambi ya Mawe location in Eastern Kenya during the long rains (LRS) of 2010 and short rains (SRS) of 2010/2011, to evaluate effect of tillage and cropping system on performance of sorghum (Sorghum bicolor L. Moench) and cowpea (Vigna unguiculata (L.) Walp). The experimental layout was a randomised complete block design with a split arrangement. The main plots were three tillage practices (Tie-ridge-TR, Sub soilingSR and Ox plough-OP) and split plots were four cropping systems (sole sorghum, sole cowpea, sorghum-cowpea intercrop and sorghum rotation). Surface roughness, soil moisture and yields were measured. The Agricultural Production System Simulator model (APSIM) was used to assess the potential impact of climate change on sorghum and cowpea yields. Effect of the following climatic scenarios on crop performances was considered, base temperature (To), elevated temperatures by 1oC (T1), 2oC (T2) and 3oC (T3), base rainfall (Ro) and 10% reduction in rainfall (R1 ) and their combined effect. For Cowpea, Carbon dioxide (CO2 ) fertilisation at 450ppm and 700ppm and their combination with T2 and T3 and R1 were also examined. Soil surface roughness as a result of different tillage practices had a significant effect (p< 0.05) on soil moisture. Soil surface roughness was 72, 29 and 25% for TR, SR and OP, respectively. Cropping season and tillage-cropping system interaction also resulted in significant difference in soil moisture (p< 0.05). Maximum (1.96 t ha-1) and minimum (0.36 t ha-1) grain yield were achieved under TR during the SRS and OP during the LRS, respectively. Intercropping led to a significant (p<0.05) reduction in yield by more than 50% in cowpea (OP and SR) and sorghum (OP) grain yields during the SRS. However, intercropping evaluation using Land Equivalent Ratio method showed that it was still superior to mono-cropping as both biomass and grain yields were greater than one. The APSIM model output and the observed yields for sorghum biomass showed no significant difference (p= 0.54), indicative of a good model performance. However, simulated yields during LRS exceeded the observed yields despite having high correlation coefficient (R2=0.74). The model performance on predicting cowpea grain yield was poor (R2=0.02). There was only a 6% difference in sorghum grain yield during SRS. Temperature and rainfall changes resulted in an average percentage reduction in biomass and grain yields (t ha-1) of 5.5 and 5.6 for sorghum and 17 and 23 for cowpea. Elevated CO2 concentration of 450 ppm had a positive effect on cowpea biomass production, showing an increase of 7 and 0.8% in biomass following temperature increase of 2oC and 3oC, respectively. Higher temperatures accelerated crop growth, leading to early maturity.