Sorghum water use efficiency and yield variations discerned by 13C isotopic technique under managed agricultural practices in Upper Eastern Kenya

Current knowledge on using ¹³C discrimination as an indirect measure of yield and water use efficiency (WUE) under different soil moisture conditions and soil fertility inputs in C4 crop species has considerable uncertainty. The objective of this study was to test for (i) the effect of selected soil water conservation measures and soil fertility inputs on sorghum yield, water use efficiency, and ¹³C discrimination, (ii) evaluate the relationship between various measures of water use efficiency and ¹³C discrimination, between sorghum yield and ¹³C discrimination; (iii) sorghum stem diameter and WUE and, the use of stem diameter and ¹³C discrimination as potential yield and WUE proxy. We implemented a field trial on-station for five seasons in the semi-arid areas of Upper Eastern Kenya. The experiment was designed in a randomized complete block design (RCBD) with three levels of nitrogen fertilization (120 kg ha⁻¹, 60 kg ha^−1, and 30 kg ha⁻¹) application with four replications. The selected soil water conservation measures and soil fertility management were minimum tillage, mulching, tied ridging, and Managing Beneficial Interactions in Legume Intercrops (MBILI) along a control (no input). Water use efficiency was determined using carbon discrimination analysis and gravimetric technique. The leaves and post-harvest grain samples were analyzed for %N, %C, and δ¹³C on an Isotope Ratio Mass Spectrometer (IRMS). A clear and significant (p≤ 0.05) treatment effect was observed on the ¹³C isotopic discrimination and sorghum yield and growth attributes over the five seasons. The highest (4.85 Mg ha^-1) grain yield was observed with minimum tillage with crop residue treatment. The δ¹³C values ranged from -13.14to -11.86‰for the sorghum grain. Treatments under minimum tillage with residue and tied ridges and the MBILI intercrop had significantly (p≤ 0.05) higher sorghum grain yield, WUE, stem diameter, chlorophyll content, and high δ¹³C values. The ¹³C discrimination was significantly (p≤ 0.05) associated with yield, WUE, stem diameter, and leaf chlorophyll. In the treatment with high N rate, the equation relating ¹³C discrimination to yield was Yield (Mg ha^-1) = 1.4822δ¹³C + 20.879; R² = 0.3518. A significant positive relationship (R² = 0.31) was observed between grain N fertilizer use efficiency and grain δ¹³C in sorghum harvested from plots with high N rate treatments. There was also a correlation (R² = 0.341; p=0.001) between WUE and sorghum stem diameter. Based on these results, we conclude that grain ¹³C discrimination values at maturity and stem diameter are a potential complementary criterion for assessing sorghum yield performance and WUE under different soil moisture and nutrient availability conditions. Therefore, it can be deduced that minimum tillage with crop residue with a high fertilizer application rate (120N/ha) improves sorghum grain yield, WUE, and higher grain δ¹³C values. The high grain δ¹³C values observed with minimum tillage with crop residue over the five seasons indicate that plants suffered less water stress under minimum tillage with crop residue treatment. Therefore, grain δ¹³C discrimination and stem diameter can be used as water use efficiency proxy with C4 crops like sorghum.