Effects of fertilizer and weeding on stable isotope composition (¹³C–¹⁸O) in different carbohydrate pools across the cassava canopy

Intrinsic water use efficiency (iWUE) is a critical characteristic for optimizing cassava (Manihot esculenta Crantz) performance under climate change. Stable isotope composition provides a valuable tool for estimating iWUE, yet the key drivers of isotope variation across the cassava canopy remain unclear. In this study, conducted at 17 farms across three agroecological zones in the Eastern Democratic Republic of Congo, we examined how agronomic practices (fertilizer application and weeding) influence carbon (δ¹³C) and oxygen (Δ¹⁸O) isotope composition at different canopy positions and in carbohydrate pools during the bulk root initiation stage. Physiological and morphological variables were measured at noon across the upper, middle, and lower canopy of cassava plants grown on-farm during the rainy season. These variables were related to δ¹³C and Δ¹⁸O in bulk leaf material, extracted cellulose, and soluble sugars.

Fertilizer application increased δ¹³C of soluble sugars (+0.6 ‰, p < 0.1) and bulk (+0.3 ‰, p < 0.1) in the drier zone, suggesting enhanced iWUE under fertilized conditions. Path analysis showed that leaf nitrogen concentration became increasingly correlated with δ¹³C from the upper to the lower canopy, while the influence of stomatal conductance declined. In upper-canopy leaves, higher stomatal conductance was associated with elevated vapour pressure deficit (VPD), possibly due to co-varying increased light intensities. Assumptions of the dual isotope approach related to Δ¹⁸O were not met, and therefore require further investigation. These findings provide new insights into the drivers of iWUE in cassava, highlighting the roles of canopy position and agronomic practices. This knowledge can inform strategies to improve cassava resilience and productivity under climate change.