Decoupled water and nitrogen use strategies in apple trees in the loess deposits

The relationship between nitrogen and water has been extensively studied from the perspective of soil conditions; however, it is unclear how plant nitrogen uptake responds to the water uptake patterns. We measured nitrate dual stable isotopic compositions in soils and trees for the 16- and 22-year-old apple orchards (abbreviated as A16 and A22) to analyze the coupling relationship between water uptake and NO 3 ⁻-N uptake by apple trees in a dry (2022) and a wet year (2023). Apple trees in both orchards mainly absorbed NO 3 ⁻-N from 2 to 6 m (44–70%) in both dry and wet years. Increased absorption of 0–2 m soil water in the wet year corresponded to greater dependence on 2–6 m NO 3 ⁻-N (51% for 2022 vs 61% for 2023). The fractions of NO 3 ⁻-N uptake from 0 to 2 m and 2–6 m were both negatively correlated with the fractions of soil water uptake. Apple trees prioritized adjustments in water uptake over NO 3 ⁻-N acquisition in response to changing climatic conditions, as evidenced by the absolute slopes of the fitted lines between water and NO 3 ⁻-N uptake fractions being less than 1, with values in the 2–6 m greater than those in the 0–2 m. Below thresholds of NO 3 ⁻-N content (20 mg kg −1 for A16 and 80 mg kg −1 for A22), the fractions of 0–2 m soil water increased significantly with NO 3 ⁻-N content. Our findings highlight how apple trees’ NO 3 ⁻-N uptake coordinates with changes in their water uptake patterns, which may help optimize nitrogen fertilizer management and reduce NO 3 ⁻-N accumulation in vadose zone.