Dual-labeling with 15N and H2 18O to investigate water and N uptake of wheat under different water regimes

Water and nitrogen are essential for plant growth and yield. Plants depend on surface roots for nutrient uptake, but frequently rely on deep root systems for water uptake, especially in semi-arid, rain fed crop production systems. We used H-2 O-18 and (NH4)-N-15 (+) tracers in pots to determine water and NH4 (+) uptake at depth by two wheat genotypes watered from the surface or from the bottom. Root traits and transpiration rates were related to water and NH4 (+) uptake. We observed a significant positive relationship between transpiration rate and water uptake measured by H-2 O-18 tracer (R-2 = 0.91), confirming that the H-2 O-18 tracer method was successful. Surface soil drying (bottom watering) decreased both water and NH4 (+) uptake from the top soil. However, increased water availability with bottom watering increased water uptake from the bottom soil layer, but not NH4 (+) uptake from the bottom soil layer. Water uptake was positively related to total root length, while NH4 (+) uptake was positively related to root biomass. With surface soil drying, plants grew more and longer roots in the bottom soil layer, shifting water uptake from the top to the bottom soil, while N uptake was reduced in the top soil because of a decrease in root biomass. Different root traits need to be considered when optimizing water and NH4 (+) uptake by wheat in water deficient seasons.