Precision subsurface drip irrigation increases yield while sustaining water-use efficiency in Mediterranean poplar bioenergy plantations

Bioenergy production in poplar Short Rotation Coppice plantations (SRC) is strongly limited in drought prone areas due to the high crop water requirement. Appropriate scheduling of subsurface drip irrigation (SDI) could be a practice for ensuring adequate biomass production with reduced water inputs while maintaining high water-use efficiency. We tested SDI in a commercial SRC cultivated with the hybrid poplar clone Monviso under Mediterranean environmental conditions. We applied two irrigation treatments during the summer season, i.e. a control irrigation treatment with an average amount of 115 mm (CI) and a double irrigation treatment for an average amount of 239 mm (DI) over two growing seasons of the second triennial rotation. We analyzed tree growth, yield, shoot diameter increments (PDI) and carbon isotope composition (δ13C) in both litterfall and tree-rings. We also measured soil moisture at 10, 20, 30, 40, 60 and 100 cm soil depths to explore more efficient irrigation scheduling. The results showed that CI and DI recovered 23-49 and 43-90% of the April-September precipitation deficit over the two growing seasons, respectively. We observed higher yield increments in DI compared to CI, with mean yields of 11.4 and 20.4 Mg ha−1 for CI and DI respectively. DI significantly affected biomass quality (biomass allocated to shoots with greater dimensions); however, stem moisture and shoot basal density did not significantly change after the irrigation treatments. δ13C in tree-rings showed non-significant differences after CI and DI applications for two growing seasons. Congruently, the analysis of litterfall δ13C did not show significant differences comparing the two irrigation regimes. Thus, the isotopic analyses indicate a constancy of intrinsic water-use efficiency (iWUE), irrespective of the watering regime. We found significant positive linear relationships (R2 from 0.89 to 0.96) between PDI and soil moisture at 30 and 40 cm soil depths for both CI and DI when compared to the rest of the monitored soil layers. We suggest, therefore, the monitoring of soil moisture at 30-40 cm as a reference for scheduling irrigation practices during the growing season. In conclusion, DI significantly increased the overall plantation yield while sustaining the same iWUE observed in the deficit irrigation regime (CI).