The effects of subsurface irrigation at different soil water potential thresholds on the growth and transpiration of Populus tomentosa in the North China Plain

In order to find the optimal subsurface drip irrigation (SDI) scheduling for mature triploid Populus tomentosa plantations in the North China Plain, a field experiment was conducted in 2010 and 2011 to investigate the effects of SDI at different soil water potential (SWP) thresholds on the growth and transpiration of a P. tomentosa plantation when it was six and seven years old. The experiment included three SWP treatments, which initiated irrigation when the SWP at 20cm depth and 10cm distance from a drip emitter reached -25 (T-25), -50 (T-50) and -75 (T-75) kPa, respectively. A control non-irrigation treatment (CK) was also included. Long-term SWP, soil water content (SWC), transpiration, tree growth, meteorological factors and groundwater level were monitored. Results showed that SDI influenced the SWC only in 0-80cm of soil. From April to July, on average, the cumulative stand-level transpiration on a ground area basis (E) and growth of basal area at breast height accounted for 81% and 93% of their corresponding whole-season values, while the cumulative reference crop potential evapotranspiration (ET0) was 43% higher than the rainfall. In contrast, from August to October, the growth rate of P. tomentosa was very slow, while the cumulative rainfall was 36% higher than ET0 and the average groundwater level was relatively high (125cm). Relative to CK, the E under the SWP treatments was significantly (P<0.05) increased by 20-73%. Decreasing the SWP irrigation threshold from -25 to -50kPa significantly reduced E by 31% (P<0.05), but decreasing the threshold from -50 to -75kPa did not further reduce P. tomentosa E. Relative to CK, T-25, T-50 and T-75 increased the annual above-ground dry biomass (ADB) increment by 54% (P<0.05), 34% (P<0.05) and 24% (P > 0.05) in 2010, and by 28% (P > 0.05), 29% (P > 0.05) and 32% (P < 0.05) in 2011, respectively. However, no significant difference in ADB increment was detected among the SWP treatments. Based on these results, it can be concluded that when planting P. tomentosa at sites with similar characteristics to ours in the North China Plain: (1) SDI could be promoted in the cultivation of P. tomentosa to improve tree growth; (2) a range of -50 to -75kPa at a depth of 20cm and 10cm distant from a drip emitter is recommended as the irrigation threshold for scheduling SDI in P. tomentosa plantations and (3) irrigation should be applied between April and July, while drainage should be implemented between August and October.