OPTIMIZING THE SEEDING RATE IN SUBSOILING FOR DRYLAND WHEAT (TRITICUM AESTIVUM L.): A KEY TO IMPROVING WATER USE EFFICIENCY AND NITROGEN USE EFFICIENCY IN THE LOESS PLATEAU OF CHINA

The effects of seeding rate and tillage on water use and nitrogen use efficiencies were studied based on a field experiment carried out from 2012 to 2016. The results showed that subsoiling significantly increased the soil water storage before sowing in all years. The water consumption (ET) during the four years was, on average, 461 and 301 mm for subsoiling (SS) and CK, respectively. The average yield of SS was 4821 kg/ha, which was 944 kg/ha higher than that of CK. The average yield was highest at the mid-seeding rate for both SS and CK, with values of 5115 and 4394 kg/ha, respectively. Since the nitrogen fertilizer application rate was the same, a higher yield indicates a higher nitrogen use efficiency (NUE). Contribution analysis showed that the higher seeding rate led to a lower contribution of nitrogen uptake efficiency (UPE) and a higher contribution of NHIp to NUEp. In summary, subsoiling during summer fallow period increased the soil water retention and improved the wheat yield and WUE. It also increased nitrogen uptake and NUEp. The mid-seeding rate (approximately 90 kg/ha) under subsoiling is optimal for improving water use efficiency (WUE) and NUE in the Loess Plateau.