WATER USE EFFICIENCY IN DRYLAND FARMING OF SOYBEANS IN THE SANJIANG PLAIN, CHINA

Water use efficiency, carbon fluxes, and turbulent fluxes at a dryland soybean site were studied at the Sanjiang Ecological Station in China. The carbon and turbulent energy fluxes were measured by an open path eddy-covariance technique in 2005, along with complementary measurements of soil moisture, leaf area index and biomass, and meteorological parameters. The cropland was reclaimed from wetlands in 1993 and is now used for dry land agriculture. Results showed that the maximum of carbon absorbance (-0.96mg.CO2.m-2.s-1) appeared at the seed filling stage. The diurnal variations of latent heat flux (LE) and sensible heat flux (H) had the same diurnal variation characteristics and showed the shape of an inverted "V" during the soybean growing season, with LE close to zero and H negative at nighttime; both reached a maximum at midday following the diurnal variation of solar radiation. The ratio of LE/Rn was high during the tillage period because of strong soil evaporation, and then decreased quickly. The ratio of LE/Rn increased with the development of the soybean plants and reached a peak value of 0.68 at the seed filling stage. In contrast, the trend of H/Rn was inversely related with LE/Rn, and the ratio of ΔQs/Rn increased in the spring time and decreased in the autumn with the decline in air temperature. Evapotranspiration was the main consumption component of solar radiation from anthesis to maturity stages. Water use efficiency (WUE) of soybean plants throughout the growing season showed a pattern of reverse "U" type, WUE increased with sunrise and decreased with sunset, with little change from 6:00-17:00 hr. Thus, there was no clear peak value. The WUE increased with the soybean development after seedling emergence. The maximum values of WUE were 0.0004, 0.0169,0.0266, 0.0241 and 0.0085 mg.mg'1 at seeding, branching, anthesis, pod-setting, seed filling- maturity stages respectively. The WUE of soybean was larger than the WUE of Carex lasiocarpa wetland in June, July and August. There was a positive correlation between LAI and WUE; the Pearson correlation coefficient is 0.5076(n=59). However there was only a significant correlation between above ground biomass and WUE during the period of increasing LAI. At the crop level, WUE of biomass and grain reached 17.14 kg.hm-2.mm-1 and 7.12 kg.hm-2.mm-1 respectively; it was similar to the common spring soybean water use efficiency 6.2 kg.hm-2.mm-1, and slightly higher than the results in Hailun.