Effects of the conversion of marshland to cropland on water and energy exchanges in northeastern China

To study the effect of land use change from marshland to croplands (rice and soybean cultivation), the water and energy fluxes in marshland, rice and soybeans were measured in northeastern China by the eddy covariance method for two growing seasons (May-October in 2005 and 2006). The rice (flooded until early September) and soybean (dry land) croplands were reclaimed from marshlands over 15 years ago. The annual precipitation in 2005 (480mm) was below the 1981-2004 average (564±130mm), the precipitation in 2006 (655mm) was significantly above average. The conversion of marshland to rice cropland resulted in an enhancement in latent heat flux (LE) and a decrease in sensible heat flux (H), which caused an increase in the ratio of LE and net radiation (LE/Rn) from 0.6 to 0.8 on average, and a decrease in H/Rn from 0.4 to 0.2 on average for the growing season. However, where the marshland was converted to soybean cropland, the latent heat flux was lower in 2005 under dry conditions, but higher in 2006 under wet conditions than it had been previously been, which was the opposite of the trend of H. Therefore, LE/Rn for soybean dropped to about 0.2 in the dry month of June 2005, and rose to 0.7 in the wet month of June 2006. H/Rn for soybeans shows opposite variation in LE/Rn in both years. The maximum evapotranspiration (ET) was about 4.5, 6-7 and 5mmday⁻¹ for marshland, rice and soybeans, respectively. Rn, air temperature (Ta), vapor pressure deficit (VPD) and leaf area index (LAI) were important environmental and biological factors controlling the seasonal dynamics of LE and H for the three ecosystems, which explains about 80% of LE and 50% of H for marshland and rice; and 30-50% of LE and 60% of H for soybeans. Among them, Rn was the primary variable controlling the variations of LE and H for marshland and rice. However, for soybeans, VPD played a more important role in LE under dry conditions.