Influence of density correction on latent heat, CO₂ flux and energy balance closure in winter wheat/summer maize rotation fields

In the past two decades, the eddy covariance technique has been used as a normalized measure method of exchange of CO₂, water vapor and heat between vegetation and the atmosphere. To understand the variation characteristics of CO₂, water vapor and heat fluxes of winter wheat/summer maize rotation system, an experiment was conducted in the period from June 2013 to June 2014 at Qingdao Modern Agricultural Demonstration Farm of Qingdao Agricultural University. The CO₂, water vapor and heat fluxes were measured during both winter wheat and summer maize growing seasons using the eddy covariance method and the values corrected by two density correction methods (WPL correction and Liu correction), and then the difference between the two methods compared. An energy balance closure was also computed during bare and vegetation cover periods of both winter wheat and summer maize based on the two density correction methods. It was found that both of the two methods (WPL correction and Liu correction) increased the amount of latent heat flux. WPL correction method increased latent heat flux by about 6% for summer maize season and 2% for winter wheat season, while Liu correction method increased latent heat flux by less than 1% for summer maize season and about 2% for winter wheat season. WPL correction method reduced the amount of CO₂ flux by less than 3% for summer maize and 4% for winter wheat. Also the Liu correction method reduced the amount of CO₂ by about 2% for summer maize and 3% for winter wheat. There was little difference (about 1%) between the two correction methods for the correction of both latent heat and CO₂ fluxes. The two correction methods had the potential to increase energy balance closure. WPL correction obviously increased energy balance closure by about 2.53%9.76% for bare soil, by 4.05% for summer maize and by 1.35% for winter wheat. Then Liu correction increased energy balance closure by less than 2.53% for bare soil and 1.35% for both summer maize and winter wheat seasons. This suggested that the degree of correction by WPL method was greater than that by Liu method. Energy balance closure during bare soil period was obviously higher than that during vegetation cover period. The order of the energy balance closure was: bare soilⅠ(before maize seed emergency) > bare soil Ⅱ (before winter wheat seed emergency) > summer maize vegetation period > winter wheat vegetation period.