Short- and long-term effects of soil moisture on soil respiration in an apple orchard

Although CO2 flux observations by eddy covariance technique have many advantages, the nocturnal CO2 efflux remains uncertain. To complement these observations and clarify the carbon budget/dynamics of an apple orchard, we applied an open chamber method and measured the soil respiration rate in September 2006-July 2010 for 237 days. We analyzed the temperature and moisture dependency of the soil respiration rate. The daily soil respiration rate, when normalized by soil temperature, was maximum when soil moisture was at the field capacity, and decreased linearly with the increase and decrease of the soil suction (pF). Excluding the negative effect of the pF, the exponential relationship between the daily soil respiration and the soil temperature became slightly closer, with the determination coefficient increasing from 0.76 to 0.84. The temperature sensitivity indicator Q(10) fluctuated inter-annually from 2.0 to 3.1, and seemed to depend on the soil moisture status of each year. We also constructed a mathematical model to calculate the daily soil respiration from the soil temperature and pF. The mean annual soil respiration estimated by the model was 616 gC m(-2), being 208 gC m(-2) lower in 2008 than in 2009. We sampled mowed grass and found that the carbon accumulated was 140 gC m(-2) lower in 2008 than in 2009. The low annual soil respiration in 2008 could be almost entirely explained by the reduced mowed grass production likely caused by the drought conditions of that year.