Soil respiration and the contribution of root respiration of cotton ( Gossypium hirsutum L.) in arid region

In northwest China, cotton field is the main cropping system in dry-land agriculture system, and the root respiration of cotton is an important component for the local carbon cycling. Our objectives were to characterize the temporal variation of soil respiration (Rt) and the contribution of root respiration of cotton. A field study was conducted to estimate root respiration by gap information method, and divide the components of root respiration (Rr) and heterotrophic respiration (Rh). Soil respiration was measured by using CIRAS chamber from August to October in 2012. Mean values of Rt and Rh were 2.65 ± 1.43 and 0.86 ± 0.51 µmol m−2 s−1, respectively. Rr estimated from gap information method was 1.98 ± 1.15 µmol m−2 s−1, and the contribution of root respiration (Rr %) was 71.37 ± 7.97% during the study period. Overall, Rr was the main component of soil respiration in cotton field. Soil abiotic and biotic factors played critical roles in the dynamics of soil respiration. Soil respiration increased exponentially with soil temperature, and soil temperature at 10 cm depth could explain 84.9, 70.5 and 82.0% of variations for Rt, Rh and Rr, respectively. The values of Q10 were 2.25, 1.97 and 2.36 for Rt, Rh and Rr, respectively, indicating that root respiration was more sensitive to soil temperature than heterotrophic respiration. R10 also showed a hyperbolic relationship with soil water content, which explained 50.4 and 56.4% of variations for Rt10 and Rr10, respectively. However, we found no significant correlation between soil water content and Rh10, indicating that excessive soil water mainly depresses root activity. In addition, soil respiration and CO2 concentration were significantly influenced by irrigation. After farmland irrigation, the reduction of CO2 concentration at 5 and 10 cm depths was probably due to the limitation of soil CO2 production and exchange with atmosphere.