Impact of temperature and defoliation (simulated grazing) on soil respiration of pasture grass (Cenchrus ciliaris L.) in a controlled experiment

A controlled experiment was conducted on Cenchrus ciliaris L. grass (exotic to Australia) commonly grown in Queensland pastures to investigate the impact of defoliation (simulated grazing), temperature and soil moisture on total soil respiration, and to isolate different components of total soil respiration i.e. the root, root free soil and rhizosphere respiration. The six types of treatments i.e. control (soil only without grass (C1)), control with grass but no defoliation (C2) grown for 9 months, non-defoliated treatments with grass grown for 4months (D0), and three defoliation treatments (grass defoliated once, D1; twice, D2; and thrice, D3 during growth) were maintained over 9 months. Our results suggested that defoliation had no effect on total soil respiration. However, soil temperature accounted for significant changes in total soil respiration across all thedefoliation and C2 treatments but not in D0, and the greatest change in soil respiration in response to temperature was noted at the third stage of defoliation, suggesting that defoliation increased the sensitivity of soil respiration to temperature. Root respiration was significantly (P <0.05) related to root biomass and greaterroot biomass contributed mainly to increased rate of total soil respiration. The greater sensitivity of total soil respiration to temperature in D1, D2, D3 and C2 treatments and the greater contribution of root respiration in total soil respiration suggests that the root respiration, rather than the total soil respiration, is likely to be more sensitive to change in temperature. With rising ambient temperature and consequently soil temperature, soil CO₂ emissions may increase in a pasture with greater root biomass than that with lesser root biomass.