Soil respiration is a major process affecting the global carbon cycle and nutrient flux in the terrestrial ecosystem. It is the major pathway for exchange of gases from soil to atmosphere, influencing atmospheric temperature and ultimately contributing to global warming. Soil carbon is returned from the soil to the atmosphere through soil respiration, which represents one of the largest fluxes in the terrestrial C cycle^1–3. The main sources of terrestrial flux of CO₂ are decomposing soil organic matter, respiration from heterotrophic soil organisms and autotrophic live root respiration.

Soil temperature and soil moisture are the most important environmental factors controlling soil respiration in ecosystems. However, soil respiration and associated edaphic factors have not been widely studied in semi-arid regions. In this study, soil respiration was measured in a riparian zone and the effect of soil temperature and soil moisture on soil respiration was examined under five forest species of the semi-arid region.

The mean daily soil respiration rate was 1.82, 2.08, 2.35, 2.27 and 2.07 g C m^–2 d^–1 in Jatropha curcas, Leucaena leucocephala, Acacia nilotica, Azadirachta indica and Prosopis juliflora sites respectively. It was found that soil respiration was significantly and positively correlated with soil moisture. A univariate model of sub-surface soil moisture could explain 77 per cent of temporal variation in soil CO₂ efflux, irrespective of species and sites. The logarithmic model could best explain the relationship between soil respiration and soil moisture at 10–20 cm of soil depth (P < 0.001). There were negative correlations between soil respiration and soil temperature under majority of species.

Overall, across all species, soil temperature poorly explained 26 per cent variation in soil respiration as independent variable. For soil respiration rate–temperature relationship, a bell-shaped function gave the best fit in this ecosystem. Under all the species, soil respiration increases with increase in soil temperature up to 33°C (± 2); thereafter it decreases gradually under all species. There was strong evidence that deficit of soil moisture rather than soil temperature was the main regulating factor of soil respiration under the semi-arid ecosystems. 

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