Invasion of a semi-arid shrubland by non-native annuals increases soil respiration rates and alters sensitivity to moisture and temperature

Background/Question/Methods

Soil carbon (C) storage and respiration (CO₂release) of natural ecosystems plays an important role in the global C cycle. Ecosystem C storage is strongly influenced by vegetation type and soil  respiration rates. This study investigated how shifts in dominant vegetation affect the relative contribution of plant roots and microbes to total soil respiration. In Southern California invasion by exotic annuals converts shrublands to annual dominated vegetation, fundamentally altering their structure and function. Shrub to annual conversion alters above and belowground C allocation, phenology, soil microbial communities, soil moisture and temperature. Understanding how plant and microbe respiration respond to vegetation changes will lead to a firmer grasp on the mechanisms underlying changes in C storage.

Respiration was measured under shrubs and annuals from 2010-2011 to capture seasonal transitions in soil temperature and moisture. Respiration, soil moisture and temperature were measured continuously at four hour intervals. Root exclusion treatments were used to quantify heterotrophic respiration; total respiration was measured in undisturbed areas. The effect of invasion on total respiration (Rt) and heterotrophic respiration (Rh) responses to seasonal changes in temperature and moisture was evaluated.

Results/Conclusions

Invasion increased root and microbial respiration rates resulting in higher Rt and Rh. Invasion altered temperature and moisture sensitivity such that respiration rates showed a more rapid temperature response and had a higher temperature threshold compared to shrubs. When soils were dry Rh declined more rapidly in invaded areas than under shrubs.

During the growing season Rt was almost double Rh in both vegetation types. At the end of the growing season Rt and Rh converge at low respiration levels. This system is water limited and soil moisture declines as soil temperature increases. Thus soil moisture mediates the seasonal response of respiration to temperature.  Respiration rates increased with temperature when soils were wet and decreased when soils were dry. Under wet conditions temperature was a better predictor of Rh than Rt indicating other factors, such as phenology and rooting depth, are important for Rt. 

Soil respiration in this invaded shrubland showed strong seasonal patterns controlled by interactions between soil moisture and temperature. Invasion of native shrublands by exotic annuals increased both Rt and Rh indicating changes in plant C allocation and decomposition rates of soil C. Annual invasion changed seasonal patterns of respiration due to altered sensitivity to temperature and moisture. Encroachment of annuals into shrubland has important implications for the future of C storage in this system.