ABSTRACT
Soil microbes critically affect plant and ecosystem responses to climate change by modulating organic C decomposition and nutrient availability for plants. Experimental evidence accumulated over the last several decades has clearly shown that climate change factors such as CO2 enrichment in the atmosphere can significantly alter plant growth [1], [2] and the availability of organic C, N and cation nutrients for microbes [3], [4], [5]. Ozone is a greenhouse gas with demonstrated inhibitory effects on plant growth and resource allocation belowground [6], [7]. Although less wellstudied, O3 is considered to have an impact on soil microbial processes [6]. Alterations in soil microbes can, in turn, profoundly influence soil C processes and the long-term potential of terrestrial ecosystems as a C sink to mitigate anthropogenic sources of atmospheric CO2. However, predicting what these changes will be is hampered by our limited understanding
The current prevailing hypothesis, building on the assumption that soil microbes are generally C limited [8], predicts that elevated CO2 increases soil microbial biomass and activities due to enhanced soil C availability [9], [10], [11], whereas O3 reduces them due to lower C allocation belowground [6], [12], [13]. This broad hypothesis has been extensively tested over the past two decades for CO2 but less so with O3 [4], [10], [12], [13], [14], [15], [16]. Though C availability to microbes has been commonly reported to increase under elevated CO2 [5], [17], [18] and to decrease under elevated O3 [6], [13], [19], results of soil microbial responses to elevated CO2 and O3 have been inconsistent [6], [11], [20], [21]. In a meta-analysis study, de Graaff et al. (2006) found that elevated CO2 increased microbial biomass C and microbial respiration by 7.7% and 17.1%, respectively, across 40 studies that mainly included herbaceous species. In the meantime, Hu et al. (2006) reviewed 135 studies examining elevated CO2 effects on a suite of soil microbial parameters such as microbial biomass and respiration and found that microbial biomass C and microbial respiration increased under elevated CO2 in 19 of 40 studies and 20 of 38 studies, respectively, but remained unchanged or even decreased in the remainder. Despite considerable efforts in the past two decades, there is a lack of conceptual understanding of why and how these inconsistencies in CO2 and O3 effects on microbes occur.
