Elevated atmospheric CO2, fine roots and the response of soil microorganisms: a review and hypothesis

There is considerable uncertainty about how rates of soil carbon (C) and ni trogen (N) cycling will change as CO2 accumulates in the Earth's atmosphere . We summarized data from 47 published reports on soil C and N cycling unde r elevated CO2 in an attempt to generalize whether rates will increase, dec rease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbia l immobilization and net N mineralization, because these pools and processe s represent important control points for the below-ground how of C and N. T o determine whether differences in C allocation between plant life forms in fluence soil C and N cycling in a predictable manner, we summarized respons es beneath graminoid, herbaceous and woody plants grown under ambient and e levated atmospheric CO2. The below-ground pools and processes that we summa rized are characterized by a high degree of variability (coefficient of var iation 80-800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration we re more rapid under elevated CO2, indicating that (1) greater plant growth under elevated CO2 enhanced the amount of C entering the soil, and (2) addi tional substrate was being metabolized by soil microorganisms. However, mic robial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elev ated CO2, contributing to a high degree of variability within and between p lant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling w ill change as the atmospheric CO2 concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our abil ity to predict the response of soil microorganisms to rising atmospheric CO 2, and that understanding differences in fine-root longevity and biochemist ry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO2.