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

Abstract

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO # accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO # in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO # . The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80-800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO # , indicating that (1) greater plant growth under elevated CO # enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO # , contributing to a high degree of variability within and between plant 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 will change as the atmospheric CO # concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO # , and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO # .

Despite strong seasonal responses, soil microbial consortia are more resilient to long-term changes in rainfall than overlying grassland

“…Microbial composition and function are sensitive to variability and extremes in soil conditions (Stark and Firestone, 1996;Gulledge and Schimel, 1998;Fierer et al, 2003), and researchers working within global change experiments have found that environmental perturbations can impact both individual bacterial groups (Horz et al, 2004(Horz et al, , 2005 and aggregate communitylevel properties such as biomass and respiration (Zak et al, 2000). We detected changes in microbial abundance and composition in response to climatic amendment, but sampling repeatedly across seasons and years found that these responses were short lived and left little legacy.…”

Carbon and oxygen isotope analysis of leaf biomass reveals contrasting photosynthetic responses to elevated CO<sub>2</sub> near geologic vents in Yellowstone National Park

“…We hypothesize that decreased photosynthetic capacity observed in our case could be due to severe reduction in plant available nitrogen in this forest stand due to phenomenon known as progressive nitrogen limitation (PNL). PNL develops because elevated CO 2 leads to N immobilization by plants and microbes which deplete soils of N, causing slower rates of N mineralization progressively reducing the mineral N available for plant uptake in the long term (Gill et al, 2002;Zak et al, 2000;Luo et al, 2004).…”

Estimating the effect of nitrogen fertilizer on the greenhouse gas balance of soils in Wales under current and future climate

“…The SOC fluxes were changed from sinks at baseline climate to small sources under future climate scenarios, with a small difference between the two scenarios (±2 %). The future increased plant photosynthesis due to high CO 2 concentration increases plant growth, belowground C input and substrate, leading to greater root and microbial activities and respiration (Zak et al 2000). Previous studies indicate that prediction of soil C fluxes in response to climate change should consider changes in biotic factors, e.g.…”

“…Temperature is one of the main driving factors affecting C flux from soils (Jabro et al 2008). The increase in plant growth and aboveground biomass produces more litter fall and may in the short term lead to higher C loss through soil respiration (Zak et al 2000;Deng et al 2010), but also to longer-term SOC accumulation. Both soil organic matter decomposition and microbial response to other perturbations, such as fertilization, temperature and rainfall, can increase (Wennman and Katterer 2006).…”