Atmospheric CO 2 and the Composition and Function of Soil Microbial Communities

Abstract

Abstract. Elevated atmospheric CO 2 has the potential to increase the production and alter the chemistry of organic substrates entering soil from plant production, the magnitude of which is constrained by soil-N availability. Because microbial growth in soil is limited by substrate inputs from plant production, we reasoned that changes in the amount and chemistry of these organic substrates could affect the composition of soil microbial communities and the cycling of N in soil. We studied microbial community composition and soil-N transformations beneath Populus tremuloides Michx. growing under experimental atmospheric CO 2 (35.7 and 70.7 Pa) and soil-N-availability (low N ϭ 61 ng N·g Ϫ1 ·d Ϫ1 and high N ϭ 319 ng N·g Ϫ1 ·d Ϫ1 ) treatments. Atmospheric CO 2 concentration was modified in large, open-top chambers, and we altered soil-N availability in open-bottom root boxes by mixing different proportions of A and C horizon material. We used phospholipid fatty-acid analysis to gain insight into microbial community composition and coupled this analysis to measurements of soil-N transformations using 15 N-pool dilution techniques. The information presented here is part of an integrated experiment designed to elucidate the physiological mechanisms controlling the flow of C and N in the plant-soil system. Our objectives were (1) to determine whether changes in plant growth and tissue chemistry alter microbial community composition and soil-N cycling in response to increasing atmospheric CO 2 and soil-N availability and (2) to integrate the results of our experiment into a synthesis of elevated atmospheric CO 2 and the cycling of C and N in terrestrial ecosystems.After 2.5 growing seasons, microbial biomass, gross N mineralization, microbial immobilization, and nitrification (gross and net) were equivalent at ambient and elevated CO 2 , suggesting that increases in fine-root production and declines in fine-root N concentration were insufficient to alter the influence of native soil organic matter on microbial physiology; this was the case in both low-and high-N soil. Similarly, elevated CO 2 did not alter the proportion of bacterial, actinomycetal, or fungal phospholipid fatty acids in low-N or high-N soil, indicating that changes in substrate input from greater plant growth under elevated CO 2 did not alter microbial community composition. Our results differ from a substantial number of studies reporting increases and decreases in soil-N cycling under elevated CO 2 . From our analysis, it appears that soil-N cycling responds to elevated atmospheric CO 2 in experimental situations where plant roots have fully colonized the soil and root-associated C inputs are sufficient to modify the influence of native soil organic matter on microbial physiology. In young developing ecosystems where plant roots have not fully exploited the soil, microbial metabolism appears to be regulated by relatively large pools of soil organic matter, rather than by the additional input of organic substrates under elevated CO 2 .

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

“…In a recent experiment, we observed that elevated atmospheric CO # substantially increased net fine-root production only when soil N was in abundant supply . Although the input of organic substrates from fine-root production clearly increased under elevated CO # , we observed no change in the biomass of soil microorganisms during a 3-yr experiment with Populus tremuloides (Zak et al, 2000a). In this soil, fine root biomass was 1\100 of the labile pool of organic matter and 1\1000 of the total organic matter content.…”

“…Reports of microbial immobilization are more abundant in the literature (Table 5), but only a handful of investigators have measured gross N mineralization and microbial immobilization in concert (Berntson & Bazazz, 1997, 1998Hungate et al, 1997a,c ;Mikan et al, 2000 ;Zak et al, 2000a). Beneath grasses and intact grasslands, microbial immobilization exhibited large declines (-64%) and large increases (j501%) ; with few exceptions, these responses are not statistically significant (Table 5).…”

“…For example, rates of soil N cycling have been observed to increase (Zak et al, 1993 ;Hungate et al, 1997a,b), decrease (Diaz et al, 1993 ;Berntson et al, 1997Berntson et al, , 1998 and remain constant (Zak et al, 2000a) under elevated atmospheric CO # , even in the same experiment (Hungate et al, 1996). Understanding the factors that produce these divergent responses is important, because soil N availability controls the extent to which elevated CO # increases plant growth (McGuire et al, 1995 ;Johnson et al, 1997 ;Curtis & Wang, 1998 ;Zak et al, 2000b), which in turn influences the amount of C that ecosystems sequester from the atmosphere.…”

“…However, the flow of substrates through microbial biomass is a key factor influencing soil N availability and C storage, so understanding the turnover of microbial biomass (biomass\assimilation rate) is central to predicting a change in soil C and N cycling under elevated CO # . At present, few studies have simultaneously measured a change in microbial biomass and the assimilation of C or N (Hungate et al, 1996(Hungate et al, , 1997aBerntson & Bazzaz, 1997, 1998Mikan et al, 2000 ;Zak et al, 2000a), making it difficult to predict how elevated CO # will influence the flow of C and N through microbial biomass. Nevertheless, a relatively large number of studies have measured microbial biomass (C or N) under a wide array of experiment settings in which plants are grown under ambient and elevated CO # (Table 3).…”

“…Rising atmospheric CO # has the potential to influence this relationship, because it can increase above-and below-ground plant growth (Poorter, 1993 ;Curtis & Wang, 1998), alter the production and chemical constituents of plant litter (Cotrufo et al, 1994 ;Cotrufo & Ineson, 1995 ;King et al, 1997) and influence the types of organic substrate available for microbial metabolism in soil. Therefore, changes in litter production under elevated CO # could alter the microbial demand for N and the flow of N between soil microorganisms and plant roots, a subject that has received growing attention over the past several years (Berntson & Bazzaz, 1997, 1998Hungate et al, 1997aHungate et al, ,b, 1999Zak et al, 2000a).…”