Atmospheric CO 2 , Soil-N Availability, and Allocation of Biomass and Nitrogen by Populus tremuloides

Abstract

Abstract. Our ability to predict whether elevated atmospheric CO 2 will alter the cycling of C and N in terrestrial ecosystems requires understanding a complex set of feedback mechanisms initiated by changes in C and N acquisition by plants and the degree to which changes in resource acquisition (C and N) alter plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumnal senescence (early vs. late) under experimental atmospheric CO 2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO 2 concentrations were manipulated with open-top chambers, and soil-N availability was modified in open-bottom root boxes by mixing different proportions of native A and C horizon soil. Net N mineralization rates averaged 61 ng N·g Ϫ1 ·d Ϫ1 in low-N soil and 319 ng N·g Ϫ1 ·d Ϫ1in high-N soil. After 2.5 growing seasons, we harvested above-and belowground plant components in each chamber and determined total biomass, N concentration, N content, and the relative allocation of biomass and N to leaves, stems, and roots. Elevated CO 2 increased total plant biomass 16% in low-N soil and 38% in high-N soil, indicating that the growth response of P. tremuloides to elevated CO 2 was constrained by soil-N availability. Greater growth under elevated CO 2 did not substantially alter the allocation of biomass to above-or belowground plant components. At both levels of soil-N availability, elevated CO 2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO 2 significantly increased whole-plant N content in high-N soil (ambient ϭ 137 g N/chamber; elevated ϭ 155 g N/chamber), but it did not influence whole-plant N content in low-N soil (36 g N/chamber). Our results indicate that plants in high-N soil obtained greater amounts of soil N under elevated CO 2 by producing a proportionately larger fine-root system that more thoroughly exploited the soil. The significant positive relationship between fine-root biomass and total-plant N content we observed in high-N soil further supports this contention. In low-N soil, elevated CO 2 did not increase fine-root biomass or production, and plants under ambient and elevated CO 2 obtained equivalent amounts of N from soil. In high-N soil, it appears that greater acquisition of soil N under elevated CO 2 fed forward within the plant to increase rates of C acquisition, which further enhanced plant growth response to elevated CO 2 .

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

“…In California annual grasslands, in which plant C inputs under elevated CO # can increase or decrease microbial biomass, soil organic matter is 13-15-fold that in plant roots and belowground detritus (ambient CO # , calculated from Hungate et al (1997a,c)). Plant roots in these grassland soils are equivalent to the biomass of soil microorganisms, in contrast to our experiment in which microbial biomass was 50-90-fold that in plant roots (calculated from Pregitzer et al (2000) and Zak et al (2000b)). Similarly, microbial biomass and activity were not altered by elevated CO # in a calcareous grassland in which microbial biomass was 0.05% of the soil organic matter content (Niklaus, 1998).…”

“…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. Consequently, it will be very difficult to predict long-term changes in ecosystem C storage as atmospheric CO # increases without discerning the mechanism(s) leading to the varied response of soil N cycling.…”

“…There is considerable debate regarding the response of soil N dynamics to elevated CO # , because observations indicate that rates of soil N cycling can increase (Zak et al, 1993 ;Hungate et al, 1997a), decline (Berntson & Bazzaz, 1997, 1998 or not change (Zak et al, 2000b), even within the same ecosystem (Hungate et al, 1996). Understanding changes in rates of gross N mineralization and microbial immobilization lie at the heart of predicting whether elevated CO # will alter the amount of N available for plant uptake (i.e.…”

Dry nitrogen deposition estimates over a forest experiencing free air CO2 enrichment

“…Most investigations to date show an increase in woody biomass under elevated atmospheric CO 2 (e.g. Bazzaz & Miao, 1993;Curtis & Wang, 1998;Zak et al, 2000;Hamilton et al, 2002) and a potential role of woody ecosystems in sequestering carbon in the future (Schimel et al, 2001). However, the growth response to elevated CO 2 in woody plants appears, in many systems, to be constrained by the availability of soil nutrients, most notably N (Pan et al, 1998;Luo et al, 1999;Zak et al, 2000;Oren et al, 2001).…”

Nitrogen supply effects on leaf dynamics and nutrient input into the soil of plant species in a sub-arctic tundra ecosystem

“…Increased N supply leads to higher leaf production rates (Aerts et al 1992;Thornton and Millard 1993;Poorter and Nagel 2000;Zak et al 2000), whereas the life expectancy of leaves of single species is usually reduced in response to increased N supply (Reader 1980;Shaver 1983;Aerts 1989;Cordell et al 2001;Reich et al 2004). However, in some other studies leaf life expectancy was higher at high than at low N supply (Turner and Olson 1976;Bazzaz and Harper 1977) or there was no eVect at all (Aerts and De Caluwe 1995).…”