Global atmospheric change effects on terrestrial carbon sequestration: Exploration with a global C- and N-cycle model (CQUESTN)

Abstract

A b s t r a c tA model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past and possible future) in atmospheric CO2 concentration, temperature and N-deposition. The model is based on published estimates of pre-industrial C and N pools and fluxes into vegetation, litter and soil compartments. It was found necessary to assign low estimates of N pools and fluxes to be compatible with the more firmly established C-cycle data. Net primary production was made responsive to phytomass N level, and to CO2 and temperature deviation from preindustrial values with sensitivities covering the ranges in the literature. Biological N-fixation could be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation intensities. As for all such simulation models, uncertainties in both data and functional relationships render it more useful for qualitative evaluation than for quantitative prediction.With the N-fixation response turned off, the historic CO2 increase led to standard-model sequestration into terrestrial ecosystems in 1995AD of 1.8 Gt C yr -l. With N-fixation restoring humus C:N strongly, C sequestration was 3 Gt yr -1 in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimental data on CO2 effects. The temperature increase also caused net C sequestration in the model biosphere because decrease in soil organic matter was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere "leakiness" to N would have to increase substantially with temperature. Assuming a constant N-loss coefficient, the historic temperature increase alone caused standard-model net C sequestration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C-sequestration depends substantially on whether the deposited N is initially taken up by plants or by soil microorganisms. Assuming the latter, standard-model net sequestration in 1995 was 0.2 Gt C in 1995 from the N-deposition effect alone. Combining the effects of the historic courses of CO2, temperature and N-deposition, the standard-model gave C-sequestration of 3.5 Gt in 1995. This involved an assumed weak response of biological N-fixation to the increased carbon status of the ecosystem. For N-fixation to track ecosystem C-fixation in the long term however, more phosphorus must enter the biological cycle. New experimental evidence shows that plants in elevated CO2 have the capacity to mobilize more phosphorus from so-called "unavailable" sources using mechanisms involving exudation of organic acids and phosphatases.

CO2 flux estimates tend to overestimate ecosystem C sequestration at elevated CO2

“…In N-limited ecosystems such as the grassland investigated here, ecosystem C storage eventually becomes N-limited because C accumulation in soils requires concomitant sequestration of N (Gifford, Lutze & Barrett 1996;Rastetter et al 1992). This is also true for most natural and semi-natural ecosystems (Vitousek & Howarth 1991) in which longer-term organic C accumulation in soils is constrained to 1-12 g C m -2 a -1 (compiled data from a survey covering different biomes by Schlesinger 1996).…”

Steady state turnover time of carbon in the Australian terrestrial biosphere

“…Gifford [1994] and Gifford et al [1996] reported experimental results for r integrated over a 24 hour period of between 0.35 and 0.55 for wheat, pea, cotton, sorghum, bean, Eucalyptus and Pinus despite 2 orders of magnitude difference in plant mass and with growth temperatures ranging from 15°to 30°C. For wheat in another study, r fell between 0.39 and 0.44 [Gifford, 1995].…”

Nitrogen deposition: a component of global change analyses

“…The N that enters the soil is competed for by roots and microbes, the outcome of this competition depending on how the N is added and the initial conditions of the plants and microbial populations (Aber et al, 1989 ;Johnson, 1992). Gifford et al (1996) concluded that the assumption about how the exogenous N is initially taken up is a critical one that should be further investigated. Hudson, Gherini & Goldstein (1994) employed a very different approach to the calculation of Nstimulated C storage.…”

“…Their conclusion, based on results of CO # -manipulation and N-fertilization experiments, was that the ecosystem-level response of C assimilation to elevated CO # might depend on how N uptake by, and N recycling within, the vegetation influence the ability of plants to incorporate elevated CO # in the construction of canopy, stem, and root biomass. In TEM, N is allocated to represent the trade-off between canopy development and acclimation of tissue-level photosynthesis so that C uptake is maximized in building vegetative biomass at a specific C : N ratio which is not constant, however, because of seasonal changes in resorption and mobilization of N. Gifford et al (1996), using CQUESTN, emphasized that, because of compensations between C and N cycles, different model assumptions can lead to similar net results. For example, if a CO # stimulation of N mineralization was included (cf.…”