Hyrum B. Johnson

Nonlinear grassland responses to past and future atmospheric CO2

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to more than 500 µmol mol-1 this century from interglacial levels of less than 200 µmol mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550 µmol mol-1 (refs 9, 10), increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in Ca there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.

Increasing CO2 and plant-plant interactions: effects on natural vegetation

Plant species and functional groups of species show marked differences in photosynthesis and growth in relation to rising atmospheric CO2 concentrations through the range of the 30 % increase of there cent past and the 100 % increase since the last glaciation. A large shift was found in the compositional mix of 26 species of C3’s and 17 species of C4’s grown from a native soil seed bank in a competitive mode along a CO2 gradient that approximated the CO2 increase of the past 150 years and before. The biomass of C3’s increased from near zero to 50 % of the total while that of the C4’s was reduced 25 % as CO2 levels approached current ambient. The proposition that acclimation to rising CO2 will largely negate the fertilization effect of higher CO2 levels on C3’s is not supported. No signs of photo synthetic acclimation were evident for Avena sativa, Prosopis glandulosa, and Schizachyrium scoparium plants grown in subambient CO2. The effects of changing CO2 levels on vegetation since the last glaciation are thought to have been at least as great, if not greater, than those which should be expected for a doubling of current CO2 levels. Atmospheric CO2 concentrations below 200 ppm are thought to have been instrumental in the rise of the C4 grasslands of North America and other extensive C4 grasslands and savannas of the world. Dramatic invasion of these areas by woody C3 species are accompanying the historical increase in atmospheric CO2 concentration now in progress.

Viewpoint: atmospheric CO2, soil water, and shrub/grass ratios on rangelands.

The abundance of woody plants on grasslands and savannas often is controlled by the availability of water and its location in soil. Water availability to plants is limited by precipitation, but the distribution of soil water and period over which it is available in these ecosystems are influenced by the transpiration rates of grasses. We discuss implications of recent and projected increases in atmospheric CO2 concentration for transpiration, soil water availability, and the balance of grasses and shrubs. An increase in CO2 concentration often reduces potential transpiration/leaf area by reducing stomatal conductance. On grasslands where effects of stomatal closure on transpiration are not negated by an increase in leaf temperature and leaf area, rising CO2 concentration should slow the depletion of soil water by grasses and potentially favor shrubs and other species that might otherwise succumb to water stress. Predicted effects of CO2 are supported by results from CO2-enrichment studies in the field and are compatible with recent models of interactions between resource levels and vegetation pattern and structure.

POTENTIAL NITROGEN CONSTRAINTS ON SOIL CARBON SEQUESTRATION UNDER LOW AND ELEVATED ATMOSPHERIC CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.

Carbon isotope dynamics of free-air CO2-enriched cotton and soils

A role for soils as global carbon sink or source under increasing atmospheric CO2 concentrations has been speculative. Free-air carbon dioxide enrichment (FACE) experiments with cotton, conducted from 1989 to 1991 at the Maricopa Agricultural Center in Arizona, maintained circular plots at 550 μmol mol−1 CO2 with tank CO2 while adjacent ambient control plots averaged about 370 μmol mol−1 CO2. This provided an exceptional test for entry of carbon into soils because the petrochemically derived tank CO2 used to enrich the air above the FACE plots was depleted in both radiocarbon (14C content was 0% modern carbon (pmC)) and 13C (δ13C≈ −36‰) relative to background air, thus serving as a potent isotopic tracer. Flask air samples, and plant and soil samples were collected in conjunction with the 1991 experiment. Most of the isotopic analyses on the plants were performed on the holocellulose component. Soil organic carbon was obtained by first removing carbonate with HCl, floating off plant fragments with a NaCl solution, and picking out remaining plant fragments under magnification. The δ13C of the air above the FACE plots was approximately −15 to −19‰, i.e. much more 13C depleted than the background air of approximately −7.5‰. The δ13C values of plants and soils in the FACE plots were 10–12‰ and 2‰13C-depleted, respectively, compared with their control counterparts. The 14C content of the FACE cotton plants was approximately 40 pmC lower than tha tof the control cotton, but the 14C results from soils were conflicting and therefore not as revealing as the δ13C of soils. Soil stable-carbon isotope patterns were consistent, and mass balance calculations indicate that about 10% of the present organic carbon content in the FACE soil derived from the 3 year FACE experiment. At a minimum, this is an important quantitative measure of carbon turnover, but the presence of 13C-depleted carbon, even in the recalcitrant 6 N HCl resistant soil organic fraction (average age 2200 years before present (BP)), suggests that at least some portion of this 10% is an actual increase in carbon accumulation. Similar isotopic studies on FACE experiments in different ecosystems could permit more definitive assessment of carbon turnover rates and perhaps provide insight into the extent to which soil organic matter can accommodate the ‘missing’ carbon in the global carbon cycle.

Carbon Dioxide and Water Fluxes of C 3 Annuals and C 3 and C 4 Perennials at Subambient CO 2 Concentrations

1. The C 3 annuals, Avena sativa and Brassica kaber, and C 3 and C 4 perennials, Prosopis glandulosa and Schizachyrium scoparium, respectively, were grown in a 38-m long chamber along a continuous gradient of daytime CO 2 concentrations ([CO 2 ]) from near the current 350 μmol mol −1 to 150 (annuals) or 200 μmol mol −1 (perennials). Diurnal CO 2 and water fluxes were calculated for plant stands in five consecutive, 7.6-m lengths of the chamber arranged linearly along the [CO 2 ] gradient

Growth and Gas Exchange of Oats (Avena sativa) and Wild Mustard (Brassica kaber) at Subambient CO 2 Concentrations

A repeated sequence of monocultures and mixtures of oats (Avena sativa L.) and wild mustard (Brassica kaber (DC.) Wheeler) was grown along a daytime gradient of CO₂ concentrations ([ CO₂]) from near 330 to a minimum of <tex-math>

Carbon isotopes and carbon turnover in cotton and wheat FACE experiments

150\ \mu {\rm mol}\ {\rm mol}^{-1}

Determination of root biomasses of three species grown in a mixture using stable isotopes of carbon and nitrogen

</tex-math>. The objectives were to determine effects of subambient [ CO₂] on leaf gas exchange, biomass production, and competitive interactions of these C₃ species. A decrease in stomatal conductance did not prevent a nearly linear increase in leaf internal [ CO₂] and net assimilation of oat leaves as [ CO₂] increased. Net assimilation of oats and wild mustard increased from 5.0 and <tex-math>

A Controlled Environment Chamber for Growing Plants Across a Subambient CO 2 Gradient

2.5\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}