Growth of Eastern Cottonwoods (Populus deltoides) in elevated [CO₂] stimulates stand‐level respiration and rhizodeposition of carbohydrates, accelerates soil nutrient depletion, yet stimulates above‐ and belowground biomass production

Abstract

We took advantage of the distinctive system-level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO 2 ] on carbon flux dynamics, above-and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO 2 ] in carbon and N-replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO 2 exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO 2 exchange under different scales in space and time. Elevated [CO 2 ] significantly stimulated whole-system maximum net CO 2 influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4-year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO 2 ] treatments. After 2 years of growth at elevated [CO 2 ], early season stand respiration was decoupled from CO 2 influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO 2 ] treatments. Soil respiration was stimulated by elevated [CO 2 ] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate-induced respiration in vitro. Elevated [CO 2 ] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of aboveand belowground biomass production by elevated [CO 2 ] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.

Leaf‐ and stand‐level responses of a forested mesocosm to independent manipulations of temperature and vapor pressure deficit

“…The atmosphere surrounding the enclosed stand was mixed by fans, and outer canopy leaves in edge trees fluttered visibly. Measurements were made during the peak of the growing season after full‐canopy development, and system‐level performance before and after the experiments has been reported previously (Barron‐Gafford et al ., 2005). At the beginning of this experiment, the trees averaged 11.3 ± 1.6 m in height and 93.8 ± 11.9 mm in basal diameter.…”

Soil organic matter dynamics under soybean exposed to elevated [CO2]

“…According to van Groenigen et al (2006), increases in SOM levels are restricted when soil N supply is limiting; or when a legume is considered, if non-N nutrients restrict plant response to CO 2 fertilization. When nutrients are limiting, plant-microbe competition for N can accelerate SOM decay and ultimately degrade soils even as plant growth increases under elevated [CO 2 ] ( Barron-Gafford et al 2005). Several studies, mostly focusing on woody and herbaceous species under CO 2 fertilization, suggest that accelerated SOM turnover rates prevent SOM accumulation in systems where nutrient supply is low van Groenigen et al 2006).…”

Elevated CO2: Plant associated microorganisms and carbon sequestration

“…In general, rhizodeposition is expected to be altered under elevated CO 2 (eCO 2 ) due to changes in physiology and C status of the plant (Darrah, 1996;Barron-Gafford et al, 2005;Drigo et al, 2009). The alteration might be in the composition/availability of chemo-attractants or signal compounds, C:N ratio or nutrient availability in the rhizosphere (Kandeler et al, 2006;Haase et al, 2007) that may influence the colonization and functional behavior of the soil microorganisms in the rhizosphere (Singh et al, 2004;Phillips, 2007;Drigo et al, 2009).…”