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CO2 and Nitrogen Effects on Soil Carbon Sequestration:
The Whole is Often Greater Than the Sum of Its Parts

Atmospheric CO2 enrichment effects on soil carbon sequestration are primarily manifest during the initial phase of carbon capture by photosynthesizing plants.  Atmospheric nitrogen deposition effects, on the other hand, are felt during both the initial phase of carbon capture and throughout the final phase of litter decomposition, when vegetative remains undergo various transformations on their way to becoming recalcitrant humus.  In this essay, we concentrate on the initial phase of carbon capture, where increases in atmospheric CO2 concentration and nitrogen deposition each typically lead to enhanced rates of plant photosynthesis and growth.

In a multi-year sixteen-unit open-top chamber study of identically-reconstructed spruce/beech forest ecosystems complete with understory herbs planted in two different soils (one acidic and one calcareous), Sonnleitner et al. (2001) studied the individual effects of atmospheric CO2 enrichment (to 590 ppm from 370 ppm), soil nitrogen (N) enrichment (to 70 kg N per hectare per year from 7 kg N per hectare per year), and the combined effect of the same increases in atmospheric CO2 and soil nitrogen applied simultaneously on the growth of the spruce and beech saplings as expressed by total leaf (beech) and needle (spruce) biomass per unit ground area.

On the acidic soil, the increase in N increased total leaf and needle biomass per unit ground area by 37%.  The increase in CO2 was less effective, increasing total leaf and needle biomass by only 10%.  When the N and CO2 increases were applied together, however, the increase in biomass was more than the 51% that might have been expected from the separate treatment results (1.37 x 1.10 = 1.51). In fact, it was much more, registering fully 77%.

On the calcareous soil, on the other hand, the increase in N actually produced an 8% decrease in total leaf and needle biomass.  The CO2 increase, however, produced a leaf and needle biomass increase of 6%.  These individual results might reasonably have been expected to imply a net drop in leaf and needle biomass of about 2% when the N and CO2 increases were applied together (0.92 x 1.06 = 0.98).  However, the combined treatment produced a biomass increase of fully 19%.

"Together," as Sonnleitner et al. thus concluded, "elevated CO2 and high N had a more than additive fertilizer effect on growth."  Indeed, their synergism in the calcareous soil treatment actually changed a small negative leaf and needle biomass impact into a significant positive impact.

Lloyd (1999) observed a similar phenomenon in a study designed to model the responses of a temperate deciduous forest to increases in both atmospheric CO2 concentration and nitrogen deposition.  He calculated that the historical rise in the air's CO2 content from 1730 to the early 1980s should have increased the forest's net primary production by approximately 7%.  He also calculated that the increase in net primary production due to a proportional increase in nitrogen deposition over the same period should have been about 25%.  However, when he allowed atmospheric CO2 concentration and nitrogen deposition to increase together, the net primary production was calculated to increase by fully 40%, which was more than the 34% implied by the individual increases produced by CO2 and nitrogen alone (1.07 x 1.25 = 1.34).  Hence, Lloyd also concluded that the "effects of increased nitrogen deposition and atmospheric CO2 increase are not simply additive."

For people who look upon humanity as a natural and integral part of life on earth, these results should be edifying.  They demonstrate that not only are anthropogenic-induced increases in atmospheric CO2 concentration and nitrogen deposition positive biosphere-enhancing forces individually, they are even more effective when acting simultaneously, which is exactly what they have done as mankind has progressed through the various stages of technological development that have taken us to - and through - the Industrial Revolution.  Furthermore, as we have demonstrated in earlier essays of this series, the ongoing increase in the air's CO2 content triggers several negative feedback processes that tend to slow its own rate of rise and preclude the occurrence of inordinately high air temperatures, while concomitant nitrogen deposition tends to slow its rate of rise as well.  Hence, there would appear to be little reason to demonize man for these two important consequences of his industrial activities.  Our garden-variety sins are sufficient to do that on their own.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Lloyd, J.  1999.  The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and the combined effects of increasing CO2 concentrations and increased nitrogen deposition at a global scale.  Functional Ecology 13: 439-459.

Sonnleitner, M.A., Gunthardt-Goerg, M.S., Bucher-Wallin, I.K., Attinger, W., Reis, S. and Schulin, R.  2001.  Influence of soil type on the effects of elevated atmospheric CO2 and N deposition on the water balance and growth of a young spruce and beech forest.  Water, Air, and Soil Pollution 126: 271-290.