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Atmospheric Nitrogen Deposition:
Its Long-Term Impact On Carbon Sequestration in Soils

The most highly publicized environmental change in all of modern history is the ongoing rise in the air's CO2 content, which is believed by many to be driven by the burning of fossil fuels and to be the primary cause of much of the global warming of the past century.  Less well known is a parallel anthropogenic-induced phenomenon of similar global reach and significance: the release of nitrogen to the atmosphere.  The importance of this phenomenon resides in the assured return of the emitted nitrogen to the surface of the earth, where it can influence a number of biological processes, one of which - the sequestration of carbon in soils - is considered to be one of the best known means for slowing the rate of rise of the air's CO2 content.  Hence, it is only natural to wonder whether concomitant atmospheric nitrogen deposition helps or hinders soil carbon storage; and that is the question we broach in this essay.

The source of information we draw on most heavily in presenting this material is the literature review of Berg and Matzner (1997).  These researchers begin their story by noting that additional nitrogen (N) availability generally results in a greater uptake of N by plants, which leads to higher N concentrations in different plant parts, including the foliage that ultimately dies and falls to the ground as litter.

On the one hand, this effect of added N is a positive phenomenon; for one of the consequences of higher foliar N concentrations is the creation of more lignin, which is considerably more resistant to decomposition than are other plant litter components.  On the other hand, higher litter N concentrations generally lead to higher initial rates of CO2 loss from decomposing plant litter, which may be viewed as a negative outcome.  In the longer view of things, however, this initial enhanced loss of CO2 from the soil-litter ecosystem is more than compensated by the ultimate enhanced carbon savings provided by the original addition of N.

After something on the order of a quarter to a third of the original plant litter has decomposed and disappeared, for example, decomposition rates tend to become slower for litter of higher N concentration.  One reason for this deceleration of mass loss is that as litter decomposes, the concentration of the recalcitrant lignin it contains increases.  Another reason is that several species of fungi that possess the ability to decompose lignin via lignin-degrading enzymes do not seem to be able to produce the necessary enzymes in the presence of plentiful N-rich compounds.  This failure to synthesize the needed enzymes may be related to a scarcity of manganese, the soil concentration of which has often been observed to decline as soil N availability rises.

Still another way in which added N may lead to the sequestration of more carbon in soils is by stimulating plants to grow faster.  In many cases of N-induced growth acceleration, plants extract the micronutrient boron from the soil as fast as, or even faster than, it can be replaced by weathering processes.  This lack of boron is very important, because boron is required by a specific plant enzyme that transports phenols out of the foliage of certain plants; and a lack of boron thus results in an accumulation of phenolics in photosynthetically-active plant tissues, which ultimately leads to a greater synthesis of decomposition-resistant lignins.

Of even more importance, perhaps, is the fact that lignin actually begins to incorporate soil N into its own constitution when it is readily available.  Thus transformed by chemical reactions into N-containing compounds, it typically reacts with aromatic substances in the soil to produce recalcitrant humus, which represents the final resting state of soil carbon.  In this condition, in fact, carbon can remain sequestered in the soil for periods of time commensurate with the duration of entire interglacials.

There is also evidence to suggest, as Berg and Matzner note, "that warmer and wetter conditions favor the development of a humus more rich in N."  Consequently, since these climatic conditions are generally predicted to occur as a result of the ongoing rise in the air's CO2 content, it can be appreciated that the whole suite of processes related to N-induced increases in soil carbon sequestration may well represent another in a long list of biologically-induced negative feedback phenomena that tend to limit the degree to which the near-surface soil and air temperatures of the planet may rise in response to an impetus for warming.

When one thinks upon the situation further, one additionally realizes that many of the anthropogenic activities that release CO2 to the atmosphere also result in biospheric N fertilization.  Hence, it becomes clear that these activities are at least somewhat, and possibly largely, offsetting in terms of their potential to perturb earth's climate, in that they produce both warming and cooling influences at one and the same time.  Viewed in this light, humankind may ultimately be found to be not nearly as environmentally discordant as some of us portray ourselves to be.

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

References
Berg, B. and Matzner, E.  1997.  Effect of N deposition on decomposition of plant litter and soil organic matter in forest ecosystems.  Environmental Reviews 5: 1-25.