Global biogeochemical cycles

The following sections describe the cycles in general, the basic processes, such as emissions, deposition and chemical reactions are presented in detail in subsequent Chapters. The aim here is to point out the most important issues in the biosphere-atmosphere interaction and the part of humans in a changing climate system. 

Nitrification is the biological oxidation of ammonium (NH4) to nitrate (NOJ), with nitrite (NOJ) as an intermediate under aerobic conditions (NHj NOJ NOJ). Under oxygen-limited conditions nitrifiers can use NOJ as a terminal electron acceptor to avoid accumulation of the toxic NOJ, whereby N2O and NO are produced. During that process, because separate bacteria (Meiklejohn 2006) oxidize NHj into NOJ and NOJ into NOJ, the process can lead to the temporary accumulation of NOJ in soil and water. These bacteria are generally chemoautotrophic, requiring only CO2, H2O and O2. The nitrifying bacteria Nitro-somas, which converts NHj to NOJ, is also able to reduce NOJ. This nitrite can decompose abiotically, yielding NO or NO2, substantially favoured in acidic soils 

The accepted sequence for denitrification is demonstrated in Fig. 2.27 - the inverse process corresponds to nitrification  

HNO3 (nitric acid) HNO2 (nitrous acid) HNO (nitroxyl). 

Nitroxyl exists only as an intermediate and acts as a very weak acid (pAT = 11.4) with NO being the protolytic anion, from which NO is formed via electron transfer (and vice versa). HNO is the transfer point to NO (as just described) and to N2 and N2O in parallel pathways (Fig. 2.27). H2N2O2 (hyponitrous acid), probably produced by enzymatic dimerisation of HNO, is formally the acid in the form of its anhydride N2O (see for details Chapter 5.4.4.2). 

---

B. H. SvENSSON and R. Soderlund (eds.). Nitrogen, Phosphorus, and Sulfur-Global Biogeochemical Cycles, SCOPE Report, No. 7, Sweden 1976, 170 pp. also SCOPE Report No. 10, Wiley, New York, 1977, 220 pp, and SCOPE Newsletter 47, Jan. 1995, pp. 1-4. 

It is the determination of volatile organic compounds produced from natural products that requires separation techniques that allow isolation of stereoisomers. The most commonly determined groups are the terpene and sesquiterpene species present in essential oils, which are used as key indicators of biological factors such as the growth season, geographic location, climate, etc. These species are also released directly into the atmosphere by very many plants and trees, and make a substantial contribution to global biogeochemical cycles. 

It is often taken for granted that the oxygen content of the air is nearly constant at ca. 20% of the atmospheric volume, that most of the liquid water on the planet is aerobic (i.e. contains O2), and that most water has pH values relatively close to neutral" (close to 7). However, these circumstances are not mere coincidences but are in fact consequences of the interaction of key global biogeochemical cycles. For instance, the pH of rainwater is often determined by the relative amounts of ammonia and sulfuric acid cycled through the atmosphere, a clear example of interaction between the nitrogen and sulfur cycles. 

So far we have not gone in-depth into the nature of the transport processes responsible for fluxes of material between and within reservoirs. This section includes a very brief discussion of some of the processes that are important in the context of global biogeochemical cycles. More comprehensive treatments can be found in textbooks on geology, oceanography and meteorology and in reviews such as Lerman (1979) and Liss and Slinn (1983). 

As described in the first part of this chapter, chemical thermodynamics can be used to predict whether a reaction will proceed spontaneously. However, thermodynamics does not provide any insight into how fast this reaction will proceed. This is an important consideration since time scales for spontaneous reactions can vary from nanoseconds to years. Chemical kinetics provides information on reaction rates that thermodynamics cannot. Used in concert, thermodynamics and kinetics can provide valuable insight into the chemical reactions involved in global biogeochemical cycles. 

The soil may represent a thin film on the surface of the Earth, but the importance of soils in global biogeochemical cycles arises from their role as the interface between the Earth, its atmosphere, and the biosphere. All terrestrial biological activity is founded upon soil productivity, and the weathering of rocks that helps to maintain atmospheric equilibrium occurs within soils. Soils provide the foundation for key aspects of global biogeochemical cycles. 

Aber, J. D. and Driscoll, C. T. (1997). Effects of land use, climate variation, and N deposition on N cycling and C storage in northern hardwood forests, Global Biogeochem. Cycles 11, 639-648. 

Pollard, D., Sitch, S. and Haxeltine, A. (1996). An integrated biosphere model of land surface processes, terrestrial carbon balance, and vegetation dynamics. Global Biogeochem. Cycles 10, 603-628. 

Friedlingstein, P., Fung, I., Holland, E., John, J., Brasseur, G., Erickson, D. and Schimel, D. On the contribution of CO2 fertilization to the missing biospheric sink. Global Biogeochem. Cycles 9, 541-556. 

Hunt, E. R. Jr., Piper, S. C., Nemani, R., Keeling, C. D., Otto, R. D. and Running, S. W. (1996). Global net carbon exchange and intra-annual atmospheric CO2 concentrations predicted by an ecosystem process model and three-dimensional atmospheric transport model. Global Biogeochem. Cycles 10, 431-456. 

Kindermann, J., Wiirth, G., Kohlmaier, G. H. and Badeck, F.-W. (1996). Interannual variations of carbon exchange fluxes in terrestrial ecosystems. Global Biogeochem. Cycles 10, 737-755. 

C. J. and Schloss, A. L. (1997). Equilibrium responses of global net primary production and carbon storage to doubled atmospheric carbon dioxide Sensitivity to changes in vegetation nitrogen concentration, Global Biogeochem. Cycles 11,173-189. 

Post, W. M., King, A. W. and Wullschleger, S. D. (1997). Historical variations in terrestrial biospheric carbon storage, Global Biogeochem. Cycles 11, 99-109. 

Potter, C. S., Randerson, J. T., Field, C. B., Matson, P. A., Vitousek, P. M., Mooney, H. A. and Klooster, S. A. (1993). Terrestrial ecosystem production A process model based on global satellite and surface data. Global Biogeochem. Cycles 7, 811-841. 

Randerson, J. T., Thompson, M. V., Conway, T. J., Fung, I. Y. and Field, C. B. (1997). The contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric carbon dioxide. Global Biogeochem. Cycles 11,535-560. 

Schimel, D. S., Braswell, B. H., McKeown, R., Ojima, D. S., Parton, W. J. and Pulliam, W. (1996). Climate and nitrogen controls on the geography and time-scales of terrestrial biogeochemical cycling. Global Biogeochem. Cycles 10, 677-692. 

Wamant, P., Francois, L., Strivay, D. and Gerard, J.-C. (1994). CARAIB A global model of terrestrial biological productivity. Global Biogeochem. Cycles 8, 255-270. 

Wigley, T. M. L. (1991). A simple inverse carbon cycle model. Global Biogeochem. Cycles 5,373-382. 

Galloway J. N. et al. (1995). Nitrogen fixation Anthropogenic enhancement - environmental response. Global Biogeochem. Cycles 9,235-252. 

Jaffe, D. A. (1992). The nitrogen cycle in global biogeochemical cycles. In "Global Biogeochemical Cycles" (S. S. Butcher, R. J. Charlson, G. H. Orians, G. V. Wolfe, eds). Academic Press, New York. 

Schindler, D. W. and Bayley, S. E. (1993). The biosphere as an increasing sink for atmospheric carbon Estimates from increased nitrogen deposition. Global Biogeochem. Cycles 7,717-733. 

Grinenko, V. A. and Ivanov, M. V. (1983). Principal reactions of the global biogeochemical cycle of sulphur. In The Global Biogeochemical Sulfur Cycle, SCOPE 19" (M. V. Ivanov and J. R. Freney, eds). Wiley, Chichester. 

Fig. 19-1 Schematic of the processes that connect global biogeochemical cycles and climate. Boxes denote observables and ovals indicate processes that affect these. 

Samuel S. Butcher, Gordon H. Orians, Robert J. Charlson, and Gordon V. Wolfe. Global Biogeochemical Cycles. 1992 Brian Evans and Teng-Fong Wong. Fault Mechanics and Transport Properties of Rocks. 

---