Cycling nutrients

Biogeochemical cycling in forests includes elemental inputs, exports, and a complex set of physical, chemical and biotic processes which comprise internal nutrient cycles (Fig. 1). Any disturbance, whether anthropogenic (i.e. 

Johnson, D. W. and Lindberg, S. E. (1992). "Atmospheric deposition and forest nutrient cycling a synthesis of the integrated forest study." Springer-Verlag, New York. 

Meliilo, J. M., Steudler, P. A., Aber, J. D. and Bowden, R. D. (1989). Atmospheric deposition and nutrient cycling. In "Dahlem Workshop on Exchange of Trace Gases Between Terrestrial Ecosystems and the Atmosphere" (M. O. Andreae and D. S. Schi-mel, eds). Wiley Interscience Publishers, New York. 

Jackson, G. A. and Williams, P. M. (1985). Importance of dissolved organic nitrogen and phosphorus in biological nutrient cycling. Deep Sea Res. 32, 223-235. 

The state of communities and ecosystems can be described according to both structural and functional parameters (see Chapter 14 in Walker et al. 2000). Functional analyses include the measurement of nutrient cycling, turnover of organic residues, energy flow, and niche metrics. Structural analyses include the assessment of species present, their population densities, and their genetic composition. 

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

Enhanced nutrient cycling in both the rhizosphere and bulk soil may depend on the bacterial grazing by protozoa or nematodes with release of inorganic N. Nematodes appear to be the primary consumers of bacteria in the rhizosphere, whereas protozoa are equally prevalent in rhizosphere and bulk soil (41,97). Estimated C-to-N ratios of bacterial-feeding nematodes range from 5 1 to 10 1 (98,99) and are generally higher than those of their bacterial food source thus the excess N is excreted as ammonia (100,101) by nematodes. The estimated... 

V. THE ROLE OF MYCORRHIZAL FUNGI IN NUTRIENT CYCLING AT THE SOIL-ROOT INTERFACE... 

Mumane RJ, Cochran JK, Sarmiento JL (1994) Estimates of particle- and thorium-cycling rates in the northwest Atlantic Ocean. J Geophys Res 99 3373-3392 Mumane RJ, Cocliran JK, Buesseler KO, Bacon MP (1996) Least-squares estimates of thorium, particle and nutrient cycling rate constants from the JGOFS North Atlantic Bloom Experiment. Deep-Sea Res 1 43(2) 239-258... 

Synergistic effects such as nutrient cycling can be obtained in mixed stands. 

The pollutant or solute cycle — that may encompass the processes of advection, diffusion, volatilization, adsorption and desorption, chemical degradation or decay, hydrolysis, photolysis, oxidation, cation or anion exchange, complexation, chemical equilibria, nutrient cycles, and others (see section 3.0). 

Reduction in nitrogen excretion is often set equal to a reduction in nitrogen emission. This is only true in those cases where the excess nitrogen in the commercial fertiliser can be caught and retained in the agricultural nutrient cycle. If the nutrients are, however, discharged from the cycle and emitted... 

Fig. 6.3 Conceptual drawing of the distribution of different groups of herbaceous plants in relation to major plant nutrients. Cycles represent the distribution of grasses, herbs, and legumes. Species with mycorrhiza are able to exploit sites low in both nitrogen (N) and phosphorus (P). Highly productive species, such as ruderal plants, need conditions abundant in N and P. 

Nematodes play a major role in decomposition and nutrient cycling in soil food webs. Due to its important role in trophic interactions, they have received attention in farming systems especially in organic farming. Although nematodes represent a relatively small amount of biomass in soil, their presence across many trophic levels in soils is vitally important in soil environments and ecosystem processes (Ingham et al. 1986). 

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