Nutrient cycling in all terrestrial ecosystems, whether they are natural (forests, rangelands,
grasslands) or managed (agroecosystems, pastures, plantations), follows similar pathways.
However the size of nutrient pools, fluxes and transformations in these systems may vary by
several orders of magnitude (Hornung, 1990). Both natural and managed ecosystems are open
systems i.e. nutrients can enter or leave the system (Binkley, 1986). The question, whether
nutrient inputs and outputs are balanced or not, is closely related to the issue of sustainability
(Smaling et al., 1996). In natural ecosystems, processes that govern nutrient cycling like
primary production, uptake and decomposition tend to be balanced (Saleem, 1998; Heal and
Harrison, 1990). However, when ecosystems are managed, and food production is one of the
major objectives, nutrient transfers are influenced not only by the conditions and process
within the system, but also by circumstances and controlling forces outside the system (i.e.
anthropogenic effects, Priess et al., 2001). As a result, unequal transfers between nutrient
pools may cause the system to accumulate or deplete nutrients. Many environmental problems
like green house gas emissions, eutrophication of rivers and lakes and soil nutrient depletion
are the results of such disproportionate transfers (Brady and Weil, 2002). Balanced nutrient
input and output fluxes are a critical condition to reach sustainable ecosystem functioning and
sustainable agricultural productivity (Dumanski et al., 1991).