Figure 11.3 Carbon budgets for the decomposer food webs in conventional tillage and no-tiilage agroecosystems in the southern USA (from Beare et a!. 1992). Fluxes between web compartments are gC/m2/day; mean annual standing stocks are in parentheses (gC/nr)

introduction of intensified management. It was found, for instance, that a key factor affecting the recovery of sward diversity was soil phosphorus concentrations (Mountford el al. 1994). Available nitrogen gradually declines through immobilization, leaching and denitrification, and this is accelerated by intensive grazing (Mountford ei al. 1993). The reduction in mineral-N fluxes facilitates the re-establishment of some species from seed banks if the soil is disturbed. The available P pool, however, was very stable and showed differential effects on the re-establishment of a diverse sward on any conceivable time scale. Experimental studies suggested that removal or burial of the P-rich surface soil (i.e. with increased management inputs) promoted the re-establishment of the initial sward communities over many decades providing that seed sources were present in soil and the surrounding fields. Rare species may never recover because of their precarious population status in a managed landscape.

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