Primary production processes constitute the principal biochemical motive force for all subsequent activities of heterotrophs in soils. The inputs come from two directions: (1) from aboveground onto the soil surface, as litter, and (2) from belowground, as roots, which contribute exu-dates and exfoliated cells while the roots are alive, and then as root litter when the roots die.

A wide range of direct measurements of root production and turnover are now in use. These include various nondestructive techniques including rhizotrons and minirhizotrons, and destructive techniques including soil coring and isotopic-labeling of roots followed by destructive sampling at specified time intervals to determine dynamics (e.g., over an entire growing season).

Of equal importance to roots themselves are their generally more efficient physiological extensions—the root-fungus mutualistic association, mycorrhiza. At a cost of 5-30% of the total photosynthate translocated belowground, mycorrhiza assist in obtaining inorganic nutrients, water, and in some cases, organic nutrients over a much wider range of the soil volume than roots alone. This symbiotic association has a significant effect on other biota, namely microbes and fauna, which inhabit all soil systems.

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