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Source: Krebs (1994).

Source: Krebs (1994).

When leaves and other detritus falls to the forest floor, nutrients are removed by leaching of soluble components, consumption by soil invertebrates, and mineralization by bacteria and fungi. Rate of degradation depends largely on the lignin content of the litter. For example, the highly digestible mulberry was found to lose 64% of its weight in a year on the ground in eastern Tennessee. Oak degraded by 39%, beech only 21%. Pines and conifers also degraded slowly. Lignins break down much more slowly than cellulose or hemicellulose, the other components of wood. Few bacteria can do the job. Most lignin decomposition is done by the white rot fungi, Phanerochaete chrysosporium.

Nutrient relationships have been studied using large-scale long-term experiments. One of the best known is the Hubbard Brook Experimental Forest. Stream and rainfall flow and composition were measured for a number of watersheds there (Table 15.2). A net loss of calcium, magnesium, potassium, sodium, aluminum, sulfate, silica, and bicarbonate are presumably balanced by inputs from weathering of bedrock. Ammonium, nitrate, and chloride showed greater inputs than outputs. The nitrogen species may also be lost by transformation.

About 60% of the rainfall that falls on undisturbed Hubbard Brook watersheds eventually leaves via the streams. The area is underlain by a tight bedrock formation, so losses to groundwater are negligible. Thus, most of the loss is by evaporation and transpiration. If a watershed is logged, streamflow increases drastically. Nitrate concentration in the streams also increases greatly, since nitrification proceeds normally in the soil, but uptake by trees is eliminated. As with anions in general, nitrate is weakly bound to soil and so is rapidly leached out in runoff.

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