Plant Uptake

The amount of nutrients taken into a plant by root and foliar uptake are shown in Table 4 and Table 5. Mycorrhizae fungi associations with plant roots can enhance the availability and uptake of several essential nutrients (e.g., phosphorus, copper, calcium, and iron). Mycorrhizae greatly increase the volume of soil in contact with rooting tissue (i.e., root plus mycorrhizae tissue) and they produce organic acids, chelates, and enzymes which enhance nutrient availability and uptake. Foliar uptake can consist of ion uptake from wet deposition across the cell membrane or direct absorption of gases (e.g., SO₂ and NO₂). While foliar uptake may be important in areas with appreciable air pollution, it generally represents a very small proportion of the total nutrient uptake as indicated by the net canopy exchange values in both these tables.

The annual uptake (or nutrient demand) changes greatly over the course of development of a forest from establishment to maturity. During the period of early establishment, nutrient uptake will increase with increasing gross ecosystem production. Uptake rates will peak at approximately the time of canopy closure. Following canopy closure, gross production slows due to competition and mortality and more carbon is allocated to woody materials having a lower nutrient content (e.g., stem). Thus, uptake rates will decline from their maximum level at canopy closure and maintain a relatively constant value.

The growth requirement is defined as the total amount of a nutrient that is required each year by vegetation to meets its demands for nutrients associated with the annual increment, litterfall, and canopy leaching. The requirement is obtained by uptake and retranslocation processes:

Growth requirement = Uptake + Retranslocation

Growth requirement = Annual increment + litterfall + canopy leaching (throughfall + stemflow - atmospheric deposition).