Nutrient Cycling
Nutrient cycling is critical to both natural, complex systems and to simplified systems for maximum biomass production. This diagram shows typical inputs and losses to hybrid poplar plantation soils in Western Washington.



Note the inputs that occur over geologic time, notably the pyroclastic fallout and mud flow (lahar) deposition processes. These are, from a human chronological standpoint, infrequent events, but in the Pacifif Northwest they have dramatic influence on the nature of alluvial soils. Most riverbottom soils in Western Washington have a large percentage of ash and volcanic alluvium. The 1980 eruption of Mt. St. Helens provided an observable example of this in action. Mud flowing down the Toutle River covered a large amount of riverside lowlands along the Cowlitz and Toutle Rivers. This new soil horizon will become fertile substrate for future hybrid poplar plantations.

Nitrogen-fixing species aren't included in many of the monospecific hybrid poplar stands, eliminating an important pathway for atmospheric nitrogen to reach the soil (Norse 1990). This must be contended with by either applying biosolids or fertilizers to the system or planting species like alders(Alnus spp.) which have actinomycetes as symbiotic associates.

The nutrient losses from the system include atmospheric loss due to decomposition, leaching from the soil, and most importantly, harvesting. In many hybrid poplar stands, the whole tree is removed from the site, taking with it the foliage, small-diameter branches, and bark, components of the tree with high nutrient concentrations (Hansen 1983). This must be mitigated by addition of biosolids or fertilization with inorganic compounds.

Cycling within the system is very limited, as none of the trees reach senescence or even contribute much litterfall additions. A simplification of the system as in hybrid poplar forestry, can also result in inhibited nutrient cycling (Norse 1990).


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