Estimating Ecosystem Nitrogen Retention Using Simple Analytical Models of Soil-Plant-Water Interactions.

Nutrient limitation in terrestrial ecosystems is often accompanied with maintaining a nearly closed vegetation-soil nutrient cycle. The ability to retain nutrients in an ecosystem requires the capacity of the plant-soil system to draw down nutrient levels in soils effectually such that export concentrations in soil solutions remain low. Here we address the physical constraints of plant nitrogen uptake which may be limited by the diffusive movement of nutrients in soils, the uptake at the root/mycorrhizal surface, and from interactions with soil water flow. We combine data on root mass, root length, and root area index with parameters for solute movement into an analytical framework of soil nitrogen transport und uptake. We evaluate our simple framework under nutrient limited conditions, for which we can show that  the physical environment permits plants to lower soil solute concentration substantially. Our analysis that is based on first principles of nutrient movement and uptake, confirms that plant uptake efficiencies in soils are considerable such that water movement in soils is generally too small to significantly erode dissolved plant available nitrogen. Our predicted levels of dissolved inorganic nitrogen concentrations based on observed root properties are within the range of observed levels in headwater streams that drain nitrogen limited upland systems. Our physically based, bottom up approach corroborates the long held notion of strong nutrient retention which in turn allow fast turnover (order of days) of nutrient in soils. Our approach offers a mechanistic parameterization of plant nutrient uptake and nutrient retention in biogeochemical models designed to evaluate the dynamics of carbon and nitrogen cycles from field to continental scales.