Scaling of physical constraints at the root-soil interface to macroscopic patterns of nutrient retention in ecosystems and landscapes

Background/Question/Methods

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.  In many forests, water and nitrogen are thought to limit plant productivity but, surprisingly, we lack a basic mechanistic understanding of how ecosystem nitrogen retention, which is crucial to sustaining plant growth, depends on the capacity of roots to counteract the competing tendency for water to leach nitrogen away.  Here we address the physical constraints on the availability and loss of plant nutrients within an analytical framework that accounts for diffusive movement of nutrients in soils, kinetic uptake at the root/mycorrhizal surface, and interactions with soil water flow. We ask: what are the relative sink strengths of plant nutrient acquisition versus hydrologic leaching in controlling nitrogen concentration in soils and losses to streams?   We combine data on root mass, root length, and root area index with parameters for solute movement to predict nitrogen leaching losses at the ecosystem level and compare these results to observed patterns in nitrogen leaching levels worldwide.

Results/Conclusions

Our results show that the physical environment permits plants to lower soil solute concentrations substantially.  Our analysis that is based on first principles of nutrient movement and uptake confirms that nitrogen-limited plant uptake capacities in soils are considerable such that water movement 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 at the upper end of observed levels in soil solutes and headwater streams.  This slight discrepancy between our process-based model predictions and macroscopic patterns of N losses can be reduced substantially by accounting for uptake via mycorrhizal symbioses.  Our theoretical framework and evaluation against data suggests that hydrologic dilution is marginal, as strong nitrogen uptake capacity necessarily leads to a fast turnover in soils, and thereby constitutes the most important factor in setting soil nitrogen concentrations for a given amount of mineralization.  We discuss implications of our framework for understanding effects of spatial heterogeneity in soils and for inferring global patterns of nutrient limitation from inversion of landscape level rates of nutrient turnover and loss.