Plant root distributions and nitrogen uptake predicted by a hypothesis of optimal root foraging

• Published in: Ecology and evolution Vol. 2; no. 6; pp. 1235 - 1250
• Main Authors: McMurtrie, Ross E., Iversen, Colleen M., Dewar, Roderick C., Medlyn, Belinda E., Näsholm, Torgny, Pepper, David A., Norby, Richard J.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.06.2012; John Wiley & Sons, Inc
• Subjects: Biogeochemistry; Biomass; Carbon dioxide; Ecology; Ecosystems; Elevated CO2; Empirical equations; Food Science; Forest ecosystems; Forests; General circulation models; Hypotheses; Livsmedelsvetenskap; Nitrogen; nitrogen‐uptake efficiency; nitrogen‐uptake fraction; nitrogen‐uptake model; nitrogen‐use efficiency; optimal foraging by roots; optimal rooting depth; Original Research; Plant roots; Plantations; Research facilities; root distributions; root strategies; Rooting; Terrestrial ecosystems; Trees; United States > US; USA, Tennessee, Oak Ridge; Elevated CO2; nitrogen-uptake fraction; nitrogen-use efficiency; root distributions; nitrogen-uptake model; root strategies; optimal rooting depth; optimal foraging by roots; nitrogen-uptake efficiency
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.266

CO2‐enrichment experiments consistently show that rooting depth increases when trees are grown at elevated CO2 (eCO2), leading in some experiments to increased capture of available soil nitrogen (N) from deeper soil. However, the link between N uptake and root distributions remains poorly represented in forest ecosystem and global land‐surface models. Here, this link is modeled and analyzed using a new optimization hypothesis (MaxNup) for root foraging in relation to the spatial variability of soil N, according to which a given total root mass is distributed vertically in order to maximize annual N uptake. MaxNup leads to analytical predictions for the optimal vertical profile of root biomass, maximum rooting depth, and N‐uptake fraction (i.e., the proportion of plant‐available soil N taken up annually by roots). We use these predictions to gain new insight into the behavior of the N‐uptake fraction in trees growing at the Oak Ridge National Laboratory free‐air CO2‐enrichment experiment. We also compare MaxNup with empirical equations previously fitted to root‐distribution data from all the world's plant biomes, and find that the empirical equations underestimate the capacity of root systems to take up N. The relationship between root distributions and N uptake is analysed using a new optimizations hypothesis (MaxNup) for root foraging in relation to spatial variability of soil N, according to which a given total root mass is distributed vertically in order to maximise annual N uptake. MaxNup makes predictions of the optimal vertical profile of root biomass, maximum rooting depth and N‐uptake efficiency (i.e. the fraction of available soil N taken up annually by roots).