Independently foraged water and nitrogen both important in determining grassland species abundances

Although root distributions are assumed to be important for plant growth and coexistence, it remains difficult to quantify how root distributions affect resource uptake and plant landscape abundance.

In a water- and nitrogen (N)-limited grassland system, Minnesota, USA, we injected water and N tracers to five depths (5–150 cm) during peak growing season. Tracer concentrations in 11 dominant species were measured to describe functional root distributions. Seasonal water and N uptake into these functional root distributions was estimated using depth-specific resource availability. Plant biomass produced from seasonal water and N uptake was estimated using water- and N-use efficiencies. Finally, biomass production was compared to plant landscape abundance.

Differences in functional root distributions resulted in seasonal water uptake between 52 and 67 cm yr⁻¹, and N uptake between 1.0 and 3.1 g m⁻² yr⁻¹ among species. Biomass production caused by water or N uptake was correlated with plant landscape abundance (R² = 0.37, 0.35, for water and N). When combined, biomass production from water and N uptake was better correlated with plant landscape abundance (R² = 0.75).

We show how root systems forage independently for different resources. Consistent with resource availability, water uptake patterns were shallower than N uptake patterns. Root uptake of these two limiting resources affected plant growth and landscape abundance. We provided a quantitative link between functional root distributions, resource uptake, biomass production, and plant landscape abundance. This research represents an important advance from research that describes niche differences among root systems.