Optimal Plant Carbon Allocation Implies a Biological Control on Nitrogen Availability

Abstract:

The degree to which nitrogen availability limits the terrestrial C sink under rising CO₂ is a key uncertainty in carbon cycle and climate change projections. Results from ecosystem manipulation studies and meta-analyses suggest that plant C allocation to roots adjusts dynamically under varying degrees of nitrogen availability and other soil fertility parameters. In addition, the ratio of biomass production to GPP appears to decline under nutrient scarcity. This reflects increasing plant C exudation into the soil (C_ex) with decreasing nutrient availability. C_ex is consumed by an array of soil organisms and may imply an improvement of nutrient availability to the plant. Thus, N availability is under biological control, but incurs a C cost. In spite of clear observational support, this concept is left unaccounted for in Earth system models.

We develop a model for the coupled cycles of C and N in terrestrial ecosystems to explore optimal plant C allocation under rising CO₂ and its implications for the ecosystem C balance. The model follows a balanced growth approach, accounting for the trade-offs between leaf versus root growth and C_ex in balancing C fixation and N uptake. We assume that C_ex is proportional to root mass, and that the ratio of N uptake (N_up) to C_ex is proportional to inorganic N concentration in the soil solution. We further assume that C_ex is consumed by N₂-fixing processes if the ratio of N_up:C_ex falls below the inverse of the C cost of N₂-fixation.

Our analysis thereby accounts for the feedbacks between ecosystem C and N cycling and stoichiometry. We address the question of how the plant C economy will adjust under rising atmospheric CO₂ and what this implies for the ecosystem C balance and the degree of N limitation.