COS 91-5
Nitrogen vs. phosphorus limitation of terrestrial produtivity: A global scale analysis based on the carbon costs of resource uptake
There is widespread understanding that the biogeochemical cycles of carbon (C), nitrogen (N) & phosphorus (P) influence terrestrial productivity. Among the least well understood of these constraints is the C cost of nutrient uptake. The flux of C belowground (TBCF) is one of the largest fluxes of C in terrestrial ecosystems, reflecting the need for water and nutrients to support growth and productivity. To address the C costs of nutrient uptake at the global scale we collected data on TBCF, annual rates of N mineralization and P pools from studies across the globe.
Results/Conclusions
We show that the C cost of N uptake--defined as the quantity of C in TBCF divided by the annual rate of net N mineralization-- is highest in boreal and temperate coniferous forests and lowest in tropical forests, with costs ranging between 4-27gC allocated belowground for every gN mineralized annually per square meter of land surface. By contrast, the C cost of P uptake was highest in the tropics and lowest in the boreal zone. Overall these patterns appear consistent with biogeochemical theories suggesting that N limitation is greatest at high latitude and P limitation is greatest at low latitude. Analysis of the partitioning of gross primary production (GPP) above- vs. belowground, however, suggests a more nuanced interpretation of the controls over terrestrial productivity. TBCF was positively correlated with GPP across biomes. The proportion of GPP allocated to TBCF was, however, inversely related with latitude with approximately 35% of GPP allocated belowground in the tropics and >60% allocated belowground in boreal forests. Under the assumption that 50% of GPP allocated above and belowground indicates co-limitation of above and belowground resources, it appears that the relative cost of N uptake in boreal and temperate coniferous forests far exceeds that of P uptake in the tropics where light appears to be more limiting. These data suggest that nitrogen and to a lesser extent light limit boreal forest productivity whereas in lowland tropical forests P supply is clearly important but not the major control over their productivity. This analysis is also usefulfor models in three ways: (1) publicly available benchmark data sets on TBCF, annual rates of net N mineralization and P supply, (2) biome-specific partitioning coefficients for C allocation above vs. belowground, and (3) semi-explicit cost functions for use in the calibration and validation of models explicitly interested in understanding how coupled biogeochemical cycles constrain terrestrial productivity.