Nitrogen Availability and Summer Drought, but not N:P Imbalance Drive Carbon Use Efficiency of a Mediterranean Tree-Grass Ecosystem

Abstract. All ecosystems are simultaneously a source and a sink of atmospheric carbon (C). A change in their balance of net and gross ecosystem carbon uptake, ecosystem-scale carbon use efficiency (CUE_ECO), is a change in their ability to buffer climate change. 

Anthropogenic nitrogen (N) deposition is increasing N availability, potentially shifting terrestrial ecosystem stoichiometry towards phosphorus (P) limitation. Depending on how gross primary production (GPP, plants alone) and ecosystem respiration (R_ECO, plants and heterotrophs) are limited by N, P, or associated changes in other biogeochemical cycles, CUE_ECO may change. Seasonally, CUE_ECO also varies as GPP is more coupled to the growing season than respiration.

We worked in a Mediterranean tree-grass ecosystem (locally called ‘dehesa’) characterized by mild, wet winters and summer droughts. We examined CUE_ECO from eddy covariance fluxes over six years under control, +N and +NP fertilized treatments on three timescales: annual, seasonal (determined by vegetation phenophase) and two-weekly aggregations. Finer aggregation allowed consideration of responses to specific vegetation and meteorological conditions. We predicted that CUE_ECO should be increased by wetter conditions, and by NP fertilization.

Milder and wetter years with proportionally longer growing seasons increased CUE_ECO, as did N fertilization, regardless of whether P was added.  Using a generalized additive model, whole ecosystem vegetation status and water deficit indicators, which both varied with treatment, were the main determinants of biweekly differences in CUE_ECO.  The direction of water effects depended on the timescale considered and occurred alongside treatment-dependent water depletion.