Preprints
https://doi.org/10.5194/egusphere-2023-2434
https://doi.org/10.5194/egusphere-2023-2434
23 Oct 2023
 | 23 Oct 2023
Status: this preprint is open for discussion.

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

Richard Nair, Yunpeng Luo, Tarek El-Madany, Victor Rolo, Javier Pacheco-Labrador, Silvia Caldararu, Kendalynn A. Morris, Marion Schrumpf, Arnaud Carrara, Gerardo Moreno, Markus Reichstein, and Mirco Migliavacca

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 (CUEECO), 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 (RECO, plants and heterotrophs) are limited by N, P, or associated changes in other biogeochemical cycles, CUEECO may change. Seasonally, CUEECO 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 CUEECO 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 CUEECO should be increased by wetter conditions, and by NP fertilization.

Milder and wetter years with proportionally longer growing seasons increased CUEECO, 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 CUEECO.  The direction of water effects depended on the timescale considered and occurred alongside treatment-dependent water depletion. 

Richard Nair, Yunpeng Luo, Tarek El-Madany, Victor Rolo, Javier Pacheco-Labrador, Silvia Caldararu, Kendalynn A. Morris, Marion Schrumpf, Arnaud Carrara, Gerardo Moreno, Markus Reichstein, and Mirco Migliavacca

Status: open

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Richard Nair, Yunpeng Luo, Tarek El-Madany, Victor Rolo, Javier Pacheco-Labrador, Silvia Caldararu, Kendalynn A. Morris, Marion Schrumpf, Arnaud Carrara, Gerardo Moreno, Markus Reichstein, and Mirco Migliavacca
Richard Nair, Yunpeng Luo, Tarek El-Madany, Victor Rolo, Javier Pacheco-Labrador, Silvia Caldararu, Kendalynn A. Morris, Marion Schrumpf, Arnaud Carrara, Gerardo Moreno, Markus Reichstein, and Mirco Migliavacca

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Short summary
We studied a Mediterranean ecosystem to understand carbon uptake efficiency and its controls. These ecosystems face potential nitrogen-phosphorus imbalances due to pollution. Analysing six years of carbon data, we assessed controls at different timeframes. This is crucial for predicting such vulnerable regions. Our findings revealed N limitation on C uptake, not N:P imbalance, and strong influence of water availability. whether drought or wetness promoted net C uptake depended on timescale.