Effects of a large scale stoichiometric imbalance manipulation on the ecosystem functioning of a Mediterranean tree-grass ecosystem: The MANIP experiment

Recent studies have shown that human induced Nitrogen (N) and Phosphorus (P) stoichiometric imbalance affect essential ecosystem processes, and might be particularly important in water-limited ecosystems. Here we present results from an large scale manipulation of a Mediterranean tree-grass ecosystem (Majadas del Tietar, Spain). We will show how ecosystem functioning (e.g. light, water use efficiency - WUE, albedo) and phenology respond as consequence of N and NP manipulation.

A cluster of eddy covariance (EC) flux towers has been set up beside a long-term EC site (Control site). The sites were selected in a way to have similar pre-treatment conditions. Two out of three EC footprint areas (18 Ha) were fertilized with N and NP. To interpret the variations in fluxes measured we monitored spatial and temporal variations in hyperspectral properties of the canopy,digital repeat photography, phenology, root growth, plant traits, and tree transpiration with sap-flow meters, and soil sampling.

The results show an increase in net ecosystem production (NEP) both in the N and the NP treatments compared to control. The results show an increase of evapotranspiration in both fertilized treatments, but the WUE is larger at the NP treatment, emphazising the role of P in the modulation of WUE. The fertilization stimulated more gross primary productivity (GPP) rather than total ecosystem respiration (TER), therefore increasing the capability of the ecosystem to act as carbon sink and increasing apparent carbon use efficiency. The effects of fertilization are pronounced in spring and winter and negligible in summer. An increase of greenness and an earlier green-up of grass in the N and NP sites in autumn after the first rainfall are observed. In particular the response of NEP and greenness to onset of precipitation in fall is stronger for NP treatment.

Results suggest that an increase of N availability can have a relevant impact on the studied tree-grass ecosystem. Compensating the induced N/P imbalance by adding P, marginally impacted the C fluxes, while increased the WUE of the ecosystem. Differences in temporal variations of ecosystem level greenness and near infrared reflectance also might indicate an impact of N and NP availability of the temporal variability of ecosystem fluxes and potentially impact surface albedo.