A long term experimental project aims at understanding responses in different eddy covariance (EC) sites from nutrient loading (primarily N and P) and water availability, on ecosystem-level carbon, water fluxes, and their interannual variability. Three main focuses for the project are to understand: ecosystem-physiological response to changing N/P stoichiometry and water availability, structural and physiological phenology of tree-grass systems, and soil processes and properties.
The experimental sites were established in spring 2014. The project is broken up into two questions. The first is focused on the impact of N/P availability on ecosystem functioning and structure and mainly carried out in the “WET” cluster located in Majadas del Tietar, Spain (~650 mm precipitation year). The second is focused on the impact of different water availability on the ecosystem functioning and phenology looking at ecosystem fluxes and phenology in the “WET” and “DRY” sites.
The WET cluster is associated to an existing long-term EC site which provides more information about site history and available time series, this helps establish a ‘baseline’ of magnitude and interannual variability of C, water, and energy fluxes. Sites are selected in a way to have similar nutrient conditions, soil chemical-physical properties, canopy structure, and stoichiometry of the different vegetation pools. The “WET” site is Mediterranean savannah in Majadas del Tietar (Extramadura, Spain) (Figure 1). The mean annual temperature is 16˚C, mean annual precipitation is about 650 mm most of which it is from November to May, with a very dry summer. The sites consists of low intensity grazing cows, single oak trees, and open grassland area. The herbaceous stratum is mainly composed of annual species with a fraction of grasses, forbs, and legumes at the peak of the growing season of 84%, 15%, and ~1%, respectively. The different plant forms show distinct phenology during the year. The soil texture of the upper soil horizons down to a depth of 30 to 60 cm is sandy loam or loamy sand (SL or LS following FAO guidelines for soil description). In the deeper subsoil, the clay content is increasing, leading to a clay loam (CL) texture. Average N/P ratio of the vegetation measured in November 2013 is approximately 14 for trees and 9.1 for herbaceous plants.
In the N-ADD and NP-ADD 2 eddy covariance flux towers were installed in March 2014. In the first phase of the experiment, currently on-going, EC measurements are collected without any manipulation of the ecosystem in order to evaluate potential systematic differences between C and water fluxes in different sites. After the evaluation of this first phase, two footprint areas will be manipulated by adding N, and NP. At each site digital cameras to monitor phenology and grazing activity, spectrometers, and sap flow meters are being installed. At the main site 3 automatic weighed lysimiter stations and a sub-canopy EC system were also installed.
Figure 1: Map of the MaNiP experimental site and location of the 3 EC sites in the ”WET“ cluster. Red lines represent the footprint climatology according to Göckede et al., (2008). The area of the Small Scale Manipulation Experiment (SMANIE) is also reported.
In the DRY cluster the plan is to establish a mobile EC tower and after the first year of measurements establish a permanent system and a moving tower. The screening of the sites conducted during summer and fall 2014 allowed to identify potential locations. Currently one location has been selected and a meterological station was installed in November 2014 to characterize precipitation and wind regimes, necessary to plan the installation of the new eddy covariance site.