## 2 Materials and Methods
### Nutrient Manipulation Experiment, Gross Primary Production and Ancillary Data  
A nutrient manipulation experiment focused on the herbaceous layer was established in early spring 2014 and 2015. The set-up consisted of four 20 m \(\times\) 20 m width randomized blocks. Within each block we established four plots (9 m \(\times\) 9 m) with 2 m of buffer between treatments (Figure S1). We established four treatments (for details, see [37]): control (C) with no fertilization, N addition with one application of 100 kg N ha\({}^{-1}\) as potassium nitrate (KNO\({}_{3}\)) and ammonium nitrate (NH\({}_{4}\)NO\({}_{3}\)), P addition with 50 kg P ha\({}^{-1}\) as monopotassium phosphate (KH\({}_{2}\)PO\({}_{4}\)), and nitrogen-phosphorous (NP) addition, with 100 kg N ha\({}^{-1}\) and 50 kg P ha\({}^{-1}\) as NH\({}_{4}\)NO\({}^{3}\) and KH\({}_{2}\)PO\({}_{4}\), respectively. Carbon Dioxide (CO\({}_{2}\)) fluxes between the herbaceous layer and the atmosphere were measured in 32 collars of 60 cm \(\times\) 60 cm for each field campaign around noon local solar time (Table 1). At each collar, GPP (\(\mu\)mol CO\({}_{2}\)m\({}^{-2}\)s\({}^{-1}\)) was estimated as the difference between net ecosystem CO\({}_{2}\) exchange (NEE) measured with transparent chambers and ecosystem respiration (Reco) measured with opaque chambers. Measures CO\({}_{2}\) and water vapor mole fractions (W) were collected at 1 Hz by means of an infrared gas analyzer (IRGA LI-840, Lincoln, NE, USA) connected to the chambers. The flux calculations and corrections were conducted using the self-developed R package \"_RespChamberProc_\" ([[https://github.com/bgtw/RespChamberProc](https://github.com/bgtw/RespChamberProc)]([https://github.com/bgtw/RespChamberProc](https://github.com/bgtw/RespChamberProc))). Air temperature (Ta,\({}^{\circ}\)C) was measured with a thermistor probe (Campbell Scientific, Logan, UT, USA). Soil moisture content (%) at 5 cm depth was determined with an impedance soil moisture probe (Theta Probe ML2x, Delta-T Devices, Cambridge, UK). Vapor pressure deficit (VPD, hPa) was computed using relative humidity and Ta. Incident PAR (\(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) was measured with a quantum sensor (Li-190, Li-Cor, Lincoln, NE, USA) mounted outside of the chamber. Surface temperature (Ts, \({}^{\circ}\)C) was measured with infrared thermometer installed in the chambers (Tc, IRTS-P, Apogee, UT, USA). The meteorological conditions for each field campaign are reported in Table 1. Destructive sampling of the vegetation in four parcels (0.25 m \(\times\) 0.25 m each) within each plot was conducted to estimate LAI and green to dry biomass ratio [37]. The abundance of each functional group such as fraction of graminoids (%graminoids), forbs (%forbs), and legumes (%legumes) was determined. The Shannon biodiversity index (H) among plant functional types was determined as in [39]. N% and P% in plant tissues were determined as described in [37]. Carbon isotopic signature (\(\delta^{13}\)C) for the vegetation was determined from dried samples using a DeltaPlus isotope ratio mass spectrometer (Thermo Fisher, Bremen, Germany) coupled via a ConFlowIII open-split to an elemental analyzer (Carlo Erba 1100 CE analyzer; Thermo Fisher Scientific, Rodano, Italy). \(\delta^{13}\)C was calculated using the measured ratio between \({}^{13}\)C and \({}^{12}\)C in the sample and in a calibrated in-house-standard (Acetanilide: \(-30.06\pm 0.05\)%) as in [40,41] (Equation (4) and Figure S2):