Appendix A. Plant-species traits that affect ecosystem functioning.
(a) Relative growth rate: Plant relative growth rate represents potential growth of a species independently of abundance; it is an important trait by which species can have different impact on primary production (Eviner and Chapin 2003). To assess relative growth rate, we created monocultures of the six species in 5 × 5 m plots, by removing the other species present. For each species, we estimated the relative growth rate as a biomass ratio between January and March of the previous year corresponding to the peak and end of consecutive growing season. In each plot, we used a non-destructive method to estimate biomass, which is based on a positive correlation between vegetation cover and biomass (Flombaum and Sala 2007). We estimated species cover on 4 parallel lines equally spaced (Canfield 1941). Each monoculture plot had three replicates.
(b) Potential soil respiration: We estimated plant species effects on CO2 release from the soil to the atmosphere. Potential soil respiration estimates microorganism activity without water or temperature limitations. We sampled the first 5 cm of soil beneath a typical individual of each species (the same individuals were used for variables described in points d, e, and f). Each soil sample was sieved with a 2 mm mesh; a 50 g soil subsample was moistened to field capacity and placed in a 0.25-L jar with a trap containing a solution of 0.1 M NaOH. Jars were placed in the dark at 25 °C for 21 days, and traps were replaced after one week. The solution was titrated with 0.15 M BaCl and 0.1 M HCl to estimate amount of CO2 respired. Afterwards, soils were dried at 100 °C and weighed. We considered each jar a replicate and expressed potential respiration as micrograms of carbon per day and gram of dry soil. Each species had ten replicates.
(c) Rooting depth: Root length gives an indication of the plant capacity to absorb soil moisture (Gordon and Rice 1993, Eviner and Chapin 2003). We used the maximum depth recorded for each species based on previous descriptions for Río Mayo locality (Soriano et al. 1987, Fernández Alduncin and Paruelo 1988, Sala et al. 1989, Golluscio et al. 2006).
(d) Plant phenology: Plant annual growth cycle can have an important effect on seasonal water demand and carbon fixation (Eviner and Chapin 2003). We recorded number of months in which plants of the studied species had green leaves during one year.
(e) Soil thermal amplitude: Plant species can influence soil microclimate through canopy cover (Hogg and Lieffers 1991, Eviner and Chapin 2003). In arid ecosystems, plant effects on soil temperature can impact soil biological processes because they are concentrated in the upper soil layers. We recorded soil temperature in the upper 5 cm under five individuals of each species and bare soil along the growing season at 9:00 and 14:00 hours. We estimated soil thermal amplitude as the daily difference in soil temperature, and averaged values for each sampled individual (n = 5).
(f) Potential soil net nitrification and net ammonification: Plant species can have direct impacts on soil nutrient availability by modifying the nitrogen net mineralization process, which is the balance between soil nitrogen mineralization and immobilization. In a growth chamber without temperature or water restriction, we assessed potential net nitrogen mineralization. We sampled the upper 5 cm of soil underneath each individual species. Each sample was sieved to 2 mm, and a 10 g subsample of moistened soil was placed in a 0.1 l jar sealed with a perforated parafilm, wetted and maintained to field capacity, and dark incubated at 25 °C. Before and after two weeks of incubations, we extracted soil inorganic nitrogen with a 2 N KCl solution, and determined nitrate and ammonium concentration in the solution with a colorimetric method using an Alpkem® autoanalyzer (O-I Corporation, College Station, Texas, USA). Potential soil net nitrification rate was the difference of nitrate concentration before and after the incubation divided by 14 days; potential soil net ammonification rate was the same for ammonium. Positive or negative differences in inorganic nitrogen concentration suggested net release or immobilization respectively. Each species had 5 replicates.
(g) Nitrogen form relative preference: Plant preference for certain forms of nitrogen could influence availability of soil inorganic nitrogen (McKane et al. 2002, Weigelt et al. 2005). We assessed plant-species relative preference for inorganic nitrogen with a greenhouse hydroponic experiment, where we provided plants with a nutrient solution containing equal amounts of ammonium and nitrate. We considered that a species preferred a nitrogen-form if the ratio of ammonium and nitrate in the solution at the end of the experiment differed from the unit. We collected plants from all six dominant species from the field and randomly assigned them to 25-mL flasks with a strength Johnson’s modified solution [0.002 M KH2PO4, 0.002 M K2SO4, 0.001 M MgSO4, 0.004 M CaSO4, 0.001 M Ca(NO3)2, 0.001 M (NH4)2SO4]. We determined nitrate and ammonium concentration in the solution at the end of the experiment with a colorimetric method using an Alpkem autoanalyzer (O-I Corporation, College Station, Texas, USA). Each species had ten replicates.
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