Environmental variables
Temperature and precipitation data were obtained from a meteorological
station located 2 km from the study site (http://sipas.inta.gob.ar/).
The volumetric soil water content was continuously determined by sensors
using the dielectric constant of the surrounding soil medium
(ECH2O, Dielectric aquameter,
Decagon Devices, Inc). In each plot, sensors were installed at 10, 40,
100 and 200 cm depth. The sensors were connected to CR1000 automatic
data acquisition systems (Campbell Scientific Inc.) powered by batteries
charged by a solar panel. Data were recorded every 30 min. Soil
psychrometers (PST-55, Wescor, Logan, UT) were used to continuously
monitor soil water potential at 40 cm depth, which was measured every 30
min and data were recorded with a datalogger (CR-7, Campbell Scientific
Inc.).
Data for pH, total nitrogen, inorganic nitrogen, available phosphorus,
and carbon content in the upper soil layer were obtained in November
2018. Three soil cores (5 cm diameter, 5 cm depth) were randomly taken
from each plot and mixed well to obtain a composite sample (n = 5). The
pH was determined in a 1:2.5 solution of distilled and deionized
H2O. Soil organic matter, total nitrogen and available
phosphorous were analyzed by the Soil Laboratory (INTA Chubut,
Argentina). Total nitrogen was determined by the Kjeldahl method
(Bremner, 1996), inorganic
nitrogen by the distillation method (Bremner & Keeney, 1965),
phosphorus by the Olsen method (Olsen et al., 1954), and organic matter
by the wet oxidation method (Walkley & Black, 1934). The C content of
the organic matter was then determined using the factor proposed by Van
Bemmelen, of 1,724, which assumes that 58% of the organic matter is
composed of C (Tabatabai, 1996). The C/N ratio was also calculated by
dividing the carbon content by the nitrogen content (Bremner, 1996).
Vegetation cover and leaf nitrogen
content
Changes in vegetation cover between 2012 (before the start of the
experiment) and 2019 (i.e., 6 years after the start of the experiment)
were determined using the intercept point method (Mueller‐Dombois &
Ellenberg, 1974). For this, the plant interception was determined every
one meter along the two diagonals of each plot. Subsequently, the
percent cover of each species and bare soil in each plot was calculated
annually by dividing the number of positive touches for each species or
bare soil by the total number of touches. It should be noted that
ephemeral herbaceous species, mainly present during October and
November, were not taken into account in the vegetation cover analysis.
Leaves were collected in December 2017 for analyses of nitrogen
concentrations. Fully expanded and healthy leaves from different
individuals of each species and in each plot were collected to obtain a
composite sample per plot and species. Total leaf N content was measured
using the Kjeldahl technique.