Response of plant species to water and nitrogen addition: plant
cover and mortality
The large increase in the abundance of P. humilis observed after
7 years of soil nitrogen addition supported our hypothesis that N
enrichment favors the growth of nitrophilous species. On the other hand,
nitrogen addition had a significant negative effect on P.
ligularis by almost completely reducing its presence in those plots,
while persistent tussocks were smaller. A plausible explanation for
these results may be that P. humilis is a better competitor on
nitrogen-rich soils that P. ligularis and P. speciosawhich perform better in poor soils. Pappostipa humilis has been
positively associated with the shrub A. volckmannii , a N-fixing
species (Armas et al., 2008). The leaves of this shrub species had a
higher leaf N content than the other shrub species in our study, and
therefore a higher amount of N in the soil next to plants of this
species is consistent with the hypothesis that high soil N levels favor
the establishment of P. humilis .
On the other hand, the response of P. ligularis and P.
speciosa could be a consequence of the combination of acidification and
ammonium toxicity. In a previous study at the same site, Carbonell
Silletta et al. (2022) found a lower soil pH and a large increase in the
NH4-N/NO3-N ratio in the +N plots
compared to the C and +W plots. Several studies have shown that these
soil changes are detrimental to plant germination, growth and survival
and have been proposed as some of the potential mechanisms explaining
biodiversity loss in response to N enrichment (Bai et al., 2010; Liu et
al., 2019; Y. Zhang et al., 2014). Some responses of grasses found in
our study contradict those found by Campana et al. (2022). These authors
observed that P. ligularis responded positively to nitrogen
addition increasing its biomass, while P. humilis was not
affected and showed a tendency to decrease aboveground biomass. The
discrepancy between these two studies could be due to the different
amount and frequency of N addition.
We hypothesized that grasses are more sensitive to water addition than
shrubs due to differences in root distribution (Bucci et al., 2011).
However, the +W treatment did not affect any of the grass species
studied here. Poa ligularis has been previously mentioned as a
more plastic species than P. speciosa , increasing its biomass
under optimal water conditions (Couso & Fernández, 2012). On the other
hand, Golluscio et al. (1998) observed short-term physiological
responses to summer water addition in P. speciosa . In our study,
the only species that responded to water treatment was the shrubA. volckmannii which substantially increased its productivity.
Our results indicate a low plasticity of Patagonian steppe species to
water addition. Although in a recent study in the same area Cavallaro et
al. (2023) observed that these species are relatively plastic to water
addition, the magnitude of plasticity was higher in functional traits
than in morphological traits. Increased leaf carbon assimilation under
the +W treatment observed by Cavallaro et al. (2023) was not reflected
in higher productivity in the present study. Photoassimilates were
probably allocated to roots or other functions (e.g., anti-herbivory
defenses) rather than to increase ANPP.
Soil nitrogen addition also caused mortality of some shrub individuals,
mainly S. filaginoides , which resulted in a lower ANPP for this
species. Soil N addition in excess of plant nutritional N demand could
trigger a nitrogen saturation response (Wallace et al., 2007).
Therefore, the advantage of N addition on plant productivity may
decrease with continuous N enrichment (Tian et al., 2016). This response
is species-specific and, therefore, species differences to N saturation
could explain the similar total shrub ANPP found in this study, despite
increased mortality in some species. Across global grasslands, the mean
ANPP saturation threshold has been estimated at 150 kg
ha-1 yr-1 (Peng et al., 2020),
higher than that applied in the present study.
Plant functional groups and
plant community ANPP responses to experimental water and nitrogen
addition
Several studies indicate that
arid ecosystems are more restricted by water than nutrients or are
co-limited by both (Guo et al., 2022; Hall et al., 2011; Lü et al.,
2014; Robertson et al., 2009). However, our results contradict this
idea, at least for grasses, due to the lack of response of ANPP to water
addition. This response was also found by Swindon et al. (2019) in
another semiarid steppe. However, in that study, ANPP increased when
water addition was combined with nitrogen addition. We observed non
co-limitation between these resources, as grass ANPP increased in
similar magnitude under +N and +NW. A similar water content in the
shallow soil layers over the years, regardless of water inputs (natural
or experimental), could explain the lack of response of grasses in our
study. According to soil water release curves, the shallow soil layers,
where grasses have most of their roots (Bucci et al., 2011), had water
potentials close to 0 MP during a large part of the growing season of
all study years. We suggest that grasses in this steppe are not limited
by water until December, which is the peak of the growing season.
The increase in grass ANPP in response to nitrogen addition has also
been observed in other studies (Gherardi & Sala, 2015; Henry & Aherne,
2014; Kowaljow et al., 2010; Tang et al., 2017; Yahdjian et al., 2014).
Decreases in abundance and tussock size of P. ligularis andP. speciosa were offset by changes in P. humilisabundance, such that grass ANPP was higher in the +N and +NW plots
relative to the other treatments. Although grass ANPP increased
significantly with nitrogen addition, this response can be interpreted
as a negative effect for this grassland where livestock grazing is the
main land use (Adler et al., 2004) and a highly palatable species asPoa ligularis is replaced by a species of low forage value
(P. humilis ) (Cenzano et al., 2013; Oñatibia & Aguiar, 2016).
The lack of response of shrubs to the +W treatment is probably due to
the amount of water applied in each irrigation pulse. Apparently, it was
not enough to increase soil moisture below the 10 cm layer where the
shrubs explore most of the soil profile. Shrub ANPP only increased
significantly during the wet year, regardless of the treatments. These
findings suggest that shrubs, more than grasses, may be restricted by
soil water availability and only respond to higher precipitation when it
causes an increase in water content in deeper soil layers. Although in
this study site less than 10% of the roots are below 1 m depth, they
contribute about 35% of the total water use (Pereyra et al., 2017).
Therefore, the higher shrub ANPP during the wet year could be correlated
with soil water recharge, root distribution, and hydraulic efficiency of
deeper shrub roots. Consistent with our findings, Burek et al. (2023,
under revision) found at the same study site, using the eddy covariance
method, a higher net carbon exchange in 2017 (283.9 g C
m−2 yr−1) than in 2015 (218.6 g C
m−2 yr−1), which was related to
higher soil water content throughout the soil profile (0-2 m depth).
Unlike grasses, shrub ANPP was insensitive to nitrogen addition. These
results are in agreement with studies in similar steppes (Fernández et
al., 2018; Reichmann et al., 2013; Yahdjian et al., 2014). Although ANPP
did not change under the +N treatment, shrubs increased N uptake under
nitrogen addition resulting in a large increase in leaf N content. These
leaf changes did not scale up to alter the biomass production of woody
species because nitrogen use efficiency substantially decreased in all
nitrogen addition treatments. Similar results were reported by Lü et al.
(2014) in temperate semiarid steppes of China. The results of
community-level ANPP determinations using destructive methods and
allometric relationships were consistent with the findings obtained
using field spectral reflectance sensors placed at 9 m height and
satellite imagery. Both approaches indicated that soil nitrogen
enrichment had a positive effect on aboveground plant productivity. The
higher NDVI in the +W plots relative to the C plots could be explained
by the presence of forbs and some minor species which were not sampled
for ANPP determination using destructive methods and allometric
relationships.