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.