Relationship between aboveground productivity and annual water inputs (ambient precipitation and experimental water addition)
Arid and semiarid ecosystems are considered highly sensitive to precipitation changes (Haverd et al., 2016; Huxman et al., 2004; Maestre et al., 2012). Several studies show a linear relationship between interannual ANPP and precipitation (Sala et al., 2012; Wang et al., 2014). Recently, however, Knapp et al. (2017) proposed that with the inclusion of precipitation extremes, the common linear relationship may change to a nonlinear one, reflected in “concave up” or “concave down” ANPP-precipitation relationships. Consistent with the linear model, we found symmetric relationships with ANPP of each functional group and total ANPP under all treatments, even when our data set included an anomalous year. In previous studies in a similar Patagonian steppe (Fernández et al., 1991; Jobbágy & Sala, 2000), at least grass ANPP was not associated with precipitation.
The sensitivity of grass ANPP to annual water inputs was higher in the +N and +NW treatments than in the C and the +W treatment. Therefore, our results suggest that increasing N availability can enhance ANPP response to increased precipitation, as shown by other studies (Ma et al., 2020; Meng et al., 2022; Zhang et al., 2021). When water availability is not a limiting factor for plant growth, as was observed for grasses in our study, plants often exhibit increased biomass production in response to improved soil N availability. Differences in the slopes of the ANPP-water input relationships suggest that grasses in the control plots were generally less sensitive to annual precipitation than shrubs. It provides evidence that soil water redistribution during wet years benefits deep-rooted species more than grasses, as grasses are less water-limited because shallow soil layers are recharged during the fall and winter seasons.
At the community level, although N enrichment increased productivity, it did not improve the sensitivity of ANPP to precipitation, probably due to the combined effects of precipitation and nitrogen addition on shrubs and grasses, highlighting the importance of community structure. The lower intercept value of the function fitted to the +W treatment compared to the control suggests higher nutrient limitation by increasing plant N demand or N losses (Mudge et al., 2017; Ren et al., 2017). However, in previous works in the Patagonian steppe, Carbonell Silletta et al. (2022) found no change in soil nutrient availability with water addition and Cavallaro et al. (2023) found increased rates of transpiration and photosynthesis in +W plots. This suggests that by increasing water input without nitrogen addition, more photosynthates are allocated to other functions rather than aboveground growth.

Conclusions

Our results show that nitrogen addition rather than water addition improves community ANPP, mainly due to the effects on the grasses and to the shift in the dominance of P. ligularis to P. humilis . Our study revealed that the relationship between ANPP and annual precipitation can be described by linear models across dry and wet years. Grasses and shrubs responded differently to increased precipitation, with shrubs being more sensitive than grasses to wet years. However, under increased nutrient availability, the response of grass ANPP to annual water input was amplified (i.e., steeper slope of the ANPP-water input relationship), whereas shrubs showed no change in ANPP with nitrogen addition. Therefore, the higher sensitivity of shrub ANPP to increased precipitation compared to that of grasses suggests that under climate change scenarios with increased precipitation, shrubs could dominate the community, whereas if this change occurs with high N deposition, an encroachment of grasses by a single species may take place in the long term.
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Table 1. Family, life form, leaf phenology, plant height and maximum rooting depth for species of Patagonian steppe included in this study