Relationships between ANPP and precipitation (ambient +
experimental)
The interannual variability in water inputs ranged from 92 to 233 mm in
the plots without experimental water addition and from 122 to 263 in
plots with water addition. A positive relationship with the same
adjusted slope (0.21) was found between annual water input and shrub
ANPP in all treatments (Fig. 7A). The intercept of the ANPP-water input
relationship was lower in +NW than in C (p =0.004), while in +N
and +W there was no difference compared to C
(p >0.05) (Fig. 7A). Grass ANPP also was positively
and linearly correlated to water input (Fig. 7B), although the nitrogen
addition (i.e., +N and +NW treatments) had a higher positive effect
(p≤0.004) (Fig. 7B). The +W treatment did not affect the response of
grass ANPP to water input compared to C (p >0.5)
(Fig. 7B). The relationship between total ANPP and water input was
significantly modified by the treatments. The +N and +NW treatments
enhanced the intercept of the function fitted to the ANPP-water input
relationship (p<0.001), while the +W treatment decreased it
(p =0.03) (Fig. 7C).
Discussion
Seven-years monitoring of a large-scale field nitrogen and water
addition experiment in a Patagonian steppe showed that plant
productivity, cover and mortality responded differentially to soil
nitrogen addition and water addition. This response also differed
between and within functional groups (grasses and shrubs). Although
water availability is considered the most important environmental factor
limiting productivity in drought-prone ecosystems, our results indicate
that addition of 25% more water than historical annual precipitation
resulted in a total ANPP similar to the ANPP in the control plots, while
the nitrogen addition had a positive effect on ANPP.
The total ANPP in control plots reported in our study was similar to
that estimated for other semiarid steppes (e.g., Bai et al., 2010; Gao
et al., 2011). The range of ANPP values obtained in our study in the
control plots (41-98 g m-2 yr-1) is
consistent with that reported by Jobbágy & Sala (2000) for a similar
Patagonian steppe (21-75 g m-2yr-1). We determined the ANPP in early December, while
that in previous studies developed in the Patagonian steppes (e.g.,
Gherardi & Sala, 2015; Jobbágy & Sala, 2000; Yahdjian & Sala, 2006),
ANPP has been estimated in January. Despite the similar range of ANPP
between our study and the others, we consider late November or early
December as the most accurate measurement period to estimate ANPP since
the maximum NDVI and MSAVI2 were observed in this period. Therefore,
measurements done in January instead at the end of November or early
December could result in relatively low ANPP estimates in the Patagonian
steppe.