Grasses and shrubs had different responses to nitrogen fertilization in the arid Patagonian Steppe

Background/Question/Methods

Nitrogen limits plant growth in almost all terrestrial ecosystems, even in arid and semiarid ecosystems. Vegetation in these ecosystems is usually made up of grasses and shrubs, which have different rooting patterns and strategies to acquire water and nutrients. Shallow-rooted grasses absorb N from a relatively N-rich layer but have low leaf N concentrations relative to shrubs. Therefore, it is difficult to predict the N fertilization response of these two groups of plants. The objectives of this study were to assess nitrogen limitation and the differential response to nitrogen availability of grasses and shrubs, which represent two contrasting plant functional groups. We performed a nitrogen fertilization experiment during two years in the Patagonian steppe, South of Argentina. We established twenty 9 m² plots of natural vegetation and fertilized half of these plots with 5 g/m² of ammonium nitrate, so we had 10 replicates of each treatment. After two years of manipulations, we evaluated aboveground net primary production (ANPP, estimated as peak biomass),% N of green leaves and nitrogen response efficiency (NRespE) of the dominant grasses and shrubs, which was estimated as the ratio of ANPP and soil mineral N availability.

Results/Conclusions

Grasses and shrubs showed different responses to N addition. Biomass of all grasses significantly (P=0.007) increased with N fertilization after two years of N addition whereas shrubs biomass did not change. Percent N of green leaves significantly (P=0.002) increased as a result of N fertilization in all grass species but only increased slightly in one of the shrub species, Senecio filaginoides. Percent N of green leaves was inversely related to foliar N responses to N addition (P=0.07). Nitrogen response efficiency (NRespE) was, on average, significantly (P=0.045) higher for shrubs than grasses and decreased with N fertilization for shrubs, but remained constant for grasses.

Our results suggest that in the Patagonian steppe, grasses are more N limited than shrubs which was evident by grasses biomass increment with N fertilization, which was not evident for shrubs. These different responses of grasses and shrubs to N addition may cause changes in the distribution of these functional groups as a consequence of global N deposition and future increases in reactive N. This, in turn, may have consequences for rangeland management and livestock production, which is a major ecosystem service of arid lands.