No abstract

Water availability is one of the factors affecting plant growth and development, especially in arid and semiarid environments. Changes in precipitation due climate change alter water availability to plants impacting on plant physiology. Numerous studies have focused on plant response to reduced precipitation and less on the effects of increased precipitation. The main objective of this study was to evaluate biophysical and physiological leaf traits in response to experimental water addition in four dominant shrubs and one grass species in a Patagonian steppe, during the dry season. The experiment consisted of two treatments: control and water addition, increasing the average annual rainfall by 25% during 6 years. We measured leaf wettability, water status, transpiration, photosynthesis, stomatal conductance, water use efficiency and foliar water uptake (FWU). In addition, we determined the phenotypic plasticity index of these evaluated traits. We expected lower FWU and higher transpiration and photosynthesis rates due changes in leaf surface properties under water addition treatment. All study species responded significantly to treatment with higher loss of water per transpiration and lower FWU. Also, all species increased photosynthesis rate and water use efficiency (WUE). However, water potential and leaf wettability did not change with higher precipitation. Thus, higher phenotypic plasticity was observed in functional than in morphological traits. Since functional traits were more sensitive than leaf surface traits, plants may quickly take advantage when environmental conditions tend to be more favourable to growth. Our findings suggest that plants of Patagonian steppe have adaptive ability to respond to environmental changes through plastic responses.

Aims: Soil respiration and N-mineralization are key processes in C and N cycling of terrestrial ecosystems. Both processes are limited by soil temperature, moisture and nutrient content in arid and cold ecosystems, but how nutrient addition interacts with water addition requires further investigation. This study addresses the effects of water and N+P additions on soil respiration and mineralization rates in the Patagonian steppe.Methods: We measured soil respiration and N-mineralization throughout seasons in control, fertilized, irrigated and irrigated-fertilized plots. We also analyzed root density and soil physico-chemical properties.Results: The soil CO2 effluxes in the Patagonian steppe were controlled by soil temperature, soil water content and root density. Increases in water addition had no effects on soil respiration, except when combined with N+P addition. However, soil nutrient enrichment without water addition enhanced soil respiration during the plant growing season. We found a linear positive relationship between root density and soil respiration, without interaction with treatments. N+P addition had negative impacts on N-mineralization, resulting in a strong N-immobilization. However, soil ammonium and nitrate content increased with N+P addition all over the seasons.Conclusion: Moderate increases in the precipitation through small pulses lead to no long-term response of soil processes in arid and cold Patagonian ecosystems. However, soil CO2 effluxes are likely to increase with nutrient additions, such as anthropogenic N deposition, and microbial biomass could retain more nutrients in the soil. Therefore, high levels of N enrichment in arid ecosystems may strengthen the positive feedback between C cycle and climate change.