CONICET | Buscador de Institutos y Recursos Humanos

Water availability has been identified as the main control of ecosystem functioning in arid and semiarid ecosystems. However, nitrogen has been shown to limit net primary production in many deserts ecosystems. As nitrogen availability is mainly controlled by internal cycling, potential N losses from ecosystem have a crucial importance for the nutrient balance and primary production. Our general objective was to estimate precipitation pulses effects on nitrogen losses and nitrogen turnover in vegetated patches and bare soil in an arid ecosystem, the Patagonian steppe. We designed a rainfall manipulation experiment in the Río Mayo Experimental site in Chubut, southern Argentina, where we simulated different precipitation patterns by adding the same amount of water in evenly spaced three small rainfall events or in one single large rainfall event, three times during a growing season. We also had control plots, without water addition. We monitored the effect of rainfall pulses on nitrogen mineralization and nitrogen losses by ammonia volatilization, denitrification, and nitrate and ammonia lixiviation form the uppermost soil layer, in vegetated patches and bare soil during the growing season. We found that soil inorganic nitrogen content in the Patagonian steppe was higher in vegetated patches than in bare soil (P<0.001), but N fluxes were not consistently higher under plants. Nitrate and ammonia lixiviation were the main inorganic nitrogen losses from the upper soil layer (0-10 cm), while gaseous losses were less important. Precipitation input stimulated nitrate lixiviation and denitrification, with small consecutive pulses showing smaller responses than large pulses (P<0.1). By contrast, water pulses did not stimulate ammonia lixiviation or ammonia volatilization. Keywords: Patagonian steppe, net N mineralization, spatial heterogeneity, soil NO3- and NH4+, denitrification, nitrate lixiviation, ammonia volatilization