Responses of nitrogen fluxes to N and water pulses in an arid ecosystem in the Patagonian Steppe

L. YAHDJIAN; O. E. SALA

Memphis, Tennessee, USA

Congreso; 91st Annual Meeting of the Ecological Society of America; 2006

Water availability has been identified as the main control of ecosystem functioning in arid ecosystems. However, there is evidence that nitrogen availability also accounts for part of primary production patterns in water-limited ecosystems. Our objective was to explore the interactive responses to water and nutrient availability of different soil processes that, in turn can explain N limitation in arid ecosystems. We designed a factorial manipulative experiment in the Patagonian Steppe, South of Argentina, where we manipulated water pulses input (a 15 mm event was added in two levels of pulse size-number addition, and a control) and nutrient availability (two levels of N availability: fertilized with 50 Kg N.Ha-1.year-1 and control) in plots of natural vegetation. We evaluated nitrogen transformation and losses in vegetated patches of shrub surrounding by grasses and in bare soil during a year. Soil inorganic nitrogen was significantly higher (p<0.05) in vegetated patches (7.5 g N/g dry soil ± 1.8) than in bare soil (2.2 g N/g dry soil ± 0.5) showing spatial nutrient heterogeneity. However, these differences were not translated into N fluxes. We found that water addition triggered pulses of denitrification that were higher in bare soil than in vegetated patches (p<0.04). Pulses were higher in winter, the rainy season in our study site, than during spring and summer. Inorganic N leaching was related to water and N availability (p<0.05). However, water pulses did not significantly stimulate N mineralization. These preliminary results show that water pulses stimulate N losses from soil that can be substantial when N availability is high and plants are not able to use it like during winter period.