Litter decomposition and nutrient release in the Patagonian steppe: carbon and nitrogen turnover decoupled?.

AMY T. AUSTIN; LUCÍA VIVANCO; MARINA GONZALEZ POLO

Memphis, Tennessee, Estados Unidos de América

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

The Ecological Society of America

Litter decomposition and nutrient release in water-limited ecosystems result from the interaction of direct abiotic and biotic controls and are a critical link in determining nutrient availability and retention, and quality of soil organic matter. Evidence from various experiments in the Patagonian steppe suggest that the controls on carbon and nitrogen turnover in this ecosystem are distinct, and that traditional models from mesic ecosystems which link carbon and nutrient turnover may not apply. The patchy vegetation structure of the Patagonian steppe is reflected in high spatial heterogeneity in soil characterstics in vegetated and unvegetated areas. Measurements of both static biogeochemical parameters such as soil carbon and nitrogen content, as well as rates of turnover including beta-glucosidase enzyme activity (P<0.01) and net nitrogen mineralization (P<0.001) all demonstrate significantly higher rates and concentration of nitrogen below shrub patches when compared to unvegetated patches. At the same time, litter decomposition shows precisely the opposite pattern, with maximum rates of mass loss occurring in bare soil or standing dead positions and minimum rates below shrubs. Direct manipulative experiments of solar radiation and soil resource availability support this result of differential controls on organic matter loss and nutrient release. Aboveground litter decomposition was strongly affected by intercepted solar radiation (P<0.001) but not through additions of soil substrate C and N, while nutrient release from litter demonstrated the opposite pattern, with no significant effect of photodegradation and significant short-term immobilization of N with additions of inorganic N and C+N (P<0.01). Taken together, these results suggest that carbon and nutrient mineralization in this ecosystem may have different primary controls, with carbon loss largely driven by abiotic photodegradation, and nutrient mineralization mediated biotically, occurring in islands that are protected from the photooxidative effects on organic matter.