Activity of soil enzymes in grazed ecosystems of northeastern Chubut Province, Argentina.

LUCIANO PRIETO; MÓNICA BERTILLER; MARCELA FERRERO; NELDA LILA OLIVERA

Rosario

Congreso; V Congreso Argentino de Microbiología General; 2008

Sociedad Argentina de Microbiología General (SAMIGE)

Soil enzymes have an important role in nutrient cycling and organic matter decomposition. We analyzed dehydrogenase, ß-glucosidase, protease, alkaline and acid phosphatase activities in the soil, and their relationships with soil organic carbon (C), total nitrogen (N) and pH at three sites across a grazing pressure gradient in northeastern Patagonia. At each site, five surface soil samples were taken from both plant-covered patches (PCP) and inter-canopy areas (IC) randomly selected. A sharp change in dehydrogenase, ß-glucosidase, and alkaline phosphatase activities existed between PCP and IC, with significantly higher values in the former. Strong positive correlations were observed among such variables and between these and soil organic C. At all sites, alkaline phosphatase activity was 2 to 3-times larger than acid phosphatase activity. Moreover, for the latter there were no significant differences among sites. With increasing grazing pressure, C and N decreased significantly in PCP, and protease activity in IC. The C/N ratio increased significantly with grazing pressure in bare interspaces, but remained unchanged in canopy soils. The pH values increased with grazing, in both PCP and IC. This agrees with findings of other studies explaining this increase by animal excrement and urine input, followed by a rapid urea-N hydrolysis. The high dehydrogenase, ß-glucosidase, and alkaline phosphatase activities in PCP may be explained by preferential resource accumulation, soil moisture, and milder climatic conditions beneath the plant canopy promoting microbial processes. The low acid phosphatase activity could be attributed to the high soil pH, which may favor microorganisms producing alkaline-active enzymes. The decrease in protease activities observed in soils with increasing grazing, which agrees with results obtained in other semi-arid areas, suggests that N mineralization and N availability may be affected by grazing. This is also supported by a diminution in N. In addition, intense grazing induces changes in vegetation resulting in plant litter with more recalcitrant organic material that could explain the increase in C/N ratio. The variations in the patterns of enzyme activities with grazing, such as those observed in this study, are valuables to assess the effects of different management regimens in semi-arid environments.