Factors affecting bacterial productivity in soils on isolated inland nunatanks in continental Antarctica

Heterotrophic bacterial production rates in soils on inland nunataks (rocky outcrops in the glacial ice sheet) in western Dronning Maud Land, Antarctica, were investigated by field and laboratory experiments using 3H-thymidine (3H-TdR) and 3H-leucine (3H-Leu) incorporation. Bacterial densities and productivities, and nutrient and physical parameters of soils from four different habitats were compared: soil from beneath moss beds, soil from the nests of the snow petrel (Pagodroma nivea), exposed unvegetated soil (polygon soil) 5 m away from nests, and exposed polygon soil on nunataks without the nests of breeding birds. Organic content, and nitrogen and carbon concentrations were significantly higher (ANOVA, P < 0.05) in soils from nests of the snow petrel than in all other soil types. Moisture levels in soils from nest entrances and beneath mosses were much higher than in soils from unvegetated exposed polygons. Polygon soils from nunataks without breeding birds exhibited the lowest nutrient content, but did not differ significantly (P> 0.05) from polygon soils from nunataks with breeding birds. Exposed polygon soils reached the highest midsummer midday temperatures (> 20 degrees C), while moss and nest soils remained much cooler. However, nest and moss soils were relatively thermally stable, compared to widely fluctuating exposed polygon soils. Soils from nests exhibited significantly (P < 0.05) higher rates of bacterial production than did any other habitat (32.3 +/- 42.2 and 18.8 +/- 13.8 cells x 10(6) gDW-1 h-1, for 3H-thymidine and 3H-leucine incorporation, respectively), whereas productivity in moss soils (9.5 +/- 5.6 and 7.9 +/- 1.5 cells x 10(6) gDW-1 h-1, for 3H-thymidine and 3H-leucine incorporation, respectively) was more than twofold greater than in either polygon soil habitat. Soils in nests and moss beds thus represent bacterial "hotspots" in an extensive matrix of exposed polygon soils, supporting denser bacterial populations and enhanced bacterial productivity. Whereas increased bacterial productivity in nest soils may be attributed to enhanced soil nutrient levels, productivity in moss soils appears to be limited by nutrients, and in polygon soils is further dampened by desiccation and relatively large daily temperature fluctuations. Stepwise multiple regression analysis revealed no correlation between bacterial productivity and physical factors measured. Weak but significant correlations were obtained between bacterial productivity and nutrient availability. This suggests that during the most favorable conditions in midsummer, bacterial growth is limited primarily by resource availability. However, water availability and temperature are likely to play important but intermittent regulatory roles.