Environmental controls of the diversity, activity, and function of soil nematodes in the McMurdo Dry Valleys of Antarctica

Abstract

The McMurdo Dry Valleys of Antarctica are one of the most extreme terrestrial environments on Earth. The polar desert soils of the dry valleys are frozen for most of the year, poorly-weathered, ahumic, often saline and desiccated, yet they contain simple low-diversity communities of microbes and their invertebrate grazers (primarily nematodes). The objective of this research was to study ecosystem processes and invertebrate distribution in soils, incorporating studies of nematode activity as an indicator of biotic function. Nematodes are capable of employing an inactive, ametabolic, anhydrobiotic survival strategy in response to adverse environmental conditions. Understanding where and when soil nematodes in the dry valleys are anhydrobiotic and inactive is important to determining how the extreme environment influences biological processes such as decomposition across the landscape. First, I studied nematode activity with respect to soil moisture, electrical conductivity (as a proxy for salinity), water potential, and temperature over seasonal and diurnal temporal scales. My objective was to understand how these factors interact and influence nematode activity. For these experiments, I developed a sampling method to fix the status of nematodes in field samples so that nematode activity was not altered by transport and storage of soils. Nematodes communities in the soils studied were often found with high proportions (> 60%) in anhydrobiosis (indicated by coiled morphology). Anhydrobiosis was most strongly correlated to soil moisture content and water potential in the soils studied, with more nematodes inactive in drier soils. In the driest soils with less than 2% soil moisture content, however, coiling of nematodes was not associated with moisture content, water potential, or electrical conductivity, suggesting that unmeasured factors are influencing activity. Nematode activity did not vary greatly over seasonal (spring to fall) and diurnal temporal scales, but addition of moisture in a soil manipulation experiment and from natural snow melt was a strong trigger for emergence from anhydrobiosis. Second, I studied nematode anhydrobiosis in soils and sediments collected across a dry valley stream channel. I predicted that the transfer of moisture and salts in the transition zone between soils and sediments would affect the structure and activity of invertebrate communities. Diversity, but not abundance, of invertebrates was correlated to moisture in these samples. Assemblages of nematodes, rotifers, and tardigrades were found in the wettest samples, beneath flowing stream waters, where productivity was highest. In contrast, in the driest soils studied, communities consisted almost entirely of a single nematode species, Scottnema lindsayae. Nematode anhydrobiosis was correlated positively to declining moisture, suggesting that this survival strategy is important for survival of Scottnema in the dry soil habitat. Finally, I studied nematode activity in conjunction with field and microcosm studies of decomposition, a process regulated by soil biota. In microcosms, decomposition of cotton strips, soil microbial respiration, and nitrification were all accelerated by the addition of water over a 9-month incubation at 10°C, suggesting that soil micro- and macrobiota in dry valley soils are capable of functioning similarly to organisms in temperate soil food webs. Decomposition and soil microbial respiration were detectable at ambient soil moisture levels (< 1% gravimetric), but respiration was very low (0.0018 μmol CO2 g-1 soil d-1). In the field, decomposition of cotton strips was negligible after two years in soils. Soil warming and annual moisture amendment treatments did not stimulate decomposition at these sites. Many cotton strips appeared to gain strength, however, suggesting that these strips may have been in a very early stage of decomposition during which microbial colonization and activity could strengthen strips. Nematode abundance, activity, and community structure were unchanged by treatment throughout the experiment. The results of these experiments suggest that the activity of soil biota and the functions they perform are limited in the dry valleys, particularly by low soil moisture, but also by the interactive effects of low temperatures that limit the biological availability of water. Nematode activity, and the function of the entire soil food web as well, are probably confined to short periods of time following rare snow fall events during the austral summer. Survival strategies, such as the anhydrobiotic strategy employed by dry valley nematodes, are an important aspect of the ecology of soil biota in this extreme environment. The ability to employ an anhydrobiotic strategy allows nematodes to survive in the driest habitats, although their contribution to ecosystem function as microbial grazers may be most limited in these soils.