Net primary production in mineral soils of a Polar desert

Cryptobiotic communities are important sources of soil organic matter in ecosystems where aridity and stochastic environmental conditions favor organisms capable of withstanding long periods of inactivity and rapid responses to pulses of water. We are investigating controls over the accumulation and turnover of organic matter in the McMurdo Dry Valleys of Antarctica, a polar desert devoid of vascular flora and with limited taxa of bryophytes, lichens and algae. Previous studies have described multiple sources of organic matter to dry valley soils, including marine and lacustrine inputs, as well as aeolian derived material from lithic communities in sandstone formations, but few studies have examined in situ photoautotrophic communities or measured net primary productivity (NPP) in the arid mineral soils. Here we report results from laboratory ¹⁴C assimilation assays and in situ gas exchange measurements of NPP in Antarctic soil ecosystems. We considered an array of soil and sediment habitat types that encompassed likely ranges of productivity including: bare mineral soils, hypolithic environments, stream sediments and moss swards. Rates of NPP ranged from below detection to 10 mg C m^-2 d^-1. These rates of production are low compared with temperate desert cryptobiotic crusts, but represent significant inputs of carbon to systems where organic matter content is <50 g C/m² in the surface 5 cm of soil. Rates of potential and in situ NPP varied significantly among habitat types, and coincided with spatial and temporal variability in soil hydrology. Soil chlorophyll a content and gravimetric soil water accounted for greater than 90% of the variance in rates of potential carbon assimilation in the laboratory ¹⁴C assays. These results challenge previous descriptions of dry valley soils as solely heterotrophic environments and suggest that cycling of recently fixed-carbon is important to soil food webs.