Land-use type changes the belowground food-web in an arid, urban ecosystem

Background/Question/Methods

Arid ecosystems experience high rates of land-use change associated with urban development including the installation of managed xeriscapes and irrigated turfgrass lawns in residential and commercial areas.  Previous research shows that regular application of water and fertilizers (N and P) in mesic, turfgrass lawns modifies soil microbial community structure, distribution, and function, which can alter N cycling pathways in arid cities. It is unclear how land-use modifications affect belowground microflora and fauna in urban areas (e.g., the soil food web). The aim of our study is to resolve the questions: 1) Who are the major groups of soil flora and fauna in an urban belowground ecosystem and, 2)  How does food web structure change during the dry and monsoon seasons in a urban area? In the summers of 2011 and 2012, we collected a series of 108 soil cores, split between the dry and monsoon seasons, in metro Phoenix. Soils were processed for flora and fauna, nutrient dynamic quantification, and microbial community molecular analyses. 

Results/Conclusions

Our results indicate that mesic lawns harbor increased soil moisture and soil organic matter over two seasons (dry and monsoon) relative to arid sites. There was a significant seasonal increase in food web biomass across all sites (p<0.05). The number of trophic groups were higher in the mesic sites, across both seasons, relative to the arid sites (p<0.05). Our analyses indicate that mesic lawns support different microarthropod communities, including increased abundances of fungal feeding mites (Cryptostigmata and Mesostigmata) and a mostly predatory sub-order (Prostigmata). Furthermore, mesic sites support a significant increase in nematodes, protozoa, and soil microbes (bacteria and fungi), relative to the arid sites (p<0.05). Visual depictions and of soil food webs at each site show that mesic food webs are more complex than their arid counterparts.   Mesic, turfgrass lawns support double the number of trophic levels and ~4-8x more belowground biomass than arid systems Thus, grass lawns represent a lush landscape that supports a unique soil food web compared to arid, urban soils. Further studies should focus on elucidating the resource costs of 'greening' an arid, urban city and whether these costs are sustainable in terms of diversity and soil function.