Decomposers and the City
By Alessandro Ossola PhD, The University of Melbourne, Australia and US National Academy of Science, Engineering and Medicine NRC Associate c/o NRMRL-USEPA, Cincinnati, OH.
Cities are generally associated with grey jungles of concrete, glass and steel. Where the urban land is still profitable for new development, little chance is left for the soil to persist unsealed. When this happens, soil is generally heavily compacted, polluted, its biota depleted and ecological functioning greatly reduced. Urban soil faces a complex and uncertain future of irrigation, fertilization, transportation, remediation and molestation which is rarely observed in natural and agricultural soils. Urban soil is ultimately associated with a mere substrate where to build new above- and below-ground structures and infrastructures to connect our modern cities. Despite their intrinsic complexities, cities and the soils underneath represent useful pre-constructed laboratories to extend our knowledge about soil functioning, its biodiversity and resilience towards a variety of stressors and habitat management practices.
Figure 1, Figure 2. Fungi colonizing Eucalyptus globulus litter in litter bags exposed in an urban park characterized by complex vegetation and high litter mass in Melbourne, Australia.
In a paper published this week in Ecosystems, we asked whether relatively small changes in the complexity of vegetation, litter and soil characteristics driven by urban green spaces management were translated into differences in superficial organic matter microbial decomposition and detritivore comminution. Since the 90’s, microbial decomposition processes have been measured in cities and towns mostly in forest remnants along urban-rural gradients. Surprisingly, to date no studies measured superficial decomposition or comminution processes in areas, such as urban parks, which make up most of the green space in cities worldwide. This is likely due to the intrinsic difficulties and risks (e.g. people, mowers, animals, etc) in undertaking litter bags experiments in these habitats. In our study, we found that the simplification of urban habitats in Melbourne, Australia, and particularly of the understory vegetation, significantly decreased decomposition and comminution rates of two substrates in litter bags (i.e. Eucalyptus globulus leaves and pea straw). This is likely determined by drier conditions under simple urban vegetation, where water availability might represent a stronger limiting factor for the activity of both microbes and detritivores compared to temperature. The soil function in structurally complex urban parks was comparable to that of woodland remnants, and the age since land use change from agricultural land to urban park (40-100 years) did not affect decomposition and comminution processes. This suggests that urban soils can recover their function towards that of remnant ecosystems if enough time has transpired and proper management measures are taken. The volume of the understory vegetation was positively related to the species richness of macrofauna detritivores, which in turn significantly enhanced comminution rates. More than 70% of detritivores sampled were European exotic species. The relation between species richness of numerous soil organisms and soil processes have been largely investigated through mesocosm experiments in the last century, but rarely observed or tested in the field.
Figure 3. Relationship between urban understory volume, species richness of macrofauna detritivores and Eucalyptus globulus litter mass remaining after 1 year litter bag experiment.
While the ecological exploration of cities started decades ago, we have just begun to investigate the functional role of urban organisms and particularly the soil biota. Worldwide, cities can represent useful laboratories to test new hypotheses and questions about the role of biophysical factors, human management, environmental legacies, soil biodiversity and their complex interactions upon soil functioning. Time is ripe to dig into!