Are urban soils similar to natural soils of river valleys?
Purpose Urban soils and soils of river valleys are constituted of heterogeneous materials that have been manipulated, disturbed or transported at different spatial and temporal scales. Despite these similarities, little is known about soil evolution in urban soils and their comparison with natural soils remains therefore highly useful. We hypothesized that, according to their degree of perturbation, urban soils and natural soils of river valleys have similar soil processes related to their structure, physical and chemical characteristics. Materials and methods Using a synchronic approach, we compared two soil gradients, one located in the natural reserve of the Allondon River (canton of Geneva, Switzerland) and the other in and around the city of Neuchâtel, Switzerland. A total of five alluvial and 18 urban soil profiles were described according to vegetation type and alluvial terraces formed at different distances from the river for the river valley ecosystem and to soil age for the urban ecosystem. Correlations between soil gradients and classical physical (soil depth, particle-size distribution, coarse fraction) and chemical (Corg, pHH2O, Ptot, Ntot, CaCO3, CEC and C/N ratio) parameters of soils were first tested in order to identify similarities and differences among soil gradients. Data of soil properties were then clustered hierarchically in order to identify soil group classification. Results and discussion Our results showed similarities and differences between soil gradients. In the urban context, soil thickness was positively correlated to soil age, while the coarse fraction, sand content and C/N were negatively correlated to soil age gradient. In soils of the river valley, most of the chemical variables were either negatively (pHH2O and CaCO3) or positively (CEC, Corg and Ntot) correlated to soil distance from the river. These differences between gradients can be mainly explained by parent material, depositional conditions and land use which can influence soil processes. However, alluvial soils were well clustered with two identified urban soil groups according to soil maturity. Evolved alluvial soils far from the river were grouped with natural and near natural urban soils. Conversely, “young” perturbed alluvial soils were most clustered with human-made soils. Conclusions From the two selected soil gradients, soils on alluvial sediments are similar to urban soils in some characteristics. However, parent material, depositional conditions and soil and vegetation interactions on soil processes (e.g. matter cycle, energy flux) still need more investigation. This study contributes to the development of a natural soil reference for urban soils.
Mille ans d'extension urbaine à Neuchâtel : évolution des paysages et des sols
Indicateurs biologiques et services écosystémiques des sols urbains : propositions de gestion
Patterns of earthworm, enchytraeid and nematode diversity and community structure in urban soils of different ages
Annelids (Lumbricidae and Enchytraeidae) and nematodes are common soil organisms and play important roles in organic matter decomposition, nutrient cycling and creation of soil structure and porosity. However, these three groups have rarely been studied together and only few studies exist for urban soils. We studied the diversity and community composition of annelids and nematodes in soils spanning more than two centuries of urban soil development in Neuch^atel (Switzerland) and assessed the relationships 1) among these three groups and 2) between each group and environmental (physical, chemical and functional) characteristics of soils and soil age. While the groups of environmental variables were correlated (Mantel tests) no correlation was found between pairs of soil fauna groups and between each soil fauna group and environmental variables. More specifically, redundancy analyses showed that earthworm assemblages were best correlated with soil bulk density and with soil depth, the latter being positively correlated with soil age. Enchytraeid assemblages and the proportion of enchytraeid r-strategists were respectively best correlated with soil carbonate content and negatively correlated with soil age. Nematodes assemblages were best correlated with soil water content. Moreover, relationships between pairs of soil biota groups, and between each group and environmental (physical, chemical and functional) variables, varied along the soil age gradient (moving window analysis). This study provides new knowledge on urban soil biodiversity and how environmental conditions can influence soil diversity and community patterns in the urban context. The contrasted community patterns of earthworms, enchytraeids and nematodes in urban soils of different ages and their different ecological roles suggest that they represent potential complementary indicators of soil quality and functioning such as soil formation and organic matter dynamics.