An Examination of Soil Microbial Communities and Litter Decomposition along an Urbanization Gradient

Abstract

Urban landscape managers increasingly recognize the critical role of soil organisms in regulating plant nutrient availability through decomposition. Naturalized forested soils are regarded as being balanced in annual organic contributions and the microorganisms responsible for nutrient cycling. Unfortunately, land conversion processes such as urbanization can potentially alter the ability of microorganisms to supply nutrients to plants by changing the organic additions and edaphic properties of soil. However, because the degree of variation in the composition and structure of soil biological communities is not clear in urban areas, baseline information regarding the impact of urban land management practices on soil microbial communities is essential to improve our ability to manage urban soils and the plants they support. This study explored soil bacterial and fungal communities along an urbanization gradient over five urban land-uses: street side terraces, new (< 5 years) residential landscapes receiving intensive management, old (> 25 years) residential landscapes receiving intensive management, old residential landscapes receiving minimal management, and rural forested lands in metropolitan Milwaukee, WI, USA. The objectives were to: 1) determine if urbanization initiates changes bacterial and fungal community composition and microbial biomass across five urban land-uses, 2) determine if urbanization initiates changes biological activity across five urban land-uses, 3) determine if urbanization initiates changes the soil physical and chemical environment across five urban land-uses, and 4) determine if differences in the bacterial and fungal compositions are related to differences in the soil’s physical and chemical characteristics. Microbial biomass did not differ along the urbanization gradient. The PLFA/FAME produced microbial communities showed statistical differences in biodiversity index values by land-use; however, analysis of similarities (ANOSIM) and multidimensional scaling (MDS) indicated highly similar communities with limited separation. The TRFLP inferred bacterial and fungal communities showed some land-use differences based on ANOSIM, MDS, and analysis of biodiversity indices. However, the discrete land-use clusters grouped closely from large-scale similarities in community profiles. Therefore, observed differences in microbial community composition reflected only a small portion of the total microbial biomass. The decomposition of transposed leaf litter showed significant decline in C:N ratio over time, but no statistical differences by land-use were observed. The soil edaphic properties (bulk density, soil organic matter, pH, Mg, Na, total N, and C:N ratio) displayed significant differences across the urbanization gradient. Additionally, Mg and total N we correlated to changes in the bacterial communities. Consequently, in locations where parent material and soil forming processes are homogeneous, urbanization and landscape management appears to have less impact on soil microbiology than initially expected.