A Study of Soil Organic Matter and Its Controlling Factors in Portland, Oregon

Traditionally, local above- and below-ground plant and microbial communities, temperature and precipitation, topography and texture and composition of parent material have been thought to govern the soil processes that lead to soil organic matter accumulation over decades or centuries. Soil organic matter is a substantial global reservoir of carbon and disturbance of equilibrated soils often leads to efflux of significant quantities of CO2. Anthropogenic influences shift the inputs, disturb the structure and alter the biochemistry of soil, profoundly disrupting soil-forming processes. Urbanization leads to soil organic matter equilibria that are different from those in naturally forming soils. Measurement of soil organic matter in diverse cities suggests that they differ in their capacity to accumulate soil organic matter. Here I quantify soil organic matter and examine the limitations of its accumulation within cities by comparing differences in soil organic matter and natural and anthropogenic characteristics at the neighborhood, city and regional scales in Portland, Oregon to that in the Pacific Northwest. I found that each Portland neighborhood has distinct urban characteristics and tree and shrub community composition, but soil organic matter content is indistinguishable among them. Across Portland, neither vegetation structure nor urban factors appear to directly influence soil organic matter content. Rather, microbial biomass, bulk density and total nitrogen appear to be important factors controlling soil organic matter content in Portland. The amount of soil organic matter stored in Portland's soils is statistically indistinguishable from Pacific Northwest soils, in contrast to other temperate cities