Heterogeneity in soil chemistry relates to urbanization while soil homogeneity relates to plant invasion in small temperate deciduous forests

Context Soil heterogeneity versus homogeneity patterns are observed within and across urban landscapes at multiple scales. To fully evaluate human-mediated influences on soil properties and processes, we need to understand spatial patterns and variation in soil characteristics within a single ecosystem patch type (e.g., forests) in and near cities. Objectives Our research objectives were to: (1) identify soil characteristics important in driving variation in soil chemistry within urban forests, and (2) examine whether urbanization and invasion gradients were related to variation in soil chemistry within these forests. Methods We measured soil chemical properties within 36 forests across the U.S. mid-Atlantic. The forests are spatially distributed across an urbanization gradient and comprise a gradient of non-native plant invasion. Results Urbanization was related to more variation in soil chemistry, whereas plant invasion was related to less variation in soil chemistry within our forests. Soil Ca and Mg concentrations increase with plant invasion yet are less variable within invaded forests most likely due to invasive plants taking up and concentrating these elements. Soil pH, Ca, Mg, Zn, and Cu increase in forests surrounded by greater urbanization, however, these elements are more variable within urban forests likely due to edge effects altering element deposition. Conclusions Our results demonstrate that while urbanization and invasion can increase soil chemical concentrations, they differentially alter variation in soil chemistry within urban forests. Plant invasion and urban environmental conditions need separate consideration in future conceptual models of urban ecological theory since non-native invasive plants influence soil chemistry independent of other urban factors.