Spatial and seasonal variability in soil nutrients and decomposer activities in a tropical urban watershed
Tropical forests in and around urban centers face a suite of anthropogenic disturbances that may be muted or absent in more remote forests. For example, urban–proximate forests are likely to experience soil degradation, forest fragmentation, atmospheric nitrogen (N) deposition, and encroachment by non-native species. These factors can have competing effects on soil nutrient levels. For example, erosion is likely to remove topsoil and nutrients, whereas N deposition and the spread of non-native N fixing plants could elevate N in soils. Here, we measured soil nutrient levels and decomposer enzyme activities in urban and suburban moist forest stands over the course of one year in the San Juan, Puerto Rico metropolitan area. A suite of biological and physical forest characteristics, as well as landscape-scale urban features, were assessed as predictors. We hypothesized that proximity to the urban center and the presence of non-native N fixers are linked to elevated soil N. We predicted elevated levels of soil N would also be linked to higher decomposer enzyme activities, linking soil N, carbon (C) and phosphorus (P) dynamics in these disturbed forests. Finally, we expected that decomposer activity would follow seasonal patterns, with the highest activity during wetter months.
We found that soil N varied greatly among urban-proximate forests, with the highest levels under all-native canopies, followed by non-native N-fixer canopies, and the lowest soil N under other introduced canopy species. While there was not an overall relationship between proximity to the urban center and soil N, proximity to the urban center was linked to the regeneration of non-native N-fixers in the understory. In general, mineral N in urban forest soils was elevated relative to more rural and remote forests in Puerto Rico. Across the urban stands, soil mineral N was significantly positively associated with decomposer enzymes that acquire C and P from organic matter. Soil nutrients and enzyme activities followed seasonal patterns in soil moisture. These results suggest that urban activity has a strong influence on tropical forest soil N dynamics, with the potential for cascading effects on C and P cycling.