Chase S LeCroy’s research while affiliated with University of California, Los Angeles and other places

Urban areas are expanding rapidly in tropical regions, with potential to alter ecosystem dynamics. In particular, exotic grasses and atmospheric nitrogen (N) deposition simultaneously affect urbanized landscapes, with unknown effects on properties like soil carbon (C) storage. We hypothesized that: (H1.) Soil nitrate (NO3 (-) ) is elevated nearer to the urban core, reflecting N deposition gradients. (H2.) Exotic grasslands have drier soils, elevated NO3 (-) , and decreased soil C relative to secondary forests, with higher N promoting decomposer activity. (H3.) Exotic grasslands have greater seasonality in soil NO3 (-) versus secondary forests, due to higher sensitivity of grassland soil moisture to rainfall. We predicted that NO3 (-) would be related to dissolved organic C (DOC) production via changes in decomposer activity. We measured six paired grassland/secondary-forest sites along a tropical urban-to-rural gradient during three dominant seasons (hurricane, dry, and early wet). We found that: (1.) Soil NO3 (-) was generally elevated near the urban core, with particularly clear spatial trends for grasslands. (2.) Exotic grasslands had lower soil C than secondary forests, which was related to elevated decomposer enzyme activities and soil respiration. Unexpectedly, soil NO3 (-) was negatively related to enzyme activities, and was higher in forests than grasslands. (3.) Grasslands had greater soil NO3 (-) seasonality versus forests, but this was not strongly linked to shifts in soil moisture or DOC. Our results suggest that exotic grasses in tropical regions are likely to drastically reduce soil C storage, but that N deposition may have an opposite effect via suppression of enzyme activities. However, soil NO3 (-) accumulation here was higher in urban forests than grasslands, potentially due to an interplay of aboveground N interception and soil processes. Net urban effects on C storage across tropical landscapes will likely vary depending on rates of N deposition, the mosaic of land covers, and responses by local decomposer communities. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.

Background/Question/Methods 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. Results/Conclusions 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.