Abstract
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 tropical urbanized landscapes, with unknown effects on properties like soil carbon (C) storage. We hypothesized that (H1) soil nitrate ( NO 3 − ) is elevated nearer to the urban core, reflecting N deposition gradients. (H2) Exotic grasslands have elevated soil NO 3 − and decreased soil C relative to secondary forests, with higher N promoting decomposer activity. (H3) Exotic grasslands have greater seasonality in soil NO 3 − vs. secondary forests, due to higher sensitivity of grassland soil moisture to rainfall. We predicted that NO 3 − would be positively 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 the three dominant seasons (hurricane, dry, and early wet). We found that (1) soil NO 3 − was generally elevated nearer to 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 NO 3 − was negatively related to enzyme activities, and was lower in grasslands than forests. (3) Grasslands had greater soil NO 3 − seasonality vs. 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 NO 3 − accumulation here was higher in urban forests than grasslands, potentially related to of aboveground N interception. Net urban effects on C storage across tropical landscapes will likely vary depending on the mosaic of grass cover, rates of N deposition, and responses by local decomposer communities.