Soil respiration across an urban-rural tropical gradient: Variability among forest and invasive grass sites

Background/Question/Methods

The abandonment of degraded agricultural land in Puerto Rico has led to large-scale forest regeneration in the region; forest recovery occurring contemporaneously with significant urban growth. As a result of this landscape fragmentation from urbanization, patches of secondary forests now exist within a mosaic of urban infrastructure and may be reacting differently to environmental changes along an urban-rural gradient. In order to explore how these changes, in general, may be affecting carbon (C) loss in tropical secondary forest soils recovering from intensive agricultural use, urban and remote secondary forests were studied in the San Juan/Rio Piedras watershed. In this study, measures of soil CO­­₂ efflux (collected using a Li-Cor 6400 infrared gas analyzer) were paired to measures of soil temperature and moisture in urban and remote secondary forest and grass sites. Based on existing knowledge on the controls of soil respiration, the hypotheses driving this study were that: First, on average, more urban and smaller forest fragments, regardless of cover type, would see higher carbon fluxes relative to larger, more continuous forests as a result of edge effects, Second, CO­­₂ efflux would be higher and more homogenous within grass sites and lower and more heterogeneous within forest sites because of differences in species composition and ecosystem characteristics and Lastly, that soil moisture is the best predictor controlling soil CO₂ flux because of the role of water in promoting or restricting soil microbial activity.

Results/Conclusions

Soil respiration, on average, was not significantly different for forest versus grass sites, but there were differences in soil respiration patterns for these two cover types. Average forest soil respiration was 6.3 ± 0.5 µmolCO₂ m²s^-1, and average grassland respiration was 7.1 ± 0.6 µmolCO₂ m²s^-1. Respiration was significantly lower after rainfall events in both cover types. There was also a significant interaction between soil temperature and cover type: soil respiration increased with soil temperature in grass sites, but not in forest sites. Soil temperature across the urban-rural gradient ranged from 22.5 C to 27.7 C, with no significant differences between the two cover types. For both cover types soil respiration was highest in sites closer to the urban center, with steeper declines away from the urban center in forest sites. These data suggest that mechanisms driving rates of soil respiration may differ among urban forest and grass areas.