Temporal patterns of soil respiration in tropical forest plantations in lowland Costa Rica

Background/Question/Methods

Soil respiration is the largest terrestrial source of atmospheric CO₂ but, in comparison to CO₂ fixation by photosynthesis, our understanding of factors that control rates of CO₂ production within soils is poor. As a result, our ability to model soil CO₂ emissions across spatial and temporal scales is limited. We measured soil CO₂ emissions hourly in four reps each of plantations of six tree species during both rainy and dry periods in lowland Costa Rica, and coupled these to long-term survey measurements to address several questions: Are there discernible diel patterns to soil CO₂ emissions? If so, what factors are responsible for them? Do daytime-only survey measurements generate biased estimates of annual soil CO₂ emissions? Does soil respiration vary across years, as these fully stocked, 20-year-old plantations mature?

Results/Conclusions

A significant relationship between soil CO₂ efflux and hour of day was observed among all plantations and measurement dates, but not within individual plantation types. This diel pattern in soil-CO₂ emissions paralleled a significant diel pattern in soil temperature. Soil CO₂ emissions varied significantly with temperature despite that mean soil temperatures varied little. Normalization of measured fluxes to 25°C-flux rates eliminated all observable diel patterns in CO₂ emissions. Multiple analyses indicated that rain events depressed soil CO₂ emissions. They did not, however, significantly affect the soil temperature-efflux relationship. In this rainforest environment rain occurs at all hours of the day. The impact of rain events likely precluded the observation of any diel CO₂ efflux pattern that might derive from daytime-only canopy photosynthesis, should one exist. In this site, soil-CO₂ emissions over hourly time steps were apparently influenced by recent weather rather than by CO₂ production rates within soils. Soil CO₂ production and emission rates may not be temporally synchronized over short time periods. Daytime survey measurements of soil respiration provided unbiased estimates of mean daily soil-CO₂ emissions. Based on survey measurements, the magnitude of soil respiration varied significantly among plantation types (tree species). These differences among plantation types persisted across four years, supporting the conclusion that individual tree species had distinct effects on the magnitude of annual soil respiration in sites with similar land-use histories, soils, reliefs and climates. Such species-identity effects are difficult to estimate a priori.