Abstract
Aboveground litter production in forests is likely to increase as a consequence of elevated atmospheric carbon dioxide (CO2) concentrations, rising temperatures, and shifting rainfall patterns. As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics. Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of the global carbon cycle; nevertheless, the effects of increasing aboveground litter production on belowground carbon dynamics are poorly understood. We used long-term, large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO2 production. Over the second to the fifth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 20% lower in the litter removal and 43% higher in the litter addition treatment compared to the controls but litter addition did not change microbial biomass. We predicted a 9% increase in soil respiration in the litter addition plots, based on the 20% decrease in the litter removal plots and an 11% reduction due to lower fine root biomass in the litter addition plots. The 43% measured increase in soil respiration was therefore 34% higher than predicted and it is possible that this ‘extra’ CO2 was a result of priming effects, i.e. stimulation of the decomposition of older soil organic matter by the addition of fresh organic matter. Our results show that increases in aboveground litter production as a result of global change have the potential to cause considerable losses of soil carbon to the atmosphere in tropical forests.
Highlights
Changes in litter quantity as a consequence of global climate change are becoming increasingly likely; recent FACE experiments have shown that litterfall increases with elevated atmospheric CO2 concentrations [1,2,3,4,5] and predicted changes in rainfall distribution patterns [6] and temperature [7] may affect litterfall by altering leafing phenology
The 20% reduction in soil respiration observed in the litter removal treatment from the second year of treatments until the end of the study is similar to the 28% decrease reported in plots in young regrowth forest in Brazil after one year of litter removal, where controls included the litter layer in CO2 efflux measurements [24], but lower than the 51% decrease after seven years of litter removal in lower montane forest in Puerto Rico [25]
We can attribute the decrease in our study principally to a reduction in heterotrophic respiration due to the withdrawal of fresh substrate, as there were no differences in fine root biomass in the upper 100 mm of the mineral soil between CT and L- plots in 2004 or in 2006 (Fig. 3)
Summary
Changes in litter quantity as a consequence of global climate change are becoming increasingly likely; recent FACE experiments have shown that litterfall increases with elevated atmospheric CO2 concentrations [1,2,3,4,5] and predicted changes in rainfall distribution patterns [6] and temperature [7] may affect litterfall by altering leafing phenology. We believe that an increase in aboveground litterfall may have a large impact on belowground carbon and nutrient cycling, as annual litterfall is closely correlated with soil respiration on a global scale [21,22], and the amount of litter on the forest floor affects soil nutrient status, soil water content, soil temperature, and pH [9], all of which can influence soil respiration rates. Our results show that an increase in aboveground litterfall caused a disproportionate increase in soil respiration, reduced the amount of carbon allocated to fine root biomass and has the potential to cause substantial losses of carbon belowground
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