Increased Litterfall in Tropical Forests Boosts the Transfer of Soil CO2 to the Atmosphere

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.

Changes in soil carbon and nutrients following 6 years of litter removal and addition in a tropical semi-evergreen rain forest

“…Treatments were assigned on a stratified random basis using total litterfall per plot in 2002 (i.e. the three plots with highest litterfall were randomly assigned to treatments, then the next three and so on) (Sayer et al, 2007). The plots were geographically blocked, litter from a particular litter removal plot was always added to a particular litter addition plot and there was a nearby control plot.…”

Priming and microbial nutrient limitation in lowland tropical forest soils of contrasting fertility

“…While we are cautious in making 'real world' inferences based on results from our laboratory study, the implication that the controls on microbial sources of soil CO 2 efflux shift between labile C and nutrients, with priming occurring when there are large inputs of labile C relative to nutrients, is supported by observations in tropical forests. Two studies hypothesized a release of primed soil organic matter within soil CO 2 efflux following the addition of C to soils: Sayer et al (2007) to explain increased soil CO 2 efflux from soils that received experimentally increased litterfall, and Cleveland et al (2010) to explain increased soil CO 2 efflux from soils that received more concentrated dissolved organic matter following a drought treatment. Two further studies in tropical forests where nutrients were added without C further support our interpretation that priming occurs when there are large inputs of labile C relative to nutrients; both studies provide evidence in support of suppressed priming in the presence of excess nutrients.…”

A new approach to trenching experiments for measuring root–rhizosphere respiration in a lowland tropical forest

“…The removed litter was immediately added to five further plots, where it was spread out as evenly as possible (Lþ plots); five plots were left undisturbed as controls (CT plots). Treatments were applied in a stratified random design according to mass of pre-treatment litterfall (Sayer et al, 2007).…”

“…Four soil respiration collars were installed in each of the 15 litter manipulation plots in 2005 by sinking PVC tubes (20 cm inner diameter and 12 cm height) into the ground to 2 cm depth (Sayer et al, 2007). In February 2007, one 2-m  2-m subplot was marked out at a randomly chosen location without stems along the edge of each litter manipulation plot and a fifth respiration collar was installed as described above in the centre of each subplot; In May 2007, respiration was measured over all the collars using a Li-8100 soil CO 2 flux system (LI-COR, Lincoln, USA) and then the subplots were trenched to 0.5 m depth, the inner side of the trenches were lined with four layers of construction plastic and the trenches were refilled.…”

“…We had expected the lower relative contribution of rooterhizosphere respiration to total belowground respiration in the Lþ plots (17%) compared to the control plots (38%) as fine root biomass was shown to be c. 30% lower in the Lþ plots (Sayer et al, , 2007 and rooterhizosphere respiration is thought to be directly related to fine root biomass (Rout and Gupta, 1989). The lower relative contribution of rooterhizosphere respiration is apparent in both the before-and-after calculations and the classic trenching calculations, which allows us to conclude that our new approach can be used successfully to compare rooterhizosphere respiration in soils with different root densities.…”