Climate Warming and Soil Carbon in Tropical Forests: Insights from an Elevation Gradient in the Peruvian Andes

Abstract

The temperature sensitivity of soil organic matter (SOM) decomposition in tropical forests will influence future climate. Studies of a 3.5-kilometer elevation gradient in the Peruvian Andes, including short-term translocation experiments and the examination of the long-term adaptation of biota to local thermal and edaphic conditions, have revealed several factors that may regulate this sensitivity. Collectively this work suggests that, in the absence of a moisture constraint, the temperature sensitivity of decomposition is regulated by the chemical composition of plant debris (litter) and both the physical and chemical composition of preexisting SOM: higher temperature sensitivities are found in litter or SOM that is more chemically complex and in SOM that is less occluded within aggregates. In addition, the temperature sensitivity of SOM in tropical montane forests may be larger than previously recognized because of the presence of "cold-adapted" and nitrogen-limited microbial decomposers and the possible future alterations in plant and microbial communities associated with warming. Studies along elevation transects, such as those reviewed here, can reveal factors that will regulate the temperature sensitivity of SOM. They can also complement and guide in situ soil-warming experiments, which will be needed to understand how this vulnerability to temperature may be mediated by altered plant productivity under future climatic change.

Keywords: decomposition; soil microorganisms; soil organic matter; temperature sensitivity; tropical lowland forest; tropical montane forest.

Figure 1.

The Kosñipata elevation transect, Manu National Park, Peru. Images show (a) the highest- (3644 meters above sea level, m asl) and lowest-elevation (194 m asl) sites; (b) all sites from 3644 m asl to 1500 m asl viewed facing approximately northeast from the top of the transect; (c) a photograph of the transect of the same view as shown in 1B. Abbreviation: km, kilometers.

Figure 2.

Carbon stocks (a) and available nutrients (b) in tropical forest soils along the Kosñipata transect. Data were determined from five samples within 1-hectare plots at each elevation. Carbon in the organic horizon (ranging from 1 to 23 centimeters in depth) and mineral horizons was determined to a 50-cm depth from the soil surface and is presented on an area basis. Mineralized nitrogen and resin-extractable phosphorus were determined to a 10-cm depth from the soil surface using in situ resin bags. Nutrient data are log-transformed to more clearly show elevation transitions (Nottingham et al. 2015b). The error bars represent one standard error. Abbreviations: kg, kilograms; m², square meter; m asl, meters above sea level; mg, miligram.

Figure 3.

Ecosystem properties and processes—each of which has its own intrinsic temperature sensitivity (Q₁₀)—that may interact to determine the overall apparent Q₁₀ of soil-carbon degradation.

Figure 4.

The relationship between the Q₁₀ values of soil organic carbon degradation during the first 2 years following translocation and the relative portions of carbon stored in particulate organic matter (physically unprotected; r = –.96, p < .01). The four points represent the four sites included in the soil translocation experiment (situated at 210, 1000, 1500, and 3030 meters above sea level), where Q₁₀ values were determined by respiration responses following translocation among sites and soil physical fractions were determined for soil from each site (Zimmermann et al. 2012).

Figure 5.

(a) Total abundance of phospholipid fatty acids (PLFA) and (b) the ratio of bacterial to fungal PLFA in soils across the Kosñipata elevation transect. Trends indicate the shift in the relative importance of fungal versus bacterial biomass in the microbial decomposer community along the transect (Whitaker et al. 2014b). The error bars represent one standard error. Abbreviations: dwt, dry weight; g, grams; m asl, meters at sea level; nmol, nanomoles.

Figure 6.

The complexity of proposed climate-warming effects on soil carbon (C) in lowland and montane tropical forests. The thermal adaptation of soil microorganisms and changes in plant productivity, rainfall, and atmospheric carbon dioxide (CO₂) will modulate these responses, but the mode of adaptation is uncertain. To understand the impacts of warming in lowland tropical forest, we need in situ experiments to simulate warming. Driving processes are represented by the dashed arrows; fluxes of energy or nutrients are represented by solid arrows (weighted by their relative importance in montane and lowland tropical forests); the boxes are organisms or resource pools. Abbreviations: N, nitrogen; NPP, net primary production; P, phosphorus; SOM, soil organic matter.