Biological controls on soil respiration and its climatic response across a large tropical elevation gradient

Lead Research Organisation: University of Manchester
Department Name: Life Sciences

Abstract

This project will advance our ability to quantify the influence of climatic warming on the emission of CO2 from soil by investigating how soil biological and functional diversity (roots and microbes), and soil chemical properties, limit respiration processes in soil. This work will be the first of its kind to address this question over a large elevation gradient and in a tropical region where biodiversity and biogeochemical cycling of carbon are very high. The carbon balance of an ecosystem is strongly dependent on the balance between photosynthesis and respiration. Globally, respiration on land is at present very slightly smaller than photosynthesis, meaning that terrestrial ecosystems are thought to be a 'sink' for atmospheric carbon dioxide, slowing the continual rise in carbon dioxide (CO2) concentration in the atmosphere. The largest fraction of total respiration from land comes from the decomposition of organic matter in soil. This decomposition leads to emissions of CO2 to the atmosphere. The rate of decomposition may increase under climatic warming, possibly accelerating climate change over this century, so we need urgently to understand what the risk of this happening is. Our study site is in the tropical rain forests of Peru, ranging in altitude from 3000 m to 220 m above sea level. The soil carbon stock is large, particularly at high elevation and so represents a risk in the sense that this carbon could be broken down and emitted as CO2 under climatic warming. Our preliminary data suggest that there are large differences in the temperature sensitivity of soil CO2 emissions in these forests, with high sensitivity at high elevations. This project aims to understand these differences in sensitivity by examining controls over the decomposition of organic matter that are exerted by the physical environment and also by roots, and by the decomposing microbes in soil. Our study site is ideally suited to address this question because it spans a natural temperature gradient of 12-26 degrees Celsius. We will use this in two ways: (i) to observe natural differences in CO2 emissions at different elevations and temperatures and (ii) to examine the effects of transplanting soil from one elevation and 're-planting' it at another. We have performed part (ii) for 4 sites across our elevation gradient and now haave an exceptional opportunity to study the effects, and to advance our understanding of short- and long-term climatic warming of soil CO2 emissions. Our approach will be to observe the temperature response characteristics of soil CO2 emission in natural and transplanted soil. We will make high temporal resolution measurements over 2.5 years, further manipulating the soil to see the effects of removing roots and mycorrhizal fungi from the decomposing system. We will measure the physical environment and the chemical complexity of the soil carbon. We will also measure the biological diversity of microbes in the soil using leading edge membrane- and DNA-based techniques. Finally we will use a laboratory experiment to trace the types of carbon compound that different microbes use from different sites along the study transect. Here we will 'feed' the soil with a stable (safe) carbon isotope and trace where that carbon is used and emitted - ie how much labeled CO2 is emitted and which organisms use it in their metabolism. This will give us valuable information to inform our analysis of the data we get from field measurements. In our analysis we will statistically examine what microbes/root functions are most important for constraining the response by soil respiration to climatic change and use our laboratory data to provide mechanistic interpretations of our statistical analysis. Combined we will develop a new understanding of the response by soil respiration to climatic warming and we will test how important biological diversity is for controlling and constraining that response, and its effect on climatic change.

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/G018367/1 17/05/2010 31/01/2013 £49,836
NE/G018367/2 Transfer NE/G018367/1 01/02/2013 30/11/2013 £11,181
 
Description Our work on this project was part of a larger consortium led by Patrick Meir of Edinburgh University. We report here on two major components of the project that we had a significant role in, as detailed below.

The Andes are predicted to warm by 3-5 °C this century with the potential to alter the processes regulating carbon (C) cycling in these tropical forest soils. This rapid warming is expected to stimulate soil microbial respiration and change plant species distributions, thereby affecting the quantity and quality of C inputs to the soil and influencing the quantity of soil-derived carbon dioxide (CO2) released to the atmosphere. Using soils taken from a 3200-m elevation gradient located in south-east Andean Peru, including tropical lowland, premontane and montane forest, we explored how climate change will impact microbial communities in tropical soils and the consequences of this for carbon losses to the atmosphere by respiration.

In the first experiment we determined how soil microbial communities and abiotic soil properties differed with elevation. We then examined how these differences in microbial composition and soil abiotic properties affected soil C-cycling processes, by amending soils with C substrates varying in complexity and measuring soil heterotrophic respiration (RH). We found consistent patterns of change in soil biotic and abiotic properties with elevation. Microbial biomass and the abundance of fungi relative to bacteria increased with elevation, and these differences in microbial community composition were strongly correlated with greater soil C content and C:N (nitrogen) ratios. We also found that RH increased with added C substrate quality and quantity and was positively related to microbial biomass and fungal abundance. Statistical modelling revealed that RH responses to changing C inputs were best predicted by soil pH and microbial community composition, with the abundance of fungi relative to bacteria, and abundance of gram-positive relative to gram-negative bacteria explaining much of the model variance. Overall, our results showed that the relative abundance of microbial functional groups is an important determinant of respiration responses to changing C inputs along an extensive tropical elevation gradient in Andean Peru.

In a second experiment we examined how substrate quality, soil nutrient availability and microbial community composition influenced priming effects in tropical soils. We did this by amending soils with 13C labelled substrates and then measured microbial respiration and assimilation of substrate- and soil organic matter (SOM)- derived C. We found that substrate quality was the single strongest control on priming, determining the relative contribution of substrate and SOM mineralization to the overall flux. Microbial community composition was also important in regulating the magnitude of priming, with specific microbial groups assimilating substrate C to different degrees dependant on substrate quality. This study provides evidence that changes in plant-derived C inputs to soils in response to climate change will be important in regulating C sequestration and losses in C-rich tropical forest soils.
Exploitation Route See lead report from PI
Sectors Environment