Changes to soil organic carbon lability following ingestion by Cubitermes fungifaber (Isoptera: Termitidae)

Studentships 2009

Project Description

The world’s soils contain an estimated 1,550 Gt organic carbon (C), nearly three times the size of the terrestrial biotic C pool (560 Gt), and more than twice that of the atmospheric C pool (760 Gt). Among the various fractions to which soil organic carbon (SOC) may be sequestered, only the passive pool, bound intimately to soil minerals and with a mean residence time of 2,000-10,000 years, has sufficient stability to serve as a suitable long-term store of organic C. 

Despite the widely acknowledged recalcitrance of this mineral-bound OC, many soil organisms are well adapted to extract nutrients from even these most refractory forms of soil organic matter. In tropical rainforests for example, soil-feeding termites, comprising as much as 95% of all soil insect biomass, and with densities up to 100 g m-2, obtain nutrients wholly from ingested soil organic matter. 

In the termite gut this soil is subject to extreme and sustained alkaline conditions (pH~12), inducing near complete desorption of OC from the mineral colloids and thus facilitating extraction and assimilation of phosphorous and other nutrients. An environmentally relevant consequence of this mode of nutrient acquisition is the concomitant and large increase in SOC lability, which may lead ultimately to increased SOC mineralization and CO2(g) release. Despite partial re-stabilisation of SOC on expulsion from the gut, as a consequence of pH reduction and reformation of organo-mineral associations, the SOC will not have regained its pre-ingestion recalcitrance, leading to an increased pool of labile C. Given that the population of soil feeding termites in the African tropical rainforest alone are estimated to ingest each year approximately 3 x 108 kg soil, increasing the lability of at least 3 x 106 kg SOC, the action of these macroinvertebrates has considerable potential to further increase, albeit indirectly, concentrations of atmospheric CO2

Objectives

(i) Establish experimental mesocosms under rainforest canopy in Gabon, incorporating C. fungifaber with representative and unmodified (termite-free) local soil.


(ii) Develop molecular level models describing the architecture of organo-mineral associations before and after soil ingestion using, primarily, X-ray diffraction and FTIR spectroscopy combined with multiple complementary wet chemical techniques.


(iii) Determine lability of SOC before and after soil ingestion as measured by the 813C signature of the CO2 emission and that of oxidised SOC. Discrimination against 13C within the termite gut will enable determination of the direct effects of termite ingestion on SOC lability.


(iv) Predict changes to SOC lability across landscapes and between climatic regions as a consequence of termite activity

Student Training

The student will receive comprehensive training in: (i) termite physiology and ecology, including international fieldwork to establish the mesocosms; (ii) various mineralogical and geochemical techniques, such as X-ray diffraction, UV-vis and FTIR spectroscopic methods, essential for the characterisation of soils; (iii) soil microbiology and stable isotope approaches, and hands-on experience in gas chromatography and mass spectrometry.

Supervisors

Dr Bill Dubbin (b.dubbin@nhm.ac.uk); Dr Paul Eggleton (p.eggleton@nhm.ac.uk); Dr Elizabeth Baggs (e.baggs@abdn.ac.uk)