Scientific Papers

Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls

Cynthia M. Kallenbach(1,2), Serita D. Frey(1) & A. Stuart Grandy(1) Nature Communications 11 January 2016

(1) Department of Natural Resources and the Environment, University of New Hampshire, Durham, New Hampshire 03824, USA. (2) Soil and Crop Sciences Department, Colorado State University, Fort Collins, Colorado 80523, USA. Correspondence and requests for materials should be addressed to C.M.K. (email: kallenbachcm@gmail.com).

Soil organic matter (SOM) and the carbon and nutrients therein drive fundamental submicron- to global-scale biogeochemical processes and influence carbon-climate feedbacks. Consensus is emerging that microbial materials are an important constituent of stable SOM, and new conceptual and quantitative SOM models are rapidly incorporating this view. However, direct evidence demonstrating that microbial residues account for the chemistry, stability and abundance of SOM is still lacking. Further, emerging models emphasize the stabilization of microbial-derived SOM by abiotic mechanisms, while the effects of microbial physiology on microbial residue production remain unclear. Here we provide the first direct evidence that soil microbes produce chemically diverse, stable SOM. We show that SOM accumulation is driven by distinct microbial communities more so than clay mineralogy, where microbial-derived SOM accumulation is greatest in soils with higher fungal abundances and more efficient microbial biomass production.

Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage

Colin Averill(1), Benjamin L. Turner(2) & Adrien C. Finzi(3) Nature 2014

(1)Department of Integrative Biology, Graduate Program in Ecology, Evolution and Behavior, University of Texas at Austin, Austin, Texas 78712, USA. (2)Smithsonian Tropical Research Institute, Apartado
0843-03092, Balboa, Ancon, Republic of Panama. (3)Department of Biology, Boston University, Boston, Masachusetts 02215, USA.

Soil contains more carbon than the atmosphere and vegetation combined. Understanding the mechanisms controlling the accumulation and stability of soil carbon is critical to predicting the Earth’s future climate. Recent studies suggest that decomposition of soil organic matter is often limited by nitrogen availability to microbes and that plants, via their fungal symbionts, compete directly with free-living decomposers for nitrogen. Ectomycorrhizal and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes,
allowing them greater access to organic nitrogen sources than arbuscular mycorrhizal (AM) fungi. This leads to the theoretical prediction that soil carbon storage is greater in ecosystems dominated by EEM fungi than in those dominated by AM fungi. Using global data sets, we show that soil in ecosystems dominated by EEM-associated plants contains 70%more carbon per unit nitrogen than soil in ecosystems dominated by AM-associated plants. The effect of mycorrhizal type on soil carbon is independent of, and of far larger consequence than, the effects of net primary production, temperature, precipitation and soil clay content. Hence the effect of mycorrhizal type on soil carbon content holds at the global scale. This finding links the functional traits of mycorrhizal fungi to carbon storage at ecosystem to global scales, suggesting that plant–decomposer competition for nutrients exerts a fundamental control over the terrestrial carbon cycle.

Potential effect of melanised endophytic fungi on levels of organic carbon within an Alfisol

T. T. Mukasa Mugerwa(a),(b) and P. A. McGee(a)
(a) Macleay Building A12, School of Life and Environmental Sciences, The University of Sydney, NSW 2006, Australia.
(b) Corresponding author Email: t.mukasamugerwa@sydney.edu.au

Levels of organic carbon within agricultural soils in Australia continue to decline predominantly due to intensive cultivation. Such practices place sustainable use of agricultural soils at risk. The aim of the present study was to test whether selected melanised endophytic fungi could enhance organic carbon in an experimental soil. In a compartmental pot study, 20 melanised endophytic fungi significantly increased carbon in an aggregated carbon-rich Alfisol over 14 weeks, with increases of up to 17% measured. Two of these fungi increased organic carbon within microaggregates.

This study demonstrates that some melanised endophytic fungi have the potential to increase levels of organic carbon within an experimental soil. Melanin, a polyaromatic compound present within the cell walls of melanised endophytic fungi, may have contributed towards increases in organic carbon, particularly if protected within soil aggregates.

Deposition of aromatic carbon within aggregates would leave this carbon less susceptible to oxidation and contribute towards long-term carbon storage in soils.