Soil carbon decomposition is regulated by microbial accessibility and diversity

Jonathan Nuñez1,2, Gabriel Moinet1,3, Gwen-Aelle  Grelet1, John  Hunt1, Matthew  Turnbull2, Tim  Clough4, David  Whitehead1

1Manaaki Whenua-landcare Research, Lincoln, New Zealand, 2School of Biological Sciences, University of Canterbury, Christchurch, New Zealand, 3Soil Biology Group, Wageningen University and Research , Wageningen, The Netherlands, 4Lincoln University, Lincoln, New Zealand

Most studies addressing relationships between soil carbon dynamics and microbial diversity have not considered the role of carbon accessibility in regulating net carbon balance. We manipulated carbon accessibility to microbial decomposition by adding different proportions of carbon-binding active mineral surfaces to a constant mass of soil to investigate the linkages between soil microbial communities and cycling of stable soil carbon. Three proportions of an active mineral (0, 15, and 50%) were added to three soils with contrasting carbon concentrations of 3, 4.5 and 6%. We used four independent methods to determine differences in carbon accessibility, comprising (a) soil respiration rate, (b) enrichment of ¹³C isotopic signature of respired CO₂, (b), (c) soluble carbon extracted in cold and hot water, and (d) aluminium and iron pyrophosphate concentrations as a proxy for the degree of organo-mineral binding interactions at 1, 4 and 8 days after the active mineral surfaces were added to the soils. DNA was extracted and the diversity and abundance of bacterial communities were estimated from bacterial 16s gene abundance.

All four methods showed that additions of the active mineral to the soils proportionally reduced carbon accessibility by up to 80% with a constant trend observed for the three soils and no significant differences between days after addition. The observed proportional δ¹³C enrichment from -28‰ (no active mineral addition) to -22‰ (50% mineral addition) was interpreted as a shift in the substrates used for decomposition, from labile C to microbially-processed, presumably older stable C. We present relationships between bacterial diversity and abundance and cycling of soil C in relation to its accessibility, providing new insights to reveal a mechanistic understanding of processes linking microbial accessibility and regulation of soil carbon decomposition.

Biography: Jonathan is from Colombia and has been studying for a PhD degree in New Zealand since May 2017 working on mechanistic linkages between soil carbon and nitrogen dynamics, contributing to a wider research programme to find solutions for farmers to reduce carbon and nitrogen losses. Jonathan completed his master’s degree at the National University of Colombia in association with the International Centre for Tropical Agriculture (CIAT), where he developed methods to characterise the biological nitrification inhibition activity in soils under tropical forages.

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