Soil science

Event Date: 
Wednesday, August 26, 2015 - 18:15 - 18:30
Institution: 
CSIRO
Title: 

Effects of temporal pH shifts on ammonia oxidiser community structure and function

Abstract: 

Soil nitrification, the oxidation of ammonia to nitrate, is and driven by bacterial and archaeal autotrophic ammonia oxidisers (AOB and AOA) that carry out the first, rate limiting, step of oxidising ammonia to nitrite.  Previous work has suggested that adaptation and selection in AOA and AOB communities is, to some extent, pH driven.  Acidophilic, acido-neutral, and alkalinophilic groups have been identified by environmental surveys of amoA genes.  These studies of the role of pH in determining ammonia oxidiser community structure and activity have largely relied on spatial pH gradients.  In many managed soil systems (e.g., agricultural systems) edaphic factors (e.g., pH, N concentrations) vary widely temporally and the implications of short term temporal shifts in factors thought to govern oxidiser community structure, and therefore our ability to manipulate edaphic factors to direct community structure, are not well understood.   We investigated the roles of pH in driving nitrifier activity (potential) and community structure over a crop growing season (6 sampling points) in agricultural soils by comparing unamended soils with soils amended with lime to create a temporal pH gradient.  Liming induced a rapid and sustained change in the pH of surface soils (0-10cm), with pH in these soils increasing from 4.8 to 6.5, while in subsurface soils pH increased to a lesser degree after liming (4.3 – 4.5).  After liming, potential nitrification rates increased significantly throughout the production season in both surface and subsurface soils.   TRFLP analysis of total bacterial and archaeal communities showed significant partitioning of the broader communities with soil depth, pH treatment and time, suggesting that microbial communities respond rapidly to changes and that temporal variation in community structure is an important, if often overlooked, factor in assessing microbial diversity patterns. These changes were greater for bacterial, than archaeal, communities. We then utilised amoA gene microarrays to investigate specific AOA and AOB community responses to temporally induced pH changes.  Despite significant changes to ammonia oxidiser function, we saw only very weak changes in community structure of AOA and AOB, suggesting that over shorter temporal periods soil communities are resilient to environmental change and that niche partitioning of ammonia oxidiser communities is likely to be spatially, rather than temporally, governed.

Event Date: 
Wednesday, October 31, 2012 - 06:15 - 06:30
Institution: 
CSIRO
Title: 

Impact of nutrient addition on microbial community actively decomposing wheat residues and sequestration of carbon in soil

Abstract: 

 
Soils represent a significant terrestrial carbon reservoir and there is considerable interest in increasing or maintaining its size.   Recent work has suggested that microbially mediated carbon sequestration into soils may be maximised by ensuring adequate supply of other nutrients (nitrogen, phosphorous, sulphur), thus maintaining high microbial carbon use efficiency.  This is particularly important in cropping agriculture systems where appropriate crop-residue management may result in substantial soil carbon and general soil quality improvements.
A soil microcosm experiment was conducted to evaluate the effects of nutrients on the decomposition of wheat residues; specifically the effects on active bacterial community and carbon sequestration.  Stable Isotope Probing (SIP) of 16S rDNA using Phylochip microarrays was employed to investigate the bacterial community associated with the decomposition of 13C-labelled wheat straw and incorporation of the carbon into soil humus.  Respiration and carbon, nitrogen, phosphorous and sulphur transformations were measured over a 56 day incubation to assess priming effects, both gross and net humification efficiency and carbon sequestration through microbiological action.  Soils that received wheat residues only always showed an increase in “new” carbon (13C), although they did not always show an increase in “total” (12C +13C) carbon.  Soils receiving nutrients (nitrogen, phosphorous and sulphur) with wheat residues did show an increase in both new and total carbon pools, suggesting availability of nutrients other than carbon influence soil carbon sequestration.  Bacterial communities responsible for residue decomposition and carbon sequestration were different for nutrient added and no-nutrient treatments, indicating microbial carbon use efficiency is also influenced by the composition of the active community.  Our results suggest that soil carbon:nitrogen:phosphorus:sulphur (C:N:P:S) stoichiometry and bacterial community composition play important roles in determining potential levels of carbon sequestration in agricultural soils.

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