Javier Izquierdo
javier@microbio.umass.edu

Curriculum Vitae [pdf]

I am interested in the influence of soil aggregation on the arrangement of microbial communities in soil, and the distribution of function across these different soil microenvironments. Water-stable soil aggregates play an important role in the stability of soils and in the sequestration of organic matter. Different aggregate size classes create completely different soil microenvironments that vary in temporal stability, niche density and micronutrient availability. These environments can be separated and studied in detail using molecular methods. Because soil is such a complex environment, this approach enables us to obtain information on soil community structure that remains largely unknown. Below, you may read about some of the ongoing projects related to these questions.


RESEARCH

Distribution of nifH genes in arctic, temperate and tropical soils

We have studied the nifH gene distribution across water-stable soil fractions of an arctic tundra soil using DGGE, RFLP and clone libraries. Additionally, we have compared the distribution of these genes with the distribution across microenvironments in tropical soils from Venezuela and temperate soils from Massachusetts. An increase in the prominence of alpha-proteobacterial nifH genes is evident in the microaggregate fractions (75 and 250 μm in diameter) in all three soils. However, we have identified a large uncultured cluster of nifH genes in the microaggregate fractions of the arctic tundra soil.

Distribution of nitrogen processes in agricultural soils

We have examined the distribution of nifH, narG and amoA genes across four soil size fractions in three agricultural plots. These plots are located at Hampshire College and represent an excellent opportunity to study the distribution of these functional genes in three different treatments: a 10-year organic plot, a newly-tilled plot and an uncultivated grassland. We have determined that soil microaggregates are not able to protect communities of diazotrophs, nitrate reducers and ammonia oxidizers from a tillage disturbance. However, in long-term tilled and long-term uncultivated soils, a larger degree of overlap was found across communities in each aggregate fraction.

We are currently examining the long-term effect of this tillage event and the recovery of these communities.



Community analysis of individual water-stable microaggregates

I have also been able to compare the 16S rDNA gene pools of individual water-stable microaggregates using clone libraries. Microaggregates are believed to be the most stable structures in soil. My goal is hence to determine if the community remains the same or varies from one aggregate to another. For this project, I designed a method of DNA extraction to obtain PCR-amplifiable DNA from a single water-stable microaggregate. The distributions of the most dominant 16S rDNA genes in three individual microaggregates were compared and they were determined to be statistically similar. We are currently examining variation in nifH community composition across individual microaggregates, and correlating these to our previous findings at the aggregate fraction scale. Electron micrograph of an individual microaggregate from our study plots and detail of microcolony on its surface. A larger picture can be also obtained from The UMass Gallery of Biological Imagery.

Publications

Izquierdo, J.A. and K. Nüsslein (In review) Effect of tillage on the distribution of genes involved in the nitrogen cycle across aggregate fractions of agricultural ecosystems.

Izquierdo, J.A. and K. Nüsslein (2006) Distribution of extensive nifH gene diversity across physical soil microenvironments. Microbial Ecology 51(4): 441-452


Funding

Lotta M. Crabtree Scholarship in Agriculture - Spring 2005



POSTERS and PRESENTATIONS

Please, click on the picture or the [pdf file] link to download a copy of each presentation.

	Micro-scale separation of microbial communities involved in the nitrogen cycle across soil aggregate fractions Oral presentation at the Boston Bacterial Meeting, June 16-17, 2005. Cambridge, MA [pdf file]
	Shifts in the microbial community involved in the nitrogen cycle of agricultural soil microenvironments Poster presented at the Soil Science Society of America Annual Meeting, October 31-November 4, 2004. Seattle, WA [pdf file]
	Diversity of nifH, narG and amoA genes in agricultural soil microenvironments Poster presented at the American Society for Microbiology General Meeting, May, 23-27, 2004. New Orleans, LA. [pdf file]
	Diazotrophic diversity in soil microenvironments from three different latitudinal regions. Poster presented at the American Society for Microbiology General Meeting, May 18-22, 2003. Washington, D.C. [pdf file]

FURTHER READING

If you would like to find further reading on these topics, there is a great number of soil aggregate references available on the web, as well as several websites on soil health, including a thorough piece on factors affecting soil aggregate stability.

The following peer-reviewed scientific journals (among many others) provide a wealth of information and touch on the current state of our understanding of some the questions I am trying to address with my research:

  Applied and Environmental Microbiology
  Microbial Ecology
  Environmental Microbiology
  Soil Science Society of America Journal
  Soil Biology and Biochemistry


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