The Effects of Long Term Agricultural Production on Soil Microbial Diversity

Abstract

Soil microorganisms are important facilitators of ecosystem functions including: crop and pesticide residue decomposition, carbon sequestration, Nitrogen (N) fixation and nutrient cycling. Therefore, the abundance and diversity of soil microorganisms may act as a buffer against stress and change, ensuring that soils remain productive. The Rotation ABC Long term soil experiment (LTSE) at Lethbridge, Alberta provides an opportunity to use modern molecular tools to study microbial dynamics in response to over a century of agricultural land management. My goals were to assess cumulative effects of N fertilizer and different cropping intensities (CI) on soil microbial community abundance and diversity, both in soils collected in 2012 and in the soils archived over 100 years of wheat production. Soil microbial community abundance and diversity of soils collected in 2012 were influenced by N fertilization and CI. Phospholipid fatty acid (PLFA) analysis of the 2012 soils revealed a positive relationship between N fertilizer application and CI on the total active biomass. Quantitative PCR analysis showed that N functional gene abundances were affected by significant interactions between N fertilizer and CI, and the abundances of denitrifier genes (nosZ and nirK) were also strongly influenced by higher levels of CI in the 2012 soils. A short term soil storage experiment indicated that storage time and fertilizer treatment (45 kg N ha-1, 20 kg P ha-1 vs. unfertilized) influenced the amount of DNA extracted. In addition, the results suggested that N and P fertilized soils had greater bacterial diversity than unfertilized soils. Through the use of 16S rRNA gene profiling with these soils the results suggested that bacterial diversity and richness was lowest in the oldest samples, implying a shift over decades of agricultural production. Soils collected after the introduction of N fertilizer displayed an increase in N function gene abundances, indicating an increase in N cycling potential. Overtime, the changes in agricultural land management led to increased plant and soil productivity, resulting in a positive relationship between years under agricultural land management and soil bacterial diversity and abundance.