How do grassland management history and bacterial micro-localisation affect the response of bacterial community structure to changes in aboveground grazing regime?

Abstract

In a context of frequent intensification or de-intensification of management in grasslands, a better understanding of how quickly soil microbiota responds to changes in management is required. The kinetics of changes in the structure of the bacterial community (using ribosomal intergenic spacer analysis) was studied in grassland mesocosms after changes of aboveground grazing regime, taking into account bacteria micro-localisation by separating the bacteria located inside stable aggregates (inner soil fraction) and the bacteria easily washed out, i.e. mainly located in macropores (outer soil fraction). Four treatments were used: (i) control grazed mesocosms, (ii) control ungrazed mesocosms, (iii) application of grazing on previously ungrazed mesocosms, (iv) cessation of grazing on previously grazed mesocosms. Each grazing event was simulated by application of synthetic sheep urine and plant clipping. Application of grazing led to a change in the structure of the whole soil bacterial community within 5 months, whereas changes were observed only 12 months after cessation of grazing. Changes in plant species composition and soil organic carbon content observed after cessation of grazing were found to be possible drivers of the changes in the bacterial community structure. However, after application of grazing, changes of the bacterial community structure occurred prior to changes in plant species composition and soil organic carbon content, suggesting that supply of urine and/or impact of labile carbon were likely the main drivers of changes. After 12 months, the application of grazing significantly affected the bacterial community structure in both inner and outer soil fractions. Conversely, 12 months after cessation of grazing, community structure was affected only for bacteria located in the outer fraction. This study shows that the bacterial community structure responded faster and more deeply after application than after cessation of grazing, and may be driven by different environmental factors between both scenarios. This study also shows that, 2 years after the changes in grazing regime, the bacterial community structure was determined by both the past and new grazing regimes.