Impacts of organic and conventional crop management on diversity and activity of free-living nitrogen fixing bacteria and total bacteria are subsidiary to temporal effects

A three year field study (2007-2009) of the diversity and numbers of the total and metabolically active free-living diazotophic bacteria and total bacterial communities in organic and conventionally managed agricultural soil was conducted at the Nafferton Factorial Systems Comparison (NFSC) study, in northeast England. The result demonstrated that there was no consistent effect of either organic or conventional soil management across the three years on the diversity or quantity of either diazotrophic or total bacterial communities. However, ordination analyses carried out on data from each individual year showed that factors associated with the different fertility management measures including availability of nitrogen species, organic carbon and pH, did exert significant effects on the structure of both diazotrophic and total bacterial communities. It appeared that the dominant drivers of qualitative and quantitative changes in both communities were annual and seasonal effects. Moreover, regression analyses showed activity of both communities was significantly affected by soil temperature and climatic conditions. The diazotrophic community showed no significant change in diversity across the three years, however, the total bacterial community significantly increased in diversity year on year. Diversity was always greatest during March for both diazotrophic and total bacterial communities. Quantitative analyses using qPCR of each community indicated that metabolically active diazotrophs were highest in year 1 but the population significantly declined in year 2 before recovering somewhat in the final year. The total bacterial population in contrast increased significantly each year. Seasonal effects were less consistent in this quantitative study

Soil Functions—An Introduction

It is now widely recognised that soils not only provide food but in addition, they deliver a wide range of ecosystem services to society. The EU Thematic Strategy on Soils (2006) identified seven ‘environmental, economic, social and cultural functions’. In research studies from Ireland, these ecosystem services have been rearranged into the following five main soil functions for agricultural land (1) primary production, (2) water purification and regulation, (3) carbon storage and sequestration, (4) habitat for intrinsic and functional biodiversity and (5) the cycling and provision of nutrients. In addition, soils also provide two ancillary functions, namely: (6) a platform for infrastructure (e.g. roads, buildings) and (7) an outdoor archive of archaeological heritage. In principle, all soils perform each of these functions simultaneously. However, the extent to which each function is delivered depends in the first instance, on land use. For example, the functionality of arable soils is characterised by primary production and nutrient cycling. Second, the functionality of soils depends on soil properties. The dominant soil properties in Atlantic climates relate to soil moisture dynamics, specifically the occurrence of excess soil water. Quantifying soil functions is difficult as each soil function encompasses a set of processes which may be altered through management which could alter the delivery of another function, resulting in potentially a synergy or a trade-off between functions. Therefore, proxy-indicators are used to estimate the extent to which a soil provides the five functions. The functionality of soils worldwide is threatened by unsustainable management practices and in Europe, there are eight main threats to soil functionality