Soil microbial diversity and community composition during conversion from conventional to organic agriculture

It is generally assumed that the dependence of conventional agriculture on
artificial fertilizers and pesticides strongly impacts the environment,
while organic agriculture relying more on microbial functioning may
mitigate these impacts. However, it is not well known how microbial
diversity and community composition change in conventionally managed
farmers’ fields that are converted to organic management. Here, we
sequenced bacterial and fungal communities of 34 organic fields on sand
and marine clay soils in a time series (chronosequence) covering 25 years
of conversion. Nearby conventional fields were used as references. We
found that community composition of bacteria and fungi differed between
organic and conventionally managed fields. In the organic fields, fungal
diversity increased with time since conversion. However, this effect
disappeared when the conventional paired fields were included. There was a
relationship between pH and soil organic matter content and the diversity
and community composition of bacteria and fungi. In marine clay soils,
when time since organic management increased, fungal communities in
organic fields became more dissimilar to those in conventional fields. We
conclude that conversion to organic management in these Dutch farmers’
fields did not increase microbial community diversity. Instead, we
observed that in organic fields in marine clay when time since conversion
increased soil fungal community composition became progressively
dissimilar from that in conventional fields. Our results also showed that
the paired sampling approach of organic and conventional fields was
essential in order to control for environmental variation that was
otherwise unaccounted for.