permaculture

[Lawrence London <lfljvenaura@gmail.com>]

Crop Rotation Works - Here's Why
By News Staff | July 21st 2013 10:18 AM

The ancient Romans were the first to officially discovered that rotating
crops improves plant nutrition and inhibits the spread of disease.

While it's common wisdom today, science is often about confirming why
nature works the way it does. A new paper details profound effect crop
rotation has on enriching soil with bacteria, fungi and protozoa.

Soil was collected from a field near Norwich and planted with wheat, oats
and peas. After growing wheat, it remained largely unchanged and the
microbes in it were mostly bacteria. However, growing oat and pea in the
same sample caused a huge shift towards protozoa and nematode worms. Soil
grown with peas was highly enriched for fungi.

 The scientists also grew an oat variety unable to produce normal levels of
avenacin, a compound that protects roots from fungal pathogens. They
expected the soil to contain higher levels of fungi as a result, but
instead found it contained a greater diversity of other eukaryotes such as
protozoa. All organisms on our planet can be divided between prokaryotes
(which include bacteria) and eukaryotes (which include humans, plants and
animals as well as fungi). After only four weeks of growth, the soil
surrounding wheat contained about 3% eukaryotes. This went up to 12-15% for
oat and pea. The change of balance is likely to be even more marked in the
field where crops are grown for months rather than weeks.

 The findings of the study could be used to develop plant varieties that
encourage beneficial microbes in the soil. The scientists are already
investigating the possibility of engineering cereal crops able to associate
with the nitrogen-fixing bacteria normally associated with peas.

"Changing the crop species massively changes the content of microbes in the
soil, which in turn helps the plant to acquire nutrients, regulate growth
and protect itself against pests and diseases, boosting yield," said
co-author Professor Philip Poole from the John Innes Centre.
"The soil around the roots was similar before and after growing wheat, but
peas and oats re-set of the diversity of microbes."

Analysis has previously relied on amplifying DNA samples. This limits
scientists to analyzing one taxonomic group at a time such as bacteria. It
also means that everything present in that group is analyzed rather than
what is playing an active role. Every gram of soil contains over 50,000
species of bacteria so the task is enormous.

There are relatively fewer actively expressed genes - RNA. It is now
possible to sequence RNA across kingdoms so a full snapshot can be taken of
the active bacteria, fungi, protozoa and other microbes in the soil. The
research was carried out in collaboration with the University of East
Anglia and The Genome Analysis Centre on Norwich Research Park.

"By sequencing RNA, we can look at the big picture of active microbes in
the soil," said PhD student Tom Turner from the John Innes Centre.  "This
also allows us to work out what they are doing there, including how they
might be helping the plants out."

"Our work helps explain the experience of farmers in the field," said
Poole. "The best seed needs to be combined with the best agronomic
practices to get the full potential benefits. While continued planting of
one species in monoculture pulls the soil in one direction, rotating to a
different one benefits soil health."

Seeds can be inoculated with bacteria before planting out, just like humans
taking a dose of friendly bacteria. But this does not achieve the diversity
or quantity of microbes found in this study.

"Small changes in plant genotype can have complex and unexpected effects on
soil microbes surrounding the roots," said Poole. "Scientists, breeders and
farmers can make the most of these effects not only with what they grow but
how they grow it."

The research was made possible with funding from the Earth and Life Systems
Alliance at the University of East Anglia and from the core strategic grant
to the John Innes Centre from the Biotechnology and Biological Sciences
Research Council (BBSRC).

Reference: 'Comparative metatranscriptomics reveals Kingdom level changes
in the rhizosphere microbiome of plants' by Thomas Turner et al., ISME
Journal.
http://www.science20.com/news_articles/crop_rotation_works_heres_why-116804