Healthy soil and sustainable growing

["TradingPostPaul" <tradingpost@riseup.net>]

what else are we missing, indeed ...


Bacteria get the upper hand on modern crops
http://www.nature.com/news/2007/071016/full/news.2007.168.html
New soya-bean breeds don't cope well with swindling bacteria.

Heidi Ledford
Modern soy may find it harder to grow in unfertilized fields.Modern soy may
find it harder to grow in unfertilized fields.

Decades of breeding high-yielding soya-beans in fertilized soils has
produced varieties that have lost the ability to interact effectively with
soil bacteria, researchers have found. The results suggest that modern
breeds of soya beans, and possibly other crops, may now be less suited for
growth in low-nutrient conditions, such as the unfertilized fields of
developing countries.

“I would say it’s worrying,” says Ford Denison, a plant ecologist at
the University of Minnesota in St Paul and a co-author on the study.
“It’s something that I would like plant breeders to recognize.”

Soya beans do not typically require fertilizer because they live together
with soil bacteria that ‘fix’ atmospheric nitrogen into more useful
forms, such as ammonium, that can be used by the plant. In exchange, the
plants provide carbon to the bacteria.

But fixing nitrogen is costly, and not all bacteria put much effort into
the process. Soils contain some strains of bacterial 'swindlers', which
live off plant roots without giving much nitrogen in return.

Previous research has shown that soya beans tend to fight back against such
swindlers, providing less carbon to cheaters and extra carbon to the more
generous bacteria. This encourages the growth of beneficial bacteria1.

Modern agricultural practices, however, have partially relieved soya beans
from having to fight this battle. Although the plants are not directly
fertilized, residual fertilizer is often left in the soil after rotating
other crops through the same field. And farmers sometimes inoculate soya
bean seeds with strains of nitrogen-fixing bacteria that are particularly
adept at fixing nitrogen.
Lost talent

Toby Kiers, now at VU University in Amsterdam, and her colleagues wondered
whether these conditions might have affected the ability of soya beans to
distinguish helpful bacteria from cheaters. The researchers gathered six
soya bean variants from throughout 60 years of crop breeding history, and
grew them in unfertilized fields. They tested the soya beans by providing
them with mixtures of bacteria containing good nitrogen fixers and closely
related bacteria that couldn’t fix nitrogen at all.

Seed production in the more modern breeds of soya bean declined more than
in older breeds when cheating bacteria were present, suggesting that older
breeds are able to cope better with the presence of such cheaters, they
found2. The results are published this week in the Proceedings of the Royal
Society Lond.

    “Modern plant breeding has allowed this trait to slip through the
cracks.”

Tim Crews

“The difference between old and new was enough to eliminate the yield
benefit of all of these decades of breeding,” says Denison. Plant
breeders may not have detected the decline in performance, he notes,
because fertilized soils and inoculation with robust nitrogen-fixers may
have masked the trait. Alternatively, other benefits of the new breeds,
such as disease resistance, may have outweighed the detriments of reduced
bacterial selection in some regions, he adds.

Nevertheless, those bacterial relationships are important in non-fertilized
soils, as is often found in developing nations. Soils also vary in the
quality of their nitrogen-fixing bacteria, and some regions have a high
proportion of ‘cheaters’. Under these conditions, the ability of soya
beans to direct their efforts to rewarding bacteria is probably more
important.
Important partners

Breeders can now cross new, higher-yielding breeds with old varieties to
try to recoup the ability to cope with cheating bacteria, notes Tim Crews,
an ecologist at Prescott College in Arizona, who was not involved with the
work.

The results provide a reminder that breeders should consider ecological
relationships when they select their crops and breed for high performance,
says Crews. “Modern plant breeding has allowed this trait to slip through
the cracks,” he says.

Previous research has suggested that interactions of some crops, including
wheat, corn and soya beans, with certain soil fungi may also have become
less beneficial over time. This could be a consequence of using fertilizers
in breeding programmes, notes Denison.

“It raises the question in my mind: what else are we missing?” says
Crews. “We could be doing a lot more with a lot less.”

    *
      References
         1. Kiers, E. T., Rousseau, R. A., West, S. A. & Denison, R. F.
Nature 425, 78-81 (2003).
         2. Kiers, E. T., Hutton, M. G. & Denison, R. F. Proc. R. Soc.
Lond. B. doi: 10.1098/rspb.2007.1187 (2007).