Worms and weeds to flourish under global warming

By SIMON COLLINS science reporter



Tired of weeding the garden? Well, brace yourself because it is set to get worse.

New research shows that with more carbon dioxide being released into the air, plants are growing faster. But weeds grow even faster.

An AgResearch experiment in which excess carbon dioxide was pumped into the air over a paddock near Bulls for six years found that the grass grew faster - but weeds and nematodes (soil worms) grew even more.

AgResearch pastoral ecologist Paul Newton told a seminar on global warming in Wellington last week that overseas research pointing to a "greening" of the world caused by rising carbon dioxide levels was mostly based on laboratory experiments rather than tests in actual environments such as pasture.

Surprisingly, it also found that feeding sheep with grass that was enriched with extra carbon dioxide actually reduced the amount of methane belched out by the sheep, partially offsetting the "greenhouse" effect of the carbon dioxide.

The Bulls experiment is part of a network of more realistic open-air tests now under way around the world, and is unique in including grazing animals.

Using tanks of carbon dioxide, scientists increased the level of the gas in the air over the paddock to a steady 475 parts per million - 28 per cent higher than the present global level of 370ppm, and about the level that the Earth is expected to experience in about 30 years because of increasing emissions from cars and industry.

They put sensors in the soil to measure nutrients and masks on the sheep to measure how much methane they belched out.

They found that the extra carbon dioxide:

* Increased grass growth 8 per cent.

* Increased growth of broadleaf weeds and legumes more than any other plants.

* Boosted the number of diverse plant species.

* Doubled the number of root-feeding soil worms.

* Reduced the amount of nitrogen available in the soil by 35 per cent.

* Reduced methane emissions from the sheep by 10 per cent.

"It doesn't change yield much, but it does change species numbers and the availability of soil nutrients," Dr Newton said.

"Biodiversity might be a good thing, but you might be gaining species that are really weedy species or nuisance species.

"For pasture, more of an issue is that we are seeing reduced availability of nutrients for plants, such as phosphorous and nitrogen. It's being locked up by putting more carbon below ground.

"Pasture growth is not worse, because it may be more efficient at using those materials at high carbon dioxide.

"But that certainly could be an explanation of why we don't see big increases in yield."

Dr Newton said the same effects had been found in pastures surrounding some springs in Northland which produce a naturally high level of carbon dioxide.

For farmers, the experiment suggests that they might need to put on more nitrogen fertiliser as the global level of carbon dioxide rises.

For the country, the biggest surprise was the reduced methane belched out by sheep - a result that puzzles scientists.

AgResearch is doing further research to solve the conundrum.


GLOBAL WARMING

* Car exhausts and industrial emissions have pushed up the average level of carbon dioxide in the air from 280 parts per million to 370ppm in the past 200 years.

* Rising levels of "greenhouse gases" such as carbon dioxide and methane are trapping more of the sun's heat in the lower atmosphere.

* Average temperatures rose 0.6C during the 20th century and are forecast to rise between 1.4C and 5.8C this century.

* Rising levels of carbon dioxide will also help plants to grow faster - but may help weeds more than pasture.