NISKU, Alta. – Ask most farmers which essential nutrient their crop uses the most and you’ll probably hear nitrogen.
But zero till researcher Dwayne Beck said they’d be wrong.
“The nutrient taken up in the largest quantity by plants is carbon. We’ve never thought ‘what can I do to encourage more carbon in my field,’ ” said Beck, the keynote speaker at the Alberta Reduced Tillage Linkages Direct Seeding Advantage conference, held recently at Nisku.
Beck said a 75 bushel wheat crop will have about 3,000 pounds of carbon in the residue it produces, compared to about 50 lb. of nitrogen. And the carbon removed in the wheat kernels far exceeds that of the nitrogen.
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But what does carbon have to do with crop yield? Lots, according to Beck, who manages the Dakota Lakes Research Farm near Pierre, South Dakota.
Elevating carbon dioxide levels in a crop canopy might lead to better water use efficiency and higher yields. A way to accomplish this is through zero tillage and leaving higher levels of crop residue on the soil.
The elevated carbon dioxide levels increase the efficient use of water in plants for a simple reason.
“When I increase the CO2 concentration, it allows the partial closure of the stomata. The plant opens the stomata only to let CO2 in. It doesn’t open them to let water out. If it has more CO2 than it needs, it partially closes the stomata, which restricts water loss during transpiration,” said Beck.
Beck said many agricultural scientists believe soils with higher levels of organic carbon are more productive because of better water holding capacity, better structure and more cation (electrical) exchange capacity. But few have considered that there may be an impact from how well plants use the carbon dioxide.
He said in tilled systems, where most carbon dioxide cycling occurs three or four days after the tillage, there’s little chance to manage that carbon to suit the plant’s needs.
That may not be true for no till farmers, whose carbon cycles later in the season, as it does in native range and under other natural conditions.
“One of the good things about climate change and the increase of carbon in the atmosphere is that it makes plants grow better. We know that if we increase CO2 levels, then things change,” he said.
“Maybe the carbon dioxide levels in that crop canopy can be changed by management. If I can keep my residue from decomposing until after the canopy is established, then that CO2 that results from the residue decomposing will enhance the carbon dioxide in that canopy.”
Beck said in the early part of this decade, Jerry Hatfield, a U.S. Department of Agriculture soil scientist, made observations from no till research he was doing at Ames, Iowa.
Hatfield noted that carbon dioxide and water vapour measured within a corn canopy in the summer of 2001 varied depending on the time of day. Carbon dioxide levels in the lower canopy increased to nearly 900 parts per million during the night, then rapidly decreased as sunlight penetrated the crop canopy in the morning.
By mid afternoon, levels were down to less than 300 ppm and Hatfield suggested these lower carbon dioxide levels may be limiting crop growth.
Examining the patterns of carbon dioxide and water vapour suggested the soil may be a significant source of carbon dioxide when the crop canopy covers the soil surface.
“Hatfield said that up to 40 percent of the carbon dioxide in a no till system can come from the soil and the residue itself. It doesn’t have to just blow in,” said Beck.
“Higher water use efficiency in these high residue rotations may not be just due to soil property. It may be because we’re changing the carbon dioxide concentration in the canopy.”
Beck said carbon cycling could possibly be manipulated by rotation, residue management, nitrogen application methods and other farming activities, with the goal of raising carbon dioxide in the crop canopy when the crops need carbon the most.
He said because soils with higher organic matter normally produce higher yields than soils with low organic matter, and fields recently converted from perennial crops or sod to annual crops often produce superior yields, the response could be partly caused by carbon cycling.
“Nitrogen use efficiency also generally improves. You won’t necessarily get better protein, but you will get better nitrogen use efficiency. You know in years where you have drought, it takes the same amount of nitrogen to produce less bushels of higher protein wheat,” he said.
Beck summed up by saying that farmers take sunlight, water and carbon dioxide and turn them into products they can sell.
Managing carbon may some day be just as important as the other nutrients in producing a crop.
