By Donna Fleury
As the biomass produced by the pea variety increases, the amount of N fixed also increases.
Growers include field pea in their rotations for many reasons, including the ability of pea to fix its own nitrogen (N) and for the benefits to the subsequent crop. However, researchers are still trying to quantify these benefits. As well, the trend to semi-leafless field pea varieties has researchers questioning if pea varieties with different leaf types will fix different amounts of N, and what the impact is on subsequent wheat crops.
“We wanted to conduct a study using commercially available varieties and to see if we could find any differences in the amount of N fixed with the range of biomass they produced,” explains Dr. Yoong Soon, research scientist, Nutrient Cycling Research at the Agriculture and Agri-Food Canada Research Station in Beaverlodge, Alberta. “We also wanted to determine the influence of these varieties on soil N balance and subsequent crops, such as wheat.”
Trials for the three year study were conducted at Beaverlodge and Fort Vermillion, Alberta. The Grande pea variety was the only normal leaf variety commercially available for the study, the rest were semi-leafless varieties including Carrera, Eiffel and Swing. Barley was grown as a check. Unfortunately, three of the four site years of the study had below average growing season rainfall, which impacted the study results. “From the research, we did find that there was a significant variation in the amount of N fixed by the different varieties,” says Soon. “This study confirms the well known fact that as the biomass produced by the pea variety increases, the amount of N fixed also increases.”
The Grande variety showed a neutral soil N balance, which means that the amount of N fixed by Grande is equal to the amount of N removed by the pea seed. “However with the semi-leafless varieties, there was a varying amount of N removed by the pea crop,” explains Soon. “In most cases, the semi-leafless varieties had a negative N balance, which means they were not fixing enough N to satisfy their requirements to produce N in the seed.”
Some of the semi-leafless varieties had the same N balance as the barley reference crop used. The amount of N removed by the pea varieties was about the same as barley, particularly in the drier years when pulse crops and rhizobia were stressed by a lack of soil moisture and very little N fixation occurred.
The second component of the study was to look at the impact of the N balance on the subsequent wheat crops. “Our results show that irrespective of whether the N balance was zero or negative, the pea always benefited the subsequent crop in terms of production and protein content of the wheat,” explains Soon. “Therefore, something else is influencing the response of the wheat following pea. Although we couldn’t pinpoint it in this study, there are previous studies and studies done elsewhere in the world showing that the N mineralization may be coming from the roots left by the pea, releasing it for use by the subsequent crop.”
The research did not show a significant difference between pea varieties in the subsequent year. However, Soon notes that the actual average of the numbers indicates a trend for the wheat that follows the Grande pea variety to be a slightly higher yield of maybe 10 percent. “So although it may not be statistically significant from a scientific point of view, a 10 percent increase could make an economic difference to a grower.” Overall, the research results confirm there are some benefits to growing peas in rotation and for the subsequent wheat crop. Even where the soil N balance was negative, wheat following pea mostly had higher seed protein content and yield than wheat following barley.
“Although this study did give us certain indications, we didn’t feel that we ended up with the right range of dry matter production by the pea to confirm whether or not there was a differential effect between pea varieties. Therefore, we have now initiated a new study with a greater variety of different types of pulse crops and varieties that will give us a greater range of dry matter production and hopefully the range in N fixation,” says Soon.
The new study includes various pea varieties and fababeans with a wide range of dry matter production. This new study, which started in 2007 and is funded by the Alberta Pulse Growers Commission, will run for four years. This will allow researchers to track the effects of the N released beyond the first year after pea crops to subsequent crops grown two and three years after peas.
“One of the varieties we grew in 2007 produced a lot of biomass and because we had such wet conditions, the crops were very heavy and lodged quite badly. We expect that the effects and yield of subsequent crops will be quite noticeable following a crop with this much biomass.” Fababeans are being grown as a green manure crop, as the conditions in the Peace do not usually allow for it to reach maturity. “For the Peace area, peas are a good crop and
the only pulse crop we can grow to maturity,” adds Soon. “Pea crops are a good rotation option that do influence the soil N balance and benefits the subsequent crops in rotation.”