One Pulse, Two Pulse, Three Pulse, More - PulsePoint
December 17, 2018
Recent research paints an interesting and complex picture of what makes a sustainable rotation
by Lyndsey Smith
If some pulses in rotation is good then more pulses is better, right? As with most agronomic questions, it depends.
Agriculture and Agri-Food Canada research scientist Dr. Yantai Gan has been leading a complex, multi-site, multi-year project searching for answers on the impact of several pulse crops in a four-year rotation, what changes if you change the pulse species, and how pulse crops in rotation shift the soil and root microbial ecosystems to the good or bad.
Previous work has spelled out the rooting depth/water use relationship with pulse crops in rotation, Gan explains, as pulses are known for being shallow rooted, leaving water at deeper soil layers for future crops to access. Also, pulse crops leave nitrogen-rich residues (straw and roots) behind that benefit subsequent non-pulse crops, but what else do pulse crops do to influence soil biology and crop yields in the following year or years?
“Our research shows that about 30 per cent of the yield advantage in a subsequent wheat crop is due to the nitrogen and water‑use impact of the preceding pulse crop,” Gan says. “What contributes to the other 70 per cent of the yield benefit?”
The answer lives within changes in population of microbes within the root, on the root surface (rhizosphere), as well as in the rooting zone (soil).
Comparing a one pulse in four years rotation, to two-in-four, and three-in-four year rotation, Gan and collaborators measured typical agronomic performance, such as yield of both the pulse and non-pulse crops. They also analyzed the roots and the soil that adhered on the root surface by brushing the soil from the roots to collect samples.
What science is just beginning to identify, isolate, and quantify, are the millions of fungal, bacterial, and mycorrhizae species living within soil. Taking it one step further, research is looking at understanding how these fungi, bacteria, and mycorrhizae interact with crop roots growing in the soil, soil environments, and cropping practices.
Unlocking some of these answers is actually quite key to establishing sustainable pulse crop rotations. As Gan explains, research suggests that a diverse four- to six-year crop rotation that includes two or more pulse crops is beneficial — but we also know that fungal pathogens that cause root rot diseases of pulse crops begin to increase in the soil, if host crops are grown too closely in rotation.
That said, some of the pulse species have the capacity to build up antagonistic bacteria in the soil, Gan says. While perhaps counterintuitive, in dry conditions and climates, those antagonistic bacteria naturally fight off fungal pathogens, he says. Such is the case with a lentil-durum alternate-year rotation grown near Swift Current since 1979 — the level of soil fungal pathogens of lentils are actually kept in check because of antagonistic bacteria that thrive in a root zone that includes a lentil crop every second year.
In relation to this on-going study, Gan says results are just as clear in wetter conditions. Plots at Indian Head showed very dramatically that where pulse root rots are of a real concern, adding more pulses in rotation resulted in devastating levels of root rot in peas.
The very notion of sustainability as it relates to pulse rotations is finding that balance of the soil-feeding and flourishing aspects of adding one, two, or more pulses in a four- to six-year rotation, against the very real possibility of encouraging pathogens to multiply. Ultimately, what is sustainable — which includes economic considerations — will rest squarely on climatic conditions in conjunction with diverse rotations.
Gan’s research shows that pea yields benefit most from a diverse rotation that includes canola, cereals, and perhaps one other type of pulse, especially in those areas where higher moisture is expected.
And what about those beneficial antagonistic bacteria in a lentil/durum rotation? Could the future include a time when we inoculate pulses with nitrogen-fixing species plus antagonistic bacteria? Gan says that laboratory and greenhouse-based research has proved promising, and that field trials will be conducted in 2019 to test the most effective antagonistic bacteria they have identified. Understanding and identifying those soil microbes that can be either friend or foe is quite complex.