Legumes such as field peas have emerged as a viable option for farmers to diversify their cropping rotations. An added benefit of legumes in rotation is their ability to fix nitrogen through a symbiotic relationship with rhizobia bacteria. The nitrogen fixed by this interaction not only helps the legume crop, but also the subsequent crop as the fixed nitrogen is released to the soil as legumes decompose at the end of the season. This process allows farmers to be less reliant on expensive nitrogen fertilizers for both crops. However, as field peas continue to expand into the northern Plains, where dry, hot summers are common, it is evident that heat and drought stress are major limiting factors to both production and adoption by farmers. The problem of drought stress is two-fold in that it not only affects yield, but it also severely limits nitrogen fixation by rhizobia.
It is possible that native rhizobia, acclimated to the local climate and soils, are more effective at nitrogen fixation under stress conditions. This project intends to 1) examine the paired use of drought tolerant field peas and native rhizobia versus commercial rhizobia to maintain nitrogen fixation in field peas during drought stress, 2) identify the most effective inoculation methods for nitrogen fixation, and 3) create an open-source culture method that farmers can utilize on their own farms to be used as either a standalone inoculant or in combination with commercial inoculant.
1) Evaluate the use of native rhizobia to maintain nitrogen fixation in field peas during drought stress and improve crop performance
2) Identify the most drought tolerant FP varieties and the most effective inoculation methods for nitrogen fixation in field peas and ensure that farmers are employing these techniques
3) Educate farmers on the value of effective nitrogen fixation and the importance of legumes in diversified cropping rotations
4) Create an open-source culture method that farmers can utilize on their own farms to be used as either a standalone inoculant or in combination with commercial inoculants
Legumes such as field peas (FP) have emerged as a viable option for farmers to diversify rotations. In fact, SARE
has recently funded several projects examining the value of FP in intensified farming rotations, which replaces the
traditional wheat-fallow system. An added benefit of legumes is their ability to fix atmospheric nitrogen through
symbiotic relationships with ‘rhizobia’ bacteria. This fixed nitrogen helps the legume crop, and also the
subsequent crop as the nitrogen is released to the soil during plant decomposition. This process allows farmers to
be less reliant on expensive nitrogen fertilizers for both crops. To our knowledge, SARE has not funded similar
projects or projects related to rhizobia development.
However, as FP acreage expands across the northern Great Plains, where dry, hot summers are common, it is
evident that heat and drought stress are major limiting factors to production and adoption by farmers. The
problem of drought stress in FP is two-fold. It not only affects yield, but also limits nitrogen fixation by rhizobia.
Drought stress leads to a reduction in nodulation due to limited root hair growth which consequently reduces the
amount of N-fixation (Bordeleau, 1994). Studies have shown that even moderate drought conditions can lead to a
50% or greater reduction in N-fixation (Kirda, 1989). Consequently, farmers lose money on poor yields and lose
the soil health benefits of nitrogen fixation.
We currently conduct FP variety trials at five locations across the state. The past two growing seasons have been defined by drought and this project stems from our experiences with these variety trials and producer concerns.
There has been some concern that inoculants are failing under drought stress, hence the need for more drought
tolerance in both plant and inoculant. Moreover, during our field tours, farmers remarked that if drought is severe,
a modestly productive legume would still be beneficial as a soil health builder even when not producing a viable
yield. This would be preferable to other crops such as failed wheat or corn. In other words, where a harvest is not
viable due to drought, field peas are a preferable option to wheat due to the low C:N, which degrades rapidly and
provides a valuable green manure.
Native rhizobia populations arise in the soil by two means, A) from compatible strains nodulating wild legumes or
B) commercial strains remaining in the soil from previous cropping seasons (Vessey and Chemining’wa, 2006).
The failure of commercial inoculants under drought conditions is not surprising. It is well established that when
microorganisms are introduced into a new environment they often fail to survive because the environment is too
different from their native growing conditions or they cannot compete well with the native community and thus fail
to become established (Mendes, 2013). This is particularly true for rhizobia where it is recognized that inoculation
efficacy can be improved by using strains that were isolated from environments similar to where they will be used
(Lupwayi, 2006). In spite of this, currently, most commercial rhizobia inoculants are sold as ‘one size fits all.’ The
same rhizobia strains used in South Dakota are also used in South Carolina.
We envision an integrated approach that matches drought tolerant FP varieties with compatible, drought tolerant
rhizobia adapted to local growing conditions so both nitrogen fixation and yield can be maintained during drought
and heat stress. In order to improve rhizobia survival and activity, we will evaluate the interaction between
commercial and native rhizobia to identify suitable crop variety-rhizobial strain pairs for improved performance
under drought and heat stress. A combination of commercial and native rhizobia may work in concert.
Commercial rhizobia could maximize nitrogen fixation during optimal conditions while native rhizobia will
supplement nitrogen fixation during stress conditions. While the examination of native rhizobia is not a new
concept, evaluating native and commercial inoculants together is. Typically, research on native rhizobia focuses
on the value of the native rhizobia to obviate the necessity of applying commercial inoculants (e.g. Chemining’wa
and Vessey, 2006). But most studies fail to culture the native rhizobia and apply it at similar rates that a
commerical inoculant is applied at. Therefore, while the native population may be effective at nodulating the target
legume, it is not present in sufficient quantities to compete with a commercial inoculant. Moreover, there is little if
any information in the literature that examines how different rhizobial strains might work symbiotically under
varying abiotic stressors.
The hypotheses for this research are: 1) native rhizobia are better adapted to their local environment and can
maintain nitrogen fixation more effectively than commercial rhizobia under stress conditions when supplied in
adequate concentrations. And 2) specific legume variety-rhizobial strain combinations will perform differently due
to compatibility and competitiveness.
Native rhizobia cultured from drought-prone regions can outcompete commercial inoculants under drought conditions in field pea production.
Soil has been gathered from 24 unique sites across Montana, North Dakota and South Dakota. The sites represent different (never cultivated) biomes including high-altitude, mountainous regions, forest, undisturbed prairie and shrub-steppe grassland. Uninoculated field peas were grown in each of these soils under lab conditions and harvested after 8 weeks to determine colonization by native rhizobia. Nearly all soils produced nodules. Of these nodules, 92 rhizobia samples were successfully cultured. These samples will then be screened for drought tolerance and candidate rhizobia will be tested for further greenhouse and field studies in 2020.
To date, we do not have conclusive results. Currently, we have 92 rhizobia candidates. It is unclear how many of these samples are unique. Genetic sequencing will be utilized to determine how many species we actually have. Further, we have now identified 5 candidates that appear to offer some drought resistance. These samples, along with any others we find as we work through the full sample list, will be tested under drought conditions in controlled greenhouse studies and in the field in the coming year.
Educational & Outreach Activities
Field days and demonstrations were described in the previous section