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Pulse crops, including lentil (Lens culinaris Medik.) and pea (Pisum sativum L.), can improve the sustainability of Northern Great Plains cropping systems, largely through biological N fixation. Greater N fixation amounts can help producers to increase yield while decreasing N fertilizer inputs for the following crop. There may be potential to breed greater N‐fixing pulse varieties, yet little is known about varietal differences in N fixation. Nitrogen fixation of pea and lentil varieties was quantified at two sites in Montana from 2019 to 2021 using an N difference approach and the 15N natural abundance method. Riveland and CDC Richlea were frequently high N‐fixing lentil varieties, both fixing ca. 130 kg N ha−1 in the site‐year with the most favorable growing conditions. No pea variety had consistently greater N fixation than others, despite N fixation ranging from 88 to 135 kg N ha−1 in one site‐year among varieties. Nitrogen fixation by lentil had an inverse relationship with days to flowering but was not correlated with days to maturity. Nitrogen fixation by pea was positively correlated with days to maturity but was not correlated with days to flowering. Breeding lentil and pea for high N fixation by selecting high N‐fixing varieties is likely difficult, as varieties performed differently under variable environmental conditions. Breeding efforts based on traits, such as days to flowering, could be more successful. There were more positive correlations between N fixation parameters and seed yield for pea than for lentil, suggesting that breeding for greater yields could increase N fixation for pea but not lentil. Core Ideas: Variation in environmental conditions strongly influenced N fixation by lentil and pea varieties.Differences in N fixation among varieties were up to 21 kg N ha−1 for lentil and 47 kg N ha−1 for pea.Two lentil varieties often fixed more N than others, while no pea varieties performed consistently well.Breeding lentil for shorter days to flower could increase N fixation, suggested by an inverse relationship.Breeding pea for greater yield or longer days to maturity could increase N fixation, suggested by positive relationships. [ABSTRACT FROM AUTHOR]

Cover crops during summer fallow have been rarely researched in the semiarid northern Great Plains. This study was conducted during 2012–2019 at four Montana locations and included four functional groups (Brassica family, fibrous‐rooted crops, legumes, and tap‐rooted crops). Eleven treatments included sole functional groups, a Full Mix, the Full Mix minus each functional group, pea, and chemical fallow. Wheat (Triticum aestivum L.) was grown after each cover crop year with three nitrogen (N) fertilizer rates. Cover crops were terminated with herbicide at first flower stage of pea (Pisum sativum L.) 57 to 66 days after planting. Shoot biomass averaged 2.0 Mg ha−1 over eight site‐years representative of dryland farming in Montana. Using equal overall plant densities, treatments with six species averaged 13% greater biomass than two species. Measured at termination to a 0.9‐m depth, Fallow held greater soil water than cover crop treatments, with Fallow averaging 57 mm greater than the Full Mix. Soil nitrate averaged 49 kg N ha−1 greater after Fallow than the Full Mix; the Legume treatment averaged 26 kg N ha−1 greater than the Minus Legume treatment. Wheat yield on Fallow averaged 0.85 Mg ha−1 greater than the Full Mix in 5 of 10 site‐years, mainly at the driest site‐years. The Legume treatment elevated wheat protein over the Minus Legume treatment by an average of 15 g kg−1. Cover crops grown during summer fallow reduced soil nitrate‐N, soil water, and wheat yields compared with chemical fallow, especially in the major wheat growing region of north central Montana. Core Ideas: Soil improvement via cover crop growth is an uncertain trade‐off with soil water and nitrogen use.May–June rainfall best predicted cover crop biomass (Y = 757 + 12.4X; Adj. R2 = 0.72; p < 0.01).Cover crop treatments that included six crop species averaged 13% greater biomass than those with only two species.An eight‐species cover crop mix averaged 57 mm less soil water and 49 kg ha−1 less soil N compared with fallow.Wheat yield on fallow averaged 0.85 Mg ha−1 (22%) greater than cover crops for 5 of 10 site‐years. [ABSTRACT FROM AUTHOR]

The use of killed cover crop mulch for weed suppression, soil erosion prevention and many other soil and crop benefits has been demonstrated in organic no-till or zero-till farming systems in eastern US regions and in Canada. Implements have been developed to make this system possible by terminating cover crops mechanically with little, if any, soil disturbance. Ongoing research in the US northern Great Plains is being conducted to identify cover crop species and termination methods for use in organic zero-till (OZ) systems that are adapted to the crop rotations and climate of this semi-arid region. Current termination strategies must be improved so that cover crop species are killed consistently and early enough in the growing season so that subsequent cash crops can be grown and harvested successfully. Delaying termination until advanced growth stages improves killing efficacy of cover crops and may provide weed-suppressive mulch for the remainder of the growing season, allowing no-till spring seeding of cash crops during the next growing season. Excessive water use by cover crops, inability of legume cover crops to supply adequate amounts of N for subsequent cash crops and failure of cover crops to suppress perennial weeds are additional obstacles that must be overcome before the use of killed cover crop mulch can be promoted as a weed control alternative to tillage in the US northern Great Plains. Use of vegetative mulch produced by killed cover crops will not be a panacea for the weed control challenges faced by organic growers, but rather one tool along with crop rotation, novel grazing strategies, the judicious use of high-residue cultivation equipment, such as the blade plow, and the use of approved herbicides with systemic activity in some instances, to provide organic farmers with new opportunities to incorporate OZ practices into their cropping systems. Emerging crop rotation designs for organic no-till systems may provide for more efficient use of nutrient and water resources, opportunities for livestock grazing before, during or after cash crop phases and improved integrated weed management strategies on organic farms. [ABSTRACT FROM AUTHOR]

We conducted this study to quantify long-term cropping related changes in soil organic carbon (SOC) stocks and characterize the optical properties of dissolved organic matter (DOM) after a decadal on-farm experiment in Montana, USA. Soil samples (0–50 cm) were collected from minimum till (MT) and no-till (NT) fields under fallow-winter wheat (Triticum aestivum L.; F-W) and pea-winter wheat (Pisum sativum L.; P-W) rotations. Stocks of SOC (0–50 cm) averaged 65.6 Mg C ha−1 and 60.6 Mg C ha−1 for P-W and F-W, respectively. The net SOC accretion rate for P-W equated to 0.61 Mg ha−1 yr−1 relative to F-W. We used absorbance spectroscopy and excitation-emission matrices to characterize DOM composition of samples collected from MT F-W and NT P-W. The two cropping systems exhibited similar estimates of aromaticity (absorbance at 254 nm; 0.33–0.39 a.u.) and humification index (1.83–1.86). Parallel factor (PARAFAC) analysis revealed humic-like (C1, C2), monolignol/amino acid-like (C3), and amino acid-/tannin-like (C4) components with equivalent fluorescent intensities among MT F-W and NT P-W. Fluorescence efficiencies increased with depth, suggesting a shift from larger, plant-like material to smaller, microbial-derived precursors. Overall, we found DOM composition to be minimally affected by cropping systems in this semiarid climate of the northern Great Plains. • Annual cropping sequestered more SOC than fallow-based systems over a 10 year period. • The accretion of SOC within annual cropping occurred at a rate of 0.61 Mg ha−1 yr−1. • Soil DOM bulk optical properties did not respond to changes in decadal SOC stocks. • PARAFAC analysis identified four different fluorescent components of the EEMs. • Sub-soil DOM was comprised of low molecular size constituents. [ABSTRACT FROM AUTHOR]