Biological nitrogen fixation (BNF) is a key ecosystem service rendered by legumes, but quantitative information is lacking regarding BNF in the Texas Rolling Plains (TRP), an easterly portion of U.S. Southern Great Plains where cool-season legumes can typically be grown as cash or cover crops in dryland conditions. Using three diverse cropping systems studies (Site 1, long-term cotton [Gossypium hirsutum L.]cover crop systems; Site 2, short-term cotton-cover crop systems; and Site 3, transitional organic wheat [Triticum aestivum L.] system), our research objective was to quantify and compare BNF and associated parameters among four cool-season legumes (Austrian winter pea [Pisum sativum subsp. Arvense], hairy vetch [Vicia villosa], crimson clover [Trifolium incarnatum], and lentil [Lens culinaris]) in different settings. At Sites 1 and 2, winter pea had growth of 2.73 to 3.44 Mg ha −1 , the greatest percent N derived from the atmosphere (%Ndfa; 57.2-72.6%) and, depending on site, the greatest level of BNF (67-74 kg N ha −1 ). Results from the organic system (Site 3) demonstrated winter pea was also productive in growth and BNF when intercropped with wheat in N-poor soil, with exceptionally high %Ndfa (92.7%). Hairy vetch and lentil differed in growth and %Ndfa, but had comparable BNF (46-51 kg N ha −1 ) at Sites 1 and 2. Growth of crimson clover was poor. This data provides insights on how cool-season legumes differ in BNF in the TRP in a year when legume growth was average for the region. The TRP has a volatile climate, however, indicating further study is needed to fully understand the BNF benefit cool-season legumes can provide in the region. INTRODUCTIONCool-season legumes, grown as cash and cover crops, are a valuable component of cropping systems. Legumes can contribute to long-term accumulation of soil organic matter, Abbreviations: BNF, biological nitrogen fixation; %Ndfa, percent nitrogen derived from the atmosphere; TRP, Texas Rolling Plains.

Guar [Cyamopsis tetragonoloba (L.) Taub.], also known as clusterbean, is a crop plant grown in semi-arid regions worldwide for the galactomannan gum of its seed, and as a forage, vegetable, and green manure. Despite the importance of guar and its products, uniform growth stage descriptions have not been established for the crop. Such descriptions allow for improved documentation and communication of the growth and phenological development of crops by researchers, producers, and others. Following models of other warm-season, indeterminant-growth legumes, such a system was developed for guar based on visually observable and sequential vegetative (V) and reproductive (R) plant events. The system was evaluated in two locations in dryland and irrigated conditions, using three morphologically contrasting guar varieties. The V stages begin with emergence (VE) and then are determined by counting the nodes on the main stem of the plant, with the cotyledonary node as zero [V0 to V(N)]. The R stages include R1 (First Flower), R2 (First Pod), R3 (First Seed or Full Pod), R4 (Full Seed), R5 (First Maturity), R6 (50% Maturity), and R7 (Harvest Maturity). The V and R stages may be reported alone or concurrently and applied at plant or crop scales. The stage descriptions apply to all growth morphologies of guar (i.e., branched and non-branched) and accommodate the indeterminant growth habit of the plant. Season-long data is reported from field studies on crop V and R stage progression, as well as data on crop growth, including biomass component partitioning, productivity, and canopy development.

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Guar (Cyamopsis tetragonoloba [L.] Taub.) is a leguminous crop plant produced in semiarid regions worldwide, primarily for industrial uses of guar gum. Little research has been conducted on critical belowground traits of the plant, particularly on spatial and temporal development of the root system, including nodules. Providing such analysis was the objective of this research. We observed root and nodule character traits at regular time intervals in 130-cm-tall soil columns sectioned by depth in 2 years. In this tap-rooted plant, root weight was greatest at the soil surface (0-15 cm) and decreased with depth.Root length density (RLD) from 0 to 15 cm and 15 to 30 cm soil depths was similar. The RLD and root weight were generally minimal in deeper soil layers, although deeper roots seem to play an important role in water uptake for guar, especially in drought conditions.Compared to most crops, guar has fine rooting and a small root-to-shoot ratio. At all soil depths analysed, ≥86% of root length was comprised of roots <1 mm in diameter, although most were very fine roots <0.5 mm. The root-to-shoot ratio ranged from 0.055 to 0.17 g g −1 over the growing seasons. Significant accumulation of nodule weight occurred by 60 days, as did the first observations of nodule senescence, but nodule growth continued after this time. Most nodules and nodule weight were concentrated near the soil surface and the average weight per nodule decreased with depth, although smaller nodules were present at all depths. Because guar is typically grown in dryland conditions in hot semi-arid regions, nodules near the soil surface may often be susceptible to shedding or loss of function due to dehydration and/or heat stress and the plant may rely on deeper nodules for N fixation. The basic biological information provided in this report is needed to hasten progress in guar genetic improvement and management.