Abstract
Interest in sustainable alternatives to synthetic nitrogen (N) for switchgrass (Panicum virgatum L.) forage and bioenergy production, such as biological N2 fixation (BNF) via legume-intercropping, continues to increase. The objectives were to: (i) test physical and chemical scarification techniques (10 total) for common vetch (Vicia sativa L.); (ii) assess whether switchgrass yield is increased by BNF under optimum seed dormancy suppression methods; and (iii) determine BNF rates of common and hairy vetch (Vicia villosa L.) via the N-difference method. Results indicate that chemical scarification (sulfuric acid) and mechanical pretreatment (0.7 kg of pressure for one minute) improve common vetch germination by 60% and 50%, respectively, relative to controls. Under optimum scarification methods, BNF was 59.3 and 43.3 kg·N·ha−1 when seeded at 7 kg pure live seed ha−1 for common and hairy vetch, respectively. However, at this seeding rate, switchgrass yields were not affected by BNF (p > 0.05). Based on BNF rates and plant density estimates, seeding rates of 8 and 10 kg pure live seed (PLS) ha−1 for common and hairy vetch, respectively, would be required to obtain plant densities sufficient for BNF at the current recommended rate of 67 kg·N·ha−1 for switchgrass biomass production in the Southeastern U.S.
Highlights
Legumes are agronomically beneficial because they fix atmospheric nitrogen (N2 ) through a symbiotic relationship with Rhizobia bacteria, which form nodules in leguminous roots
The specific objectives of this study were to: (i) determine the efficacy of physical and chemical seed scarification for common vetch germination; (ii) determine whether or not switchgrass yields are increased by vetch intercrops; and (iii) determine N-fixation rates of common and hairy vetch via the N-difference method in switchgrass production systems
Alamo was planted in spring 2007 at 9 kg·ha−1 pure live seed (PLS) at three field sites, two at the East Tennessee Research and Education Center (ETREC): the Plant Sciences Unit
Summary
Legumes are agronomically beneficial because they fix atmospheric nitrogen (N2 ) through a symbiotic relationship with Rhizobia bacteria, which form nodules in leguminous roots. These beneficial bacteria enhance soil fertility by increasing N through rhizodeposition, which reduces the amount of synthetic N fertilizer needed for switchgrass growth [1]. Annual switchgrass yields average 15.9 Mg·ha−1 in Agronomy 2017, 7, 39; doi:10.3390/agronomy7020039 www.mdpi.com/journal/agronomy. Agronomy 2017, 7, 39 the upper Southeast [6], with only modest responses to greater N fertilization [7]. Switchgrass N fertilization is recommended at an annual rate of 67 kg·ha−1 [8], or approximately half the rate for corn (Zea mays L.) [9]
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