Abiotic and Biotic Limitations to Nodulation by Leguminous Cover Crops in South Texas

Abstract

Many farms use leguminous cover crops as a nutrient management strategy to reduce their need for nitrogen fertilizer. When they are effective, leguminous cover crops are a valuable tool for sustainable nutrient management. However, the symbiotic partnership between legumes and nitrogen fixing rhizobia is vulnerable to several abiotic and biotic stressors that reduce nitrogen fixation efficiency in real world contexts. Sometimes, despite inoculation with rhizobial strains, this symbiosis fails to form. Such failure was observed in a 14-acre winter cover crop trial in the Rio Grande Valley (RGV) of Texas when three legume species produced no signs of nodulation or nitrogen fixation. This study examined the role of nitrogen, phosphorus, moisture, micronutrients, and native microbial communities in the nodulation of cowpea (Vigna unguiculata L. Walp) and assessed arbuscular mycorrhizal fungi as an intervention to improve nodulation. Results from two controlled studies confirm moisture and native microbial communities as major factors in nodulation success. Micronutrients showed mixed impacts on nodulation depending on plant stress conditions. Nitrogen and phosphorus deficiencies, however, were not likely causes, nor was mycorrhizal inoculation an effective intervention to improve nodulation. Inoculation method also had a major impact on nodulation rates. Continued research on improved inoculation practices and other ways to maximize nitrogen fixation efficiency will be required to increase successful on-farm implementation.

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

“…We must not forget that competitiveness of a Rhizobium strain and how efficiently it fixes nitrogen is determined by its genetics and physiology, in interaction with the legume host genotype ( Laguerre et al, 2003 ; Burghardt et al, 2018 ; Boivin et al, 2020 ) and the inoculation context, determined by soil influence ( Batista et al, 2015 ) and climatic conditions ( Frey and Blum, 1994 ; Vuong et al, 2017 ). The population of rhizobia in different soils is heterogeneous and varies quantitatively and qualitatively, responding to different abiotic and biotic factors ( Graham, 2008 ; Kasper et al, 2019 ). A given elite strain may thrive in one climate or soil type but fail in a different environment to which it is poorly adapted.…”

“…These factors should be taken into account when evaluating the success of beneficial microbes ( Sessitsch et al, 2002 ; Busby et al, 2017 ; Onishchuk et al, 2017 ; diCenzo et al, 2019 ). Nodule formation, and therefore rhizobial competitiveness, is affected by soil type and its physicochemical properties (i.e., temperature, pH, and moisture; Hungria and Franco, 1993 ; Frey and Blum, 1994 ; Anyango et al, 1995 ; Montañez et al, 1995 ; Zahran, 1999 ; Rao et al, 2002 ; Rathi et al, 2018 ), nutrient availability and the ability of microbes to use them ( Rynne et al, 1994 ; Kyei-Boahen et al, 2017 ; Kasper et al, 2019 ), the population of native rhizobia and the remaining soil microbiome ( Meade et al, 1985 ; Siefert et al, 2018 ; Han et al, 2020 ), previous inoculation history ( Laguerre et al, 2003 ; Batista et al, 2015 ), and/or the choice of inoculation method ( Danso and Bowen, 1989 ; López-García et al, 2009 ). The degree to which the rhizobial strains adapt to the local soil conditions will strongly influence the competition between strains.…”

Red harvester ant (Order: Hymenoptera) preference for cover crop seeds in South Texas

“…(MacMahon, 2000;Taber 1999). In addition to the use of cover crops, LRGV farmers may inoculate cover crop seeds with nitrogen-fixing bacteria to facilitate root nodulation to further benefit the soil (Rai et al, 2021;Kasper et al, 2019;Kasper 2019). As such treatments may influence ant foraging decisions, the second objective was to determine if seed inoculation treatments used for increased germination rates would alter the previously established cover crop seed preferences.…”

Rhizobial adhesives enhance nodule formation in sunn hemp

“…Occasionally, this interaction between rhizobia and plant root parts fails to form nodules due to several environmental conditions such as soil temperature, soil moisture, salinity, nutrient deficiency, soil pH (Kasper et al, 2019) and competition with native rhizobia (Miller and May, 1991). Additionally, inoculating with the right strain in high numbers that can outcompete the native-naturalized rhizobia under field conditions thus preventing their impact on the inoculated strain is important (Brockwell et al, 1995; Brockwell et al, 1987; Irisarri et al, 2019; Nambiar et al, 1987; Smith et al, 1981; Somasegaran et al, 1988).…”

“…Sunn hemp is drought tolerant and has potential to build organic matter levels and sequester soil carbon (Sheahan, 2012). It is known to form symbiotic associations with Bradyrhizobium japonicum and fixes nitrogen in the root nodules (Kasper et al, 2019). In addition, sunn hemp is reported to be resistant to root knot nematode (McSorley, 1999) and can be a strong host of mycorrhizal fungi (Soti et al, 2016).…”