Long‐term agricultural management does not alter the evolution of a soybean–rhizobium mutualism

Abstract

Leguminous crops, like soybeans, often rely on biologically fixed nitrogen via their symbiosis with rhizobia rather than synthetic nitrogen inputs. However, agricultural management practices may influence the effectiveness of biological nitrogen fixation (BNF). While the ecological effects of agricultural management on rhizobia have received some attention, the evolutionary effects have been neglected in comparison. Resource mutualism theory predicts that evolutionary effects are likely, however. Both fertilization and tillage are predicted to cause the evolution of rhizobia that provide fewer growth benefits to plant hosts and fix less nitrogen. This study capitalized on a Long-Term Ecological Research experiment that manipulated agricultural management practices in a corn-soybean-wheat row crop system for 24 yr to investigate whether four different management practices (conventional, no-till, low chemical input, and certified organic) cause rhizobia populations to evolve to become more or less cooperative. We found little evidence that 24 yr of varying management practices affect the net growth benefits rhizobia provide to soybeans, although soybean plants inoculated with soils collected from conventional treatments tended to have lower BNF rates than plants inoculated with soils from the no-till, low input, and organic management treatments. These findings suggest that rhizobia will continue to provide adequate growth benefits to leguminous crops in the future, even in intensively managed systems.

How do less‐expensive nitrogen alternatives affect legume sanctions on rhizobia?

“…However, these models assume plants can consistently sanction even moderately less‐beneficial strains when soil N is available, which have not been widely reported. In field studies, while long‐term addition of nitrogen may decrease the relative abundance of beneficial rhizobial genotypes, perhaps by decreasing legume abundance (Weese, Heath, Dentinger, & Lau, 2015), Schmidt, Weese, and Lau (2017) found little effect of crop management on rhizobial mutualism, suggesting that elevated levels of soil nitrogen does not select for poor‐fixing rhizobia. However, crop management treatments alter multiple parameters, such as soil structure and organic content, that could confound the effects of fertilizer on rhizobial evolution.…”

Novel plant–microbe interactions: Rapid evolution of a legume–rhizobium mutualism in restored prairies

“…Growing in the presence of soy hosts could have selected for strains that are less beneficial for alternate hosts like Chamaecrista, as different legume species have been shown to be compatible with the same rhizobia but vary in the benefits they derive from them (Pahua et al, 2018). And while the Lux site now has lower soil nitrogen availability, prior fertilizer inputs could have led to the evolution of less beneficial mutualists that might still persist today (Corkidi, Rowland, Johnson, & Allen, 2002, Johnson, 1993, Weese et al, 2015, but see Schmidt, Weese, & Lau, 2017).…”

Light availability and rhizobium variation interactively mediate the outcomes of legume–rhizobium symbiosis

“…Typically researchers have studied how rhizobia vary in terms of their effects on whole-plant traits (Burdon et al, 1999;Heath, 2010;Barrett et al, 2012;Porter and Simms, 2014) or the instantaneous rate of N fixation via acetylene reduction assays (McNeil, 1982;Minchin et al, 1983;Tan and Tan, 1986). Ecosystem ecologists and ecophysiologists have long used isotope abundances (natural or enriched) to study biological N fixation in the field (Shearer and Kohl, 1986;Mead and Preston, 2011;Yelenik et al, 2013;Craine et al, 2015) or greenhouse (Menge et al, 2015;Taylor and Menge, 2018), but mutualism research increasingly features the use of isotope abundance (natural or enriched) to study the trade of benefits in resource mutualisms (Ruess et al, 2013;Regus et al, 2017;Schmidt et al, 2017;Taylor and Menge, 2018). Ruess et al (2013) estimated nodule respiration, N fixation, and Frankia strain identity in a field survey of Alnus tenuifolia and found that Frankia strains vary in terms of both N fixation and respiratory cost.…”