DISCUSSION
Concurrent developments in evolutionary theory and molecular methodology over the past two decades have revealed a great deal of evolutionary dynamism in RNS, including: evolution of cheating in microbes and countermeasures in host plants (Denison, 2000; Denison et al ., 2003; Denison & Kiers 2004, 2011; Kiers et al ., 2003, 2006; Sachs et al ., 2004; Simms & Taylor, 2002; Simms et al ., 2006; West et al ., 2002), context-dependence in the mutualistic outcome of symbiosis (Halloway et al ., 2022; Heath & Tiffin, 2007), influence of quasi-economic forces on the evolution of bacterial behavior (Hammerstein & Noe, 2016; Noe & Hammerstein, 1995; Werneret al ., 2014), and evolutionary switches in microbial partners between parasitic, mutualistic, and free-living lifestyles (Sachset al ., 2010, 2014; Sachs & Simms, 2006). Recent controlled inoculation studies with alder species have demonstrated the ability to select specific genotypes of Frankia from soil, and also suggest differential abilities among bacterial genotypes to convert selection by hosts into increased representation in soil assemblages (Ben Tekayaet al ., 2018; Vemulapally et al . 2022a,b). To date, however, no field studies that we know of have investigated concurrent changes in soil and nodule assemblages across environmental gradients at local spatial scales in any naturally occurring RNS. Our study examined variation in soil-dwelling Frankia in locations previously characterized with respect to nodule-dwelling Frankia across three ecologically-important sources of variation: successional change across a primary chronosequence, changes in nutrient availability known to trigger host autoregulation mechanisms, and presence of a specific host species. We found significant effects of all three ecological drivers, and also discovered several clades of apparently non-symbioticFrankia .