Arbuscular mycorrhizal fungi along secondary forest succession at the eastern periphery of Amazonia: Seasonal variability and impacts of soil fertility

Abstract

As anthropic transformation of Amazonian rainforests into degraded lands continues, the consequences of this on arbuscular mycorrhizal fungi (AMF) remain in the dark. This paper describes changes in AMF (glomerospore abundances, species composition and diversity and glomalin contents) along secondary forest succession, and we explore the impacts of seasonality and of soil texture and chemistry. Research was conducted in a shifting cultivation region at the eastern periphery of Amazonia, in ‘young’ (3–4 yrs old – SEC1) and ‘mid-aged’ (6-8yrs old –SEC2) degraded secondary forest regrowth, and in mature (>120yrs old – SEC3) rainforest, both in dry (November 2015) and in rainy (May 2016) seasons. We identify a total of 36 AMF species, corresponding to 24.1% of Braziĺs and 12.5% of the worldwide known AMF species richness. The genera Glomus and Acaulospora predominated in both seasons and over all successional stages, in terms of species richness and of relative abundances and frequencies. Though AMF species diversity did differ significantly between young / mid-aged degraded secondary forests and >120 yrs-old mature rainforests, differences were altogether small and – contrary to vegetation species diversity – higher in stages SEC2 + 3 than in SEC1. Neither easily-extractable nor total glomalin contents differed significantly between seasons or successional stages, though they did contribute substantially (overall 2.5% average) to total soil organic carbon. BCA suggests that AMF species composition was driven mainly by seasonality (15.9%) and only marginally (though likewise significantly) by succession (8.0%). A subset of soil variables (pH, OM, Al, CEC, Ca) related strongly (r2 = 0.46) with glomerospore abundance and AMF species composition. Our results support the view of a remarkably resilient limited set of AMF species with only subtle differences between young (degraded) secondary regrowth and mature rainforests. Thus, the successional trajectories of vegetation recovery after repeated shifting cultivation cycles are likely not limited by AMF species availability.