Free-living soil diazotrophs provide more nitrogen to pastures after land use conversion from the Amazon rainforest

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Rachel E Danielson, Land, Air, and Water Resources, University of California Davis, DAVIS, CA, Kyle M. Meyer, Department of Integrative Biology, University of California Berkeley, Berkeley, CA, Jordan M. Sayre and Jonathan Y. Lin, Microbiology, University of California, Davis, Davis, CA, Klaus Nusslein, Microbiology, University of Massachusetts, Amherst, Amherst, MA, Brendan J.M. Bohannan, Institute of Ecology and Evolution, University of Oregon, Eugene, OR, Jorge L. Mazza Rodrigues, Land, Air, and Water Resources, University of California Davis, Davis, CA

Background/Question/Methods The Amazon Rainforest is a global diversity hotspot and crucial carbon sink, but approximately 20% of its total extent has been deforested- primarily for the establishment of cattle pasture. Understanding the impact of this large-scale disturbance on soil microbial community composition and activity is crucial to understand consequential shifts in nutrient or greenhouse gas cycling, as well as adding to the body of knowledge concerning how these complex communities respond to human disturbance. In this study, we sought to determine the impact of rainforest conversion on microbial communities involved in asymbiotic nitrogen fixation. Surface soils (0-10cm) from three rainforests and adjacent pastures each were collected in Rondônia, Brazil. Soil DNA and RNA were extracted and amplified targeting the asymbiotic nitrogen fixation community, incubations were performed to determined nitrogen fixation rates, and relevant soil chemical parameters were measured.
Results/Conclusions Measuring both the natural abundance of 15N in total soil N, as well as incorporation of enriched 15N2 under incubation has revealed that conversion of primary forest to cattle pasture results in a significant 47x increase in the rate of nitrogen fixation by free-living diazotrophs. Quantitative PCR of a gene encoding the nitrogenase reductase enzyme correspondingly reveals a 23x increase of genes in pasture compared to forest soils, and shows significant correlation with asymbiotic fixation rates (⍴ = 0.66). Additionally, genetic sequencing of both nifH genes and transcripts shows a significant increase in the diversity of present (3.31x) and transcriptionally-active (2.12x) diazotrophs within pasture soil communities. Principal components of DNA-based community ordination constrained by land use type also correlated strongly with community abundance (⍴ = 0.79). With respect to soil chemical measurements, levels of both organic and inorganic nitrogen tended to be lower in pastures compared to forests, with ammonium accounting for 85% of inorganic N as oppose to 4% in forest soils, which were dominated by nitrate. However no significant or consistent differences in total (27.7g kg-1 in forests, 23.4g kg-1 in pasture), extractable, permanganate-oxidizable, or loss-on-ignition carbon are present between the two land use types. Molybdenum, a key micronutrient for nitrogen fixers, was below detection limit (<75mg kg-1) in pasture soils and 1.09mg kg-1 in forest soils. This study indicates a significant change in nitrogen cycling and diazotroph community composition with the conversion of Amazon Rainforest. This may have important implications for the sustainability of cattle pastures once established, since nitrogen is a crucial nutrient for forage grass productivity.