DOI: 10.5061/dryad.kwh70rz54
title: Wild legumes maintain beneficial soil rhizobia populations despite decades of nitrogen deposition

description: Natural landscapes are increasingly impacted by nitrogen enrichment from
 aquatic and airborne pollution sources. Nitrogen enrichment in the
 environment can eliminate the net benefits that plants gain from
 nitrogen-fixing microbes such as rhizobia, potentially altering
 host-mediated selection on nitrogen fixation. However, we know little
 about the long-term effects of nitrogen enrichment on this critical
 microbial service. Here, we sampled populations of the legume Acmispon
 strigosus and its associated soil microbial communities from sites
 spanning an anthropogenic nitrogen deposition gradient. We measured the
 net growth benefits plants obtained from their local soil microbial
 communities and quantified plant investment into nodules that house
 nitrogen-fixing rhizobia. We found that plant growth benefits from
 sympatric soil microbes did not vary in response to local soil nitrogen
 levels, and instead varied mainly among plant lines. Soil nitrogen levels
 positively predicted the number of nodules formed on sympatric plant
 hosts, although this was likely due to plant genotypic variation in nodule
 formation, rather than variation among soil microbial communities. The
 capacity of all the tested soil microbial communities to improve plant
 growth is consistent with plant populations imposing strong selection on
 rhizobial nitrogen fixation despite elevated soil nitrogen levels,
 suggesting that host control traits in A. strigosus are stable under
 long-term nutrient enrichment.
description: We sampled soil from six field
 sites along a nitrogen deposition gradient in California, USA and
 chemically analyzed each soil (WSI_soil_data.csv). We prepared slurries of
 each soil and used these to inoculate <em>Acmispon
 </em>seedlings in a greenhouse. To check for the presence of
 microbes in the soil slurries, we plated several dilutions of the soil
 slurries onto selective media and counted the colonies that formed
 (WSI_culture_data.csv). For our greenhouse experiment, half of the
 <em>Acmispon </em>plants were inoculated with live soil
 slurries (containing soil nutrients and microbes), and half were
 inoculated with sterilized soil slurries (containing soil nutrients only).
 We grew the inoculated plants for several weeks and then collected plant
 biomass data (WSI_plant_data.csv). We also examined the root nodules that
 formed on the plants inoculated with live soil slurries and scored the
 colors of these nodules, which can indicate nitrogen-fixing activity
 (WSI_nodcolor_data.csv).
description: We provide four datasets as csv files and one R code file that
 contains code for analyzing the data in these files. Metadata for all four
 datasets is located in the README file. The file contents are as
 follows: WSI_soil_data.csv contains soil
 chemical data from our analyses of bulk field soil and soil slurries. We
 also include nitrogen deposition data gleaned from other
 publications. WSI_nodcolor_data.csv contains nodule
 color scores for each plant in the experiment following the scoring guide
 in Figure S2. WSI_plant_data.csv contains biomass and
 harvest data for each plant in the experiment.
 WSI_culture_data.csv contains colony count data from the soil
 slurries (also called "filtrates") used to inoculate
 plants. Wendlandt_et_al_2022_Oecologia_code.R
 contains R code for importing, processing, and analyzing data from the
 above four datasets. It also contains code for producing figures from
 these data.