This file explains the variables in the dataset “Roley_etal_2018_Nfix_minnit_SG.csv” that accompanies the publication: Roley SS, Duncan DM, Liang D, Garoutte A, Jackson RD, Tiedje JM, Robertson, GP. 2018. Associative nitrogen fixation (ANF) in switchgrass (Panicum virgatum) across a nitrogen input gradient. PLOS ONE. In the descriptions below, “Labeled” refers to samples that were incubated with 15N2, while “Control” refers to samples incubated with unenriched N2. Dataset: Roley_etal_2018_Nfix_minnit_SG.csv site = the location of experiment, either MI (Kellogg Biological Station, Hickory Corners, MI) or WI (Arlington Agricultural Research Station, Arlington, WI) N_addn = the amount of fertilizer nitrogen added to each plot annually, in kg N/ha/yr replicate = field replicate time = the growth stage of switchgrass at sampling; PRE (prior to fertilizer addition), POST (2 weeks post-fertilizer addition), PEAK (peak biomass, at switchgrass flowering), and SEN (post-senescence) sample.date = the date the field samples were taken, in MM/DD/YYYY format t_d.C = the incubation time, in days, for Control samples atm_15N_at_pct.C = the 15N atom percent of the atmosphere for Control samples DW_g.C = the dry weight, in g, of the Control samples t_d.L = the incubation time, in days, for Labeled samples atm_15N_at_pct.L = the 15N atom percent of the atmosphere for the Labeled samples DW_g.L = the dry weight, in g, of the Labeled samples AEatm = the 15N atom percent excess of the headspace of the Labeled samples Material = the sample material; this data set includes soil only d15N.C = the delta 15N value of the Control soils, expressed per mil 15N_at.pct.C = the 15N atom percent of the Control soils pctN.C = the percent of the Control soils that is nitrogen d15N.L = the delta 15N value of the Labeled soils 15N_at.pct.L = the 15N atom percent of the Labeled soils pctN.L = the percent of the Labeled soils that is nitrogen AEsoil = the 15N atom percent excess of the Labeled soils, calculated as 15N_at.pct.L - 15N_at.pct.C pct_Ndfa = the percent of N in the soil from fixation TN_g = the total nitrogen in the sample, in g fix_rate_gN_d = fixation rate, expressed as g N/vial/d fix_rate_gN_gsoil_d = fixation rate, expressed as gN/g soil/d fix_rate_ugN_gsoil_d = fixation rate, expressed as ugN/g soil/d experimental_treatment =Treatment names according to plot map, found at: https://lter.kbs.msu.edu/maps/images/current-switchgrass-nitrogen-experiment.pdf initial_date = the first day of the mineralization/nitrification incubation year = the year the samples were taken harvest = the harvest history of the experimental plot; only one harvest per year (H1) plots were used final_date = the last day of the mineralization/nitrification incubation days = the length of the mineralization/nitrification incubation, in days soil_grav_moist = the soil gravimetric moisture, in g H20/g dry soil soil_water_ml = the volume of water in the soil used in incubation, in milliliters V_ml = the total volume of liquid (1M KCl + water), in milliliters soil_dw_g = the dry weight of the soil used in the mineralization/nitrification incubations, in grams initial_ppm.NH4 = the ammonium concentration on the first day of the incubation, expressed as mg N/L final_ppm.NH4 = the ammonium concentration on the last day of the incubation, expressed as mg N/L initial_gsoil.NH4 = the ammonium concentration on the first day of the incubation, expressed as ug N/g soil final_gsoil.NH4 = the ammonium concentration on the last day of the incubation, expressed as ug N/g soil initial_ppm.NO3 = the nitrate concentration on the first day of the incubation, expressed as mg N/L final_ppm.NO3 = the nitrate concentration on the last day of the incubation, expressed as mg N/L initial_gsoil.NO3 = the nitrate concentration on the first day of the incubation, expressed as ug N/g soil final_gsoil.NO3 = the nitrate concentration on the last day of the incubation, expressed as ug N/g soil length = the length of the soil core, in cm netmin = net mineralization rate, expressed as ug N/g soil/d netnit = net nitrification rate, expressed as ug N/g soil/d pct_nit = percent of the total mineralization that was nitrified, calculated as 100 x netnit/netmin inorg_N_gsoil = inorganic N, expressed as ug N/g soil inorg_N_ppm = inorganic N, expressed as mg N/L BD_g_cm3 = the mean bulk density of previous soil coring efforts, expressed as g/cm3 areal_min_g_m2_d = net mineralization rate, expressed as g N/m2/d areal_nit_g_m2_d = net nitrification rate, expressed as g N/m2/d