Switchgrass (Panicum virgatum) is a perennial prairie grass native to North America and a candidate crop for cellulosic bioenergy. Its yield is non-responsive to nitrogen (N) fertilizer addition, partly because it is able to obtain N via associative N fixation. Many details of switchgrass N fixation remain unknown, including how fixation rates respond to seasonality, soil N availability, and soil type. To determine how fixation responds to these variables, we measured N fixation rates, using 15N2 incubations, in switchgrass soils and roots that were grown across three N fertilization rates (0, 56, and 196 kg N ha-1 yr-1) and two soil types: fertile mollisols at Arlington Agricultural Research Station (ARL), WI and moderately fertile alfisols at Kellogg Biological Station (KBS), MI. To determine seasonal changes, we sampled during four key time periods: pre-fertilizer addition, post-fertilizer addition, plant peak biomass, and post-senescence. In addition, to assess seasonal changes in other potential N sources, we measured soil potential net mineralization and nitrification at the same sites and times via 28-day aerobic incubations.
N fertilization decreased soil N fixation rates at KBS, where rates decreased from 0.9 μg N g soil-1 d-1 prior to fertilizer addition to undetectable post-fertilizer addition. Fixation rates in unfertilized plots were consistent through time, ranging from 0.5-1.1 μg N g soil-1 d-1 throughout the growing season. At ARL, response to fertilizer addition was less evident; rates ranged from 0.6-1.25 μg N g soil-1 d-1 and exhibited no seasonal or fertilizer treatment differences. Root fixation rates were more temporally dynamic than the soil rates. At KBS, they were below detection, except at peak plant biomass, when the plots fertilized at 0 or 56 kg N ha-1 yr-1 fixed N at 1.25 μg N g root-1 d-1. At ARL, root N fixation declined with fertilizer addition, ranging from 0.9 μg N g soil-1 d-1 in the unfertilized plots undetectable in plots receiving 196 kg N ha-1 yr-1. Net mineralization and nitrification increased with N fertilization rate at KBS and ARL, except in the pre-fertilizer addition time period, when rates were consistent across treatments. In general, fixation rates appeared to decrease with soil N availability, and increase with plant N demand, whereas mineralization and nitrification appeared to increase with N availability.