Century-scale wood nitrogen isotope trajectories from an oak savanna with variable fire frequencies

Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Individual fires often increase short-term nitrogen (N) availability to plants, but repeated burning causes ecosystem N losses and can ultimately decrease soil organic matter and N availability. However, these effects remain poorly understood due to limited long-term biogeochemical data. Here, we evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (delta N-15) across space and time at one of the longest running prescribed burn experiments in the world (established in 1964). We developed multiple delta N-15 records across a burn frequency gradient from precisely dated Quercus macrocarpa tree rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied with respect to the temporal onset of burning and burn frequency but were consistent in overstory species representation, soil characteristics, and topography. Burn frequency ranged from an unburned control stand to a high-fire-frequency stand that had burned in 4 of every 5 years during the past 55 years. Because N stocks and net N mineralization rates are currently lowest in frequently burned stands, we hypothesized that wood delta N-15 trajectories would decline through time in all burned stands, but at a rate proportional to the fire frequency. We found that wood delta N-15 records within each stand were remarkably coherent in their mean state and trend through time. A gradual decline in wood delta N-15 occurred in the mid-20th century in the no-, low-, and medium-fire stands, whereas there was no trend in the high-fire stand. The decline in the three stands did not systematically coincide with the onset of prescribed burning. Thus, we found limited evidence for variation in wood delta N-15 that could be attributed directly to long-term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood delta N-15 results may instead reflect decadal-scale changes in vegetation composition and abundance due to early- to mid-20th-century fire suppression.