Carbon and nitrogen cycling are resistant to fire in nutrient-poor grassland

We used a long-term experiment with four different fire frequencies, annual burns (B1), biennial burns (B2), burn every 4 yr (B4), and no burn (BC) over a 27-yr period. We quantified temporal changes in vegetation dynamics, aboveground and belowground carbon (C) and nitrogen (N) pools, and we examined the cumulative effects of fires on N cycling in a nutrient-poor, old field grassland at Cedar Creek, MN. Compared with fires in fertile grasslands with high productivity, fire in this nutrient-poor and low-productivity old field grassland caused only minor shifts in plant functional groups and did not change net primary productivity (NPP) or N cycling rates. We also found that fire frequency did not affect ecosystem C pools or N pools, and the soil C was accumulating at 28 times of the rate of N accumulation for the period 2000 to 2010. This N accumulation in the soil, combined with the low-productivity and the dominant C4 grasses, which have relative low litter N concentration and thereby low fire-induced N losses, makes this successional grassland resistant to fire-induced N cycling and C and N pool changes for at least decades after agricultural abandonment.