Increased burn frequencies may be detrimental to soils & productivity in savanna grasslands

TPOS notes: A recent paper in the journal Nature made headlines for suggesting that an increase in the frequency of burning is detrimental to soils and plant productivity. The meta-analysis titled, "Fire frequency drives decadal changes in soil carbon and nitrogen and ecosystem productivity" (available online Dec. 11), analyzed soils data from 48 long-term research sites from around the globe, representing savanna grasslands, broadleaf forests, needleleaf forests, and boreal forests.

The study primarily reports the comparison of sites with elevated fire frequencies (average fire frequency 4 times greater than the historic fire frequency) compared to sites protected from fire. Analysis of intermediate sites (at which fire frequencies were closer to mimicking the historic fire frequency) suggests that the changes associated with elevated burn frequencies were "... attributable both to C and N accumulation during fire protection, and to C and N loss during increased burning."

Media coverage included, "More frequent fires reduce soil carbon and fertility, slowing the regrowth of plants" at phys.org.

Abstract:

Fire frequency is changing globally and is projected to affect the global carbon cycle and climate. However, uncertainty about how ecosystems respond to decadal changes in fire frequency makes it difficult to predict the effects of altered fire regimes on the carbon cycle; for instance, we do not fully understand the long-term effects of fire on soil carbon and nutrient storage, or whether fire-driven nutrient losses limit plant productivity. Here we analyse data from 48 sites in savanna grasslands, broadleaf forests and needleleaf forests spanning up to 65 years, during which time the frequency of fires was altered at each site. We find that frequently burned plots experienced a decline in surface soil carbon and nitrogen that was non-saturating through time, having 36 per cent (±13 per cent) less carbon and 38 per cent (±16 per cent) less nitrogen after 64 years than plots that were protected from fire. Fire-driven carbon and nitrogen losses were substantial in savanna grasslands and broadleaf forests, but not in temperate and boreal needleleaf forests. We also observe comparable soil carbon and nitrogen losses in an independent field dataset and in dynamic model simulations of global vegetation. The model study predicts that the long-term losses of soil nitrogen that result from more frequent burning may in turn decrease the carbon that is sequestered by net primary productivity by about 20 per cent of the total carbon that is emitted from burning biomass over the same period. Furthermore, we estimate that the effects of changes in fire frequency on ecosystem carbon storage may be 30 per cent too low if they do not include multidecadal changes in soil carbon, especially in drier savanna grasslands. Future changes in fire frequency may shift ecosystem carbon storage by changing soil carbon pools and nitrogen limitations on plant growth, altering the carbon sink capacity of frequently burning savanna grasslands and broadleaf forests.

Citation:

Pellegrini, A. F., Ahlström, A., Hobbie, S. E., Reich, P. B., Nieradzik, L. P., Staver, A. C., ... & Jackson, R. B. (2017). Fire frequency drives decadal changes in soil carbon and nitrogen and ecosystem productivity. Nature.

doi:10.1038/nature24668

Corresponding author: Adam F. A. Pellegrini (afapelle "at" stanford.edu)