Annual burning of a tallgrass prairie inhibits C and N cycling in soil, increasing recalcitrant pyrogenic organic matter storage while reducing N availability

Abstract

Grassland ecosystems store an estimated 30% of the world's total soil C and are frequently disturbed by wildfires or fire management. Aboveground litter decomposition is one of the main processes that form soil organic matter (SOM). However, during a fire biomass is removed or partially combusted and litter inputs to the soil are substituted with inputs of pyrogenic organic matter (py-OM). Py-OM accounts for a more recalcitrant plant input to SOM than fresh litter, and the historical frequency of burning may alter C and N retention of both fresh litter and py-OM inputs to the soil. We compared the fate of these two forms of plant material by incubating (13) C- and (15) N-labeled Andropogon gerardii litter and py-OM at both an annually burned and an infrequently burned tallgrass prairie site for 11 months. We traced litter and py-OM C and N into uncomplexed and organo-mineral SOM fractions and CO2 fluxes and determined how fire history affects the fate of these two forms of aboveground biomass. Evidence from CO2 fluxes and SOM C:N ratios indicates that the litter was microbially transformed during decomposition while, besides an initial labile fraction, py-OM added to SOM largely untransformed by soil microbes. Additionally, at the N-limited annually burned site, litter N was tightly conserved. Together, these results demonstrate how, although py-OM may contribute to C and N sequestration in the soil due to its resistance to microbial degradation, a long history of annual removal of fresh litter and input of py-OM infers N limitation due to the inhibition of microbial decomposition of aboveground plant inputs to the soil. These results provide new insight into how fire may impact plant inputs to the soil, and the effects of py-OM on SOM formation and ecosystem C and N cycling.

Fire affects root decomposition, soil food web structure, and carbon flow in tallgrass prairie

“…These results were in agreement with the observed higher aboveground litter respiration in the AB as compared to the IB site (Soong and Cotrufo, 2015). Yet, in a root decomposition study by Reed et al (2009) there were no significant main effects of burning on root decomposition; however, low precipitation may have masked the effects of burning on decomposition for that study.…”

“…Faster decomposition in annually burned prairie soil could be due to the indirect effects of burning on the soil community composition or to the direct effects on soil conditions (i.e., heat, moisture, nutrients), which would impact decomposition processes (O'Lear et al, 1996). For example, relative to unburned tallgrass prairie soils, the soil conditions of frequently burned areas are often N-limited (Blair, 1997;Ojima et al, 1994), causing microbes to scavenge for N before beginning decomposition (Soong and Cotrufo, 2015;Craine et al, 2007). N mining by microbes in N-limited areas has been shown to increase decomposition rates in other areas (Craine et al, 2007).…”

“…Frequent fires can have large effects on plant productivity, plant community composition, and root properties (Kitchen et al, 2009;Knapp et al, 1998), which can significantly alter ecosystem processes such as litter decomposition and C cycling (Ojima et al, 1994;Johnson and Matchett, 2001;Soong and Cotrufo, 2015). Litter decomposition is an important component of belowground C cycling and root litter C provides a major energy source for soil biota (Eisenhauer and Reich, 2012).…”

“…The 20-year burn treatment had a soil pH of 6.1. For specific soil characterization data for these sites including % C, % N, pyrogenic organic C content and bulk density, see Soong and Cotrufo (2015). Soil from the annual spring burn treatment area will be referred to as annually burned (AB) and the 20-year burn as infrequently burned (IB) for the remainder of this paper.…”

Stability of grassland soil C and N pools despite 25 years of an extreme climatic and disturbance regime

“…In this grassland, although the majority of C in senesced aboveground plant biomass is released to the atmosphere during fire events,~4% is left behind as pyrogenic C, which has a long turnover time in the soil [Knicker et al, 2012;Soong and Cotrufo, 2014], thus limiting C losses. Additionally, increased soil respiration in irrigated plots [Knapp et al, 1998a] indicates increased microbial activity in these soils.…”