Low-intensity frequent fires in coniferous forests transform soil organic matter in ways that may offset ecosystem carbon losses.

Funder: Sequoia Parks ConservancyFunder: Gordon and Betty Moore Foundation; Id: http://dx.doi.org/10.13039/100000936The impact of shifting disturbance regimes on soil carbon (C) storage is a key uncertainty in global change research. Wildfires in coniferous forests are becoming more frequent in many regions, potentially causing large C emissions. Repeated low-intensity prescribed fires can mitigate wildfire severity, but repeated combustion may decrease soil C unless compensatory responses stabilize soil organic matter. Here, we tested how 30 years of decadal prescribed burning affected C and nitrogen (N) in plants, detritus, and soils in coniferous forests in the Sierra Nevada mountains, USA. Tree basal area and litter stocks were resilient to fire, but fire reduced forest floor C by 77% (-36.4 Mg C/ha). In mineral soils, fire reduced C that was free from minerals by 41% (-4.4 Mg C/ha) but not C associated with minerals, and only in depths ≤ 5 cm. Fire also transformed the properties of remaining mineral soil organic matter by increasing the proportion of C in a pyrogenic form (from 3.2% to 7.5%) and associated with minerals (from 46% to 58%), suggesting the remaining soil C is more resistant to decomposition. Laboratory assays illustrated that fire reduced microbial CO2 respiration rates by 55% and the activity of eight extracellular enzymes that degrade cellulosic and aromatic compounds by 40-66%. Lower decomposition was correlated with lower inorganic N (-49%), especially ammonium, suggesting N availability is coupled with decomposition. The relative increase in forms of soil organic matter that are resistant to decay or stabilized onto mineral surfaces, and the associated decline in decomposition suggest that low-intensity fires may promote mineral soil C storage in pools with long mean residence times in coniferous forests

The Long-Term soil productivity study after three decades

In 1989, the Long-Term Soil Productivity (LTSP) concept, and eventual study design, began with a conversation about the needs of forest managers in the USDA Forest Service to meet the requirements of the National Forest Management Act (NFMA) of 1976. This Act mandated that the productive capacity of forests be maintained on federally managed lands while also maintaining soil productivity. Soon after initiating the LTSP study within the USDA Forest Service, it was expanded to include partnerships with national and international researchers and managers to establish and maintain more than 100 LTSP and affiliated sites across North America (Fig. 1), as well as at several other international locations, including New Zealand, China, and Chile.This article is published as Page-Dumroese, Deborah S., Dave M. Morris, Miranda T. Curzon, and Jeffery A. Hatten. "The Long-Term soil productivity study after three decades." Forest Ecology and Management 497 (2021): 119531. doi: https://doi.org/10.1016/j.foreco.2021.119531.Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted

Fuel moisture influences on fire-altered carbon in masticated fuels: An experimental study

Biomass burning is a significant contributor to atmospheric carbon emissions but may also provide an avenue in which fire-affected ecosystems can accumulate carbon over time, through the generation of highly resistant fire-altered carbon. Identifying how fuel moisture, and subsequent changes in the fire behavior, relates to the production of fire-altered carbon is important in determining how persistent charred residues are following a fire within specific fuel types. Additionally, understanding how mastication (mechanical forest thinning) and fire convert biomass to black carbon is essential for understanding how this management technique, employed in many fire-prone forest types, may influence stand-level black carbon in soils. In this experimental study, 15 masticated fuel beds, conditioned to three fuel moisture ranges, were burned, and production rates of pyrogenic carbon and soot-based black carbon were evaluated. Pyrogenic carbon was determined through elemental analysis of the post-fire residues, and soot-based black carbon was quantified with thermochemical methods. Pyrogenic carbon production rates ranged from 7.23% to 8.67% relative to pre-fire organic carbon content. Black carbon production rates averaged 0.02% in the 4-8% fuel moisture group and 0.05% in the 13-18% moisture group. A comparison of the ratio of black carbon to pyrogenic carbon indicates that burning with fuels ranging from 13% to 15% moisture content resulted in a higher proportion of black carbon produced, suggesting that the precursors to black carbon were indiscriminately consumed at lower fuel moistures. This research highlights the importance of fuel moisture and its role in dictating both the quantity and quality of the carbon produced in masticated fuel beds. Citation: Brewer, N. W., A. M. S. Smith, J. A. Hatten, P. E. Higuera, A. T. Hudak, R. D. Ottmar and W. T. Tinkham (2013), Fuel moisture influences on fire-altered carbon in masticated fuels: An experimental study, J. Geophys. Res. Biogeosci., 118, 30-40, doi:10.1029/2012JG002079.This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/jgr/.Keywords: Severity, Molecular structure, Storage, Soil organic matter, Scots pine forests, Black carbon, Woody debris, Pyrogenic carbon, Burning efficiency, Charcoal formatio

Soil pore network response to freeze-thaw cycles in permafrost aggregates

Climate change in Arctic landscapes may increase freeze–thaw frequency within the active layer as well as newly thawed permafrost. Freeze-thaw is a highly disruptive process that can deform soil pores and alter the architecture of the soil pore network with varied impacts to water transport and retention, redox conditions, and microbial activity. Our objective was to investigate how freeze–thaw cycles impacted the pore network of newly thawed permafrost aggregates to improve understanding of what type of transformations can be expected from warming Arctic landscapes. We measured the impact of freeze–thaw on pore morphology, pore throat diameter distribution, and pore connectivity with X-ray computed tomography (XCT) using six permafrost aggregates with sizes of 2.5 cm3 from a mineral soil horizon (Bw; 28–50 cm depths) in Toolik, Alaska. Freeze-thaw cycles were performed using a laboratory incubation consisting of five freeze–thaw cycles (−10 °C to 20 °C) over five weeks. Our findings indicated decreasing spatial connectivity of the pore network across all aggregates with higher frequencies of singly connected pores following freeze–thaw. Water-filled pores that were connected to the pore network decreased in volume while the overall connected pore volumetric fraction was not affected. Shifts in the pore throat diameter distribution were mostly observed in pore throats ranges of 100 µm or less with no corresponding changes to the pore shape factor of pore throats. Responses of the pore network to freeze–thaw varied by aggregate, suggesting that initial pore morphology may play a role in driving freeze–thaw response. Our research suggests that freeze–thaw alters the microenvironment of permafrost aggregates during the incipient stage of deformation following permafrost thaw, impacting soil properties and function in Arctic landscapes undergoing transition