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Abstract Increasing fine root carbon (FRC) inputs into soils has been proposed as a solution to increasing soil organic carbon (SOC). However, FRC inputs can also enhance SOC loss through priming. Here, we tested the broad-scale relationships between SOC and FRC at 43 sites across the US National Ecological Observatory Network. We found that SOC and FRC stocks were positively related with an across-ecosystem slope of 7 ± 3 kg SOC m⁻²per kg FRC m⁻², but this relationship was driven by grasslands. Grasslands had double the across-ecosystem slope while forest FRC and SOC were unrelated. Furthermore, deep grassland soils primarily showed net SOC accrual relative to FRC input. Conversely, forests had high variability in whether FRC inputs were related to net SOC priming or accrual. We conclude that while FRC increases could lead to increased SOC in grasslands, especially at depth, the FRC-SOC relationship remains difficult to characterize in forests. 

Abstract Wildfires have the potential to dramatically alter the carbon (C) storage potential, ecological function, and the fundamental mechanisms that control the C balance of Pacific Northwest (PNW) forested ecosystems. In this study, we explored how wildfire influences processes that control soil C stabilization and the consequent soil C persistence, and the role of previous fire history in determining soil C fire response dynamics. We collected mineral soils at four depth increments from burned (low, moderate, and high soil burn severity classes) and unburned areas and surveyed coarse woody debris (CWD) in sites within the footprint of the 2020 Holiday Farm Fire and in surrounding Willamette National Forest and the H.J. Andrews Experimental Forest. We found few changes in overall soil C pools as a function of fire severity; we instead found that unburned sites contained high levels of pyrogenic C (PyC) that were commensurate with PyC concentrations in the high severity burn sites—pointing to the high background rate of fire in these ecosystems. An analysis of historical fire events lends additional support, where increasing fire count is loosely correlated with increasing PyC concentration. An unexpected finding was that PyC concentration was lower in low soil burn severity sites than in control sites, which we attribute to fundamental ecological differences in regions that repeatedly burn at high severity compared with those that burn at low severity. Our CWD analysis showed that high mean fire return interval (decades between fire events) was strongly correlated with low annual CWD accumulation rate; whereas areas that burn frequently had a high annual CWD accumulation rate. Within the first year postfire, trends in soil density fractions demonstrated no significant response to fire for the mineral-associated organic matter pool but slight increases in the particulate pool with increasing soil burn severity—likely a function of increased charcoal additions. Overall, our results suggest that these PNW forest soils display complex responses to wildfire with feedbacks between CWD pools that provide varying fuel loads and a mosaic fire regime across the landscape. Microclimate and historic fire events are likely important determinants of soil C persistence in these systems. 

Abstract Wildfire is a disturbance expected to increase in frequency and severity, changes that may impact carbon (C) dynamics in the soil ecosystem. Fire changes the types of C sources available to soil microbes, increasing pyrogenic C and coarse downed wood, and if there is substantial tree mortality, decreasing C from root exudates and leaf litter. To investigate the impact of this shift in the composition of C resources on microbial processes driving C cycling, we examined microbial activity in soil sampled from an Oregon burn 1 year after fire from sites spanning a range in soil burn severity from unburned to highly burned. We found evidence that postfire rhizosphere priming loss may reduce soil C loss after fire. We measured the potential activity of C‐acquiring and nitrogen (N)‐acquiring extracellular enzymes and contextualized the microbial resource demand using measurements of mineralizable C and N. Subsurface mineralizable C and N were unaltered by fire and negatively correlated with hydrolytic extracellular enzyme activity (EEA) in unburned, but not burned sites. EEA was lower in burned sites by up to 46%, but only at depths below 5 cm, and with greater decreases in sites with high soil burn severity. These results are consistent with a subsurface mechanism driven by tree mortality. We infer that in sites with high tree mortality, subsurface EEAs decreased due to loss of rhizosphere priming and that inputs of dead roots contributed to mineralizable C stabilization. 

Forest ecosystems are important global soil carbon (C) reservoirs, but their capacity to sequester C is susceptible to climate change factors that alter the quantity and quality of C inputs. To better understand forest soil C responses to altered C inputs, we integrated three molecular composition published data sets of soil organic matter (SOM) and soil microbial communities for mineral soils after 20 years of detrital input and removal treatments in two deciduous forests: Bousson Forest (BF), Harvard Forest (HF), and a coniferous forest: H.J. Andrews Forest (HJA). Soil C turnover times were estimated from radiocarbon measurements and compared with the molecular‐level data (based on nuclear magnetic resonance and specific analysis of plant‐ and microbial‐derived compounds) to better understand how ecosystem properties control soil C biogeochemistry and dynamics. Doubled aboveground litter additions did not increase soil C for any of the forests studied likely due to long‐term soil priming. The degree of SOM decomposition was higher for bacteria‐dominated sites with higher nitrogen (N) availability while lower for the N‐poor coniferous forest. Litter exclusions significantly decreased soil C, increased SOM decomposition state, and led to the adaptation of the microbial communities to changes in available substrates. Finally, although aboveground litter determined soil C dynamics and its molecular composition in the coniferous forest (HJA), belowground litter appeared to be more influential in broadleaf deciduous forests (BH and HF). This synthesis demonstrates that inherent ecosystem properties regulate how soil C dynamics change with litter manipulations at the molecular‐level. Across the forests studied, 20 years of litter additions did not enhance soil C content, whereas litter reductions negatively impacted soil C concentrations. These results indicate that soil C biogeochemistry at these temperate forests is highly sensitive to changes in litter deposition, which are a product of environmental change drivers. 

Protecting existing soil carbon (C) and harnessing the C sequestration potential of soils require an improved understanding of the processes through which soil organic matter accumulates in natural systems. Currently, competing hypotheses exist regarding the dominant mechanisms for soil C stabilization. Many long-standing hypotheses revolve around an assumed positive relationship between the quantity of organic inputs and soil C accumulation, while more recent hypotheses have shifted attention toward the complex controls of microbial processing and organo-mineral complexation. Here, we present the observed findings of soil response to 20 years of detrital manipulations in the wet, temperate forest of the H.J. Andrews Experimental Station. Annual additions of low-quality (high C:N content) wood litter to the soil surface led to a greater positive effect on observed mean soil C concentration relative to additions of higher-quality (low C:N content) needle litter over the 20-year study period. However, high variability in measurements of soil C led to a statistically non-significant difference in C concentration between the two treatments and the control soil. The observed soil C responses to these two addition treatments demonstrates the long timescale and potential magnitude of soil C responses to management or disturbance led changes in forest litter input composition. Detrital input reduction treatments, including cutting off live root activity and the aboveground removal of surface litter, led to relatively small, non-significant effects on soil C concentrations over the 20-year study period. Far greater negative effects on mean soil C concentrations were observed for the combined removal of both aboveground litter and belowground root activity, which led to an observed, yet also non-significant, 20% decline in soil C stocks. The substantial proportion of remaining soil C following these dramatic, long-term reductions in above- and belowground detrital inputs suggests that losses of C in these forest soils are not readily achieved over a few decades of reductions in detrital input and may require far greater periods of time or further perturbations to the environment. Further, the observed soil C responses to detrital manipulations support recent hypotheses regarding soil C stabilization, which emphasize litter quality and mineral stabilization as relevant controls over forest soil C.