Research

Forest disturbance effects on soil nutrient turnover and fluxes

Natural and manmade disturbances such as pests, storms, and timber removal result in altered patterns of litter and root inputs to soil carbon. These inputs fuel the microorganisms that break down carbon for storage, release it to the atmosphere as carbon dioxide, and release nutrients for plant growth. Carbon inputs also serve a structural role in soil aggregate formation and water retention. We are curious how forest disturbances, particularly storm-related increases in deadwood and debris, alter soil nutrient cycling from both structural and microbial processes.

Oxygen limitations from hill to valley

Water limits the diffusion of oxygen 10,000 times compared to air. Thus, wet soils such as floodplains – up to 20% of the global sink of terrestrial organic carbon – are typically considered places of carbon storage. However, these wet and high-carbon soils are also big emitters of potent greenhouse gases nitrous oxide and methane and host carbon loss pathways via fermentation and metal-induced decomposition. On the flip side, “upland” unsaturated soils are known to host anaerobic microbes and processes despite a constant presence of oxygen at bulk scales. We use this natural gradient in moisture and carbon to test for unexpected greenhouse gas production based on the categorization of soils as wet or dry – i.e., oxygen-depleted or oxygen-rich. This work will inform current gaps in our understanding of soil carbon fluxes.

2. Environmental controls on decomposition

Metal-catalyzed decomposition

While water-saturated soils are great preservers of carbon, even minute exposure to oxygen can set off a cascade of reactions capable of decomposing organic matter through both microbial and abiotic (metal-mediated) processes. We study the contribution of microorganisms such as fungi and bacteria to organic matter degradation versus redox-active metals such as Fe(II) and Mn(III) along different wetland to dryland continuums. We also consider grass and tree dominated systems and soils that differ in abundance of decomposers, Fe, and Mn. While metal-based oxidation may contribute to 50% of carbon dioxide production in arctic streams, these processes have not yet been assessed in temperate floodplain or wetland systems. This work aims to establish which decomposition processes are relevant where and to highlight the biogeochemical features needed to accurately represent wetland carbon cycling.

Inherent controls of soil organic carbon on decomposition

Soil organic carbon (SOC) has long been thought to accrue in soil due to “recalcitrance”. This operational property of SOC was thought to arise predominantly through humification – a process of joint decomposition and reconfiguration into chemically stable, irregular organic structures that resisted enzymatic decomposition. The concept of humus has since lost traction in favor of environmental limitations on decomposition. However, the chemistry of organic carbon theoretically still affects its likelihood for decomposition. Certain organic linkages, such as those found in lignin or woody debris, are predominantly broken by enzymes or compounds requiring oxygen. Thus, these compounds are expected to accumulate in water-logged, oxygen-limited environments. Another intrinsic control on SOC decomposition is its oxidation state: just like sulfur or nitrogen or metals cycle through oxidation states, carbon in organic compounds ranges in oxidation state from -4 to +4. The more positive, the more energy this carbon provides to a microbe respiring it. We test whether these “energetic” and “enzymatic” limitations on SOC decomposition play an important role in determining the long-term storage potential of carbon in different soils.

3. Measuring redox variability in soils and sediments

Redox status determines the fate of nutrients and contaminants in soils. Oxygen presence or absence controls decomposition rate by orders of magnitude and determines whether nutrients and contaminants are locked away on soil minerals or released into the environment. However, we struggle to visualize redox variability at the relevant scale of microns to centimeters and have relied on meter to kilometer estimates that do not capture crucial environmental processes. We are working with colleagues at SLAC and elsewhere to highlight this disconnect, develop processes to measure and describe redox heterogeneity in soils, and ultimately use these metrics to better understand ecosystem-level elemental cycling.