My dissertation is entitled When is 2+2≠4? Interactive priming of pyrogenic organic matter, soil organic carbon, and plant roots in natural and managed ecosystems. I defended in May 2014, and officially received my Ph.D. in July 2014.

Investigating mechanisms for negative priming of soil organic carbon by pyrogenic carbon using NanoSIMS and LA-AMS (published in incubation study paper in Environmental Science and Technology): Many studies have seen that, over longer timescales, black C additions to soil decrease soil C decomposition. One possible mechanism for this is that the soil C is stabilized on the black C, either directly or mediated through interactions with soil minerals. Our proposal to the Environmental and Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Lab was  accepted, so I travelled there in January 2014 to use nanoscale secondary ion mass spectrometry (nanoSIMS) to see whether we find evidence for these effects in the final soil samples from my incubation trial. I am very excited to be collaborating with the staff at the EMSL!

Interactive priming of black C, soil organic C, and plant root C: A field study (completed, in prep): The overarching objective of my Ph.D. research is to investigate BC-SOC priming effects in systems with plants present, with a focus on the mechanisms of priming. In my greenhouse study with BC and corn plants as treatments, I found that negative priming may occur over short timescales in this system. In the incubation trial, I investigated how the relative lability of SOC and BC drive these priming effects. This project built on my previous findings, scaling my research questions beyond the lab and greenhouse, to the field. In addition, using high-throughput 16S sequencing, I determined that BC additions to soil significantly alter the soil bacterial community, although not as much as the addition of fresh biomass, and am in the process of delving deeper into these data.

When does 2 + 2 ≠ 4? Testing mechanisms for microbially-mediated priming of soil carbon by black carbon DBC (published in Environmental Science and Technology): We aim to determine whether the relative amounts of labile SOC and dissolved BC (DBC) can be used to predict the magnitude and direction of “priming” in a soil system. This study will investigate potential mechanisms for short-term priming in a 5-week incubation study. The BC is produced from ¹³C-labelled maple twigs, which will facilitate distinguishing the source of soil CO₂ emissions using an isotopic partitioning approach.

Carbon isotopes, two ways: Using ¹³C to distinguish soil organic carbon-derived soil CO₂ emissions from root respiration under black carbon treatments (published in Soil Biology and Biochemistry): Black carbon (BC) is thought to be a very stable form of carbon. However, its interactions with non-BC soil organic carbon (SOC) remain poorly understood, with studies showing both positive and negative “priming effects” on SOC decomposition under BC applications. This 12-week greenhouse pot trial was designed to investigate BC-SOC priming effects and the identification of an optimal ¹³C proxy for BC emissions using stable isotope partitioning. Significant BC losses occurred in the first week. We find that there may be evidence for a “negative priming” effect of BC on SOC in the system (SOC losses are lower than predicted) during week 1, but this effect disappears over time, indicating it may be due to transient effects driven by labile BC. In addition, I have identified that the ∂¹³C signatures of sub-components of BC differ significantly, showing the importance of choosing isotopic proxies for BC respiration carefully.