Photooxidation of pyrogenic organic matter reduces its reactive, labile C pool and the apparent soil oxidative microbial enzyme response

Abstract

The surface chemistry of pyrogenic organic matter (PyOM) is altered by a variety of abiotic and biotic oxidative and sorption/desorption processes in the environment. Exposure of PyOM to high energy light prior to addition to soil or sediment, or while entrained in the atmosphere, may induce significant surface photooxidation, i.e., photochemical weathering, altering its environmental reactivity. We report on a 30-day soil incubation experiment testing the effects of the photochemical weathering of a 13 C-enriched ponderosa pine PyOM, produced by pyrolysis at 450°C, on PyOM and soil organic carbon (SOC) mineralization. PyOM C mineralization was measured for both the photochemically weathered (i.e. UV treated PyOM or PyOMUV) and PyOM not exposed to high-energy light (i.e. PyOMW serving as a dark control). PyOMW exhibited a 3.7 times faster C mineralization rate across the 30-d study, which was driven by a large early mineralization of accessible/labile C during the first 6 days. In contrast, PyOMUV had faster C mineralization rates in the later part of the experiment (days 11-30). Overall, PyOMUV had a 13% lower net C mineralization than the untreated PyOMW where the MRT of accessible PyOMUV-C and PyOMW-C was calculated at 25.7 ± 6.8 d and 1.7 ± 0.2 days, respectively. Both forms of PyOM promoted a similar net reduction in native SOC mineralization (i.e., negative priming) of approximately 50% relative to the unamended, control soil. Addition of either PyOM form resulted in an equivalent minor decrease in the concentration of extractable soil lignin phenols and substituted fatty acids chemistry with respect to the unamended soil. At 30 d, soil phenol oxidase and peroxidase enzyme activities were higher with additions of either form of PyOM compared with the unamended control soils with PyOMUV exhibiting lower activities than PyOMW. Our results indicate that PyOM photochemical weathering can impart important changes to short-term PyOM reactivity and soil microbial 3 activity, which could have important implications for soil systems by ultimately lowering turnover rates for both NSC and PyOM-C.

Interacting Controls of Pyrolysis Temperature and Plant Taxa on the Degradability of PyOM in Fire-Prone Northern Temperate Forest Soil

“…This is in keeping with our previous study that showed that artificially aged PyOM, which was thought to have significant amounts of leachable C, induced the enhanced oxidative enzyme response from white-rot fungi in a single culture while not being degraded itself [22]. We attributed the lack of decomposition in Reference [22] to the absence of a soil microbial consortia (present here in this study), which may benefit from the oxidative enzyme response of fungi can could lead to the further solubilization of PyOM for direct metabolism by bacterial groups [17][18][19]23].…”

“…It is assumed that the first-order control on PyOM reactivity in soil is determined by the pyrolysis of the initial plant material, whereby PyOM becomes progressively condensed and aromatized with increasing pyrolysis temperature [5,13,14]. While PyOM may persist for long periods of time in soil due to its acquired physicochemistry and the promotion of stabilization through soil aggregation [15,16], it is susceptible to both abiotic and biotic decomposition processes, which are influenced by the soil microbial community composition [17][18][19], soil edaphic properties [8], and exposure to chemical and photo oxidants [20][21][22][23]. Direct microbial degradation has been extensively documented.…”

“…Direct microbial degradation has been extensively documented. Decay studies of PyOM have demonstrated that the complex consortia of bacteria and fungi present in soil are capable of PyOM degradation through the release of oxidative and hydrolytic enzymes [19,[23][24][25]. However, microbial degradation varies based upon source plant taxa, temperature of production, and the proportion, chemistry, and availability of unaltered biomass within PyOM [26][27][28].…”

Charcoal Increases Microbial Activity in Eastern Sierra Nevada Forest Soils

“…This priming could be due to charcoal providing a refuge for microbes, thereby allowing them to avoid predation by larger soil fauna [14]. Previous studies have shown mixed effects of charcoal on priming and noted that conflicting reports regarding effects of priming may be due to soil types used, length of the incubation, and pre-incubation conditions, but no specifics were given how these factors may have affected priming [31]. Whether or not priming was important in our study could have been addressed by using isotopically labeled charcoal and measuring the isotopic signature of the respired CO 2 .…”

Chemical Characteristics of Macroscopic Pyrogenic Carbon Following Millennial-Scale Environmental Exposure

“…The ultimate state would be a neardominance of SPAC, potentially followed by complete chemical breakdown and loss of the PyC, if the SPAC itself is eventually exposed to degradation in either particularly aggressive chemical environments, or by exposure to chemical attack (e.g., oxidation) over very extended time periods (Ascough et al, 2011a). This may not be directly linear, as the rate of alteration should be a function of the intensity of degradation processes, both abiotic (e.g., photooxidation, Wang et al, 2017), and biotic (Wang et al, 2016;Woo et al, 2016). These clearly could vary through time if environmental conditions (e.g., soil pH, moisture regime, etc.)…”