Abstract
Abstract. Predicting the effects of pyrogenic organic matter (OM) addition (either natural or intentional as in the case of biochar amendment) on soil chemistry and crop yields has been hampered by a lack of understanding of how pyrogenic OM evolves in the environment over time. This work compared the physicochemical characteristics of newly made and 15-month-field-aged biochars and biochar–soil mixtures. After aging, biochars made by pyrolysis of wood and grass at 250, 400 and 650 °C exhibited 5-fold increases in cation exchange capacity (CEC), on average; appearance of anion exchange capacity (AEC); and significant decreases in pH, ash content and nanopore surface area. Cross polarization 13C nuclear magnetic resonance (NMR) analyses indicated relative increases in O-containing functional groups, including substituted aryl, carboxyl and carbonyl C, and losses of O-alkyl groups. Similar chemical trends were observed for soil–biochar mixtures, suggesting the same biochar aging processes occurred in the soil environment. However, there was evidence for a role of soil OM–microbe–biochar interaction during aging. Field aging of soil with biochar resulted in large increases in C and N content (up to 124 and 143%, respectively) and exchange capacity (up to 43%) beyond that calculated by the weighted addition of the properties of biochar and soil aged separately. These beneficial interactive effects varied with soil and biochar type. Scanning electronic microscopy (SEM) images of biochar particles aged with soil showed colonization by microbes and widespread OM coatings. Thus, sorption of both microbially produced and soil OM are likely processes that enhanced biochar aging. Thus, biochar's full beneficial effects on soil properties likely increase over time, and proper assignment of C sequestration credits to biochar users will require consideration of soil–biochar interactions.
Highlights
Pyrogenic organic matter (OM), or black carbon, represents a large but poorly understood portion of global C cycling that may have played a role in climate changes, soil fertility and, pollutant fate and transport
As with freshly produced biochars (Mukherjee et al, 2011; Zimmerman, 2010), the physicochemical characteristics of aged biochar varied most clearly with highest treatment temperature (HTT). Both fresh and aged biochars exhibited significant increases in C, N and H content with HTT (Table 3 and Supplement Table S1). For both fresh and aged biochars, pH, ash content and surface area increased with HTT, while volatile matter (VM) decreased with HTT
This was true for C content that increased and cation exchange capacity (CEC) that decreased from grass to pine to oak (Table 3)
Summary
Pyrogenic organic matter (OM), or black carbon, represents a large but poorly understood portion of global C cycling that may have played a role in climate changes, soil fertility and, pollutant fate and transport. The characteristics of biochar that make it well suited to these purposes include its abundant and refractory OM (Glaser et al, 2001a, 2001b, 2002, 2004), high pH, high cation exchange capacity (CEC; Mukherjee et al, 2011), high surface area (SA) and high organic compound sorption affinity (Chen et al, 2012; Hale et al, 2011; Kasozi et al, 2010; Smernik, 2009) These characteristics have been shown to vary a great deal among parent biomass types and biochar production conditions (Mukherjee, 2011; Mukherjee et al, 2011). Literature reviews have concluded that crop yields with added biochar are highly variable
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