Impact of six lignocellulosic biochars on C and N dynamics of two contrasting soils

Abstract

Both soil and biochar properties are known to influence greenhouse gas emissions from biochar-amended soils, but poor understanding of underlying mechanisms challenges prediction and modeling. Here, we examine the effect of six lignocellulosic biochars produced from the pyrolysis of corn stover and wood feedstocks on CO 2 and N 2 O emissions from soils collected from two bioenergy cropping systems. Effects of biochar on total accumulated CO 2 -C emissions were minimal (<0.45 mg C g À1 soil; <10% of biochar C), consistent with mineralization and hydrolysis of small labile organic and inorganic C fractions in the studied biochars. Comparisons of soil CO 2 emissions with emissions from microbially inoculated quartz-biochar mixtures ('quartz controls') provide evidence of soil and biochar-specific negative priming. Five of six biochar amendments suppressed N 2 O emissions from at least one soil, and the magnitude of N 2 O emissions suppression varied with respect to both biochar and soil types. Biochar amendments consistently decreased final soil NO 3 À concentrations, while contrasting effects on pH, NH 4 + , and DOC highlighted the potential for formation of anaerobic microsites in biochar-amended soils and consequential shifts in the soil redox environment. Thus, results implicated both reduced substrate availability and redox shifts as potential factors contributing to N 2 O emission suppression. More research is needed to confirm these mechanisms, but overall our results suggest that soil biochar amendments commonly reduce N 2 O emissions and have little effect on CO 2 emissions beyond the mineralization and/ or hydrolysis of labile biochar C fractions. Considering the large C credit for the biochar C, we conclude that biochar amendments can reduce greenhouse gas emissions and enhance the climate change mitigation potential of bioenergy cropping systems.

Impact of Biochar Organic and Inorganic Carbon on Soil CO2and N2O Emissions

“…Nonadditive effects of biochar C pool impacts on CO 2 production may additionally arise from priming of native SOC and/or added corn stover C. As evidenced by the large discrepancy between OC added (0.007–0.010 mg C g −1 soil) and the CO 2 –C produced in excess of the controls (0.31–0.35 mg C g −1 soil), the bicarbonate extracts likely caused a persistent positive priming effect. By contrast, previous research suggested that the untreated biochars (CfBu and WgBu) did not significantly prime the SOC of the Exira soil studied here (Fidel et al, 2017a). Such priming interactions between native SOC and labile biochar OC are likely to depend in part on native SOC properties in addition to biochar OC properties and interactions among biochar OC pools (Cross and Sohi, 2011; Wang et al, 2016; Whitman et al, 2013, 2014).…”

“…The nonadditive relationships among CO 2 produced by soil treated with biochar extracts and washed biochars observed here suggest that soluble biochar LOC and IC pools can be stabilized by chemical and/or physical interaction with the recalcitrant biochar matrix. The untreated biochars examined here have previously been shown to not have significant interactions with native SOC and/or corn stover OC (Fidel et al, 2017a); however, this may not be the case in soils with different SOC pool sizes and compositions (or soils receiving different residues). Therefore, special attention should be given to biochar OC × native SOC interactions that may occur in other contexts involving various biochars, soils, and fertilizers.…”

“…The prepared samples were first incubated for 60 d at 20°C without fertilizer, during which time CO 2 and N 2 O production rates were measured on Days 0, 1, 4, 7, 13, 21, 32, 42, and 48, and the soil was gradually allowed to dry to −1/3 bar matric potential (equivalent to 29–31% gravimetric moisture content) by periodically opening the vials (Fidel et al, 2017a).…”

“…The fertilizer was a mixture of NH 4 NO 3 and K 2 HPO 4 added in an amount equivalent to 72, 42, and 54 mg kg −1 of N, P and K, respectively. The samples were mixed and then incubated for an additional 80 d at 20°C, during which time moisture was maintained at −1/3 bar matric potential equivalent (29–31% gravimetric moisture content as determined by pressure plate, see Fidel et al, 2017a) by adding deionized water periodically as needed, and CO 2 and N 2 O production rates were quantified on Days 0, 1, 2, 3, 4, 8, 9, 18, 25, 31, 52, 66, and 80.…”

“…The fertilizer was a mixture of NH 4 NO 3 and K 2 HPO 4 added in an amount equivalent to 72, 42, and 54 mg kg -1 of N, P and K, respectively. The samples were mixed and then incubated for an additional 80 d at 20°C, during which time moisture was maintained at -1/3 bar matric potential equivalent (29-31% gravimetric moisture content as determined by pressure plate, see Fidel et al, 2017a) by adding deionized water periodically as needed, and CO 2 and N 2 O production rates were quantified on Days 0, 1, 2, 3, 4, 8, 9, 18, 25, 31, 52, 66, and 80. Between each set of gas sampling events, soil samples were kept covered with gray butyl septa (no crimp cap) to minimize moisture loss, and then briefly uncovered for 10 to 15 min to allow gas exchange. For each set of sampling events, serum vials were sealed with gray butyl septa and crimp caps, and 11.5-mL gas samples were collected using a syringe three times over the course of 16 to 48 h, as appropriate for the gas production rate.…”