Understanding the dominant controls on biochar decomposition using boosted regression trees

Abstract

Summary Extensive studies have been carried out to investigate the decomposition of biochar. Biochar properties, soil characteristics and incubation conditions are considered key factors that control the rate of biochar decomposition. However, the relative contributions of these factors to its decomposition remain unknown. Approximately 812 individual measurements of the rate of biochar decomposition were collected from 23 studies involving 13C or 14C isotope techniques to evaluate the effects of incubation conditions, soil characteristics and biochar properties on its decomposition with a boosted regression tree (BRT) model. The BRT model accounted for 95% of the variation in biochar decomposition. Incubation conditions, soil characteristics and biochar properties accounted for 41, 31 and 28% of variation in the rate of biochar decomposition, respectively. The most important single predictor of biochar decomposition was incubation time (contributing 33% to the rate), followed by soil carbon: nitrogen (C:N) ratio (9%), pyrolysis time (9%), soil N content (8%) and biochar C content (7%). The rate of decomposition of biochar decreased with time for incubation times less than 1 year. Soil with a large C content and a small C:N ratio resulted in a large rate of biochar decomposition. Type of feedstock and biochar N content had little or no effect on biochar decomposition. Our results provide further insight into the factors that affect biochar decomposition and quantify the relative contributions of these factors to it. Highlights We studied the dominant factors that control biochar decomposition with BRT modelling. We quantified relative contributions of incubation conditions, soil characteristics and biochar properties to biochar decomposition. Rate of biochar decomposition explained by incubation conditions (41%), soil characteristics (31%) and biochar properties (28%). The most important predictor of biochar decomposition was incubation time.

Potentials, Limitations, Co-Benefits, and Trade-Offs of Biochar Applications to Soils for Climate Change Mitigation

“…However, there is a large difference between the assumption of stable carbon in LCA studies and the 3%/97% split noted by Wang and colleagues, which is based on a meta-analysis of field/laboratory experiments [20]. This may be due to a lack of compiled carbon stability data at the time of the LCA studies as the meta-analysis of biochar stability are from 2016 and 2018 [18,20] or to a choice of keeping conservative assumptions. Thers and colleagues [249] showed that carbon sequestration is increased at higher pyrolysis temperature by 10% from 400 • C to 800 • C.…”

“…Biochar decomposition follows a two-pool behavior, with a labile fraction that is quickly degraded and a recalcitrant fraction respectively estimated at 3 and 97% in a meta-analysis [20]. Based on the meta-analysis [18], average decomposition rates of biochar quickly drop from 0.6433%/day to 0.0024%/day after one year, due to depletion of the labile pool and then slow degradation of the recalcitrant pool [18]. Table 2 presents key production parameters increasing biochar recalcitrance: pyrolysis time (>3 h) and temperature (>400 • C).…”

“…An increasing number of studies assesses the role of biochar as a negative emissions technology, with a quantification of technical, economic, and sustainable large-scale deployment potential [5,14,16,17]. Individual studies focus on different aspects of its production systems and on one or more environmental implication(s), such as the long-term stability and effect on soil organic carbon (SOC) [18][19][20][21], its effect on soil physical and hydraulic properties [22][23][24][25], soil degradation [26,27], agricultural yield [28][29][30][31][32], greenhouse gas balance (GHG) [33][34][35][36][37][38][39][40][41][42][43][44], nitrogen availability and emissions [37][38][39][40][45][46][47], that of phosphorus [48][49][50], biochar's toxicity [51][52][53], remediation potential [54]…”

Inclusion of biochar in a C dynamics model based on observations from an 8-year field experiment

“…Another relevant aspect affecting the calculation of biochar degradation rates and its effect on soil organic matter, both in laboratory and field experiments, is the duration of the study (Chao et al, 2018). It has been observed that short-term trials result in lower biochar mean residence time (Leng et al, 2019a), due to the decomposition of the labile biochar fraction and an overestimation of the positive priming effect, which normally occurs in the first few months.…”

Inclusion of biochar in a C-dynamics model based on observations from a 8 years field experiment

“…Therefore, field experiments are essential to understand the dynamics of biochar in soils. Yet, despite their importance, field scale experimental studies on soil biochar degradation are still scarce (Jones et al 2012;Gurwick et al 2013).Another relevant aspect affecting the calculation of biochar degradation rates and its effect on soil organic matter, both in laboratory and field experiments, is the duration of the study (Chao et al 2018). It has been observed that short term trials result in lower biochar mean residence time (Leng et al 2019a), due to the decomposition of the labile biochar fraction, and an overestimation of the positive priming effect, which normally occurs in the first few months.…”