The BPCA analyses suggest that BC MED produced at a reported temperature between 500–750°C reached a higher condensation degree than our reference 682°C BC LAB . Another Miscanthus biochar produced by Pyreg was analyzed by Wiedner and colleagues [ 36 ] using the BPCA method. Similar to our findings, they found high levels of B6CA, i.e. 85% B6CA, 10% B5CA, 5% B4CA, 0% B3CA. The degree of condensation of this biochar was reported to be higher than all other materials tested [ 36 ]. The total BPCA content of our BC LAB and BC MED are similar to those obtained for grass biochars prepared at 700–900°C [ 33 , 37 ]. Our results suggest that the medium-scale pyrolysis process affected the condensation more than the aromatization degree of BC MED vs. BC LAB .

Our chemical oxidation values were close to those reported for a wheat-derived gasification char, which was resistant at 72% to chemical oxidation by potassium dichromate [ 20 ]. This latter study used a methodology similar to ours, only with a slightly shorter reaction time, i.e. 12 vs 15.5 h. In general, oxidation methods reported in the literature follow variable protocols, making it difficult to compare results among individual studies. Oxidation utilizing hydrogen peroxide and thermogravimetric analysis have also been used to estimate biochar stability [ 35 ]. Our chemical oxidation data suggest that BC LAB and BC MED were equally carbonized.

Laboratory analyses pointed towards equivalent degrees of stability and aromaticity for the medium-scale and the laboratory biochars. The H/C atomic ratio of BC MED was slightly lower than that of BC LAB , i.e. 0.18 vs 0.24 ( Table 1 ). Similar to our results, Keiluweit and colleagues [ 34 ] reported H/C atomic ratio of 0.2 for grass biochar produced at 700°C, but did not test higher HTT. However, 0.2 is not the lowest limit for biochar produced with Miscanthus, as Budai and colleagues [ 12 ] report H/C atomic ratio of 0.1 for biochar produced in the laboratory at 800°C. Therefore, the H/C atomic ratio suggests that BC MED reached a carbonization degree comparable to that of BC LAB , i.e. a slow-pyrolysis biochar produced in the laboratory at 682°C.

Laboratory incubations confirmed the high stability of BC MED , which was suggested by H/C ratio, BPCA and chemical oxidation methods. BC MED mineralized by only 0.10% after 90 days, which is consistent with results of Luo and colleagues [ 38 ] who observed a 0.16% mineralization of 700°C Miscanthus biochars in an 87-day incubation. Lower temperature Miscanthus biochars have been reported to display higher mineralization rates, from 0.73% in 87 days for a 350°C biochar [ 38 ] to 1.1% in 200 days for a 575°C biochar [ 39 ]. Here, we could not estimate a precise MRTs based on our short-time laboratory incubation, but even the most conservative first-order kinetics model suggested it to be longer than 220 years ( S1 Fig ). Even if a laboratory MRT could be obtained it could not be extrapolated to field conditions, notably because incubation conditions are artificial and we used a standard soil type. Living roots can promote biochar mineralization [ 17 ] and soil type affects biochar mineralization rates [ 40 ]. What the incubations tell us is that BC MED is highly stable and therefore worthy of field investigation. Incubations are also useful to compare the decomposition kinetics of different biochars [ 30 ]. Here we show that the stability of Miscanthus biochar produced in a medium-scale pyrolyzer actually exceeds that of biochar produced at a laboratory scale, which suggests that the large volume of feedstock in the pyrolyzer was not a limitation for obtaining a well carbonized product.

Mineralization of BC in a two-year field trial

Mineralization rate of BC MED in the field approximated 0.5% per growing season (Table 6), which implies that the annual rate is probably lower than 1% for the entire year under the cold-climate conditions prevailing in Norway. We acknowledge that the average 0.5% mineralization rate per growing season is only an estimate. However, we found no obvious source of bias on this estimate and therefore consider it fairly robust. Although our soil respiration fluxes were obtained with a simple manual chamber system, our results appear consistent with literature values. We measured on average a soil CO 2 efflux of about 275 g CO 2 -C m-2 over 4 months in 2012, while the annual soil respiration from all croplands averages 544 g C m-2 yr-1 [41]. Our soil respiration data appear similar or higher to those compiled for field crops in Sweden, Canada and Russia [42].

For soil respiration alone, the absence of a significant difference between our biochar treatments and the control appears consistent with recent reports. For example, Schimmelpfennig and colleagues [43] report that throughout an 18-month monitoring period, a field having received Miscanthus biochar had lower cumulative CO 2 emissions than biochar-free controls. In a recent meta-analysis, Sagrilo and colleagues [15] indicate that soil CO 2 efflux from biochar treated soils are not significantly higher than from no-biochar controls when the ratio of biochar-C to SOC is lower than 2. Across application dose, these authors report no increase in soil CO 2 efflux with biochar addition when the biochar is produced with a pyrolysis retention time > 30 minutes or at a temperature above 550°C, or when it has a surface area > 50 m2 g-1. In addition, none of the 8 field studies included in the review of Sagrilo and colleagues [15] displayed significant higher CO 2 fluxes with biochar addition to soil. These findings suggest that biochar decomposition in the field is slow. However, actual quantification of the decomposition rate is crucial, as there is for example a large difference between a 1% and a 5% biochar decomposition rate, although both are likely to produce non-significant CO 2 responses in the field, being possibly hidden by negative priming effects and root respiration responses. Therefore, isotopic tracing of C sources is needed to estimate the actual biochar mineralization rate in the field [16], as was conducted for one growing season in the present study.

Our biochar mineralization estimates computed from δ13C and soil respiration measurements are in the lower range of the limited set of studies having attempted a similar assessment. A mineralization rate of 9% was reported for maize biochar after 245 days [17]. However, biochar in the latter study had an atomic H/C ratio of 0.49, which is higher than our 0.18 value. In Australia, mineralization rates of Eucalyptus biochar ranged from 2% to 7% per year depending on soil type and climate [40]. This high mineralization rate might be due to the high H/C ratio of the Eucalyptus biochar, i.e. 0.63, which is higher than the H/C threshold of 0.6 for proposed for non-stable biochars [23]. Our results are similar to those of Major and colleagues [44], who reported a biochar mineralization rate of 2.2% over 2 years, i.e. about 1% per year, in tropical conditions, using a biochar made of mango tree wood with H/C atomic ratio of 0.26. Also, Maestrini and colleagues [45] reported an in situ annual mineralization rate of 0.5% for pinewood biochar in a temperate forest soil.

Estimating a MRT from the measured biochar mineralization rate in the field is the most crucial yet most uncertain step for assessing the C-storage potential of different biochar products in soil. Having measured a 2% mineralization for biochar over 12 months in an arenosol, Singh and colleagues [40] applied one-, two- and infinite-pool decomposition models and inferred that the corresponding MRT was comprised between 44 and 1079 years, which clearly exemplifies the large uncertainty associated with converting annual mineralization rates into MRT. Major and colleagues [44] observed a mineralization rate of 2.2% over two years, and extrapolated this value to a MRT of 3200 years using a two-pool model. This long MRT was a result of a three-fold decrease in biochar mineralization rate from year one to year two in their study. Our estimated mineralization rate for the 2012 season was slightly lower than that of Major and colleagues [44], i.e. 0.8 vs. 1.1% per year. However, we cannot apply a two-pool model to our results because we have no indication that such two pools actually existed in our case. Laboratory incubation (Fig 1) did not reveal any significant pool of mineralizable C for BC MED at the beginning of the incubation. By contrast, the feedstock displayed a pronounced two-pool behavior, with 45% being mineralized in 90 days, which might explain why feedstock mineralization rates in the field in 2012 were fairly low. We used a one-pool model with constant mineralization rate of 0.8% per year, which yields a conservative MRT estimate for BC MED of 125 years. Although this value barely exceeds the conventional 100-year threshold for permanent removal, large gains in terms of C storage in soil can still be achieved with a pyrolysis process transforming crop residues into biochar with 1% y-1 mineralization rate [46].

In conclusion, our biochar produced in a medium-scale pyrolyzer: 1) scored high on stability indices in the laboratory, 2) had similar to higher stability indices than a laboratory-produced biochar, and 3) mineralized at an estimated 0.8% per year under field conditions. The corresponding MRT for field conditions exceeds 100 years, but is only an extrapolation. Based on laboratory re-incubations, Spokas [47] argues that field-incorporated biochar might become intrinsically more susceptible to mineralization. Others have argued the opposite, that the real MRT might greatly exceed the projected MRT because biochar is not composed of one or two pools but of a continuum of increasingly recalcitrant fractions [40]. Ascertaining the long-term dynamics of this response calls for long-term monitoring of biochar field experiments having isotopic C tracing possibilities.