Identification of long-term carbon sequestration in historically charcoal-enriched soils

Charcoal is the solid product obtained by heating biomass in oxygen limiting conditions. It consists mainly of aromatic carbon (C) that is highly resistant against biodegradation, its estimated residence time in soil is at least several centuries. Charcoal has, therefore, been proposed as a soil amendment (then referred to as ‘biochar’) for enhanced soil C sequestration. In addition, it has been proposed that biochar can indirectly enhance the soil C sequestration by reducing the C-mineralization rates and by promoting crop growth, i.e. by increasing the annual C input to soil. Most data on effects of biochar on C-turnover in the field is based on studies of maximally a few years and positive crop yield responses to biochar are mostly observed in soil with inherently low soil fertility. The overall goal of this study is to quantify and understand the long-term effects of biochar on soil C sequestration in temperate agricultural soils. Specifically, this study wants to identify to what extent and how biochar enhances the sequestration of crop-derived C, the so-called indirect C sequestration effects of biochar. It is speculated that the potential build-up of crop-derived C is related to a yet unknown combination of increased crop yield (and, hence, higher return of crop-C to soil) and of decreased mineralization of residue derived C. The long-term effects were based on experiments and observation in soil under historical charcoal kilns in Belgium. These kilns were abandoned since late 19th century, and are nowadays visible as black spots on arable fields due to elevated charcoal levels.To quantify the effect of historical charcoal presence on sequestration of crop-derived C, we sampled soils from such black spots (3.5 % organic carbon, OC) and adjacent soils (2.0 % OC) from fields that have been yearly cropped with maize for ca. the last 20 years. Three different quantification methods were used to estimate the charcoal-C concentration in soil; dichromate oxidation (Cr2O7), chemo-thermal oxidation (CTO-285) and differential scanning calorimetry (DSC). The estimated charcoal-C concentrations varied with the method (CTO-285>DSC>Cr2O7), indicating the large uncertainty related with charcoal quantification in soils. Using the distinct C isotopic signature (δ13C) of maize-C (a C4 plant), it was shown that soils from black spots contained 1.6-1.7 times more maize-derived C than adjacent soils, the conclusion being unaffected by the choice of charcoal quantification method. Moreover, a significant positive correlation between charcoal-C concentration and maize-derived C concentration was found.The hypothesis of reduced mineralization of maize-derived C in charcoal-enriched soils was further tested by residue mineralization studies in laboratory conditions. Mineralization of 13C enriched maize-straw, added to both soil types (black spots and adjacent soils), was quantified over an incubation period of 227 days. The concentration of 13C retained in soil after incubation was on average 1.26 times higher in black spots compared to adjacent soils, corresponding with 62 vs. 70% of the added 13C mineralized, respectively. The respiration dynamics revealed that the initial mineralization of the residue was unaffected by the charcoal whereas prolonged incubation widened the differences in mineralization. Soil drying and rewetting revealed 1.3 times lower dissolved organic C (DOC) concentrations in pore waters from charcoal-enriched soils, and recovery of added DOC was negatively correlated with the charcoal-C concentration in the soil, both observations suggesting that sorption of DOC is the underlying mechanism for lower C mineralization and that the specifically larger sorption of more aromatic and more recalcitrant DOC explains the reduced mineralization on the long term. This hypothesis was further explored by setting-up mineralization experiments with different DOC substrates, ranging between low to high aromaticity. The 24h DOC mineralization was up to a factor 1.8 higher in adjacent soils compared to charcoal-enriched soils and this factor increased for DOC substrates that were more aromatic, indicating preferential stabilization of aromatic C. The same experiment was repeated in soils that had been sterilized and re-inoculated to minimize effects of different soil microbial communities. Here, effects of charcoal on reduced DOC mineralization were even more pronounced than in soil with the native and, hence, adapted communities, suggesting that microbial adaptation to aromatic-like components overcomes, and partially counteracts, the effects of charcoal on lower bioavailability of aromatic DOC. This charcoal effect on soil microbial community composition (DNA) and activity (RNA) could, however, not be confirmed by DGGE analysis.The hypothesis of higher crop yield in the presence of charcoal was examined through estimation of the maize yield in both soil types in three consecutive years (2014-2016). On average, maize yield was 23 % higher in black spots compared to control soils, and the yield increase was highest in the driest growing season, suggesting that changes in soil moisture retention is the underlying mechanism. Determination of the plant available water content confirmed this hypothesis, as this was 13 % higher in soils from black spots than in adjacent soils. Moreover, plants grown on black spots showed less indications of water stress during growth, as revealed by the leaf-δ13C. Soils from black spots showed higher CEC (16.0 vs. 13.0 cmolc kg-1), lower pH (6.2 vs. 6.4), and higher concentrations of available Ca and Mg relative to adjacent soils. These higher concentrations of available nutrients could, however, not be related to the higher crop yields, as plant nutrient composition was unaffected.The observed effects of charcoal on yield increase (23 %) and on reduced mineralization rate of the resistant plant material (factor 1.2 lower) were implemented in RothC modelling. This showed that both factors underestimated the factor 1.6-1.7 enhanced maize derived C in soils under black spots after about 20 years of cultivation; model calibration to fit these observations showed that reduced mineralization has the highest relative contribution to the soil C sequestration. The modelling with these adjusted parameter was then extrapolated to 130 years, mimicking the 130 years of long-term cultivation on charcoal amended soils. The predicted long-term sequestration of SOC was about 10 g SOC/kg soil larger in charcoal amended soil than in unamended soils; this build-up is larger than that observed in the field for C3-C. This suggest that some C-substitution effects are taken place and that the observed enhanced C4-stocks may overestimate the long-term indirect effects of BC on C-sequestration.In summary, this study shows that historical charcoal presence can effectively increase C sequestration in temperate soils, both directly as it is a recalcitrant form of C as indirectly, as it increases the sequestration of crop-derived C. The higher build-up of maize-derived C in historically charcoal-enriched soils is the result of a combined effect of lower crop-residue degradation and enhanced crop yield. The sorption of aromatic DOC is proposed as the most important mechanism for the lower C mineralization, whereas increased soil water availability is most likely responsible for the higher crop yields. The extent of long-term soil C sequestration capacity of biochar cannot be determined yet because of limited accuracy of available BC quantification techniques.

Publication year:2017