Long-term biochar and soil organic carbon stability - Evidence from field experiments in Germany.

Organic soil amendments (OSA) with long residence times, such as biochar, have a high potential for soil organic carbon (SOC) sequestration. The highly aromatic structure of biochar reduces microbial decomposition and explains the slow turnover of biochar, indicating long persistence in soils and thus potential SOC sequestration. However, there is a lack of data on biochar-induced SOC sequestration in the long-term and under field conditions. We sampled two long-term field experiments in Germany, where biochar was applied 12 and 14 years ago. Both locations differ in soil characteristics and in the types and amounts of biochar and other OSA. Amendments containing compost and 31.5 Mg ha-1 of biochar on a loamy soil led to a SOC stock increase of 38 Mg ha-1 after OSA addition. The additional increase is due to non-biochar co-amendments such as compost or biogas digestate. After eleven years, this SOC stock increase was still stable. High biochar amount additions of 40 Mg ha-1 combined with biogas digestate, compost or synthetic fertilizer on a sandy soil led to an increase of SOC stocks of 61 Mg ha-1; 38 Mg ha-1 dissipated in the following four years most likely due to lacking physical protection of the coarse soil material, and after nine years the biochar-amended soils showed only slightly higher SOC stocks (+7 Mg ha-1) than the control. Black carbon stocks on the same soil increased in the short- and mid-term and decreased almost to the original stock levels after nine years. Our results indicate that in most cases the long-term effect on SOC and black carbon stocks is controlled by biochar quality and amount, while non-biochar co-amendments can be neglected. This study proves that SOC sequestration through the use of biochar is possible, especially in loamy soils, while non-biochar OSA cannot sequester SOC in the long term.

Sauna, sweat and science II - do we sweat what we drink?

Inspired by a previous 'Sauna, sweat and science' study [Zech et al. Isot Environ Health Stud. 2015;51(3):439-447] and out of curiosity and enthusiasm for stable isotope and sauna research we aimed at answering the question 'do we sweat (isotopically) what we drink'? We, therefore, pulse-labelled five test persons in a sauna experiment with beverages that were 2H-enriched at about +25,600 ‰. Sweat samples were collected during six sauna rounds and the hydrogen isotope composition δ2Hsweat was determined using an isotope ratio mass spectrometer. Before pulse labelling, δ2Hsweat - reflecting by approximation body water - ranged from -32 to -22 ‰. This is ∼35 ‰ enriched compared to usual mid-European drinking water and can be explained with hydrogen-bearing food as well as with the respiratory loss of 2H-depleted vapour. The absence of a clearly detectable 2H pulse in sweat after pulse labelling and δ2Hsweat results of ≤+250 ‰ due to a fast 2H equilibration with body water are moreover a clearly negative answer to our research question also in a short-term consideration. Given that the recovery of the tracer based on an isotope mass balance calculation is clearly below 100 %, we finally answer the question 'where did the rest of the tracer go?'

Substitution of mineral fertilizers with biogas digestate plus biochar increases physically stabilized soil carbon but not crop biomass in a field trial.

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B0), with 1 Mg biochar ha-1season-1 for two growing seasons (B2), or with 40 Mg biochar ha-1 application (B40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 µm aggregate-occluded and mineral-associated OM. CO2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha-1), while biochar had no effect. B40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 µm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha-1 with digestate resulted in 20% more SOC in the <20 µm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B0 and B2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates.