Possibilities to stabilize organic matter in soil using various biochars
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Soil organic matter (SOM) has an important role in soil fertility and agricultural productivity. Carbon (C) sequestration in soil can be an important step towards soil organic matter (SOM) stabilization and conservation. The traditional practices used to increase SOM do not sequester significant quantities of C into soil because most of the organic matter is not stable and is mineralized very fast. With the discovery of Terra Preta, it is evident that charcoal/biochar can be used to increase SOM. The potential positive effects of biochars on soils and plant production have been well elaborated. With increasing interest of biochar production using various biomass materials, it is necessary to study their mineralization and stabilization potential in various soils. In the present study, three biochars (a charcoal produced by burning of wood at 550°C, a hydrothermal carbonization coal (HTC) of bark, and a low-temperature conversion coal (LTC) of sewage sludge) in comparison to wheat straw were selected with the objectives to investigate the C mineralization kinetics. Prior to the incubation experiment, Fourier transformation infrared spectroscopy (FTIR) and N-fractionation were performed to characterize the C and N bonds in the biochars and straw. Charcoal was highly carbonized compared with the HTC and LTC. The LTC biochar contained more N in the heterocyclic-bound N fraction as compared with the charcoal, HTC and straw. The selected soils for the incubation experiment were sampled from various areas in Hesse, Germany. The Ferralsol (a mixture of various soil horizons) was a relict tertiary Ferralsol. In comparison, the Luvisol derived from loess (topsoil and subsoil) was selected because this is the most important soil type in Western Europe for crop production.In the first experiment, the above-described soils were incubated at 25°C with the biochars, straw, and without amendment (control) over a period of 730 d. Carbon mineralization was analyzed as alkali absorption of CO2 released at regular intervals. Soil samples taken after 5 d, 365 d, and 735 d of incubation were analyzed for soluble organic C and mineral N. To examine the reactions occurring on biochar surfaces during the first year of incubation, SOM density fractionation was performed. Total C and N in the density fractions were determined using an elemental analyzer. The bulk soil samples and SOM density fractions were scanned using FTIR to observe chemical changes on the surfaces of density fractions. In a second experiment, the effect of soil pH on C mineralization of straw and HTC was investigated. The Ferralsol was selected as test soil in this experiment. In the third experiment, wheat straw (8 t ha-1) was mixed with the biochar-amended (365 d and 0 d aged-soils) and control soils. Carbon mineralization was measured at different time intervals for a time scale of 90 d. Carbon mineralization kinetics was calculated using cumulative CO2 release data. The results show that wheat straw was mineralized very fast and had a lower half-life of C compared with the biochars. Among the biochars, HTC showed some C mineralization when compared with charcoal and LTC but less than straw. The kinetics show that half-lives of C were higher in the charcoal and LTC treatments in the Ferralsol than in the Luvisol topsoil and subsoil, possibly due to high concentration of Fe-oxide in the Ferralsol. The LTC-C had a higher half-life, possibly due to N-binding in a heterocyclic form. The results of density fractionation showed a higher proportion of applied C in the free fraction (FF). The FTIR spectra of the FF indicated surface depositions of alumino-silicate minerals on biochars, which might have increased the stabilization of biochars in the soils. Followed by the FF, higher amounts of C were found in intra-aggregate fraction (IAF). No surface depositions occurred in IAF. Except LTC, no treatment increased the C content of the heavy fraction (HF). The mechanisms involved in the increase of TC in the HF with LTC are unclear and need further investigations.In the second experiment, it was shown that soil pH did not affect mineralization of HTC and straw. The results of the third experiment show that biochar ageing in soils influenced C mineralization of added straw. The half-lives of C in various treatments were significantly higher in 365 d biochar-aged soils. Stabilization of straw C was observed in the charcoal added-soils, while increased mineralization or priming was documented in HTC and LTC added-soils. The results suggest an interaction between biochar aged in soil and applied straw. However, to investigate the detailed mechanisms of C-stabilization and C priming, further research is needed using C-labeled biochars and straw.Verknüpfung zu Publikationen oder weiteren Datensätzen
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Giessen : VVB Laufersweiler