Browsing by Author "Olivier, Charl Francois"

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    An investigation into the degradation of biochar and its interactions with plants and soil microbial community
    (Stellenbosch : Stellenbosch University, 2011-12) Olivier, Charl Francois; Rozanov, Andrei Borisovich; Botha, Alfred; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Biochar (charcoal) is lauded by many scientists as an effective way to remove carbon dioxide from the atmosphere and storing it in a very stable form in the soil for hundreds to thousands of years, whilst promoting soil fertility and productivity. Considering that no significant amounts of charcoal are presently accumulating in the environment, despite considerable amounts produced globally in natural and man-made fires, this study focuses on understanding the degradation of biochar and its interactions with plants and soil organisms. The following experiments were conducted to achieve this goal. Controlled chemical oxidation of biochar, using different concentrations of hydrogen peroxide, was conducted in an attempt to mimic the enzymatic degradation of biochar by basidiomycetes. The changes occurring in biochars structure and chemistry were assessed afterwards. Furthermore, aerobic and anaerobic digestion of biochar was conducted in vitro, and in vivo to investigate the changes occurring in biochar‘s elemental composition and chemistry during oxidation and factors that play a determining role in the rate of biochar degradation. The influence of biochar in soil on free-living and symbiotic microbial communities as well as its impact on total plant biomass production and root development was assessed in three greenhouse pot trials using wheat and green beans as test plants It was proven that biochar is almost fully H2O2-degradable, mostly through hydroxylation and carboxylation reactions which led to the formation of various short chained carboxylic acids, surface saturation with acidic functional groups as determined by the surface acidity measurements and proven by the increase in the intensity of FT-IR peaks associated with carboxyl and phenolic C-O groups. Furthermore, hydrogen peroxide treatment resulted in preferential removal of volatile organic carbons and led to the purification of biochar as evident by the new, more intense and sharper peaks in the region of 1600-1000 cm-1. These FT-IR peaks are considered as the more recalcitrant fraction of biochar and were shown to be mostly associated with transformation products of lignin and cellulose formed during pyrolysis. The incubation trial confirmed that biochar cannot be utilized as a sole carbon source without the addition of nutrients or glucose, to activate microbial activity within the columns. Furthermore, abiotic oxidation can be facilitated by oxidative soil minerals such as birnessite, but oxidation with atmospheric oxygen did not result in the evolution of CO2 from biochar. The average CO2 production in pot trials without plants in both the fertilized and unfertilized treatments increased linearly (R2= 0.80; 0.79 respectively) with increasing biochar application rates when biochar was the main carbon sources. Anaerobic degradation of biochar by a methanogenic consortium was much more efficient in utilizing biochar as a carbon source, compared to aerobic digestion. The anaerobic digesters maintained a chemical oxygen demand (COD) removal efficiency of 30% per week with continuous production of CO2, whilst methane production was very erratic. We proposed that better control over pH and alkalinity as well as an increase in hydraulic retention time would improve both the COD removal efficiency and methane production. Field incubations resulted in various degrees of oxidation at different incubation sites. An increase in the oxygen content and a decreased in the carbon content of biochar‘s elemental composition and also an increase in the surface acidity due to a larger amount of carboxyl acid groups on the surface as seen in the increase in the FT-IR peak at 1700 cm-1 confirmed that biochar are susceptible to oxidation under field conditions. We came to the conclusion that oxidation and mineralization of biochar in this trial occurred at a faster rate in soils with a higher microbial activity. The pot trials, confirmed that biochar does not serve as a fertilizer even though it did increase total biomass production between biochar application rates of 0.05-2.5 % (w/w). For agricultural purposes the addition of biochar should always be applied together with NPK fertilizer. In both the wheat and green bean trials it was confirmed that biochar application rates of 0.05-0.5% (w/w) on the sandy, slightly acidic soil used in this trial resulted in the greatest biomass production and fertilizer use efficiency. Biochar additions resulted in considerable increases in soil pH and C/N ratios which were considered as the main reasons for the decrease in microbial biomass in the unfertilized green bean treatments as it made the uptake of N more limited. The addition of fertilizer however, alleviated N-supply constraints and as a result promoted microbial growth at all biochar application rates of pot trial 1. However, biochar did not promote mycorrhyzal colonization and caused a decrease in the mycorrhizal colonization of roots with increasing biochar application rates and within biochar layers. Biological nitrogen fixation, however, reacted positively to the addition of biochar. High biochar application rates significantly enhanced the plants reliance on these symbiotic relationships. We hypothesized that biochar physically immobilized N into its microvoids through capillary suction and then served as a physical barrier between plant roots and absorbed N. However, immobilzation of N by microbes could also have contributed to the decrease in N uptake if one takes into account that microbial activity was higher (respiration data) at the higher biochar application rates. Further investigations are needed to warrant this hypothesizes.