The effect of composted biochar on compost properties and mineralisation

Botha, Ockert Guillaume (2016-12)

Thesis (MScAgric)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Pyrolized carbon, also known as biochar, is a widely used soil conditioner recognized for its adsorption, C sequestration and agricultural qualities. This led to the investigation into the possible use thereof by small-scale sustainable farmers as a filter for agricultural olive or wine effluent, where after the spent biochar can be incorporated into composts to sterilize it from toxins and pathogens before being used as soil amendment. However, before these used biochar filters can be applied to compost, research is required to assess the affect that biochar could have on the composting process. This research project was therefore initiated to investigate the feasibility of adding biochar to composts, specifically focusing on the effect of type and amount of biochar on the composting process and mineralisation of the composts in soils. The final aim was to construct a method for quantifying biochar content in compost and soil that can be used to assess the stability of biochar in soils. Furthermore, none of this research has previously been done in South Africa or on two locally produced biochars. The first experiment was constructed to evaluate the effect of two contrasting commercial biochars on composting; a relatively low-cost, crude, pine wood biochar produced using a low-tech slow pyrolysis technique at 450°C, and a significantly more expensive, refined eucalyptus biochar produced using a high-tech slow pyrolysis technique at 900°C. The biochars were applied at two application rates (10% and 20% dry weight) to a mixture of green and animal waste. The effect was measured through composting indices such as temperature, C/N ratio, pH and EC, and microbial activity. Results showed that the robust, low temperature pine biochar applied at 10% (d/w) is the most suitable for composting due to higher composting temperatures measured, lower C/N ratios in the final product and higher cumulative microbial activity relative to the other biochar treatments. However, all biochar and control composts were all classified as successfully matured and stabilized according to the indices used, indicating that both types of biochar and application rates can be used to produce compost. The second experiment was aimed at comparing the carbon (C), nitrogen (N) and phosphorus (P) mineralisation of the composted biochar in relation to compost with biochar and biochar only under ideal laboratory conditions. The incorporation of these treatments into the soil showed that the composting process increased the composted biochars degradability with 7.6 – 11.7% more carbon dioxide (CO2) being respired than compost with biochar of the same quantity. Biochar type and quantity influenced the mineralisation as eucalyptus char in general, and all treatments containing 20% biochar proved to be least degradable by microbes. Nitrogen mineralisation results showed that regardless of biochar type, quantity or composting, all biochar containing treatments caused net N immobilization and reduced nitrification. Phosphorus availability was found to be improved for both biochars through composting and the addition of compost, especially for eucalyptus biochar of which the amount of available P surpassed that of pine biochar although pine biochar only applications released more P. A 6-month field trial experiment was also constructed to further evaluate the five composts’ C mineralisation under natural conditions. In this experiment there was found that all biochar containing compost produced 7.6 – 20.1% less CO2 than the control compost, of which eucalyptus biochar showed the least amount of respiration. Loss on ignition results also revealed that composted eucalyptus biochar was the least degradable composts as only 7.4% and 7.8% of the total SOM was lost. Density fractionation further illustrated that composted biochar remains in the soil in particulate form longer than conventional compost and is slower to transform into the mineral fraction. No discernable difference in biochar content within the composts could be seen after field application at 50 t/ha. The final aim of developing a rapid and cost-effective quantification method with the use of near-infrared spectroscopy (NIRS), was completed by constructing a calibration range of soils and compost from both types of biochar. The spectra acquired was then used to create regression models that were used to predict biochar content in the final mature composts and field trial soils. The results showed that NIRS can be used to quantify biochar, to within the same order of magnitude, in both composts and soil mixtures, which is of great importance for C stock audits and assessing biochar decay over time. Selecting the type of biochar for water filtration, composting and soil conditioning, would be dependent on the purpose of the application. Both biochars show the ability to be successfully composted and used as soil amendment with good C sequestration capabilities. However, pine biochar is more suitable for the composting process and sterilization as it results in higher temperatures and increased microbial activity. Eucalyptus biochar however, would be the best option for phosphorus mineralisation and building soil carbon stocks.

AFRIKAANS OPSOMMING: Biochar is ʼn grond verbeteringsmiddel wat wydbekend is vir adsorpsie, koolstof vaslegging en verskeie ander landboukundige gebruike. Hierdie eienskappe het na die ondersoek vir die moontlike gebruik daarvan deur kleinskaalse, boere gelei. Die doel is om dit as `n volhoubare filter vir olyf- of wynuitvloeisel te gebruik, waarna dit in kompos toegedien kan word om dit van gifstowwe en patogene te steriliseer. Voordat die gebruikte biochar filters egter toegevoeg kan word in komposhope, is navorsing nodig om te bepaal wat die invloed van biochar op die komposproses en komposkwaliteit sal wees. Hierdie navorsingsprojek was dus tot stand gebring om die haalbaarheid van gekomposteerde biochar te ondersoek met spesifieke fokus op die effek wat die tipe en hoeveelheid biochar op die afbreek en mineralisasie van die kompos in die grond sal hê. Die finale doel was ook om ʼn metode te skep vir die kwantifisering van biochar in kompos en grond sodat die stabiliteit van biochar beoordeel kan word. Die eerste eksperiment was opgestel om die effek van twee kontrasterende, kommersiële biochars op kompos te evalueer. Die een biochar is relatief goedkoop, ru en geproduseer uit dennehout deur middel van stadige pirolise by 450°C, terwyl die ander een aansienlik duurder, meer verfynd en uit bloekomhout teen 900°C geproduseer is. Die biochars was in twee toedieningshoeveelhede (10% en 20% droë gewig) in 'n mengsel van groen materiaal en beesmis toegedien. Die effek van die biochar op die kompos is deur middel van verskeie kompos indekse soos temperatuur, C/N verhouding, pH, EG, en mikrobiese aktiwiteit gemeet. Resultate het getoon dat die ru, lae temperatuur dennehout biochar, toegedien teen 10% (D / w), die mees geskikte is vir kompos aangesien hoër kompos temperature gemeet is, laer C/N verhoudings in die finale produk was en hoër kumulatiewe mikrobiese aktiwiteit in vergelyking met die ander biocharbehandelings gevind is. Al die biochar-ryke komposhope was egter geklassifiseer as volledig gestabiliseer wat daarop dui dat beide tipes biochar en die toedieningshoeveelhede gebruik kan word om suksesvolle kompos te vervaardig. Die tweede eksperiment was daarop gemik om die mineralisasie van koolstof (C), stikstof (N) en fosfor (P) onder ideale laboratoriumtoestande van die gekomposteerde biochar met kompos saam met biochar, en slegs biochar te vergelyk. Die toediening van hierdie behandelings in die grond het getoon dat gekomposteeerdebiochar tussen 7,6-11,7% meer koolstofdioksied (CO2) in vergelyking met kompos met dieselfde hoeveelheid biochar vrystel. Die hoeveelheid en tipe biochar het ook ‘n invloed gehad aangesien mineralisasie van bloekombiochar in die algemeen laer was, terwyl behandelings met 20% biochar die minste afbreekbaar was. Stikstof mineralisasie resultate het getoon dat, ongeag van die tipe of hoeveelheid biochar in die kompos, alle biochar-ryke behandelings netto immobilisasie van N veroorsaak. Fosforbeskikbaarheid het verbeter deur kompostering en die byvoeging van kompos, veral vir bloekomhoutbiochar waarvan die hoeveelheid beskikbare P dennehoutbiochar se hoeveelheid oortref het. ’n Ses maande veld-eksperiment is ook opgestel om koolstofmineralisasie van die vyf komposte onder natuurlike omstandighede verder te evalueer. In hierdie eksperiment is daar bevind dat alle gekomposteerde biochar behandelings 7,6-20,1% minder CO2 as die kompos beheerdes geproduseer het, waarvan bloekombiochar die minste gerespireer het. Verlies op ontstekingsresultate het ook getoon dat gekomposteerde bloekombiochar die minste afbreekbaar was aangesien net 7,4% en 7,8% van die totale OM verlore gegaan het. Digtheidsfraksionering het ook verder getoon dat gekomposteerde biochar langer in die grond bly as konvensionele kompos. Die finale doel was om 'n vinnige en koste-effektiewe kwantifiseringsmetode te skep deur gebruik te maak van naby infrarooi spektroskopie (NIRS). Dit voeris uitgevoer deur 'n kalibrasie reeks te konstrueer met ‘n verskeidenheid van biochar hoeveelhede in beide grond en kompos. Die spektra wat verkry is, was daarna gebruik om ‘n regressiemodel te skep, wat dan gebruik was om biocharinhoud te voorspel in die finale kompos en veldgronde. Die resultate het getoon dat NIRS wel gebruik kan word om biochar te kwantifiseer binne dieselfde orde grootte in beide kompos en grondmengsels. Hierdie resultate is van groot belang vir koolstofvoorraad oudits en die beoordeling van biochar verval met verloop van tyd. Die selektering van ʼn tipe biochar vir waterfiltrasie en kompos- en grondkondisionering is gevind om afhanklik te wees van die wyse van toediening. Beide biochar’s het die vermoë om suksesvol gekomposteer te word en as grondwysiging met 'n goeie K sekwestrasie vermoë gebruik te kan word. Dennebiochar blyk meer geskik te wees vir die komposterings proses in terme van sterilisasie, aangesien dit tot hoër temperature en verhoogde mikrobiese aktiwiteit lei. Bloekombiochar sou egter die beste opsie wees vir fosformineralisasie en die bou van grondkoolstof

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