Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by Author "Aghoghovwia, Makhosazana Princess"

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    Effect of different biochars on inorganic nitrogen availability
    (Stellenbosch : Stellenbosch University, 2018-03) Aghoghovwia, Makhosazana Princess; Hardie-Pieters, Ailsa G.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Biochar (a fine pyrolysed organic material) is an amendment used to increase and sustain productivity, reduce environmental pollution and sequester carbon (C) in soils. Successes were reported in improving acidic, sandy and highly weathered soil. However, the effects are strongly influenced by biochar physico-chemical characteristics, which vary widely depending on feedstock and pyrolysis conditions. The main objective of this study was to determine the effects of six biochars (commercially-produced in South Africa under various pyrolysis conditions from maize stover, grape pip, grape skin, pine wood, rubber tyre and sugarcane pith) on nitrogen (N) interactions in a sandy soil. The physico-chemical properties of the above biochars were characterised, three main experiments were conducted to study the effects of biochar addition to soil on (1) inorganic (ammonium and nitrate) N adsorption and desorption of added ammonium and nitrate in aqueous solution; (2) soil C and N mineralisation; and (3) leaching of inorganic N fertiliser. Maize stover and grape skin chars were suggested to be imperfect biochars due to low total C contents. Characterisation suggested that the sugarcane pith char was either not a suitable raw material for biochar or it was not actually a biochar due to its low stability and high chemical reactivity. However, its high ash content (66%) suggests good nutrient delivery as a soil amendment. Pine wood biochar was the most recommendable because of its low ash (3.5%), high total C (80%) and high surface area (344 m2 g-1), which all aid nutrient and water holding. However, the grape pip biochar had a low surface area (9.8 m2 g-1) and the highest fixed-C content (87%) which can be good for soil C storage. This work shows that despite many positive effects of biochar application to soil reported in literature, the negative effects of such applications on N availability are clear. All six biochars had a stronger nitrate removal affinity (82-89%) compared to ammonium (33-39%). It was also shown that adsorbed nitrate was not desorbable (0.01-0.23%) compared to adsorbed ammonium removal which was around 50% desorbable with KCl. Based on the shape of the adsorption isotherms, physisorption was the suggested mechanism for this behaviour. Competing reactions such as redox reactions in nitrate adsorption and volatilisation of ammonium were also suggested to have influenced the adsorption study results. Laboratory incubation studies showed that biochars enhanced N immobilisation along with increase in absolute and suppression of relative soil respiration. Pine wood and sugarcane pith biochars were found to reduce inorganic N availability the most due to net N immobilisation. The following biochar property may be linked to N immobilisation: inherent inorganic N in the soil-biochar system. Suppression of relative soil respiration may be due to biochar fixed-C content. Sugarcane pith char had the least effect on relative respiration because of its low fixed-C content (15.6%). However, the remaining biochars were substantially limiting the relative CO2 emissions. Rubber tyre char was the best performer in this regard with 75% lower cumulative relative CO2 emissions compared to the control. Among the plant-derived biochars, grape pip had the lowest CO2 released with 59% lower cumulative relative CO2 release. The leaching column experiment showed that application of biochars at 2.5% (w/w) to sandy soil reduced cumulative leaching of NH4+ and NO3- by 15-26% and 11-54%, respectively, compared to unamended soil. Using 15N labelled ammonium nitrate, it was found that 0.77-10.81% of applied fertiliser N remained in soil-biochar treatments after leaching. Only the pine wood and sugarcane pith biochar treatments significantly increased N fertiliser retention by 136 and 157% compared to the control soil. Whereas, the rubber tyre biochar treatment significantly reduced N fertiliser retention by 81%. The study concludes that all six biochars make inorganic N less available by mechanisms such as nitrate capture which is related to aromaticity and metal content of the chars and by enhancing biological immobilisation.