Effect of different biochars on inorganic nitrogen availability

Aghoghovwia, Makhosazana Princess (2018-03)

Thesis (PhDAgric)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Biohoutskool (‘n fyn gepiroliseerde organiese materiaal) is ‘n grondverbeteringsmiddel wat gebruik word om produktiwiteit te verhoog en vol te hou, omgewingsbesoedeling te verminder en koolstof (C) in gronde te sekwestreer. Sukses was aangetoon in die verbetering van suur-, sanderige- en hoogs verweerde grond. Maar, die effekte word egter sterk beïnvloed deur die biohoutskool fisies-chemiese eienskappe, wat hoogs wissellend is, afhangende van biomassa en pirolise toestande. Die hoof doel van hierdie studie was om die effekte van ses houtskole (wat kommersieël in Suid Afrika onder verskillende pirolise toestande vanuit mieliestoppels, druiwepitte, druiweskille, dennehout, rubberbande en suikerriet-sponsweefsel geproduseer word) op stikstof (N) interaksies in die sandground te bepaal. Die fisies-chemiese eienskappe van die bogenoemde biohoutskole was karakteriseer, drie hoof eksperimente was onderneem om die effek van biohoutskool byvoeging op grond op (1) anorganiese (ammonium en nitraat) N adsorpsie en desorpsie van bygevoeg ammonium en nitraat in waterige oplossing te bestuurder; (2) grond C en N mineralisering; en (3) uitloging van anorganiese N kunsmis te bepaal. Mieliestoppel- en druiweskille houtskole was voorgestel as grensgeval biohoutskole as gevolg van die lae totale-C inhoud. Die eienskappe van suikerriet-sponsweefsel houtskool het aangedui dat dit nie ‘n geskikte rou materiaal vir biohoutskool was nie of dat dit nie eintlik ‘n biohoutskool was nie as gevolg van die lae stabiliteit en hoë chemiese reaktiwiteit. Maar, sy hoë as inhoud (66%) dui egter op goeie voedingstof-lewering as ‘n grondverberingsmiddel. Dennehout biohoutskool was die meeste aanbevelende weens sy lae as (3.5%), hoë totale C (80%) en hoë oppervlakarea (344 m2 g-1), wat goeie voedingstof- en waterhuishouding bevorder. Maar die druiwepit biohoutskool het ‘n lae oppervlakarea (9.8 m2 g-1) en die hoogste vaste-C inhoud (87%) wat goed vir C-berging en as ‘n mikrobiese voedselbron kan wees. Hierdie werk toon dat ten spyte van die baie positiewe effekte van biohoutskool wat in die literatuur aangetoon is, die negatiewe effekte van sulke toediening op N beskikbaarheid ook duidelik is. Hierdie effekte sluit in N immobilisasie en onderdrukking van grondrespirasie. Hierdie effekte was gekwantifiseer en meganismes was ondersoek vir biohoutskole vir van verskillende kwaliteit. Dennehout en suikerriet-sponsweefsel biohoutskole was die beste om anorganiese N beskik baarheid te verminder as gevolg van netto N immobilisasie. Die volgende biohoutskooleienskap kan aan N immobilisasie gekoppel word: inherente anorganiese N in die grond-biohoutskool-stelsel. Onderdrukking van grondrespirasie kan wees as gevolg van biohoutskool vaste-C inhoud. Suikerriet-sponsweefsel houtskool was nie vir die onderdrukking van koolstofdioksied (CO2) respirasie as gevolg van sy lae vaste-C inhoud geskik nie. Maar, die biohoutskole wat oorbly het as goeie CO2-sorbente gedien, wat die CO2-vrystelling beperk. Rubberband houtskool was die bes-presterende biohoutskool met 75% laer kumulatiewe CO2 as die kontrole. Tussen die plant-afgeleide biohoutskole, het druiwepit die laagste CO2 vrygestel met 59% laer kumulatiewe CO2. Die logingskolom-eksperiment het gewys dat biohoutskole die kumulatiewe loging van ammonium en nitraat met 15-26% en 11-54%, relatief tot suiwer sand, verminder. Biohoutskole het betekenisvol N kunsmis behou met 0.77-10.81% in die grond-biohoutskool behandelings na uitloging. Die N kunsmis wat oorgebly het was dubbel in die dennehout en suikerriet-sponsweefsel houtskole. Daar was 18-24% totale uitruilbare N wat in die grond oorgebly het na uitloging. Die studie het tot die gevolgtrekking gekom dat al ses biohoutskole die anorganiese N kunsmis onbeskikbaar gemaak het as gevolg van sy hoë nitraat vasleggingsvermoë weens hoë biohoutskool-aromatisiteit, wat nitraat kan verminder en redoks toestande kan bevorder.

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