Temporal perspective of humification of organic matter
Humustumisprosessissa kuollut orgaaninen aines hajoaa maaperässä. Osa tästä orgaanisesta aineesta mineralisoituu hiilidioksidiksi ja vedeksi lopun jäädessä maaperään ja muodostaa humusaineita, joita ovat humushappo, fulvohappo ja humiini. Humusaineilla, erityisesti humushappojakeella, on myönteisiä vaikutuksia maaperään sekä kasvien kasvuun. Huolimatta yli 200 vuoden tutkimuksista yksiselitteistä teoriaa humushappojen kemiallisesta rakenteesta tai niiden synteesistä ei vielä ole esitetty. Osa teorioista painottaa humushappojen rakenteen keskeisinä tekijöinä ligniiniä ja aromaattisuutta, osa teorioista taas korostaa hiilihydraattien ja alifaattisuuden merkitystä.Koivulan tutkimuksessa ns. hiiltymissarjasta (komposti, turve, hiili) eristetyt humushapot ryhmiteltiin humustumisen alku-, keski- ja loppuvaiheeseen kuuluviksi. Abioottisten eli elottomien tekijöiden vaikutusta humustumisen alkuvaiheeseen tutkittiin kompostoimalla erilliskerättyä biojätettä käyttäen tuhkaa kompostoinnin lisäaineena. Kompostointia käytettiin myös synteettisenä menetelmänä siten, että ligniinivapaista lähtöaineista tuotettiin kompostoimalla humushappoja. Tämä osoitti, että ligniinin läsnäolo ei ole humustumisprosessissa välttämätöntä.Koivulan mukaan orgaanisen aineen hajoamista – humustumista – on tapahtunut siitä lähtien kun maapallolle syntynyt elävä orgaaninen aine on alkanut kuolla. Hiilihydraatit ovat kehittyneet ennen ligniiniä, jonka hajotus on ajan myötä tullut osaksi olemassa olevaa humustumisprosessia. Ligniini ei ole siis dramaattisesti muuttanut tätä prosessia. Teoriaa tukee se, että ligniinivapaa selluloosakomposti tuotti humushappoja ja niiden hajotustuotteissa oli myös aromaattisia yhdisteitä.Koivulan tutkimus osoittaa, että ligniinin merkitystä humustumisprosessille on painotettu liikaa ja vastaavasti hiilihydraattien merkitystä painotettu liian vähän. Potentiaalisen kasvi- ja mikrobiperäisen aineksen osuus humustumisessa tulisi huomioida mahdollisimman tasapuolisesti.
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Dead organic matter is stabilised in the humification process to humic substances (HS). Functionally the most important group of HS is humic acids (HA), which have essential functions in soil, sediments and water. Humification involves both biological and abiotic processes, but the (bio)chemical synthesis as well as the structures of HA are unknown. Numerous theories of humification have been presented over the years. Some of these emphasise aromaticity as the structural unit of HA, while others emphasise aliphaticity.In this work, humification and the structure of HA were studied from a temporal perspective. HA samples were grouped according to age: early, intermediate and end stages of humification; and the respective samples were compost, peat and coal HA. Composting of cellulose with lignin-free bulking agent produced HA, indicating that humification and formation of HA can occur without a lignin contribution. Use of ash as a composting additive enhanced the rate of mineralisation and the formation of HA, indicating abiotic effects on humification during composting. HA was extracted from the above-mentioned matrices. The amount of HA extracted was greatest from peat samples and less from composts and coal samples. Degradation of HA by alkaline hydrolysis produced hydrophilic and hydrophobic phases. Structural study of HA was focused on the degradation products in the two phases, and on the importance of carbohydrate structures to HA. The significance of studying both phases was that more organic matter could be analysed. Compounds obtained in hydrophilic phase were mainly aliphatic, while compounds obtained in hydrophobic phase were mainly aromatic. Yields of aliphatic compounds were clearly higher than yields of aromatic compounds. Easily degradable carbohydrates are rapidly utilised by microbes during humification, but microbes then generate new polysaccharides. Compounds of carbohydrate origin are found throughout the coalification series. Carbohydrates are thus recycled during humification, which means that carbohydrates, too, are recalcitrant in humification, but in a different way than lignin. Lignin persists through being slow to decompose, while carbohydrates persist through recycling. The contribution of lignin to humification can also be discussed from an evolutionary perspective. Non-vascular plants were forming soils long before vascular plants had evolved. Conceivably the humification process did not change radically after lignin was evolved; rather, lignin was adopted into an existing humification and degradation system. From this it follows that, the humification process does not depend upon a lignin contribution. Cellulose compost HA produced aromatic degradation products
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University of JyväskyläISBN
951-39-1770-3ISSN Search the Publication Forum
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