Browsing by Person "Coban, Halil"

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Fate of microbial carbon derived from biogas residues applied to arable soil
(2015) Coban, Halil; Kandeler, Ellen
Soil organic matter (SOM) is the major determinant of soil fertility as it has a number of positive impacts such as improving soil physical parameters, providing nutrients for crops, and supplying energy for the microbial biomass activity in soil. Loss of organic matter is a soil threat observed worldwide. Also, bioenergy crop cultivation may accelerate SOM loss due to higher biomass harvesting compared to food crops. It is necessary to supply adequate organic matter input to arable soils in order to maintain sustainable food and biofuel production. Biogas residues (BGRs), the side-products of biogas production, are rich in microbial and plant biomass; they thus can be used as a soil conditioner and contribute to replenishing the carbon (C) pool in soil. However, our knowledge on the contribution of BGRs particularly the microbial residues present in it to SOM formation is limited, even though scientific interest on SOM formation via microbial inputs is growing. Therefore, the objective of this thesis were i) developing an approach to label microbial biomass of biogas residues, ii) tracing the fate of labelled BGRs in arable soil, iii) determining the C flux within microbial food web, and iv) determining the impacts of other soil conditioners on the mineralization of BGRs. In the first study a method was developed to label the autotrophic microorganisms in a biogas reactor using KH13CO3-amended cow manure as substrate. Analyses of phospholipid fatty acids (PLFA) and ether lipids confirmed the successful labelling of microorganisms, especially Gram-positive bacteria and methanogenic archaea. After removal of unused labelled carbonates by an acid fumigation approach, the labelled BGRs were incubated in soil for 378 days. The fate of 13C was traced in CO2 and in bulk soil with a mass balance having 93% mean recovery. Results showed that about 40% of the C derived from BGRs was rapidly mineralized within the first seven days, and mineralization reached 65% at the end of experiment. The data could be fitted to a two-pool exponential degradation model assuming two C pools each decaying exponentially. The proportions of readily degrading and stable C pools were determined to be 51% and 49%, respectively, with half-lives of 3 days and 1.9 years, respectively. The long half-life of the stable C pool in BGRs may indicate a mid-term contribution to SOM. In addition, the mineralization of SOM was enhanced by BGR-application, i.e. priming effects were detected, thus their extensive application should be avoided. A differential fatty acid approach was used in the second study for the separation of C input from BGRs to living biomass and non-living SOM. Phospholipid fatty acids (PLFA) as indicators of living biomass were compared with total fatty acids (t-FA), which are found also in necromass. Using PLFA as biomarkers of specific microbial groups, C redistribution within the microbial food web was determined. Results showed that BGRs increased the microbial biomass in soil. The sum of 13C-labelled PLFA and t-FA decreased during incubation to 60% and to 33%, respectively. The level of enrichment was different for the individual PLFA and indicated that Gram-negative bacteria were predating on Gram-positive bacteria. A contribution of ether lipids was also detected indicating C flow from decaying methanogens. This study confirmed that microbial biomass in BGRs applied to arable soil significantly contributes to SOM formation. After determining the fate of microbial C derived from BGRs in arable soil, the impacts of other soil conditioners on the mineralization of BGRs were tested in the third study. For this, labelled BGRs were incubated in soil both alone and together with compost, biochar and untreated manure. The amount of C mineralized to CO2 and the degradation rate constant of stable C pool were not affected by any of the co-amendments. However, manure resulted in a higher mineralization rate constant of the readily degrading C pool. C flow within microbial food web was from Gram-positive bacteria and methanogenic archaea to mainly Gram-negative bacteria and slightly to fungi in all treatments. This study showed that co-amending BGRs with other soil conditioners brings neither benefits nor harms in terms of the formation or the mineralization of soil organic matter. The proposed labelling approach using KH13CO3 may be useful for tracing the fate of BGRs. The enrichment in both bacteria and archaea were sufficient to be measured in an incubation experiment lasting for more than one year. However, there are disadvantages of the proposed approach such as presence of highly enriched residual carbonates. The fumigation method should be optimized for a complete removal of the highly labelled residual carbonates which will increase the precision of the overall approach.