Turnover of bioethanol residues in soil and effects on soil microbiota

Summary

Regarding the use of residual material from bioethanol conversion, a series of experiments were performed in collaboration with Landcare Research in NZ. The work included two NZ soils from organic farming systems (Templeton and Ballantray, respectively) and treatments with three types of biomaterials: 1) residual material from bio-ethanol conversion (based on rye straw; 2) untreated rye straw; 3) grass-clover mixture - as well as a control with no amendment. The dried and ground biomaterials were supplied at 2 and 10 mg kg-1 dry soil, incubated in 1-L glass vials and harvested destructively after 3, 5 and 21 days. In this way, four experimental series were performed, each with three sequential harvests. The soil was analyzed for various microbial parameters: bacterial colony-forming units (not in all experiments), phospholipid fatty acid composition (PLFA; genetic diversity), catabolic response profiling (CRP; functional diversity); content of labile organic carbon (hot-water extractable C) and mineral N concentration and loss of organic C via accumulated respiration.
The results showed that right after application of the experimental materials to the soil, the microbial population reacted mainly to the amount and quality of readily available organic compounds, which in the present experiment was measured as the cold- and hot-water-extractable organic carbon (CWEC and HWEC, respectively). In both soils the initial level of CWEC was somewhat higher (5-15%) when bioethanol residues were added at 2 mg g-1 dry soil. However, when adding 10 mg g-1 dry soil, the CWEC was 2-3 time higher than the other treatments which more or less similar. This shows very clearly that the residual material from bioethanol conversion contained high amounts of available organic matter which can be utilized by the soil microbiota. This was also revealed by the soil respiration which was similarly enhanced after addition of bioethanol residues. Initial CWEC values after addition of untreated rye grass and grass-clover were also higher (10-30%) than in the controls resulting in intermediate values of soil respiration. The basic level of soil mineral nitrogen was rather high in both soils and addition of all the fertilizer material caused an initial decrease in especially nitrate, although most pronounced when bioethanol residues were added at 10 mg g-1 dry soil. This indicates that the high content of available C in the bioethanol residues apparently caused a fast immobilization of mineral N in microbial biomass as the soil concentration of nitrate was only about 25% of the values in the controls. Untreated rye straw and grass-clover showed nitrate concentration comparable to the controls, but did also decrease with time (especially with grass-clover). These trends were also apparent in the treatments with 2 mg material added per gram soil, but obviously not so clear cut. Overall, the results indicates that the amount of organic material available to the soil microbiota has strong influence on the concentration of soil mineral N. In this respect, the residues from bioethanol conversion seem to contain a large pool of organic C (considerably larger that crude rye straw and grass-clover) which can be readily utilized by the soil microorganism. This also seems to cause a significant immobilization of mineral N in microbial biomass, which may constitute a considerable factor when competing with plants for soil mineral N.