Effects of biogas residues on respiration and denitrification in arable soil
evaluation of methods, microbial activity and agronomic implications
Biogas residueAnaerobic digestionPig slurryPDARespirationOrganic fertilizersBiogasRötrestFlytgödselDenitrifikationBördighetSoil fertility
Agricultural soils constitute the base in human food production and soil content of organic matter and plant nutrients together with soil microbial activity are all important parameters for high crop yield of good quality. These parameters are dependant on proper fertilization.Anaerobic digestion of organic wastes for biogas production generates a liquid residue called biogas residue (BR). It contains organic material and plant nutrients which makes ita potential fertilizer for arable cropping. However, it also contains heavy metals and toxic organic compounds and it is therefore in need of evaluation before usage.Microbial tests were performed aiming to evaluate the agronomic traits of four different BR and to find a viable procedure for evaluating slurry fertilizers. Two experiments where soil respiration and soil potential denitrification activity (PDA) was measured at fertilizer addition were performed. As controls pig slurry (PS) was used in both experiments along with a mixture of ammonium and glucose in the respiration experiment. In two follow up experiments soil PDA at addition of trace elements, extra glucose and heat treated PS were tested. All additions were based on fertilizer mineral nitrogen content and corresponded to0-140 and 0-1120 kg NH4-N ha-1 in the respiration and PDA experiment, respectively.A respiration peak was observed for the BRs immediately or within three days after fertilizer addition. After ten days the respiration of the BR had almost stopped. There weredifferences between the BR but larger differences were observed when compared with the controls, which respired more and for a longer time. Characteristic data useful for determining the qualities of the fertilizers, were extracted from the respiration curves including (1) the utilization rate of carbon, (2) the time of respiration peak, (3) the height and (4) the slope of that peak and finally (5) the initial respiration rate. Practical difficulties with the experiment, mostly due to different additions of carbon with the fertilizers, were discussed.All four BR stimulated soil PDA at low doses but inhibited soil PDA at high rates of addition. Differences between PS and BR were distinct as PS stimulated soil PDA at highrates of addition. The follow up experiment indicated that this was due to denitrifying bacteria and enzymes present in the PS itself. This has not previously been shown but offers a potential explanation to the often observed evolution of N2O gas at PS fertilization. The heavy metal contents of the BR were proposed as the reason for their inhibition of PDA.Overall the BR stimulated microbial processes in the soil to a lesser extent than the controls.Since microbial activity often results in immobilization of plant nutrients, it might be easier to predict nutrient dynamics for BR compared to PS when added to soil. Furthermore, the BR behaved differently probably depending on their substrates and the anaerobic digestion process. Together with a number of other microbial tests, incubation experiments and C compound analyzes, soil respiration and soil PDA can be used for evaluating slurry fertilizers.