Exploring the mechanisms behind elevated microbial activity after wood ash application

Wood ash fertilization increases the pH and concentration of dissolved organic carbon (DOC) in the soil solution and enhances the activity of soil microorganisms. However, it is unknown whether DOC or pH is primarily responsible for the increase in microbial activity. We designed an experiment to separate the effects of DOC and/or pH on soil microbial activity using suspensions of humus extracts and bacteria that had not previously been exposed to wood ash fertilization. After a 3-week incubation, DOC extracts were obtained from control (DOCC) and ash (DOCA) treatments with carbon concentrations of 9.1 and 32.5 mg C l−1, respectively. These extracts were supplied to bacterial suspensions at concentrations of 0 and 5 mg C l−1. We controlled for pH by matching adjustments, i.e. the original pH of the DOCC extract was 4.5 and its adjusted pH was 6.9, whereas the DOCA extract was pH 6.9 originally and pH 4.5 adjusted. The relative bacterial growth rate (RBGR), as measured by 3H-thymidine incorporation, increased in suspensions of 5 mg C l−1 DOC as compared to control suspensions of 0 mg C l−1. At pH 6.9, RBGR was higher for both DOC extracts than at pH 4.5. These results suggest that both DOC and pH influence microbial activity. As the growth rate at pH 6.9 with DOCA was higher than with DOCC, the quality of the DOC extract must also play a role since the carbon concentration was controlled for. The decrease in relative abundance of hydrophobic and hydrophilic acids in DOCA compared to DOCC indicates a quality shift. As measured by DGGE banding patterns, the bacterial community structure changed over the course of the 24-h experiment in the following three trials, all of which received 5 mg C l−1: DOCC at pH 6.9 and DOCA at pH 4.5 and 6.9. These results demonstrate that both the DOC origin (control vs. ash) and the pH influence a subset of the bacterial community.