Responses of soil foodweb organisms in the first year following clearcutting and application of chloropicrin to control laminated root rot

Abstract

Alteration in soil foodweb organism populations (numbers, biomass and activity) were monitored following clearcutting of a mature 70-year old Douglas-fir forest and compared with an uncut mature Douglas-fir stand. Changes in soil foodweb organisms were assessed following application of chloropicrin to stumps in the clearcut to reduce infection by Phellinus weirii, which causes laminated root rot. The effect of chloropicrin was measured using two different spatial scales: (1) around treated stumps, and (2) within 0.2 ha areas where all stumps within the area were treated with chloropicrin. A 10-fold decrease in total fungal biomass, between 2- and 3-fold reductions in total bacterial biomass, a 2-fold reduction in nematode numbers and a nearly 10-fold reduction in the ratio of total fungal to total bacterial biomass were observed between 9 months to a year following clearcutting. Active fungal biomass in the 20% chloropicrin treatment was reduced 1 year (October) after application. Reduction in amoebae numbers were observed between April and June in the 100% chloropicrin treatments, while no effects of chloropicrin on total fungal biomass, total bacterial biomass, flagellate, ciliate or nematode numbers were observed. The ratio of total fungal to total bacterial biomass was reduced as compared to the control in the 100% chloropicrin-treated infected areas in April following chloropicrin treatment. Chloropicrin decreased soil foodweb organisms in only one out of 45 random soil samples from 0.2 ha areas. In samples collected from around stumps, three points out of 180 had significant decreases in all organism numbers as compared to non-chloropicrin treated stumps. Chloropicrin-sensitive tomato and alfalfa seedlings, planted at 2 m from stumps to monitor the release of chloropicrin from stumps, indicated there was no significant release of chloropicrin in the first year following clearcutting and chloropicrin application.