The influence of high application rates of polyacrylamide on microbial metabolic potential in an agricultural soil

Water soluble anionic polyacrylamide (PAM) is a highly effective erosion preventing and infiltration enhancing polymer, when applied at rates of 1-10 g m(-3) in furrow irrigation water. PAM greatly reduces sediment, nutrients, pesticides and coliform bacteria in irrigation runoff. There has been some concern about the potential for PAM accumulation to affect microbial ecology. We ran a long-term study applying massive quantities of PAM to soil and monitored its impact on soil microbial potential. In June, July and August, we measured active soil bacterial and fungal biomass and microbial diversity in soils receiving 0 (control), 2691 and 5382 kg active ingredient (ai) PAM ha(-1). Active bacterial biomass in soil was 20-30% greater in the control treatment than in soil treated with 2691 or 5382 kg ai PAM ha(-1) in June and August, but not July. Active fungal biomass in soils was 30-50% greater in the control treatment than soil treated with 2691 or 5382 kg ai PAM ha(-1) in June and July, but not August. Active microbial biomass in soil was 27-48% greater in the untreated control than soil treated with 2691 or 5382 kg ai PAM ha(-1) except in June. Whole soil fatty acid profiles showed no discernible change in the soil microbial community due to either of the PAM treatments at any sampling time. Analysis of nutritional characteristics using Biolog GN plates, however, yielded an apparent separation of the non-amended control soils from those plots receiving the high PAM application rate in June, but not in July or August. In contrast, comparisons of the three sampling times by both the fatty acid and Biolog analyses indicated that the microbial metabolic potential present in June were different from those sampled in July and August. Although PAM application to soil or irrigation water in some cases may reduce active bacterial and fungal biomass it does not seem to appreciably affect the soil microbial metabolic potential.