Effects of polyacrylamide and organic matter on microbes associated to soil aggregation of Norfolk loamy sand

Polyacrylamide (PAM) has been reported to increase aggregation and improve soil physical properties in loamy sand soils, but nothing is known about the effects of PAM on microbes involved in aggregate formation. We studied the effects of PAM (0, 30 and 120 mg kg−1) and organic matter (wheat [Triticum aestivum] and pecan [Carya illinoinensis]) incubated in a Norfolk soil (a blend of 90% E horizon and 10% Ap horizon) for 96 days at 10% water content on microorganisms that have the ability to aggregate soil. We used an in vitro soil sedimentation assay to test the aggregative ability of the predominant, heterotrophic bacteria from the culturable portion of microaggregates (0.25-0.05 mm), fatty acid methyl ester (FAME) profiling to identify the isolates, and DNA sequencing to find their position in a phylogeny with known taxa. Among all the identified bacterial species, 19 were soil aggregators. There was a PAM effect across all residue types indicating that the highest proportion of soil aggregating isolates was at 120 mg kg−1 PAM. This suggests that soil conditions created by the addition of 120 mg kg−1 PAM favored the growth of bacteria functioning as soil aggregators. There was evidence of interaction between PAM and residue type. Differences were found in the PAM effect but only when no residue was added. The amount of soil aggregating basidiomycete fungi from the different aggregate size fractions was also determined using an enzyme-linked immunosorbent assay (ELISA). There was a higher amount of soil aggregating basidiomycete fungi in macroaggregates (2.00-1.00 and 1.00-0.50 mm classes) generated from PAM and/or wheat residue amended soil compared to the control soil, but no difference was found in treatments with pecan residue added. This suggests that PAM and PAM with wheat can be utilized as a source of nutrient for the basidiomycetes, but pecan appeared to inhibit their growth. This study is the first to provide evidence that adding PAM to soil favored the growth and survival of specific fungi and bacterial species functioning as soil aggregators in vitro.