Persistent Effects of Soil Amendments on Microbial Communities in a Dryland Wheat-Fallow Rotation.

Application of diverse carbon amendments for five years affected soil carbon accretion and soil carbon/nitrogen ratios in a Pacific Northwest dryland wheat rotation.  The plots were re-sampled seven years post-amendment to test for long-term effects on microbial communities. Amendments, added at 250 g carbon m^-2 to main plots of continuous fallow or annual winter wheat, included cotton, sucrose, wheat residue, composted wheat residue, Brassica residue, wood sawdust, alfalfa foliage, manure, and dry biosolid.  Fallow main plot amendments also included perennial grass and winter brassica crops.  The microbial communities from the top 10 cm of soil were analyzed for activity related to carbon-cycling (glucosidase, galactosidase), phosphorous-cycling (acid phosphatase), sulfur-cycling (arylsulfatase), and nitrogen cycling (β-glucosaminidase, arylamidase), in addition to fungal and bacterial abundance and diversity.  Although the sum activity was similar between the wheat and fallow main plots, amendment-based differences were observed in the fallow plots in which activity was greater in grass-cropped soil than other amendments excluding alfalfa, manure and compost.  Fungal abundance, based on qPCR analysis, was significantly greater in the wheat compared to fallow main plots. Wood was the only amendment in the wheat main plots that increased the fungal abundance from the no-amendment control; however, both sugar and grass enhanced fungi in the fallow main plots.  No effects were observed for bacterial numbers.  Ordination of the Bray-Curtis diversity indices of T-RFLP data demonstrated that sugar, wood and grass had persisting effects on fungal diversity, and biosolids and brassica cropping affected bacterial communities.  Overall, the results suggest that wood, sugar, biosolid amendment, and growth of different plant species can impart persisting effects on abundance, diversity and activity of soil microbial communities several years after the treatments end.