Soil amendments yield persisting effects on the microbial communities-a 7-year study

Soil microbial communities are sensitive to carbon amendments and largely control the decomposition and accumulation of soil organic matter. In this study, we evaluated whether the type of carbon amendment applied to wheat-cropped or fallow soil imparted lasting effects on the microbial community with detectable differences in activity, population size, or community structure after a period of seven years post-amendment. The microbial communities from the top 10 cm of soil were analyzed for activity related to C-cycling (glucosidase, galactosidase), P-cycling (acid phosphatase), S-cycling (arylsulfatase), and N-cycling (β-glucosaminidase, arylamidase), in addition to fungal and bacterial abundance and structure. The amendments were applied at similar carbon rates for five years under annual wheat or continuous fallow and included cotton linters, sucrose, wheat residue, composted wheat residue, brassica residue, wood sawdust, alfalfa feed pellets, manure, biosolid and a no treatment control. Two crops, brassica and grass, were in the fallow treatments. The majority of the communities in the amended soils were not distinguishable from the no-treatment control. For amendments and crops that produced changes, significant differences in the population size and community structure were observable for fungi but not bacteria. Wood, sugar, and grass cropping produced the most pronounced effects on enzyme activity, fungal abundance and structure. Overall, the species of the planted crop had a significant effect on the soil enzyme activity and population size of fungi, with the greatest values under grass compared to wheat or brassica. The microbial communities were differentially affected by C source amendments in which the persistency of change and the aspect of the community affected (i.e. function, size, structure, kingdom) were dependent on amendment type.