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a Danish Institute of Agricultural Sciences, Dept. Crop Physiology and Soil Science, P.O. Box 50, DK-8830 Tjele
b USDA-ARS, Land Management and Water Conservation Unit, 215 Johnson Hall,Washington State University, Pullman, WA 99164-6421
* Corresponding author (soren.o.petersen{at}agrsci.dk)
In arable systems, seasonal fluctuations of microbiological properties can be significant. We hypothesized that adaptation to soil environmental conditions may contribute to the variation observed, and this was examined by characterization of different microbial community attributes under a range of soil conditions. Soil was sampled from no-till and chisel-tilled fields within a long-term experiment in eastern Washington during growth of spring wheat (Triticum aestivum). The range of soil environmental conditions covered was extended by amendment of crop residues. Soil samples were characterized with respect to biomass N and biomass P, substrate utilization dynamics, phospholipid fatty acid (PLFA) profiles and whole-soil fatty acid (MIDI-FA) profiles, and with respect to soil environmental variables (bulk density, soil organic C [SOC], temperature, moisture, and inorganic N and P). Bacterial and fungal lipid biomarkers were negatively correlated (P < 0.001), confirming that these subsets of fatty acids are associated with contrasting components of the microbial biomass. Biomass N was closely associated with soil conditions, notably N availability. The proportion of substrates used with no apparent lag phase decreased during summer and was negatively correlated with lipid stress indicators. Cyclopropyl fatty acids accounted for more than 60% of the variation in bacterial PLFA. These observations suggest that adaptation to environmental stresses was partly responsible for the microbial dynamics observed. Tillage practice had little effect on the relationships between soil conditions and microbiological properties. The results showed that MIDI-FA included a significant background of nonmicrobial material and was less sensitive to soil environmental conditions than PLFA.
Abbreviations: 16:1
7t/c, trans/cis ratio of hexadecenoic acid • cy17:0/16:17c and cy19:0/18:17c, cyclopropyl/precursor ratios • 18:26c, linoleic acid, a fungal biomarker • bactPLFA, a subset of PLFA found in prokaryotes only • MIDI-FA, fatty acids extracted by the MIDI procedure • PLFA, phospholipid fatty acids • SOC, soil organic C • 
T, diurnal temperature range at time of sampling
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