Dynamics of a Soil Microbial Community under Spring Wheat

Bossio et al., 1999). Microorganisms are intimately associated with their physical and chemical environment, In arable systems, seasonal fluctuations of microbiological properand it is therefore conceivable that the temporal dynamties can be significant. We hypothesized that adaptation to soil environmental conditions may contribute to the variation observed, and ics observed will partly reflect adaptation to environthis was examined by characterization of different microbial commumental variables rather than competition between difnity attributes under a range of soil conditions. Soil was sampled from ferent components of the microbial biomass. Many no-till and chisel-tilled fields within a long-term experiment in eastern aspects of soil microbial communities are affected by Washington during growth of spring wheat (Triticum aestivum ). The prevailing conditions with respect to substrate quality range of soil environmental conditions covered was extended by and stresses. For example, the C/N/P ratio can vary with amendment of crop residues. Soil samples were characterized with nutrient availability (Srivastava et al., 1989; Drury et respect to biomass N and biomass P, substrate utilization dynamics, al., 1991), membrane lipid composition can vary with phospholipid fatty acid (PLFA) profiles and whole-soil fatty acid temperature (Petersen and Klug, 1994; Navarrete et al., (MIDI-FA) profiles, and with respect to soil environmental variables 2000) and substrate utilization patterns can vary with the (bulk density, soil organic C [SOC], temperature, moisture, and inorganic N and P). Bacterial and fungal lipid biomarkers were negatively number of active organisms (Winding and Hendriksen, correlated (P 0.001), confirming that these subsets of fatty acids 1997). Effects of adaptation should be distinguished are associated with contrasting components of the microbial biomass. from successional changes when the response of microBiomass N was closely associated with soil conditions, notably N bial communities to management or seasonal changes availability. The proportion of substrates used with no apparent lag is interpreted. phase decreased during summer and was negatively correlated with The present study described different aspects of a soil lipid stress indicators. Cyclopropyl fatty acids accounted for more microbial community and possible effects of soil enthan 60% of the variation in bacterial PLFA. These observations vironmental conditions on these microbiological propsuggest that adaptation to environmental stresses was partly responsierties across a growing season of spring wheat. The ble for the microbial dynamics observed. Tillage practice had little range of soil conditions was extended by sampling from effect on the relationships between soil conditions and microbiological properties. The results showed that MIDI-FA included a significant field plots under no-till and chisel till, respectively, and background of nonmicrobial material and was less sensitive to soil with or without residues at the soil surface. Reduced environmental conditions than PLFA. tillage frequently increases soil compaction (Mielke et al., 1986) and will delay the turnover of soil organic matter (Papendick and Moldenhauer, 1995), while crop T microbial community of arable soils is a comporesidues can modify the soil environment by reflecting nent with multiple functions of importance to soil irradiation, and by creating a boundary layer of stagnant fertility, such as catalysis of nutrient transformations, air (Hammel, 1996). (temporary) storage of nutrients, formation and stabilization of soil structure, and control of plant pathogens MATERIALS AND METHODS (Anderson and Domsch 1989; Oades, 1993; Smith, 1994; The study site was the Palouse Conservation Field Station Kennedy and Papendick, 1995). Activity and growth near Pullman, WA. The soil was a Palouse silt loam (fineof microorganisms is restricted by soil environmental silty, mixed mesic, Pachic Ultic Haploxerolls) (Myrold et al., conditions, e.g., temperature, moisture, pore-size distri1981). In the experimental year (1998), May and June were bution and nutrient availability and therefore, indislightly cooler, and July and August were considerably warmer rectly, by cultivation practices. than normal. Precipitation was close to normal (annual precipSeasonal fluctuations of microbial characteristics can itation is 500–550 mm). For this study, two fields from a longbe comparable with, or larger than effects of manageterm experiment under no-till management and chisel tilled ment practices such as spring burn and N fertilization to a 10-cm depth, respectively, were selected. Both had been under spring wheat or winter wheat for the past 20 yr, and 3 Mg (Ajwa et al., 1999), fertilization and drainage (Bardgett ha 1 straw had been returned to the soil each fall. Tillage took et al., 1999), or organic vs. conventional farming systems place 1 wk prior to fertilization (N/P/S at a rate of 16:20:14 S.O. Petersen, Danish Institute of Agricultural Sciences, Dept. Crop Physiology and Soil Science, P.O. Box 50, DK-8830 Tjele; P.S. Frohne Abbreviations: 16:1 7t/c, trans/cis ratio of hexadecenoic acid; cy17:0/ and A.C. Kennedy, USDA-ARS, Land Management and Water Con16:1 7c and cy19:0/18:1 7c, cyclopropyl/precursor ratios; 18:2 6c, liservation Unit, 215 Johnson Hall,Washington State University, Pullnoleic acid, a fungal biomarker; bactPLFA, a subset of PLFA found man, WA 99164-6421. Received 23 June 2000. *Corresponding author in prokaryotes only; MIDI-FA, fatty acids extracted by the MIDI ([email protected]). procedure; PLFA, phospholipid fatty acids; SOC, soil organic C; T, diurnal temperature range at time of sampling. Published in Soil Sci. Soc. Am. J. 66:826–833 (2002).