Short-term dynamics of nitrogen, microbial activity, and phospholipid fatty acids after tillage

Little is known about the short-term effects (hours to days) of tillage on labile pools of C and N, or microbial activity and community composition. W e examined the effects of rototillage on microbial biomass C (MBC) and N (M BN), respiration (i.e., soil CO2 production in 1-h incubations), CO2 efflux from the soil surface, inorganic N, nitrification potential, denitrificati on rate, and phospholipid fatty acids (PLFA), A fallow silt loam soil was r ototilled in the field and soil cores were immediately obtained from tilled and adjacent control soils. The soil cores were then incubated at constant temperature and sampled throughout a 2-wk period, Tilled soil had higher C O2 efflux than the control soil, This increase occurred immediately after t illage and lasted for 4 d. Respiration was similar in both soils until the fourth day after tillage, and then declined in the tilled soil. Tilled soil showed increases in MEN, nitrate, and denitrification rates, suggesting th at tillage makes available previously protected organic N. The overall redu ction in respiration together with the lack of response in B;IBC, however, suggests that tillage did not make available significant amounts of readily decomposable C. These combined C and N dynamics suggest that low C/N ratio compounds may have been mineralized following tillage. Denitrification rat es increased in the tilled soil even though the bulk of the soil had reduce d respiration and bulk density, Tillage caused temporary changes in PLFA pr ofiles, suggesting changes in soil microbial community structure. Phospholi pid fatty acid 18:1 omega 7t, which marks the presence of eubacteria, decre ased in the tilled soil. In contrast, 19:0 cy, a marker for anaerobic eubac teria, increased in the tilled soil. Our results show that tillage causes s hortterm changes in nutrient dynamics that may potentially result in N loss es through denitrification and nitrate leaching, as well as C losses throug h degassing of dissolved CO2. These changes are accompanied by concomitant shifts in microbial community structure, suggesting a possible relationship between microbial composition and ecosystem function.