Short‐Term Dynamics of Nitrogen, Microbial Activity, and Phospholipid Fatty Acids after Tillage

Abstract

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. We examined the effects of rototillage on microbial biomass C (MBC) and N (MBN), respiration (i.e., soil CO2 production in 1-h incubations), CO2 efflux from the soil surface, inorganic N, nitrification potential, denitrification rate, and phospholipid fatty acids (PLFA). A fallow silt loam soil was rototilled 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 CO2 efflux than the control soil. This increase occurred immediately after tillage 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 MBN, nitrate, and denitrification rates, suggesting that tillage makes available previously protected organic N. The overall reduction in respiration together with the lack of response in MBC, 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 rates increased in the tilled soil even though the bulk of the soil had reduced respiration and bulk density. Tillage caused temporary changes in PLFA profiles, suggesting changes in soil microbial community structure. Phospholipid fatty acid 18:1 ω7t, which marks the presence of eubacteria, decreased in the tilled soil. In contrast, 19:0 cy, a marker for anaerobic eubacteria, increased in the tilled soil. Our results show that tillage causes short-term changes in nutrient dynamics that may potentially result in N losses through denitrification and nitrate leaching, as well as C losses through 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.