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

soil to the atmosphere and it is measured at the soil surface (Rolston, 1986). Increases of soil CO2 efflux Little is known about the short-term effects (hours to days) of are not always preceded by increases in soil respiration tillage on labile pools of C and N, or microbial activity and community composition. We examined the effects of rototillage on microbial (Calderón et al., 2000). Tillage is known to induce shortbiomass C (MBC) and N (MBN), respiration (i.e., soil CO2 production lived increases in CO2 efflux from soil (Reicosky and in 1-h incubations), CO2 efflux from the soil surface, inorganic N, Lindstrom, 1993; Reicosky et al., 1995, 1997; Rochette nitrification potential, denitrification rate, and phospholipid fatty acids and Angers, 1999; Ellert and Jantzen, 1999). It has been (PLFA). A fallow silt loam soil was rototilled in the field and soil hypothesized that tillage may expose previously procores were immediately obtained from tilled and adjacent control tected organic matter that may serve as a substrate for soils. The soil cores were then incubated at constant temperature and microbial growth (Rovira and Greacen, 1957). Yet, sampled throughout a 2-wk period. Tilled soil had higher CO2 efflux breaking up the soil structure may reduce or stress mithan the control soil. This increase occurred immediately after tillage crobial populations and consequently reduce respiration and lasted for 4 d. Respiration was similar in both soils until the (Petersen and Klug, 1994; Calderón et al., 2000). Thus, fourth day after tillage, and then declined in the tilled soil. Tilled soil showed increases in MBN, nitrate, and denitrification rates, suggesting although tillage can increase the efflux of CO2 from that tillage makes available previously protected organic N. The overthe soil surface, it may not always stimulate microbial all reduction in respiration together with the lack of response in activity. Degassing of dissolved CO2 from the soil soluMBC, however, suggests that tillage did not make available significant tion and pore space upon soil disturbance and aeration amounts of readily decomposable C. These combined C and N dynammay partially explain the temporary efflux of CO2 after ics suggest that low C/N ratio compounds may have been mineralized tillage (Reicosky et al., 1997). following tillage. Denitrification rates increased in the tilled soil even Changes in microbial activity and nutrient pools may though the bulk of the soil had reduced respiration and bulk density. be associated with simultaneous changes in microbial Tillage caused temporary changes in PLFA profiles, suggesting community structure. Analysis of PLFA is a useful assay changes in soil microbial community structure. Phospholipid fatty because (i) the concentration of total PLFA can be used acid 18:1 v7t, which marks the presence of eubacteria, decreased in the tilled soil. In contrast, 19:0 cy, a marker for anaerobic eubacteria, as an index of viable microbial biomass since phosphoincreased in the tilled soil. Our results show that tillage causes shortlipids are rapidly degraded after cell death, (ii) multivarterm changes in nutrient dynamics that may potentially result in N iate analysis of the PLFA profiles can be used to detect losses through denitrification and nitrate leaching, as well as C losses changes in community composition, and (iii) certain through degassing of dissolved CO2. These changes are accompanied fatty acids may be used as molecular markers for specific by concomitant shifts in microbial community structure, suggesting a taxa and as indicators of microbial stress (Bossio and possible relationship between microbial composition and ecosystem Scow, 1995; White et al., 1996; Vestal and White, 1989). function. The complex interactions between C and N dynamics, microbial activity, and CO2 efflux after tillage deserve further investigation. The purpose of this research was A soils under a long-term regime of frequent to measure the short-term (hours and days) effect of tillage usually suffer from losses in organic matter, field rototillage on respiration (as measured by soil CO2 increased nitrification, and deteriorated soil structure, production in sealed containers), CO2 efflux from the thus reducing agricultural sustainability (Doran, 1982; soil surface, soil inorganic N, denitrification, microbial Elliott, 1986). However, little is known about the shortbiomass C (MBC) and N (MBN) by chloroform fumigaterm effects (hours to days) of tillage on labile C and tion-extraction, and microbial community structure N pools, microbial activity and community structure, (PLFA profiles). We tested the hypothesis that tillage and nitrate that is prone to loss through denitrification makes available previously protected organic matter and leaching. and as a result, causes changes in microbial composition Soil respiration is the production of CO2 by metabolizas well as nutrient mineralization and, as a result, immoing organisms within the soil matrix (Anderson, 1982). bilization by microbes. A bare fallow silt loam soil from In contrast, CO2 efflux is the movement of CO2 from an intensively managed vegetable field was used. Our experimental approach was to till the soil in situ, then Francisco J. Calderón, Edificio Julio Garcia Diaz 102, Departamento obtain soil cores of tilled and control soils, which were de Biologia, Universidad de Puerto Rico, PO Box 23360, San Juan, then sampled and analyzed throughout a 2-wk period PR 00931-3360; Louise E. Jackson, Dep. of Vegetable Crops, Univ. of California, One Shields Avenue, Davis, CA 95616; Kate M. Scow under controlled environmental conditions. and Dennis E. Rolston, Dep. of Land, Air and Water Resources, Univ. of California, One Shields Avenue, Davis, CA 95616. This work MATERIALS AND METHODS was completed while the senior author was at Univ. of CaliforniaDavis. Received 25 Oct. 1999. *Corresponding author (fcaldero@ Site Characteristics goliath.cnnet.clu.edu). The experiment was carried out in April 1998 on an agricultural field in the Salinas Valley, California. The soil was a Published in Soil Sci. Soc. Am. J. 65:118–126 (2001). 118 Published January, 2001