Abstract During freeze–thaw events, biophysical changes occurring in soils can affect processes such as mineral- ization, nitrification and denitrification which control in- organic N balances in agro-ecosystems. To evaluate the impact of these climatic events on soil biochemical prop- erties, a study was conducted comparing soil denitrifica- tion enzyme activity (DEA), dissolved organic C (DOC) and inorganic N levels before and after the winter season in plots under: (1) continuous corn (Zea mays L.) (CC) with annual chisel plow and disking, (2) corn–soybean (Glycine max L.) (CS) rotation with chisel plow every other year prior to planting soybean, and (3) corn– soybean–wheat (Triticum aestivum L.)/hairy vetch (Vicia villosa Roth) (CSW-V) with ridge tillage during the corn and soybean crops, and dairy manure application during the corn year. Soil cores were collected in late autumn and immediately after spring thaw at 0–5, 5–10, 10–15, and 15–30 cm depths. Regardless of management prac- tices, freeze–thaw events resulted in significant (2–10 times) increases in NH 4 + -N, NO 3 – -N (P<0.001) and DOC (P<0.01) levels at all soil depths. Following freeze–thaw, DEA remained unchanged in the 5–30 cm depth but dropped significantly (P<0.01) in the 0–5 cm soil layer. In that layer, soils which had been chisel plowed during the previous growing season lost 78–84% of the DEA recorded during the fall, whereas in the plots amended with manure during the previous season, the loss of activity was 40–45%. These data indicate that frequent tillage, compared with manure additions, is more conducive to overwinter loss of DEA in surface layers of soils subject to freeze–thaw cycles. Keywords Denitrification · Freeze–thaw · Tillage · Nitrate · Dissolved organic C Introduction During the winter months, temperate region soils under- go important biophysical changes brought about by freeze–thaw events. These changes include disruption of soil aggregates (Bullock et al. 1988) and the release of aggregate-protected organic C (Edwards and Cresser 1992) resulting in increased availability of substrate and enhanced microbial activity in soils. Soil denitrification is a microbially-mediated anaerobic process regulated by various factors including organic C and nitrate (NO 3 – -N) availability, denitrifier population, redox status and tem- perature. The intensity of each of these factors changes both with time and space, hence the highly variable char- acter of the soil denitrification process (Parkin 1987; van Kessel et al. 1993). Increased availability of substrates resulting from soil freezing could stimulate the activity of denitrifiers. This view is supported by reports of large and often ephemeral fluxes of nitrous oxide (N 2 O) emis- sion at spring thaw (Christensen and Tiedje 1990; Burton and Beauchamp 1994). Several studies (Roth and Fox 1990; Heaney et al. 1992; McCracken et al. 1994; Ryan et al. 2000) have shown significant increases in soil mineral N between the fall and winter seasons, but these N gains tend to quickly disappear at the onset of the spring season. Early spring N loss has been attributed to leaching (Campbell et al. 1984; Liang et al. 1991) and to denitrification (Heaney et al. 1992; Rover et al. 1998). While reports of early spring bursts of N 2 O from soils (Christensen and Tiedje 1990; Burton and Beauchamp 1994) and occur- rence of denitrification at low and sub-zero temperatures (Dorland and Beauchamp 1991; Rover et al. 1998) sug- gest that denitrification is an important mechanism for P.-A. Jacinthe ( ✉ ) · W.A. Dick School of Natural Resources, The Ohio State University, Wooster, OH 44691, USA e-mail: jacinthe.1@osu.edu Tel.: +1-614-2925678, Fax: +1-614-2927432 L.B. Owens North Appalachian Experimental Watershed, USDA-ARS, Coshocton, OH 43812, USA Present address: P.-A. Jacinthe, School of Natural Resources, The Ohio State University, 210 Kottman Hall, Columbus, OH 43210, USA Biol Fertil Soils (2002) 36:1–9 DOI 10.1007/s00374-002-0499-8 ORIGINAL PAPER P.-A. Jacinthe · W. A. Dick · L. B. Owens Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices Received: 4 May 2001 / Accepted: 9 April 2002 / Published online: 25 June 2002 © Springer-Verlag 2002