Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices

Abstract.

During freeze–thaw events, biophysical changes occurring in soils can affect processes such as mineralization, nitrification and denitrification which control inorganic N balances in agro-ecosystems. To evaluate the impact of these climatic events on soil biochemical properties, a study was conducted comparing soil denitrification 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 practices, freeze–thaw events resulted in significant (2–10 times) increases in NH₄ ⁺-N, NO₃ ^–-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.