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CHANGES IN SOIL ORGANIC CARBON AND NITROGEN AFTER SIX YEARS OF CORN PRODUCTION

Corn (Zea mays L.) production practices have significant effects on soil organic nitrogen (N) and carbon (C) levels. The effects of plant population, irrigation, and fertilization rate on soil organic C, total N, and organic N fractions after 3 and 6 years of continuous corn on a Chicot sandy clay loam soil (fine-loamy, mixed, frigid Typic Hapludalf) were studied. Soil organic C content increased by 7% after 3 years of continuous corn and by 18% after 6 years of continuous corn. The C/N ratio increased after the first 3-year period. No change in C/N ratio was found after the second 3-year period. The greater rate of fertilization (400 kg N ha−1, 132 kg P ha−1, 332 kg K ha−1) compared to the normal rate of fertilization (170 kg N ha−1, 44 kg P ha−1, 141 kg K ha−1) had a greater level of soil total N over 3 years. This increased soil total N was still found after 6 years of corn production. After 3 years of corn production, the relative amount of hydrolyzable NH4+-N and amino acid-N in acid hydrolyzates of soil organic N decreased. High rates of fertilization reduced the relative amounts of total hydrolyzable-N and unidentified-N and increased non-hydrolyzable-N compared with the normal fertilizer rate.