Effect of cropping systems on nitrogen mineralization in soils

Understanding the effect of cropping systems on N mineralization in soils i s crucial for a better assessment of N fertilizer requirements of crops in order to minimize nitrate contamination of surface and groundwater resource s. The effects of crop rotations and N fertilization on N mineralization we re studied in soils from two long-term field experiments at the Northeast R esearch Center and the Clarion-Webster Research Center in Iowa that were in itiated in 1979 and 1954, respectively. Surface soil samples were taken in 1996 from plots of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oa ts (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had rec eived 0 or 180 kg N ha(-1) before corn and an annual application of 20 kg P and 56 kg K ha(-1). N mineralization was studied in leaching columns under aerobic conditions at 30 degrees C for 24 weeks. The results showed that N mineralization was affected by cover crop at the time of sampling. Continu ous soybean decreased, whereas inclusion of meadow increased, the amount of cumulative N mineralized. The mineralizable N pool (N-o,) varied considera bly among the soil samples studied, ranging from 137mg N kg(-1) soil under continuous soybean to >500 mg N kg(-1) soil under meadow-based rotations, s ampled in meadow. The results suggest that the N-o and/or organic N in soil s under meadow-based cropping systems contained a higher proportion of acti ve N fractions.