Nitrogen transformations in calcareous soils amended with pig slurry under aerobic incubation

The mineralization and nitrification processes in pig slurry were investigated in Spain in 1990 in three calcareous soils, differing mainly in content and type of clay, two classified as Typic Calciorthids and one as a Xerollic Calciorthid. Two different amounts of slurry were mixed with the soils, one equivalent to 273 mg N/kg soil and the other equivalent to 546 mg N/kg soil. The mixtures of soil and slurry were aerobically incubated for 16 weeks at 28 °C. A mean initial immobilization of 22·19% of the total-N added was detected in soils treated with the lower rate of slurry. The amount of mineralized N after 112 days of incubation was between 36 and 47% of the total-N added with the lower slurry application to the three soils. The net amount of N mineralized throughout the experiment was therefore within the range 14–23% of the total-N added. The nitrification of the NH4-N added to the soil began after 7 days of incubation, and followed a first-order kinetic model. The mean values of potential nitrate formation were 61 and 51% of the total-N added in soils amended with 273 and 546 mg N as pig slurry per kg soil respectively. The rate constants were higher in the treatments with the lower slurry application (0·043–0·083 per day). Therefore, in these treatments, the nitrification process was faster and total nitrate levels were higher than in soils receiving double that amount of slurry. The NO3-N evolution was parallel to the total-N evolution from the 14th day of incubation onwards. The N-mineralization process was limited by the transformation of the organic-N into ammonium (ammonification).