Microbiological activity and N transformations in a soil subjected to aggregate extraction amended with pig slurry

Pig slurry as a fertilizer source has been extensively used in agriculture; however, in order to reduce the risks of contaminating the water supplies given its high level of N sources, it is important to understand the N transformations occurring in the soil where it is applied. In this study, incubations were performed at 25 °C for a period of 63 to 73 d to evaluate the effect of different doses of pig slurry on the global microbiological activity and the N dynamics in time, with an emphasis on N mineralization and nitrification in a soil subject to aggregate extraction. The slurry was applied in doses equivalent to: 0, 162, 244, and 325 m³ ha^-1, constituting four treatments: T0, T50, T75, and T100, respectively. The microbiological activity and the contents of NH₄ ⁺-N and NO3 ^--N were measured. Increasing doses of slurry produced an increase in the evolution of the accumulated CO₂, with 63.5, 115.0, 112.7, and 125.7 mg 100 g^-1 soil for T0, T50, T75, and T100 respectively. A similar situation was observed in the initial contents of NH₄ ⁺-N, which were 22.4, 30.3, 44.3, and 60.7 mg kg^-1 in each treatment, respectively. On the other hand, the increase in NO3 ^--N contents were only noticed by the end of the incubation period and corresponded to 28.6, 69.0, 95.3, and 109.8 mg kg^-1. In addition, the net N mineralization was predominant in all treatments with slurry during the measurement period, being 9.1, 45.4, 58.1, and 52.7 mg kg^-1 for T0, T50, T75 and T100, respectively, at the end of the trial. The mineralization rate of the organic C decreased when increasing the dose of slurry and the mineralization rate of the organic N resulted to be low, which would indicate a high contribution of material resistant to degradation by the slurry, which could have a long term effect in the soil.

Keywords
Degraded soil; microbial activity; N mineralization; N immobilization; pig slurry