Organic N mineralization and transformations in soils treated with animal waste in relation to carbonate dissolution and precipitation

Addition of organic residues and their composts to enhance soil fertility results in organic N mineralization and transformations that may have profound effects on chemical processes. Our hypothesis is that carbonate precipitation and dissolution reactions are driven by proton consumption and release by organic N mineralization and nitrification. In the present study we evaluated the role of N-rich animal waste on carbonate dissolution/precipitation in a range of soil types. To discriminate the effects of organic C mineralization from nitrification we added a nitrification inhibitor, dicyandiamide (DCD), which slows NH4+ oxidation without interfering with soil C respiration. Calcareous and non-calcareous soils were incubated with a pasteurized chicken manure (PCM) amendment with and without nitrification inhibitor DCD and control soil with and without DCD. Soil solution pH, extractable NH4+ and NO3−, CO2 emission and the concentrations of major cations in soil solution were determined. PCM application without DCD resulted in a pH increase in the first few days of the incubation followed by a pH decrease. DCD application effectively inhibited the nitrification process and the reduction in soil pH. The high correlation between nitrification and the reduction in pH indicated that the nitrification process was the main source of protons released, which led to carbonate dissolution and CO2 emission from CaCO3. Application of DCD to PCM reduced CO2 emission in PCM amended soils, especially in the earlier part of the incubation of calcareous soils. Furthermore, we found a linear 0.7:1 relation between the sum of cations (mostly Ca2 + plus Mg2 +) released to solution due to proton production by organic N mineralization and nitrification that occurred in all soils with PCM applied, without DCD. Whereas, application of DCD to the PCM treatment inhibited nitrification and there was little or no proton production.