Does ammonium-based N addition influence nitrification and acidification in humid subtropical soils of China?

The impacts of ammonium-based N ( NH4+ - N) addition on soil nitrification and acidification were investigated in terms of kinetic mechanisms and major factors controlling these soil processes for terrestrial ecosystems in subtropical China. Soil samples were collected from an upland soil derived from a sandstone parent ( SU), a brush-land soil from a granite ( GB), and a forest soil from a quaternary red earth ( QF) in a typical subtropical region of China. The samples were incubated at 30 degrees C with soil moisture content of 60% water holding capacity ( WHC) for 35 days, after adding ammonium sulphate, urea, and ammonium bicarbonate at rates of 0, 100, and 250 mg N kg(-1), respectively. Nitrification in SU soil ( pH 6.27) followed a first-order reaction model ( P < 0.001). Addition of ammonium sulphate, urea and ammonium bicarbonate significantly ( P < 0.05) stimulated nitrification. As a result, the soil was significantly acidified ( P < 0.05) and the soil pH at the end of incubation decreased with increasing N addition. In contrast, nitrification in QF ( pH 4.46) and GB ( pH 4.82) soils followed a zero-order reaction model ( P < 0.001) and hence the addition of NH4+ did not directly affect soil nitrification. However, the chemical input directly changed initial pH of GB and QF soils, resulting in either a decrease or an increase in NO3- production, dependent on the impact of the chemicals applied. At the end of incubation, the pH of QF and GB soils was significantly higher ( P < 0.05) in treatments with NH4+-input than without NH4+ - input. These results indicated that for some acid soils nitrification was not controlled by available NH4+ - N and that NH4+-N-input was not necessary to stimulate soil nitrification. And so no acceleration of soil acidification occurred. In order to characterize nitrification intensity in these humid soils and its effect on acidification, nitrification without N-amendment is a better indicator than with N-amendment.