Ball milling pretreatment affects the content of fixed ammonium in soils in response to the content of exchangeable ammonium

Abstract

Analysis of fixed ammonium (NH4+) in soils by the most common method requires the subsample to be ground to pass through a 100-mesh sieve. We tested a hypothesis that the ball milling pretreatment for the analysis of fixed NH4+ may influence the analytical results depending on the content of exchangeable NH4+. Five paddy soils collected from the surface layer in Japan were treated with 15N-labeled ammonium sulfate at 0, 100, 500, or 1000 mg N kg−1, incubated aerobically at 30°C for 7 days, and air-dried. Part of each soil was then ground in a vibration ball mill at 1200 rpm for 10 min as a pretreatment (BMP treatment). The samples with and without the BMP treatment were analyzed for exchangeable NH4+ and total fixed NH4+. The samples that received 0 or 1000 mg N kg−1 were also analyzed for nonexchangeable 15N (total fixed 15NH4+ plus organic 15N) and hot KCl-extractable organic N. At 0 mg N kg−1, the BMP treatment significantly decreased total fixed NH4+ in all soils and significantly increased exchangeable NH4+ in two of the five soils. The fixation of exchangeable NH4+ started to occur when its content is greater than around 300 mg N kg−1, and its ratio to total fixed NH4+ is greater than around 1.0. At 1000 mg N kg−1, the BMP treatment significantly decreased exchangeable NH4+ and significantly increased total fixed NH4+ in all soils especially two clayey, vermiculitic soils. On the average of all soils applied 1000 mg N kg−1, the increase in total fixed NH4+ explained 55% of the decrease in exchangeable NH4+, and the decrease in exchangeable NH4+ was almost equal to the increase in nonexchangeable 15N. The rest of the NH4+ unaccounted by the fixation was partly due to the transformation of exchangeable NH4+ to hot KCl-extractable organic N, because the content of hot KCl-extractable organic N was significantly increased by the BMP treatment. In conclusion, the milling treatment prior to the analysis of fixed NH4+ in soils might cause either underestimation or overestimation depending on the content of exchangeable NH4+.