Nitrogen Fertilizer Effects On Soil Mineralogy in An Agroecosystem.

Nitrogen (N) fertilizer inputs are necessary for crop production in no-till agroecosystems.  Past studies have noted changes in basic soil properties at the soil surface as N fertilizer rates increase under no-till management.  However, the impact of N fertilizer rates on soil mineralogy is unclear.  This study was conducted to examine mineralogy of clay fractions  from surface soil samples under long-term, no-till management subjected to various ammonium nitrate fertilizer rates (0, 168, and 336 kg N/ha yr).  The mineralogy in the control plots (0 kg N/ha yr) consisted of hydroxy-interlayered vermiculite (HIV), illite (IL), kaolinite, quartz, and a mixed-layer mineral.  There was a significant decrease in the integrated peak areas for HIV and IL as applied N fertilizer increased to 168 and 336 kg N/ha yr (p<0.01).  Furthermore, the HIV peak at 1.4 nm failed to collapse upon potassium saturation and heating to 550 degrees Celsius in the 168 and 336 kg N/ha yr treatments.  Persistence of the 1.4 nm peak indicates the presence of an aluminum chlorite-like mineral unique to the clay fraction of soil under these treatments.  This change in soil mineralogy with increasing N fertilizer addition is due, in part, to soil acidification because aluminum chlorite is known to form under acidic conditions and this site has a history of soil acidification.  These results underscore the need to monitor soil minerals in agroecosystems subject to N fertilizer inputs.