Changes in chemical fractions of nitrogen during incubation of soils with histories of large organic matter increase under pasture

Abstract

A lateritic podzolic soil and a solodized solonetz soil, with histories of organic matter increase under tropical and temperate pastures respectively, were incubated over a period of a year with a weekly wetting and drying cycle. Decreases in organic matter (7 and 11%), total nitrogen (12 and 10%), and non-distillable acid-soluble nitrogen (30 and 25%) occurred, but there were no significant changes in residual nitrogen. Although there were marked increases in mineral nitrogen at the beginning of the incubation in both soils, nitrification was relatively poor in both soils. In a short-term (4 weeks) incubation of a lateritic podzolic soil and a grey clay soil the addition of labelled nitrogen followed by fractionation allowed nitrogen transformations to be examined precisely. Although differences in total nitrogen (labelled + unlabelled) with time were not statistically significant (P = 0.05) in the sandy soil (lateritic podzolic) a loss (28%) of total labelled nitrogen did occur. Parallel with this was an increasing incorporation of labelled nitrogen in all three organic fractions and a disappearance of ammonium nitrogen without the appearance of an equivalent amount of nitrate nitrogen. On the other hand, in the clay soil there was no loss of total labelled nitrogen with time and little incorporation of labelled nitrogen in the organic fractions after the first week. There was little change in the pool of labelled mineral nitrogen between the first and fourth weeks, but virtually complete and quantitative conversion from ammonium nitrogen to nitrate nitrogen. Some of the results point to varying biological lability of chemical fractions of soil nitrogen. These fractions are unlikely to be of much use as indices for sensitive measurement of nitrogen changes in the field, however, possibly because of the continuing accretion and losses which occur in the field.