Nitrogen mineralisation in fallow and wheat soils under field and laboratory conditions

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Abstract
Net soil nitrogen (N) mineralisation and its implications in soil under fallow and wheat were investigated in field and pot experiments. Wheat significantly enhanced net soil N mineralisation compared with that under fallow. In the field, mineralisation depended on the stage of wheat growth, beginning only after sufficient root development had occurred, and continuing to the booting stage. However, it was not related to differences in soil heat accumulation and moisture content between fallow and wheat. Soil ammonium-N (NH4-N) and nitrate-N (N03-N) levels under fallow followed a seasonal pattern, decreasing over the winter and early spring, and increasing over late spring and summer. There was no apparent lag in net soil N mineralisation under fallow in the spring compared with that under wheat. Total amounts of soil N mineralised over 9 months were 3.3 and 2.0% under fallow and wheat respectively. Mean rate of nitrification increased with increasing depth of soil sampled under fallow. Nitrifier numbers did not change over the same sampling period when large changes in NH4-N and NO3-N occurred, and thus nitrifier numbers do not provide a reliable estimate of NH4-N formation. In growth cabinets, the response of microbial populations to some moisture treatments differed in soil under fallow and wheat. Increases in daytime temperature of 7°c from 17°c did not affect net soil N mineralisation. Under fallow, different sequences of fluctuating temperatures also showed no effect.