CHANGES IN MINERAL N AND NUMBERS OF BACTERIA AND ACTINOMYCETES DURING TWO YEARS UNDER WHEAT-FALLOW IN SOUTHWESTERN SASKATCHEWAN

Abstract

The aim of this study was to identify, in situ, some of the microbial responses to environmental conditions previously noted in experiments in the laboratory and field. Soil samples were taken from a Brown Chernozem under a wheat-fallow rotation at 2-wk intervals during spring and autumn and at 4-wk intervals in winter and summer for a 2-yr period. Nitrate-N and exchangeable NH₄-N, and numbers of bacteria and actinomycetes by plate count, were measured in 0- to 2.5-cm, 2.5- to 15-cm and 15- to 30-cm soil layers. Changes in microbial numbers and mineral N were correlated with soil depth, available carbon and environmental conditions. Bacterial numbers ranged between 14 and 119 million per gram of soil in the 0- to 2.5-cm layer, between 9 and 47 million in the 2.5-to 15-cm layer and were 4 million in the 15- to 30-cm soil layer. Bacteria:actinomycetes ratios were 3:1 in the 0- to 2.5-cm layer, 2:1 in the 2.5- to 15-cm layer and 1:1 in the 15- to 30-cm layer. Exchangeable NH₄- and NO₃-N as high as 20 and 280 ppm, respectively, were found in the top 2.5 cm. Different processes with similar or opposing effects often occurred simultaneously, thus making interpretation difficult. However, we identified (i) the stepwise nature of the ammonification-nitrification process; (ii) the importance of crop residues in microbial growth, and denitrification; (iii) the flush in microbial growth when a dry soil is moistened; (iv) the importance of the tilled layer as the prime site of soil microbial activity; and (v) the rapid decrease in microbial population and activity below the tilled soil layer. There was also evidence of possible upward movement of NO₃ due to temperature gradient (as soil froze), and due to evaporation.