108 Influence of Plant Residues on Nitrogen Fixation 



Quantitative bacteriological analyses of these soils (30) showed that 

 treatment with starch or sugar leads to very marked increases of the 

 bacterial flora and that under field conditions these increases may persist 

 for upwards of 8-9 months. Further work in the laboratory confirmed 

 these results and also indicated that this increased growth occurs at the 

 expense of the easily available nitrogen compounds of the soil. When 

 equal quantities of sugar are supplied to two portions of soil, the one 

 stored at 10'' and the other at 30°, the former not only retains the power 

 for denitrification for a longer period, but also tends to have a higher 

 bacterial content — possibly due in part to the absence of competition 

 by nitrogen fixing organisms at this temperature. This is illustrated in 

 the following table. The denitrification power was tested by incubating 

 25 grms. of the respective soils with 50 c.c. of 0-2 per cent, solution of 

 sodium nitrate for 48 hours at 30°. 



Treatment 



Bacteria (millions per grm. soil) 3 weeks 

 after treatment ... 



Nitrate reduction (expressed as ragrm. 

 nitrogen) 3 weeks after treatment ... 



Ammonia and nitrate (pts per million dry 

 soil) 3 months after treatment 



Soil+ 0-2 Soil+ 0-2 Soil + 1-0 



per cent, per cent, per cent 

 Soil sugar sugar sugar 



alone 10° C. 30° C. 30° C. 



12-5 16ie 24-8 61-5 



nil 10-3 8-2 14-6 



30-4 19-2 28-9 7-2 



Thus, under the most favourable conditions for its destruction the 

 presence of 0-2 per cent, sugar confers the power of denitrification on 

 soils, stored at 30°, for more than three weeks, and only after a period 

 of three mouths does the nitrate content of the soil so treated approach 

 that of the untreated. 



The addition of sugar induced similar changes in two other soils, one 

 of wliich possessed a low, and the other a high, initial nitiate content. 

 Treatment resulted in the production of high bacterial numbers and 

 these were accompanied by a marked reduction in the amount of soil 

 nitrates (equal to 46 parts per million in the case of soil II). Not- 

 withstanding these pronounced changes, the accumulation of nitrates 

 again occurred in less than eight weeks at laboratory temperature and 

 the differences in the total ammonia and nitrate content of the untreated 

 and treated at the end of the experiment were in each case less than the 

 losses which were originally occasioned by the addition of sugar. 



