Nonsymbiotic nitrogen fixation in soils of a semi-arid region of North China

Studies have been conducted in a semi-arid part of North China having conditions generally considered to be very favorable for an active nonsymbiotic nitrogen fixation to determine to what extent nitrogen is added to soils in this region by this process. The response of field crops to applications of nitrogenous fertilizers was tested in 17 field experiments. The yield of intertilled summer crops was increased by fertilization in a majority of cases by 20 to 30%. Increases of the grain yield of winter wheat did not exceed .5% in any of the five experiments in which this crop was grown. The changes in total nitrogen content of a soil over a period of years of cropping was determined in an experiment using small artificial plots cropped for six successive seasons without the addition of nitrogeneous materials. Results showed that the soil underwent a definite loss of nitrogen and that the amount so lost was approximately the same as that removed in the crops harvested. In a second experiment designed to measure the amount of nitrogen added to the soil by fixation, soil was placed in large pots sunk in the ground, and a nitrogen balance was drawn up after treatment and cropping for 2 years. In one of the three series of this experiment a definite unaccounted for loss of nitrogen occurred. In no case was a positive gain by fixation shown. In laboratory tests it was found that a greater number of macroscopic colonies of Azotobacter appeared on soil plaques of Honeoye silt loam and Dunkirk silty clay loam than on those made with China soils; but nitrogen was found to be fixed at an appreciably more rapid rate by the last-mentioned soil than by the other two in tests with soil in tumblers and in solution cultures. It is doubted whether conclusions can be drawn from this work which will apply to all the common conditions of cropping and management of this soil. It seems to be indicated, however, that the summer crops grown on soils not receiving fertilizer additions, or with additions only of high carbonaceous organic matter, the amount of nitrogen added by nonsymbiotic fixation is not sufficient to constitute all appreciable part of that required by the growing crop.