Influence of Crop Growth on Mineralization of Nitrogen in the Soil

M of nitrogen under crops appears to differ in rate from that in fallow soil. There is conflicting evidence on whether the rate under crops is more or less rapid, and it also remains unknown whether the rate under crops is constant or variable at different stages of crop growth. Moreover, it has been claimed that cropping may result in ,a loss of mineral nitrogen not entirely accounted for by that taken up by the crop or by changes in the organic nitrogen content of the soil. Lyon, Bizzell, and Wilson (10, 11, 12, 13), in experiments with oats and maize, demonstrated that the accumulation of nitrate under these crops was less than in corresponding fallow soil, even after allowance had been made for the nitrogen taken up by the crop. Prescott (22) verified these observations in similar experiments with maize and wheat. Mineral nitrogen formed in the cropped soils was approximately half that formed in the fallow soils. It was not determined whether there was an actual depression of mineralization or an apparent one caused by denitrification and loss of gaseous nitrogen to the air. Pinck, Allison, and Gaddy (21) believed that there may be some loss of nitrogen due to metabolic processes occurring within growing plants. Not all workers have agreed that crops depress mineralization. Lohnis (9) and Greaves, Stewart, and Hirst (5) believed that plant growth favored mineralization of nitrogen. Lyon and co-workers' (W, 11, 12, 13) believed that maize might stimulate mineralization in the earlier stages of growth. Starkey (24) found that nitrates accumulated in soils taken from near the roots more rapidly than in soils taken some distance away. He concluded that cropped soils have a greater nitrifying capacity than do fallow soils. However, soils taken from near the roots may contain more root material than those taken some distance away, and consequently the release of mineral nitrogen would be greater from soil taken from within the root zone simply because such soil contains more readily decomposable material. The results obtained would not give true estimates of the nitrifying capacities of the soils under growing crops. In general the rate of mineralization of nitrogen in soils to which plant materials have been added depends upon the carbon/nitrogen ratio of the added material (16, 19). Numerous workers have noted the greater accumulation of nitrate nitrogen after the growth of legumes than after non-legumes. It has been suggested that increased release of mineral nitrogen under legumes is due to the relatively higher nitrogen content of the v material introduced into the soil by the roots of legumes (13).