What Happens to Fertilizer Nitrogen

Only complete denitrification returning N2 to the atmosphere has no unwelcome environmental consequences, but it can be a significant source of economic losses because of an excessive reduction of nitrates that would otherwise be available to soils. High denitrification rates are favored by high temperature, high soil water 

Leaching rates depend on levels of fertilization, compounds used (NH3 leaches very little in comparison to readily soluble NO3), soil thickness and permeability, temperature, and precipitation. The single most important land management factor 

Recent concerns about soil erosion and its effects on crop productivity have not been matched by reliable information abont the actnal extent and intensity of the process or, more accurately, about soil losses in excess of natural denudation. Quantification of these impacts is complicated by the fact that in some areas much of the eroded soil is not lost to food production very littie or no eroded sediment leaves the fields in areas with gentle relief without any major surface oudet, and when the sediment leaves the land most it may be deposited downstream as colluvium or alluvium.  

Neither translocation to roots nor root exudates explain the loss. A major part of the losses must be due to the shedding of various plant parts (pollen, flowers, leaves), leaching of nitrogen from aging leaves, and to herbivory these losses (except for windborne pollen) are mostly internal redistributions of the nutrient, as the litter, leaching, and herbivory return the nutrient to soils. But most of the nitrogen lost from tops of plants is due to volatilization of NHs.  

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A rather complete picture of what happens to fertilizer nitrogen added to cropped soil in greenhouse pots is shown in Table 23.11, taken from the data of Legg and Allison (1959, 1960). Immobilization of added tracer nitrogen increased 4—5 times as the nitrogen addition increased 16-fold. Soil nitrogen mineralization (column 9) was essentially a constant as fertilizer nitrogen additions were increased. 

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