ABSTRACT
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Nitrogen in Intensive Agricultural Systems . . . . . . . . . . . . . . . . . . . . 228
Nitrogen Inputs and Transformations in Conventional Intensive Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Total and Available Soil Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Nitrogen Inputs into Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 Biological Nitrogen Fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Mineralization of Soil Organic Nitrogen. . . . . . . . . . . . . . . . . . . . . . . 234 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
Nitrogen Losses from Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Denitrification and Nitrate Reduction. . . . . . . . . . . . . . . . . . . . . . . . . 237 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Ammonia Volatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Leaching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Economic and Environmental Consequences of Nitrogen
Losses from Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Nitrogen Use Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Improved Nitrogen Use Efficiency Practices . . . . . . . . . . . . . . . . . . . 244
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
INTRODUCTION
In agroecosystems, management is generally aimed at optimizing the nutrient economy of planted crops to achieve maximum yield. Tivy (1992) maintained that the yield of agricultural crops is a function of a more complex set of interacting variables than is primary biological production in natural (unmanaged) ecosystems. These include (1) the environmental conditions under which the crop is grown, (2) yield potential of the particular crop and cultivar, and (3) management of the crop and its soil environment. Of these variables, some from the first two groups represent the ultimate determinants of maximum biological primary productivity, while the third group comprises factors which can be regarded as powerful tools for manipulating (increasing) commercial or utilizable crop yield.
