Forms of Inorganic Nitrogen.

In the last 20 yr. there have been several notable changes in routine analysis of inorganic N. The first is the widespread use today of autoanalyzers with colorimetric methods for NH₄⁺, NO₂^-, and NO₃^- analysis of soil extracts. These autoanalyzers have largely replaced steam distillation. Similarly, micro-Kjeldahl methods for total soil N have been replaced to a large extent by automated N analyzers (ANA) which use combustion. Micro-diffusions of ¹⁵N-labeled inorganic N have become much more common. Diffusing onto, for example, an acidified glass fiber disk, allows use of ANA-mass spectrometry. Research on extractable soil profile NO₃^- has steadily increased the last 20 yr. Concern for surface and groundwater contamination is driving much of this large body of research, which is only partly reviewed here. Hypoxia in the Gulf of Mexico is blamed, in part, on N from cropped fields in the Mississippi River watershed. One encouraging sign of the last 10-15 yr. is the increasing adoption of the pre-sidedress N test (PSNT) for corn. This was developed in the northeastern U.S.A. but is now being used to some extent in 20 states. The PSNT can predict whether sidedress N fertilizer is needed in 15 to 30-cm tall corn. It works particularly well on manured soils, and as a result minimizes un-necessary N fertilization. Spectral reflectance for assessing need for in-season N by crops is also rapidly expanding. Canopy reflectance will not replace soil NO₃^- tests, but can complement soil testing. Research on emissions of N₂O and NO_x from fertilized and natural soils has increased in volume in the last 20 yr. This reflects newer technology, such as portable chemiluminescence NO_x detectors, and the increasing interest in the contribution of soils and N fertilizers to climate change.