Nitrogen-Inorganic Forms

Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NOz) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH20H), hyponitrous acid (H2N20 2), and nitramide (NH2N02), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g., Rodrigues, 1954; Dhariwal & Stevenson, 1958; Stevenson & Dhariwal, 1959; Bremner & Harada, 1959; Bremner, 1959; Schachtschabel, 1960, 1961; Young, 1962), and by estimates that the proportion of soil N in this form can exceed 50% for some subsurface soils (Stevenson & Dhariwal, 1959; Young, 1962). In such cases, NHt is said to be fixed, and fixed NHt has subsequently been defined as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), such as 1 or 2 M KCI or 0.5 M K2S04, in contrast to exchangeable NHt, which is extractable at room temperature with such a solution. Existing information indicates that fixed NHt occurs largely, if not entirely, between the layers of 2: I-type clay minerals, particularly vermiculite and illite (hydrous mica), and that fixation results from entrapment of NHt in ditrigonal voids in the exposed surfaces upon contraction of the clay lattice (Nommik & Vahtras, 1982). The term, nonexchangeable NHt, has been used by Bremner (1965) and Keeney and Nelson (1982) in previous editions of this publication as a more precise alternative to fixed NHt. The same term is used in the present treatment, with specific reference to NHt determined by the method described in "Determination of Nonexchangeable Ammonium," which involves digestion with an HF-HCI solution following treatment of the soil with alkaline KOBr to remove exchangeable NHt and labile organic-N compounds.