INTRODUCTION

Nitrogen is unique among the essential nutrients of higher plants in terms of its roles in biological systems and its complex cycling. In addition, it is the element most commonly limiting to crop production and the one most demanding of management skills. Soil organic matter has a pivotal role in cycling nitrogen contents of crop residues and animal manures to the mineral forms that are used by higher plants. In Chapter 7, we found that the level of organic matter reflects the relative rates of carbon and nitrogen inputs and decomposition on the one hand and mineralization on the other. This chapter is concerned with several microbiological and physical processes important in nitrogen cycling and management of nitrogen in agriculture.

THE NITROGEN CYCLE

Oxidation–reduction states of nitrogen

Nitrogen is present in soil–plant systems at stable oxidation states ranging from + 5 (oxidized) to – 3 (reduced). Arrows in Fig. 8.1 indicate the transformations, most of which require biological catalysis, that occur in nitrogen cycles. Nitrogen in soil organic matter and in proteins and nucleic acids of living organisms is reduced at the – 3 level. Most plants produce protein and other reduced nitrogen compounds beginning with mineral nitrogen absorbed from soil as nitrate (+ 5) or ammonium (– 3) ions. Ammonium ions enter directly into biosyntheses of amino acids and other compounds while nitrate must first be reduced to that level (nitrate reduction) through the addition of 8 electrons.