What governs nitrogen loss from forest soils?

Abstract

The predominance of organic nitrogen in stream waters and soil solutions is no proof of plant uptake of organic nitrogen, and could indeed be brought about by the uptake of only inorganic nitrogen, as Addiscott and Brookes claim and standard thinking would have it. Nor did I suggest otherwise¹. Yet I maintain that “some standard thinking about how nature deals with nitrogen in soils and waters needs to be re-evaluated”.

van Breemen replies

Standard thinking is best summarized by published diagrams of the terrestrial nitrogen cycle — with one exception² that I know of, such representations in recent soil-science textbooks^3,4,5,6 ignore two features of the nitrogen cycle that have come to light: dissolved organic nitrogen as a potentially important loss term for soil nitrogen⁷, and the apparently widespread ability of plants (including crop plants) to take up dissolved organic nitrogen^8,9.

Addiscott and Brookes suggest that dissolved organic nitrogen reaching stream water is rather inert. Maybe so, but it has hitherto been largely ignored and we know little about it. The free amino acids present in low concentrations in soil and stream waters probably reflect a small, dynamic pool⁸ on the way from a large pool of dissolved high-molecular-mass organic nitrogen to microorganisms, plants or ammonium. Plants might get a better share of that pool than we once thought.

Editorial note: See also addendum from S. S. Perakis and L. O. Hedin on page 665 of this issue.