Interactions between fertilizer nitrogen and soil nitrogen—the so‐called ‘priming’ effect

TLDR

The priming effect, or added nitrogen interaction (ANI), refers to the observation that plants receiving fertilizer nitrogen often absorb more soil nitrogen than those that do not, and ANIs can be real—due to increased root exploration—or apparent, arising from processes such as pool substitution, microbial immobilization, denitrification, plant uptake, or isotope displacement. This paper theoretically examines how ANIs influence the interpretation of 15 N‑labelled fertilizer experiments. The authors use a simple model of 15 N‑labelled fertilizer uptake to relate ANIs, A values, and fertilizer uptake efficiencies. The model shows that a positive apparent ANI alters the A value as fertilizer rates rise and makes 15 N‑based uptake efficiency appear lower than that estimated by the non‑isotopic difference method.

Abstract

SUMMARY Experiments with 15 N labelled fertilizers often show that plants given fertilizer N take up more N from the soil than plants not given N—the priming effect or ‘added nitrogen interaction’(ANI). This paper is a theoretical study of ANIs and how they can affect the interpretation of experiments with 15 N labelled fertilizers. ANIs can be ‘Real’, if for example, fertilizer N increases the volume of soil explored by roots, or ‘apparent’, caused by pool substitution or by isotope displacement reactions. Pool substitution is the process by which added labelled N stands proxy for native unlabelled N that would otherwise have been removed from that pool. Microbial immobilization of N, whether driven by the decomposition of soil organic matter or by the decomposition of plant roots, can lead to pool substitution and is the dominant cause of apparent ANIs. Denitrification and plant uptake of N can also, under special circumstances, lead to pool substitution and thus give rise to apparent ANIs. Isotope displacement reactions, in which the added labelled N displaces native unlabelled N from a ‘bound’ pool, can lead to apparent ANIs but are only likely to be of significance in exceptional circumstances. The relationship between ANIs, ‘A’ values and N fertilizer uptake efficiencies are examined by means of a simple model for uptake of 15 N‐labelled fertilizer by a crop. A positive ‘apparent’ ANI is accompanied by an ‘A’ value that changes as fertilizer applications increase. Likewise, a positive ‘apparent’ ANI also causes fertilizer uptake efficiency to appear lower when measured by the uptake of 15 N than when measured by the non‐isotopic ‘difference’ method.