Quantifying Soil–plant Nutrient Dynamics in Nitrogen-fixing Trees and Shrubs under Soil Amendments and Bio-inoculants: A Critical Appraisal of Measurement Methods in Containerised Nursery and Field Systems

Abstract

Nitrogen-fixing trees and shrubs are planted widely in nursery-raised restoration programmes, mixed-species plantations and agroforestry systems, and soil or substrate amendments and bio-inoculants are applied routinely to improve their nutrition. Confidence in the reported effects of these interventions rests on measurement systems whose validity conditions the interventions themselves disturb, and this dependence has not been examined systematically. This critical narrative review appraises the methods used to quantify soil–plant nutrient dynamics in woody nitrogen-fixing systems across containerised nursery and field domains. Literature was identified in open scholarly indexes and agricultural and biomedical databases, supplemented by citation searching and institutional technical sources, for the period 1985 to 14 June 2026, and was selected and appraised for its bearing on measurement validity rather than for effect estimation. Five arguments emerge. Methods for estimating symbiotic nitrogen input each hold one quantity constant, and amendments or inoculation alter that quantity, with predictable directions of bias. Extraction-based measures of nutrient availability lose their calibration to plant uptake when sorptive amendments are present, and the best-documented case shows soil test results and plant responses moving in opposite directions. Diffusion-based and sink-based measures of phosphorus availability outperform conventional extraction in high-sorption soils, which is the soil class most relevant to restoration planting. Colonisation percentages, functional gene abundances and acetylene-dependent activity are surrogates whose conversion to nutrient fluxes is neither linear nor stable, yet they are frequently reported as fluxes. Containerised and field systems are distinct measurement domains rather than two scales of one system, so container results support statements about mechanism and capability but not about magnitude. Major unresolved questions concern host-specific isotopic fractionation parameters for the species in widest use, the separation of inoculant establishment from inoculant effectiveness, and the persistence of nursery treatment effects after outplanting. A substantial and identifiable portion of the apparent variability in this literature is methodological rather than biological, and most remedies require changes to design and reporting rather than new instrumentation.