Arbuscular mycorrhizal fungi mitigated nitrogen leaching by enhancing soil nitrogen retention in Camellia oleifera Abel. soils

Nitrogen leaching is a major pathway of nitrogen fertilizer loss. Although arbuscular mycorrhizal (AM) fungi are known to reduce nitrogen leaching by improving plant nitrogen uptake, the soil-based mechanisms remain unclear. A pot experiment was conducted using a randomized complete block design, with four nitrogen levels (0, 3.75, 7.5, and 15 mM) and two AM fungal inoculation treatments. The study evaluated plant nitrogen uptake, hyphal length density, glomalin-related soil protein (GRSP) content, soil aggregation, soil nitrogen content, and nitrogen leaching loss in Camellia oleifera soils. Results showed that both NH 4 + -N and NO 3 − -N in leachate increased with increasing nitrogen application. AM fungi reduced NH 4 + -N (by 19.04% and 44.96%) and total nitrogen (TN, by 20.74% and 24.70%) in the leachate under high nitrogen conditions (7.5 and 15 mM). Under high nitrogen levels, AM fungi significantly increased soil easily extractable and total GRSP contents, NO 3 − -N, TN, and the proportion of 0.05–1.00 mm aggregates, while reducing soil dissolved total nitrogen (DTN) content, and the proportion of 0.002–0.05 mm aggregates. Redundancy analysis (RDA) showed that hyphal length density, DTN, soil NH 4 + -N, NO 3 − -N, and aggregate distribution were strongly associated with nitrogen leaching. Partial least squares path modeling indicated that soil DTN explained 96% of the variation in nitrogen leaching; AM fungi likely reduce soil DTN through increased soil aggregation via GRSP secretion and direct nitrogen absorption by extraradical hyphae. These findings highlight that changes in soil structure play a key role in reducing nutrient loss mediated by AM fungi. IMPORTANCE Nitrogen fertilizers are essential for crop productivity, but low nitrogen use efficiency remains a major challenge in agriculture. Nitrogen leaching is a key contributor to nitrogen loss. Arbuscular mycorrhizal (AM) fungi are known to reduce fertilizer loss by enhancing nitrogen uptake in host plants. This study explored how AM fungi affect nitrogen leaching, plant nitrogen uptake, hyphal development, glomalin-related soil protein (GRSP) production, soil aggregation, and soil nitrogen content in Camellia oleifera soils. The results show that AM fungi can reduce nitrogen in soluble form and subsequently decrease nitrogen leaching, not only by facilitating nitrogen uptake via extraradical hyphae but also by altering soil structure—especially by promoting aggregate formation through the secretion of GRSP. The findings underscore that the functional significance of AM fungi should not be limited to their interactions with plant components, as their role in soil amelioration is equally important.