Evaluating Soil–Root Interaction of Hybrid Larch Seedlings Planted under Soil Compaction and Nitrogen Loading

Although compacted soil can be recovered through root development of plantedseedlings, the relationship between root morphologies and soil physical properties remain unclear.We investigated the impacts of soil compaction on planted hybrid larch F1(Larix gmeliniivar.japonica×L. kaempferi, hereafter F1) seedlings with/without N loading. We assumed that N loadingmight increase the fine root proportion of F1seedlings under soil compaction, resulting in less effectsof root development on soil recovery. We established experimental site with different levels of soilcompaction and N loading, where two-year-old F1seedlings were planted. We used a hardnesschange index (HCI) to quantify a degree of soil hardness change at each depth. We evaluated rootmorphological responses to soil compaction and N loading, focusing on ectomycorrhizal symbiosis.High soil hardness reduced the total dry mass of F1seedlings by more than 30%. Significantpositive correlations were found between HCI and root proportion, which indicated that F1seedlingcould enhance soil recovery via root development. The reduction of fine root density and itsproportion due to soil compaction was observed, while these responses were contrasting under Nloading. Nevertheless, the relationships between HCI and root proportion were not changed by Nloading. The relative abundance of the larch-specific ectomycorrhizal fungi under soil compactionwas increased by N loading. We concluded that the root development of F1seedling acceleratessoil recovery, where N loading could induce root morphological changes under soil compaction,resulting in the persistent relationship between root development and soil recovery.