Objectives Nematodes are one of typical soil health indicators. We explored the effect of long-term phosphorus application on soil nematodes community and metabolic footprint, to provide a theoretical support for the P nutrient management benefiting the sustainability of alfalfa production in Loess Plateau.

Methods The research was based on the long-term alfafa (Medicago sativa) phosphorus fertilization experiment, located in Gansu Agricultural University and started since 2014. Soil samples were collected from alfalfa fields with P₂O₅ application rates of 0, 60, 120 and 180 kg/hm² per three years. Nematodes were isolated separately using the shallow tray method to isolate and identify nematode communities . The ecological function index and metabolic footprint of soil nematodes were calculated, and the indicator species of nematodes were clarified.

Results A total of 4575 soil nematodes were isolated from all the phosphorus treatment plots. They belonged to 19 families and 27 genera, including 9 genera of both bacterial feeders and plant-parasites, 4 genera of fungal feeders and 5 genera of omnivores-predators. Plant-parasite nematodes accounted for the highest proportion (43.00%−57.33%), and Helicotylenchus, with a proportion of 20.67%−39.33%, were the dominate genera in all the P treatment plots. In addition, Acrobeloides (10.33%), Aprutides (11.00%) and Filenchus (10.00%) were the dominant genera in CK; Acrobeloides (11.00%) in P60 treatment, Aprutides (10.00%) in P120 treatment and Mesorhabditis (10.00%) in P180 treatment. Long-term phosphorus addition had no significant effect on soil nematode abundance, but significantly affected nematode diversity, richness and community composition. Phosphorus addition altered soil physicochemical properties, resulting in species-selective soil nematode communities. P60 treatment was isolated the greatest proportion of positive selective species of bacterial feeders, omnivores-predators, plant-parasites and c-p nematodes. P60 treatment possessed six indicator species Aulolaimus, Chiloplacus, Eucephalobus, Ditylenchus, Xiphinema and Laimydorus, while P120 treatment had only three indicator species Panagrolaimus, Pratylenchus and Campydora, CK and P180 treatment even did not have one. The plant parasitic nematode maturity index (PPI) reached peaks in P120 treatment. The nematode diversity index (H′), nematode channel ratio (NCR) and maturity index (MI) reached peaks in P60 treatment, while the plant parasitic nematode to plant parasite index maturity index ratio (PPI/MI) and Simpson index (λ) hit the lowest in P60 treatment as well. The floristic analysis showed that soil nematodes of CK were distributed in quadrant A, B and C, while those of P60, P120 and P180 were all in quadrant B. Soil nematode functional footprints were P60>CK>P180>P120, and soil nematode enrichment index increased with the addition level of phosphorus, indicating that the soil nematode community in P60 was less disturbed and the nematode community structure was mature and stable, because of higher abundance of omnivores-predator nematodes.

Conclusions Phosphorus nutrient significantly causes the change of soil nematode communities and their metabolic footprints, leading to the variation of soil food web structure. Suitable phosphorus addition promotes soil food webs towards more complex and stable, so conducing to the sustainable use of alfalfa artificial grassland.