Planting a deep-rooted green manure (GM) (more than 1.0 m depth) greatly improves soil fertility and reduces the loss of nutrients. However, few studies have examined the response of soil nitrogen (N) distribution in the soil profile and subsoil N recovery to the long-term planting and incorporation of deep-rooted GM. Based on a 12-year (2009–2021) experiment of spring maize-winter GMs rotation in the North China Plain (NCP), this study investigated the effects of different GMs that were planted over the winter, including ryegrass (RrG, Lolium L.) (>1.0 m), Orychophragmus violaceus (OrV, Orychophragmus violaceus L.) (>0.8 m), and hairy vetch (VvR, Vicia villosa Roth.) (>1.0 m), on the spring maize yield, N distribution in the deep soil profile, N use efficiencies, functional gene abundances involving soil nitrification–denitrification processes and N₂O production. Compared with the winter fallow, the maize yield significantly increased by 11.6% after 10 years of green manuring, and water storage in 0–200 cm soil profile significantly increased by 5.0–17.1% at maize seedling stage. The total N content in the soil layer at 0–90 cm increased by 15.8–19.7%, while the nitrate content in the deep soil layer (80–120 cm) decreased by 17.8–39.6%. Planting GM significantly increased the N recovery rate (10.4–32.7%) and fertilizer N partial productivity (4.6–13.3%). Additionally, the topsoil N functional genes (ammonia-oxidizing archaea amoA, ammonia-oxidizing bacterial amoA, nirS, nirK) significantly decreased without increasing N₂O production potential. These results indicated that long-term planting of the deep-rooted GM effectively reduce the accumulation of nitrates in the deep soil and improve the crop yield and N use efficiencies, demonstrating a great value in green manuring to improve the fertility of the soil, increase the crop yield, and reduce the risk of N loss in NCP.

Vicia villosa Roth is an excellent green manure crop and offers promising applications in green agricultural production and bioactive substance exploitation. In this study, crude polysaccharides (VVP) were prepared from V. villosa using water extraction and alcohol precipitation. Single-factor experiments combined with Box–Behnken response surface methodology were employed to optimize the decolorization process of VVP with AB-8 macroporous resin. The optimal conditions were determined as follows: a resin-to-sample ratio of 1:140 (m/m), decolorization temperature of 78 °C, and a decolorization time of 300 min. Under these conditions, the decolorization rate reached 67.36% and the polysaccharide retention rate was 61.85%, with a relative error of only 2.1% between the model-predicted and experimental values. In vitro antioxidant assays showed that VVP possessed strong antioxidant activity. At a concentration of 2 mg/mL, the DPPH radical scavenging rate of VVP was 91.9%, and the ABTS+ radical scavenging rate reached 96% at 8 mg/mL. Moreover, VVP promoted crop growth and markedly alleviated the inhibition of salt stress on root development. To the best of our knowledge, this is the first report demonstrating that V. villosa crude polysaccharide extract simultaneously possesses strong antioxidant activity, salt-stress alleviation capacity, and plant growth-promoting effects, highlighting their potential as natural antioxidants and agricultural biostimulants.