To investigate the effects of nitrogen addition on water use efficiency and related enzyme activities of P. massoniana seedlings under continuous drought and re-watering, and to provide reference for exploring whether nitrogen addition improves drought resistance of P. massoniana.

One-year-old P. massoniana seedlings were used as experimental materials, and a nitrogen application group (N) and a control group (CK, no nitrogen fertilizer applied) were set up. Continuous drought and re-watering experiments were conducted through potted plants. Samples were collected at 0, 20, 40, and 60 days during continuous drought, as well as two days after re-watering. Soil water content, physiological and biochemical indices, and water consumption were measured for drought resistance evaluation.

1) The nitrogen application group increased soil moisture content, stem water potential, amino acid content, and water consumption of seedlings during the severe extreme drought stage (20-60 days), while reducing soluble protein, superoxide anion, and malondialdehyde content during the extreme drought stage (40-60 days); 2) During 40-60 days of drought, the nitrogen application group showed a significant increase in amino acid, proline content, and CAT activity; 3) After re-watering, the soluble protein, superoxide anion, and malondialdehyde content, as well as the activities of superoxide dismutase and catalase, were significantly reduced in the nitrogen application group, while the control group maintained high levels, indicating weak recovery ability after drought; 4) The comprehensive evaluation of drought resistance was conducted on physiological and biochemical indicators using the membership function method. The results showed that during the extreme drought stage (40-60 days), the nitrogen application group exhibited strong drought resistance.

Nitrogen application can improve soil water retention during drought periods, increase the content of osmoregulatory substances in P. massoniana seedlings, and enhance antioxidant enzyme activity, thereby reducing cell dehydration and mitigating oxidative damage to seedlings, thus improving their drought resistance and post-drought recovery ability under drought stress.

To explore the mechanism of Ectomycorrhiza fungi enhancing aluminum tolerance of Pinus massoniana, and provide theoretical basis for the study of aluminum resistance mechanism of P. massoniana.

Using the hydroponic method, different concentrations of Al³⁺ were treated (0, 0.2, 0.4, 0.8, 1.2 mmol·L^-1) to determine the growth, osmoregulation substances, malondialdehyde, chlorophyll content, and antioxidant enzyme activity of two types of P. massoniana seedlings.

1) Aluminum stress inhibited the growth of P. massoniana seedlings, and inoculation with Sl significantly promoted the high growth and biomass accumulation of P. massoniana seedlings. 2) Compared with non inoculation, inoculation with Sl reduced the content of malondialdehyde (MDA), and showed a trend of first decreasing, then increasing, and then decreasing with the increase of Al³⁺ concentration. However, it increased the activity of peroxidase (POD) and superoxide dismutase (SOD), which increased the most when the concentration of Al³⁺ was 0.2 mmol·L^-1 and 0.4 mmol·L^-1, respectively. 3) Compared with non inoculation, inoculation with Sl significantly increased the chlorophyll content of P. massoniana seedlings, with a maximum increase of 50.88% at 0.2 mmol·L^-1. Afterwards, with the increase of Al³⁺ concentration, there is a trend of first increasing and then decreasing. When the concentration of Al³⁺ was 0.2 mmol·L^-1, the chlorophyll content of P. massoniana seedlings inoculated with Sl increased by 108.06% compared to the aluminum free treatment. 4) Compared with non inoculation, inoculation with Sl significantly reduced the soluble sugar content and proline content in the leaves of P. massoniana seedlings (except for 0.4 mmol·L^-1), with the largest decrease at 0.2 mmol·L^-1. Afterwards, with the increase of Al³⁺ concentration, the overall trend showed a first decrease and then an increase. However, it increased the accumulation of soluble proteins and had the greatest promoting effect at 1.2 mmol·L^-1, an increase of 30.13% compared to the aluminum free treatment. In addition, MDA showed a highly significant positive correlation with POD activity, proline, and soluble sugar (P < 0.01), with the highest correlation with soluble sugar being 0.775; SOD was highly significantly positively correlated with chlorophyll (P < 0.01), and negatively correlated with proline and soluble sugar (P < 0.01).

Within a certain concentration range, inoculation with Sl significantly enhances host plant enzyme activity, improves leaf light energy utilization efficiency and osmotic substance content, enhances its aluminum resistance, and thereby promotes the growth of mycorrhizal P. massoniana seedlings. The promotion effect is greatest when the concentration of Al³⁺ is 0.2 mmol·L^-1.