Objective Effects of applying biochar and Trichoderma harzianum on mitigating aeolian, improving water retention, and maintaining fertility of sandy soil on the coast of Dongshan Island were studied.

Method A pot experiment growing Brassica chinensis L. on soil collected from the beaches on Dongshan Island was conducted under 4 conditions including control without amendment (CK) and treatments with addition of 1% biochar (T1), 0.25% T. harzianum agent (T2) or T1+T2 (T3). Physiochemical properties and N-acetyl-β-D-glucosaminidase (NAG), β-glucosidase (β-GC), and leucine aminopeptidase (LAP) activities in the soil as well as plant biomass, total nitrogen, total phosphorus, nitrogen, phosphorus of the vegetables grown in the pots were measured. Trichoderma colonization and correlations among various indicators were analyzed.

Result Among all tested conditions, T3 rendered the greatest water-holding capacity of the soil at 208.17g·kg⁻¹, with available water content 1.4 times that of CK without significant changes on wilting coefficient. After 21d of cultivation, Trichoderma population in T3 grew to 1.02×10⁶ CFU·g⁻¹, which was significantly higher than in T2 of 5.55×10⁵ CFU·g⁻¹. Compared with CK, T3 also significantly raised the soil organic matter, total nitrogen, alkali-hydrolysable nitrogen, and available potassium by 104.41%, 96.77%, 87.55%, and 159.07%, respectively, while significantly reduced the nitrate nitrogen by 21.59%. The NAG activity in T3-treated soil was enhanced by 62.15% relative to CK, whereas the highest β-GC and LAP activities in T1-treated counterparts increased by 50.48% and 148.32%, respectively. Under T3, the fresh and dry weights of Shanghai green increased by 57.77% and 58.43%, respectively, and nitrogen and phosphorus accumulation by 92.94% and 94.89%, respectively, over those of CK. A partial least squares structural equation modeling revealed that the soil physicochemical properties indirectly drove the biomass increase by promoting plant N and P accumulation at a path coefficient of 0.559 and activating soil enzyme activities at a path coefficient of 0.563. The “soil physicochemical properties → plant N and P accumulation → biomass” was the key yield-enhancing route with an indirect effect of 0.455.

Conclusion The application of biochar in combination with T. harzianum not only significantly promoted the Trichoderma colonization in rhizosphere but also enhanced the water-holding capacity, preserved the nutrient pool, and activated the chitin turnover-related NAG in sandy soil resulting in an elevated vegetable yield.