Background The Loess Plateau is one of the regions globally afflicted by severe soil erosion. The inherent instability of its soil structure and limited water retention capacity significantly impede local agricultural development. Lignin, as a widely employed soil conditioner, plays a crucial role in addressing this issue.

Methods Laboratory tests and model simulation were conducted to investigate the effects of lignin application on soil structure and water transport in typical soils in loess areas. The samples were taken from Changwu Experimental Station, Institute of Water Conservation, Chinese Academy of Sciences. The application ratios of lignin were set to be 0, 0.2%, 0.3%, 0.4% and 0.5%. The influences of different lignin application amounts on each index were studied by measuring soil shear force, soil aggregate and one-dimensional vertical infiltration test. The average value of 3 repeated tests was taken as the final result of the test data.

Results 1) Adding lignin in soil improved the stability of soil structure. The soil shear strengths of all treatment groups increased significantly with the increased of lignin application amount, and the soil shear strengths of all treatment groups increased by 13.6%, 18.2%, 31.8% and 50.0% compared with CK control group, respectively. Lignin significantly increased the mass fraction of super-large aggregates ( > 2 mm), large aggregates (0.25 − < 2 mm) and micro-aggregates (0.053 − < 0.25 mm) in soil, and reduced the mass fraction of clay particles ( ≤ 0.053 mm). Compared with CK control group, with the increase of lignin application ratio, lignin reduced the mass fraction of clay particles (≤ 0.053 mm). The super-large aggregates of > 2.000 mm increased by 26.7%, 33.3%, 106.7% and 133.3%, respectively. The aggregate size of 2.000 − > 0.250 mm increased by 13.3%, 18.3%, 26.3% and 40.0%, respectively. The microaggregates of 0.250 − > 0.053 mm increased by 26.7%, 48.5%, 78.2% and 83.2%, respectively. The clay particles ≤ 0.053 mm decreased by 15.2%, 42.8%, 41.5% and 47.5%, respectively. The fractal dimension was negatively correlated with the amount of lignin applied, which decreased by 1.0%, 1.3%, 2.1% and 3.1% compared with CK, respectively. 2) Lignin significantly reduced the infiltration capacity of soil water. With the increase of lignin application, the cumulative infiltration, infiltration rate and wetting front migration distance decreased significantly. The wetting front and infiltration rate simulated by the HYDRUS-1D model changed well with time, with R² > 0.95, RMSE value smaller and MAE value < 0.04 in all treatment groups. With the increase of lignin application, the reciprocal air suction α and saturated water content θ_s gradually increased, while the residual water content θ_r, hydraulic conductivity K_s and shape coefficient n gradually decreased.

Conclusions Lignin can improve soil structure, reduce soil water infiltration rate, and enhance soil water retention ability. To a certain extent, it can improve soil erosion, increase grain production and improve land productivity in the Loess Plateau area.