Abstract:

The study aims to explore the effect of partial replacement of chemical fertilizer by organic fertilizer on the yield of lettuce in newly reclaimed red soil. According to the principle of equal replacement of 20% chemical fertilizer nitrogen, five treatments, including no fertilizer (CK), conventional fertilizer (CF), biological organic fertilizer + 80% chemical fertilizer (B), refined organic fertilizer + 80% chemical fertilizer (O) and biological organic fertilizer + refined organic fertilizer + 80% chemical fertilizer (B + O), were set to study their effects on soil physical and chemical properties, and lettuce nutrient uptake, yield and sustainability index. Structural equation model was used to analyze the relationship between soil organic matter and nutrient supply and yield. Boundary line method was used to reveal main factors affecting the yield of lettuce. The results showed that compared with CF, B and B+O treatment significantly increased the yield by 28.8% and 13.8%, respectively. Compared with CF, lettuce nitrogen and potassium uptake under B, O and B+O treatment significantly increased by 12.9%-29.0% and 28.1%-40.6%, yield sustainability index significantly increased by 10.4%-23.8%, soil organic matter and aggregates significantly increased by 27.0%-53.0% and 25.0%-32.4%, respectively. Soil bulk density significantly decreased by 7.2%-10.5%, the contents of soil alkali-hydrolyzed nitrogen, available phosphorus and available potassium under O and B+O treatment were not significantly different from those under CF. Structural equation model analysis showed that organic fertilizer substitution mainly increased soil organic matter to improve the structure of soil aggregates and ensure the supply of soil nutrients, and then increased the lettuce nutrient absorption and yield. Boundary line analysis showed that the boundary yield reached the maximum when the soil organic matter was 21.05 g/kg, alkali-hydrolyzed nitrogen was 88.07 mg/kg, available phosphorus was 45.17 mg/kg, and available potassium was 210.51 mg/kg. However, the effect of the addition of refined organic fertilizer on soil available nutrient supply was not obvious. Therefore, soil organic matter is the main factor that limiting the boundary yield increase, the replacement of 20% nitrogen fertilizer by biological organic fertilizer could significantly increase the content of soil organic matter, ensure the supply of soil nutrients, and promote the yield increase of lettuce.

Key words: organic fertilizer, newly cultivated red soil, yield, structural equation model, boundary line analysis