Objective Effects of partially substituting chemical fertilizer with organic one on the growth of Brassica rapa var. parachinensis and the soil organic carbon (SOC) stability were studied to optimize vegetable farming operation.

Method A field experiment was conducted using chemical fertilizer alone (CF) and 4 treatments with organic fertilizer substituting 20%, 30%, 40%, and 50% of chemical fertilizer nitrogen at equivalent nitrogen inputs in growing Brassica rapavar. parachinensis. Cabbage yield and SOC fractions and mineralization were measured.

Result Of all treatments, substituting 30% of the chemical fertilizer with an organic fertilizer achieved the highest annual cabbage yield of 136208.71 kg·hm⁻², and further increase on the replacement reversed the trend. Organic fertilizer substitution for chemical fertilizer could increase the average soluble sugar content of Brassica rapavar. parachinensis by 11.22%, the average vitamin C content by 9.15%, and decrease the average nitrate content by 19.94%. In the topsoil, the organic matter increased by 5.83% and the total nitrogen by 12.5% over CF, but the available nutrients declined. On the other hand, the labile SOC fractions and mineralization rose with a 12.49% increase on the SOC instability index. The structural equation model indicated that the substitution did not directly affect the content but indirectly through alternating the fractions and mineralization of SOC.

Conclusion Replacing 30% of inorganic with organic fertilizer materially reduced the chemical use and facilitated carbon sequestration without sacrificing the cabbage yield. However, it also raised the SOC instability in the farm soil, therefore, warranted careful evaluation prior to application.