Effects of Partial Replacement of Chemicals with Organic Fertilizer on Growth of Brassica rapa and Organic Carbon Stability in Soil

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 different replacement proportions of chemical with organic fertilizer at equivalent nitrogen inputs in growing choy sum cabbages. 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 136,208.71kg·hm⁻². However, further increase on the replacement reversed the trend, even though the cabbage quality continued to improve over CF. 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.