Feasibility of mitigating the secondary salinization in greenhouse soil by carbon regulator

【Objectives】Over application of nitrogen fertilizer leads to high level nitrate accumulation in the top layer, which has been considered as the main cause of secondary salinization in greenhouse soils. Based on the principle of carbon/nitrogen interactions, a carbon regulator, composed of crop straw and fast decomposing microbial strains, was developed and added to a salinized greenhouse soil to investigate the effect of the regulator on reducing soil soluble salt content. 【Methods】 Two pot experiments were carried out and 2.5 kg of satirized soil from a greenhouse was added to each pot. In the salt monitoring experiment, 150 g of carbon regulator was added to each pot and the electro-conductivity (EC) of soil was measured during the 10 days incubation. In the salt regulator dosage experiment, six rates of the regulator, 0 (CK), 30 (T1), 60 (T2), 90 (T3), 120 (T4), 150 (T5) and 180 (T6) g/pot were added and the composition and concentration of soluble salts, the EC of soil solution were determined after short term (7 d) and longer term (90 d) incubation. 【Results】 The soil EC was elevated by the addition of carbon regulator at the beginning of the experiment, then declined with the incubation continuing. The dynamic of soil EC with the incubation time fitted with a quadratic curve: y=3.7768 - 0.2681x + 0.0138x2 (r=0.9966**) and the EC was stable after 7 d incubation. In the dosage experiment, the salt contents at the 7th day of incubation in all treatments significantly declined, compared with that in CK, and the salt content declined more in the treatments with higher rates of regulator addition. After 90 days' incubation, the salt content was observed further declined in all treatments with regulator addition, especially in T3 and T4. In T4, the salt content deceased by 23.8% after 7 day's incubation and further reduced by 9.14% at the end of incubation. The total salt content in T4 reduced by 32.96%. These data indicated the long term and apparent effect of the regulator. The addition of regulator could reduce nitrate content in the soil with both the short and long term incubation, and the reduction of NO-3 in soil solution could be as high as 97.10% by adding carbon regulator, which suggested that adding carbon regulator to reduce NO-3 content in salinized soil was feasible. Furthermore, the addition of carbon regulator caused a significant increase of available K, and soluble K+ concentration of soil solution, which was approximately 10 times higher in T6 than that in CK. But the effect on available P was elevated slightly. All the change pattern of available N, P and K showed similar trends in both the long term (90 d) and short term (7 d) incubation, but those of NH+4 and NO-3 were decreased after longer term incubation. 【Conclusions】The recommended rates of carbon regulator were between 36 g/kg (T3) and 48 g/kg (T4). Further study is needed to verify the mitigation effect of the regulator on the secondary salinization in field scale.