Combined application of different rates of chemical and organic fertilizer increase soil nitrification potential and pH in greenhouse tomato cultivation

Objectives The effects of nitrogen (N) application rate and combination of chemical and organic fertilizer on soil nitrification potential and pH were studied based a 9-years of tomato positioning experiment, to provide theoretical basis for improving N supply capacity of greenhouse soil and slowing down soil acidification.

Methods The long-term field experiments of greenhouse tomato cultivation started in 2013 in Shenyang, Liaoning Province, with cropping system of mono-tomato. Four urea N application level of 0, 187.5, 375.0, 562.5 kg/hm², denoted as N0, N1, N2, N3, were setup, and extra four treatments were setup by combining with organic fertilizer 75000 kg/hm² in each N level treatment (MN0, MN1, MN2, MN3), with a total of 8 treatments. At the 1st and 2nd fruit expanding period (S1 and S2), harvest period (S3), and the fallow period after harvest (S4) in 2021, 0−10 cm and 10−20 cm soil samples were collected for the determination of nitrification potential (NP), pH, and the NH₄⁺-N and NO₃⁻-N content. The soil organic carbon (SOC) and total N (TN) after harvest were also determined for the calculation of mineral N (N_min) to TN ratio (N_min/TN).

Results  The application rate of chemical N fertilizer, as well as combination with organic fertilizer, and their interaction significantly affected soil NP, pH and NO₃⁻-N content. Compared with the sole chemical fertilizer treatment, the combined application of chemical and organic fertilizer decreased the NP at S1 stage, but increased the NP across S2–S4 stages. N1 treatment had similar soil pH with N0, but N2 and N3 treatment had significantly lower pH at 0–20 cm soil layer, and that N3 treatment was significantly lower than that of N2 treatment (P<0.05). However, there was no significant difference in soil pH among the four MN treatments at S1 and S2 stages, and the pH at S3 and S4 stages decreased gradually with the increase of N application level, but the decrease range was significantly less than that of sole chemical fertilizer treatments. The decreasing rates of soil pH in sole chemical fertilizer treatments and the combination treatments were 0.00–0.20 ∆pH/yr and 0.00–0.08 ∆pH/yr, respectively. Combined application treatments significantly increased soil NO₃⁻-N content across whole growing periods and soil NH₄⁺-N content after harvest, the soil NO₃⁻-N content of sole chemical fertilizer and combination treatment was 4.43–197.35 mg/kg and 35.16–400.04 mg/kg, respectively. Compared with the sole application of chemical fertilizer, the combined treatment significantly increased soil SOC and TN, and decreased soil N_min/TN (P<0.05). In sole chemical fertilizer treatment, NP was positively correlated with NO₃⁻-N content and pH, and negatively correlated with NH₄⁺-N content. Under the combination treatment, soil NP was positively correlated with SOC, TN and NO₃⁻-N in S2 stage, and negatively correlated with pH in S2 stage and N_min/TN in fallow period. Soil pH of chemical fertilizer treatment was negatively correlated with NH₄⁺-N and NO₃⁻-N content at S3 and S4, pH was negatively correlated with NH₄⁺-N and NO₃⁻-N content at S1, S2 and S4 stages of the combination treatments, and soil pH was negatively correlated with N_min/TN during fallow period both in chemical fertilizer alone and the combination treatments, and there was a negative correlation between soil pH and TN in the combination treatment (P<0.05).

Conclusions Compared with sole chemical fertilizer treatment, chemical and organic fertilizer combination could increase soil SOC and TN and NO₃⁻-N content (nitrification potential) in the middle and later stage of tomato growth, reduce N_min/TN, and effectively retard the decline rate of soil pH. The combined application of urea N 187.5 kg/hm² and organic fertilizer 75000 kg/hm² is recommended as its extrusive effect in N supply capacity and stable soil pH.