Increased microbial biomass in coastal saline fields of North China Plain by application of biochar and organic manure

Objectives Changes of microbial biomass carbon and nitrogen (MBC, MBN) contents were studied by applying different rates of biochar and organic manure to coastal saline soils in Binzhou, Shandong Province, which could provide scientific basis for improving soil environmental quality and sustainable utilization of coastal saline soils.

Methods An experiment of six treatments with three replications was established: CK, C1 biochar 5 t/(hm²·a), C2 biochar 10 t/(hm²·a), C3 biochar 20 t/(hm²·a), M1organic manure 7.5 t/(hm²·a) and M2 organic manure 10 t/(hm²·a). All treatments received the same amounts of N200 kg/(hm²·a) and P₂O₅ 120 kg/(hm²·a), while the N and P₂O₅ deficiencies with biochar and organic fertilizer were supplemented by adding urea and diammonium phosphate. Biochar, organic manure and basal fertilizer were evenly applied to growing seasons of maize and wheat. At different growth stages of these crops, soil samples of 0–20 cm and 20–40 cm were collected by soil sampling auger to determine soil MBC, MBN, soil pH, soil water content, NH₄⁺-N and NO₃^–-N.

Results Both the biochar and organic manure treatments increased soil MBC and MBN. After 5 days of basal fertilization, biochar and organic manure significantly increased the contents of soil MBC and MBN. But the effects of top dressing on soil MBC and MBN were not significant. For the biochar treatments, the ranges of MBC were 64.1–570.0 μg/g, and those for the organic manure treatments were 90.6–451.3 μg/g. The treatments of C3, M1 and M2 significantly increased MBC in 0–40 cm soil by 40.9%–118.4%, while the C1 and C2 treatments only significantly increased MBC in 20–40 cm soils by 47.7%, 60%. For the biochar treatments, the ranges of MBN were 5.3–92.5 μg/g, and those for the organic manure treatments were 4.2–163.9 μg/g. However, the biochar treatments did not show significant differences compared with CK, while M1 and M2 increased soil MBN contents significantly by 56.4%–162.3%. The effect of biochar and organic manure on soil pH was significant, and biochar significantly reduced soil pH in 20–40 cm layer, while organic fertilizer significantly reduced soil pH in 0–40 cm layer. The correlation analysis results showed that soil pH had significant negative correlations with soil MBC and MBN. And both soil MBC and MBN showed significant positive correlation with soil mineral nitrogen. For crop yields, besides M1 in the maize season showed decrease significantly, the effects of biochar and organic manure on maize and wheat yields were not significant. Bacteria was dominated in the early stage of maize season, while fungus was the main components in the later stage. The MBC/MBN in the wheat season fluctuated greatly.

Conclusions Biochar and organic manure significantly impacted soil MBC and MBN, and increased soil MBC and MBN in coastal saline soils. The soil MBC and MBN had significant negative correlations with soil pH and positive correlations with soil mineral nitrogen, which indicated that the application of biochar and organic manure had some positive effects on coastal saline land and increased the soil mineral nitrogen, which was beneficial to the improvement of soil environmental quality.