Objectives Soil microorganisms are generally limited by the availability of carbon (C), nitrogen (N) and phosphorus (P). Studying the effects of different fertilization methods on soil physico-chemical properties, soil microbial biomass, and enzyme activities will improve our understanding on the roles of soil nutrients and microbes, and provide theoretical basis for improving soil fertility.
Methods An experiment was conducted on a black soil in Heilongjiang Province. Maize crop was cultivated in 2019 planting season with 9 different treatments viz: no fertilizer (CK), Farmers’practice (FP); optimum nutrient treatment (OPT); N-omission based on OPT treatment (–N); and organic nitrogen (horse manure) substituted for chemical nitrogen fertilizer by 10% (M1); 20% (M2); 30% (M3); 40% (M4) and 50% (M5). After the maize harvest, soil samples were collected at 0–20 cm depth to determine soil moisture, pH, organic carbon, total nitrogen, available phosphorus, available potassium, soluble carbon (DOC), and soluble nitrogen, microbial biomass carbon, microbial biomass nitrogen, and soil enzyme activities (acid phosphatase, β-D-glucosidase, L-leucine aminopeptidase, β-N-acetylglucosaminidase).
Results Compared with OPT treatment, the contents of available nutrients (soluble organic carbon; available phosphorus; available potassium) significantly increased by 15.5%–46.6%, 1.4%–18.5% and 2.4%–18.8% respectively, and microbial biomass (microbial biomass carbon; microbial biomass nitrogen) increased by 1.4%–19.9% and 0.04%–22.7% respectively with the increase of organic substitution ratio in organic nitrogen substitution treatments. PCA analysis showed that there were significant differences in enzyme activities among CK, FP, OPT, –N and chemical nitrogen substituted with organic nitrogen (M1–M5). RDA analysis further showed that DOC (F = 23.8, P = 0.002) was the main factor affecting the change of enzyme activities by explaining 48.8% of the difference among different treatments. The scatter plot of enzyme stoichiometry showed that the soil microorganisms in the test site were limited by P, and the soil microorganisms under FP treatment were also limited by C. Compared with CK, organic fertilizer substitution significantly improved the BG/AP ratio, but there was no significant difference between different substitution ratios.
Conclusions In this study area, the soil microorganism is limited by both Cand Punderno fertilization condition. Both conventional fertilization and optimized fertilization aggravated the carbon limitation of soil microorganism. The substitution of organic fertilizer for chemical nitrogen fertilizer significantly improved the soil nutrients and biological fertility, and significantly reduce the C and P limitations of soil microorganisms. However, the reduction effect of phosphorus limitation did not increase with the increase in the proportion of organic fertilizer replacing chemical nitrogen fertilizer. Considering the slow release of nutrients inorganic fertilizer, the specific proportion of organic fertilizer substitution still needs to be observed through long-term experiments.
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