Coated and stable nitrogen fertilizers improve physico-chemical and biological quality and delay acidification in brown soil

Objective Coated and stable nitrogen fertilizers have different release characteristics of N in soil. We studied their effects on soil physico-chemical properties and enzyme activities related to nitrogen conversion, to evaluate their influence in long-term application.

Methods The long-term experiment was established in 2007 in Shenyang Agro-ecosystem Experimental Station. Urease inhibitor (N-butyl thiophosphate-triamine, N), hydroquinone (H), nitrification inhibitor (3, 4-dimethylpyrazole phosphonate, DM), dicyandiamide (D) were added into urea to make products. Coated urea were polymer-coated urea (PCU) and sulfur-coated urea (SCU). A total of 9 treatments were set up in the test: urea (U), H+U, N+U, D+U, DM+U, H+D+U, N+DM+U, SCU, PCU. Soil samples from 0−20 cm depth were collected in 2021 to analyze the physical and chemical properties, activities of soil nitrogen conversion-related enzymes, and soil microbial biomass C and N content.

Results Compared with the soil before test (2007), the long-term application of stable urea and coated urea increased the organic matter, total N, total P, total K, available P, and available K in brown soil, the organic matter increased by 34%–48%, the highest total N was recorded in SCU and U treatments (1.26 g/kg); stable urea treatments recorded higher soil pH, with the highest pH in DM+U treatment (5.83), while SCU treatment, with soil pH 4.87, decreased soil pH by 8%, leading to serious soil acidification. Coated or stable urea increased soil microbial biomass carbon to varying degrees, and DM proved to be suitable for microbial nitrogen fixation. H+U had the strongest inhibition effect on urease activity, and two coated urea treatments significantly stimulated urease activity. The coated release-stable urea enhanced nitrification potential activity and nitrification of brown soil, improved nitrate reductase activity, except N+DM+U treatment.

Conclusions The application of stable/coated urea improves brown soil fertility and soil microbial quantity. Except sulfur-coated urea, all the other stable/coated urea effectively delay the acidification of brown soil, reduce the loss of urea N, so are suitable for long-term application in dry brown soil.