Nitrogen-driven shifts in soil extracellular enzyme systems and stoichiometric indicators in a tropical ecosystem

Abstract

Nitrogen (N) fertiliser form is a key regulator of soil microbial processes, yet its role in shaping soil extracellular enzyme and as a stoichiometric indicator remains unclear. We conducted a two-year field experiment (2023‒2024) in a tropical perennial durian orchard in Hainan, China, to examine how contrasting N forms influence soil biochemical properties and microbial nutrient acquisition strategies. Six fertilisation regimes were tested, namely control (CK), urea (URT), ammonium sulfate (AMT), calcium nitrate (NT), slow-release N (SRT) and bio-organic fertiliser (BFT). The bio-organic and slow-release N treatments significantly supplemented organic carbon in soils (50%‒113%), cation exchange capacity (26%‒134%), and microbial biomass C and N (22%‒206%), and maintained a relatively constant β-glucosidase (β-G): urease: alkaline phosphatase (ALP) ratio (coordinated nutrient acquisition) by the microbes. Synthetic N fertilisers particularly the nitrate-based and ammonium-based ones, in contrast, altered the stoichiometry of enzymes and deactivated enzyme phosphatase and reinforced phosphorus inhibition, which was followed by a successively growing decline in soil carbon retention. The enzyme-substrate relations (e.g., β-G-SOC, r = 0.76; p < 0.01) were improved under BFT and SRT, but not synthetic N inputs. Multivariate findings indicated that there was segregation in the first year of treatment and partial convergence in the second year, which is a sign in microbial acclimation to the continuous nutrient feeds. Combined, these findings show that N form modulates the patterns of microbial allocation and nutrient fixation systems in tropical soils, and that ecoenzymatic stoichiometry provides a sensitive biochemical quantification of fertilisation responses of soil functional processes.

Graphical abstract

Keywords

ecoenzymatic stoichiometry / microbial biomass / soil organic carbon / tropical perennial systems

Highlight

	● Bio-organic and slow-release N improved soil C, CEC, and microbial biomass.
	● Ammonium and nitrate fertilisers induced P limitation via reduced phosphatase.
	● Balanced enzyme ratios reflected optimal microbial nutrient acquisition.
	● Fertiliser type shaped whether enzyme–nutrient linkages were coupled or decoupled.
	● Nitrogen form determines nutrient retention pathways in tropical soils.

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