关键词: 砖红壤, 母岩, 溶解性有机质, 光谱特征, 微生物养分限制, 酶化学计量学

Abstract:

Parent rock is a key inherent geological factor regulating soil organic matter cycling and microbial metabolic strategies in tropical ecosystems, as it fundamentally shapes soil formation, mineral composition, and nutrient availability, yet its regulatory mechanisms on the molecular composition of dissolved organic matter (DOM) and subsequent microbial nutrient limitation patterns in tropical ferralitic soils remain poorly elucidated. This study focused on ferralitic soils under rubber plantations in Hainan Island, South China, which developed from five typical parent rocks including basalt, granite, metamorphic rock, volcanic ash, and sandy shale. By integrating comprehensive soil physicochemical analysis, DOM spectral characterization techniques, and microbial extracellular enzyme stoichiometry, we systematically investigated how lithological background modulates the spectral features of DOM, and further clarified the intrinsic links between DOM molecular properties and microbial nutrient limitation, as well as the cascade coupling pathway of “mineral-DOM-microorganism” driven by parent rock. In each parent rock distribution area, three replicate plots (50 m × 50 m) were set up, and surface soil samples (0‒20 cm) were collected using the systematic five-point sampling method. Standard methods were adopted to determine basic soil physicochemical properties including total organic carbon (TOC), dissolved organic carbon (DOC), pH, alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), and available potassium (AK). Microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) were measured via the chloroform fumigation extraction method. DOM was extracted with ultrapure water using the salt-free oscillation method, and its spectral characteristics were analyzed by ultraviolet-visible (UV-Vis) absorption spectroscopy and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor analysis (PARAFAC). UV-Vis indicators including SUVA₂₅₄ and SUVA₂₆₀ were used to reflect DOM aromaticity, while fluorescence indices such as fluorescence index (FI), biological index (BIX), and humification index (HIX) were calculated to characterize DOM source, biogenic contribution, and humification degree. PARAFAC identified four fluorescent components of DOM: tryptophan-like (C1), tyrosine-like (C2), humic acid-like (C3), and fulvic acid-like (C4). The activities of six extracellular enzymes involved in carbon, nitrogen, and phosphorus acquisition were assayed by fluorometric microplate methods, including β-1,4-glucosidase (BG), cellulase (S-CL), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (ACP), and alkaline phosphatase (ALP). Microbial nutrient limitation was comprehensively evaluated by ecoenzymatic stoichiometry vector analysis and threshold elemental ratio (TER) analysis, where vector length indicated the degree of carbon limitation and vector angle distinguished nitrogen versus phosphorus limitation (angle>45° for phosphorus limitation). Redundancy analysis (RDA) and partial least squares structural equation modeling (PLS-SEM) were employed to quantify the contributions of different factors to microbial nutrient limitation and reveal the key regulatory pathways. Results showed that parent rocks significantly shaped the physicochemical properties of ferralitic soils and formed distinct DOM pools with characteristic spectral features. Volcanic-derived soils (basalt and volcanic ash) had the highest TOC, AN, AP, AK contents and microbial biomass, with DOM exhibiting the strongest aromaticity and humification degree (HIX>10, SUVA₂₅₄>3.0 L·mg⁻¹·m⁻¹), in which the humic acid-like component C3 was the dominant fluorescent fraction (up to 42.3%). Sandy shale soils had the lowest nutrient contents and microbial biomass, and their DOM showed the highest biological index (BIX>1.0) and the lowest aromaticity, indicating the dominance of labile microbially derived components. Granite soils were enriched in the tryptophan-like component C1 (relative content reaching 38.5%) but displayed significantly suppressed humification with a low HIX value. Metamorphic rock soils had moderate fluorescence indices and a relatively balanced distribution of the four DOM fluorescent components. Soil extracellular enzyme activities also showed obvious lithological differences: volcanic ash and metamorphic rock soils had higher BG and ACP activities, while sandy shale soils exhibited the lowest activities across all enzymes, with ACP activity less than 3% of that in other soils. Microbial nutrient limitation patterns diverged markedly with parent rock types, and all sampling sites showed phosphorus limitation with vector angles exceeding 45°, yet the limitation types and degrees varied significantly. Granite and basalt soils presented typical strong phosphorus limitation with vector angles ranging from 48° to 51° and positive R_C꞉P−TER_C:P values. Sandy shale soils were dominated by extreme carbon limitation with the highest vector length (1.69‒1.71) among all sites, and the negative R_C꞉P−TER_C:P value indicated that the strong carbon energy crisis masked the potential phosphorus limitation. Volcanic ash and metamorphic rock soils had the weakest overall nutrient limitation, showing moderate phosphorus limitation accompanied by slight carbon limitation. Correlation analysis confirmed the close intrinsic links between DOM characteristics and microbial nutrient limitation: vector length was significantly and negatively correlated with TOC and the tyrosine-like component C2 (p<0.05), and vector angle was significantly and negatively correlated with AN and the tryptophan-like component C1 but positively correlated with BIX (p<0.05). RDA explained 92.3% of the total variation in microbial nutrient limitation, with DOM fluorescent components and spectral properties being the dominant driving factors, contributing 35.5% and 25.0% of the independent variation, respectively, which far exceeded soil physicochemical properties (20.3%) and microbial biomass (13.5%). PLS-SEM further verified the cascading regulatory pathway driven by parent rock: parent rock first modulates DOM chemical properties, including aromaticity and humification degree, which then affect soil available nutrient status, and ultimately regulate microbial carbon and phosphorus limitation. High-stability DOM characterized by high HIX, high SUVA₂₆₀, and high relative content of C3 was identified as the key direct driver of both microbial carbon and phosphorus limitation. This study demonstrates that parent rock lithology fundamentally governs microbial nutrient limitation patterns in tropical ferralitic soils by regulating the molecular composition and spectral characteristics of DOM. Volcanic rocks promote the accumulation of aromatic and humified DOM, inducing moderate carbon limitation coupled with strong phosphorus limitation; sandy shale, with its nutrient-poor sandy texture, leads to insufficient labile DOM, resulting in extreme carbon limitation that overshadows potential phosphorus demand; granite-derived soils exhibit suppressed humification and enrichment of protein-like DOM components, presenting the weakest overall nutrient limitation. DOM spectral parameters including HIX, BIX, and specific fluorescent components can serve as effective biogeochemical indicators for diagnosing microbial nutrient status in tropical ferralitic soils. The established “mineral-DOM-microorganism” cascade coupling framework deepens the mechanistic understanding of soil carbon-nutrient coupling processes in tropical ecosystems, and provides important theoretical support for precise nutrient management and carbon sequestration assessment in rubber plantations in tropical laterite areas.

Key words: laterite, parent rock, dissolved organic matter, spectral characteristics, microbial nutrient limitation, enzyme stoichiometry