Comparative Assessment of Soil Microbial Biomass Across Land-Use Types

Thesis Abstract

Soil microbial biomass is a key driver of nutrient cycling and soil health, yet its magnitude and composition across land-use types remain incompletely understood in agroecosystems and natural landscapes. This study addresses the problem of variable microbial biomass responses to land-use change and management, and how these differences influence soil fertility, carbon sequestration potential, and ecosystem resilience under global change pressures. The aim is to quantify and compare soil microbial biomass across cropland, pasture, forest, and urban green spaces, and to relate these differences to soil physical-chemical properties, antecedent land-use history, and management practices. Specific objectives are (1) to estimate microbial biomass carbon and nitrogen across four land-use types using substrate-induced respiration (SIR) and fumigation-extraction methods; (2) to evaluate how soil organic carbon, pH, texture, moisture, and soil nutrients mediate microbial biomass using regression analyses and structural equation modeling; (3) to test the consistency of microbial biomass patterns with the soil carbon–nutrient dynamics framework proposed by the microbial efficiency-mass ratio theory and the resource-rummer model; and (4) to provide land-use specific implications for soil health and carbon management. The methodology employs a cross-sectional design across representative sites within a temperate region. The population comprises surface soils (0–20 cm) from four land-use categories cropland under conventional and reduced-tillage practices, permanent pasture, mixed-species temperate forest, and urban green space. A stratified random sampling scheme yields 40 soil cores per site, totaling 160 samples per land-use type (640 samples overall) to ensure robust comparisons. Data collection integrates laboratory measurements of microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) via the chloroform fumigation-extraction method, and substrate-induced respiration for corroborative biomass estimates. Complementary analyses include soil organic carbon by dry combustion, total nitrogen, pH, cation exchange capacity, texture by hydrometer, bulk density, moisture content, and inorganic N forms. Secondary data on land-management intensity, rotation length, fertilizer inputs, mowing frequency, and historical land-use are compiled from site records and interviews with land managers. Data analysis proceeds with descriptive statistics to summarize key variables, followed by analysis of variance (ANOVA) to detect overall differences across land-use types. Multiple regression and hierarchical linear modeling examine relationships between microbial biomass and soil properties, while structural equation modeling (SEM) assesses causal pathways linking land-use, soil attributes, and microbial biomass. Model selection relies on AIC and fit indices (CFI, RMSEA). The study also tests for interaction effects between management practices and soil properties. Expected findings include higher MBC and MBN in forest soils compared with cropland and urban soils, with pasture exhibiting intermediate values, moderated by soil organic matter and pH. It is anticipated that soil moisture and texture will significantly influence microbial biomass, with finer textures and higher moisture supporting greater biomass in forest and pasture soils. The SEM is expected to reveal that land-use indirectly controls microbial biomass through soil organic carbon and pH, aligning with the microbial carbon use efficiency framework and the resource-based view of soil microbial communities. The results will identify key thresholds in soil properties beyond which microbial biomass responds markedly to land-use changes. The study's contribution to knowledge lies in providing a comparative, multi-land-use quantification of soil microbial biomass anchored in robust laboratory measurements and advanced modeling, clarifying the mediating role of soil properties in biomass responses, and informing land-management strategies for soil health, carbon sequestration, and sustainable productivity. The findings will be pertinent to theories of microbial ecology in soils, including the microbial efficiency–mass ratio and resource-based models, and will offer empirical benchmarks for policy and practice in land-use planning. The main conclusion is that land-use type and management significantly shape soil microbial biomass, primarily through alterations in soil organic carbon and pH, and that restoration or avoidance of soil acidity and organic matter depletion is critical for sustaining microbial-mediated soil functions. Recommendations include promoting organic matter inputs, adopting conservation tillage or reduced-tillage systems in cropland, maintaining perennial vegetation in urban greenspaces to support microbial resilience, and integrating microbial biomass monitoring into soil health assessment protocols to guide land-use decisions and carbon management strategies.