Work

Microbial quotient responses to altitudinal and edaphic variability in Himalayan apple orchard soil

Year: 2026

Type: article

Abstract: Introduction Soil biological health is a critical determinant of sustainable fruit production in mountain ecosystems, yet microbial dynamics in high-altitude orchards remain poorly understood. Whereas total organic carbon (TOC) changes only slowly, the microbial quotient (MQ)—the ratio of microbial biomass carbon (MBC) to TOC—responds early and sensitively to shifts in organic matter turnover and soil biological quality. Methods This study quantified MQ across four altitude classes (<1,550, 1,550–1,650, 1,650–1,850 and >1,850 m a.s.l.), three orchard age classes (<8, 8–14, and >14 years) and three soil depths (0–25, 25–50, and 50–75 cm) in apple orchards of the Jammu Himalaya, using 144 stratified samples. MBC and TOC were measured and MQ computed as (MBC/TOC) × 100; dehydrogenase (DHA) and alkaline phosphatase activities were determined as supporting indicators. Data were analysed by three-way ANOVA, Tukey HSD, Pearson correlation, principal component analysis (PCA) and regression. Results MQ averaged 3.57 ± 0.08% (range 2.06–8.28%) and declined significantly with altitude, from 4.13% at <1,550 m to 3.11% at >1,850 m ( p < 0.005), the inverse of the MBC and TOC trends. MQ was lowest in surface soil (2.82%) and highest in the 25–50 cm layer (4.14%; p < 0.005) and decreased modestly with orchard age ( p = 0.010); significant altitude × depth and altitude × age interactions were detected. MQ correlated strongly and negatively with TOC ( r = −0.745; R 2 = 0.556) but was decoupled from MBC ( r = −0.082, ns), indicating that MQ carries information about the substrate-normalised size of the biomass pool that absolute MBC does not. PCA separated a biomass-substrate axis (PC1, 48.3%) from an MQ-dominated axis. Discussion Because microbial carbon-use efficiency, community composition and necromass were not measured directly, the efficiency interpretation of these gradients is advanced as a hypothesis consistent with the data rather than as a demonstrated mechanism. The study nevertheless establishes MQ as a sensitive, substrate-normalised descriptor of biological soil quality and provides the first MQ baseline for mountain apple orchards of the north-western Himalaya.

Source: Frontiers in Microbiology

DOI: 10.3389/fmicb.2026.1957054