Microbial Quotient Responses to Altitudinal and Edaphic Variability in Himalayan Apple Orchard Soil

Abstract

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. This study quantified MQ across four altitude classes (<1550, 1550-1650, 1650-1850 and >1850 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. MQ averaged 3.57 ± 0.08% (range 2.06-8.28%) and declined significantly with altitude, from 4.13% at <1550 m to 3.11% at >1850 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; R2 = 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. 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.