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

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 (<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.

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.

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.