Soil microbial biomass and nutrient limitation in high altitude treeline ecotones of central Nepal Himalaya

Microbial biomass and nutrient content in soils are crucial indicators of ecosystem health and soil productivity as they reflect intricate relationship among soil organic matter decomposition, nutrient cycling, and resource availability for plants and soil organisms. In the treeline ecotone of Nepal Himalaya regions, there has only been a limited research focus on belowground microbial biomass and how it varies in treeline ecotones of near-natural forest ecosystems, particularly in the context of climate change and dynamic treeline positions. With this research, we tried to fill this research gap by measuring soil microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP) along transects with elevational vegetational zones to assess the nutrient limitation in the forefront of the forest ecotone region. The main objective of our study is to disentangle the relationship between the soil microbiome and nutrient limitation as a controlling factor of tree growth.

We collected 118 soil samples from two slope sectors (northeast and northwest) each with four elevational vegetational zones (3910 to 4260 meter above mean sea level) upper dwarf shrub heath: UD, lower dwarf shrub heath: LD, upper krummholz: UC, lower krummholz: LC. Each zone consisted of four 20x20 m² plots, from which composite samples representing soil horizons were taken. MBC and MBN were measured using fumigation-extraction methods. For MBP, after fumigation-extraction, we slightly modified the quantifying method using Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES).

Our result showed low microbial biomass content in soils of the treeline ecotone indicating nutrient limitation which might influence the growth patterns of vegetation and ecosystem dynamics in the alpine treeline of  Himalaya. Specifically, there was a significant decline in MBC values with increasing soil depth, (p-value ≈ 0), with the highest mean MBC of 561 µg g^-1 dry soil in the O-horizon followed by progressively lower values in the Ah-horizon: 277 µg g^-1, E-horizon: 112 µg g^-1 and Bh-horizon: 56.9 µg g^-1. Similar decreasing trends were observed for mean MBN and MBP. Elevational zone wise variation followed the order of LD > UD> LC> UC for mean MBC and LD>UD>LC>UC for mean MBN. Unlike MBC and MBN, MBP showed significant differences (p-value= 0.011) among four elevational zones and in the decreasing order of LC > UD > LD > UC with mean MBP values of 184 µg g⁻¹, 106 µg g⁻¹, 90 µg g⁻¹ and 86.8 µg g⁻¹ dry soil, respectively. The ratio of MBC and MBN in the UD elevational zone was high, which might be related to the very low MBN content in the soils. MBC and MBN had a strong positive correlation (r = 0.85). Higher microbial biomass values in the higher altitude zones LD and UD than in LC and UC indicate an active microbial pool in open (higher amount of sun radiation) and dwarf  shrub vegetation zone compared to closed canopy in the Rhododendron campanulatum krummholz zone. These findings contribute to a better understanding of nutrient limitations and their role in treeline shift dynamics within the  krummholz dominated upper treeline ecotone in the study area.