Soil Respiration in Temperate and Tropical Ecosystems: The Role of Abiotic and Biotic Factors

Abstract

Soil respiration (Rs) represents a major carbon flux from terrestrial ecosystems to the atmosphere and an important component of ecosystem carbon cycling. The magnitude and temporal variation of Rs are influenced by a combination of environmental and biological factors. Despite the important role of Rs in the global C cycle, field-based observations across diverse ecosystems remain limited, constraining our understanding of how environmental and biological factors regulate soil CO2 efflux. The current project explored the variations of Rs across temperate dairy grassland, temperate Pinus radiata plantation forest, and tropical forest systems through comparative field-based measurements under natural conditions. The objective was to identify how Rs co-varied with temperature, moisture, and broader site context, and to examine the extent to which these relationships differed among ecosystems.

In New Zealand dairy grasslands, observed Rs rates at the sampled sites were higher than the global temperate grassland values used for comparison. Seasonal variation was apparent with higher Rs in summer than in winter. Soil temperature and soil type were more strongly associated with Rs variation than soil moisture, although these relationships differed among sites and should be interpreted within the limits of short-term sampling and site-specific conditions.

In Pinus radiata plantation forests, seasonal Rs was more strongly related to soil moisture than soil temperature. Stand age was associated with shifts in soil microbial community composition, including changes in dominant bacterial and fungal groups, while stand-age-related differences in Rs were less consistent. These findings suggest that the changes in microbial community across stand development were more clearly observable than a straightforward trend in Rs with stand age under the sampled conditions.

In the tropical forest study, Rs varied among forest types and landscape positions. Plantation and native forest sites showed overlapping ranges of Rs, but the observed patterns were affected by co-varying environmental gradients, such as elevation, moisture, and site context. Vegetation type should therefore be interpreted as an integrated indicator of multiple environmental and site factors rather than as a standalone driver.

In summary, this thesis shows that Rs responses varied depending on the context across all three ecosystems. Temperature and moisture were significant factors influencing Rs, but their apparent impact depended on site conditions, soil characteristics, and broader environmental setting. Within clearly defined interpretive limits, this thesis offers comparative evidence that supports more cautious, context-aware interpretation of soil respiration patterns across ecosystems, rather than universal or mechanistic claims.