Soil depth as a driver of microbial and carbon dynamics in a planted forest (Pinus radiata) pumice soil

Abstract

Abstract. Forest soils are fundamental in regulating the global carbon (C) cycle; their capacity to accumulate large stores of C means they form a vital role in mitigating the effects of climate change. Understanding the processes that regulate forest soil C dynamics and stabilisation is important to maximise the capacity and longevity of C sequestration. Compared with surface soil layers, little is known about soil C dynamics in subsoil layers, sensu those below 30 cm depth. This knowledge gap creates large uncertainties when estimating the distribution of global soil C stocks and assessing the vulnerability of soil C reserves to climate change. This study aimed to dive deep into the subsoils of Puruki Experimental Forest (New Zealand) and characterise the changes in soil C dynamics and the soil microbiome down to 1 m soil depth. ITS and 16S rRNA sequencing and quantitative real-time PCR were used to measure changes in soil microbial diversity, composition, and abundance. Stable (δ13C) and radioactive (14C) C analyses were performed to assess depth-driven changes in the stability and age of soil C. Our research identified large declines in microbial diversity and abundance with soil depth, alongside significant structural shifts in community membership. Importantly, we conservatively estimate that more than 35 % of soil C stocks are present in subsoil layers below 30 cm. Although the age of soil C steadily increased with depth, reaching a mean radiocarbon age of 1571 yr BP (years before present) in the deepest soil layers, the stability of soil C varied between different subsoil depth increments. These research findings highlight the importance of quantifying subsoil C stocks for accurate C accounting. By performing a broad range of analytical measures, this research has comprehensively characterised the abiotic and biotic properties of a subsoil environment – a frequently understudied but significant component of forest ecosystems.

Distribution of Genes and Microbial Taxa Related to Soil Phosphorus Cycling across Soil Depths in Subtropical Forests

“…According to previous studies [53,55], the archaea taxa associated with soil P cycling was more abundant in deeper mineral soil than in surface organic soil (Figure 2). Several reasons have been documented for the increasing abundance of archaea taxa with soil depth, including their adaptation to nutrient-limited conditions as slow-growing oligotrophs, adaption to chronic energy stress, preference as methanogens for anaerobic conditions, and function as ammonia oxidizers to stimulate autotropic nitrification in deeper soil [53,54,59,60]. In addition, the total relative abundance of fungi taxa that harbored soil P cycling genes was lower than 0.01% (Figure 2).…”

“…The bacteria phyla Actinobacteria, Bacteroidetes, and Candidatus_Eremiobacteraeota decreased in abundance with depth (Figure 2), which may be attributed to their copiotrophic behavior [52,53]. The significant increases in the abundance of Acidobacteria, Verrucomicrobia, and Chloroflexi with depth (Figure 2) have been previously evidenced by their tolerance for nutrient-poor conditions [54,55].…”

“…Furthermore, copiotrophic taxa exhibit fast growth rates with high soil C availability, whereas oligotrophic taxa grow slowly by metabolizing recalcitrant C and nutrient-poor substrates [57]. The variations in copiotrophic and oligotrophic taxa with soil depth may be directly influenced by the decreases in the available soil P concentration, N:P ratio, and SOC, or they are indirectly affected by the increase in soil pH in mineral soil [6,53,58]. For example, the abundance of oligotrophic Acidobacteria significantly decreased with the soil N:P ratio (R 2 = 0.35, p < 0.05) in this study.…”