Large, old pools of carbon and microbial communities are present deep in soils under a temperate planted forest

Abstract. Forest soils are fundamental in regulating the global carbon (C) cycle; their capacity to accumulate large stores of C means they are vital in mitigating the effects of climate change. Understanding the processes that regulate forest soil organic C (SOC) dynamics and stabilisation is important to maximise the capacity and longevity of C sequestration. Compared to surface soil layers, little is known about the SOC dynamics in subsoil layers, sensu those below 30 cm depth. This knowledge gap creates large uncertainties when estimating the global distribution and vulnerability of SOC reserves to climate change. This study aimed to dive deep into the subsoils of Puruki Experimental Forest (New Zealand) and characterise the incremental changes in SOC dynamics and the soil microbiome down to 1 metre 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 SOC stability and age. We conservatively estimate more than 35 % of total C stocks are present in subsoil layers below 30 cm. Although C age steadily increased with depth, reaching a mean radiocarbon age of 1571 yBP (years before present) in the deepest soil layers, the stability of SOC varied between different subsoil depth increments. Declines in soil carbon were associated with lower microbial diversity, abundance, and significant shifts in community membership. These research findings highlight the importance of quantifying subsoil C stocks for accurate systems-level global and local C budgets and modeling. Furthermore, performing a broad range of analytical measures (i.e. ¹³C & ¹⁴C natural abundance, and microbiome analysis) is vital to assess the vulnerability of subsoil C to climate change.