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Lignin is a key component of soil dissolved organic carbon (DOC) and is recently suggested to track 14C-young DOC components. However, direct evidence is still lacking to prove this hypothesis in the soil. Here, utilizing molecular radiocarbon dating, we present the first 14C dataset on dissolved lignin through a Podzol soil profile. Dissolved lignin and hydroxy phenols had similar 14C content as soil organic carbon (SOC) and DOC in the surface organic layer. However, in contrast to SOC, both DOC and dissolved lignin phenols exhibited consistent and higher Δ14C values in the mineral soils. Coupled with lignin phenol concentration data, our results suggest that dissolved lignin comprises a key DOC component throughout this Podzol profile and is mainly supplied by surface leachates with young 14C ages. Image 1 • We present the first 14C dataset on dissolved lignin along a Podzol profile. • Dissolved phenols exhibited similar Δ14C values to SOC and DOC in surface organic layer. • Unlike SOC, DOC and dissolved lignin were enriched in 14C in mineral soils. • Surface leachate was the main source of DOC components in this Podzol profile. • Dissolved lignin may exhibit larger 14C variability in other (non-Podzol) profiles. [ABSTRACT FROM AUTHOR]

Subsoils of alpine grasslands on the Qinghai-Tibetan Plateau represent a tremendous yet poorly investigated reservoir of soil organic carbon (SOC) on a global “hotspot” of warming. Compared with the temperature sensitivity of SOC decomposition, microbial anabolism of new carbon and priming of native SOC remain poorly constrained under warming-enhanced labile carbon input in these subsoils. Here we employed an innovative approach to investigate the sequestration of freshly added carbon in microbial necromass versus SOC priming in the top- (0–10 cm) and subsoils (30–40 cm) from a field experiment that simulated varied warming scenarios in an alpine grassland on the Qinghai-Tibetan Plateau. The 13 C composition of microbial necromass-derived amino sugars was analyzed in tandem with respired CO 2 and dissolved SOC components (including dissolved lignin) in an 86-day laboratory incubation with 13 C-labeled glucose. A higher fraction of freshly added carbon was respired while a smaller proportion was stabilized as amino sugars in the subsoil relative to the topsoil, leading to a much lower microbial carbon accumulation efficiency at depth. Meanwhile, a higher relative priming effect was observed in the subsoil (47± 14%) compared to the topsoil (14± 4%), suggesting a higher vulnerability to substrate-induced SOC loss at depth, although such changes may be associated with higher glucose addition rate (relative to SOC) in the subsoil. Furthermore, enhanced winter warming significantly reduced degradable SOC (assessed by SOC mineralization and dissolved lignin content) in the subsoil and potentially intensified nitrogen limitation under labile carbon additions, which further decreased microbial carbon accumulation (in the form of amino sugars) in the subsoil without affecting the topsoil. These results collectively indicate a limited microbial carbon sequestration potential and a higher vulnerability to warming-induced substrate changes in the subsoil of this alpine grassland, which warrants better understanding to predict soil carbon responses to climate warming on the Qinghai-Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Microbial necromass carbon (MNC) is key to soil organic carbon (SOC) storage. However, mechanisms regulating MNC accumulation on large scales are poorly understood. Here we provide the first batch of regional-scale MNC data based on amino sugars for the Qinghai-Tibet Plateau alpine grasslands. We show that Qinghai-Tibet grasslands have similar microbial biomass carbon (MBC) but lower MNC concentrations in SOC than Mongolian and other grasslands. The low contribution of MNC to SOC is mainly attributed to high aridity and low net primary productivity of the Qinghai-Tibet grasslands. Our findings highlight climatic and plant influences on MNC accumulation at regional scales. • Qinghai-Tibet grasslands have lower microbial necromass carbon (MNC) in SOC. • The MNC content of grasslands is regulated by aridity at global scale. • Low MNC in Qinghai-Tibet grasslands is attributed to the low NPP and high aridity. [ABSTRACT FROM AUTHOR]

Drainage poses a major threat to the tremendous soil organic carbon (SOC) reservoir in wetlands. However, drainage-induced carbon loss may be mitigated by the accumulation of mineral-associated organic carbon (MAOC) via both microbial processes and iron (Fe)-organic matter (especially lignin) interactions, which remains under-investigated in wetlands. Here using a novel analytical approach, we quantitatively examined the response of MAOC to drainage and the driving mechanisms in four distinct wetlands. Contrary to the prevailing assumption, MAOC consisted a significant fraction (7%∼91%) of wetland SOC, and increased with mineral-bound (microbial-dominated) sugars, but decreased with Fe-bound lignin phenols in SOC assessed after removal of reactive soil minerals. Furthermore, mineral-bound sugars increased with clay and reactive aluminum instead of Fe, and overrode Fe-bound lignin regarding contribution to MAOC after long-term drainage. These results indicate that microbial processes leading to microbial sugars accumulation played a prominent role in wetland MAOC accrual after long-term drainage, especially in soils rich in reactive Al and clay. Our study highlights the prevalent yet under-investigated microbial regulation on MAOC accrual in wetlands during long-term drainage. Given the large stock and persistence of MAOC in wetlands, its increase may partly compensate for particulate organic carbon loss during drainage, and may underpin wetland SOC stabilization in the long term. [Display omitted] • Mineral-associated organic carbon (MAOC) consisted 7–91% of SOC in four wetlands. • MAOC increased with microbial-dominated sugars mainly bound to clay and Al. • MAOC decreased with Fe-bound lignin phenols in SOC. • Mineral-bound sugars overrode Fe-bound lignin in MAOC after long-term drainage. [ABSTRACT FROM AUTHOR]