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Subsoils contain >50% of soil organic carbon (SOC) globally yet remain under-investigated in terms of their response to climate changes. Recent evidence suggests that warmer, drier conditions in alpine grasslands induce divergent responses in SOC decomposition and carbon accrual in top- versus subsoils. However, longer term effects on microbial activity (i.e., catabolic respiration vs. anabolic growth) and belowground carbon cycling are not well understood. Here we utilized a field manipulation experiment on the Qinghai-Tibetan Plateau and conducted a 110-day soil incubation with and without 13 C-labeled grass litter to assess microbes' role as both SOC "decomposers" and "contributors" in the top- (0-10 cm) versus subsoils (30-40 cm) after 5 years of warming and drought treatments. Microbial mineralization of both SOC and added litter was examined in tandem with potential extracellular enzyme activities, while microbial biomass synthesis and necromass accumulation were analyzed using phospholipid fatty acids and amino sugars coupled with 13 C analysis, respectively. We found that warming and, to a lesser extent, drought decreased the ratio of inorganic nitrogen (N) to water-extractable organic carbon in the subsoil, intensifying N limitation at depth. Both SOC and litter mineralization were reduced in the subsoil, which may also be related to N limitation, as evidenced by lower hydrolase activity (especially leucine aminopeptidase) and reduced microbial efficiency (lower biomass synthesis and necromass accumulation relative to respiration). However, none of these effects were observed in the topsoil, suggesting that soil microbes became inactive and inefficient in subsoil but not topsoil environments. Given increasing belowground productivity in this alpine grassland under warming, both elevated root deposits and diminished microbial activity may contribute to new carbon accrual in the subsoil. However, the sustainability of plant growth and persistence of subsoil SOC pools deserve further investigation in the long term, given the aggravated N limitation at depth., (© 2021 John Wiley & Sons Ltd.)

Subsoil contains more than half of soil organic carbon (SOC) globally and is conventionally assumed to be relatively unresponsive to warming compared to the topsoil. Here, we show substantial changes in carbon allocation and dynamics of the subsoil but not topsoil in the Qinghai-Tibetan alpine grasslands over 5 years of warming. Specifically, warming enhanced the accumulation of newly synthesized ( 14 C-enriched) carbon in the subsoil slow-cycling pool (silt-clay fraction) but promoted the decomposition of plant-derived lignin in the fast-cycling pool (macroaggregates). These changes mirrored an accumulation of lipids and sugars at the expense of lignin in the warmed bulk subsoil, likely associated with shortened soil freezing period and a deepening root system. As warming is accompanied by deepening roots in a wide range of ecosystems, root-driven accrual of slow-cycling pool may represent an important and overlooked mechanism for a potential long-term carbon sink at depth. Moreover, given the contrasting sensitivity of SOC dynamics at varied depths, warming studies focusing only on surface soils may vastly misrepresent shifts in ecosystem carbon storage under climate change., (© 2019 John Wiley & Sons Ltd.)

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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]

• Long-term drainage induced divergent changes of SOC contents in fen and bog. • Both plant- and microbial-dominated soil fractions increased in drained fen. • Microbial sugars increased at the expense of plant sugars in drained bog. • Drainage enhanced soil "microbial carbon pump" both in fen and bog. Drainage-induced changes in wetland soil organic carbon (SOC) composition and origin are poorly investigated compared to SOC stocks. Here, using soil fractionation and neutral sugars, we find that long-term drainage increased both plant- and microbial-dominated (i.e., light and mineral-associated, respectively) fractions in a fen while microbial residues increased at the expense of plant residues in a drained bog, accompanied by divergent changes of SOC contents. These findings highlight stimulated soil "microbial carbon pump" in drained wetlands, whose efficiency deserves further investigation related to wetland SOC persistence. [ABSTRACT FROM AUTHOR]