Xiaojuan Feng

Differential mobilization of terrestrial carbon pools in Eurasian Arctic river basins

Mobilization of Arctic permafrost carbon is expected to increase with warming-induced thawing. However, this effect is challenging to assess due to the diverse processes controlling the release of various organic carbon (OC) pools from heterogeneous Arctic landscapes. Here, by radiocarbon dating various terrestrial OC components in fluvially and coastally integrated estuarine sediments, we present a unique framework for deconvoluting the contrasting mobilization mechanisms of surface vs. deep (permafrost) carbon pools across the climosequence of the Eurasian Arctic. Vascular plant-derived lignin phenol 14C contents reveal significant inputs of young carbon from surface sources whose delivery is dominantly controlled by river runoff. In contrast, plant wax lipids predominantly trace ancient (permafrost) OC that is preferentially mobilized from discontinuous permafrost regions, where hydrological conduits penetrate deeper into soils and thermokarst erosion occurs more frequently. Because river runoff has significantly increased across the Eurasian Arctic in recent decades, we estimate from an isotopic mixing model that, in tandem with an increased transfer of young surface carbon, the proportion of mobilized terrestrial OC accounted for by ancient carbon has increased by 3–6% between 1985 and 2004. These findings suggest that although partly masked by surface carbon export, climate change-induced mobilization of old permafrost carbon is well underway in the Arctic.

Molecular-level methods for monitoring soil organic matter responses to global climate change

Soil organic matter (SOM) is one of the most complex natural mixtures on earth. It plays a critical role in many ecosystem functions and is directly responsible for sustaining life on our planet. However, due to its chemical heterogeneity, SOM composition at molecular-level is still not completely clear. Consequently, the response of SOM to global climate change is difficult to predict. Here we highlight applications of two complementary molecular-level methods (biomarkers and nuclear magnetic resonance (NMR)) for ascertaining SOM responses to various environmental changes. Biomarker methods that measure highly specific molecules determine the source and decomposition stage of SOM components. However, biomarkers only make up a small fraction of SOM components because much of SOM is non-extractable. By comparison, (13)C solid-state NMR allows measurement of SOM in its entirety with little or no pretreatment but suffers from poor resolution (due to overlapping of SOM components) and insensitivity, and thus subtle changes in SOM composition may not always be detected. Alternatively, (1)H solution-state NMR methods offer an added benefit of improved resolution and sensitivity but can only analyze SOM components that are fully soluble (humic type molecules) in an NMR compatible solvent. We discuss how these complementary methods have been employed to monitor SOM responses to: soil warming in a temperate forest, elevated atmospheric CO(2) and nitrogen fertilization in a temperate forest, and permafrost thawing in the Canadian High Arctic. These molecular-level methods allow some novel and important observations of soil dynamics and ecosystem function in a changing climate.

Importance of Oceanian small mountainous rivers (SMRs) in global land-to-ocean output of lignin and modern biospheric carbon

The land-to-ocean export of particulate organic carbon (POC) connects carbon flow from the atmosphere through land to the ocean, of which the contemporary fraction that reaches the deep sea for burial may effectively affect atmospheric CO2. In this regard, small mountainous rivers (SMRs) in Oceania, a global erosion hotspot driven by torrential typhoon rain and active earthquakes are potentially important. Here we measured typhoon lignin discharges for Taiwan SMRs. We found that the particulate lignin export in 96 hours by a single SMR amounting to ~20% of the annual export by Mississippi River. The yearly particulate lignin discharge from Taiwan Island (35,980 km2) is governed by the frequency and magnitude of typhoon; thus, the historical lignin export ranged widely from 1.5 to 99.7 Gg yr−1, which resulted in a 10–100 times higher areal yield relative to non-Oceanian rivers. The lignin-derived modern POC output from Oceania region is 37 ± 21 Tg C yr−1, account for approximately 20% of the annual modern POC export from global rivers. Coupled with the hyperpycnal pathway, the forested watersheds of SMRs in Oceania may serve as a giant factory to rapidly produce and efficiently convey modern POC into deep sea for sequestration.

Iron-mediated soil carbon response to water-table decline in an alpine wetland

The tremendous reservoir of soil organic carbon (SOC) in wetlands is being threatened by water-table decline (WTD) globally. However, the SOC response to WTD remains highly uncertain. Here we examine the under-investigated role of iron (Fe) in mediating soil enzyme activity and lignin stabilization in a mesocosm WTD experiment in an alpine wetland. In contrast to the classic ‘enzyme latch’ theory, phenol oxidative activity is mainly controlled by ferrous iron [Fe(II)] and declines with WTD, leading to an accumulation of dissolvable aromatics and a reduced activity of hydrolytic enzyme. Furthermore, using dithionite to remove Fe oxides, we observe a significant increase of Fe-protected lignin phenols in the air-exposed soils. Fe oxidation hence acts as an ‘iron gate’ against the ‘enzyme latch’ in regulating wetland SOC dynamics under oxygen exposure. This newly recognized mechanism may be key to predicting wetland soil carbon storage with intensified WTD in a changing climate.

Soil gross nitrogen transformations along the Northeast China Transect (NECT) and their response to simulated rainfall events

Climate changes are predicted to increase extreme rainfall events in semiarid and arid region in Northern Hemisphere. Nutrient cycles will be affected by the precipitation changes but so far only very little is known how soil N transformations may respond. Here we investigated gross soil N transformation rates and their response to simulated rainfall events across Northeast China Transect (NECT). The results showed that gross N mineralisation rate, nitrification rate and nitrification to mineralisation ratio significantly increased as the humidity index decreased along NECT, resulting in NO3− as the predominant inorganic N form. These characteristics could increase the risk of NO3− losses but at the same time reduce the risk of N losses via volatilization in the semiarid and arid region. The soil-plant ecosystems have developed effective N conservation strategies in the long term with respect to the prevailing climate in arid region. However, compared to humid soils more dramatic changes of soil N transformation rates are likely to occur in arid soils, after sudden soil moisture increases. Soil N conservation mechanisms in arid regions were drastically affected when the heavy rainfall frequently occurred. Arid ecosystems are expected to be more vulnerable than humid ecosystems in response to extreme rainfall events.

Soil N transformations and its controlling factors in temperate grasslands in China: A study from 15N tracing experiment to literature synthesis

Temperate grasslands in arid and semiarid regions cover about 40% of the total land area in China. So far, only a few studies have studied the N transformations in these important ecosystems. In the present study, soil gross N transformation rates in Inner Mongolia temperate grasslands in China were determined using a 15N tracing experiment and combined with a literature synthesis to identify the soil N transformation characteristics and their controlling factors in a global perspective. Our results showed that the rates of gross N mineralization and immobilization NH4+ were significantly lower, while autotrophic nitrification rates were significantly higher in Chinese temperate grassland soils compared to other regions in the world. In particular, the primary mineral N consumption processes, i.e., immobilization of NO3− and NH4+, and dissimilatory nitrate reduction to ammonium, were on average much lower in temperate grassland soils in China, compared to other temperate grassland regions. The reduced heterotrophic activity and microbial growth associated with lower soil organic carbon and arid climate (e.g., mean annual precipitation) were identified as the main factors regulating soil N cycling in the studied regions in China. To restrict NO3− accumulation and associated high risks of N losses in these arid and semiarid ecosystems in China, it is important to develop the regimes of soil organic C and water management that promote the retention of N in these grassland ecosystems.

Different composition and distribution patterns of mineral‐protected versus hydrolyzable lipids in shrubland soils

Mineral protection is known as an important mechanism stabilizing soil organic carbon (SOC). However, the composition, sources and variations of mineral-protected SOC remain poorly constrained. To fill this knowledge gap, we used hydrofluoric acid to demineralize soil matrix and compared the sources and distribution of mineral-protected lipids (ML) versus hydrolysable lipids (HL) of four typical Chinese shrubland soils. ML was found to represent a sizable fraction (9–32%) of total aliphatic lipids (including n-alkanols, n-alkanoic acids, α,ω-alkanedioic acids, hydroxyalkanoic acids and mid-chain-substituted acids) in all soils. Based on carbon chain length and branch positions, microbial- and plant-derived lipids were distinguished. No significant difference was found in the ratio of microbial- to plant-derived lipids in ML versus HL, implying that plant and microbial inputs are equally important for the mineral-associated soil lipids. However, ML contained a higher proportion of non-specific lipids, especially at depths. Furthermore, to evaluate key environmental variable(s) controlling the distribution of different lipid components, a multiple stepwise regression analysis was conducted. Notably, ML was mainly affected by SOC-to-nitrogen ratio instead of mineralogical properties, implying that the accrual of mineral-associated soil lipids relies strongly on organic matter properties. Collectively, our findings provide novel insights on sources and accumulation mechanisms of mineral-protected soil lipids. SOC decomposition and subsequent accretion of degradation products appear to be vital for the sequestration of mineral-associated soil lipids and warrant better recognition in the investigations of stable soil carbon accumulation mechanisms.

Distribution of fatty acids in the alpine grassland soils of the Qinghai-Tibetan Plateau

As an important biomarker, fatty acids (FAs) have been extensively used to trace the origin of organic matter in sediments and soils. However, studies of the distribution and abundance of FAs in alpine grassland soils are still rare, especially on the Qinghai-Tibetan Plateau (QTP), the highest plateau in the world, which contributes sediments to many large rivers in Asia. This study investigates the composition, distribution and source of FAs with increasing soil depths from 17 typical alpine grassland sites in the QTP. The most abundant FAs included the ubiquitous C16 FA and even-numbered long-chain FAs (C20–C30), indicating mixed inputs from microbial and higher plant sources. Source apportionment showed that higher plants were the dominant contributor of FAs (approximately 40%) in QTP soils. The abundance of FAs decreased with soil depth, with the highest value (1.08±0.09 mg/g OC) at a 0–10 cm depth and the lowest value (0.46±0.12 mg/g OC) at a 50–70 cm depth, due to much lower plant inputs into the deeper horizons. The total concentration of FAs was negatively correlated to the mean annual temperature (MAT; P<0.05) and soil pH (P<0.01), suggesting that the preservation of FAs was favored in low-MAT and low-pH soils on the QTP. The abundance of fresh OC source FAs increased significantly with the mean annual precipitation (MAP; P<0.05), indicating that high MAP facilitates the accumulation of fresh FAs in QTP soils. Other environmental parameters, such as the soil mineral content (aluminum and iron oxide), microbial community composition as well as litter quality and quantity, may also exert a strong control on the preservation of FAs in QTP soils and warrant further research to better understand the mechanisms responsible for the preservation of FAs in QTP soils.

Divergent accumulation of microbial necromass and plant lignin components in grassland soils

The means through which microbes and plants contribute to soil organic carbon (SOC) accumulation remain elusive due to challenges in disentangling the complex components of SOC. Here we use amino sugars and lignin phenols as tracers for microbial necromass and plant lignin components, respectively, and investigate their distribution in the surface soils across Mongolian grasslands in comparison with published data for other grassland soils of the world. While lignin phenols decrease, amino sugars increase with SOC contents in all examined grassland soils, providing continental-scale evidence for the key role of microbial necromass in SOC accumulation. Moreover, in contrast to clay’s control on amino sugar accumulation in fine-textured soils, aridity plays a central role in amino sugar accrual and lignin decomposition in the coarse-textured Mongolian soils. Hence, aridity shifts may have differential impacts on microbial-mediated SOC accumulation in grassland soils of varied textures.It remains unclear how microbes and plants contribute to soil organic carbon (SOC) accrual. Here, using biomarkers, the authors show that microbial necromass and plant-derived lignin components have divergent accumulation mechanisms and that microbial necromass plays a key role in SOC accumulation.

Large‐Scale Distribution of Molecular Components in Chinese Grassland Soils: The Influence of Input and Decomposition Processes

Chinese grasslands hold a third of the national soil organic carbon (OC) stocks but remain poorly investigated in terms of soil molecular components and their distribution patterns. Such information is important for understanding mechanisms governing grassland soil OC dynamics and its response to global changes. Here employing solvent-extractable compounds as a group of widely used biomarkers, we present a large-scale study on the distribution of different soil OC components (including plantand microbial-derived carbohydrates and aliphatic and cyclic lipids) in the surface soils of Chinese grasslands, spanning from temperate grasslands in the arid/semiarid regions to alpine grasslands on the Qinghai-Tibetan Plateau. We show that alpine grassland soils are more enriched with carbohydrates and plant-derived compounds relative to the temperate counterparts due to temperature-inhibited decomposition. While plant belowground biomass plays a key role in explaining the spatial variation of compounds in the alpine grasslands, climatic variables do in the temperate region. In particular, aliphatic lipids accumulate with increasingmean annual temperature in the temperate grasslands due to a preferential decay of labile soil OC, whereas they decrease in the alpine grasslands owing to dilution by an enhanced plant input of nonlipid components. Collectively, these results demonstrate different mechanisms governing the distribution of solvent-extractable compounds in grassland soils, with climate-mediated decomposition processes dominating in the temperate grasslands and plant inputs being more important in the alpine region. In the context of climate change, alterations to soil OC input and decomposition processesmay have varied impacts on soil carbon cycling in these two regions.