Soil organic carbon and labile and recalcitrant carbon fractions attributed by contrasting tillage and cropping systems in old and recent alluvial soils of subtropical eastern India

Conservation agriculture-based sustainable intensification (CASI) technologies comprising zero-tillage with crop residue retention (>30%) on the soil surface, diversified cropping systems, and balanced nutrient management are recognized as operative and efficacious strategies to ensure food security in the parts of South Asia. The present investigation was a component of CASI technologies undertaken in the farmers’ field of Malda (old alluvial Inceptisol) Coochbehar (recent alluvial Entisol) district, West Bengal (subtropical eastern India). This study was conducted to evaluate the short-term impact of contrasting tillage (zero and conventional) and cropping systems (rice–wheat and rice–maize) on total organic carbon (TOC) and its fractions, viz., labile pool-1 (LP1), labile pool-2 (LP2) and recalcitrant carbon (RC) fractions after 4-year trial of conservation agriculture (CA) in the old and recent alluvial soils. Soil samples were collected from three depths (0–5, 5–10, and 10–20 cm), and thus, our study was focused on two factors, viz., cropping system and tillage. Results pointed that TOC along with LP1, LP2, and RC fractions under rice–maize (RM) cropping system were significantly (p<0.05) greater (15–35%) over rice–wheat (RW) system as a result of higher residue biomass addition. Zero-tillage (ZT) improved the C fractions by 10–20% over conventional tillage (CT) in all aspects. TOC and its fractions were observed to be greater under the ZT system in the topmost soil depths (0–5 and 5–10 cm), but the same system failed to improve these at 10–20 cm. Interestingly, the CT increased all the fractions at 10–20 cm depth due to the incorporation of crop residues. The concentration of TOC along with its fractions decreased with increasing soil depth was evident. Comparatively, all the C fractions, including TOC were maximum in soils from Malda sites as compared to Coochbehar sites because of a higher amount of residue biomass application, higher clay content, and greater background content of C in these soils. All the studied C fractions showed a significant correlation (r = >0.635; p<0.01) with TOC among all the soil depths in both the districts but the relationship with soil texture showed some interesting results. TOC fractions were significantly correlated (p<0.01) with clay particles indicating that its higher stabilization with clay in old alluvial Inceptisol (Malda); while in recent alluvial Entisol (Coochbehar), sand particle showed its strong relation with TOC fractions. Higher stratification ratio (SR) in the ZT system suggested that the concentration of TOC and its fractions are confined to the upper soil layers whereas in the case of CT, by and large, the distribution of these was comparatively high in subsequent soil depths due to residue incorporation effect. The concentration of C fractions in soils followed the order: TOC > RC > LP2 > LP1. The present investigation concluded that ZT under the RM system increases the turnover rates of C in both soil types but the amount of clay influences the stabilization/storage of C.

Towards understanding kraft lignin depolymerisation under hydrothermal conditions

Kraft lignin depolymerisation using hydrothermal liquefaction suffers from the formation of char, resulting in a decreased product yield as well as causing operational problems. While this may be mitigated by the addition of capping agents such as phenol and isopropanol, other reaction parameters, for example reaction time and temperature, are also important for the product yields. In this work, the effect of short reaction times on the hydrothermal liquefaction of kraft lignin in an alkaline water and isopropanol mixture was investigated at 1–12 min and 290 °C. The results show that there were swift initial reactions: the major ether bonds in the lignin were broken within the first minute of reaction, and the molecular weight of all product fractions was halved at the very least. Longer reaction times, however, do not cause as pronounced structural changes as the initial reaction, indicating that a recalcitrant carbon-carbon skeleton remained in the products. Nevertheless, the yields of both char and monomers increased slowly with increasing reaction time. The swift initial depolymerising reactions were therefore followed by slower repolymerisation as well as a slow formation of monomers and dimers, which calls for careful tuning of the reaction time.

Soil Aggregate-Associated Carbon Fraction Dynamics during the Process of Tea (Camellia sinensis L.) Planting in Southern Guangxi, China

Revealing the variation in soil aggregate-associated organic carbon (Corg) in tea plantations of various planting ages is crucial to shed more light on the accumulation and decomposition of soil Corg in the tea-planting period. This study measured the concentrations of soil Corg, active carbon (Cact), and recalcitrant carbon (Crec) in different-sized aggregates obtained from tea plantations of various planting ages (8, 17, 25, and 43 years old) at the soil depths of 0–20 and 20–40 cm in southern Guangxi, China. According to the wet-sieving approach, soil aggregates were classified as macro- (>0.25 mm) and micro- (<0.25 mm) aggregates, and the former were further divided into coarse (>2 mm), medium (2–1 mm), and fine (1–0.25 mm) fractions. Based on the mean weight diameter (MWD), the stability of soil aggregates was the highest in the 17-year-old tea plantations, and it was closely related to the concentration of soil Cact (0–20 cm: R2 = 0.9744, p < 0.05; 20–40 cm: R2 = 0.8951, p < 0.05), but not Corg (0–20 cm: R2 = 0.1532, p > 0.05; 20–40 cm: R2 = 0.4538, p > 0.05), during the tea-planting process. In the 0–20 and 20–40 cm soil layers, the coarse and medium macro-aggregates had higher concentrations of Corg, Cact, and Crec, regardless of the tea-planting age; meanwhile, the soil Cact/Crec ratio, indicating the Corg availability, increased as aggregate size increased, implying that the soil Corg was younger and more labile in coarse macro-aggregates relative to finer aggregates. Moreover, the tea-planting age significantly affected the Corg, Cact, and Crec reserves in both soil layers. To be specific, continuous tea planting facilitated the accumulation of soil Corg and Crec, but their reserves’ increase rates decreased over time; meanwhile, the soil Cact reserve increased during the early (from 8 to 17 years) tea-planting stage and later decreased. Therefore, during the middle (from 17 to 25 years) and late (from 25 to 43 years) tea-planting stages, maintaining the soil as an Cact pool plays a vital role in facilitating the formation and stabilization of soil aggregates in southern Guangxi, China.

Vegetation and microbes interact to preserve carbon in many wooded peatlands

Peatlands have persisted as massive carbon sinks over millennia, even during past periods of climate change. The commonly accepted theory of abiotic controls (mainly anoxia and low temperature) over carbon decomposition cannot fully explain how vast low-latitude shrub/tree dominated (wooded) peatlands consistently accrete peat under warm and seasonally unsaturated conditions. Here we show, by comparing the composition and ecological traits of microbes between Sphagnum- and shrub-dominated peatlands, that slow-growing microbes decisively dominate the studied shrub-dominated peatlands, concomitant with plant-induced increases in highly recalcitrant carbon and phenolics. The slow-growing microbes metabolize organic matter thirty times slower than the fast-growing microbes that dominate our Sphagnum-dominated site. We suggest that the high-phenolic shrub/tree induced shifts in microbial composition may compensate for positive effects of temperature and/or drought on metabolism over time in peatlands. This biotic self-sustaining process that modulates abiotic controls on carbon cycling may improve projections of long-term, climate-carbon feedbacks in peatlands.

Re-visiting a long-term Free Air CO2 Enrichment (FACE) experiment in a Danish heathland/grassland ecosystem (CLIMAITE) reveals highly dynamic soil carbon

&lt;p&gt;The feedback of the terrestrial carbon cycle to global climate change is among the largest uncertainties in climate change research. To test the potential ecosystem effects of future climate scenarios, a field-scale FACE (Free Air CO&lt;sub&gt;2&lt;/sub&gt; Enrichment) experiment combined with increased temperatures and extended summer drought was performed in the period 2005&amp;#8211;2013 on a temperate heathland/grassland ecosystem in Denmark (the CLIMAITE project). A major finding from the original experiment was that the soil carbon pool increased by approximately 20% under elevated CO&lt;sub&gt;2&lt;/sub&gt; over the 8 years of the study*.&lt;/p&gt;&lt;p&gt;The FACE treatment was in effect also an in situ labeling experiment because the added CO&lt;sub&gt;2&lt;/sub&gt; was depleted for &lt;sup&gt;13&lt;/sup&gt;C &amp;#65288;&lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2FACE&lt;/sub&gt;=-29&amp;#8240;&amp;#65289;compared to ambient atmospheric CO&lt;sub&gt;2&lt;/sub&gt;&amp;#65288;&lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2AIR&lt;/sub&gt;=-8&amp;#8240;&amp;#65289;. Therefore, the isotopic signal of the remaining soil carbon can be used to investigate the turnover of soil carbon during the time since the end of the original study.&lt;/p&gt;&lt;p&gt;During the growing season in 2020, seven years after the CO&lt;sub&gt;2&lt;/sub&gt; fumigation experiment was terminated, soil samples were extracted in all plots using the same sampling strategy as in previous samplings. Interestingly, the direct soil C pool measurements showed that the extra soil carbon, which was stored during the eight years with elevated CO&lt;sub&gt;2&lt;/sub&gt; had been lost again over the course of the following seven years. The isotopic composition of the different soil layers had also changed back towards the values measured in control plots, although still being slightly more depleted for &lt;sup&gt;13&lt;/sup&gt;C. Still, the convergence of the isotopic composition in the different treatments confirms the trend observed from the direct C pool measurements and also hints that a part of the more recalcitrant carbon taken up during the elevated CO2 experiment is still there while most of the labile/less recalcitrant carbon has been decomposed and reemitted to the atmosphere. The results show that the soil carbon pool in the ecosystem is extremely dynamic and may change fast in response to changes in major ecosystem drivers, and in particular is highly sensitive to the atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentration.&lt;/p&gt;&lt;p&gt;*Dietzen CA, Larsen KS, Ambus P, Michelsen A, Arndal MF, Beier C, Reinsch S, Schmidt IK (2019) Accumulation of soil carbon under elevated CO&lt;sub&gt;2&lt;/sub&gt; unaffected by warming and drought. Global Change Biology, 25: 2970&amp;#8211;2977. doi: 10.1111/gcb.14699.&lt;/p&gt;

Organic carbon fractions in temperate mangrove and saltmarsh soils

Coastal wetlands, such as mangrove and saltmarsh environments, can store significant amounts of soil organic carbon (SOC); however, most studies focus on tropical and subtropical environments. We assessed SOC stocks and fractions in temperate mangrove (two sites) and saltmarsh (sites SM1, SM2 and SM3) environments in southern Australia. The SOC fractions were separated according to particulate organic carbon (POC), humic carbon (HC) and recalcitrant carbon (RC) by size fractionation. Saltmarsh sites generally had the highest SOC content (up to 12.4% SOC). The POC fraction was the highest at the surface in the saltmarsh site and decreased relative to the HC and RC fractions with depth. Conversely, the proportion of POC at the mangrove sites did not decrease with depth, forming up to 76% of the SOC. The vertical displacement of soil of up to 5.8 mm year–1 at the saltmarsh sites, measured using root ingrowth bags, suggest significant contributions of POC via root materials. Retention of these POC inputs are likely to be related to waterlogging, which decreases decomposition rates – with much lower soil moisture content at SM1, where the lowest POC content occurred below the surface, compared with SM2 and SM3.

Source and quantity of carbon influence its sequestration in Rostherne Mere (UK) sediment: a novel application of stepped combustion radiocarbon analysis

We explored the roles of phytoplankton production, carbon source, and human activity on carbon accumulation in a eutrophic lake (Rostherne Mere, UK) to understand how changes in nutrient loading, algal community structure and catchment management can influence carbon sequestration in lake sediments. Water samples (dissolved inorganic, organic and particulate carbon) were analysed to investigate contemporary carbon sources. Multiple variables in a 55-cm sediment core, which represents the last ~ 90 years of accumulation, were studied to determine historical production rates of algal communities and carbon sources. Fluctuations in net primary production, inferred from sedimentary diatom abundance and high-performance liquid chromatography (HPLC) pigment methods, were linked to nutrient input from sewage treatment works (STW) in the catchment. Stepped combustion radiocarbon (SCR) measurements established that lake sediment contains between 11% (~ 1929 CE) and 69% (~ 1978 CE) recalcitrant carbon, with changes in carbon character coinciding with peaks in accumulation rate and linked to STW inputs. Catchment disturbance was identified by radiocarbon analysis, and included STW construction in the 1930s, determined using SCR analysis, and recent nearby highway construction, determined by measurements on dissolved organic carbon from the lake and outflow river. The quantity of autochthonous carbon buried was related to diatom biovolume accumulation rate (DBAR) and decreased when diatom accumulation rate and valve size declined, despite an overall increase in net carbon production. HPLC pigment analysis indicated that changes in total C deposition and diatom accumulation were related to proliferation of non-siliceous algae. HPLC results also indicated that dominance of recalcitrant carbon in sediment organic carbon was likely caused by increased deposition rather than preservation factors. The total algal accumulation rate controlled the sediment organic carbon accumulation rate, whereas DBAR was correlated to the proportion of each carbon source buried.

Shotgun proteomics reveals putative polyesterases in the secretome of the rock-inhabiting fungus Knufia chersonesos

Knufia chersonesos is an ascomycotal representative of black fungi, a morphological group of polyextremotolerant melanotic fungi, whose ability to resort to recalcitrant carbon sources makes it an interesting candidate for degradation purposes. A secretome screening towards polyesterases was carried out for the fungus and its non-melanized mutant, grown in presence of the synthetic copolyester Polybutylene adipate terephthalate (PBAT) as additional or sole carbon source, and resulted in the identification of 37 esterolytic and lipolytic enzymes across the established cultivation conditions. Quantitative proteomics allowed to unveil 9 proteins being constitutively expressed at all conditions and 7 which were instead detected as up-regulated by PBAT exposure. Protein functional analysis and structure prediction indicated similarity of these enzymes to microbial polyesterases of known biotechnological use such as MHETase from Ideonella sakaiensis and CalA from Candida antarctica. For both strains, PBAT hydrolysis was recorded at all cultivation conditions and primarily the corresponding monomers were released, which suggests degradation to the polymer’s smallest building block. The work presented here aims to demonstrate how investigations of the secretome can provide new insights into the eco-physiology of polymer degrading fungi and ultimately aid the identification of novel enzymes with potential application in polymer processing, recycling and degradation.