Labile Organic Carbon Content Research Articles

Nitrogen deposition-induced stimulation of soil heterotrophic respiration is counteracted by biochar in a subtropical forest

Both atmospheric nitrogen (N) deposition and biochar application can markedly impact soil heterotrophic respiration (RH), an important component of the global carbon cycle. However, the interactive effects of N deposition and biochar application on soil RH in subtropical forest ecosystems remain unclear. Here, we conducted a three-year (2019–2022) field trial within a bamboo forest in subtropical China to examine the responses of soil physicochemical and microbial properties to N deposition and biochar application, and to elucidate how biochar regulates N deposition-induced change in soil RH. Nitrogen deposition stimulated soil RH by 8.1–9.8 % annually over three years compared to the control, and this stimulation was mitigated (by 8.1–8.9 % annually) with biochar addition. In the context of N deposition, the decrease of soil RH by biochar application was not through changing soil temperature, moisture or labile organic carbon content. Biochar treatment reduced the abundances of bacterial glycoside hydrolase family 48 gene (GH48) and fungal glycoside hydrolase family 7 cellobiohydrolase I gene (cbhI) and the activities of β-glucosidase and cellobiohydrolase (CBH), but increased the abundance of cbbL gene and activity of RubisCO enzyme. Furthermore, the RH was correlated positively (P < 0.01) with β-glucosidase and CBH activities and negatively (P < 0.01) with RubisCO enzyme activity. Structural equation modeling revealed that the biochar-induced reduction of soil RH under N deposition was associated with decreases in the abundance of cbhI gene and the activity of CBH in soils. We highlight that management practices can mitigate soil carbon loss in forests through modulating soil microbial functions under atmospheric N deposition, and that biochar application in Moso bamboo forests has the potential to reduce RH by approximately 7.6 × 106 t CO2 yr−1 under atmospheric N deposition.

Carbon and Phosphorus in Volcanic Soils of Mt. Kerinci (Sumatra) After Long Term Tea and Paddy Cultivation

ABSTRACT Volcanic soils are well-known to have a high agronomical value, and in Sumatra, Mt. Kerinci volcanic soils produce one of the highest quality tea and rice. Declining soil organic carbon (C) and high phosphorus (P) retention are two main issues in volcanic soils in the area. Thus, this study aimed to study the dynamic of C) and P content in volcanic soils and to find the main controls of their variation from geochemical signatures. The soils of Mt Kerinci present a unique environment as one part of the area was converted from forest to tea plantation for about 100 years, and a nearby field had been used for paddy rice cultivation for about 200 years. The study was conducted in the Mt Kerinci area covering 5,922 ha in the north comprise of paddy soils (ps) and north tea soils (nts) and the south slope (south tea soils= sts). Soil samples were collected on a grid of 500 × 500 m under tea and paddy (n = 206 samples) at 0–20 and 20–40 cm depth. All soils have similar pH in H2O, but pH in KCl paddy soils (ps) > north tea soils (nts) > south tea soils (sts). The average content of soil organic Carbon (SOC) in paddy soils was 50% less than in the soils of tea plantations. The labile organic carbon content in paddy soils were lower than in soils of tea plantations. Most C was chemically bound to the metal complex. Higher P retention occurred in all soils. The CaO/TiO2 index indicated a weathering sequence of ps > nts > sts. Soil weathering controlled P retention with an increasing P retention as the soils weathered. Overall, the study suggests that cultivation in volcanic soils influenced the geochemical properties of the soils and subsequently their C and P contents.

The role of blue carbon stocks becomes more labile with mangrove development

Soil labile organic carbon (LOC) is a crucial component in carbon cycling in coastal wetlands and serves as an important indicator of SOC. Despite this, little is known about the stabilization of SOC and its LOC during mangrove development. The objective of our study was to quantify SOC and LOC in the soil at depths ranging from 0 to 100 cm across four sites, including a mudflat and three mangrove sites of varying ages (15-, 45-, and 80-yr old) were selected in Yingluo Bay, China. The concentration of SOC, POC (particulate organic carbon), DOC (dissolved organic carbon), MBC (microbial biomass carbon), and KMnO4-C (potassium permanganate-oxidizable carbon) were measured. The CMI (soil carbon management index) was also calculated to precisely and directly reflect the dynamic changes of soil carbon pools. Mangrove natural expansion showed a significant positive effect on both SOC and LOC. The concentration of KMnO4-C in the top 50 cm of soil layer showed a significant increase in three different mangrove forest sites. The POC, DOC and MBC were also significantly increased in response to mangrove development. The 80-yr old mangrove site had the highest SOC concentrations, LOC concentrations, soil carbon lability Index (LI) and CMI among all sites. These findings suggested that the development of mangroves increases soil organic carbon fractions through vegetation production and enhances long-term carbon sequestration rates by expanding soil labile carbon pools. As a management option, promoting the natural expansion of mangroves can maximize the sequestration potential of soil organic carbon.

Eutrophication decreases Halophila beccarii plant organic carbon contribution to sequestration potential

Seagrass ecosystems rank among the most effective blue carbon sinks in climate change mitigation and greenhouse gas removal. Nutrient pollution has emerged as a leading threat to seagrass decline and has diminished the carbon sequestration potential in recent decades. Changes in the nutrient regime can also impact the organic carbon compositions (labile and refractory organic carbon compositions) of seagrass tissues, with important implications for determining the quantity and quality of carbon sequestration. However, there is still little information about the impact of nutrient loading on seagrass plant refractory organic carbon composition (ROC), which hinders our ability to reveal the driving mechanisms of anthropogenic factors that decrease seagrass organic carbon sequestration capability. Here, a multidisciplinary approach was employed to investigate the organic carbon variations of Halophila beccarii at five seagrass meadows with contrasting nutrient loading levels. The results showed that H. beccarii plant nitrogen (N) content ranged from 2.21% to 5.65%, which well reflected the external nutrient loading levels. High nutrient loading elevated labile organic carbon content, like free amino acids and soluble sugars. Nevertheless, ROC content (cellulose-associated organic matter) decreased with increasing nutrient loading, which presented a significant negative linear correlation with plant N content. These results provide evidence that eutrophic conditions enhance H. beccarii plant quality (high N and labile organic carbon) and consequently decrease plant ROC sequestration potential. This suggests that reducing nitrogen input to seagrass meadows would aid in increasing seagrass carbon storage.

Potential role of nitrite-dependent anaerobic methane oxidation in methane consumption and nitrogen removal in Chinese paddy fields

Nitrite-dependent anaerobic methane oxidation (n-damo), catalyzed by bacteria closely related to Candidatus Methylomirabilis oxyfera, links the global carbon and nitrogen cycles. Currently, the contribution of n-damo in controlling methane emissions and nitrogen removal, and the key regulatory factors of this process in Chinese paddy fields are poorly known. Here, soil samples from 20 paddy fields located in different climate zones across China were collected to examine the n-damo activity and bacterial communities. The n-damo activity and bacterial abundance varied from 1.05 to 5.97 nmol CH4 g−1 (dry soil) d−1 and 2.59 × 105 to 2.50 × 107 copies g−1 dry soil, respectively. Based on the n-damo activity, it was estimated that approximately 0.91 Tg CH4 and 2.17 Tg N could be consumed annually via n-damo in Chinese paddy soils. The spatial variations in n-damo activity and community structure of n-damo bacteria were significantly (p < 0.05) affected by the soil ammonium content, labile organic carbon content and pH. Furthermore, significant differences in n-damo activity, bacterial abundance and community composition were observed among different climate zones. The n-damo activity was found to be positively correlated with the mean annual air temperature. Taken together, our results demonstrated the potential importance of n-damo in both methane consumption and nitrogen removal in Chinese paddy soils, and this process was regulated by local soil and climatic factors.

Sloping Farmlands Conversion to Mixed Forest Improves Soil Carbon Pool on the Loess Plateau.

Vegetation restoration is considered a potentially useful strategy for controlling soil erosion and improving soil organic carbon (SOC) in arid and semiarid ecosystems. However, there is still debate regarding which vegetation restoration type is the best choice. In this study, four vegetation restoration types (i.e., grasslands, shrubs, forests and mixed forests) converted from sloping farmlands were selected to explore the SOC variation among the four types and to investigate which soil factors had the greatest effect on SOC. The results showed while the magnitude of effect differed between vegetation restoration type, all studied systems significantly increased SOC and labile organic carbon contents (p < 0.01), soil nutrients such as total nitrogen (TN) (p < 0.01), available nitrogen (AN) (p < 0.01), total phosphorus (TP) (p < 0.05) and available phosphorus (AP) (p < 0.05), soil enzyme activities such as phosphatase (p < 0.01), soil microbial biomass carbon (MBC) (p < 0.05), and basal respiration (BR) (p < 0.05), but had significant negative correlationswith polyphenol oxidase (p < 0.05). However, the effects of vegetation restoration of farmland converted to natural grasslands, shrubs, forests and mixed forests varied. Among the types studied, the mixed forests had the largest overall positive effects on SOC overall, followed by the natural grasslands. Soil nutrients such as N and P and soil microbial activities were the main factors that affected SOC after vegetation restoration. Mixed forests such as Robinia pseudoacacia and Caragana korshinskii are the best choice for farmland conversion on the central of the Loess Plateau.

Changes in soil organic carbon stocks from reducing irrigation can be offset by applying organic fertilizer in the North China Plain

Elaborate water and fertilization management to increase soil organic carbon (SOC) accumulation is important in maintaining high agricultural productivity and a sound ecological environment. Here, a field experiment was conducted for eight years to investigate the effects of irrigation regimes and fertilizer types on SOC stocks and labile organic carbon concentrations (particulate organic carbon, POC; dissolved organic carbon, DOC; and microbial biomass carbon, MBC). The irrigation regimes were no irrigation (W0); presowing irrigation (W1); presowing and jointing irrigation (W2); and presowing, jointing, and anthesis irrigation (W3). The fertilizer types were no fertilizer (C0), mineral fertilizer only (C1), half mineral plus half cattle manure (C2), and cattle manure only (C3). Thus, sixteen treatments were created and laid out in a split block design with triplicates. Both irrigation and manure application distinctly enhanced SOC accumulation. POC, DOC, and MBC were mainly affected by manure application in surface soil (0–20 cm) and by irrigation in deep soil (40–60 cm). The interaction between irrigation and fertilization on SOC and labile organic carbon concentrations was not significant in the 0–60 cm soil layer. The SOC stocks and relative SOC sequestration rate increased with increasing irrigation and cattle manure inputs. There was no significant difference in SOC stocks between W2C2 (108.15 Mg C ha−1) and W3C1 (109.05 Mg C ha−1), as well as between W1C2 (90.25 Mg C ha−1) and W2C1 (87.07 Mg C ha−1), indicating that in W2 and W1, replacing half of the nitrogen with organic manure can result in equal SOC stocks caused by increasing irrigation under mineral fertilizer alone. This study emphasizes the important role that irrigation plays in SOC management and provides theoretical support to improve the SOC stock by reducing irrigation and applying cattle manure in the North China Plain.

Using Radiocarbon to Assess the Abundance, Distribution, and Nature of Labile Organic Carbon in Marine Sediments

AbstractPositive 14C gradients have recently been observed within the surface mixed layer of several continental‐margin sediments. The best explanation for these positive 14C gradients is the occurrence and rapid degradation of labile organic carbon (LOC) in the upper 5–10 cm of the seabed. Based on a two‐component model for sedimentary organic matter (i.e., a planktonic labile component and an older refractory component), bulk 14Corg data were used to determine the abundances of LOC within the surface mixed layers of three cores from the West Antarctic Peninsula (WAP) shelf and one core from San Clemente Basin (California Borderland). LOC contents in surface samples from the four stations varied from 0.5 to 1.1 mg/cm3, comprising 20% (San Clemente Basin) to 80% (WAP, Sta. G) of the total organic carbon. By incorporating a steady state diagenetic model and particle‐mixing bioturbation coefficients, the LOC profiles were used to determine LOC turnover times (LOC τ) and LOC e‐folding depths. The LOC τ values for the West Antarctic Peninsula sediments varied from 0.09 to 0.59 years, whereas the LOC τ value from the San Clemente Basin core was 63 years. The LOC e‐folding depths for the WAP stations varied from 0.8 to 3.4 cm, in contrast to the LOC e‐folding depth in San Clemente Basin, which was 4.0 cm. LOC characteristics from the four cores examined in this study were compared to LOC data in the literature as a means of substantiating the overall 14Corg‐based approach and justifying model assumptions.

Responses of Soil Labile Organic Carbon and Carbon Management Index to Different Long-Term Fertilization Treatments in a Typical Yellow Soil Region

The capacity of soil to store carbon (C) and emit carbon dioxide (CO2) into the atmosphere primarily depends on soil management practices. It is essential to understand the impact of management strategies on the soil organic carbon (SOC) content and labile organic carbon (LOC) fraction. The impacts of 24‑year-long organic and inorganic nitrogen (N) treatments on SOC, KMnO4-oxidizable organic carbon and its fractions (highly labile organic carbon (HLOC), moderately labile organic carbon (MLOC), low labile organic carbon (LLOC) and nonlabile organic carbon (NLOC)), and a carbon management index (CMI) were investigated under a continuous maize cultivation system in a long-term experiment in Guizhou, Southwest China. Six fertilizer treatments were included: no fertilizer input (CK), chemical fertilizer alone (NPK), 25% N through farmyard manure (FYM) plus 75% N through chemical fertilizer (1/4N-M+3/4N-CF), 50% N through FYM plus 50% N through chemical fertilizer (1/2N-M+1/2N-CF), FYM plus chemical fertilizer (MNPK) and FYM alone (M). We used the LOC content and CMI value to assess the effects of long-term combinations of FYM and chemical fertilizers at different rates on the SOC pool in various soil layers (0–20, 20–40, 40–60, 60–80, 80–100 cm) and to identify the most suitable integrated treatment. The results showed that the application of organic fertilizer generally increased the SOC content, the LOC fraction, and the CMI values in different layers, especially the surface layer, compared to the CK and NPK fertilization treatments. The SOC content and LOC fraction decreased with increasing soil depth. The significant relationship between the LOC fraction, CMI value, LOC available ratio of carbon (LOC-AR), and soil parameters showed that these values can be used to sensitively assess soil quality and SOC changes in the system. Considering the comprehensive effects on the SOC content, LOC fraction, CMI value, AR value, etc., the 1/4N-M+3/4N-CF and 1/2N-M+1/2N-CF treatments showed the greatest influence on carbon sequestration and soil productivity; therefore, these could be the best options for maize cropping systems in this region.

Effects of long-term super absorbent polymer and organic manure on soil structure and organic carbon distribution in different soil layers

Super absorbent polymer (SAP) and organic manure (OM) may improve soil structure and change soil organic carbon (SOC) composition and agroecosystem functioning. However, the understanding of the effects of SAP and OM on SOC composition, specifically in deeper soil layers, is still not clear. The objectives of this study were to examine the effects of long-term application of SAP and OM (8 years) on changes in SOC and soil structure down to a soil depth of 100 cm. Therefore, in order to investigate the stability of soil structure and the distribution of organic carbon at different soil depths (0-10 cm, 10-20 cm,…, 90-100 cm) under long-term application of SAP, OM, and a control treatment, soil structure and soil organic carbon content were analyzed in mixed soil samples and undisturbed soil samples. The results indicated that with depth, the proportion of large aggregates (0.5-2.0 mm) decreased gradually, while the proportion of small aggregates (<0.25 mm) increased gradually. Compared with the control, SAP treatment was conducive to the increment of>0.5 mm soil aggregates in the 0-30 cm and 40-60 cm soil layers (P < 0.05) and 0.25-0.5 mm in the 0-50 cm soil layer (P < 0.05), and while the OM treatment had the same effect in the 0-30 cm soil layer (P < 0.05). The total organic carbon content (TOC) and labile organic carbon content (LOC) of bulk soil increased (0-20 cm) initially with depth, and then decreased (20-70 cm) approximately. In the 0-50 cm soil layer, the TOC and LOC under SAP treatment were higher than those under OM treatment. The SAP treatment was more beneficial to the increase of the TOC and LOC of 0.5-2.0 mm and 0.25-0.5 mm aggregates in the 10-40 cm soil layer compared with the control and OM treatment (P < 0.05). The SAP treatment was also more beneficial in increasing the contribution rate of organic carbon (CROC) in> 0.5 mm aggregates in 0-40 cm soil depth, while the OM treatment was more beneficial in increasing the CROC of > 0.5 mm aggregates in 0-30 cm soil depth. The SAP treatment improved the stability of the soil structure in 0-30 cm and 40-60 cm depths, and the OM treatment had the same effect in the 0-30 cm depth. Compared with the control and OM treatments, SAP treatment has shown to be the most beneficial in improving soil structure and increasing organic carbon content.

Altitudinal changes of soil organic carbon fractions of evergreen broadleaved forests in Guanshan Mountain, Jiangxi, China

We investigated soil total organic carbon (TOC), recalcitrant organic carbon (ROC), and labile organic carbon (LOC) of evergreen broadleaved forests at different altitudes (400, 600, 800, 1000 and 1200 m) in Guanshan National Nature Reserve, Jiangxi Province, with the aim to understand their altitudinal distribution. The results showed that soil TOC, ROC and LOC contents were the highest in the surface layer and decreased with soil depth. With the increases of altitude, contents of soil TOC, ROC, readily oxidizable organic carbon (ROOC), microbial biomass carbon (MBC), and particulate organic carbon (POC, 0-20 cm depth) increased with a peak at 1000 m and then decreased, whereas soil water-soluble organic carbon (WSOC) contents and POC contents in 20-40 cm layer did not change. In 0-10 cm soil layer, the proportions of ROC to TOC at 800 and 1200 m were significantly higher than those at 400 and 1000 m, while the proportions of LOC to TOC were the highest at 400 m. The proportions of ROC and LOC to TOC in 10-40 cm layer showed a low-high-low tendency along the altitude, with peaks at 1000 and 600 m, respectively. Soil organic carbon fractions were positively correlated with soil moisture, microbial biomass nitrogen, and soluble organic nitrogen. A positive correlation was observed between LOC and ammonium concentration. Our results suggested that altitude significantly affected the distribution of soil organic fractions, with soil ROC, ROOC and MBC being more sensitive to altitudinal changes. Soil ROC and LOC at high altitude were prone to decomposition and transformation under conditions with sufficient water and nitrogen, which reduced soil carbon stability. It was essential to study the dyna-mics of soil organic carbon in high altitude forests under global warming scenarios.

Effects of different organic amendments on soil organic carbon and its labile fractions in the paddy soil of a double rice cropping system

Application of organic amendments is an effective approach for improving soil organic carbon and soil fertility. To investigate the effects of different organic amendments on soil organic carbon and its labile fraction content, a batch of incubation experiments was conducted on the fluvo-aquic soil in Dongting Lake region, Hunan Province. There were six treatments, including soil amended with rice straw, soil amended with Chinese milk vetch, soil amended with bio-organic fertilizer, soil amended with pig manure, and soil amended with rice straw-derived biochar, with unamended soil as control. Each treatment had the same amount of carbon input. After 180 days of incubation, application of organic amendments increased soil labile organic carbon content. Application of bio-organic fertilizer, pig manure and rice straw-derived biochar significantly increased soil organic carbon content by 26.1%, 9.7% and 30.7%, respectively. There was no significant change in soil organic carbon content in rice straw and Chinese milk vetch treatments which were more favourable to the accumulation of soil dissolved organic carbon and microbial biomass carbon. Pig manure was more favourable to the accumulation of soil dissolved organic carbon. Bio-organic fertili-zer could benefit the accumulation of soil microbial biomass carbon and readily oxidizable organic carbon. Rice straw-derived biochar could promote the accumulation of soil microbial biomass carbon and light fraction organic carbon. Compared with rice straw, soil carbon pool management index was increased by 31.8%, 111.6%, 62.2% and 50.7% in Chinese milk vetch, bio-organic fertilizer, pig manure and rice straw-derived biochar treatments, respectively. The performance of bio-organic fertilizer, pig manure, and rice straw biochar was better than rice straw and Chinese milk vetch from the perspective of soil carbon sequestration and soil carbon pool management index.

Influence mechanisms of long-term fertilizations on the mineralization of organic matter in Ultisol

The influence mechanisms of long-term fertilizations with chemical fertilizer (NPK), a mixture of chemical fertilizer and straw (NPKS), animal manure (AM), and no fertilizer (Ck) on the mineralization of soil organic matter (SOM) in an Ultisol was studied in this study. The cumulative amount and rate of SOM mineralization were stimulated by the examined fertilizations, especially AM treatment. The order Ck ≤ NPK < NPKS < AM was found in the content of the soil total organic carbon, dissolved organic carbon, microbial biomass carbon, and organic carbon in each pool. The proportion of Fe/Al-associated organic carbon in the soil total organic carbon showed the opposite order. The fertilizations, especially the AM and NPKS treatments, increased the relative abundance of Actinobacteria, Proteobacteria and Ascomycota as well as the activity of β-glucosidase and invertase. Structural equation modeling showed that the activity of the hydrolases, the content of labile organic carbon, and the composition and diversity of the microbes were positive factors in controlling SOM mineralization. Among the positive factors, the activity of the hydrolases was the most important, and the content of labile organic carbon was more important than the composition and diversity of the microbes. The information implied that the stimulation of fertilizations on the mineralization of organic matter in Ultisol progressed mainly through the increase in microbial activity.

Integrated use of straw mulch with nitrogen fertilizer improves soil functionality and soybean production

Mulching can effectively maintain and improve soil health and functionality. The mechanisms, however, have not been fully elucidated. Therefore, the effects of temperature on the biogeochemical properties of soil were investigated in the present study in relation to nitrogen management and soil functionality. The results of the 3-year field experiments showed that integrated straw mulch (S) and nitrogen fertilizer (N) treatments enhanced the activities of soil urease, invertase, alkaline phosphatase, and catalase by >1.8, 2.1, 2.0 and 1.4 fold, respectively, compared with the control treatment. Furthermore, these treatments increased soil available N by 28%, phosphorus by 45%, and potassium by 55%. In general, the soil organic carbon, dissolved carbon and labile organic carbon content in the treated plot were approximately 1.2–2.9 folds greater than in the control plot. These improvements in soil fertility and carbon indices increased the biomass and grain yield of soybean (67 and 75%, respectively) during the three-year study period. During 2015–2017, the straw mulch and nitrogen fertilizer treatment increased the soil moisture (23%) and decreased the soil temperature (8%) in comparison to the control in the 0–0.2 m soil depth and, therefore, improved soil enzyme activities, nutrient availability, and carbon stocks, and ultimately, soil functionality and sustainability, in the semiarid region.

Biological Activity of Sod-Podzolic Soils in a Long-Term Experiment Involving Various Agrotechnical Methods

The effects from the multiyear use of grain–fallow–plow crop rotation schemes comprising legumes, permanent cultivation of winter rye and barley, and bare fallow regime on the ecological and physiological state of microbial communities in a sod-podzolic heavy loamy soil were examined in the course of a long-term, stationary experiment. Biological activity parameters of soil microbiocoenoses, including the potential intensity of CO2 emission, microbial biomass gross respiration rate, metabolic coefficient, and nitrogen fixation, were assessed. It is shown that the application of organic fertilizers (manure) in the crop rotation scheme with 28.6% legumes ensures the best humus state of the soil and a significant increase in the total nitrogen content. It is established that the share of microbial biomass (Cmic) in the total organic matter of the sod-podzolic heavy loamy soil is 1.50–3.24%. The application of mineral fertilizers results in a slight decrease in the microbial biomass carbon content and increase in the labile organic carbon content. The metabolic activity of microbial communities decreases in all long-term experimental scenarios regardless of the land use techniques. The permanent intensive cultivation of crops, either with or without crop rotation, disturbs the microbiocoenosis stability in soils of the Cis-Ural region.

Labile C dynamics reflect soil organic carbon sequestration capacity: Understory plants drive topsoil C process in subtropical forests

AbstractThe huge background pool of soil organic carbon (SOC) is likely to impede the ready detection of SOC changes. We propose to explore SOC changes by monitoring the dynamics of soil labile organic carbon (LOC); namely if LOC could be largely retained in soils rather than respired rapidly, the SOC would be ready to be sequestered. The effects of the two major functional groups of plants, that is, canopy trees and understory plants, on SOC accumulation were then illustrated with this LOC‐based approach. The characteristics of LOC and SOC of topsoils (0–20 cm) in a field manipulation experiment with 5‐yr treatments of understory removal and tree girdling in both a young and a mature Eucalyptus plantations were examined. The concentration and potential turnover time of soil LOC were used to indicate the state of vegetation‐induced C accumulation in soils, which were estimated by a sequential fumigation‐incubation procedure. Soil natural abundances of 13C and 15N were measured to reflect the proportion of newly retained LOC in soils. We found that, in the young plantation, understory removal did not significantly affect both soil LOC and SOC concentrations, but significantly increased the potential turnover time of soil LOC. In contrast, in the mature plantation, understory removal significantly decreased soil LOC and SOC concentrations, but did not significantly alter the potential turnover time of soil LOC. However, tree girdling did not significantly affect SOC concentration, soil LOC concentration, or potential turnover time in either the young plantation or the mature plantation. These results demonstrated that understory plant‐derived C was one of the major components of LOC pool in topsoils, and it may be readily mineralized in the young plantation but accumulated as an important fraction of SOC in the mature plantation. This study suggests that the LOC‐based approach is potentially useful in monitoring SOC changes and improves our understanding of how plant functional groups and soil fertility status could jointly affect LOC and SOC dynamics. In considering the great contribution of understory plants to SOC processes, we propose that understory plants should be maintained in subtropical plantation ecosystems.

Effects of winter cropping mode on soil organic carbon and carbon management index of double rice paddy

To explore the effects of winter crops recycling into paddy field on soil carbon pool, we cultivated rape, Chinese milk vetch, ryegrass and potato in winter season after harvesting double-season rice, with fallow as control (CK). The contents of soil organic carbon (SOC) and labile organic carbon (LOC) in the harvest of early and late rice were measured, and the stable carbon, activity of carbon pool, carbon pool index, carbon pool activity index and carbon pool management index (CMI) were calculated. Results showed that winter cover crops returning increased SOC content by 1%-8% and 3%-18% after harvest of early rice and late rice, respectively in comparison with CK. Cultivating rape, Chinese milk vetch and ryegrass all increased the LOC content, with enhancement of 16.2%-84.2% and 24.4%-28.1% after harvest of early rice and late rice, respectively. The CMI of all winter crop treatments presented a growth ratio of 1.4%-41.8% compared to CK. In conclusion, the cultivation of winter crop and recycling to field promoted soil carbon fixation and soil quality, among which the ryegrass and Chinese milk vetch showed better comprehensive effect.

Effect of polymer materials on soil structure and organic carbon under drip irrigation

Polymer materials have broad potential applications for saving water and increasing crop yields in irrigated agricultural systems. However, little is known about how polymers affect the distribution and transformation of soil organic carbon. The purpose of this study is to evaluate the effects of humic acid (H) (an alkali extract of cottonseed meal), modified polymer (P) (a mixture of anionic polyacrylamide, polyvinyl alcohol, and manganese sulfate), and composite polymer (HP) (a 1:1 mixture of H and P) applied with drip irrigation on soil aggregate stability and organic carbon stability. The results demonstrated that the H and P treatments significantly increased soil aggregate stability and organic carbon content, compared with the HP treatment. Compared with the controls for corpped soils (CK) and uncorpped soils (CK-NP), the DR0.25 (>0.25 mm soil mechanical-stable aggregate), W-MWD (mean weight diameter of water stable aggregate), and W-GMD (geometric mean diameter of water stable aggregate) of the H treatment increased by 3.9%, 33.1%, and 23.2%, respectively (p < 0.05); the WR0.25 (>0.25 mm soil water stable aggregate), W-MWD, and W-GMD of the H-NP treatment (H treatment for uncorpped soils) increased by 44.7%, 38.0%, and 37.6%, respectively (p < 0.05); the WR0.25 and W-GMD of the P treatment increased by 29.8% and 26.8%, respectively (p < 0.05); and the W-GMD of the P-NP treatment (P treatment for uncorpped soils) increased by 16.5% (p < 0.05). Although the HP treatment also increased the organic carbon content of soil aggregates, it was less effective for improving soil aggregate stability. During the first 20 days of incubation, the mineralisation rate of the CK was the highest. Then, the mineralisation rates for the H and HP treatments increased more rapidly than that of the CK (p < 0.05), with increases of 27.6% and 54.4%, respectively. Results from field experiments (soil microbial biomass carbon content) supported this result. Compared with the CK, the H and P treatments rapidly increased soil labile organic carbon (LOC). The LOC content for the H treatment was 28.9% and 21.6% higher than that of the CK after 30 and 90 days, respectively (p < 0.05), whereas the LOC content for the P treatment was 32.8% and 20.2% higher than that of the CK after 60 and 90 days, respectively (p < 0.05). Redundancy analyses revealed that cultivation vs fallow treatments affected how polymer materials transform soil organic carbon. Cultivation allowed humic acid to improve soil aggregate mechanical-stability, promoting microbial decomposition of carbon and increasing organic carbon content. Modified polymer improved soil aggregate water-stability and reduced aggregate destruction, increasing the proportion of soil organic carbon that can be easily oxidised.

Labile organic carbon dynamics in continental shelf sediments after the recent collapse of the Larsen ice shelves off the eastern Antarctic Peninsula: A radiochemical approach

Labile organic carbon (LOC) dynamics (i.e., of recently produced, planktonic material) and sediment dynamics were studied in the seabed using naturally occurring 14Corg and 210Pb measurements in the region where the Larsen Ice Shelves A and B were floating almost two decades ago. A non-steady-state diagenetic model was used to estimate sediment mixing coefficients as well as LOC fluxes to the seabed and LOC turnover times (i.e., mean residence times) in a suite of 14 sediment cores from the continental shelf, including a glacial trough. At four of the stations, cores were collected during 2007 and 2011 cruises, enabling a time-series approach for understanding the evolution of sedimentary processes and LOC dynamics in the deposits below a collapsed ice shelf. Sediment mixing coefficients, based on non-steady-state 210Pb profiles, varied between 0.01 cm2 y−1 and 1.6 cm2 y−1 in these post-ice shelf sediments. These values were similar to those found in polar deep-sea environments, where sedimentary conditions are less dynamic than in shallower provinces. LOC, whose abundance decreased uniformly with depth, was detected to depths ranging from 2 to 16 cm, with LOC seabed inventories varying from 1.5 to 22 mg LOC cm−2. Excess 210Pb and LOC fluxes were relatively uniform across the study area suggesting that similar particle fluxes have taken place within the Larsen system since the disintegration of the various ice shelves. The LOC mean residence time at the different stations varied from 3 y to >60 y. The 14Corg approach, calculating LOC content based on a two-end member model with planktonic 14Corg as the labile carbon end member, most closely correlated with the lipid content of the sediment, which has been considered one of the best descriptors of reactive organic matter readily available to benthic consumers. We suggest that the irregular combination of sea ice coverage, organic matter production and supply to the sea floor introduce scatter in the determination of sediment and LOC dynamics such that short-term temporal (<5 years) and spatial trends could not be readily resolved.

A simple approach to estimate daily loads of total, refractory, and labile organic carbon from their seasonal loads in a watershed.

Loads of naturally occurring total organic carbons (TOC), refractory organic carbon (ROC), and labile organic carbon (LOC) in streams control the availability of nutrients and the solubility and toxicity of contaminants and affect biological activities through absorption of light and complex metals with production of carcinogenic compounds. Although computer models have become increasingly popular in understanding and management of TOC, ROC, and LOC loads in streams, the usefulness of these models hinges on the availability of daily data for model calibration and validation. Unfortunately, these daily data are usually insufficient and/or unavailable for most watersheds due to a variety of reasons, such as budget and time constraints. A simple approach was developed here to calculate daily loads of TOC, ROC, and LOC in streams based on their seasonal loads. We concluded that the predictions from our approach adequately match field measurements based on statistical comparisons between model calculations and field measurements. Our approach demonstrates that an increase in stream discharge results in increased stream TOC, ROC, and LOC concentrations and loads, although high peak discharge did not necessarily result in high peaks of TOC, ROC, and LOC concentrations and loads. The approach developed herein is a useful tool to convert seasonal loads of TOC, ROC, and LOC into daily loads in the absence of measured daily load data.