Light Fraction Carbon Research Articles

Are bacterial communities and aggregation in fragile soils influenced by the management system?

Light-textured soils are widely distributed globally and, despite their limitations, have been integrated into agricultural production systems. This study aimed to assess how management systems—conventional tillage (CT) and no-till (NT)—affect aggregate formation pathways (physicogenic and biogenic) and bacterial communities. Two management systems (NT and CT) and three cover crops were evaluated: CJ: Crotalária (Crotalaria juncea (40 ​kg ​ha−1); M: Millet (Pennisetum glaucum - 60 ​kg ​ha−1); and C: Cocktail (Crotalária - Crotalaria juncea - 10 ​kg ​ha−1, Jack bean - Canavalia ensiformis - 75 ​kg ​ha−1, and Millet - Pennisetum glaucum - 30 ​kg ​ha−1). Undisturbed soil samples were collected from the crop row at a depth of 0.00–0.10 ​m. Aggregates with diameters between 9.7 and 8.0 ​mm were classified as biogenic or physicogenic. In addition to the chemical attributes of the aggregates, total organic carbon (TOC) and its fractions (mineral-associated organic carbon, MAOC; particulate organic carbon, POC; and free light fraction carbon, FLFC) were quantified. The structure and bacterial composition of the aggregates were also characterized. A higher proportion of biogenic aggregates (53–64%) was observed compared to physicogenic aggregates (36–47%). Cover crops exhibited significant differences in pH, calcium (Ca2+), base saturation, phosphorous (P), and percentage of base saturation. The management systems differed significantly for Ca2+ and P, with CT showing higher values than NT. The management system influenced organic matter accumulation and stabilization in the aggregates, with MAOC content being significantly lower in CT. POC and TOC were also significantly lower in physicogenic aggregates under CT. Bacterial community richness, diversity, and structure were significantly influenced by the management system, with greater richness and diversity in NT compared to CT. Network analysis revealed NT had more nodes and edges (65 and 406, respectively) than CT (52 and 357, respectively. Phyla abundance differed between the systems, with Firmicutes and Entotheonellaeota more abundant in CT, while WPS_2, GAL15, Bdellovibrionota, and Myxococcota were more abundant in NT. Despite the relatively short period of NT implementation (5 years), it had a positive effect on the bacterial community, which may subsequently influence nutrient and carbon content and their fractions in the aggregates.

Management impacts on organic carbon under continuous perennial grass, perennial grass-legume mixture, and annual cereals on a thick Black Chernozemic soil

Impacts of annual and perennial pasture management on soil organic carbon (SOC) and equivalent SOC stocks (equal soil mass basis) were investigated in two trials [CAESA (1994–1997) and BMP (2008–2012) trials] conducted on the same experimental paddocks at Lacombe, AB. The original site was broken from perennial grass in 1992, and the CAESA trial established in 1993. Between 1994 and 1997, half of the paddocks included winter triticale and a mixture of triticale and spring barley; half included smooth and meadow bromegrass; and each paddock was light, medium, or heavily grazed. The BMP trial (2008–2012) on the same paddocks included fertilized, direct seeded barley as silage; grazing and haying of unfertilized meadow bromegrass, fertilized meadow bromegrass, and meadow bromegrass and alfalfa mixture; and unfertilized oldgrass that was continuous since 1994. Between trials (1998–2007), all paddocks received no fertilizer. In the 0–15 cm depth, SOC under oldgrass was constant between 1994 and 2012 and averaged 88 Mg C ha−1. Under barley silage, SOC decreased from 89 to 72 Mg C ha−1 by 2012. Between 1994 and 2012, SOC decreased in all treatments re-established on original annual forage (1994–1998) but not to the level of barley silage. Light fraction carbon was the highest under oldgrass and the lowest under barley silage. Overall, oldgrass with no fertilizer inputs maintained a constant SOC, although annuals reduced SOC stocks. Re-establishment of perennial grass with grazing may therefore reduce SOC loss, whereas haying perennial grass may not reduce SOC loss.

Biogenic and physicogenic aggregates as indicators of quality in soils with sandy texture in areas of organic agriculture

ABSTRACT Sandy texture soils have a great expression in agricultural areas worldwide. In the Baixada Fluminense, soils with a sandy texture on the surface horizons are striking, and a good part of these areas is destined for producing vegetables using conventional cultivation methods. The sandy texture is one of the great challenges for agriculture due to the low water retention capacity provided to the soil, the rapid decomposition of organic matter, and the intense loss of nutrients by leaching. In these areas, the action of erosive processes is sometimes observed, whether water or wind erosion. The practices carried out in conventional agriculture can accentuate these processes. This study aimed to evaluate the influence of different soil management systems, with different vegetation covers, on the pathways of aggregate formation, the nutrient contents contained therein, and the organic matter fractions, with the objective of using these properties as indicators of soil quality. The study was carried out in an organic production unit, with no-till system (NT) and conventional system (CT), three vegetal covers were evaluated, namely; seed cocktail 1 (C1) (Crotalaria (Crotalaria juncea) (20 kg ha -1 ), Jack Bean (Canavalia ensiformis) (150 kg ha -1 ) and millet (Pennisetum glaucum) (60 kg ha -1 )), and seed cocktail 2 (C2) (with 50 % of the amount of seeds used in C1), and spontaneous plants (S. P). Undisturbed samples were collected at the layers of 0.00-0.05 and 0.05-0.10 m, and, from these samples, aggregates with a diameter between 9.7 and 8.0 mm were classified according to the formation route (Biogenic or Physicogenic). From these, the chemical properties were quantified (pH, Ca 2+ , Mg 2+ , Al 3+ , P, Na + , K + ), and also the carbon fractions (total organic carbon – TOC, mineral-associated organic carbon – MAOC), particulate organic carbon – POC, and free light fraction carbon – LFC). Based on the results, it was verified that the percentage of biogenic aggregates was higher than the physicogenic one in the layer of 0.00-0.05 m, not being verified influences of the vegetal coverage or the management system. Chemical properties did not differ significantly between training pathways. The CT, for the most part, was the system in which the highest values of chemical properties were observed, and in general, the C2 and S.P coatings were the ones that provided the greatest improvements for chemical properties and carbon content.

Soil fertility thresholds driven by sand content indicate drylands degradation phases on the Tibetan Plateau

AbstractUnderstanding the process of dryland degradation is crucial to combating desertification and maintaining land sustainability. To identify important indicators of dryland degradation, we conducted a field survey of 46 drylands spanning an aridity gradient across the Tibetan Plateau. Our random forest model suggested that soil sand content was the most important predictor of variation in soil fertility (carbon, nitrogen, and phosphorus). Most soil fertility showed a nonlinear and abrupt decline with increasing soil sand content, and displayed three sequential thresholds. First, soil organic carbon sharply decreased beyond a soil sand content threshold of 47%. Second, exceeding a subsequent threshold of 70%, the total nitrogen and light fraction carbon declined dramatically with increasing soil sand content. Finally, soil heavy fraction carbon, inorganic carbon, and total carbon were drastically reduced when the soil sand content exceeded 80%. The three soil fertility thresholds along the gradient of the soil sand content indicated a sequence of dryland degradation phases. Our findings provide an approach for detecting the severity of dryland degradation and suggest vulnerable areas of degradation on the Tibetan Plateau.

Response of soil organic carbon fractions to legume incorporation into cropping system and the factors affecting it: A global meta-analysis

Recently, there has been increased advocacy for the adoption of rational cropping systems to increase soil organic carbon (SOC) levels and health and ensure food security. The intercropping and crop rotation of legumes with other crops is believed to improve soil environment and influence SOC dynamics. However, a comprehensive assessment of the effects of legume incorporation into cropping systems (LCS) on SOC fractions is still lacking. Therefore, this study aims to elucidate the effects of LCS on SOC fractions under different climatic conditions, soil properties, and agronomic practices, based on meta-analysis of 85 publications. LCS significantly increased the concentrations of microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon, light fraction carbon (LFC), heavy fraction carbon, labile carbon pool, and recalcitrant carbon pool by 21.4 %, 7.2 %, 9.1 %, 29.6 %, 6.4 %, 7.1 %, and 7.9 %, respectively, compared with cropping systems without legume. LCS-induced increase in the contents of SOC fractions was stronger under relatively suitable climatic conditions (mean annual temperature > 15 °C, mean annual precipitation > 1000 mm, and aridity index (AI) > 0.65) and soils with more severe nutrient limitation (soil organic matter < 10 g kg−1, pH < 6.6, and subsoils). Specifically, AI was positively correlated with the response ratios of labile organic carbon fractions, whereas initial soil pH was negatively correlated with MBC, DOC, and LFC. Additionally, low nitrogen fertilizer application rates (< 120 kg ha−1) and intercropping favored an increase in SOC fractions under legume cultivation. Overall, these results provide insights into the ecological benefits of legumes and highlights the importance of developing site-specific strategies to effectively manage SOC dynamics under legume cultivation for sustainable agricultural practices.

Labile Soil Organic Matter Pools Are Influenced by 45 Years of Applied Farmyard Manure and Mineral Nitrogen in the Wheat—Pearl Millet Cropping System in the Sub-Tropical Condition

Labile soil organic matter pools (LSOMp) are believed to be the most sensitive indicator of soil quality when it is changed rapidly with varied management practices. In sub-tropical climates, the turnover period of labile pools is quicker than in temperate climates. Organic amendments are of importance in improve the LSOMp for a temperate climate and may be helpful in sub-tropical climates as well. Hence, the status of LSOMp was studied in long term farmyard manure (FYM) amended soils under wheat (Triticum aestivum L.) and pearl millet (Pennisetum glaucum L.) cropping systems in sub-tropical arid conditions. At the same time, we also attempt to determine the impact of mineral nitrogen (N) application in these pools. In this study, dissolved organic matter (DOM), microbial biomass (MB), and light fraction (LF) were isolated in the management practices involving different modes and rates of FYM applications along with the application of nitrogenous fertilizer. C and N contents of the labile pools were analyzed in the soil samples at different periods after FYM applications. Among the different pools, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) were changed significantly with different rates and modes of FYM application and mineral N application. Application of FYM at 15 Mg ha−1 in both the seasons + 120 kg ha−1 mineral N resulted in significantly higher MBC and DOC as compared to all of the other treatments. This treatment also resulted in 13.75% and 5.8% more MBC and DOC, respectively, as compared to the amount of MBC and DOC content in the control plot where FYM and mineral N were not applied. Comparing the labile organic matter pools of 45 years of FYM amendment with initial values, it was found that the dissolved organic carbon, microbial biomass carbon, and light fraction carbon were increased up to the maximum extent of about 600, 1200, and 700 times, respectively. The maximum amount of DOM (562 mg kg−1 of DOC and 70.1 mg kg−1 of DON), MB (999 mg kg−1 of MBC and 158.4 mg kg−1 of MBN), LF (2.61 g kg−1 of LFC and 154.6 g kg−1 of LFN) were found in case of both season applied FYM as compared to either summer or winter applied FYM. Concerning the different rates of FYM application, 15 Mg ha−1 FYM also resulted in a significantly higher amount of DOM, MB, and LF as compared to other FYM rates (i.e., 5 Mg ha−1 and 10 Mg ha−1). Amongst different pools, MB was found to be the most sensitive to management practices in this study. From this study, it was found that the long-term FYM amendment in sub-tropical soil along with mineral N application can improve the LSOMp of the soil. Thus, it can be recommended that the application of FYM at 15 Mg ha−1 in summer and winter with +120 kg ha−1 mineral N can improve SOC and its labile pools in subtropical arid soils. Future studies on LSOMp can be carried out by considering different cropping systems of subtropical climate.

Soil organic matter of aggregates physicogenic and biogenic in areas under no-tillage system in the Cerrado, Brazil

The aim of this study was to evaluate i) the different cover crops contribution used in no-tillage system (NT) to biogenic aggregation; and ii) the influence of aggregate formation pathways on the compartmentalization and the soil organic carbon origin. Two areas managed under NT with different implementation times (6 and 18 years, NT06 and NT18, respectively) and cover crops were evaluated, totaling six sampling areas: NT06, millet (NT06MI); NT06, brachiaria (NT06BR); NT06, sunn hemp (NT06SH); NT18, millet (NT18MI); NT18, brachiaria (NT18BR); NT18, and sunn hemp (NT18SH). In each sampling area, five pseudo-replicates were collected in the 0.00-0.05 and 0.05-0.10 m layers. The samples were air-dried and sieved using sieves with 9.7 and 8.0 mm mesh, and the aggregates retained within this interval were selected. The percentage of each type of aggregate (physicogenic and biogenic) was quantified. Total organic carbon (TOC) and the natural abundance of δ13C (‰) were analyzed and the physical fractionations of SOM were performed: particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) and density fractionation (free light fraction carbon, FLFC). Physicogenic aggregates were quantified in greater proportion, except for the areas of NT06BR and NT18BR in the 0.00-0.05 m layer. The biogenic aggregates showed the highest contents of TOC, POC, MAOC, FLFC and more negative values of δ13C. The use of grasses, especially Brachiaria spp., as cover plants in NT after 6 and 18 years of adoption favors the formation of aggregates through the biogenic pathway and they influence the compartmentalization and origin of stored organic carbon.

Available nutrients and labile carbon fractions in soil irrigated with sodic water

An ongoing long-term field experiment established in the year 1994 at Vegetable Science Research Farm, CCS HAU, Hisar, India under different vegetable cropping systems was selected to study the effects of sodic water irrigation, gypsum and farmyard manure on labile soil organic carbon fractions and available nutrients in soil. The present study was conducted after 20 years of experimentation during 2014-2015. The field was irrigated with high RSC (11.5 me/l) sodic water having three treatment levels each of gypsum, viz G0 control, G1=50 % neutralization of RSC, G2=100% neutralization of RSC, as well as FYM (F0 = control, F1= 10 tons/ha, F2 =20 tons/ha) in a sandy loam Typic Ustochrept. Results showed that the pH of soil was very high under F0G0, and it decreased with the use of FYM and gypsum as amendments, however a reverse trend was observed in electrical conductivity. There was an increase in different carbon fractions and available nutrients when FYM was applied over no FYM application. Application of gypsum increased microbial biomass carbon, light fraction carbon and available nutrients (N, P, S) content in the soil but dissolved organic carbon and available K content was found decreased. It can be concluded that application of FYM @20 t/ha along with gypsum @100% neutralization of RSC of irrigation water should be applied for maintaining soil quality and productivity.

Effects of Organic Wastes on Soil Organic Carbon and Surface Charge Properties in Primary Saline-alkali Soil

High salinity and low fertility have restricted crop production in primary saline-alkali soils. Soil organic carbon (SOC) and surface charge characteristics affect the soil fertility and soil colloid characteristics of primary saline-alkali soils, respectively. In this paper, the SOC and surface charge properties of primary saline-alkaline soil under organic wastes applications were assessed. Five treatments were involved in this experiment: chemical fertilizer combined with sheep manure (SM), corn straw (CS), fodder grass (FG), and granular corn straw (GS), while chemical fertilizer only was used as control (CK). The content of SOC was significantly different under different organic wastes application (p &lt; 0.05). Treatment GS recorded the highest content of SOC compared with the other treatments. In addition, the content of each SOC density fraction increased after the application of organic wastes. Similarly, the application of organic wastes, increased the proportion of organic carbon in free light fraction (Fr-FLOC) and organic carbon in occluded fraction (Oc-FLOC) in the soil however the proportion of organic carbon in heavy fraction (HFOC) decreased. In this study, we found that treatment GS has a greater impact on soil surface charge properties than other treatments, and through redundancy analysis (RDA) the content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were identified as the main factors affecting the surface charge properties of soil organic carbon. In conclusion, GS is the recommended organic waste for ameliorating primary saline-alkali soil, as compared to the other organic waste treatments.

Seasonal changes in labile organic matter as a function of environmental factors in a relict permafrost region on the Qinghai-Tibetan Plateau

Labile organic matter plays an important role in permafrost carbon cycling, however, little is known about the effects of permafrost on labile organic matter dynamics. Here, we examined seasonal changes in the labile organic matter including light fraction carbon (LFC), water extractable organic carbon (WEOC), microbial biomass carbon (MBC) and nitrogen (MBN) contents in a relict permafrost region on the eastern edge of the Qinghai-Tibetan Plateau (QTP), China. We selected three different areas including permafrost, boundary, and seasonally frozen ground area. The results showed that seasonal changes in labile organic matter were not the result of soil heterogeneity. Among all three areas, depth strongly correlated with labile organic matter content. LFC was significantly associated with the soil organic carbon (SOC) content. The MBC contents, which were the lowest in permafrost area but highest in seasonally frozen ground area, were strongly affected by temperature. Multiple linear regression models showed that temperature was a significant predictor for labile organic matter in the permafrost area, but the effects of temperature were weaker in the boundary and seasonally frozen ground areas. Our results suggested that permafrost degradation could decrease production but increase decomposition rates of labile organic matter in permafrost regions, and this process should be taken into consideration in permafrost carbon cycle models.

English

Forest management impacts on the soil carbon pool and soil properties. This study analyzed the changes of soil organic carbon (SOC) fractions, soil properties, and forest floor in a Chinese fir (Cunninghamia lanceolata) plantation after two years of thinning in Eastern China. The experiment consisted of three thinning treatments: unthinned, moderate thinned (stand stem density reduced by approximately 21%), and heavy thinned (stand stem density reduced by approximately 36%) stands. Concentrations of soil dissolved organic carbon (DOC), light fraction carbon (LFOC), and total SOC in the topsoil (0 to 20 cm) increased in the heavy thinned stands, and no significant effect for heavy fraction carbon (HFOC) was detectable in the soil profile. Some soil properties were also altered after thinning, which included a decrease in soil bulk density (BD), and an increase in soil temperature (ST), soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), available potassium (AK), and pH. The change of these soil properties mainly occurred in the topsoil layers (0 to 20 cm) in the heavy thinned treatment. The SOC and soil labile carbon fractions were positively correlated with ST, TN, AN, AP, and AK. Moreover, the increase of SOC fractions and the availability of soil nutrients maybe due to the increased input from an abundant well-decomposed forest floor after thinning. These results suggest that a 36% of thinning operations had a marked impact on the soil labile carbon pool and soil fertility in the topsoil in a Chinese fir plantation during a short-time period. Thus, changes to soil carbon stability and soil fertility should be considered when developing forest management plans. Key words: Thinning, soil organic carbon fraction, soil nutrient, soil carbon stability.

Influencia del clima, uso del suelo y profundidad sobre el contenido de carbono orgánico en dos pisos altitudinales andinos del departamento Norte de Santander, Colombia

El incremento y estabilización del carbono orgánico del suelo (COS) representa una alternativa viable para la mitigación del efecto invernadero. Pero el COS es severamente afectado por los cambios del suelo, existiendo controversias sobre cuáles tipos de uso de la tierra favorecen el secuestro de carbono. El clima tiene efecto importante sobre la dinámica del COS, por tanto el efecto generado por el uso y manejo del suelo es distinto en climas fríos y cálidos. Para entender como los factores clima, uso del suelo y profundidad del suelo, afectan la dinámica del COS en dos lugares del departamento de Norte de Santander en Colombia, se evaluaron propiedades físicas (arenas, limos y arcillas, densidad aparente) y propiedades químicas (pH, conductividad eléctrica, capacidad de intercambio catiónico y saturación de bases). Se determinó el contenido de carbono orgánico total (COT), fracción ligera (CO de FL), fracción húmica (CO de FH) y biomasa microbiana (CO de BM). Estas evaluaciones se realizaron en dos climas contrastantes (frío y cálido), tres tipos de uso del suelo (bosque, pasturas y cultivo intensivo) y tres profundidades (0 a 5 cm, 5 a 10 cm y 10 a 20 cm). Se encontró que hay mayor carbono orgánico en todos los compartimientos (COT, CO de BM, CO de FL y CO de FH) del suelo en clima frío. Los usos del suelo en bosque y pastura, son más favorables para el almacenamiento de COS en clima frío, mientras en cultivo intensivo genera menor contenido de COT, CO de FL y CO de BM. El índice de humificación y el CO de FH fueron mayores en cultivo intensivo de clima cálido.

Free light fraction carbon and nitrogen, a physically uncomplexed soil organic matter distribution within subtropical grass and leucaena–grass pastures

Quantifying the size and turnover of physically uncomplexed soil organic matter (SOM) is crucial for the understanding of nutrient cycling and storage of soil organic carbon (SOC). However, the C and nitrogen (N) dynamics of SOM fractions in leucaena (Leucaena leucocephala)–grass pastures remains unclear. We assessed the potential of leucaena to sequester labile, free light fraction (fLF) C and N in soil by estimating the origin, quantity and vertical distribution of physically unprotected SOM. The soil from a chronosequence of seasonally grazed leucaena stands (0–40 years) was sampled to a depth of 0.2m and soil and fLF were analysed for organic C, N and δ13C and δ15N. On average, the fLF formed 20% of SOC and 14% of total N stocks in the upper 0.1m of soil from leucaena rows and showed a peak of fLF-C and fLF-N stocks in the 22-year-stand. The fLF δ13C and fLF δ15N values indicated that leucaena produced 37% of fLF-C and 28% of fLF-N in the upper 0.1m of soil from leucaena rows. Irrespective of pasture type or soil depth, the majority of fLF-C originated from the accompanying C4 pasture-grass species. This study suggests that fLF-C and fLF-N, the labile SOM, can form a significant portion of total SOM, especially in leucaena–grass pastures.

Microbial and organic matter patterns in a prescribed burned and thinned managed forest ecosystem

AbstractPrescribed burning and thinning were implemented in an Alabama forest (Bankhead National Forest) as a management strategy to control pest and disease outbreaks and also to increase forest productivity. However, using fire as a control mechanism in this forest may alter soil nutrient cycling, soil organic matter (SOM), and soil microbial populations. There is the need for continuous research on forest ecosystems to bridge knowledge gaps in our understanding of SOM transformations and microbial processes in a repeatedly burned forest ecosystems. The objectives of this study were to assess and document the impact of prescribed burning and thinning on soil labile organic matter fractions, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), potential carbon mineralized (PCM), enzymatic activity, and energy (ATP) potentials. Labile organic carbon was isolated using the density method, whereas enzyme activities were determined as described in Bottomley et al. (Methods of soil analysis: part 2—Microbiological and biochemical properties, Madison, Wisconsin, USA: Soil Science Society of America, 1994). Microbial biomass C and N (MBC and MBN) were determined using the fumigation‐incubation method. Treatment applications had some effects on MBC and MBN although these effects were not statistically significant (P ≤ 0.05). Light fraction carbon (LFC) and light fraction nitrogen (LFN) were significantly (P ≤ 0.05) affected by treatments. Irrespective of treatment, xylanase activity was the highest (3244 ± 327–5223 ± 567 μmol/g 24 h−1), whereas amylase activity was the lowest (12.57 ± 8.9–116 ± 42.8 μmol/g 24 h−1). Correlation analysis revealed that amylase, β‐glucosidase, and NAGase correlated with particulate organic carbon (POC), particulate organic nitrogen (PON), and LFC, whereas cellulase, xylanase, and invertase had no correlation with the labile organic matter fractions. Compared to the referenced treatment plot, burned and thinned plots had reduced MBC, MBN, and increased PCM although this was not statistically significant. Although enzyme activities within plots were significant, no significant enzyme activities between plots were observed except with amylase. The lack of statistical significance on microbial activities, MBC, and MBN support our hypothesis that prescribed burning and thinning temporarily affected microbial indices, and these parameters are expected to return to pretreatment levels after a period of time.

Soil enzyme response to permafrost collapse in the Northern Qinghai-Tibetan Plateau

Permafrost degradation can result in the formation of permafrost collapsed ground features, and thus greatly alter soil variables such as moisture, pH, soil carbon and nitrogen content, and the biogeochemical cycling of soil carbon. However, little is known about the biogeochemical processes in these features within mountainous-permafrost areas. We examined activities of six different soil enzymes (invertase, amylase, catalase, polyphenol oxidase, urease, and alkaline phosphatase) in three micro-topographical settings (i.e., collapsing, collapsed, and an unaffected control site) of a typical thermokarst feature on the northern Qinghai-Tibetan Plateau. Our results show soil moisture is substantially lower within the permafrost thaw-induced collapsed feature. In addition, soil organic carbon, light fraction organic carbon and total nitrogen in the upper 10cm soil depth were lower in soils where permafrost was in the process of collapsing. Accordingly, soil enzyme activities varied considerably among the three settings, indicating biogeochemical processes have been altered by permafrost collapse. The invertase activities in collapsing soils were significantly lower than those of the control and collapsed soils for the upper 0–20cm layer. Coefficient of variation values for amylase and polyphenol oxidase were 44.1% and 6.7%, respectively. For 0–10cm soil depth, the catalase in collapsing soils were highest while the urease activities were lowest among the three settings. Statistical analysis demonstrated that light fraction carbon content, C:N ratios, and moisture were the most important predictors for enzyme activities. These results suggest that soil enzyme activities are good indicators for the decomposition of organic matter in permafrost collapse-affected areas.

Permafrost and land cover as controlling factors for light fraction organic matter on the southern Qinghai-Tibetan plateau

Permafrost degradation can stimulate the decomposition of organic soil matter and cause a large amount of greenhouse gas emissions into the atmosphere. The light fraction organic matter (LFOM) is a labile substrate for microbial decomposition and probably plays an important role in future permafrost carbon cycles. However, little is known about the distribution of LFOM and its relationship with permafrost and environmental factors. Here, we investigated the light fraction carbon (LFC) and nitrogen (LFN) contents and stocks under meadows and wet meadows with different permafrost conditions on the southern Qinghai-Tibetan Plateau. Our results showed that LFC and LFN were mainly distributed in the upper 30cm of soils, and the sites with permafrost had significantly higher contents of LFC and LFN than those from the sites without existing permafrost. The LFC and LFN decreased sharply with depth, suggesting that the soil organic matter (SOM) in this area was highly decomposed in deep soils. Soil moisture and bulk density explained approximately 50% of the variances in LFC and LFN for all the sampling sites, while soil moisture explained approximately 30% of the variance in permafrost sites. Both the C:N ratios and LFC:LFN ratios in the sites with permafrost were higher than those in the sites without permafrost. The results suggested that the permafrost and land cover types are the main factors controlling LFOM content and stock, and that permafrost degradation would lead to a decrease of LFOM and soil C:N ratios, thus accelerating the decomposition of SOM.

Stock characteristics of soil organic carbon pools under three subtropical forests in South China

Vegetation biomass and soil organic carbon (SOC) pools for the three representative forest types, i.e. conifer forest (CF), mixed conifer and broad-leaf forest (CBF), evergreen broad-leaf forest (EBF) in South China were investigated. We found that SOC stock of the three chief forest ranged from 55.54 to 151.16 MgC·ha-1, and it increased with increasing vegetation biomass under the same type forest within 100cm depth. The organic carbon contents at an equivalent level of forest maturity tended to be in the following decreasing order: EBF > CBF > CF, various active organic carbon (AOC) fractions in the 0-20cm topsoil layer tended to be in the following decreasing order: light fraction carbon (LFC) ≈ particulate organic carbon (POC) > easily oxidisable carbon (EOC) > microbial biomass carbon (MBC) > water-soluble carbon (WSC). At an equivalent level of forest maturity, there was a trend that each of these five AOC fractions increased from CF to CBF to the EBF.

Effects of fallow or planting wheat (Triticum aestivum L.) and fertilizing P or fertilizing P and N practices on soil carbon and nitrogen in a low-organic-matter soil

ABSTRACTSoil carbon (C) and nitrogen (N) are important for maintaining soil fertility, and they are considerably affected by soil use and management. In the present study, we conducted an 8-year field experiment on loessial dryland soil (Eum-Orthic Anthrosol, Food and Agriculture Organization of the United Nations (FAO)) in the southern Loess Plateau, China. We tested four soil management regimes—i.e., winter wheat (Triticum aestivum L.) cultivation with phosphorus (P) fertilization (WP), winter wheat cultivation with N and P fertilization (WNP), natural fallow (NF) and bare fallow (BF)—to evaluate their effects on soil C and N fractions. After 8 years, compared with the WNP treatment, the total soil organic nitrogen (SON) in the WP treatment decreased by 14.6% and 36.8%, and microbial biomass nitrogen (MBN) by 35.6% and 61.1%, at 0–20 and 20–40 cm soil depths, respectively. The soil heavy fraction nitrogen (HFN) and light fraction nitrogen (LFN) in the WP treatment also decreased by 36.6% and 39.4%, respectively. Furthermore, BF treatment decreased total soil organic carbon (SOC), heavy fraction carbon (HFC), LFN and MBN at both soil depths with average reductions of 43.4%. The NF treatment decreased light fraction carbon (LFC) by 17.0% at 0–20 cm soil depth, as well as MBN by 24.8% and 71.2%, and inorganic C by 29.1% and 23.8%, at 0–20 and 20–40 cm soil depths, respectively. There was no significant difference of microbial biomass C concentration among the WP, NF and BF treatments. These results confirmed that a lack of N fertilization decreased SON, BF reduced both SOC and SON, and NF decreased soil inorganic C. Therefore, the managements of a recommended rate of N fertilizer application and shortened time of bare fallow are critical for maintaining or increasing SON fraction sequestration, and natural fallow management is not a useful method for maintaining soil fertility in dryland in the Loess Plateau in China.Abbreviations: HFC: heavy fraction carbon; HFN: heavy fraction nitrogen; LFC: light fraction carbon; LFN: light fraction nitrogen; MBC: microbial biomass carbon; MBN: microbial biomass nitrogen; SOC: soil organic carbon; SON: soil organic nitrogen

Are active organic matter fractions suitable indices of management effects on soil carbon? A meta-analysis of data from the Pampas

ABSTRACTActive fractions of organic matter have been proposed as early indices of changes in soil organic carbon (organic-C) induced by management. We performed a meta-analysis of published results from 31 field experiments conducted in the Pampas in which tillage and rotation effects on organic-C, microbial biomass carbon (microbial-C), light fraction carbon (light-C), particulate carbon (particulate-C) and basal respiration (mineralized-C) were assessed. We compared the changes of organic-C and the four active fractions between management treatments sampled at the same date and depth within each experiment. Pooling all the experiments, active fractions-C varied on average 1.2- to 3.0-fold more than organic-C, depending on the fraction considered, but these average changes were significantly greater than organic-C changes only for particulate-C. This later fraction showed to be more sensitive to agricultural practices than organic-C. In experiments in which organic-C changes were lower than 15–25%, the four labile fractions may show opposite trends than organic-C. Above this threshold, changes of active fractions generally copied organic-C changes and were greater. Consequently, the active fractions may be used as indicators of changes in organic-C only when this latter variable has already suffered a huge change. In these cases, it will be easier to simply measure organic-C.

Seasonal variations in labile soil organic matter fractions in permafrost soils with different vegetation types in the central Qinghai–Tibet Plateau

Labile soil organic matter (SOM) plays a crucial role in nutrient and carbon cycling, particularly in permafrost ecosystems. Understanding its variation is therefore very important. In the present study, we evaluated the seasonal patterns of labile SOM from April 2013 to March 2014 under alpine swamp meadow (ASM), meadow (AM), steppe (AS) and desert (AD) vegetation in permafrost regions of the China's Qinghai–Tibet Plateau. The fractions (0 to 10cm depth) included dissolved organic carbon (DOC), light-fraction carbon (LFC) and nitrogen (LFN), and microbial biomass carbon (MBC) and nitrogen (MBN). These fractions showed dramatic seasonal patterns in ASM and AM soils, but were relatively stable in AD soil. Soil DOC concentrations in the ASM, AM, and AD soils increased from April to May 2013, then increased again from July to August 2013 and from February to March 2014. The LFC and LFN concentrations in all four vegetation types were higher from June to August 2013. The highest MBC and MBN concentrations in the ASM, AM, and AS soils all occurred in the summer and the ASM soil showed a second peak in October or November 2013. Seasonal changes in climatic factors, vegetation types, and permafrost features were great causes of labile SOM variations in this study. Throughout the entire sampling period, the ASM soil generally had the highest labile SOM, followed by the AM, AS, and AD soils; thus, the ASM soil is the best system conserving soil nutrient (especially labile fractions) and microbial activity. Correlation analysis indicated that these fractions were not related to soil moisture and temperature in AS or AD soils, but soil temperature and moisture were significantly related to MBC and MBN in AM soil and DOC in ASM soil. Thus, the response of the labile SOM fractions in this high-altitude permafrost soils to climate change depended strongly on vegetation types.