Stable carbon isotope ratios of water-extractable organic carbon affected by application of rice straw and rice straw compost during a long-term rice experiment in Yamagata,Japan

ABSTRACT

Hot-water- and water-extractable organic matter were obtained from soil samples collected from a rice paddy 31 years after the start of a long-term rice experiment in Yamagata, Japan. Specifically, hot-water-extractable organic carbon and nitrogen (HWEOC and HWEON) were obtained by extraction at 80°C for 16 h, and water-extractable organic carbon and nitrogen (WEOC and WEON) were obtained by extraction at room temperature. The soil samples were collected from surface (0–15 cm) and subsurface (15–25 cm) layers of five plots that had been treated with inorganic fertilizers alone or with inorganic fertilizers plus organic matter, as follows: PK, NPK, NPK plus rice straw (RS), NPK plus rice straw compost (CM1), and NPK plus a high dose of rice straw compost (CM3). The soil/water ratio was 1:10 for both extraction temperatures. We found that the organic carbon and total nitrogen contents of the bulk soils were highly correlated with the extractable organic carbon and nitrogen contents regardless of extraction temperature, and the extractable organic carbon and nitrogen contents were higher in the plots that were treated with inorganic fertilizers plus organic matter than in the PK and NPK plots. The HWEOC and WEOC δ¹³C values ranged from ?28.2% to ?26.4% and were similar to the values for the applied rice straw and rice straw compost. There were no correlations between the HWEOC or WEOC δ¹³C values and the amounts of HWEOC or WEOC. The δ¹³C values of the bulk soils ranged from ?25.7% to ?23.2% and were lower for the RS and CM plots than for the PK and NPK plots. These results indicate that HWEOC and WEOC originated mainly from rice plants and the applied organic matter rather than from the indigenous soil organic matter. The significant positive correlations between the amounts of HWEOC and HWEON and the amount of available nitrogen (P < 0.001) imply that extractable organic matter can be used as an index for soil fertility in this long-term experiment. We concluded that the applied organic matter decomposed more rapidly than the indigenous soil organic matter and affected WEOC δ¹³C values and amounts.     

Carbon isotopes of water‐extractable organic carbon in a depth profile of forest soil imply a dynamic relationship with soil carbon

Although considerable research has been conducted on the importance of recent litter compared with older soil organic matter as sources of dissolved organic carbon (DOC) in forest soils, a more thorough evaluation of this mechanism is necessary. We studied water‐extractable organic carbon (WEOC) in a soil profile under a cool‐temperate beech forest by analysing the isotopic composition (¹³C and ¹⁴C) of WEOC and its fractions after separation on a DAX‐8 resin. With depth, WEOC became more enriched in ¹³C, which reflects the increasing proportion of the hydrophilic, isotopically heavier fraction. The ¹⁴C content in WEOC and its fractions decreased with depth, paralleling the ¹⁴C trend in soil organic matter (SOM). These results indicate a dynamic equilibrium of WEOC and soil organic carbon. The dominant process maintaining the WEOC pool in the mineral soil appears to be the microbial release of water‐soluble compounds from the SOM, which alters in time‐scales of decades to centuries.     

Dynamics of water extractable organic carbon at a subtropical catchment using fluorescence excitation-emission matrix spectroscopy coupled with parallel factor analysis

The vertical and horizontal movement of water extractable organic carbon (WEOC) has a significant influence on soil organic carbon (SOC) storage and water quality. However, how the amount, components and chemical qualities of WEOC change at horizontal and vertical levels, as well as desorption in the cascade model in subtropical red soils, are not fully understood. We sampled soils from four types of land (Elaeocarpus, Masson Pine, Crape-Myrtle and a deposition site) in a subtropical catchment as replicated cores (n = 96) and investigated changes in the molecular and structural constituents of WEOC using excitation-emission matrix fluorescence and UV–visible absorbance spectroscopy. Our results showed that WEOC concentration changed remarkably in all four plots, and that the concentration coupled with the relative abundance of aromatic compounds decreased with increasing soil depth, whereas low-molecular-weight materials increased with increasing soil depth. Five fluorescent components were identified by parallel factor analysis (i.e., tryptophan-like, tyrosine-like and three humic-substance-like components). The relative abundance of these components suggested that WEOC composition shifted along vertical soil layers. Dynamic variations in fluorescent indices indicated that WEOC from the subsoil has strong biological characteristics. Additionally, there was no significant correlation between WEOC and SOC in subsoil, indicating that much of the WEOC within mineral soils is of local microbial origin rather than from local soil organic matter. Sorption processes alone or stepwise exchange processes of WEOC on the soil minerals do not readily occur, as the red soil in a subtropical area is typically unsaturated and potentially has a large sorption capacity. Once soil carbon is adsorbed, it is difficult to release it into soil water by microbial processing. Data from the deposition site suggest that components of hydrophilic WEOC with low molecular weight are preferentially transported.     

Determination of the fate of 13C labelled maize and wheat exudates in an agricultural soil during a short-term incubation

A broader knowledge of the contribution of carbon (C) released by plant roots (exudates) to soil is a prerequisite for optimizing the management of organic matter in arable soils. This is the first study to show the contribution of constantly applied ¹³C‐labelled maize and wheat exudates to water extractable organic carbon (WEOC), microbial biomass‐C (MB‐C), and CO₂‐C evolution during a 25‐day incubation of agricultural soil material. The CO₂‐C evolution and respective δ¹³C values were measured daily. The WEOC and MB‐C contents were determined weekly and a newly developed method for determining δ¹³C values in soil extracts was applied. Around 36% of exudate‐C of both plants was recovered after the incubation, in the order WEOC < MB‐C < CO₂‐C for maize and MB‐C < WEOC < CO₂‐C for wheat. Around 64% of added exudate‐C was not retrieved with the methods used here. Our results suggest that great amounts of exudates became stabilized in non‐water extractable organic fractions. The amounts of MB‐C stayed relatively constant over time despite a continuous exudate‐C supply, which is the prerequisite for a growing microbial population. A lack of mineral nutrients might have limited microbial growth. The CO₂‐C mineralization rate declined during the incubation and this was probably caused by a shift in the microbial community structure. Consequently, incoming WEOC was left in the soil solution leading to rising WEOC amounts over time. In the exudate‐treated soil additional amounts of soil‐derived WEOC (up to 110 μg g⁻¹) and MB‐C (up to 60 μg g⁻¹) relative to the control were determined. We suggest therefore that positive priming effects (i.e. accelerated turnover of soil organic matter due to the addition of organic substrates) can be explained by exchange processes between charged, soluble C‐components and the soil matrix. As a result of this exchange, soil‐derived WEOC becomes available for mineralization.     

A comparison of extraction procedures for water‐extractable organic matter in soils

The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.     

Changes in soil organic carbon stocks and soil quality: land-use system effects in northern Ethiopia

Abstract

In Tigray, Ethiopia, land degradation is a dominant environmental problem and hence the regional government has undertaken restoration measures on degraded soils since 1991. The present study was aimed to assess the impact of land uses and soil management practices on soil properties, and consequently on soil quality of degraded soils. The catchments selected were Maileba and Gum Selassa, and land uses included cultivated (CL), grazing (GL), plantation (PA) and area exclosure (AE). Replicated soil samples were collected from topsoil and profiles of four land-use types in both catchments. Soils in area exclosure showed higher soil organic carbon (SOC), total N and extractable K than grazing land, cultivated land and plantation area mainly at 0–40 cm soil depth. Estimated soil organic carbon stock at Maileba in 0–40 cm depth varied between 54 to 74 Mg C ha^?1, being lowest in cultivated land and highest in area exclosure, and the soil organic carbon stock in area exclosure represents 63% of total carbon stock stored in the profile. Soil organic carbon stock (0–40 cm) at Gum Selassa ranged between 33 to 38 Mg C ha^?1, being higher in cultivated land and lower in plantation area. Soil quality index (SQI) of area exclosure (0.794) at Maileba and cultivated land (0.721) at Gum Selassa scored highest among all land uses, and the order was area exclosure>grazing land>plantation area>cultivated land at Maileba and cultivated land>grazing land>plantation area at Gum Selassa, highlighting the effectiveness of area exclosure in restoring soil quality of degraded soils.     

SOIL ORGANIC CARBON STOCK AND FRACTIONS IN RELATION TO LAND USE AND SOIL DEPTH IN THE DEGRADED SHIWALIKS HILLS OF LOWER HIMALAYAS

The proportional differences in soil organic carbon (SOC) and its fractions under different land uses are of significance for understanding the process of aggregation and soil carbon sequestration mechanisms. A study was conducted in a mixed vegetation cover watershed with forest, grass, cultivated and eroded lands in the degraded Shiwaliks of the lower Himalayas to assess land‐use effects on profile SOC distribution and storage and to quantify the SOC fractions in water‐stable aggregates (WSA) and bulk soils. The soil samples were collected from eroded, cultivated, forest and grassland soils for the analysis of SOC fractions and aggregate stability. The SOC in eroded surface soils was lower than in less disturbed grassland, cultivated and forest soils. The surface and subsurface soils of grassland and forest lands differentially contributed to the total profile carbon stock. The SOC stock in the 1.05‐m soil profile was highest (83.5 Mg ha⁻¹) under forest and lowest (55.6 Mg ha⁻¹) in eroded lands. The SOC stock in the surface (0–15 cm) soil constituted 6.95, 27.6, 27 and 42.4 per cent of the total stock in the 1.05‐m profile of eroded, cultivated, forest and grassland soils, respectively. The forest soils were found to sequester 22.4 Mg ha⁻¹ more SOC than the cultivated soils as measured in the 1.05‐m soil profiles. The differences in aggregate SOC content among the land uses were more conspicuous in bigger water‐stable macro‐aggregates (WSA > 2 mm) than in water‐stable micro‐aggregates (WSA < 0.25 mm). The SOC in micro‐aggregates (WSA < 0.25 mm) was found to be less vulnerable to changes in land use. The hot water soluble and labile carbon fractions were higher in the bulk soils of grasslands than in the individual aggregates, whereas particulate organic carbon was higher in the aggregates than in bulk soils. Copyright © 2012 John Wiley & Sons, Ltd.     

Living and dead soil organic matter under different land uses on a Mediterranean island

Soil under six different land uses on Pianosa Island, in the Mediterranean Sea, was characterized in terms of microbial activity and organic matter quality, in order to define relationships between living and dead organic matter. Biological measurements and chemical and spectroscopic (¹³C NMR and FTIR) investigation of the extracted soil organic matter provided clues to the effects of soil management on the reciprocal interactions between living and dead organic matter. In particular, the conversion from the original bushy maquis to other land uses, such as degraded thickets of holm oak, maquis‐invaded groves of olive trees, stands of Aleppo pines and abandoned pastures, implied significant reduction of soil organic carbon (SOC) and its microbial fraction (MBC). Cropland, which is the land use with the greatest perturbation of soil, had the smallest SOC, MBC and soil respiration rate. Significant differences in extractable SOC among land uses occurred, both as total amount and as molecular mass distribution. The relatively good relationship between soil respiration and the extracted SOM‐fraction of 2–50 kDa, expressed on a molecular mass basis, suggests that this size is strongly linked to heterotrophic organisms and that it could be representative of a transitory pool of C in soil.     

Impact of land use on soil organic carbon stocks in the humid tropics of NE Tanzania

The conversion of tropical forests to agricultural land use is considered as a major cause for a decline in soil organic carbon (SOC) stocks. However, the extent and impact of different land uses on SOC stock development is highly uncertain, especially for tropical Africa due to a lack of reliable data. Interactions of SOC with the soil mineral phase can modify the susceptibility of SOC to become mineralized. Pedogenic Fe‐, Al‐oxides and clay potentially affect SOC stabilization in highly weathered soils typically found in the humid tropics. The aim of our study was to determine the impact of different land uses on SOC stock on such soils. For that purpose, 10 pedologically similar, deeply weathered acidic soils (Acrisols, Alisols) in the Eastern Usambara Mountains (Amani Nature Reserve, NE Tanzania) under contrasting land use were sampled to a depth of 100 cm. The calculated mean SOC stocks were 17.5 kg C m^?2, 16.8 kg C m^?2, 16.9 kg C m^?2, and 20.0 kg C m^?2 for the four forests, two tea plantations, three croplands, and one homegarden, respectively. A significant difference in mean SOC stock of 1.3 kg C m^?2 was detected between forest and cropland land use for the 0–10 cm depth increment. No further significant impacts of land use on SOC stocks were observed. All soils have a clearly clay‐dominated texture. They are characterized by high content of pedogenic oxides with 29 to 47 g kg^?1 measured for the topsoils and 36 to 65 g kg^?1 for the subsoils. No positive significant relationship was found between SOC and clay content. Statistically significant positive relationships existed between oxalate‐extractable Fe, Al, and SOC content for cropland soils only. Compared to data published in literature the SOC stocks determined in our study were generally high independent of the established land use. It appears that efficient SOC stabilization mechanisms are counteracting the higher disturbance regime under agricultural land use in these highly weathered tropical soils.     

Biological, chemical and thermal indices of soil organic matter stability in four grassland soils

The various ecosystem functions of soil organic matter (SOM) depend on both its quantity and stability. Numerous fractionation techniques have been developed to characterize SOM stability, and thermal analysis techniques have shown promising results to describe the complete continuum of SOM in whole soil samples. However, the potential link between SOM thermal stability and biological or chemical stability has not yet been adequately explored. The objective of this study was to compare conventional chemical and biological methods used to characterize SOM stability with results obtained by thermal analysis techniques. Surface soil samples were collected from four North American grassland sites along a continental mean annual temperature gradient, each with a native and cultivated land use. Soil organic C concentrations ranged from 6.8 to 33 g C kg⁻¹ soil. Soils were incubated for 588 days at 35 °C, and C mineralization rates were determined periodically throughout the incubation by measuring CO₂ concentration using an infrared gas analyzer (IRGA) to calculate biological indices of SOM stability. Hot-water extractable organic C (HWEOC) contents were determined before and after incubation as chemical indices. Finally, samples from before and after incubation were analyzed by simultaneous thermal analysis (i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)) to determine thermal indices of SOM stability. Long-term incubation resulted in the mineralization of up to 33% of initial soil C. The number of days required to respire 5% of initial soil organic carbon (SOC), ranged from 27 to 115 days, and is proposed as a standardized biological index of SOM stability. The number of days was greater for cultivated soils compared to soils under native vegetation, and generally decreased with increasing site mean annual temperature. HWEOC (as % of initial SOC) did not show consistent responses to land use, but was significantly lower after long-term incubation. Energy density (J mg⁻¹ OM) was greater for soils under native vegetation compared to cultivated soils, and long-term incubation also decreased energy density. The temperatures at which half of the mass loss or energy release occurred typically showed larger responses to land use change than to incubation. Strong correlations demonstrated a link between the thermal and biogeochemical stability of SOM, but the interpretation of the thermal behavior of SOM in bulk soil samples remains equivocal because of the role the mineral component and organo-mineral interactions.     

Reproducibility of a soil organic carbon fractionation method to derive RothC carbon pools

Fractionation of soil is undertaken to isolate organic carbon with distinct functional properties, such as stability and turnover times. Soil organic carbon (SOC) fractionation helps us to understand better the response of SOC to changes in land use, management or climate. However, fractionation procedures are often poorly defined and there is little information available on their reproducibility in different laboratories. In a ring trial, we assessed the reproducibility of a SOC fractionation method introduced by Zimmermann et al. (2007). The isolated fractions were linked to the model pool sizes of the Rothamsted carbon model (RothC). We found significant differences between six laboratories for all five defined fractions in three different soils with coefficients of variation ranging from 14 to 138%. During ultrasonic dispersion, the output power (energy per unit time) was identified as an important factor controlling the distribution of SOC within these five fractions, while commonly only the output energy is standardized. The amount of water used to wet‐sieve dispersed soil slurry significantly influenced the amount of extracted dissolved organic carbon (DOC). We therefore suggest using a fixed amount of power for ultrasonic dispersion (20 W) and a minimum amount of water for wet sieving (2000 ml). RothC pool sizes were predicted from the measured fractions and compared with RothC equilibrium pool size distributions. This model initialization using measured SOC fractions, however, led to an over‐estimation of stable RothC SOC pools when compared with pool size distributions derived from RothC equilibrium runs under a bare fallow soil model simulation. To improve the isolation of particulate organic matter from stable mineral‐bound organic matter, we suggest that the density should be increased from 1.8 to 2.0 g cm^?3 in the density fractionation step. We formulated a modified fractionation procedure, which aims specifically to enhance reproducibility across laboratories and to improve the match of the isolated SOC fractions with RothC's SOC pools.     

Nutrient management effects on organic carbon pools in a sandy loam soil under rice-wheat cropping

Agricultural management practices are known to influence soil organic C. While changes in total organic C (TOC) are relatively less discernible over short to medium-term, some extractable pools of TOC are considered early indicators of changes in TOC. Therefore, to devise nutrient management practices that can lead to C sequestration, it is important to study their effect on soil organic C pools that may respond rapidly to management. We studied the impact of balanced (NPK) and imbalanced (N, NP, NK and PK) application of fertilizer nutrients without and with farmyard manure (FYM) on total and labile pools of organic C viz. water soluble (WEOC), potassium permanganate oxidizable (KMnO₄-C), microbial biomass (MBC) and fractions of decreasing oxidizability after 5-cycles of rice-wheat cropping. Integrated use of NPK and FYM significantly increased TOC and extractable C pools in both surface (0–7.5 cm) and sub-surface (7.5–15 cm) soil. Majority of TOC (72%) was stabilized in less labile and recalcitrant fractions; the magnitude being higher under balanced (NPK+FYM) than imbalanced nutrient management (N+FYM). The results showed that balanced fertilizer application conjointly with FYM besides enlarging TOC pool favorably impacts soil organic matter composition under rice-wheat system.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Variations of soil organic carbon fractions in response to conservative vegetation successions on the Loess Plateau of China

Land use changes profoundly affect the equilibrium of soil organic carbon (SOC) sequestration and greenhouse gas emissions. With the current global climatic changes, it is vital to understand the influence of ecological restoration and conservation management on the dynamics of SOC under different land uses, especially in erosion-endangered Loess soils. Therefore, we investigated changes in SOC through a suit of labile fractions, namely: light fraction organic C (LFOC), heavy fraction organic C (HFOC), coarse particulate organic C (CPOC), fine particulate organic C (FPOC), and dissolved organic C (DOC), from two forests i.e., Robinia pseudoacacia (RP) and Platycladus orientalis (PO), with different ages, in comparison with farmland (FL). The SOC and STN contents significantly increased over 42 years in the RP forest where the contents of CPOC and FPOC were significantly higher than in the FL. Moreover, total SOC and its labile fractions, in the studied land use types, significantly correlated with soil CaCO₃, pH, and STN contents, indicating their key roles in SOC sequestration. The results reported here from different vegetation with different ages provide a better understanding of SOC and STN alterations at different stages of vegetation restoration. Our findings suggest that long-term natural vegetation restoration could be an effective approach for SOC sequestration and soil conservation on the Loess soil.     

Determination of the fate of regularly applied 13C‐labeled‐artificial‐exudates C in two agricultural soils

Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of ¹³C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO₂‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO₂‐C concentrations and the respective δ¹³C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO₂‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC << MBC < CO₂‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO₂‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.     

Effects of cultivation and abandonment on soil carbon content of subalpine meadows, northwest China

Purpose

Land use changes have a significant impact on soil carbon emission and sequestration worldwide. Accurate evaluation of the effect of land use change (cultivation and abandonment) on soil carbon content of subalpine meadows is required to monitor the soil carbon dynamics of rangeland ecosystems in China.

Materials and methods

Based on collection of soil cores and vegetation, investigations of four types of land use (undisturbed natural meadow, land cultivated for 20?years, land abandoned for 3?years following cultivation, and land abandoned for 10?years following cultivation) were undertaken in the headwater area of the Heihe River in northwest China. Three soil carbon fractions [soil organic carbon (SOC), light fraction organic carbon (LFOC), and microorganism biomass carbon (MBC)] were determined in the laboratory, and the relative abundances of LFOC/SOC and MBC/SOC were calculated.

Results and discussion

Repeated cultivation by ploughing reduced the carbon content of the top soil layer, resulting in more uniform vertical distribution of soil organic matter. Ten years after cessation of cultivation, the organic carbon content within the top 10-cm soil layer (0?C10?cm) had reached 90?% of the content in native meadows, equivalent to a mean annual sequestration rate of 1.73?t?C?ha^?1. The rate of LFOC restoration was faster than that of SOC restoration. The variation in the ratio of MBC to SOC (0.91?C1.07?%) was small.

Conclusions

The activity of cultivation reduced all indicators of soil carbon status, which were not completely restored to the level of natural meadow, even after abandonment of cultivation for 10?years. Nevertheless, abandonment of cultivation is a practical, even if long-term, means of improving carbon sequestration in subalpine meadow of China.     

不同土地利用方式对土壤有机无机碳比例的影响

【目的】 土壤有机碳 (SOC) 和无机碳 (SIC) 对全球碳循环和减缓气候变化具有重要作用,进一步明确二者之间相互转化关系,对准确估算土壤碳储量具有重要意义。现有研究对SOC和SIC相互关系缺乏系统量化,研究结果不一。因此,明确SOC和SIC之间相互关系,可为准确估算和模拟土壤碳的转化过程提供理论基础。 【方法】 本研究搜集了我国1990—2018年已发表的文献共41篇,从不同气候区、不同土地利用方式、不同土层深度探究了SOC和SIC比例的变化,进一步量化了二者之间的相互关系。 【结果】 不同气候区、不同土地利用方式下土壤SOC/SIC值在0—20 cm土层均大于20—100 cm土层。具体来说,在温带大陆性气候区,草地0—20 cm土壤SOC/SIC值最小 (0.53),林地 (0.90) 和农田 (0.80) 土壤较高,且三种土地利用方式下SOC和SIC呈极显著正相关关系;而在温带季风性气候区,0—20 cm土壤SOC/SIC值表现为草地 (0.82) ≈ 农田 (1.05) > 林地 (0.29),且SOC和SIC在林地、农田土壤中呈正相关关系,但在草地土壤中二者为负相关关系。另外,温带大陆性气候区20—100 cm以林地土壤SOC/SIC值最高,草地和农田次之,而在温带季风性气候区三种土地利用方式下无显著差异;SOC和SIC在林地和农田土壤中呈正相关关系,然而在草地土壤中为负相关关系。温带大陆性气候区SOC/SIC值总体以林地较大,农田、草地次之。温带季风性气候区,0—20 cm土层SOC/SIC值以草地较大,农田和林地分别次之。这可能是因为植被覆盖不同,导致了作物碳的归还量不一。同时,不同的植被覆盖还影响了土壤中的各种生物化学进程,改变了碳在土壤中的循环转化过程,进而影响了SOC和SIC含量,使得SOC/SIC值产生较大差异。 【结论】 SOC和SIC之间存在循环转化关系,且不同气候条件、不同土地利用方式、不同土壤类型对SOC和SIC循环转化存在显著影响。不同条件下SOC/SIC值存在显著差异,且二者呈现不同的相关性。本研究结果可为明确土壤碳的循环积累机制,准确估算土壤有机和无机碳库提供理论依据。      

Is thermal oxidation at different temperatures suitable to isolate soil organic carbon fractions with different turnover?

Findings of previous studies suggest that there are relations between thermal stability of soil organic matter (SOM), organo‐mineral associations, and stability of SOM against microbial decay. We aimed to test whether thermal oxidation at various temperatures (200°C, 225°C, 275°C, 300°C, 400°C, or 500°C) is capable of isolating SOM fractions with increasing stability against microbial degradation. The investigation was carried out on soils (Phaeozem and Luvisol) under different land‐use regimes (field, grassland, forest). The stability of the obtained soil organic carbon (SOC) fractions was determined using the natural‐¹³C approach for continuously maize‐cropped soils and radiocarbon dating. In the Luvisol, thermal oxidation with increasing temperatures did not yield residual SOC fractions of increasing microbial stability. Even the SOC fraction resistant to thermal oxidation at 300°C contained considerable amounts of young, maize‐derived C. In the Phaeozem, the mean ¹⁴C age increased considerably (from 3473 y BP in the mineral‐associated SOC fraction to 9116 y BP in the residual SOC fraction after thermal oxidation at 300°C). An increasing proportion of fossil C (calculated based on ¹⁴C data) in residual SOC fractions after thermal oxidation with increasing temperatures indicated that this was mainly due to the relative accumulation of thermally stable fossil C. We conclude that thermal oxidation with increasing temperature was not generally suitable to isolate mineral‐associated SOC fractions of increasing microbial stability.     

Hot water extractable phosphorus pools as indicators of soil P responses to harvest residue management in an exotic pine plantation of subtropical Australia

Purpose

This study evaluated the potential of using hot water extractable phosphorus (P) pools as a method to assess the impacts of harvest residue management on the bioavailability of P in an exotic pine plantation of southeast Queensland, Australia.

Materials and methods

This study was carried out under three harvest residue management regimes: (1) residue removal, RR₀; (2) single-level residue retention (operational level), RR₁; and (3) double level of residue retention, RR₂, established immediately following clear-cut harvesting in a randomised complete block experiment. Soil was sampled after 24 months of the residue management regimes applied and analysed for hot water extractable inorganic P (HWEIP), hot water extractable organic P (HWEOP) and total P (HWETP), in relation to hot water extractable organic C (HWEOC) and N (HWEON), calcium chloride extractable P (CaCl₂_P), bicarbonate extractable P (NaHCO₃_P) and fluoride extractable P (Bray1_P).

Results and discussion

The HWEIP and CaCl₂_P concentrations showed no significant variations amongst the treatments, while Bray1_P, NaHCO₃_P and the HWEOP P pools were only significantly greater in the RR₂ treatment than the RR₀ treatment. In contrast, the HWETP pool showed highly significant (p?<?0.005) differences amongst all the treatments. In addition, both the HWEOP and HWETP were significantly related to the HWEOC and the total C, in contrast to the lack of such relationships with soil total P, suggesting their association with the HWEOC released through residue decomposition.

Conclusions

This study showed that HWETP is a more sensitive measure of labile soil P and has the potential to be used as an indicator of management practices, particularly in cases where high spatial variations in soil P concentrations might confound P responses.     

Impacts of land‐use intensity on soil organic carbon content,soil structure and water‐holding capacity

The impact of land‐use intensity is evaluated through changes in the soil properties in different areas of the traditional central Spanish landscape. Soil organic carbon (SOC) content, bulk density, aggregate stability and water‐holding capacity (WHC) in the topsoil of active and abandoned vineyards, livestock routes (LR) and young Quercus afforested areas were analysed. These different types of land use can be interpreted as having a gradient of progressively less impact on soil functions or conservation. As soil use intensity declines, there is an increase in SOC content (from 0.2 to 0.6%), WHC (from 0.2 to 0.3 g H₂O per g soil) and aggregate stability (from 4 to 33 drop impacts). Soils beneath vines have lost their upper horizon (15 cm depth) because of centuries‐old tillage management of vineyards. Except for an increase in bulk density (from 1.2 to 1.4 g/cm³), there were no differences in soil characteristics 4 yr after the abandonment of vine management. LR can be considered sustainable uses of land, which preserve or improve soil characteristics, as there were no significant differences between topsoil from LR and that from a 40‐yr‐old Quercus afforested area. SOC content, one of the main indicators for soil conservation, is considered very low in every case analysed, even in the more conservative uses of land. These data can be useful in understanding the slow rate of recovery of soils, even after long‐term cessation of agricultural land use.