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1.
The different management regimes on grassland soils were examined to determine the possibilities for improved and/or changed land management of grasslands in Flanders (Belgium), with respect to article 3.4 of the Kyoto Protocol. Grassland soils were sampled for soil organic carbon (SOC) and for bulk density. For all grasslands under agricultural use, grazing and mowing + grazing led to higher SOC stocks compared with mowing, and grazing had higher SOC stocks compared with mowing + grazing. Overall, 15.1 ± 4.9 kg OC m–2 for the clayey texture, 9.8 ± 3.0 kg OC m–2 for the silty texture, and 11.8 ± 3.8 kg OC m–2 for the sandy texture were found for grassland under agricultural use to a depth of 60 cm. For seminatural grasslands, different results were found. For both the clayey and silty texture, mowing and mowing + grazing led to higher SOC stocks compared with grazing. The clayey texture had a mean stock of 15.1 ± 6.6, the silty texture of 10.9 ± 3.0, and the sandy texture of 12.1 ± 3.9 kg OC m–2 (0–60 cm). Lower bulk densities were found under grazed agricultural grassland compared with mown grassland but for seminatural grassland, no clear trends for the bulk density were found. The best management option for maintaining or enhancing SOC stocks in agricultural grassland soils may be permanent grazed grassland. For seminatural grassland, no clear conclusions could be made. The water status of the sampled mown fields was influencing the results for the clayey texture. Overall, the mean SOC stock was decreasing in the order clay > sand > silt. The higher mean SOC concentrations found for the sandy texture, compared to the finer silty texture, may be explained by the historical land use of these soils.  相似文献   

2.
Total soil organic‐carbon (SOC) stocks for grassland soils in Flanders (N Belgium) were determined for the Kyoto Protocol reference year 1990 and 2000 in order to investigate whether these soils have been CO2 sinks or sources during that period. The stocks were calculated by means of detailed SOC datasets, which were available at the community scale for the whole of Flanders. The total SOC stocks for Flemish grassland soils (1 m depth) were estimated at 38 Mt SOC in 1990 and 34 Mt SOC in 2000. The loss of SOC resulted from a decrease in the SOC content of grassland soils (71%) and could also partly (29%) be explained by a decline in grassland area. Significant decreases in %SOC for the 0–6 cm depth layer were found for the 1990s for the coarser‐textured soils with SOC losses ranging between –0.3% and –0.5% over the 10 y period. Specific management practices that disturb the SOC balance such as conversion to temporary grassland and a reduction of animal‐manure application are hypothesized to have contributed to the observed loss of SOC stocks. We furthermore conducted an analysis of uncertainty of the 1990 and 2000 grassland SOC–stocks calculation using Monte Carlo analysis. Probability‐distribution functions were determined for each of the inputs of the SOC‐stock calculation, enabling us to assess the uncertainty on the 1990 and 2000 SOC stocks. The frequency distributions of these simulated stocks both closely approached lognormal distributions, and their 95%‐confidence intervals ranged between 150% and 50% of the calculated mean SOC stock. The standard error on the measured decrease in SOC stocks in Flemish grassland soils during the 1990s was calculated to be 7–8 Tg SOC, which is equivalent to twice this decrease. This clearly shows that large‐scale changes in SOC stocks are uncertainty‐ridden, even when they are based on detailed datasets.  相似文献   

3.
Past land‐use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne‐Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability. We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg?1 in 2005 to 9.8 g kg?1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg?1 in 2005 to 3.5 g kg?­1 in 2013). Intriguingly, these changes were due to changes in mineral‐bound SOC rather than to changes in sand‐sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)?1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils [?0.73 t SOC (ha · y)?1], and less pronounced for loamy soils [?0.64 t SOC (ha · y)?1]; silty soils revealed the smallest reduction in SOC [?0.3 t SOC (ha · y)?1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)?1 (2003–2005) to about 133 kg C (ha · y)?1 (2005–2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics.  相似文献   

4.
Because of the large spatial and temporal variability of soil organic carbon (SOC) dynamics, a modelling approach is crucial in detailed regional analyses. Several estimates of regional scale SOC sequestration potential have been made using dynamic soil organic matter (SOM) models which have been linked to spatial databases contained within a Geographic Information System. In all these previous studies, a large‐scale model validation, which provides information on the general model performance for the study area under concern, was impossible because of lack of data. A data set of over 190 000 SOC measurements, grouped as means per community and covering the period 1989–2000 was available for Flanders in northern Belgium. In order to validate the DNDC model at a large spatial scale, we used this data set along with detailed pH, soil texture and crop areas which were all available at the municipality scale to simulate SOC stocks for the entire study area during the period 1990–2000. A minor adjustment of the initial distribution of SOC in the model's SOC pool was necessary to fit the simulated SOC stock changes to the measured decrease of −475 kg OC ha−1 year−1 (0–30 cm). Although DNDC was able to simulate the SOC stock changes well for the whole study area, the simulated decrease in the SOC stocks was overestimated for communities predominantly having sandy textures and underestimated for communities with silt loam to silt textures. This study also urges caution with the application of SOM models at regional scales after limited validation or calibration at the field scale as these do not guarantee good simulation of spatial variation in SOC changes.  相似文献   

5.
An essential prerequisite for a sustainable soil use is to maintain a satisfactory soil organic‐matter (OM) level. This might be achieved by sound fertilization management, though impacts of fertilization on OM have been rarely investigated with the aid of physical fractionation techniques in semiarid regions. This study aimed at examining changes in organic C (OC) and N concentrations of physically separated soil OM pools after 26 y of fertilization at a site of the semiarid Loess Plateau in China. To separate sensitive OM pools, total macro‐OM (> 0.05 mm) was obtained from bulk soil by wet‐sieving and then separated into light macro‐OM (< 1.8 g cm–3) and heavy macro‐OM (> 1.8 g cm–3) subfractions; bulk soil was also differentiated into light OM (< 1.8 g cm–3) and mineral‐associated OM (> 1.8 g cm–3). Farmyard manure increased concentrations of total macro‐OC and N by 19% and 25%, and those of light fraction OC and N by 36% and 46%, compared to no manuring; both light OC and N concentrations but only total macro‐OC concentration responded positively to mineral fertilizations compared to no mineral fertilization. This demonstrated that the light‐fraction OM was more sensitive to organic or inorganic fertilization than the total macro‐OM. Mineral‐associated OC and N concentrations also increased by manuring or mineral fertilizations, indicating an increase of stable OM relative to no fertilization treatment, however, their shares on bulk soil OC and N decreased. Mineral fertilizations improved soil OM quality by decreasing C : N ratio in the light OM fraction whereas manuring led to a decline of the C : N ratio in the total macro‐OM fraction, with respect to nil treatment. Further fractionation of the total macro‐OM according to density clarified that across treatments about 3/4 of total macro‐OM was associated with minerals. Thus, by simultaneously applying particle‐size and density separation procedures, we clearly demonstrated that the macro‐OM differed from the light OM fraction not only in its chemical composition but also in associations with minerals. The proportion of the 0.5–0.25 mm water‐stable aggregates of soil was higher under organic or inorganic fertilizations than under no manure or no mineral fertilization, and increases in OC and N concentrations of water‐stable aggregates as affected by fertilization were greater for 1–0.5 and 0.5–0.25 mm classes than for the other classes. Results indicate that OM stocks in different soil pools can be increased and the loose aggregation of these strongly eroded loess soils can be improved by organic or inorganic fertilization.  相似文献   

6.
Changes in land‐use and agricultural management affect soil organic C (SOC) storage and soil fertility. Grassland to cropland conversion is often accompanied by SOC losses. However, fertilization, crop rotation, and crop residue management can offset some SOC losses or even convert arable soils into C sinks. This paper presents the first assessment of changes in SOC stocks and crop yields in a 60‐year field trial, the Zurich Organic Fertilization Experiment A493 (ZOFE) in Switzerland. The experiment comprises 12 treatments with different organic, inorganic and combined fertilization regimes. Since conversion to arable land use in 1949, all treatments have lost SOC at annual rates of 0.10–0.25 t C ha?1, with estimated mean annual C inputs from organic fertilizers and aboveground and belowground plant residues of 0.6–2.4 t C ha?1. In all treatments, SOC losses are still in progress, indicating that a new equilibrium has not yet been reached. Crop yields have responded sensitively to advances in plant breeding and in fertilization. However, in ZOFE high yields can only be ensured when mineral fertilizer is applied at rates typical for modern agriculture, with yields of main crops (winter wheat, maize, potatoes, clover‐grass ley) decreasing by 25–50% when manure without additional mineral fertilizer is applied. ZOFE shows that land‐use change from non‐intensively managed grassland to cropland leads to soil C losses of 15–40%, even in rotations including legumes and intercrops, improved agricultural management and organic fertilizer application.  相似文献   

7.
Quantitative information about the amount and stability of organic carbon (OC) in different soil organic‐matter (OM) fractions and in specific organic compounds and compound‐classes is needed to improve our understanding of organic‐matter sequestration in soils. In the present paper, we summarize and integrate results performed on two different arable soils with continuous maize cropping (a) Stagnic Luvisol with maize cropping for 24 y, b) Luvic Phaeozem with maize cropping for 39 y) to identify (1) the storage of OC in different soil organic‐matter fractions, (2) the function of these fractions with respect to soil‐OC stabilization, (3) the importance and partitioning of fossil‐C deposits, and (4) the rates of soil‐OC stabilization as assessed by compound‐specific isotope analyses. The fractionation procedures included particle‐size fractionation, density fractionation, aggregate fractionation, acid hydrolysis, different oxidation procedures, isolation of extractable lipids and phospholipid fatty acids, pyrolysis, and the determination of black C. Stability of OC was determined by 13C and 14C analyses. The main inputs of OC were plant litter (both sites) and deposition of fossil C likely from coal combustion and lignite dust (only Phaeozem).  相似文献   

8.
Sandy cropland soils in NW Europe were found to contain unusually high organic‐carbon (OC) levels, and a link with their land‐use history has been suggested. This study's aim was to assess the discriminating power of physical and chemical fractionation procedures to yield information on soil‐organic‐matter (OM) stability for these soils. In relict‐ and cultivated‐heathland soils, much higher proportions of 6% NaOCl treatment–resistant but 10% HF–soluble OC (MOC) and N (32.2% and 29.9%) were measured compared to a set of “permanent"‐cropland soils without a history of heathland land use (11.9% and 8.5%). Also, the proportions of 6% NaOCl– and 10% HF treatment–resistant OC and N in the relict and cultivated heathlands (19.2% and 12.0%) were higher than in the permanent‐cropland soils (17.7% and 5.7%). Stepwise multiple linear‐regression yielded a significant relationship between the annual mineralization (g C [100 g OC]–1), soil OC (g C kg–1) content, and %MOC: Annual mineralization = 4.347 – 0.087 soil OC – 0.032 %MOC (R2 = 0.65). Combinations of incubation experiments for quantification of the labile soil OM pool with chemical fractionation may thus yield meaningful data for development of soil‐organic‐matter models with measurable pools, but their applicability will be limited to specific combinations of former land use with soil, climate, and current management.  相似文献   

9.
Land use change (LUC) is known to have a large impact on soil organic carbon (SOC) stocks. However, at a regional scale, our ability to explain SOC dynamics is limited due to the variability generated by inconsistent initial conditions between sample points, poor spatial information on previous land use/land management history and scarce SOC inventories. This study combines the resampling in 2003–2006 of an extensive soil survey in 1950–1960 with exhaustive historical data on LUC (1868–2006) to explain observed changes in the SOC stocks of temperate forest soils in the Belgian Ardennes. Results from resampling showed a significant loss of SOC between the two surveys, associated with a decrease in variability. The mean carbon content decreased from 40.4 to 34.5 g C kg?1 (10.6 to 9.6 kg C m?2), with a mean rate of C change (ΔSOC) of ?0.15 g C kg?1 year?1 (?0.023 kg C m?2 year?1). Soils with high SOC content tended to loose carbon while conversely soils with low SOC tended to gain carbon. Land use change history explained a significant part of past and current SOC stocks as well as ΔSOC during the last 50 years. We show that the use of spatially explicit historical data can help to quantitatively explain changes in SOC content at the regional scale.  相似文献   

10.
Abstract

We estimated the carbon (C) sequestration potential of organic matter application in Japanese arable soils at a country scale by applying the Rothamsted carbon (RothC) model at a 1-km resolution. After establishing the baseline soil organic carbon (SOC) content for 1990, a 25-year simulation was run for four management scenarios: A (minimum organic matter application), B (farmyard manure application), C (double cropping for paddy fields) and D (both B and C). The total SOC decreased during the simulation in all four scenarios because the C input in all four scenarios was lower than that required to maintain the baseline 1990 SOC level. Scenario A resulted in the greatest depletion, reflecting the effects of increased organic matter application in the other scenarios. The 25-year difference in SOC accumulation between scenario A and scenarios B, C and D was 32.3, 11.1 and 43.4?Mt?C, respectively. The annual SOC accumulation per unit area was similar to a previous estimate, and the 25-year averages were 0.30, 0.10 and 0.41?t?C?ha?1?year?1 for scenarios B, C and D, respectively. The system we developed in the present study, that is, linking the RothC model and soil spatial data, can be useful for estimating the potential C sequestration resulting from an increase in organic matter input to Japanese arable soils, although more feasible scenarios need to be developed to enable more realistic estimation.  相似文献   

11.
Abstract. Knowledge of the stocks and the potential range of soil organic carbon (SOC) in various land–soil combinations is an important precursor to policies aimed at linking, for example, management of SOC to greenhouse gas emission controls. To investigate the factors controlling the percentage of SOC (%SOC) of soils in England and Wales, we made a multiple regression analysis of data for the 2448 arable and ley-arable sites in the 1980 England and Wales National Soil Inventory (NSI). Clay content, average annual precipitation and depth of topsoil explained 25.5% of the variation in %SOC, when calcareous and peaty soils and those susceptible to flooding were excluded. Using 'robust' statistics, 'indicative SOC management ranges' were estimated for different physiotopes, that is, landscape units for which the environmental factors governing %SOC are similar, namely soil clay content and precipitation. These ranges describe the expected %SOC range for an arable soil in a given physiotope. They have potential to support approximate targets for the %SOC of arable soils and for estimating upper and lower limits for sequestered soil carbon in arable systems.  相似文献   

12.
Carbon fractions in soils apparently vary not only in space, but also over time. A lack of knowledge on the seasonal variability of labile carbon fractions under arable land hampers the reliability and comparability of soil organic carbon(SOC) surveys from different studies. Therefore, we studied the seasonal variability of two SOC fractions, particulate organic matter(POM) and dissolved organic carbon(DOC), under maize cropping: POM was determined as the SOC content in particle-size fractions, and DOC was measured as the water-extractable SOC(WESOC) of air-dried soil. Ammonium, nitrate, and water-extractable nitrogen were measured as potential regulating factors of WESOC formation because carbon and nitrogen cycles in soils are strongly connected. There was a significant annual variation of WESOC(coefficient of variation(CV) = 30%). Temporal variations of SOC in particle-size fractions were smaller than those of WESOC. The stocks of SOC in particle-size fractions decreased with decreasing particle sizes, exhibiting a CV of 20%for the coarse sand-size fraction(250–2 000 μm), of 9% for the fine sand-size fraction(50–250 μm), and of 5% for the silt-size fraction(20–50 μm). The WESOC and SOC in particle-size fractions both peaked in March and reached the minimum in May/June and August, respectively. These results indicate the importance of the time of soil sampling during the course of a year, especially when investigating WESOC.  相似文献   

13.
Land management in agricultural lands has important effects on soil organic carbon (SOC) dynamics. These effects are particularly relevant in the Mediterranean region, where soils are fragile and prone to erosion. Increasing interest of modelling to simulate SOC dynamics and the significance of soil erosion on SOC redistribution have been linked to the development of some recent models. In this study, the SPEROS‐C model was implemented in a 1.6‐ha cereal field for a 150‐year period covering 100 years of minimum tillage by animal traction, 35 years of conventional tillage followed by 15 years of reduced tillage by chisel to evaluate the effects of changes in land management on SOC stocks and lateral carbon fluxes in a Mediterranean agroecosystem. The spatial patterns of measured and simulated SOC stocks were in good agreement, and their spatial variability appeared to be closely linked to soil redistribution. Changes in the magnitude of lateral SOC fluxes differed between land management showing that during the conventional tillage period the carbon losses is slightly higher (0.06 g C m−2 yr−1) compared to the period of reduced till using chisel (0.04 g C m−2 yr−1). Although the results showed that the SPEROS‐C model is a potential tool to evaluate erosion induced carbon fluxes and assess the relative contribution of different land management on SOC stocks in Mediterranean agroecosystems, the model was not able to fully represent the observed SOC stocks. Further research (e.g. input parameters) and model development will be needed to achieve more accurate results. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

14.
There are few reliable data sets to inspire confidence in policymakers that soil organic carbon (SOC) can be measured on farms. We worked with farmers in the Tamar Valley region of southwest England to select sampling sites under similar conditions (soil type, aspect and slope) and management types. Topsoils (2–15 cm) were sampled in autumn 2015, and percentage soil organic matter (%SOM) was determined by loss on ignition and used to calculate %SOC. We also used the stability of macroaggregates in cold water (WSA) (‘soil slaking’) as a measure of ‘soil health’ and investigated its relationship with SOC in the clay‐rich soils. %SOM was significantly different between management types in the order woodland (11.1%) = permanent pasture (9.5%) > ley‐arable rotation (7.7%) = arable (7.3%). This related directly to SOC stocks that were larger in fields under permanent pasture and woodland compared with those under arable or ley‐arable rotation whether corrected for clay content (F = 8.500, p < .0001) or not (F = 8.516, p < .0001). WSA scores were strongly correlated with SOC content whether corrected for clay content (SOCadj R2 = .571, p < .0001) or not (SOCunadj R2 = 0.490, p = .002). Time since tillage controlled SOC stocks and WSA scores, accounting for 75.5% and 51.3% of the total variation, respectively. We conclude that (1) SOC can be reliably measured in farmed soils using accepted protocols and related to land management and (2) WSA scores can be rapidly measured in clay soils and related to SOC stocks and soil management.  相似文献   

15.
Changes in soil organic carbon (SOC) storage in agricultural land are an important part of the Land Use, Land-Use Change and Forestry component of national greenhouse gas emission inventories. Furthermore, as climate mitigation strategies and incentives for carbon farming are being developed, accurate estimates of SOC stocks are essential to verify any management-induced changes in SOC. Based on agricultural mineral soils in the Danish soil-monitoring network, we analysed management effects on SOC stocks using data from the two most recent surveys (2009 and 2019). Between 2009 and 2019, the average increase in SOC stock was 1.2 Mg C ha−1 for 0–50 cm despite a loss of 1.2 Mg C ha−1 from the topsoil (0–25 cm), stressing the importance of including deeper soil layers in soil-monitoring networks. Comparing all four national surveys (1986, 1997, 2009, 2019), the mean SOC stock of mineral soils in Denmark appears stable. The change in SOC stock between 2009 and 2019 was analysed in detail in relation to management practices as reported by farmers. We found that the effects of single management factors were difficult to isolate from co-varying factors including soil parameters and that the use of farm management data to explain changes in SOC stocks observed in soil-monitoring networks appears limited. Uncertainty in SOC stock estimates also arises from low sampling frequency and statistical challenges related to regression to the mean. However, repeated stock measurements at decadal intervals still represent a benchmark for the overall development in regional and national SOC storage, as affected by actual farm management.  相似文献   

16.
The development of successful agricultural policies in response to the Kyoto Protocol is aided by the identification of regions where the effects of soil organic carbon (SOC) sequestration measures can be maximized. We describe a modelling approach which incorporates a spatial analysis of the results from regional simulation under different management alternatives. SOC stock changes in Flemish cropland soils, simulated with the DNDC model, were previously fitted to a large data set of SOC measurements for the period 1990–2000. Using the results of this study, simulations with DNDC of SOC stock changes during the period 2006–2012 including the Kyoto commitment period were carried out at the community level for a business‐as‐usual (BAU) scenario and seven alternative agricultural management options for SOC sequestration. The baseline SOC stock decreased during that period by 0.15 t OC ha−1 year−1 compared with 0.48 t OC ha−1 year−1 in the 1990s. All alternative scenarios resulted in net SOC storage compared with the BAU scenario, but none of the individual scenarios were able to increase the average absolute SOC stock. Overall, spatial variability in SOC storage for the selected management options was strongly dependent on the current distribution of crops and associated management. The modelling approach used in this study provides a case study in regional scale modelling of SOC sequestration and is applicable to other regions in Europe with comparable intensive agriculture.  相似文献   

17.
Low soil organic carbon (SOC) levels in dry areas can affect soil functions and may thus indicate soil degradation. This study assesses the significance of SOC content in Mediterranean arable soils based on the analysis of a broad data set of 2613 soils sampled from Mediterranean grasslands and agricultural land. The distribution in values of SOC, pH, clay and carbonates was analysed according to different climatic areas (semi‐arid, Mediterranean temperate, Mediterranean continental and Atlantic) and with respect to six different land uses (grassland, cereal crops, olives and nuts, vineyards, fruit trees and vegetable gardens). The general trend was for low SOC in arable land and decreased with aridity. In wet areas (Atlantic and Mediterranean continental), acidic soils had a higher SOC content than did calcareous soils, whereas in the Mediterranean temperate area SOC had little relationship to soil pH. In low SOC arable soils, the SOC content was related to clay content. In calcareous arable soils of the Mediterranean temperate zone, SOC content was more closely related to carbonates than to clay. In contrast to the Atlantic area, Mediterranean grassland soils had much lower amounts of SOC than forest soils. Mediterranean calcareous and temperate acidic soils under grassland had SOC‐to‐clay ratios similar to or only slightly greater than that under a crop regime. In contrast, Mediterranean continental acidic soils under grassland had a much higher SOC‐to‐clay ratio than arable soils. This suggests a low resilience of the Mediterranean temperate and calcareous arable soils in terms of SOC recovery after the secession of ploughing, which may be a result of intensive use of these soils over many centuries. Consequently, we hypothesize that the Mediterranean calcareous soils have undergone significant changes that are not readily reversed after ploughing ceases. Such changes may be related to alterations in soil aggregation and porosity which, in turn, are associated with soil carbonate dynamics. Decarbonation processes (the depletion of active carbonates) may therefore be relevant to the reclamation of highly calcareous arable soils through fostering soil re‐aggregation. The article concludes by discussing the suitability of zero tillage, manuring or the introduction of woody species to increase SOC in calcareous arable soils that are highly depleted of organic matter.  相似文献   

18.
After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root‐cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra‐microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation‐resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF‐res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N/ha) from undisturbed soil and coarse fPOM N (kg N/ha), silt and clay N (kg N/ha) and its C:N ratio (R2 = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF‐res OC (% of soil organic carbon) and its C:N ratio (R2 = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio‐available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils.  相似文献   

19.
Permafrost degradation may cause strong feedbacks of arctic ecosystems to global warming, but this will depend on if, and to what extent, organic matter (OM) is protected against biodegradation by mechanisms other than freezing and anoxia. Here, we report on the amount, chemical composition and bioavailability of particulate (POM) and mineral‐associated OM (MOM) in permafrost soils of the East Siberian Arctic. The average total organic carbon (OC) stock across all soils was 24.0 ± 6.7 kg m?2 within 100 cm soil depth. Density fractionation (density cut‐off 1.6 g cm?3) revealed that 54 ± 16% of the total soil OC and 64 ± 18% of OC in subsoil horizons was bound to minerals. As well as sorption of OM to clay‐sized minerals (R2 = 0.80; P < 0.01), co‐precipitation of OM with hydrolyzable metals may also transfer carbon into the mineral‐bound fraction. Carbon:nitrogen ratios, stable carbon and nitrogen isotopes, 13C‐NMR and X‐ray photoelectron spectroscopy showed that OM is transformed in permafrost soils, which is a prerequisite for the formation of mineral‐organic associations. Mineral‐associated OM in deeper soil was enriched in 13C and 15N, and had narrow C:N and large alkyl C:(O‐/N‐alkyl C) ratios, indicating an advanced stage of decomposition. Despite being up to several thousands of years old, when incubated under favourable conditions (60% water‐holding capacity, 15°C, adequate nutrients, 90 days), only 1.5–5% of the mineral‐associated OC was released as CO2. In the topsoils, POM had the largest mineralization but was even less bioavailable than the MOM in subsoil horizons. Our results suggest that the formation of mineral‐organic associations acts as an important additional factor in the stabilization of OM in permafrost soils. Although the majority of MOM was not prone to decomposition under favourable conditions, mineral‐organic associations host a readily accessible carbon fraction, which may actively participate in ecosystem carbon exchange.  相似文献   

20.
Under semiarid climatic conditions, intensive tillage increases soil organic matter losses, reduces soil quality, and contributes to climate change due to increased CO2 emissions. There is a need for an agricultural management increasing soil organic matter. This paper presents the organic carbon (OC) and nitrogen (N) stocks, C:N ratio and stratification ratios (SRs) of these properties for olive groves soils under long‐term organic farming (OF), and conventional tillage (CT) in Los Pedroches valley, southern Spain. The results show that OF increased C and N stocks. The soil organic carbon (SOC) stock was 73·6 Mg ha−1 in OF and 54·4 Mg ha−1 in CT; and the total nitrogen (TN) stock was 7·1 Mg ha−1 and 5·8 Mg ha−1 for OF and CT, respectively. In the surface horizon (A: 0–16·9 cm in OF and Ap: 0–21·8 cm in CT) and Bw horizon (16·9–49·6 cm in OF and 21·8–56 cm in CT), SOC and TN concentrations and C:N ratios were higher in OF than in CT. Soil properties stratification in depth, expressed as a ratio, indicates the soil quality under different soil management systems. The SR of SOC ranged from 2·2 to 3·1 in OF and from 2·1 to 2·2 in CT. However, only SR2 (defined by Ap‐A/C) showed significant differences between CT and OF. The SR of TN showed similar trends to that of the SR of SOC. Organic farming contributes to a better soil quality and to increased carbon sequestration. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

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