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1.
Overgrazing has led to severe degradation and desertification of semi-arid grasslands in Northern China over the last decades. Despite the fact that vegetation is often heterogeneously distributed in semi-arid steppes, little attention has been drawn to the effect of grazing on the spatial distribution of soil properties. We determined the spatial pattern of soil organic carbon (SOC), total nitrogen (Ntot), total sulphur (Stot), bulk density (BD), pH, Ah thickness, and carbon isotope ratios (δ13C) at two continuously grazed (CG) and two ungrazed (UG79 = fenced and excluded from grazing in 1979) sites in Leymus chinensis and Stipa grandis dominated steppe ecosystems in Inner Mongolia, Northern China. Topsoils (0–4 cm) were sampled at each site using a large grid (120 m × 150 m) with 100 sampling points and a small plot (2 m × 2 m) with 40 points. Geostatistics were applied to elucidate the spatial distribution both at field (120 m × 150 m grid) and plant (2 m × 2 m plot) scale. Concentrations and stocks of SOC, Ntot, Stot were significantly lower and BD significantly higher at both CG sites. At the field scale, semivariograms of these parameters showed a heterogeneous distribution at UG79 sites and a more homogeneous distribution at CG sites, whereas nugget to sill ratios indicated a high small-scale variability. At the plant scale, semivariances of all investigated parameters were one order of magnitude higher at UG79 sites than at CG sites. The heterogeneous pattern of topsoil properties at UG79 sites can be attributed to a mosaic of vegetation patches separated by bare soil. Ranges of autocorrelation were almost congruent with spatial expansions of grass tussocks and shrubs at both steppe types. At CG sites, consumption of biomass by sheep and hoof action removed vegetation patches and led to a homogenization of chemical and physical soil properties. We propose that the spatial distribution of topsoil properties at the plant scale (<2 m) could be used as an indicator for degradation in semi-arid grasslands. Our results further show that the maintenance of heterogeneous vegetation and associated topsoil structures is essential for the accumulation of SOM in semi-arid grassland ecosystems.  相似文献   

2.
Spatial inaccessibility of soil organic carbon (SOC) for microbial decay within soil aggregates is an important stabilization mechanism. However, little is known about the stability of aggregates in semiarid grasslands and their sensitivity to intensive grazing. In this study, a combined approach using soil chemical and physical analytical methods was applied to investigate the effect of grazing and grazing exclusion on the amount and stability of soil aggregates and the associated physical protection of SOC. Topsoils from continuously grazed (CG) and ungrazed sites where grazing was excluded from 1979 onwards (UG79) were sampled for two steppe types in Inner Mongolia, northern China. All samples were analysed for basic soil properties and separated into free and aggregate‐occluded light fractions (fLF, oLF) and mineral‐associated fractions. Tensile strength of soil aggregates was measured by crushing tests. Undisturbed as well as artificially compacted samples, where aggregates were destroyed mechanically by compression, were incubated and the mineralization of SOC was measured. For undisturbed samples, the cumulative release of CO2‐C was greater for CG compared with UG79 for both steppe types. A considerably greater amount of oLF was found in UG79 than in CG soils, but the stabilities of 10–20‐mm aggregates were less for ungrazed sites. Compacted samples showed only a slightly larger carbon release with CG but a considerably enhanced mineralization with UG79. We assume that the continuous trampling of grazing animals together with a smaller input of organic matter leads to the formation of mechanically compacted stable ‘clods’, which do not provide an effective physical protection for SOC in the grazed plots. In UG79 sites, a greater input of organic matter acting as binding agents in combination with an exclusion of animal trampling enhances the formation of soil aggregates. Thus, grazing exclusion promotes the physical protection of SOC by increasing soil aggregation and is hence a management option to enhance the C sequestration potential of degraded steppe soils.  相似文献   

3.
Desertification is reversible and can often be prevented by adopting measures to control the causal processes. Desertification has generally decreased in most of the arid and semiarid areas of China during the last few decades because of the restoration of degraded vegetation and soil nutrients. However, little is known about the responses of soil nutrients in different particle‐size fractions to the restoration process and about the importance of this response to the restoration of bulk‐soil nutrients. In this study, we separated bulk‐soil samples in different sieve fractions: coarse‐fine sand (2·0–0·1 mm), very fine sand (0·10–0·05 mm) and silt + clay (<0·05 mm) fractions. Soil organic carbon (SOC), N, P and K contents stored in the silt + clay were greater than the contents of non‐protected nutrients in the coarser fractions. During the restoration of desertified land, the content and stability of bulk‐soil SOC, total N and P and available N, P and K increased with increasing nutrient contents in all fractions. Topsoil nutrients stored in coarse‐fine sand and very fine sand fractions were more sensitive than those stored in the silt + clay fraction to the fixation of mobile sandy lands and vegetation recovery. The changes of bulk‐soil nutrients and their stability were decided by the soil nutrients associated with all particle‐size fractions. Path analysis revealed that SOC and total nutrients in very fine sand and available nutrients in coarse‐fine sand were the key factors driving the soil recovery. These results will help us understand soil recovery mechanisms and evaluate the degree of recovery. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

4.
Quantifying the sensitivity of soil organic matter decomposition (SOM) to global warming is critical for predict future impacts of climate change on soil organic carbon stocks (SOC) and soil respiration, especially in semi‐arid regions such as north‐eastern Brazil, where SOC stocks are naturally small. In this study, the responses of the labile and recalcitrant carbon components and soil respiration dynamics were evaluated in three different soil types and land use systems (native vegetation, cropland and pasture) of the Brazilian semi‐arid region, when submitted to temperature increase. After 169 days of incubation, the results showed that an increase of 5°C generated an average increase in CO2 emission of 12.0%, but which could reach 28.1%. Overall, the labile carbon (LC) in areas of native vegetation showed greater sensitivity to temperature than in cropland areas. It was also observed that recalcitrant carbon (RC) was more sensitive to warming than LC. Our results indicate that Brazil's semi‐arid region presents a substantial vulnerability to global warming, and that the sensitivity of RC and of LC in areas of native vegetation to warming can enhance SOC losses, contributing to positive feedback on climate change, and compromising the productive systems of the region. However, further studies evaluating other types of soil and texture and management systems should be carried out to consolidate the results obtained and to improve the understanding about SOM decomposition in the Brazilian semi‐arid region.  相似文献   

5.
Overgrazing contributes to rangeland degradation altering plant community composition, erosion and biodiversity. Little unanimity in the literature exists on the effects of livestock grazing on soil carbon and biodiversity, in part, due to uncontrolled grazing pressure from native and feral animals. Paired paddock contrasts at three, long‐term (>8 years) study locations in the southern Australian rangelands were used to examine the effects of managing grazing intensity through the use of exclusion fencing and rotational grazing on soil organic carbon (SOC), soil nitrogen (TN), ground cover and biodiversity (flora and invertebrates). Grazing management had no effect on SOC or TN on grey soils (Vertisols), but for red soils (Lixisols), significantly higher levels of SOC were found for both the 0 to 5 and 5 to 10‐cm soil depths (0·3% and 0·27% respectively) and associated with increased TN. We found strong and consistent relationships among SOC and higher perennial (p < 0·001), higher litter (p < 0·05) cover and close proximity to trees (p < 0·05). Managing grazing intensity resulted in significantly higher perennial ground cover (p < 0·001) on Vertisols (8·9 to 11%) and Lixisols (12·5 to 15%) and higher plant diversity (both native and exotic) but negatively impacted invertebrate diversity, indicating trade‐offs between production and resources. We provide evidence that the effects of grazing management on SOC are mediated by ground cover and increased organic matter supply and/or reduced soil carbon redistribution (erosion), which indicates that the management of grazing intensity may provide a tool to avoid soil carbon loss in rangelands. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

6.
Soil organic carbon (SOC) plays an essential role in the sustainability of natural and agricultural systems. The identification of sensitive SOC fractions can be crucial for an understanding of SOC dynamics and stabilization. The objective of this study was to assess the effect of long‐term no‐tillage (NT) on SOC content and its distribution between particulate organic matter (POM) and mineral‐associated organic matter (Min) fractions in five different cereal production areas of Aragon (north‐east Spain). The study was conducted under on‐farm conditions where pairs of adjacent fields under NT and conventional tillage (CT) were compared. An undisturbed soil nearby under native vegetation (NAT) was included. The results indicate that SOC was significantly affected by tillage in the first 5 cm with the greatest concentrations found in NT (1.5–43% more than in CT). Below 40 cm, SOC under NT decreased (20–40%) to values similar or less than those under CT. However, the stratification ratio (SR) never reached the threshold value of 2. The POM‐C fraction, disproportionate to its small contribution to total SOC (10–30%), was greatly affected by soil management. The pronounced stratification in this fraction (SR>2 in NT) and its usefulness for differentiating the study sites in terms of response to NT make POM‐C a good indicator of changes in soil management under the study conditions. Results from this on‐farm study indicate that NT can be recommended as an alternative strategy to increase organic carbon at the soil surface in the cereal production areas of Aragon and in other analogous areas.  相似文献   

7.
Soil organic‐carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (Ntot) were measured before and after density fractionation in two light fractions and a mineral‐associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (14C) concentrations of mineral associated C were determined. Carbon stocks and mineral‐associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root‐biomass distribution, with 59% and 86% of the total root biomass at 0–5 cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral‐associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land‐use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high 14C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land‐use change effects on SOC are assessed.  相似文献   

8.
The continuous use of plowing for grain production has been the principal cause of soil degradation. This project was formulated on the hypothesis that the intensification of cropping systems by increasing biomass‐C input and its biodiversity under no‐till (NT) drives soil restoration of degraded agro‐ecosystem. The present study conducted at subtropical [Ponta Grossa (PG) site] and tropical regions [Lucas do Rio Verde, MT (LRV) site] in Brazil aimed to (i) assess the impact of the continuous plow‐based conventional tillage (CT) on soil organic carbon (SOC) stock vis‐à‐vis native vegetation (NV) as baseline; (ii) compare SOC balance among CT, NT cropping systems, and NV; and (iii) evaluate the redistribution of SOC stock in soil profile in relation to soil resilience. The continuous CT decreased the SOC stock by 0·58 and 0·67 Mg C ha−1 y−1 in the 0‐ to 20‐cm depth at the PG and LRV sites, respectively, and the rate of SOC sequestration was 0·59 for the PG site and ranged from 0·48 to 1·30 Mg C ha−1 y−1 for the LRV site. The fraction of C input by crop residues converted into SOC stock was ~14·2% at the PG site and ~20·5% at the LRV site. The SOC resilience index ranged from 0·29 to 0·79, and it increased with the increase in the C input among the NT systems and the SOC sequestration rates at the LRV site. These data support the hypothesis that NT cropping systems with high C input have a large potential to reverse the process of soil degradation and SOC decline. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

9.
选择岷江上游理县山地森林/干旱河谷交错带地区人工刺槐林、人工杨柳林、草地和锥花小檗灌丛这4种植被类型为研究对象,对4种植被下的土壤微生物量及呼吸熵对放牧干扰的响应进行了研究。以距牧道距离远近的不同设置了3种放牧干扰强度处理,分别对各植被类型3种放牧干扰强度的土壤进行了分析。结果表明,在各植被类型下,土壤有机碳(SOC)和土壤微生物量碳(MBC)含量随放牧干扰强度的增加而降低。各植被类型下表层土壤呼吸熵值(qCO2)值随放牧压力的增加而增大(除灌丛中度干扰外),增加幅度为15.14%~100.54%,说明放牧干扰使微生物体的周转率加快,对SOC的利用率降低,释放的CO2增多,土壤碳保存率降低。  相似文献   

10.
Enrichment of soil organic carbon (SOC) stocks through sequestration of atmospheric CO2 in agricultural soils is important because of its impacts on adaptation to and mitigation of climate change while also improving crop productivity and sustainability. In a long‐term fertility experiment carried out over 27 y under semiarid climatic condition, we evaluated the impact of crop‐residue C inputs through rainfed fingermillet (Eleusine coracana [L.] Gaertn.) cropping, fertilization, and manuring on crop yield sustainability and SOC sequestration in a Alfisol soil profile up to a depth of 1 m and also derived the critical value of C inputs for maintenance of SOC. Five treatments, viz., control, farmyard manure (FYM) 10 Mg ha–1, recommended dose of NPK (50 : 50 : 25 kg N, P2O5, K2O ha–1), FYM 10 Mg ha–1 + 50% recommended dose of NPK, and FYM 10 Mg ha–1 + 100% recommended dose of NPK imposed in a randomized block design replicated four times. Application of FYM alone or together with mineral fertilizer resulted in a higher C input and consequently built up a higher C stock. After 27 y, higher profile SOC stock (85.7 Mg ha–1), C build up (35.0%), and C sequestration (15.4 Mg C ha–1) was observed with the application of 10 Mg FYM ha–1 along with recommended dose of mineral fertilizer and these were positively correlated with cumulative C input and well reflected in sustainable yield index (SYI). For sustenance of SOC level (zero change due to cropping) a minimum quantity of 1.13 Mg C is required to be added per hectare per annum as inputs. While the control lost C, the application of mineral fertilizer served to maintain the priori C stock. Thus, the application of FYM increased the C stock, an effect which was even enhanced by additional amendment of mineral fertilizer. We conclude that organic amendments contribute to C sequestration counteracting climate change and at the same time improve soil fertility in the semiarid regions of India resulting in higher and more stable yields.  相似文献   

11.
Hedgerows have the potential to influence ecosystem function in livestock‐grazed pasture. Despite this, they are often ignored when quantifying farmland ecosystem service delivery. In this study, we assess the contribution of hedgerows to the ecosystem function of carbon (C) storage, with a particular emphasis on soil organic carbon (SOC). We measured SOC stock (kg C m?2), on an equivalent soil mass basis, at 0–0.15 m depth in pasture adjacent to 38 hedgerows (biotic) and 16 stone walls or fences (abiotic controls) across ten farms in the county of Conwy, Wales, UK. Pasture SOC stock (~7 kg C m?2) was similar adjacent to biotic and abiotic field boundaries, positively associated with soil moisture and negatively with soil bulk density (BD). For biotic boundaries, two further variables were significantly associated with SOC stock, distance from hedgerow (decrease in SOC stock) and slope orientation (upslope SOC stock greater than downslope). For pasture adjacent to hedgerows, a model combining the aforementioned variables (BD, soil moisture, distance from hedgerow, slope orientation) explained 78% of variation in SOC stock. This study demonstrates that whilst hedgerows do have subtle positive effects on SOC stock in adjacent pasture, SOC storage adjacent to field boundaries is influenced more by soil moisture content and BD than field boundary type.  相似文献   

12.
Vegetation in many arid and semi‐arid shrublands frequently occurs in patches with high plant cover (shrub patches) interspersed in a low‐cover herbaceous matrix (inter‐shrub areas). We hypothesized that (a) livestock grazing is an important determinant of such spatial patterns of vegetation, and (b) redistribution of soil resources associated with shrub patches helps in the recovery of vegetation in inter‐shrub areas. To test these hypotheses, we (a) used line transects to compare spatial variations in vegetation, soil microtopography, and soil physicochemical properties in grazed areas and areas protected from grazing since 1970, (b) added sediment and seeds to inter‐shrub areas, and (c) measured resource redistribution after a wildfire. Results were consistent with the hypotheses. They indicated greater spatial heterogeneity in vegetation, soil microtopography and soil physicochemical properties in grazed areas than in protected areas, and that addition of sediment and seeds or redistribution of soil resources from shrub patches after a wildfire enhanced re‐establishment of vegetation in degraded inter‐shrub areas. As a synthesis, a conceptual model of degradation and recovery processes in semi‐arid shrublands of Northern Patagonia is presented. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
Twelve soil‐preparation techniques used in afforestation were applied and 16 environmental variables were measured to test their effects on vegetation abundance, diversity, and cover in semiarid abandoned farmland located in southeastern Spain. The soil‐preparation techniques consisted of punctual treatments—making a hole and constructing a plantation bench with different types of machinery; linear treatments—making furrows along contour lines; and surface treatments—that prepare the soil in general all over the plantation surface. Variables measured were the surface and volume affected by the soil‐preparation treatments, distance from sample quadrats to the remnants of natural vegetation, surface runoff estimated by slope angle, and some edaphic characteristics such as gravel content, active lime, CaCO3, phosphorus, oxidizable organic matter, total soil nitrogen, pH, potassium, salinity, and clay, silt and sand content. The multivariate contrasts obtained demonstrated the existence of a significant relationship between species, samples and environmental variables. Both the distance of the quadrats from the remnants of natural vegetation and the volume of soil removed in different treatments were very weighty variables in the explanation of the variance of data, negatively affecting overall vegetation development. The accumulation or loss of surface runoff and the CaCO3 content in soil were also important variables to explain abundance and diversity data. However, not all the species responded to the variables analysed in the same way. Identification of non‐uniform spatial patterns that significantly affect vegetation suggests that afforestation policies and conservation and management strategies in these semiarid abandoned lands should always take into consideration the quality of the whole landscape. Voluminous soil treatments for afforestation should be avoided. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

14.
Detailed information on the profile distributions of agronomically important soil properties in the planting season can be used as criteria to select the best soil tillage practices. Soil cores (0–60 cm) were collected in May, 2012 (before soybean planting), from soil transects on a 30‐yr tillage experiment, including no‐tillage (NT), ridge tillage (RT) and mouldboard plough (MP) on a Brookston clay loam soil (mesic Typic Argiaquoll). Soil cores were taken every 19 cm across three corn rows and these were used to investigate the lateral and vertical profile characteristics of soil organic carbon (SOC), pH, electrical conductivity (EC), soil volumetric water content (SWC), bulk density (BD), and penetration resistance (PR). Compared to NT and MP, the RT system resulted in greater spatial heterogeneity of soil properties across the transect. Average SOC concentrations in the top 10 cm layer were significantly greater in RT than in NT and MP (P = 0.05). NT soil contained between 0.8 and 2.5% (vol/vol) more water in the top 0–30 cm than RT and MP, respectively. MP soil had lower PR and BD in the plough layer compared to NT and RT soils, with both soil properties increasing sharply with depth in MP. The RT had lower PR relative to NT in the upper 35 cm of soil on the crop rows. Overall, RT was a superior conservation tillage option than NT in this clay loam soil; however, MP had the most favourable soil conditions in upper soil layers for early crop development across all treatments.  相似文献   

15.
Grazing in outlying fields has a long history and is important in local communities worldwide. During the last few decades, grazing pressure has both decreased and increased in alpine ecosystems, but little is known about the effects on soil carbon storage. As part of a sheep grazing experiment with three sheep stocking rates of no sheep (control), 25 and 80 sheep km?2, we tested effects of grazing on soil organic carbon storage, the form of soil organic matter (SOM) and its lability (potential carbon mineralization) in organic horizons of low‐alpine grasslands in southern Norway. After 7 years of grazing, the greatest sheep density reduced soil organic carbon concentration (% SOC) and carbon stocks at equivalent soil mass as compared with the control. In contrast, the low stocking rate caused no change or a slight increase. The form of SOM, expressed as ratios of particulate organic carbon to soil organic carbon, was only slightly affected by grazing, with a small decrease and moderate increase at the greater and smaller stocking rate, respectively. The lability of SOM was not affected by grazing directly, but was significantly related to the mineral content of the O‐horizons. In general, there were large differences between the plant communities of snowbed and willow‐shrub for several soil attributes. We concluded that 7 years of grazing had limited impacts on stocks, form and lability of SOM.  相似文献   

16.
Soil organic carbon (SOC) sequestration and soil redistribution are linked to soil properties, land use, farming system and climate. In a global‐change context, landscape and climate changes are expected and will most probably have impacts on changes in the soil. Soil change was simulated from 2010 to 2100 in an 86‐ha hedgerow landscape under different scenarios of landscape and climate changes. These scenarios combined contrasting land uses, hedgerow networks and climates. Two models were combined to evaluate the impact of these scenarios on soils: LandSoil, a soil redistribution model, and a SOC model based on RothC‐26.3. A soil thickness of 105 cm was considered. The results indicate that the main factor influencing soil degradation was land‐use change: when compared with the baseline business‐as‐usual landscape, the landscape with the most intensive agricultural systems had the greatest soil erosion (+0.26 t ha?1 year?1) and reduced mean SOC stocks (?17 t ha?1 after 90 years). The second significant factor was climate change, followed by hedgerow network density. Sensitivity to climate change differed between landscapes, and the most sensitive were those with continuous winter wheat. The results indicate that a hedgerow landscape is well adapted to protect soil (regarding carbon storage and soil erosion) in a context of climate change. However, this type of landscape is highly sensitive to cropping intensification and should be protected.  相似文献   

17.
Vegetation of the Tihany Peninsula in Hungary has undergone serious changes caused by heavy trampling and overgrazing during the last century. In Tihany, the vegetation started to change towards anthropogenic associations. After grazing ceased, vegetation started to rearrange. There were no significant changes in species composition on trampled areas (on the ridge of the hill) between the examinations in 1994 and 2002. As a result of ceased grazing, the proportion of natural species was high in the grasslands of the S Nyereg Hill in 1994, while during the 2002 investigations, forest and scrub expanded at a rate that did not leave space for grassland on shallow soils and slope steppe (closing dry grasslands on hilly areas). The coverage of Elymus repens decreased, while Festuca species increased. The possible effects of plant‐cover change on soil loss were examined with the USLE (Universal Soil Loss Equation) model on the sloping areas. Eliminating harmful effects can decrease soil degradation, but the original state of soils can be restored only after a long period (hundreds of years). Further degradation of soils assists the expansion of drought‐resistant species and weeds in the associations. In 2007, soil‐thickness measurements and on‐site examinations were carried out to check the results of the former surveys and modeling.  相似文献   

18.
In this paper, we studied the spatial variability of soil organic C (SOC), inorganic N (SIN) and extractable P (Pextr) in a grazed Mediterranean‐type vegetation formation. Sampling was conducted from a gently sloping area in northern Greece.. The grazing pressure was evenly distributed over the experimental area with the exception of an overgrazed passage zone 200–300 m from steeper foothills. Soil samples, from the upper 10 cm, were collected every 10 m along four replicate lines (400 m length with a distance of 10 m between lines). Sampling took place twice (October and February). Data were analysed by geostatistical tools, and spherical models were significantly fitted to the semivariograms. SOC in both samplings and SIN in the first one displayed moderate spatial dependence which indicates the non‐random distribution of their concentration. On the contrary, Pextr and SIN in winter exhibited weak spatial dependence, whereas Pextr in autumn showed spatial independence. For the parameters exhibiting spatial pattern, two scales of dependence were revealed: a fine scale within distances shorter than 10 m and a coarse scale varying between 80 and 130 m. The coarse distribution of SOC, SIN and Pextr invoked interplay among more predictable (composition of vegetation) and unpredictable (leaching, runoff) extrinsic factors, occurring at the landscape level. Specifically, SOC as a storage agent exhibited uniform spatial pattern in both samplings. By contrast, SIN by being susceptible to leaching exhibited time‐specific dependence, whereas Pextr which was affected by surface runoff displayed limited or even spatial independence. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

19.
The aim of this study was to determine the effect of land‐use and forest cover depletion on the distribution of soil organic carbon (SOC) within particle‐size fractions in a volcanic soil. Emphasis was given to the thermal properties of soils. Six representative sites in Mexico were selected in an area dominated by Andosols: a grassland site, four forested sites with different levels of degradation and an agricultural site. Soils were fractionated using ultrasonic energy until complete dispersion was achieved. The particle‐size fractions were coarse sand, fine sand, silt, clay and particulate organic matter from the coarse sand sized fraction (POM‐CS) and fine sand (POM‐FS). Soil organic carbon decreased by 70% after forest conversion to cropland and long‐term cultivation; forest cover loss resulted in a decrease in SOC of up to 60%. The grassland soil contained 45% more SOC than the cropland one. Soil organic carbon was mainly associated with the silt‐size fraction; the most sensitive fractions to land‐use change and forest cover depletion were POM followed by SOC associated with the silt and clay‐sized fractions. Particulate organic matter can be used as an early indicator of SOC loss. The C lost from the clay and silt‐sized fractions was thermally labile; therefore, the SOC stored in the more degraded forest soils was more recalcitrant (thermally resistant). Only the transformation of forest to agricultural land produced a similar loss of thermally stable C associated with the silt‐sized fraction.  相似文献   

20.
Soil organic matter (SOM) is considered an important indicator of soil quality, which can be impacted by crop production practices such as tillage. In this study, two long‐term tillage regimes (conventional tillage [CT] and no tillage [NT], conducted for 36 years) were compared in continuous sorghum production in a sub‐tropical environment in southeast Texas. The positive effects of long‐term NT practice were more conspicuous at the soil surface compared with the deeper soil profiles. The SOC was greater (1.5 t C ha?1 greater) in the NT system compared with the CT system. Results from an incubation study indicate that the rate of C‐min at 0–5 cm soil depth was significantly greater (164 μg of CO2–C g?1 of soil greater) in NT than that of CT, but this trend was reversed at 10–20 cm depth wherein the C‐min rates were 106 μg of CO2–C g?1 of soil greater in CT compared with NT, which is likely because of soil disturbance during the study. Soil cumulative CO2‐C emissions were greater in the CT system (7.28 g m?2) than in the NT system (5.19 g m?2), which is primarily attributed to high soil temperature conditions in the CT system. Sorghum grain yield however was not influenced by the differences in SOC content in this long‐term experiment. Overall, the present study found that long‐term conservation tillage improved SOC stock and reduced carbon loss, thus had a positive impact on soil health and sustainability.  相似文献   

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