Soil Organic Carbon Accumulation in Abandoned Croplands on the Loess Plateau

The Grain to Green Program in China which began in 1999 led to the conversion of 0.64 million ha of cropland to grassland on steep sloping landscapes. However, the pattern of natural vegetation succession following cropland has not been well represented in previous regional syntheses of land use change effects on soil organic carbon (SOC). A chronosequence study focusing on the vegetation succession and soil carbon stocks was conducted in the center of the Loess Plateau. The chronosequence included fields of 0, 2, 5, 8, 9, 10, 12, 15 and 25 years of self‐restoration after cropland abandonment, as well as a natural grassland reference. Plant coverage, species richness and plant biomass increased significantly with time of cropland abandonment. Over time, the species composition more nearly resembled a natural grasslands community. Cropland abandonment replenished SOC stocks by 3.6 kg C m⁻² during the 25‐year self‐restoration, but the SOC accumulation was restricted to the upper soil profiles (0–60 cm). SOC accumulation rate was 88 g C m⁻² y⁻¹ in 0–30 cm and 55 g C m⁻² y⁻¹ in 30–60 cm soil depth, respectively. These carbon stocks were still significantly lower than those found in the natural grassland soil. Our results suggest that the recovery of plant communities and SOC stocks appears to be slow in this semiarid environment without revegetation effort along with appropriate field management, although the post‐agricultural soils have a high potential for carbon sequestration. Copyright © 2016 John Wiley & Sons, Ltd.     

Variations of Soil Organic Carbon Following Land Use Change on Deep‐Loess Hillsopes in China

Land use change is a key factor driving changes in soil organic carbon (SOC) around the world. However, the changes in SOC following land use changes have not been fully elucidated, especially for deep soils (>100 cm). Thus, we investigated the variations of SOC under different land uses (cropland, jujube orchard, 7‐year‐old grassland and 30‐year‐old grassland) on hillslopes in the Yuanzegou watershed of the Loess Plateau in China based on soil datasets related to soils within the 0–100 cm. Furthermore, we quantified the contribution of deep‐layer SOC (200–1,800 cm) to that of whole soil profiles based on soil datasets within the 0–1,800 cm. The results showed that in shallow profiles (0–100 cm), land uses significantly (p  < 0·05) influenced the distribution of SOC contents and stocks in surface layer (0–20 cm) but not subsurface layers (20–100 cm). Pearson correlation analysis indicated that soil texture fractions and total N were significantly (p  < 0·05 or 0·01) correlated with SOC content, which may have masked effects of land use change on SOC. In deep profiles (0–1,800 cm), SOC stock generally decreased with soil depth. But deep soils showed high SOC sequestration capacity. The SOC accumulated in the 100–1,800 m equalled 90·6%, 91·6%, 87·5% and 88·6% of amounts in the top 100 cm under cropland, 7‐year‐old grassland, 30‐year‐old grassland and jujube orchard, respectively. The results provide insights into SOC dynamics following land use changes and stressed the importance of deep‐layer SOC in estimating SOC inventory in deep loess soils. Copyright © 2017 John Wiley & Sons, Ltd.     

Changes in soil carbon and crop yield over 60 years in the Zurich Organic Fertilization Experiment,following land‐use change from grassland to cropland

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.     

Land‐use change in subalpine grassland soils: Effect on particulate organic carbon fractions and aggregation

Abandonment of mountain grassland often changes vegetation composition and litter quantity and quality, but related effects on labile soil organic matter (SOM) are largely unknown. The aim of this study was to investigate the impacts of grassland management and abandonment on soil carbon distribution in light (< 1.6 g cm^–3) particulate organic matter (POM) and aggregation along a gradient of management intensity including hay meadows, pastures, and abandoned grasslands. The reduction of management intensity is an interregional phenomenon throughout the European Alps. We therefore selected sites from two typical climate regions, namely at Stubai Valley, Austria (MAT: 3°C, MAP: 1097 mm) and Matsch Valley, Italy (MAT: 6.6°C, MAP: 527 mm), to evaluate effects of land‐use change in relation to climate. Free water‐floatable and free POM (wPOM, fPOM), and an occluded POM fraction (oPOM), were isolated from three water‐stable aggregate size classes (2–6.3 mm, 0.25–2 mm, < 0.25 mm) using density fractionation. Aggregate mean weight diameter slightly decreased with decreasing management intensity. In contrast to absolute POM‐C, fPOM‐C increased in aggregates at both sites with abandonment. Because the oPOM‐C was less affected by abandonment, the ratio of oPOM‐C : fPOM‐C shifted from > 1 to < 1 from meadow to abandoned grassland in aggregates at both sites and thus independent of climate. This suggests that in differently managed mountain grasslands free and occluded POM are functionally different SOM fractions. In bulk soil, the oPOM‐C : fPOM‐C ratio is better suited as an indicator for the response of SOM to management reduction in subalpine grasslands than the total soil C, absolute or relative POM‐C content.     

Variation in organic‐carbon concentration and bulk density in Flemish grassland soils

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.     

Grassland conversion along a climate gradient in northwest China: Implications for soil carbon and nutrients

Soil organic carbon (SOC) stocks and nutrient availability are key indicators of soil quality, and both can be influenced by land-use change. However, it is still unclear whether the impact of land-use change on SOC and nutrient stocks differs between ecoregions. Grasslands near the northeast border of the Qinghai-Tibetan Plateau (QTP) occur across several ecoregions that have recently been subjected to substantial land-use change. Based on long-term land-use history, we conducted a field investigation comparing soil C and nutrient stocks between natural grassland (NGL) and three types of converted grassland (agricultural grassland, AGL; farmland, FL; and abandoned farmland, AFL) in three ecoregions along a climate gradient: alpine meadow, temperate steppe and temperate desert. Compared with NGL, soil C stocks in converted grasslands were 22%–30% lower in the alpine meadow, but 60–82% higher in the temperate steppe and 6%–76% higher in the temperate desert. Converted grasslands also contained higher stocks of available nitrogen and phosphorus than NGL in the temperate steppe and desert. Soils (0–40 cm) in NGL contained 14.8 ± 0.1 kg C m⁻² in alpine meadow, 6.7 ± 0.6 kg C m⁻² in temperate steppe and 1.7 ± 0.3 kg C m⁻² in temperate desert. Together, our results indicate that the responses of soil C and nutrients to grassland conversion differed between ecoregions. Thus, to optimize soil C sequestration rates and overall soil quality, we suggest that land-use policies in this area should take into account local environmental conditions.     

Conversion of cropland into grassland: Implications for soil organic‐carbon stocks in two soils with different texture

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 (N_tot) 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.     

Carbon sequestration in secondary pasture soils: a chronosequence study in the South African Highveld

Soil restoration is a means of combating desertification in semi‐arid and arid parts of the world. There, vast areas of the cropped soil degrade, particularly because of the loss of organic matter. One approach to reverse this loss is the conversion of cropland into permanent grassland for use as pasture. This study was designed to evaluate how fast and to what degree degraded cropland may re‐sequester soil organic carbon (SOC) when converted into permanent secondary pasture. Topsoil samples (0–5, 5–10 and 10–20 cm) were taken from chronosequences of secondary pastures (1 to 31 years old) at three agro‐ecosystems in the semi‐arid Highveld of South Africa. Long‐term croplands and primary grassland used as pastures served as the controls. In bulk soil samples (<2 mm) and their clay (<2 µm), silt (2–20 µm), fine sand (20–250 µm) and coarse sand (250–2000 µm) fractions, the contents of carbon (C) and nitrogen were determined. In all three agro‐ecosystems, using a mono‐exponential model, the SOC stocks increased exponentially until a maximum was reached 10–95 years after land conversion. This gain in SOC was clearly pronounced for the top 0–5 cm of soil, but hardly detectable at 10–20‐cm depth. The sand fractions recovered organic C more rapidly but less completely than did the finer size separates. Overall, between 9.0 and 15.3 t of SOC were sequestered in the 0–20 cm of surface soil by this land conversion. Thus, the SOC recovery in the secondary pastures resulted in SOC stocks that were 29.6–93.9% greater than those in the arable land. Yet, in no agro‐ecosystem, at any soil depth, nor in any soil fraction, did the measured SOC content reach that of the primary grassland. In part this can be attributed to a slightly finer texture of the primary grassland that had not lost silt through wind erosion or had never been used as arable land because of slightly elevated clay contents. Overall it appears, however, that previous losses of SOM cannot easily be rectified, suggesting that the native primary grassland soils are only partially resilient to land‐use change.     

Distribution and controlling factors of soil organic carbon storage in the northeast Tibetan shrublands

Purpose

Although large amounts of soil organic carbon (SOC) stored in the shrublands, information about SOC storage was little on the Tibetan Plateau. This study aims to evaluate the spatial patterns and storage of SOC in the shrublands and the relationships of climatic variables and soil pH on the Tibetan Plateau.

Materials and methods

We used 177 profiles of soil samples obtained from 59 shrubland sites on the northeast Tibetan Plateau from 2011 to 2013. Ordinary least squares regressions, curve estimation, and multiple linear regressions were used to evaluate controlling factors on SOC stock. Kriging interpolation was used to upscale sit-level measurements to the whole study area.

Results and discussion

We found that SOC storage in the northeast Tibetan shrublands was 1.36 Pg C in the top 1 m with an average SOC stock of 12.38 kg m^?2. SOC stock decreased from east to west and south to north but generally increased significantly with the mean annual temperature (MAT) and the mean annual precipitation (MAP), and tended to decrease with soil pH. Although similar relationships were also observed in alpine shrublands, the trends among SOC stock, MAP, and MAT were not observed in desert shrublands. Our results indicate that a reduction in soil pH accelerates the C sequestration potential. Furthermore, global warming contributed to C sequestration in alpine shrublands, specifically, SOC stock increased 8.44 kg m^?2 with an increased unit of MAT in alpine shrublands just considering temperature effects. Meanwhile, the C sequestration was different among different regions due to the uneven increases in precipitation. However, in desert shrublands, MAP and MAT did not significantly affect SOC stock.

Conclusions

The results indicate that though a reduction in soil pH could contribute to C sequestration, MAT and MAP have different effects on SOC stock in different Tibetan Plateau shrublands. Increased MAT and MAP were 0.05 °C and 1.67 mm every year on the Tibetan Plateau, which will increase C sequestration in alpine shrublands, but might have limited impacts on desert shrublands, which help us comprehend soil C cycling in the global climate change scenario.

     

Effect of land‐use on soil nutrients and microbial biomass of an alpine region on the northeastern Tibetan plateau,China

Land‐use patterns affect the quantity and quality of soil nutrients as well as microbial biomass and respiration in soil. However, few studies have been done to assess the influence of land‐use on soil and microbial characteristics of the alpine region on the northeastern Tibetan plateau. In order to understand the effect of land‐use management, we examined the chemical properties and microbial biomass of soils under three land‐use types including natural grassland, crop‐field (50 + y of biennial cropping and fallow) and abandoned old‐field (10 y) in the area. The results showed that the losses of soil organic carbon (SOC) and total nitrogen (TN) were about 45 and 43 per cent, respectively, due to cultivation for more than 50 y comparing with natural grassland. Because of the abandonment of cultivation for about a decade, SOC and TN were increased by 27 and 23 per cent, respectively, in comparison with the crop field. Microbial carbon (ranging from 357·5 to 761·6 mg kg⁻¹ soil) in the old‐field was intermediate between the crop field and grassland. Microbial nitrogen (ranging from 29·9 to 106·7 mg kg⁻¹ soil) and respiration (ranging from 60·4 to 96·4 mg CO₂‐C g⁻¹ C_mic d⁻¹) were not significantly lower in the old‐field than those in the grassland. Thus it could be concluded that cultivation decreased the organic matter and microbial biomass in soils, while the adoption of abandonment has achieved some targets of grassland restoration in the alpine region of Gansu Province on the northeastern Tibetan plateau. Copyright © 2010 John Wiley & Sons, Ltd.     

CHANGES IN SOIL ORGANIC CARBON UNDER EUCALYPTUS PLANTATIONS IN BRAZIL: A COMPARATIVE ANALYSIS

Proper assessment of environmental quality or degradation requires knowledge of how terrestrial C pools respond to land use change. Forest plantations offer a considerable potential to sequester C in aboveground biomass. However, their impact on initial levels of soil organic carbon (SOC) varies from strong losses to gains, possibly affecting C balances in afforestation or reforestation initiatives. We compiled paired‐plot studies on how SOC stocks under native vegetation change after planting fast‐growth Eucalyptus species in Brazil, where these plantations are becoming increasingly important. SOC changes for the 0–20 and 0–40 cm depths varied between −25 and 42 Mg ha⁻¹, following a normal distribution centered near zero. After replacing native vegetation by Eucalyptus plantations, mean SOC changes were −1·5 and 0·3 Mg ha⁻¹ for the 0–20 and 0–40 cm depths, respectively. These are very low figures in comparison to C stocks usually sequestered in aboveground biomass and were statistically nonsignificant as demonstrated by a t‐test at p < 0·05. Similar low, nonsignificant SOC changes were estimated after data were stratified into first or second rotation cycles, soil texture and biome (savanna, rainforest or grassland). Although strong SOC losses or gains effectively occurred in some cases, their underpinning causes could not be generally identified in the present work and must be ascribed in a case basis, considering the full set of environmental and management conditions. We conclude that Eucalyptus spp. plantations in average have no net effect on SOC stocks in Brazil. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of land use and land cover change on environmental degradation in lake Qinghai watershed,northeast Qinghai‐Tibet Plateau

Lake Qinghai, the largest saline lake in China, covers 4234 km² (2007) with a catchment area of 29 660 km² on the northeastern margin of the Qinghai‐Tibet Plateau. The ecosystem of the lake is extremely vulnerable and sensitive to global climate change and human interference. However, little information is available on land use/cover change (LUCC) in Lake Qinghai watershed. Using a geographical information system (GIS) and remote sensing (RS), this study analysed land use and land cover change pattern in Lake Qinghai watershed between 1977 and 2004 and discussed major environmental issues in this area. LUCC analysis indicated that grassland (63 per cent) and water body (18 per cent) dominated in the watershed and the magnitude of the land use and land cover change was generally low; the percentage of the change of various land types relative to the total area was less than 1 per cent. From 1977 to 2004, cropland, sandy land, bare rock, salinized land, swampland and built‐up areas increased by 0·43, 0·35, 0·24, 0·06, 0·03 and 0·03 per cent of the total area, respectively; in contrast, water body, grassland and woodland decreased by 0·99, 0·22 and 0·05 per cent, respectively. Moreover, the area of LUCC tended to expand from places around the lake to the upper reaches of the watershed during the last three decades. The LUCC transition pattern was: woodland converted to grassland, grassland converted to cropland and water body converted to sandy land. Lake level decline and grassland degradation are major ecological and environmental problems in Lake Qinghai watershed. The level and area of the lake decreased at the rate of 6·7 cm a⁻¹ and 6·4 km² a⁻¹, respectively, between 1959 and 2007, resulting in sandy land expansion and water quality deterioration. Lake level decline and area shrinkage was mainly attributed to climate change, but grassland degradation was mainly resulted from anthropogenic activities (increasing population, overgrazing and policy). Copyright © 2008 John Wiley & Sons, Ltd.     

Soil carbon increases with long-term cattle stocking in northern temperate grasslands

Grassland management aimed at enhancing carbon (C) in soil is an important tool in mitigation of rising atmospheric CO₂, yet little is known of how grassland soil C changes with livestock stocking rate (SR). We relate soil organic and inorganic C mass (t ha⁻¹ to 60 cm depth) with cattle stocking over periods of 7–27 year for 32 paddocks distributed across nine community pastures in the mixed-grass prairie of Saskatchewan, Canada. Initial analysis comparing Akaike information criterion models showed that cattle SR explained a greater proportion of variance in soil C, particularly soil organic C, than rainfall. Soil organic C mass increased with cattle SR (R² = .293; p = .001), even when the latter was normalized to account for differences in vegetation composition and growing conditions among pastures. Normalized SR varied from 0.49 to 2.30 times recommended levels, over which SOC increased from 24.7 to 57.4 t ha⁻¹. Increases in soil organic C under greater stocking coincided with increased abundance of introduced vegetation, particularly the rhizomatous grass Poa pratensis. Inorganic soil C accounted for 34.6% of total soil C, being particularly large below 30 cm soil depth, but did not vary with stocking rate. These findings indicate that both organic and inorganic C are important pools of C in northern temperate grassland soils, with soil organic C positively associated with long-term cattle SR. Further studies are recommended to understand more fully the mechanisms regulating grazing impacts on soil C mass in northern temperate grasslands.     

Soil organic carbon in some land uses of Costa Rica

The native vegetation in the Tropics is increasingly replaced by crops, pastures, tree plantations, or settlements with contradictory effects on soil organic carbon (SOC). Therefore, the general objective was to estimate the SOC stock depth distribution to 100-cm depth in soils of Costa Rica and to assess their theoretical carbon (C) sink capacity by different management practices. A study was established in three ecoregions of Costa Rica: the Isthmian-Atlantic Moist Forest (AM), the Pacific Dry Forest (PD), and the Montane Forest (MO) ecoregions. Within each ecoregion, three agricultural land uses and a mature forest were sampled to 100-cm depth. The SOC stock in 0–100 cm depth was 114–150 Mg C ha^?1 for AM, 76–165 Mg C ha^?1 for PD, and 166–246 Mg C ha^?1 for MO. Land use had only weak effects on SOC concentrations and stocks except at PD where both were lower for soils under mango (Mangifera indica) and pasture. This may indicate soil degradation which was also supported by data on SOC stratification. However, it was generally unclear whether differences among land uses within each ecoregion already existed particularly at deeper depths before land-use change, and whether the sampling approach was sufficient to investigate them. Nevertheless, about 26–71% of Costa Rica's total C emissions may be offset by SOC sequestration in agricultural and forest soils. However, ecoregion-specific practices must be implemented to realize this potential.     

Do tillage and conversion of grassland to cropland always deplete soil organic carbon?

ABSTRACT

Conversion of grassland to cropland is widely reported to deplete soil organic carbon (SOC) largely due to tillage effects on the decomposition of SOC. However, most studies report on long-term changes in SOC following the conversion and little is known about the changes in the short term. Net ecosystem carbon budget (NECB) measures the difference between total C input (i.e., manure, above- and below-ground plant residues) and C loss through heterotrophic respiration (RH). However, most studies that report temporal SOC do not report other components of the NECB like RH, total C inputs and often do not include the cumulative annualized change of these components. This review evaluated the change in C input, RH, NECB and SOC after conversion of permanent/continuous grassland to cropland within 5 years after the conversion. We also reviewed and compared no-tillage and conventional tillage on SOC storage and accumulation. Total C input was higher in grassland than cropland largely due to high root biomass, as opposed to aboveground residue, and therefore grassland tended to have higher NECB. Despite higher NECB in grassland, the SOC stocks in cropland (cornfield) converted from grassland were greater than that in continuous grassland within first 2–3 years of conversion. The combination of manure C addition and tillage in cropland showed potential to maintain NECB and increase SOC. Within the continuous grassland C addition alone increased NECB but did not result in a corresponding increase in SOC. Residue retention and manure addition are recognized as good practices for increasing SOC, this study however, shows that combining them with occasional tillage, especially in managed grasslands, could increase the rate of SOC storage in soils.     

Grassland soil organic carbon and the effects of irrigated cropping in Alberta,Canada

High heterogeneity in the spatial distribution of soil organic carbon (SOC) in grasslands causes uncertainty in estimating its content and storage. In this study, we investigated the spatial distribution of SOC content and storage in the prairies of southern Alberta, Canada, and how it is affected by land use such as irrigated cropping and other environmental conditions such as cattle grazing, slope landscape position and dominant plant species. The mean SOC content was determined to be 11.5 g kg^–1 (range: 8.9 to 22.4 g kg^–1) in the 0–10 cm layer and 6.8 g kg^–1 (range: 4.0 to 13.3 g kg^–1) in the 10–30 cm layer; mean SOC storage was 1.59 kg C m^–2 (range: 1.23 to 2.78 kg C m^–2) in the 0–10 cm layer and 2.07 kg C m^–2 (range: 1.21 to 3.62 kg C m^–2) in the 10–30 cm layer. The SOC content was significantly affected by slope position in both the 0–10 and 10–30 cm layers, in the following order: bottom >middle > top position. Moreover, SOC storage was higher in sites dominated by shrubs than graminoid/forb communities. Thus, SOC content and storage had distinctly clustered spatial patterns throughout the study area and were significant differences between the 0–10 and 10–30 cm soil layers. Prior land-use change from arid grassland to irrigated cropland increased SOC content and storage in bulk soils.     

Characterization of organic matter fractions in the top layer of soils under different land uses in Central‐Eastern Europe

The objective of this study was to investigate differences in organic matter fractions, such as dissolved organic carbon and humic substances, in soils under different land uses. Soil samples were collected from the upper layer of arable lands and grasslands. Humic substances (HS) were chemically fractionated into fulvic acids (FA), humic acids (HA) and humins (HUM), and based on the separated fractions, the humification index (HI) and the degree of HS transformation (DT) were calculated. Dissolved organic carbon (DOC) was determined by cold (CWE) and hot water (HWE) extractions. Regardless of land use, the results indicated significant differences in soil organic carbon (SOC) and HS composition, with HA and HUM as the dominant fractions. Total SOC was higher in grassland (median = 17.51 g kg^?1) than arable soils (median = 9.98 g kg^?1); the HI and DT indices did not differ significantly between land uses (HI = 0.3–10.3 and DT = 0.2–6.2 for grasslands, p > 0.05; HI = 0.3–3.9 and DT = 0.2–20.1 for arable lands, p > 0.05). This indicates the relatively high stability of organic carbon and efficient humification processes in both land uses. Additionally, in arable soils lower CWE‐C (0.75 g kg^?1) and higher HWE‐C (2.59 g kg^?1) than in grasslands (CWE‐C = 1.13 g kg^?1, HWE‐C = 1.60 g kg^?1) can be related to farming practice and application of soil amendments. The results showed that both labile and humified organic matter are better protected in grassland soils and are consequently less vulnerable to mineralization.     

Influence of land use and tillage depth on dynamics of soil microbial properties,soil carbon fractions and crop yield after conversion of short‐rotation coppices

The effect of conversion of short‐rotation coppices (SRCs) to agricultural land on soil organic carbon (SOC), soil microbial properties and crop yield is largely unknown. The objective of this study was to assess the effects of subsequent land use and tillage depth after conversion of SRCs on (i) total SOC (ii) soil C fractions with differentiation of total harvest residues and woody harvest residues from SRC and maize by ¹³C analysis and (iii) dry matter and N yield of grassland and maize. For this purpose, field trials were established after conversion of SRCs at three sites in Germany and cultivated with maize and grassland with shallow (5 cm), medium (15 cm) and deep tillage depth (30 cm). Crops were sampled for 5 yrs, and soil samples were collected at a depth of 0–5, 5–15 and 15–30 cm. Amount of total carbon and soil carbon fractions immediately and 4 yrs after conversion of SRC were compared. Tillage depth had no effect on dry matter yield of maize and grassland. The amount of woody harvest residues decreased over time following conversion at all sites irrespective of land use or tillage depth, but SOC decreased only at one site. Microbial biomass was particularly sensitive to land use, but microorganisms reacted differently to tillage depth depending on the soil conditions. Our results reveal that decomposition of woody harvest residues is rapid and that effects of tillage and land use on different soil C‐pools are site specific.     

植被重建下露天煤矿排土场边坡土壤碳储量变化

植被重建是治理排土场边坡水土流失最直接也是最有效的生物措施,研究不同植被重建模式下土壤有机碳(SOC)和全氮(TN)含量的空间分布规律是筛选适宜排土场边坡生长的植被模式的重要条件。选取内蒙古黑岱沟露天煤矿治理15年的排土场边坡中4种植被重建模式(自然恢复地、草地、灌木林、乔木林),采集270个土壤剖面(0~100 cm)样品,研究不同重建模式下SOC储量的变化。结果表明:(1)植被重建模式显著影响剖面SOC、TN含量及分布(p0.05),0~10 cm和10~20 cm SOC、TN均呈草地灌木乔木自然恢复地,20 cm以下各土层SOC、TN虽然也表现相似的特征,但差异随土层深度增加越来越小。(2)剖面SOC密度和储量表现为原地貌区治理排土场新建排土场。经15年植被重建后,排土场边坡表现出巨大的固碳能力,1 m深度的林地和草地碳储量分别增加了5.38、11.85 t hm-2,但仅原地貌水平的1/2和3/5。(3)林地和草地的固碳速率分别为35.87、79.01 g m-2a-1,草地的固碳速率是林地的2.2倍,从土壤固碳及水土流失防治的角度考虑,建议矿区排土场边坡植被重建优先选择草地,其次灌木。     

Changes in carbon and nitrogen of Chernozem soil along a cultivation chronosequence in a semi‐arid grassland

Changes in the carbon (C) stock of grassland soil in response to land use change will increase atmospheric CO₂, and consequently affect the climate. In this study we investigated the effects of land use change on soil organic C (SOC) and nitrogen (N) along a cultivation chronosequence in the Xilin River Basin, China. The chronosequence consisted of an undisturbed meadow steppe, a 28‐year‐old cropland and a 42‐year‐old cropland (abbreviated as Steppe, Crop‐28 Y and Crop‐42Y, respectively). Crop‐28Y and Crop‐42Y were originally created on the meadow steppe in 1972 and 1958, respectively. The soil samples, in ten replications from three depth increments (0–10, 10–20 and 20–30 cm), were collected, respectively, in the two cropland fields and the adjacent undisturbed steppe. Bulk density, SOC, total N and 2 m KCl‐extractable mineral N including ammonium and nitrate were measured. Our results showed that the greatest changes in the measurements occurred in the 0–10 cm soil depth. The SOC stock in the upper 30‐cm soil decreased by 9.83 Mg C ha⁻¹ in Crop‐28Y and 21.87 Mg C ha⁻¹ in Crop‐42Y, which indicated that approximately 10 and 25% of the original SOC of the steppe had been emitted over 28 and 42 years, respectively. Similarly, the total N lost was 0.66 Mg N ha⁻¹ and 1.18 Mg N ha⁻¹, corresponding to approximately 9% and 16%, respectively, of the original N at the same depth and cropping duration as those noted for SOC. The mineral N concentration in the soil of both the two croplands was greater than that in the steppe soil, and the ammonium‐N was less affected by cultivation than the nitrate‐N. The extent of these changes depended on soil depth and cropland age. These effects of cultivation were much greater in the top 10 cm of soil than in deeper soil, and also greater in Crop‐42Y than in Crop‐28Y. The findings are significant for assessing the C and N sequestration potential of the land use changes associated with grassland conversion, and suggest that improved management practices are needed to sequester SOC and total N in the cropped soil in a semi‐arid grassland.