Effects of topography on soil organic carbon stocks in grasslands of a semiarid alpine region,northwestern China

Purpose

Soil organic carbon (SOC) in mountainous regions is characterized by strong topography-induced heterogeneity, which may contribute to large uncertainties in regional SOC stock estimation. However, the quantitative effects of topography on SOC stocks in semiarid alpine grasslands are currently not well understood. Therefore, the purpose of this research study is to determine the role of topography in shaping the spatial patterns of SOC stocks.

Materials and methods

Soils from the summit, shoulder, backslope, footslope, and toeslope positions along nine toposequences within three elevation-dependent grassland types (i.e., montane desert steppe at ~?2450 m, montane steppe at ~?2900 m, and subalpine meadow at ~?3350 m) are sampled at four depths (0–10, 10–20, 20–40, and 40–60 cm). SOC content, bulk density, soil texture, soil water content, and grassland biomass are determined. The general linear model (GLM) is employed to quantify the effects of topography on the SOC stocks. Ordinary least squares regressions are performed to explore the underlying relationships between SOC stocks and the other edaphic factors.

Results and discussion

In accordance with the present results, the SOC stocks at 0–60 cm show an increasing trend in respect to the elevation zone, with the highest stock being approximately 37.70 g m^?2 in the subalpine meadow, about 2.07 and 3.41 times larger than that in the montane steppe and montane desert steppe, respectively. Along the toposequences, it is revealed the SOC stocks are maximal at toeslope, reaching to 14.98, 31.76, and 49.52 kg m^?2, which are also significantly larger than those at the shoulder by a factor of 1.38, 2.31, and 1.44, in montane desert steppe, montane steppe, and subalpine meadow, respectively. Topography totally is seen to explain about 84% of the overall variation in SOC stocks, of which 70.61 and 9.74% are attributed to elevation zone and slope position, while the slope aspect and slope gradient are seen to plausibly explain only about 1.84 and 0.01%, respectively.

Conclusions

The elevation zone and the slope position are seen to markedly shape the spatial patterns of the SOC stocks, and thus, they may be considered as key indicating factors in constructing the optimal SOC estimation model in such semiarid alpine grasslands.

     

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.

     

Response of soil C:N:P stoichiometry,organic carbon stock,and release to wetland grasslandification in Mu Us Desert

Purpose

Wetlands in Mu Us Desert have severely been threatened by grasslandification over the past decades. Therefore, we studied the impacts of grasslandification on soil carbon (C):nitrogen (N):phosphorus (P) stoichiometry, soil organic carbon (SOC) stock, and release in wetland-grassland transitional zone in Mu Us Desert.

Materials and methods

From wetland to grassland, the transition zone was divided into five different successional stages according to plant communities and soil water conditions. At every stage, soil physical and chemical properties were determined and C:N:P ratios were calculated. SOC stock and soil respirations were also determined to assess soil carbon storage and release.

Results and discussion

After grasslandification, SOC contents of top soils (0–10 cm) decreased from 100.2 to 31.79 g kg^?1 in June and from 103.7 to 32.5 g kg^?1 in October; total nitrogen (TN) contents of top soils (0–10 cm) decreased from 3.65 to 1.85 g kg^?1 in June and from 6.43 to 3.36 g kg^?1 in October; and total phosphorus (TP) contents of top soils (0–10 cm) decreased from 179.4 to 117.4 mg kg^?1 in June and from 368.6 to 227.8 mg kg^?1 in October. From stages Typha angustifolia wetland (TAW) to Phalaris arundinacea L. (PAL), in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 19.0 to 11.8 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October. SOC stock decreased and soil respiration increased with grasslandification. The decrease of SOC, TN, and TP contents was attributed to the reduction of aboveground biomass and mineralization of SOM, and the decrease of soil C:N, C:P, and N:P ratios was mainly attributed to the faster decreasing speeds of SOC than TN and TP. The reduction of aboveground biomass and increased SOC release led by enhanced soil respiration were the main reasons of SOC stock decrease.

Conclusions

Grasslandification led to lowers levels of SOC, TN, TP, and soil C:N, C:P, and N:P ratios. Grasslandification also led to higher SOC loss, and increased soil respiration was the main reason. Since it is difficult to restore grassland to original wetland, efficient practices should be conducted to reduce water drainage from wetland to prevent grasslandification.

     

Impact of cultivation on soil organic carbon and carbon sequestration potential in semiarid regions of China

Dynamic changes in soil organic carbon (SOC) have become a popular topic in global research on organic carbon as part of the increasing attention being paid to food security and reducing greenhouse gas emissions. In this paper, the semiarid regions of China were selected as a research focus, and SOC data from 1980 to 2015 were analysed using IBM SPSS Statistics 20.0 software. SOC in farmland varied according to cultivated land type, mulching material type and planting method in the studied regions. The SOC content is 10.3–10.8 g kg⁻¹ in supplementally irrigated land and flat dry land, 7.1–8.7 g kg⁻¹ in terraced dry land and river beach land, and 6.2–6.4 g kg⁻¹ in sloping dry land. The SOC content increased to 16.1–17.4 g kg⁻¹ when crop stalks were used as mulch. The increase was only 11.5–13.5 g kg⁻¹ in soils mulched with film or sandstone. The SOC value in wheat, maize and potatoes sown on single or double ridges was 2.4%–3.2%, 35.7%–36.4% and 4.4%–4.8%, respectively, which are higher than the values for wheat, maize and potatoes sown using the flat planting method. The SOC sequestering potential also varied according to the previously noted factors and was improved from 224.1% to 383.8% depending on cultivated land type, from 96.5% to 182.3% depending on mulching material type and from 96.1% to 191.3% depending on planting method. The SOC sequestering potential can be improved by 453.2%–757.4% with the integration of the optimal cultivated land type, mulching material type and planting method. Thus, there is substantial soil carbon sequestration potential in China's semiarid regions.     

Soil organic carbon stock and fractional distribution across central-south China

The stock and stability of soil organic carbon (SOC) are critical to soil functions and global carbon cycle, but little quantitative information is available on the precise location and chemical components of SOC for soils across a wide range of climatic gradients. Here, a broad range of zonal soils were collected in forest land at topsoil (0–15 cm) and subsoil (15–30 cm) from temperate to tropical climatic gradient in central to south China. The stock and stability of SOC were determined in terms of aggregate and humic fractionation. SOC in bulk soils with a less significant geographic variation was comparably higher at Haplic Luvisoils in temperate regions (3637.61 g m⁻²) and Rhodi-Humic Ferrosols in tropical regions (3446.12 g m⁻²) than in the other experimental soils, but a consistent decreasing trend was observed along the soil profiles with the SOC stock was 1.11–1.97 times higher in the topsoil than in the subsoils. In addition, insoluble humin residue (HMr) as the dominant components of SOC ranged from 643.95 to 2696.90 g m⁻² and decreased from temperate to tropical regions, which was consistent with the zonal variation of humic acids (HAs), but contrary to the zonal variation of fulvic acids (FAs) that fluctuated in a range of 39.67–389.55 g m⁻² across the experimental sites. According to the results of partial correlation analysis, the variation of FAs stock was significantly attributed to soil pH, bulk density, iron and aluminum oxides, clay, and clay mineral content (|r|>0.61, p < 0.05), while these soil physical properties showed a contradictory effects on HAs, iron-linked humin (HMi), clay-combined humin (HMc), and HMr. Moreover, the aggregate-associated carbon stock was mainly stored in macroaggregates (36.34–76.09%) for both SOC and its chemical components, especially in topsoils, and its zonal variation was associated with that of bulk soils. In general, the redundancy analysis (RDA) revealed that mean annual precipitation (MAP) accounted for 81.8% and 13.8% of the variance in SOC chemical and physical fractionation, respectively, while the corresponding contribution of mean annual temperature (MAT) was 1.5% and 34.7%. With the increase of MAT and MAP, the chemical stability of SOC decreased in the molecular structure, and the physical protection of SOC by aggregate exhibited a unimodal trend. The obtained results would facilitate the development of regional soil carbon prediction and land management against global warming.     

Effects of permafrost thawing on vegetation and soil carbon pool losses on the Qinghai–Tibet Plateau,China

Bearing a total organic carbon (TOC) content of 9.3–10.7 kg C/m², alpine grassland soils of the Qinghai–Tibet plateau's permafrost region bear a greater organic carbon pool than do grassland soils in other regions of China or than tropical savannah soils. The easily released light fraction organic carbon (LFOC) accounts for 34–54% of the TOC and is particularly enriched in the topsoil (0–0.10 m). The LFOC in the organic carbon pool of alpine cold meadow and alpine cold steppe soils decreased at exponential and quadratic rates, respectively, as the vegetative cover decreased. When the vegetative cover of alpine cold meadows decreased from > 80 dm²/m² to 60 dm²/m², the topsoil TOC and LFOC dropped by 20.4% and 38.4%, respectively. Similarly, when the vegetative cover of alpine cold meadow decreased from 50 dm²/m² to 30 dm²/m² and < 15 dm²/m², the topsoil LFOC content dropped by 60% and 86.7%, respectively. Under climatic warming, the degradation of permafrost and vegetation have resulted in serious soil organic carbon (SOC) loss from the carbon pool. Land cover changes that occurred between 1986 and 2000 are estimated to have resulted in a 1.8 Gg C (120 Mg C/yr) loss in SOC, and a concomitant 65% decrease in the LFOC, in the 0–0.30 m soil layer in the Qinghai–Tibet plateau's permafrost regions. Since the region's ecosystems are quite sensitive to global climate changes, if global warming persists, alpine cold grassland ecosystems are expected to further degrade. Hence, the influence of global climatic change on soil carbon emissions from alpine grasslands should receive more attention.     

The devil is in the detail: Discrepancy between soil organic carbon stocks estimated from regional and local data sources in Flanders,Belgium

Storage of soil organic carbon (SOC) is an essential function of ecosystems underpinning the delivery of multiple services to society. Regional SOC stock estimates often rely on data collected during land‐use‐specific inventory schemes with varying sampling depth and density. Using such data requires techniques that can deal with the associated heterogeneity. As the resulting SOC assessments are not calibrated for the local scale, they could suffer from oversimplification of landscape processes and heterogeneity. This might especially be the case for sandy regions where typical historical land use practices and soil development processes determine SOC storage. The aims of this study were (a) to combine four land‐use‐specific SOC stock assessments to estimate the total stock in Flanders, Belgium, and (b) to evaluate the applicability of this regional‐scale estimate at the local scale. We estimated the SOC stock in the upper 100 cm of the unsealed area in Flanders (887,745 ha) to be 111.67 Mt OC, or 12.6 ± 5.65 kg OC m^?2 on average. In general, soils under (semi‐) natural land‐use types, for example forests, store on average more organic carbon than under agriculture. However, overall agricultural soils store the largest amounts of SOC due to their vast spatial extent. Zooming in on a sandy location study (13.55 km²) revealed the poor performance of the regional estimates, especially where Histosols occurred. Our findings show that a greater spatial sampling density is required when SOC stock estimates are needed to inform carbon‐aware land management rather than to provide for regional reporting.     

Effects of sheet and rill erosion on soil aggregates and organic carbon losses for a Mollisol hillslope under rainfall simulation

Purpose

Characterizations of soil aggregates and soil organic carbon (SOC) losses affected by different water erosion patterns at the hillslope scale are poorly understood. Therefore, the objective of this study was to quantify how sheet and rill erosion affect soil aggregates and soil organic carbon losses for a Mollisol hillslope in Northeast China under indoor simulated rainfall.

Materials and methods

The soil used in this study was a Mollisol (USDA Taxonomy), collected from a maize field (0–20 cm depth) in Northeast China. A soil pan with dimensions 8 m long, 1.5 m wide and 0.6 m deep was subjected to rainfall intensities of 50 and 100 mm h^?1. The experimental treatments included sheet erosion dominated (SED) and rill erosion dominated (RED) treatments. Runoff with sediment samples was collected during each experimental run, and then the samples were separated into six aggregate fractions (0–0.25, 0.25–0.5, 0.5–1, 1–2, 2–5, >?5 mm) to determine the soil aggregate and SOC losses.

Results and discussion

At rainfall intensities of 50 and 100 mm h^?1, soil losses from the RED treatment were 1.4 and 3.5 times higher than those from the SED treatment, and SOC losses were 1.7 and 3.8 times greater than those from the SED treatment, respectively. However, the SOC enrichment ratio in sediment from the SED treatment was 1.15 on average and higher than that from the RED treatment. Furthermore, the loss of <?0.25 mm aggregates occupied 41.1 to 73.1% of the total sediment aggregates for the SED treatment, whereas the loss of >?0.25 mm aggregates occupied 53.2 to 67.3% of the total sediment aggregates for the RED treatment. For the organic carbon loss among the six aggregate fractions, the loss of 0–0.25 mm aggregate organic carbon dominated for both treatments. When rainfall intensity increased from 50 to 100 mm h^?1, aggregate organic carbon loss increased from 1.04 to 5.87 times for six aggregate fractions under the SED treatment, whereas the loss increased from 3.82 to 27.84 times for six aggregate fractions under the RED treatment.

Conclusions

This study highlights the effects of sheet and rill erosion on soil and carbon losses at the hillslope scale, and further study should quantify the effects of erosion patterns on SOC loss at a larger scale to accurately estimate agricultural ecosystem carbon flux.

     

A multiple regression approach to assess the spatial distribution of Soil Organic Carbon (SOC) at the regional scale (Flanders,Belgium)

Estimates of the amount of Soil Organic Carbon (SOC) at the regional scale are important to better understand the role of the SOC reservoir in global climate and environmental issues. This study presents a method for estimating the total SOC stock using data from Flanders (Belgium). More than 6900 SOC measurements from the national soil survey (database ‘Aardewerk’) are combined with a digital land use map and a digital soil map of Flanders. The spatial distribution of the SOC stock is studied in its relation to factors such as soil texture, soil moisture (drainage class) and land use. The resulting map with a resolution of 15 m consists of different classes forming a combination of these environmental factors. The results show that the lowest SOC amount (kg m^? 2) is stored under cropland whereas the highest amount is found under grassland. Regarding the effect of soil properties, a significant correlation between SOC stock and depth of the ground water table is observed. Sandy loam soils stock the lowest SOC amount (kg m^? 2), whereas clay soils retain the highest SOC amount. First, the mean SOC amounts of the land use–soil type classes are calculated and assigned to the corresponding cells in order to obtain a total SOC stock with its spatial distribution for Flanders. Then, a multiple regression model is applied to predict the SOC value of a particular land use–soil type class on the map. This model is based on the observed relationships between SOC and land use–soil type characteristics, using the entire dataset. The first approach does not allow to obtain a (reliable) SOC value for all land use–soil type classes due to a lack of samples in some classes. A major advantage of the regression model approach is the attribution of class specific SOC values to each land use–soil type class, regardless of the number of observations in the classes. Consequently, by applying the model approach instead of the mean approach, the area for which a reliable SOC estimate could be obtained increased by 8.1% (from 9420 km² to 10179 km²) and the total predicted SOC stock increased by 10.1% (from 88.7 ± 5.6 Mt C to 97.6 ± 1.1 Mt C).     

Variation of soil organic carbon,nitrogen, and phosphorus stoichiometry and biogeographic factors across the desert ecosystem of Hexi Corridor,northwestern China

Purpose

The purposes of present study were to display the vertical distribution of soil organic carbon (SOC), nitrogen (N), and phosphorus (P) stoichiometry; identify the biogeographic characteristics of SOC, N, and P stoichiometry along an aridity gradient across the desert ecosystem of Hexi Corridor; and determine how biogeographic distribution patterns of SOC, N, and P stoichiometry are related to vegetation, soil texture, geography, and climate.

Materials and methods

We investigated the distribution and characteristics of SOC, N, and P stoichiometry based on samples collected from Hexi Corridor during 2011–2012 with total 400 plots of 80 sites. This region presents a precipitation gradient from about 250 mm in the east to less than 50 mm in the west. The measured variables included belowground and aboveground biomass, pH, bulk density, sand, clay, silt, SOC, N, and P contents. ANOVA analysis, reduced major axis, redundancy analysis, Person’s correlation, and regression analysis were used to analysis the variation of SOC, N, and P stoichiometry and related biogeographic factors.

Results and discussion

In present study, SOC, N, and P contents decreased significantly with increasing soil depth. C/N did not change significantly, while C/P and N/P decreased significantly. SOC and N, SOC and P, and N and P were well constrained within 0–100 cm. SOC, N, and P contents in 0–20 cm were higher than them in other studies. Vegetation, soil texture, climate, and geography could explain 91.6% of the total variance of soil stoichiometry. The impact of latitude and longitude on SOC, N, and P stoichiometry was mainly caused by the redistribution of precipitation, while the impact of altitude mainly resulted from the variation of temperature. With increasing aridity, SOC, N, and P contents and C/N/P ratios reduced consistently with inconsistent decrease rates.

Conclusions

Our results suggested that the interaction of vegetation structure, soil condition, and shortage of precipitation should be the main driver for the lower contents and much shallower distributions of SOC, N, and P of Hexi Corridor. The increasing aridity should be the critical factor that is responsible for the decrease of SOC, N, and P contents and C/N/P ratios. This study contributes to the understanding of soil stoichiometry in the desert ecosystem.

     

Assessment of frozen ground organic carbon pool on the Qinghai-Tibet Plateau

Purpose

Under rapid climate change, soil organic carbon (SOC) dynamic in frozen ground may significantly influence terrestrial carbon cycles. The aim of this study was to investigate the storage, spatial patterns, and influencing factors of SOC in frozen ground on the Qinghai-Tibet Plateau, which known as the earth’s Third Pole.

Materials and methods

Using the observed edaphic data from China’s Second National Soil Survey, we estimated the SOC storage (SOCS) of frozen ground (including permafrost, seasonally, and short time frozen ground) on the plateau with a depth of 0–3 m. Furthermore, the effect of vegetation and climate factors on spatial variance of SOC density (SOCD) was analyzed.

Results and discussion

The SOCD decreased from the southeastern to the northwestern part of the plateau, and increased with shorten of freezing duration. SOCS of permafrost, seasonally, and short time frozen ground were calculated as 40.9 (34.2–47.6), 26.7 (24.1–29.4), and 6 (5.6–6.4) Pg, making a total of 73.6 (63.9–83.3) Pg in 0–3 m depth on the plateau. Normalized difference vegetation index and mean annual precipitation could significantly affect the spatial distribution of SOC in permafrost and seasonally frozen ground.

Conclusions

The soil in plateau frozen ground contained substantial organic carbon, which could be affected by plant and climate variables. However, the heterogeneous landform may make the fate of carbon more complicated in the future.

     

Soil organic carbon depletion in global Mollisols regions and restoration by management practices: a review

Purpose

Mollisols are the most fertile, high-yielding soils in the world. During the past several decades, Mollisols have lost about 50% of their antecedent organic carbon (C) pool due to soil erosion, degradation, and other unsuitable human activities. Therefore, restoring soil organic C (SOC) to Mollisols via reasonable management is crucial to sustainable development and is important for environmental stability. However, the existing literature on SOC and soil quality has focused on one soil type or on a given region where Mollisols occur, and the degree of SOC depletion and stabilization in Mollisols have not been comprehensively evaluated. Overall, we propose to develop an optimum scheme for managing Mollisols, and we outline specific issues concerning SOC restoration and prevention of SOC depletion.

Materials and methods

In this review, we identify the uncertainties involved in analyses of SOC in Mollisols as related to management practices. According to the existing literature on SOC in Mollisols at the global scale, we analyzed the results of SOC depletion research to assess management practices and to estimate the C amount stabilized in Mollisols.

Results and discussion

The review shows that the SOC stocks in Mollisols in North America under cropped systems had 51?±?4 (equiv. mass) Mg ha^?1 in the top 30 cm soil layer. The SOC contents in Northeast China decreased from 52 to 24 g kg^?1 (46%) after 150 years of cultivation management. All of the Mollisols regions in the world are facing the challenge of SOC loss, and this trend could have a negative influence on global climate change. Hence, it is very important to take proper measures to maintain and enhance organic C contents in Mollisols.

Conclusions

We concluded that reasonable management practices, including no-tillage, manure and compost fertilization, crop straw returning, and mulching cultivation, are the recommended technologies. The C restoration in Mollisols is a truly win-win strategy for ensuring the security of food and soil resources while effectively mitigating global climate change. Thus, more attention should be given to protective management and land use for its impacts on SOC dynamics and soil properties in Mollisols regions.

     

How do hedgerows influence soil organic carbon stock in livestock‐grazed pasture?

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.     

Changes in soil properties following shrub encroachment in the semiarid Inner Mongolian grasslands of China

ABSTRACT

Shrub encroachment is very common in the semiarid regions of China. However, surprisingly little research has been conducted to evaluate the effects of shrubs on soil properties in these regions compared with other regions in the world. Three sites, one without shrub (state 1), one with 13% shrub coverage (state 2), and the third one with 40% shrub coverage (state 3) in the semiarid Inner Mongolian grasslands were selected to investigate the effects of the shrub Caragana microphylla Lam on soil properties. Soil samples were collected from three sites to evaluate the changes of bulk density (BD), grain size distribution, aggregate stability, soil organic carbon (SOC), total nitrogen (TN), and soil water content (SWC) with the development of shrub encroachment. The results showed that BD and soil sand declined, and silt, aggregate >2 mm and the mean weight diameter (MWD) rose from state 1 to state 3, especially in 20–60 cm depths. With the shrub encroachment, the content of SOC and TN increased, and especially, the increase trends were more obvious in the deeper soil than in the surface soil. SWC increased with the development of shrub encroachment at 20–60 cm depths. Greater and deeper water infiltration existed after heavy rainfall in states 2 and 3, suggesting that macropore ?ow appeared in shrub encroachment sites. All these results indicated that shrub encroachment improved soil physicochemical properties, particularly in deep soil.     

Decreased glomalin-related soil protein with nitrogen deposition in a 3-year-old Cunninghamia lanceolata plantation

Purpose

Glomalin-related soil protein (GRSP) is an essential component of soil organic C for maintaining soil quality and structure and plays a critical role in soil carbon (C) sequestration. However, how GRSP changes under nitrogen (N) deposition remains poorly understood.

Materials and methods

We assessed total GRSP (T-GRSP) and easily extractable GRSP (EE-GRSP) under a control (no N input), low N addition (LN, 40 kg N ha^?1 year^?1), and high N addition (HN, 80 kg N ha^?1 year^?1) treatments in 2015 and 2016 in a Chinese fir (Cunninghamia lanceolata) plantation in the subtropical China. We also analyzed soil properties contents and explored the stoichiometric ratios of soil organic C (SOC), total N (TN), and total phosphorus (TP) with GRSPs.

Results

Compared to the control, both T-GRSP and EE-GRSP were significantly reduced under the HN treatment, but had no significant difference under the LN treatment. The ratio of T-GRSP and EE-GRSP was reduced by the N addition. Soil organic C (SOC) and dissolved organic C (DOC) were significantly affected by N addition treatments. The ratios of GRSP-C to SOC and of EEGRSP-C to SOC ranged from 6.29 to 16.07% and 1.34 to 3.52%, respectively. T-GRSP and EE-GRSP were positively correlated with SOC/TN ratio, but negatively correlated with soil TN/TP and SOC/TP ratios.

Conclusion

Our results indicated that the GRSP reductions under N deposition in soil are mediated by soil C, N, and P stoichiometry, and particularly, the reduction of EE-GRSP by DOC. This study improved our mechanistic understanding of dynamics of GRSPs under increasing N enrichment in subtropical plantation ecosystems.

     

Long-term fertilization effects on carbon pools and carbon management index of loamy soil under grass–forage legumes mixture in semi-arid environment

ABSTRACT

Soil organic carbon (SOC) is a key component for sustaining crop production. A field experiment was conducted during 2004–2018 to assess the changes in soil carbon fractions under different fertilization practices in grass-legumes mixture. The result indicates that application of farmyard manure (FYM) at 80 Mg ha^–1 has increased SOC concentration leading to carbon sequestration rate of 4.2 Mg ha^–1 year^–1. Further, it has increased the proportion of labile carbon in the total SOC and have accumulated 126, 60, 83 and 95% higher very labile, labile, less labile and non-labile C stock than that of control plot, respectively, in top 30 cm soil layer. Inorganic fertilization and FYM 20 Mg ha^–1 influenced SOC concentration, SOC stock and C sequestration rate similarly. The highest carbon management index (264) was found in the treatment receiving FYM 80 Mg ha^–1 and it was positively correlated with SOC (r = 0.84**). The sensitivity index of the SOC varied from 26 to 152% and the differences were greatest in FYM treatments. The result indicates that grass-legumes mixture build-up the SOC in long term and the addition of FYM further increases it.     

Critical carbon inputs to maintain soil organic carbon stocks under long‐term finger‐millet (Eleusine coracana [L.] Gaertn.) cropping on Alfisols in semiarid tropical India

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.     

Change in SOC content in a small karst basin for the past 35 years and its influencing factors

ABSTRACT

To research soil organic carbon (SOC) in a typical small karst basin of western Guizhou in southwest China, data from the second national soil resource survey (1980) and data analysed in the laboratory in 2015 were used. This paper examines the changes in soil organic carbon density (SOCD) and soil organic carbon stock (SOCS) in the topsoil (0–20 cm) over the past 35 years based on soil types, and the primary influencing factors are also discussed. The SOCD and SOCS slightly increased over this period. The SOCD increased from 4.91 kg m^?2 to 5.13 kg m^?2, and the SOCS increased from 368.27 × 10³ t to 385.09 × 10³ t. The basin sequestered a low level of carbon during this time. Paddy fields were the key contributor to the increases, and the SOCD and SOCS of paddy fields increased by 1.61 kg m^?2 and 32.39 × 10³ t, respectively. Generally, the SOCD and SOCS in the soils from the southern part of Houzhai Basin increased considerably, and those from the northern part of the basin decreased significantly. The spatial variation of SOCD in the Houzhai Basin was mainly due to natural factors. However, the temporal change of SOC was primary caused by human activities.     

Soil organic carbon stock assessment for the different cropland land uses in Italy

The aim of this work was to quantify the soil organic C (SOC) stock in the top 30 cm of mineral soil for the whole Italian territory, according to the different land use types of the Intergovernmental Panel on Climate Change (IPCC) cropland category (arable land, agroforestry, vineyards, olive groves, orchards and rice fields), as a basis for future land use scenarios and to address mitigation policy at country level. A database for SOC stock was created with the data from the national project denominated SIAS and partly from regional map reports. All data were referred to the year 2000 since they were derived from surveys conducted from 1995 to 2005. The data were stratified according to the Italian climatic regions, the landscape position and the IPCC cropland subcategories. Taking into account the uncertainty in the estimate, the mean SOC stock values of the different subcategories show significant differences (p < 0.05) among climatic regions and landscapes, ranging from 41.9 ± 15.9 Mg C ha⁻¹ in the vineyards to 63.3 ± 27.9 Mg C ha⁻¹ in the rice fields. Generally, a small decrease of the SOC stock from the temperate regions toward the Mediterranean ones is observed. Taking into account the mean value of each subcategory and the country area they occupied in 2000, the total C stored in the upper 30 cm of soil was estimated at 490.0 ± 121.7 Tg C. The resulting estimate represents the 17% of the value reported by another study for the soil of the whole country down to 50 cm depth, suggesting the importance of preserving this large C pool. Considering the cropland category as a whole, the estimated mean SOC stock is 52.1 ± 17.4 Mg C ha⁻¹, similar to that reported for other European countries, 50–60 Mg C ha⁻¹. In conclusion, the assessment of the mean SOC stock of the different cropland land uses, landscape position and climate regions could notably help when assessing the impact of different agricultural practices and future stock change evaluation.     

Mechanisms behind soil N dynamics following cover restoration in degraded land in subtropical China

Purpose

Nitrogen (N) is an important nutrient for re-vegetation during ecosystem restoration, but the effects of cover restoration on soil N transformations are not fully understood. This study was conducted to investigate N transformations in soils with different cover restoration ages in Eastern China.

Materials and methods

Soil samples were collected from four degraded and subsequently restored lands with restoration ages of 7, 17, 23, and 35 years along with an adjacent control of degraded land. A ¹⁵N tracing technique was used to quantify gross N transformation rates.

Results and discussion

Compared with degraded land, soil organic carbon (SOC) and total N (TN) increased by 1.60–3.97 and 2.49–5.36 times in restoration land. Cover restoration increased ammonium and nitrate immobilization, and dissimilatory nitrate reduction to ammonium (DNRA) by 0.56–0.96, 0.34–2.10, and 0.79–3.45 times, respectively, indicating that restoration was beneficial for N retention. There were positive correlations between SOC content and ammonium and nitrate immobilization and DNRA, indicating that the increase in soil N retention capacity may be ascribed to increasing SOC concentrations. The stimulating effect of SOC on ammonium immobilization was greater than its effect on organic N mineralization, so while SOC and TN increased, inorganic N supply did not increase. Autotrophic and heterotrophic nitrification increased with increasing SOC and TN concentrations. Notably, heterotrophic nitrification was an important source of NO₃^??N production, accounting for 47–67% of NO₃^??N production among all restoration ages.

Conclusions

The capacity of N retention was improved by cover restoration, leading to an increase in soil organic carbon and total N over time, but inorganic N supply capacity did not change with cover restoration age.