Hierarchical responses of soil organic and inorganic carbon dynamics to soil acidification in a dryland agroecosystem,China

Soil acidification is a major global issue of sustainable development for ecosystems. The increasing soil acidity induced by excessive nitrogen （N） fertilization in farmlands has profoundly impacted the soil carbon dynamics. However, the way in which changes in soil pH regulating the soil carbon dynamics in a deep soil profile is still not well elucidated. In this study, through a 12-year field N fertilization experiment with three N fertilizer treatments （0, 120, and 240 kg N/（hm-2·a）） in a dryland agroecosystem of China, we explored the soil pH changes over a soil profile up to a depth of 200 cm and determined the responses of soil organic carbon （SOC） and soil inorganic carbon （SIC） to the changed soil pH. Using a generalized additive model, we identified the soil depth intervals with the most powerful statistical relationships between changes in soil pH and soil carbon dynamics. Hierarchical responses of SOC and SIC dynamics to soil acidification were found. The results indicate that the changes in soil pH explained the SOC dynamics well by using a non-linear relationship at the soil depth of 0-80 cm （P=0.006）, whereas the changes in soil pH were significantly linearly correlated with SIC dynamics at the 100-180 cm soil depth （P=0.015）. After a long-term N fertilization in the experimental field, the soil pH value decreased in all three N fertilizer treatments. Furthermore, the declines in soil pH in the deep soil layer （100-200 cm） were significantly greater （P=0.035） than those in the upper soil layer （0-80 cm）. These results indicate that soil acidification in the upper soil layer can transfer excess protons to the deep soil layer, and subsequently, the structural heterogeneous responses of SOC and SIC to soil acidification were identified because of different buffer capacities for the SOC and SIC. To better estimate the effects of soil acidification on soil carbon dynamics, we suggest that future investigations for soil acidification should be extended to a deeper soil depth, e.g., 200 cm.

Comparison on Soil Carbon Stocks Between Urban and Suburban Topsoil in Beijing, China

The urban population and urbanized land in China have both increased markedly since the 1980 s. Urban and suburban developments have grown at unprecedented rates with unknown consequences for ecosystem functions. In particular, the effect of rapid urbanization on the storage of soil carbon has not been studied extensively. In this study, we compared the soil carbon stocks of different land use types in Beijing Municipality. We collected 490 top-soil samples(top 20 cm) from urban and suburban sites within the Sixth Ring Road of Beijing, which cover approximately 2400 km2, and the densities of soil organic carbon(SOC), soil inorganic carbon(SIC), and total carbon(TC) were analyzed to determine the spatial distribution of urban and suburban soil carbon characteristics across seven land use types. The results revealed significant differences in soil carbon densities among land use types. Additionally, urban soil had significantly higher SOC and SIC densities than suburban soil did, and suburban shelterbelts and productive plantations had lower SIC densities than the other land use types. The comparison of coefficients of variance(CVs) showed that carbon content of urban topsoil had a lower variability than that of suburban topsoil. Further findings revealed that soil carbon storage increased with built-up age. Urban soil built up for more than 20 years had higher densities of SOC, SIC and TC than both urban soil with less than 10 years and suburban soil. Correlation analyses indicated the existence of a significantly negative correlation between the SOC, SIC, and TC densities of urban soil and the distance to the urban core, and the distance variable alone explained 23.3% of the variation of SIC density and 13.8% of the variation of TC density. These results indicate that SOC and SIC accumulate in the urban topsoil under green space as a result of the conversion of agricultural land to urban land due to the urbanization in Beijing.

Distribution of soil carbon in different grassland types of the Qinghai-Tibetan Plateau

Accurate estimate of soil carbon storage is essential to reveal the role of soil in global carbon cycle. However, there is large uncertainty on the estimation of soil organic carbon(SOC) storage in grassland among previous studies, and the study on soil inorganic carbon(SIC) is still lack. We surveyed 153 sites during plant peak growing season and estimated SOC and SIC for temperate desert, temperate steppe, alpine steppe, steppe meadow, alpine meadow and swamp, which covered main grassland in the Qinghai Plateau during 2011 to 2012. The results showed that the vertical and spatial distributions of SOC and SIC varied by grassland types. The SOC amount mainly decreased from southeast to northwest, whereas the SIC amount increased from southeast to northwest. The magnitude of SOC amount in the top 50 cm across grassland types ranked by: swamp > alpine meadow >steppe meadow > temperate steppe > alpine steppe > temperate desert, while the SIC amount showed an opposite order. There was a great deal of variation in proportion of SOC and SIC among different grassland types(from 55.17 to 94.59 for SOC and 5.14 to 44.83 for SIC). The total SOC and SIC storage was 5.78 Pg and 1.37 Pg, respectively, in the top 50 cm of soil in Qinghai Province. The mixed linear model revealed that grassland types was the predominant factor in spatial variations of SOC amount while grassland types and soil p H accounted for those of SIC amount. Our results suggested that the community shift of alpine meadow towards alpine grassland induced by climate warming would decrease carbon sequestration capacity by 6.0 kg C m2.

Moisture regime influence on soil carbon stock and carbon sequestration rates in semi-arid forests of the National Capital Region, India

Understanding the dynamics of soil carbon is crucial for assessing the soil carbon storage and predicting the potential of mitigating carbon dioxide from the atmosphere to the biomass and soil.The present study evaluated variations of soil carbon stock in semi-arid forests in India under diff erent moisture regimes.Soil organic carbon(SOC)and soil inorganic carbon(SIC)stocks were determined in diff erent moisture regimes i.e.monsoon,post-monsoon,winter and pre-monsoon seasons at 0–10 and>10–20 cm depths.SOC stock showed signifi cant variations under different moisture regimes.The highest SOC stock was during winter(22.81 Mg C ha−1)and lowest during the monsoon season(2.34 Mg C ha−1)among all the ridge forests under study.SOC and SIC stock under diff erent moisture regimes showed signifi cant negative correlation with soil moisture(p<0.05),as a sudden increase in soil moisture after rainfall results in an increase in carbon loss due to microbial decomposition of accumulated carbon during the dry period.There was an increase in annual SOC stock and a decrease(or no change in some cases),in SIC stock at both the depths during the study period.The SOC and SIC sequestration rates were estimated as any increase/decrease in the respective stock during each successive year.SOC sequestered ranged between 0.046 and 0.741 Mg C ha−1 y−1.Similarly,SIC sequestration ranged between 0.013 and 0.023 Mg C ha−1 y−1 over all ridge forests up to 20 cm depth.The Delhi ridge forests,which accounts to 0.007%of the semi-arid regions of India,contribute 0.25–0.32%of the national potential(semi-arid region)for SOC sequestration up to 20 cm depth.The estimates of the rate of C sequestration in this study provide a realistic image of carbon dynamics under present climatic conditions of semi-arid forests,and could be used in developing a database and formulating new strategies for carbon dioxide mitigation by enhancing soil C sequestration rates.

Effects of afforestation on soil carbon and its fractions:a case study from the Loess Plateau,China

Afforestation has been implemented to reduce soil erosion and improve the environment of the Loess Plateau,China.Although it increased soil organic carbon（SOC）,the stability of the increase is unknown.Additionally,the variations of soil inorganic carbon（SIC） following afforestation needs to be reconfirmed.After planting Robinia pseudoacacia,Pinus tabuliformis,and Hippophae rhamnoides on bare land on the Loess Plateau,total soil carbon（TSC） was measured and its two components,SIC and SOC,as well as the light and heavy fractions within SOC under bare lands and woodlands at the soil surface（0–20 cm）.The results show that TSC on bare land was 24.5 Mg ha^（-1） and significantly increased to 51.6 Mg ha^（-1） for R.pseudoacacia,47.0 Mg ha^（-1） for P.tabuliformis and 39.9 Mg ha^（-1） for H.rhamnoides.The accumulated total soil carbon under R.pseudoacacia,P.tabuliformis,and H.rhamnoides,the heavy fraction（HFSOC） accounted for 65.2,31.7 and 76.2%,respectively; the light fraction（LF-SOC） accounted for 18.0,52.0 and 4.0%,respectively; SIC occupied 15.6,15.3 and 19.7%,respectively.The accumulation rates of TSC under R.pseudoacacia,P.tabuliformis,and H.rhamnoides reached159.5,112.4 and 102.5 g m^（-2） a^（-1）,respectively.The results demonstrate that afforestation on bare land has high potential for soil carbon accumulation on the Loess Plateau.Among the newly sequestrated total soil carbon,the heavy fraction（HF-SOC） with a slow turnover rate accounted for a considerably high percentage,suggesting that significant sequestrated carbon can be stored in soils following afforestation.Furthermore,afforestation induces SIC sequestration.Although its contribution to TSC accumulation was less than SOC,overlooking it may substantially underestimate the capacity of carbon sequestration after afforestation on the Loess Plateau.

Microbial residues as the nexus transforming inorganic carbon to organic carbon in coastal saline soils

Soil inorganic carbon(SIC),including mainly carbonate,is a key component of terrestrial soil C pool.Autotrophic microorganisms can assimilate carbonate as the main or unique C source,how microorganisms convert SIC to soil organic carbon(SOC)remains unclear.A systematic field survey(n=94)was performed to evaluate the shift in soil C components(i.e.,SIC,SOC,and microbial residues)along a natural salinity gradient(ranging from 0.5‰to 19‰),and further to explore how microbial necromass as an indicator converting SIC into SOC in the Yellow River delta.We observed that SIC levels linearly decreased with increasing salinity,ranging from~12 g kg^(-1)(salinity<6‰)to~10 g kg^(-1)(salinity>6‰).Additionally,the concentrations of SOC and microbial residues exponentially decreased from salinity<6‰ to salinity>6‰,with the decline of 39%and 70%,respectively.Microbial residues and SOC was tightly related to the variations in SIC.The structural equation model showed the causality on explanation of SOC variations with SIC through microbial residues,which can contribute 89% of the variance in SOC storage combined with SIC.Taken together,these two statistical analyses can support that microbial residues can serve as an indicator of SIC transition to SOC.This study highlights the regulation of microbial residues in SIC cycling,enhancing the role of SIC playing in C biogeochemical cycles and enriching organic C reservoirs in coastal saline soils.

Distribution and storage of soil organic and inorganic carbon under different ecological zones in Xinjiang,China

The objective of this study was to quantify both soil organic carbon(SOC)and soil inorganic carbon(SIC)stocks in different ecological zones in Xinjiang Region,the largest arid and semi-arid region in northwest China.The specific focus was on the vertical distributions of 641 typical soil profiles(0-100 cm).The study region covered five ecological zones:I-Altai/west Junggar;II-Junggar basin;III-Tianshan mountain;IV-Tarim basin;and V-Kunlun-Altun mountains.The zones are categorized by their specific geographical locations from north to south with terrains derived from mountains to basins.The data used in the study were obtained from the first(1960s)and the second(1980s)National Soil Surveys and partially from the field survey of this study conducted in 2013.The results suggest that there are 11.74 Pg SOC and 26.71 Pg SIC total stocks in the 0-100 cm surface soil over the entire study region.The distributions of SOC and SIC were found to be non-uniform.The Tianshan mountain zone has the highest SOC stock,followed by the Tarim basin,Kunlun-Altun mountains,Altai and west Junggar(Altai/west Junggar),and Junggar basin zones.In contrast,the Tarim basin zone had the highest SIC stock,followed by the Tianshan mountain,Kunlun-Altun mountains,Junggar basin,and Altai/west Junggar zones.The SOC content decreases gradually from northwest to southeast and from mountains to deserts;while the SIC content decreases gradually from south to north.The SOC and SIC contents also change with soil depth.Within a given ecological zone,the SOC content increased with increasing soil depth,peaked at about 20-40 cm,then it decreased with the bigger depths below 40 cm.The SIC contents increased gradually from 0 to 40 cm,and then decreased gradually with increasing soil depth over the 40-100 cm depth in all ecological zones except for the Tianshan mountain area.

Influencing factors and partitioning methods of carbonate contribution to CO_(2)emissions from calcareous soils

In calcareous soils,recent studies have shown that soil-derived CO_(2)originates from both soil organic carbon(SOC)decomposition and soil inorganic carbon(SIC)dissolution,a fact often ignored in earlier studies.This may lead to overestimation of the CO_(2)emissions from SOC decomposition.In calcareous soils,there is a chemical balance between precipitation and dissolution of CaCO_(3)-CO_(2)-HCO_(3),which is affected by soil environmental factors(moisture,temperature,pH and depth),root growth(rhizosphere effect)and agricultural measures(organic materials input,nitrogen fertilization and straw removal).In this paper,we first introduced the contribution of SIC dissolution to CO_(2)emissions from calcareous soils and their driving factors.Second,we reviewed the methods to distinguish two CO_(2)sources released from calcareous soils and quantify the 13C fractionation coefficient between SIC and SIC-derived CO_(2)and between SOC and SOC-derived CO_(2),and to partition three CO_(2)sources released from soils with plants and organic materials input.Finally,we proposed methods for accurately distinguishing three CO_(2)sources released from calcareous soils.This review helps to improve the accuracy of soil C balance assessment in calcareous soils,and also proposes the direction of further investigations on SIC-derived CO_(2)emissions responses to abiotic factors and agricultural measures.

Spatial Variation of Soil Inorganic Carbon Reserves of Typical Estuarine Wetlands in Jiaozhou Bay, China

The storage of inorganic carbon in estuarine wetlands is of great significance for mitigating global warming. The Dagu River estuary and Yanghe River estuary of Jiaozhou Bay were selected as sampling areas, and data analysis was carried out by Duncan method to explore the distribution characteristics and influencing factors of soil inorganic carbon(SIC) reserves. The results showed that increasing distance from the estuary led to higher reserves in the mudflat along the coastal zone. The scouring action of seawater bodies was the main factor driving this distribution. In the vertical section, the SIC reserves in 40–60 cm depth were relatively high, accounting for 34.11% of the 0–60 cm soil depth, and resulting from the transport of water and salt in seawater. In the river flat along the vertical coastal zone, the SIC reserves first decreased and then increased with increasing distance from the sea, and the SIC reserves in 0–20 cm depth were relatively high in the vertical section, accounting for 38.18% of the 0–60 cm soil depth. These reserves were affected by synergetic factors such as oceanic factors and anthropogenic activities. The invasion of Spartina alterniflora decreased the SIC reserves of wetlands, mainly due to its root transformation and the differences of growth characteristics and years being the main reasons for the observed decreases. Aquaculture activities changed the physical and chemical properties of the soil in aquaculture ponds, and consequently changed the distribution of SIC reserves.

Spatial Distribution and Temporal Dynamics of Soil Carbon Removal Caused by Water Erosion in China

Using water erosion data from three national soil erosion remote-sensing surveys （the first：1985-1986; the second：1995-1996; the third： 2000-2001） and carbon density data from the second national soil survey （1979-1992）, we computed soil carbon removal caused by water erosion, and analyzed its spatial distribution and temporal dynamics in China. Results revealed that the total removal of soil carbon caused by water erosion was about 74.61 Tg C y^-1 , of which 51.49 Tg C y^-1 was organic carbon, and 23.12 Tg C y^-1 was inorganic carbon. The main erosion level of the whole is moderate erosion. Among the seven erosion regions, the Southwest Karst Region had the most significant removal of soil organic carbon, which was 26.48% of the total and mainly due to its moderate erosion. In contrast, about 67.62% of the soil inorganic carbon removal occurred in the Loess Plateau Region, which mainly due to its highly intense and intense erosion. As a whole, the removals of soil carbon caused by water erosion represented a decreasing trend among the three national soil erosion remote-sensing surveys. Between the first and the second survey, soil carbon removal decreased by 11.66 Tg C y^-1 , of which 81.93% was organic carbon. Compared with that in the second survey, soil carbon removal decreased by 1.65 Tg C y^-1 in the third survey, of which 1.514 Tg C y^-1 was organic carbon, and 0.134 Tg C y^-1 was inorganic carbon.

Climatic and Edaphic Controls on Soil pH in Alpine Grasslands on the Tibetan Plateau,China:A Quantitative Analysis

Soil acidity is an important parameter that can regulate ecosystem structure and function.However,a quantitative understanding of the relationships between soil pH and environmental factors remains unavailable.In this study,relationships of soil pH with both climatic and edaphic factors in alpine grasslands on the Tibetan Plateau,China were quantified using data obtained from a regional soil survey during 2001-2004.Our results showed that soil pH decreased along the gradient of both mean annual temperature and precipitation.Likewise,soil pH exhibited consistent negative correlations with soil moisture and silt content.However,soil organic and inorganic carbon contents played opposite roles in shaping patterns of soil pH:the accumulation of soil organic matter led to higher soil acidity,while the existence of soil inorganic matter was favorable for maintaining higher soil alkalinity.The variation partitioning analysis indicated that the combination of climatic and edaphic variables explained 74.3%of the variation in soil acidity.These results suggest that soil pH could be predicted from routinely-measured variables,allowing a robust pedotransfer function to be developed.The pedotransfer function may facilitate land surface models to generate more reliable predictions on ecosystem structure and function around the world.

Effects of nitrogen addition on carbonate-derived CO_(2) emission after biochar addition

●We studied the effect of nitrogen and biochar on CO_(2) emission from SOC and SIC.●Nitrogen increased SIC-derived CO_(2) by 41%but decreased SOC-derived CO_(2) by 20%.●Biochar reduced total soil-derived CO_(2) by neutralizing nitrogen-induced acidity.●We proposed a method for 3-or 4-source partitioning CO_(2) emission from calcareous soils.Biochar addition generally increases the alkalinity regeneration to resist soil acidification driven by nitrogen(N)fertilization.Calcareous soils contain soil organic carbon(SOC)and inorganic C(SIC).Owing to technical limitations in three-source partitioning CO_(2),how biochar addition affects SOC-and SIC-derived CO_(2) emission has not been clarified yet.Therefore,we conducted a 70-day incubation experiment of ammonium-N and maize-straw-derived biochar additions to investigate the N plus biochar impacts on SOC-and SIC-derived CO_(2) emission.Over the 70-day incubation,we found that the N-only addition increased the SIC-derived CO_(2) emission by approximately 41%compared with the control,but decreased the SOC-derived CO_(2) emission by approximately 20%.This suggests that the distinct responses of SIC-and SOC-derived CO_(2) emission to N-only addition come from N-induced acidification and preferential substrate(N)utilization of soil microorganisms,respectively.Compared with N-only addition,N plus biochar addition decreased the SIC-derived CO_(2) emission by 17%−20%during the first 20 days of incubation,but increased it by 54%during the next 50 days.This result suggested that biochar addition reduced the SIC-derived CO_(2) emission likely due to the alkalization capacity of biochar exceeding the acidification capacity of ammonium-N in the short term,but it may increase the SIC-derived CO_(2) emission induced by the weak acidity produced from biochar mineralization in the long term.This study is helpful to improve the quantification of CO_(2) emission from calcareous soils.