Distribution Characteristics of Soil Organic Carbon in Degraded Forest Land in the Sandstorm Area of Jingbian County,Shaanxi Province

[Objectives]To explore the distribution characteristics of soil organic carbon in degraded forest land in the sandstorm area of Jingbian County,Shaanxi Province.[Methods]The distribution characteristics and abundance of 0-20 cm shallow soil organic carbon in 5 towns in the sandstorm area in the north of Jingbian County were studied by field sampling and indoor detection.[Results]The average soil organic carbon contents in Hongdunjie Town,Haizetan Town,Huanghaojie Town,Ningtiaoliang Town and Dongkeng Town were 2.93,3.21,2.53,2.54 and 4.08 g/kg,respectively,which were all lower than the national background value(31.00 g/kg).The coefficients of variation of soil organic carbon content in Hongdunjie Town,Huanghaojie Town and Dongkeng Town were 59.04%,35.97%and 47.55%,respectively,with higher coefficients of variation and larger differences in spatial distribution.The organic carbon content of Haizetan Town and Dongkeng Town was above the abundance,accounting for 70%and 50%,which were relatively rich,while the soil organic carbon content of Hongdunjie was relatively scarce.The average content of soil organic carbon in the sandstorm area was 3.03 g/kg,which was also lower than the national background value.The coefficient of variation was 46.53%,showing high coefficient of variation and large difference in spatial distribution.In addition,20.41%of the average content of soil organic carbon in the sandstorm area was in the deficient level,and 79.59%were in the medium or above level.[Conclusions]The study of distribution characteristics of soil organic carbon in degraded forest land in the sandstorm area of Jingbian County will better serve the precise management of soil resources.

Carbon inputs regulate the temperature sensitivity of soil respiration in temperate forests

Litter and root activities may alter the temperature sensitivity(Q_(10))of soil respiration.However,existing studies have not provided a comprehensive understanding of the effects of litter and root carbon inputs on the Q_(10)of soil respiration in different seasons.In this study,we used the trench method under in situ conditions to measure the total soil respiration(R_(total)),litter-removed soil respiration(R_(no-litter)),root-removed soil respiration(R_(no-root)),and the decomposition of soil organic matter(i.e.,both litter and root removal;R_(SOM))in different seasons of pioneer(Populus davidiana Dode)and climax(Quercus liaotungensis Mary)forests on the Loess Plateau,China.Soil temperature,soil moisture,litter biomass,fine root biomass,litter carbon,and root carbon were analyzed to obtain the drive mechanism of the Q_(10)of soil respiration in the two forests.The results showed that the Q_(10)of soil respiration exhibited seasonality,and the Q_(10)of soil respiration was higher in summer.The litter enhanced the Q_(10)of soil respiration considerably more than the root did.Soil temperature,soil moisture,fine root biomass,and litter carbon were the main factors used to predict the Q_(10)of different soil respiration components.These findings indicated that factors affecting the Q_(10)of soil respiration highly depended on soil temperature and soil moisture as well as related litter and root traits in the two forests,which can improve our understanding of soil carbon–climate feedback in global warming.The results of this study can provide reference for exploring soil respiration under temperate forest restoration.

Spatial and seasonal variation in soil respiration along a slope in a rubber plantation and a natural forest in Xishuangbanna, Southwest China

Soil respiration is a key component of the global carbon cycle, and even small changes in soil respiration rates could result in significant changes in atmospheric CO_2 levels. The conversion of tropical forests to rubber plantations in SE Asia is increasingly common, and there is a need to understand the impacts of this land-use change on soil respiration in order to revise CO_2 budget calculations. This study focused on the spatial variability of soil respiration along a slope in a natural tropical rainforest and a terraced rubber plantation in Xishuangbanna, Southwest(SW) China. In each land-use type, we inserted 105 collars for soil respiration measurements.Research was conducted over one year in Xishuangbanna during May, June, July and October 2015(wet season) and January and March 2016(dry season). The mean annual soil respiration rate was 30% higher in natural forest than in rubber plantation and mean fluxes in the wet and dry season were 15.1 and 9.5 Mg C ha^(-1) yr^(-1) in natural forest and 11.7 and 5.7 Mg C ha^(-1) yr^(-1) in rubber plantation. Using a linear mixedeffects model to assess the effect of changes in soil temperature and moisture on soil respiration, we found that soil temperature was the main driver of variation in soil respiration, explaining 48% of its seasonal variation in rubber plantation and 30% in natural forest. After including soil moisture, the model explained 70% of the variation in soil respiration in natural forest and 76% in rubber plantation. In the natural forest slope position had a significant effect on soil respiration, and soil temperature and soil moisture gradients only partly explained this correlation. In contrast, soil respiration in rubber plantation was not affected by slope position, which may be due to the terrace structure that resulted in more homogeneous environmental conditions along the slope. Further research is needed to determine whether or not these findings hold true at a landscape level.

CO flux of soil respiration in natural recovering karst abandoned farmland in Southwest China

To estimate carbon sequestration potential in the karst area,soil respiration in a natural recovering karst abandoned farmland in Shawan,Puding,Guizhou,southwest China was continuously and automatically monitored for more than two years.The results show that the CO2flux of soil respiration(2.63±1.89 lmol m^-2s-^1)is higher in the karst area than in non-karst areas under similar conditions but that regional value(1.32 lmol m-2s-1)is lower because of larger rock fragment coverage(~50%).A the same time,the temperature sensitivity of soil respiration(Q10)in this study area is significantly higher than that of non-karst areas under similar conditions.Soil respiration has an obvious temporal variation,which is reflected in a significant exponential relationship between soil respiration and soil temperature,but the relationship between soil respiration and soil moisture is very complex.Especially soil respiration has an obvious spatial variation,which is likely affected by different diffusion or water-rock reaction processes.

Seasonality and moisture regime control soil respiration, enzyme activities, and soil microbial biomass carbon in a semi-arid forest of Delhi, India

Soil respiration,soil enzymes,and microbial biomass are important in carbon cycling in the terrestrial ecosystem which is generally limited by environmental factors and soil carbon availability.Hence,we tried to assess the factors affecting the functional aspects of these processes in a semi-arid climate.We monitored soil respiration(surface)using a portable infrared gas analyzer(Q-Box SR1LP Soil Respiration Package,Qubit Systems,Canada)equipped with a soil respiration chamber(Model:G 180).Soil respiration was measured at midday during each season throughout the study period.Soil enzymatic activities and microbial biomass carbon(MBC)were analyzed following the standard protocol for a year during peak time in four seasons at 0-10 cm and 10-20 cm depth.Soil respiration shows significant variation with highest in monsoon(3.31μmol CO2 m−2 s^(−1))and lowest in winter(0.57μmol CO2 m^(−2) s^(−1)).Similarly,β-glucosidase,dehydrogenase,and phenol oxidase activity ranged from 11.15 to 212.59μg PNP g^(−1) DW h^(−1),0.11 to 16.47μg TPF g^(−1) DW h^(−1),and 4102.95 to 10187.55μmol ABTS+g^(−1) DW min^(−1),respectively.MBC ranged from 17.08 to 484.5μgCg^(−1).Besides,soil respiration,soil enzymes(exceptβ-glucosidase),and MBC were significantly correlated with soil moisture.Seasonality,optimum moisture and temperature played a significant role in determining variations in soil microbiological processes(exceptβ-glucosidase activity);the carbon cycling in the study area is assisted by enzyme activity;dehydrogenase and phenol oxidase played a significant role in soil respiration;hence,this landscape is sensitive to environmental changes.

Changes of Organic Carbon in Soil under Different Land Use Patterns in Alpine Agricultural Region of Qinghai

Using organic carbon density grouping method,the change trends of soil total organic carbon(SOC),light fraction content and light fraction organic carbon under 4 land use patterns of returning cultivated land to forest(cropland,artificial forest,inter-cropping of forest and grassland and original sample plot)in alpine agricultural region of Qinghai were studied.The content of SOC was in order:intercropping of forest and grassland > original sample plot > artificial forest > cropland.There was signi...

Land Use Effects on Soil Organic Carbon, Microbial Biomass and Microbial Activity in Changbai Mountains of Northeast China

Land use changes are known to alter soil organic carbon （SOC） and microbial properties, however, information about how conversion of natural forest to agricultural land use as well as plantations affects SOC and microbial properties in the Changbai Moun- tains of Northeast China is meager. Soil carbon content, microbial biomass carbon （MBC）, basal respiration and soil carbon mineraliza- tion were studied in five selected types of land use： natural old-growth broad-leaved Korean pine mixed forest （NF）; spruce plantation （SP） established following clear-cutting of NF; cropland （CL）; ginseng farmland （GF） previously under NF; and a five-year Mongolian oak young forest （YF） reforested on an abandoned GF, in the Changbai Mountains of Northeast China in 2011. Results showed that SOC content was significantly lower in SP, CL, GF, and YF than in NF. MBC ranged from 304.4 mg/kg in CL to 1350.3 mg/kg in NF, which was significantly higher in the soil of NF than any soil of the other four land use types. The SOC and MBC contents were higher in SP soil than in CL, GF, and YF soils, yielding a significant difference between SP and CL. The value of basal respiration was also higher in NF than in SP, CL, GF, and YF. Simultaneously, higher values of the metabolic quotient were detected in CL, GF, and YF soils, indicat- ing low substrate utilization of the soil microbial community compared with that in NF and SP soil. The values of cumulative mineral- ized carbon and potentially mineralized carbon （Co） in NF were significantly higher than those in CL and GF, while no significant dif- ference was observed between NF and SP. In addition, YF had higher values of Co and C mineralization rate compared with GF. The results indicate that conversion from NF into agricultural land （CL and GF） uses and plantation may lead to a reduction in soil nutrients （SOC and MBC） and substrate utilization efficiency of the microbial community. By contrast, soils below SP were more conducive to the preservation of soil organic matter, which was reflected in the comparison of microbial indicators among CL, GF, and YF land uses. This study can provide data for evaluating soils nutrients under different land use types, and serve as references for the rational land use of natural forest in the study area.

Estimation of soil organic carbon stock and its controlling factors in cropland of Yunnan Province,China

Soil organic carbon(SOC)is the most important indicators of soil quality and health.Identifying the spatial distribution of SOC and its influencing factors in cropland is crucial to understand the terrestrial carbon cycle and optimize agronomic management.Yunnan Province,characterized by mountainous topography and varied elevation,is one of the highest SOC regions in China.Yet its SOC stock of cropland and influencing factors has not been fully studied due to the lack of adequate soil investigation.In this study,the digital mapping of SOC at 1 km resolution and the estimation of total SOC stock in cropland of Yunnan Province was undertaken using 8637 topsoil(0-20 cm)samples and a series of spatial data through Random Forest(RF)model.It was showed that across the cropland of Yunnan Province,the mean SOC density and total stock were 4.84 kg m^(-2) and 337.5 Mt,respectively.The spatial distribution indicated that relatively high SOC density regions resided in the northwest and northeast parts of Yunnan Province.Elevation(19.5%),temperature(17.3%),rainfall(14.5%),and Topographic wetness index(9.9%)were the most important factors which controlled spatial variability of SOC density.Agronomic practices(e.g.,crop straw treatments,fertilizer management)should be optimized for the sustainable development of crop production with high SOC sequestration capacity in Yunnan Province.

The Effect of Land Use on Soil Organic Carbon Stocks in Lake Victoria Crescent Agro-Ecological Zone, Uganda

The effect of land use on soil organic carbon （SOC） stocks and depth distribution of SOC was investigated in the Lake Victoria Crescent ago-ecological zone of Uganda. Soil samples were collected from six land use types at 0-30, 30-60 and 60-90 cm from profile pits dug in similar soils and slopes. Results indicated that SOC stocks significantly differed across the various land use systems. SOC also varied significantly by depth. The highest SOC and pH were recorded under natural forest-strict nature. Grassland had the lowest SOC but the highest bulk density （BD）. Phosphorous （P） was the highest in banana-coffee systems and the lowest under tea plantations. The lowest values of pH and BD were found in highly disturbed natural forest. The upper layers of the soil （0-30 cm） stored higher amounts of SOC compared to other depths （30-60 cm and 60-90 cm）. Land use therefore has a significant effect on SOC and other soil physical and chemical properties.

Carbon and nitrogen mineralization in soil of leguminous trees in a degraded pasture in northern Rio de Janeiro, Brazil

Use of legume trees can improve soil quality in degraded pastures. The aim of this study was to charac- terize C and N mineralization kinetics and estimate the potentially mineralizable C and N in soil under Mimosa caesalpiniifolia Benth. and Acacia auriculiformis A. Cunn. ex Benth. secondary forest and pasture in red-yellow latosols in southeast Brazil. We conducted a laboratory aerobic incubation experiment using a completely ran- domized design of four replicates and four types of plant cover using a modified version of the Stanford and Smith technique （1972） to study C and N mineralization potential. Potentially mineralizable N （No） ranged from 135 to 170 mg kg-1. The predominant form of mineral N for all types of plant cover was N-NO3-. M. caesalpiniifolia was the only species that had a positive influence on N min- eralization. Neither of the legumes influenced C mineral- ization in pasture or secondary forest. The model of N mineralization corresponded to a sigmoidal curve while C mineralization corresponded to an exponential curve, revealing that the N and C mineralization processes were distinct. N mineralized by M. caesalpiniifolia （216 kg ofN ha-1） was adequate to meet the N requirement for a livestock-forest system.

Research Advance on Carbon Storage of Artificial Grassland in China

As an essential part of the grassland ecological system,study on the carbon storage has great significances to the carbon reduction in grassland ecological system.The carbon storage in biomass,carbon storage in soil and soil respiration are summarized in this paper to provide scientific reference for the evaluation of carbon storage in artificial grassland.

Estimating Ecological Characteristics and Carbon Stock in Uneven-Aged Plantations of Acacia senegal L. in the Savannah Woodlands of Sudan

The aim of this study was to assess Acacia senegal trees’ characteristics as well as evaluate the carbon stock under a variety of ages in the El Demokeya forest in Sudan, where the Gum Arabic belt is located. 12 sample plots, in 2021 were randomly distributed to represent the entire area of the forest prior to the required measurements. The sample was designed as squire plots with one hectare. In each sample plot, all trees were counted, their height (m), and Diameters Breast Height (DBH in cm), respectively. The results showed the highest number of trees per ha at age 20 years old and the lowest number at age 47 years, while the highest values of DBH and volume were found at age 47 years old. As a result, the maximum and minimum values of the aboveground biomass were found in the age 47 years old and 16 years, accounting for 19.87 tons and 1.9 tons respectively. Thus, the amount of carbon stock was 11.92 tons/ha in the 35-years-old and 1.19 tons/ha in the 21-year stands. Furthermore, the average carbon stock in all plots was estimated as 18.70 tons/ha and hence the total carbon stock in the El Demokeya forest is equal to 620.11 tons. Conclusively, the characteristics of trees, amount of aboveground biomass and carbon stock in the El Demokeya forest varied among the uneven-aged plantation groups. The study recommends and encourages the protection of A. senegal in order to increase the carbon sink as well as protect the environment in the era of climatic changes.

Effect of vegetation type, wetting intensity, and nitrogen supply on external carbon stimulated heterotrophic respiration and microbial biomass carbon in forest soils

By using packed soil-core incubation experiments, we have studied stimulating effects of addition of external carbon （C） （glu- cose, 6.4 g C m 2） on heterotrophic respiration and microbial biomass C of a mature broadleaf and Korean pine mixed forest （BKPF） and an adjacent white birch forest （WBF） soil under different wetting intensities （55% and 80% WFPS, water-filled pore space） and nitrogen （N） supply （NH4C1 and KNO3, 4.5 g N m-e） conditions. The results showed that for the control, the cumulative carbon dioxide （CO2） flux from WBF soil during the 15-day incubation ranged from 5.44 to 5.82 g CO2-C m-2, which was significantly larger than that from BKPF soil （2.86 to 3.36 g CO2-C m 2）. With increasing wetting intensity, the cumulative CO2 flux from the control was decreased for the WBF soil, whereas an increase in the CO2 flux was observed in the BKPF soil （P 〈 0.05）. The addition of NH4C1 or KNO3 alone significantly reduced the cumulative CO2 fluxes by 9.2%-21.6 % from the two soils, especially from WBF soil at low wetting intensity. The addition of glucose alone significantly increased soil heterotrophic respiration, microbial biomass C （MBC）, and microbial metabolic quotient. The glucose-induced cumulative CO2 fluxes and soil MBC during the incubation ranged from 8.7 to 11.7 g CO2-C m-2 and from 7.4 to 23.9 g C m-2, which are larger than the dose of added C. Hence, the addition of external carbon can increase the decomposition of soil native organic C. The glucose-induced average and maximum rates of CO2 fluxes during the incubation were significantly in- fluenced by wetting intensity （WI） and vegetation type （VT）, and by WIxVT, NH4ClxVT and WIxVTxNH4C1 （P〈0.05）. The addition of NH4C1, instead of KNO3, significantly decreased the glucose-induced MBC of WBF soil （P〈0.05）, whereas adding NH4C1 and KNO3 both significantly increased the glucose-induced MBC of BKPF soil at high moisture （P〈0.05）. According to the differences in soil labile C pools, MBC and CO2 fluxes in the presence and absence of glucose, it can be concluded that the stimulating effects of glucose on soil heterotrophic respiration and MBC under temperate forests were dependent on vegetation type, soil moisture, and amount and type of the N added.

Calibration of CO₂Trapping in Alkaline Solutions during Soil Incubation at Varying Temperatures Using a Respicond VI

Measurements of carbon dioxide (CO2)-evolution from soils are important in evaluating biomass and activity of soil microorganisms, as well as decomposition of soil organic matter. The Respicond VI is a fully computerized system allowing continuous measurement of CO2 evolution in short- and long-term soil incubation experiments in up to 96 incubation vessels. The measurement of CO2 evolution is based on the absorption of CO2 by an electrolyte (KOH solution) producing a change in the cell conductance measured using two electrodes. In this study, the Respicond VI was recalibrated yielding 174.5 mg CO2 as constant A expressing the theoretical maximum amount of CO2 absorbed in 10 ml 0.5 M KOH. This value of A corresponds to 34.9 mg CO2 ml-1 1 M KOH. The constant A does neither depend on the investigated incubation temperatures (5°C - 25°C) nor on the concentrations of the KOH solutions (0.5, 0.1, 0.05 M KOH). To eliminate any influence of changing incubation temperatures, either induced by uncertainties in temperature control or as a part of the experimental setup, on the conductance of KOH solution, a correction procedure was developed using a factor calculated from changing conductance of KOH solutions in incubation vessels without soil.

CARBON CYCLE OF MARSH IN THE SANJIANG PLAIN

The Sanjinag Plain of China is a low plain. Its total area is 10. 89 × 104 km2 and marsh area takes up 10. 20%. Marsh is a kind of vegetation type with strong carbon-fixing ability among the terrestrial areystam and carbon cycle performing in the form of carbon dioxide. This paper discusses the fixation of atmospheric CO2 by marsh plant; the transfer of carbon from marsh to atmosphere; the change of CO2 and vertical transfer of CO2 near atmosphere of the earth; the carbon floW among marsh plants, soil and atmosphere. Some scientific data about the biological production, carbon content of marsh plants and the capacity for releasing and fixing are used to explain the carbon cycle of marsh in the Sanjiang Plain.

有机和常规管理对茶园土壤固碳的影响——以林地为对照

为探究有机和常规管理方式对茶园土壤有机碳的影响,选择云南省普洱市思茅区常规管理茶园、有机管理茶园和附近自然林地3种典型土地利用类型,通过测定0~20 cm和20~40 cm土层的土壤有机碳(SOC)、易氧化有机碳(EOC)、非活性有机碳(NLOC)、颗粒态有机碳(POC)和矿物结合态有机碳(MOC)含量,计算土壤各组分有机碳的分配比例以及土壤碳库管理指数(CPMI),研究3种土地利用方式下土壤有机碳各组分含量和质量的变化特征。结果显示:1)常规管理茶园的SOC含量和储量分别比自然林地低48.67%~51.94%和27.25%~35.71%(P<0.05),而有机管理茶园的SOC含量和储量比常规管理茶园分别高52.09%~62.86%、15.54%~20.26%(P<0.05)。2)常规管理茶园的EOC、NLOC、POC和MOC含量均低于自然林地(P<0.05),而有机管理茶园的EOC、NLOC、POC和MOC含量比常规管理茶园分别高出46.39%~57.89%、54.24%~66.15%、80.87%~121.01%和40.07%~46.28%(P<0.05)。3)与自然林地相比,常规管理茶园的POC/SOC、NLOC/SOC较低,有机管理茶园的POC/SOC、NLOC/SOC则高于常规管理茶园。4)常规管理茶园具有较高的CPAI和较低的CPMI,常规管理茶园的CPMI比自然林地低24.53%~46.12%,有机管理茶园的CPMI比常规管理茶园高67.88%~100.33%,其差异均显著(P<0.05)。以上研究结果表明,与自然林地相比,常规管理的茶园土壤有机碳含量和土壤碳库质量下降,存在一定程度的土地退化,而有机管理是提高茶园土壤碳库质量的有效措施。

间伐对秦岭松栎混交林土壤异养呼吸的影响

【目的】研究间伐对秦岭松栎混交林土壤异养呼吸的影响,可深入了解该地区间伐对森林土壤异养呼吸造成的碳损失情况,为研究区森林经营碳汇提供科学依据。【方法】以秦岭松栎混交林为研究对象,对未间伐样地(ck)、间伐4 a、间伐12 a的样地采用静态箱-气相色谱法监测生长季土壤异养呼吸速率的月变化,并计算各处理异养呼吸的温度敏感性(Q_(10))。【结果】①相比ck,间伐4 a、间伐12 a的样地土壤微生物生物量碳均显著下降(P<0.05),土壤pH均显著上升(P<0.05),土壤总有机碳质量分数在间伐4 a的样地中显著下降(P<0.05)。间伐12 a样地的理化指标相比间伐4 a的样地更接近ck。②生长季土壤异养呼吸呈现“双峰型”变化规律,峰值分别出现在6和10月。异养呼吸累计通量在间伐后略有上升但不显著,不同样地从大到小依次为间伐12 a、间伐4 a、ck。③土壤温度与异养呼吸呈极显著指数正相关(P<0.001);间伐后Q_(10)降低,不同样地Q_(10)从大到小依次为ck、间伐12 a、间伐4 a。【结论】土壤温度是影响松栎混交林异养呼吸的关键因素。间伐没有导致秦岭松栎混交林异养呼吸通量显著增大。

连续翻压黑麦草对豫中植烟土壤剖面CO_(2)日排放变化的影响

【目的】探究豫中烟区翻压黑麦草对烤烟生长过程中土壤呼吸速率、土壤剖面二氧化碳(CO_(2))日排放变化的影响及其与环境因子的关系,明确CO_(2)采集最佳观测时间。【方法】2021年在河南农业大学郑州科教园区设置2个处理(依托于长期定位试验),施氮磷钾肥(NPK)、氮磷钾+种植并翻压黑麦草(NPKG),烟苗移栽后每隔30d测定烟田CO_(2)排放日变化过程。【结果】(1)0~10 cm地温与土壤呼吸速率均表现为昼高夜低的单峰或双峰形态。NPKG处理土壤呼吸速率显著高于NPK处理,9:00和21:00矫正系数最接近1且与日平均排放无显著差异。(2)随着土层加深,CO_(2)浓度升高,CO_(2)扩散通量降低,同层土壤NPKG处理CO_(2)浓度和扩散通量高于NPK处理。0、5、10和20 cm土层CO_(2)浓度和0~20 cm土层扩散通量有明显日变化特征,90 d时CO_(2)浓度较60 d降低。(3)土壤呼吸速率与0~20 cm土层CO_(2)扩散通量呈显著或极显著正相关关系。【结论】黑麦草翻压加快土壤呼吸速率、提高了各土层CO_(2)浓度和CO_(2)扩散通量。9:00左右和21:00左右为最佳取样时间段,5 cm和10 cm土层CO_(2)浓度和5~10 cm土层扩散通量与土壤呼吸速率日变化特征相似,3个取样日CO_(2)浓度以60 d最高,30 d次之。可根据土壤CO_(2)浓度预测土壤呼吸速率。

Impact of land use conversion on soil organic carbon stocks in an agro-pastoral ecotone of Inner Mongolia

Soil organic carbon （SOC） stocks in terrestrial ecosystems vary considerably with land use types. Grassland, forest, and cropland coexist in the agro-pastoral ecotone of Inner Mongolia, China. Using SOC data compiled from literature and field investigations, this study compared SOC stocks and their vertical distributions among three types of ecosystems. The results indicate that grassland had the Largest SOC stock, which was 1.5- and 1.8-folds more than stocks in forest and cropland, respectively. Relative to the stock in 0-100 cm depth, grassland held more than 40% of its SOC stock in the upper 20 cm soil layer; forest and cropland both held over 30% of their respective SOC stocks in the upper 20 cm soil layer. SOC stocks in grazed grasslands were remarkably promoted after -〉20 years of grazing ex- clusion. Conservational cultivation substantially increased the SOC stocks in cropland, espe- cially in the 0-40 cm depth. Stand ages, tree species, and forest types did not have obvious impacts on forest SOC stocks in the study area likely due to the younger stand ages. Our study implies that soil carbon loss should be taken into account during the implementation of ecological projects, such as reclamation and afforestation, in the arid and semi-arid regions of China.

Effects of warming,wetting and nitrogen addition on substrate-induced respiration and temperature sensitivity of heterotrophic respiration in a temperate forest soil

Soil heterotrophic respiration and its temperature sensitivity are affected by various climatic and environmental factors.However,little is known about the combined effects of concurrent climatic and environmental changes,such as climatic warming,changing precipitation regimes,and increasing nitrogen(N)deposition.Therefore,in this study,we investigated the individual and combined effects of warming,wetting,and N addition on soil heterotrophic respiration and temperature sensitivity.We incubated soils collected from a temperate forest in South Korea for 60 d at two temperature levels(15 and 20℃,representing the annual mean temperature of the study site and 5℃warming,respectively),three moisture levels(10%,28%,and 50%water-filled pore space(WFPS),representing dry,moist,and wet conditions,respectively),and two N levels(without N and with N addition equivalent to 50 kg N ha^(-1)year^(-1)).On day 30,soils were distributed across five different temperatures(10,15,20,25,and 30℃)for 24 h to determine short-term changes in temperature sensitivity(Q_(10),change in respiration with 10℃increase in temperature)of soil heterotrophic respiration.After completing the incubation on day 60,we measured substrate-induced respiration(SIR)by adding six labile substrates to the three types of treatments.Wetting treatment(increase from 28%to 50%WFPS)reduced SIR by 40.8%(3.77 to 2.23μg CO_(2)-C g^(-1)h^(-1)),but warming(increase from 15 to 20℃)and N addition increased SIR by 47.7%(3.77 to 5.57μg CO_(2)-C g^(-1)h^(-1))and 42.0%(3.77 to 5.35μg CO_(2)-C g^(-1)h^(-1)),respectively.A combination of any two treatments did not affect SIR,but the combination of three treatments reduced SIR by 42.4%(3.70 to 2.20μg CO_(2)-C g^(-1)h^(-1)).Wetting treatment increased Q_(10)by 25.0%(2.4 to 3.0).However,warming and N addition reduced Q_(10)by 37.5%(2.4 to 1.5)and 16.7%(2.4 to 2.0),respectively.Warming coupled with wetting did not significantly change Q_(10),while warming coupled with N addition reduced Q_(10)by 33.3%(2.4 to 1.6).The combination of three treatments increased Q_(10)by 12.5%(2.4 to 2.7).Our results demonstrated that among the three factors,soil moisture is the most important one controlling SIR and Q_(10).The results suggest that the effect of warming on SIR and Q_(10)can be modified significantly by rainfall variability and elevated N availability.Therefore,this study emphasizes that concurrent climatic and environmental changes,such as increasing rainfall variability and N deposition,should be considered when predicting changes induced by warming in soil respiration and its temperature sensitivity.