Soil carbon stock change following afforestation in Northern Europe: a meta‐analysis

Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land‐use changes during the past decades. However, this region has not been well represented in previous large‐scale syntheses of land‐use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta‐analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0–10 cm and 0–20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land‐use, forest type, and soil textural class. Three major improvements were incorporated in the meta‐analysis: analysis of major interaction groups, evaluation of the influence of nonindependence between samples according to study design, and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0–10 cm layer. Afforestation of former grasslands had a small negative (nonsignificant) effect indicating limited SOC change following this land‐use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta‐estimates calculated for the periods <30 years and >30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta‐estimates in former land‐use, forest type, and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30‐year perspective.     

Storage and sequestration potential of topsoil organic carbon in China's paddy soils

Carbon (C) storage and sequestration in agricultural soils is considered to be an important issue in the study of terrestrial C cycling and global climatic change. The baseline C stock and the C sequestration potential are among the criteria for a region or a state to adopt strategies or policies in response to commitment to the Kyoto Protocol. Paddy soils represent a large portion of global cropland. However, little information on the potential of C sequestration and storage is available for such soils. In this paper, an estimation of the topsoil soil organic carbon (SOC) pool and the sequestration potential of paddy soils in China was made by using the data from the 2nd State Soil Survey carried out during 1979–1982 and from the nationwide arable soil monitoring system established since then. Results showed that the SOC density ranged from 12 to 226 t C ha^?1 with an area‐weighted mean density of 44 t C ha^?1, which is comparable to that of the US grasslands and is higher than that of the cultivated dryland soils in China and the US. The estimated total topsoil SOC pool is 1.3 Pg, with 0.85 Pg from the upper plow layer and 0.45 Pg from the plowpan layer. This pool size is ～2% of China's total storage in the top 1 m of the soil profiles and ～4% of the total topsoil pool, while the area percentage of paddy soil is 3.4% of the total land. The C pool in paddy soils was found predominantly in southeast China geographically and in the subgroups of Fe‐accumulating and Fe‐leaching paddy soils pedogenetically. In comparison with dryland cultivation, irrigation‐based rice cultivation in China has induced significant enrichment of SOC storage (0.3 Pg) in paddy soils. The induced total C sequestration equals half of China's total annual CO₂ emission in the 1990s. Estimates using different SOC sequestration scenarios show that the paddy soils of China have an easily attainable SOC sequestration potential of 0.7 Pg under present conditions and may ultimately sequester 3.0 Pg. Soil monitoring data showed that the current C sequestration rate is 12 Tg yr^?1. The total C sequestration potential and the current sequestration rate of the paddy soils are over 30%, while the area of the paddy soils is 26% that of China's total croplands. Therefore, practicing sustainable agriculture is urgently needed for enhancing SOC storage to realize the ultimate SOC sequestration of rice‐based agriculture of China, as the current C sequestration rate is significantly lower than the potential rate.     

Effects of long‐term straw return on soil organic carbon storage and sequestration rate in North China upland crops: A meta‐analysis

Soil organic carbon (SOC) is essential for soil fertility and climate change mitigation, and carbon can be sequestered in soil through proper soil management, including straw return. However, results of studies of long‐term straw return on SOC are contradictory and increasing SOC stocks in upland soils is challenging. This study of North China upland agricultural fields quantified the effects of several fertilizer and straw return treatments on SOC storage changes and crop yields, considering different cropping duration periods, soil types, and cropping systems to establish the relationships of SOC sequestration rates with initial SOC stocks and annual straw C inputs. Our meta‐analysis using long‐term field experiments showed that SOC stock responses to straw return were greater than that of mineral fertilizers alone. Black soils with higher initial SOC stocks also had lower SOC stock increases than did soils with lower initial SOC stocks (fluvo‐aquic and loessial soils) following applications of nitrogen‐phosphorous‐potassium (NPK) fertilizer and NPK+S (straw). Soil C stocks under the NPK and NPK+S treatments increased in the more‐than‐20‐year duration period, while significant SOC stock increases in the NP and NP+S treatment groups were limited to the 11‐ to 20‐year period. Annual crop productivity was higher in double‐cropped wheat and maize under all fertilization treatments, including control (no fertilization), than in the single‐crop systems (wheat or maize). Also, the annual soil sequestration rates and annual straw C inputs of the treatments with straw return (NP+S and NPK+S) were significantly positively related. Moreover, initial SOC stocks and SOC sequestration rates of those treatments were highly negatively correlated. Thus, long‐term straw return integrated with mineral fertilization in upland wheat and maize croplands leads to increased crop yields and SOC stocks. However, those effects of straw return are highly dependent on fertilizer management, cropping system, soil type, duration period, and the initial SOC content.     

黄土丘陵区土壤有机碳固存对退耕还林草的时空响应

研究了黄土丘陵区土壤有机碳固存对退耕还林草的时空响应特征,分析了退耕还林草对土壤有机碳的近期影响和长期效应。结果表明,1)从黄土丘陵区退耕还林草的土壤固碳效应整体而言,相对于坡耕地,退耕还林和退耕撂荒具有显著的土壤碳增汇效应,而退耕还草、退耕还果没有明显土壤碳增汇效应。以天然草地土壤有机碳密度为目标,撂荒地表层土壤有机碳增汇潜力可达8.3 t/hm2。2)以10a为界,退耕还林草的近期土壤碳增汇效应不明显,而10a后土壤碳增汇效应逐渐明显,退耕还林、还灌、撂荒和坡耕地的固碳效应差异显著。3)在评估黄土丘陵区退耕还林草的土壤固碳效应时应当注重长期固碳效应。4)退耕还林草的土壤固碳效应主要受还林草方式及年限的影响,二者分别可解释55.6%和24.1%的有机碳变异性;地形因子可解释8.5%的有机碳变异性。在评估该区退耕还林的土壤固碳效应时应当充分考虑退耕年限和地形因子的影响。5)人工刺槐林地、人工柠条林地以及撂荒地深层土壤(100—200 cm)有机碳密度占2 m土体有机碳密度的35%—40%,而且随着植被恢复深层土壤有机碳密度显著增加。6)在估算黄土丘陵区退耕还林土壤固碳效应时应该考虑深层碳累积。如果按1 m土层的土壤有机碳密度计算,会严重低估退耕还林草的土壤固碳量。     

Changes in cropland topsoil organic carbon with different fertilizations under long-term agro-ecosystem experiments across mainland China

Topsoil soil organic carbon (SOC) data were collected from long-term Chinese agro-ecosystem experiments presented in 76 reports with measurements over 1977 and 2006. The data set comprised 481 observations (135 rice paddies and 346 dry croplands) of SOC under different fertilization schemes at 70 experimental sites (28 rice paddies and 42 dry croplands). The data set covered 16 dominant soil types found in croplands across 23 provinces of mainland China. The fertilization schemes were grouped into six categories: N (inorganic nitrogen fertilizer only), NP (compound inorganic nitrogen and phosphorus fertilizers), NPK (compound inorganic nitrogen, phosphorus and potassium fertilizers), O (organic fertilizers only), OF (combined inorganic/organic fertilization) and Others (other unbalanced fertilizations such as P only, K only, P plus K and N plus K). Relative change in SOC content was analyzed, and rice paddies and dry croplands soils were compared. There was an overall temporal increase in topsoil SOC content, and relative annual change (RAC, g kg⁻¹ yr⁻¹) ranged −0.14–0.60 (0.13 on average) for dry cropland soils and −0.12–0.70 (0.19 on average) for rice paddies. SOC content increase was higher in rice paddies than in dry croplands. SOC increased across experimental sites, but was higher under organic fertilization and combined organic/inorganic fertilizations than chemical fertilizations. SOC increase was higher under balanced chemical fertilizations with compound N, P and K fertilizers than unbalanced fertilizations such as N only, N plus P, and N plus K. The effects of specific rational fertilizations on SOC increase persisted for 15 years in dry croplands and 20 years in rice paddies, although RAC values decreased generally as the experiment duration increased. Therefore, the extension of rational fertilization in China’s croplands may offer a technical option to enhance C sequestration potential and to sustain long-term crop productivity.     

Depth‐dependent soil organic carbon dynamics of croplands across the Chengdu Plain of China from the 1980s to the 2010s

Agricultural soils have tremendous potential to sequester soil organic carbon (SOC) and mitigate global climate change. However, agricultural land use has a profound impact on SOC dynamics, and few studies have explored how agricultural land use combined with soil conditions affect SOC changes throughout the soil profile. Based on a paired soil resampling campaign in the 1980s and 2010s, this study investigated the SOC changes of the soil profile caused by agricultural land use and the correlations with parent material and topography across the Chengdu Plain of China. The results showed that the SOC content increased by 3.78 g C/kg in the topsoil (0–20 cm), but decreased in the 20–40 cm and 40–60 cm soil layers by 0.90 and 1.26 g C/kg respectively. SOC increases in topsoil were observed for all types of agricultural land. Afforestation on former agricultural land also caused SOC decreases in the 20–60 cm soil layers, while SOC decreases only occurred in the 40–60 cm soil layer for agricultural land using a traditional crop rotation (i.e. traditional rice–wheat/rapeseed rotation) and with rice–vegetable rotations converted from the traditional rotations. For each agricultural land use, SOC decreases in deep soils only occurred in high relief areas and in soils formed from Q4 (Quaternary Holocene) grey‐brown alluvium and Q4 grey alluvium that had a relatively low soil bulk density and clay content. The results indicated that SOC change caused by agricultural land use was depth dependent and that the effects of agricultural land use on soil profile SOC dynamics varied with soil characteristics and topography. Subsoil SOC decreases were more likely to occur in high relief areas and in soils with low soil bulk density and low clay content.     

Effects of tillage and nitrogen fertilizers on CH4 and CO2 emissions and soil organic carbon in paddy fields of central China

Quantifying carbon (C) sequestration in paddy soils is necessary to help better understand the effect of agricultural practices on the C cycle. The objective of the present study was to assess the effects of tillage practices [conventional tillage (CT) and no-tillage (NT)] and the application of nitrogen (N) fertilizer (0 and 210 kg N ha(-1)) on fluxes of CH(4) and CO(2), and soil organic C (SOC) sequestration during the 2009 and 2010 rice growing seasons in central China. Application of N fertilizer significantly increased CH(4) emissions by 13%-66% and SOC by 21%-94% irrespective of soil sampling depths, but had no effect on CO(2) emissions in either year. Tillage significantly affected CH(4) and CO(2) emissions, where NT significantly decreased CH(4) emissions by 10%-36% but increased CO(2) emissions by 22%-40% in both years. The effects of tillage on the SOC varied with the depth of soil sampling. NT significantly increased the SOC by 7%-48% in the 0-5 cm layer compared with CT. However, there was no significant difference in the SOC between NT and CT across the entire 0-20 cm layer. Hence, our results suggest that the potential of SOC sequestration in NT paddy fields may be overestimated in central China if only surface soil samples are considered.     

Fine-scale spatial distribution of soil organic carbon and its fractions after afforestation with Pinus sylvestris and Salix psammophila in a semiarid desert of China

半干旱沙漠中樟子松和沙柳造林后土壤有机碳及其组分的小尺度空间分布 半干旱沙漠造林有助于改善土壤功能以及增加土壤有机碳(SOC)固定,但人们对造林后SOC及其不稳定(LOC)组分的小尺度空间分布了解甚少。本研究以毛乌素沙地东南缘樟子松(Pinus sylvestris)和沙柳 (Salix psammophila)为研究对象,量化了距离树体20、80、150和240 cm处SOC、LOC组分及其相关变量的小尺度空间分布。研究结果表明,沙柳和樟子松造林显著提高了SOC、总氮(TN)、可溶性有机碳 (DOC)、微生物碳(MBC)和易氧化有机碳(ROOC)含量;在距离树体20 cm处,0–100 cm土层樟子松SOC 储量比沙柳高27.21%;在距离树体80和150 cm处,沙柳SOC储量分别比樟子松高5.50%和5.66%;与流 沙地相比,在距离树体20、80、150 和240 cm处,沙柳和樟子松SOC储量显著增加了94.90%、39.50%、 27.10%和18.50%;沙柳和樟子松ROOC分别占SOC的14.09%和18.93%。总之,造林促进了半干旱流沙地SOC的积累,樟子松比沙柳分配更多的有机质到距离树体<80 cm范围内的土体中。     

Effect of different fertilization modes on soil organic carbon sequestration in paddy fields in South China: A meta-analysis

Evidences have shown that fertilizer application could affect soil organic carbon (SOC) content in cropland. South China is the main production region of rice, in which many different fertilization practices have been widely used during the last several decades, but the effect of different fertilization modes on SOC sequestration in paddy fields in this region was scarcely studied. Based on 274 samples taken from 44 paddy field experimental sites in South China, a meta-analysis was performed to quantify the relative annual change of SOC content (RAC) and SOC sequestration duration in paddy fields under five fertilization modes (inorganic nitrogen fertilization, N; inorganic nitrogen and phosphorus fertilization, NP; inorganic nitrogen, phosphorus, and potassium fertilization, NPK; organic fertilization, O; and inorganic combined with organic fertilization, OF). The results showed that the RAC under the five fertilization modes was 0–0.4 g kg⁻¹ yr⁻¹, with increments of 0.19 and 0.23 g kg⁻¹ yr⁻¹ in double cropping systems and triple cropping systems, respectively. The RAC under the O and OF treatments was higher than that under the N, NP and NPK treatments and was the highest (0.32 g kg⁻¹ yr⁻¹) under OF among the five treatments. The SOC accumulation rate decreased with time, and the SOC sequestration duration under N, NP, NPK, O and OF was approximately 23, 28, 46, 64 and 55 years, respectively. The accumulation enhancement rate of SOC over the whole SOC sequestration period under the N, NP, NPK, O and OF treatments was approximately 12.9%, 23.4%, 29.3%, 47.2% and 55.1%, respectively. The OF treatment performed the largest potential for SOC sequestration in paddy fields in South China under long-term conditions.     

滩涂围垦和土地利用对土壤微生物群落的影响

土壤微生物在生态系统营养物质循环过程,特别是碳、氮循环过程中扮演着重要的角色。上海市崇明岛位于长江入海口,因其土壤发育时间较短、土地利用历史背景清晰、土壤本底均一,不同土壤围垦年代的土壤,代表了土壤发育年代的不同时期。以空间变化代替时间变化,对崇明岛稻田和旱地6个不同围垦年代土壤的磷酸脂肪酸(PLFA)指纹图谱研究表明,湿地滩涂围垦16a后土壤微生物总PLFA、细菌PLFA、革兰氏阳性菌(G+)PLFA和革兰氏阴性菌(G-)PLFA含量显著降低。随着围垦时间的逐步增加,PLFA含量逐步上升。经过长时间的农业种植,G+PLFA在围垦120a和300a稻田和旱地土壤中没有显著性差异;而总PLFA、细菌和G-PLFA在围垦75、120a和300a的土壤中含量趋于稳定且没有显著性差异。围垦16a和40a稻田土壤中总PLFA和G+PLFA显著高于旱地土壤;围垦40a稻田土壤中细菌和G-PLFA显著高于旱地土壤。不同围垦年代土壤总PLFA、细菌PLFA与土壤总氮、粘土含量成显著的正相关关系。河口湿地围垦后微生物数量的变化与土壤营养含量存在强烈相关关系,提示土壤围垦及演替过程中微生物与土壤肥力之间的紧密关系,对探讨土壤演替过程中微生物群落的变化具有重要意义。     

Dynamics and Sources of Soil Organic C Following Afforestation of Croplands with Poplar in a Semi-Arid Region in Northeast China

Afforestation of former croplands has been proposed as a promising way to mitigate rising atmospheric CO₂ concentration in view of the commitment to the Kyoto Protocol. Central to this C sequestration is the dynamics of soil organic C (SOC) storage and stability with the development of afforested plantations. Our previous study showed that SOC storage was not changed after afforestation except for the 0–10 cm layer in a semi-arid region of Keerqin Sandy Lands, northeast China. In this study, soil organic C was further separated into light and heavy fractions using the density fractionation method, and their organic C concentration and ¹³C signature were analyzed to investigate the turnover of old vs. new SOC in the afforested soils. Surface layer (0–10 cm) soil samples were collected from 14 paired plots of poplar (Populus × xiaozhuanica W. Y. Hsu & Liang) plantations with different stand basal areas (the sum of the cross-sectional area of all live trees in a stand), ranging from 0.2 to 32.6 m² ha⁻¹, and reference maize (Zea mays L.) croplands at the same sites as our previous study. Soil Δ_C stocks (Δ_C refers to the difference in SOC content between a poplar plantation and the paired cropland) in bulk soil and light fraction were positively correlated with stand basal area (R ² = 0.48, p<0.01 and R ² = 0.40, p = 0.02, respectively), but not for the heavy fraction. SOC_crop (SOC derived from crops) contents in the light and heavy fractions in poplar plantations were significantly lower as compared with SOC contents in croplands, but tree-derived C in bulk soil, light and heavy fraction pools increased gradually with increasing stand basal area after afforestation. Our study indicated that cropland afforestation could sequester new C derived from trees into surface mineral soil, but did not enhance the stability of SOC due to a fast turnover of SOC in this semi-arid region.     

耕作方式对紫色水稻土有机碳和微生物生物量碳的影响

以位于西南大学的农业部紫色土生态环境重点野外科学观测试验站始于1990年的长期定位试验田为对象,研究了冬水田平作(DP)、水旱轮作(SH)、垄作免耕(LM)及垄作翻耕(LF)等4种耕作方式对紫色水稻土有机碳(SOC)和微生物生物量碳(SMBC)的影响。结果表明,4种耕作方式下SOC和SMBC均呈现出在土壤剖面垂直递减趋势,翻耕栽培下其降低较均匀,而免耕栽培下其富集在表层土壤中。同一土层不同耕作方式间SOC和SMBC的差异在表层最大,随着土壤深度的增加,各处理之间的差异逐渐减小。在0—60 cm剖面中,SOC含量依次为:LM(17.6 g/kg)>DP(13.9 g/kg)>LF(12.5 g/kg)>SH(11.3 g/kg),SOC储量也依次为:LM(158.52 Mg C/hm2)>DP(106.74 Mg C/hm2)>LF(93.11 Mg C/hm2)>SH(88.59 Mg C/hm2),而SMBC含量则依次为:LM(259 mg/kg)>SH(213 mg/kg)>LF(160 mg/kg)>DP(144 mg/kg)。与其它3种耕作方式比较,LM处理显著提高SOC含量和储量以及SMBC含量。对土壤微生物商(SMBC/SOC)进行分析发现,耕作方式对SOC和SMBC的影响程度并不一致。SMBC与SOC、全氮、全磷、全硫、碱解氮、有效磷均呈现极显著正相关(P<0.01),与有效硫呈显著正相关(P<0.05);表明SMBC可以作为表征紫色水稻土土壤肥力的敏感因子。     

长期施肥对双季稻种植下土壤有机碳库和固碳量的影响

研究了长期施用化肥和猪粪(PM)、稻草(RS)对双季稻集约化种植下30年期间(1981-2010年)土壤有机碳(SOC)及其组分的影响.结果表明：化肥平衡施用处理(NPK)的SOC、颗粒有机C(POC)和KMnO₄氧化C(KMnO₄C)组分高于化肥非平衡施用处理(NP和NK);猪粪、稻草与化肥(NK+PM、NP+RS和NPK+RS)长期配合施用处理的SOC、POC和KMnO₄ C组分显著增加.连续种植30年60季水稻后,猪粪与NK配施处理0～45 cm土层的SOC(84.71 t C·hm^-2)、POC(8.94 t C·hm^-2)和KMnO₄ C(21.09 t C·hm^-2)数量最高,其次是NPK+RS处理;NK+PM处理(485 kg C·hm^-2·a^-1)的固C量最高,其次是NPK+RS处理(375 kg C·hm^-2·a^-1).化肥与猪粪、稻草配施处理SOC的固C效率(CSE)明显高于单施化肥处理;施肥处理POC的固C效率(0.4%～1.2%)低于KMnO₄C(3.0%～8.3%).采用腐殖化常数值(h)和Jenkinson方程的衰减常数(k)可以预测不同处理2010年的SOC储量,通过Jenkinson方程可以计算维持1981年的SOC储量水平所需要的C投入量(A_E).双季稻种植下,长期连续施用NK+PM、NP+RS和NPK+RS处理的SOC含量增加是由于年C输入量高于A_E所致.在南方亚热带双季稻种植区,化肥与猪粪、稻草长期配施将促进水稻土有机碳的固定.     

Digging deeper: A holistic perspective of factors affecting soil organic carbon sequestration in agroecosystems

The global magnitude (Pg) of soil organic carbon (SOC) is 677 to 0.3‐m, 993 to 0.5‐m, and 1,505 to 1‐m depth. Thus, ~55% of SOC to 1‐m lies below 0.3‐m depth. Soils of agroecosystems are depleted of their SOC stock and have a low use efficiency of inputs of agronomic yield. This review is a collation and synthesis of articles published in peer‐reviewed journals. The rates of SOC sequestration are scaled up to the global level by linear extrapolation. Soil C sink capacity depends on depth, clay content and mineralogy, plant available water holding capacity, nutrient reserves, landscape position, and the antecedent SOC stock. Estimates of the historic depletion of SOC in world soils, 115–154 (average of 135) Pg C and equivalent to the technical potential or the maximum soil C sink capacity, need to be improved. A positive soil C budget is created by increasing the input of biomass‐C to exceed the SOC losses by erosion and mineralization. The global hotspots of SOC sequestration, soils which are farther from C saturation, include eroded, degraded, desertified, and depleted soils. Ecosystems where SOC sequestration is feasible include 4,900 Mha of agricultural land including 332 Mha equipped for irrigation, 400 Mha of urban lands, and ~2,000 Mha of degraded lands. The rate of SOC sequestration (Mg C ha^?1 year^?1) is 0.25–1.0 in croplands, 0.10–0.175 in pastures, 0.5–1.0 in permanent crops and urban lands, 0.3–0.7 in salt‐affected and chemically degraded soils, 0.2–0.5 in physically degraded and prone to water erosion, and 0.05–0.2 for those susceptible to wind erosion. Global technical potential of SOC sequestration is 1.45–3.44 Pg C/year (2.45 Pg C/year).     

Increase in soil organic carbon stock over the last two decades in China's Jiangsu Province

Estimates of regional and national topsoil soil organic carbon (SOC) stock change may help evaluating the soil role in mitigation of greenhouse gas (GHG) emissions through carbon (C) sequestration in soils. However, understanding of the exact mitigation role is often constrained by the uncertainty of the stock estimation associated with different methodologies. In this paper, a soil database of topsoil (0–20 cm) SOC measurements of Jiangsu Province, China, obtained from a soil survey in 1982, and from a geological survey in 2004, was used to analyze the variability of topsoil SOC among soil groups and among soil regions, and to estimate the change in SOC stocks that have occurred in the province over the last two decades. The soil survey data was obtained from measurements of 662 690 randomly collected samples, while the geological survey data was from 24 167 samples taken using a 2 km × 2 km grid. Statistical analysis was conducted on SOC values for 1982 and 2004 for different categories of soil groups, soil regions, and administrative municipalities, respectively. Topsoil SOC storage was then calculated and the provincial topsoil SOC stock was estimated for each sampling time. There were remarkable differences in SOC levels between soil groups and soil regions and different municipalities. The grid sampling with the geological survey in 2004 yielded smaller variability of topsoil SOC averages, both with soil groups and with soil spatial distribution than the random sampling method used in 1982. Variation of SOC was greater with soil groups than with soil regions in both sampling times, although it was less variable across soil taxonomic categories than within a spatial category. Little variance of the SOC level with soil groups could be explained by clay content. However, the prevalence of paddy fields in the total cropland area governed the regional and municipal average SOC levels. The average provincial topsoil SOC content increased from 9.45 g kg⁻¹ in 1982 to 10.9 g kg⁻¹ in 2004, and the total provincial topsoil SOC stock was enhanced from 149.0±58.1 Tg C in 1982 to 173.2±51.4 Tg C in 2004, corresponding to a provincial average SOC sequestration rate of 0.16±0.09 t C ha⁻¹ yr⁻¹. The SOC sequestration trend for the last two decades could be, in part, attributed to the enhanced agricultural production, symbolized by the grain yield per hectare. The results of SOC stock changes suggest a significant C sequestration in soils of Jiangsu, China, during 1980–2000, with paddy management playing an important role in regional SOC storage and sequestration capacity.     

连续免耕对不同质地稻田土壤理化性质的影响

以我国南方地区典型的单季水稻生产大田为研究对象,按土壤质地分为壤质和粘质两个系列,探讨不同质地稻田土壤理化性质变化对连续免耕的响应规律。结果表明,在无秸秆覆盖条件下,随着免耕年限的增加,壤质和粘质稻田土壤的耕层均有紧实度提高的趋势,特别是粘质土壤,导致耕层变浅。与常年翻耕土壤相比,免耕6a后壤质水稻土0—20 cm土层的紧实度值平均增加了32%,而粘质的平均增加了90%。在相同免耕年限条件下粘质稻田土壤容重的增加也比壤质土壤的明显。壤质土壤0—10 cm土层有机质和碱解氮含量随免耕年限延长而提高,而在粘质土壤则显著降低。无论是壤质还是粘质土壤,连续免耕多年后土壤速效磷均在耕层(0—20 cm)富集,而速效钾则相反。总体而言,壤质水稻土对免耕的适宜性要优于粘质土壤;应根据土壤质地的不同选择性地实施免耕技术,并结合秸秆覆盖,以实现免耕稻田土壤的可持续利用。     

Assessing the Impact of Afforestation on Soil Organic C Sequestration by Means of Sequential Density Fractionation

Afforestation is a prevalent practice carried out for soil recovery and carbon sequestration. Improved understanding of the effects of afforestation on soil organic carbon (SOC) content and dynamics is necessary to identify the particular processes of soil organic matter (SOM) formation and/or decomposition that result from afforestation. To elucidate these mechanisms, we have used a sequential density fractionation technique to identify the transfer mechanisms of forest derived C to soil fractions and investigate the impact of afforestation on SOC sequestration. Surface soil samples from continuous maize crop land (C4) and forest land (C3), which had been established 5, 12 and 25 yr, respectively, on the Northeast China Plain were separated into five density fractions. SOC, nitrogen (N) concentration and δ13C data from the three forests and adjacent cropland were compared. Afforestation decreased SOC concentration in the < 2.5 g cm-3 fractions from 5 yr forest sites, but increased SOC content in the < 2.0 g cm-3 fractions from 25 yr forest sites. Afforestation did not affect soil mass distribution, SOC and N proportional weight distributions across the density fractions. The < 1.8 g cm-3 fractions from 12 and 25 yr forests showed higher C/N and lower δ13C as compared to other fractions. Incorporation of forest litter-derived C occurred from low density (< 1.8 g cm-3) fractions to aggregates of higher density (1.8-2.5 g cm-3) through aggregate recombination and C transport in the pore system of the aggregates. Some forest litter-derived C could transfer from the light fractions or directly diffuse and adsorb onto mineral particles. Results from this study indicate that microaggregate protection and association between organic material and minerals provide major contribution to the SOC sequestration in the afforested soil system.     

The impact of agricultural land use changes on soil organic carbon dynamics in the Danjiangkou Reservoir area of China

Aims

Over recent decades, a large uncultivated area has been converted to woodland and shrubland plantations to protect and restore riparian ecosystems in the Danjiangkou Reservoir area, a water source area of China’s Middle Route of the South-to-North Water Transfer Project. Besides water quality, afforestation may alter soil organic carbon (SOC) dynamics and stock in terrestrial ecosystems, but its effects remain poorly quantified and understood.

Methods

We investigated soil organic C and nitrogen (N) content, and δ ¹³C and δ ¹⁵N values of organic soil in plant root-spheres and open areas in an afforested, shrubland and adjacent cropped soil. Soil C and N recalcitrance indexes (RIC and RIN) were calculated as the ratio of unhydrolyzable C and N to total C and N.

Results

Afforestation significantly increased SOC levels in plant root-spheres with the largest accumulation of C in the afforested soil. Afforestation also increased belowground biomass. The C:N ratios in organic soil changed from low to high in the order the cropped, the shrubland and the afforested soil. The RIC in the afforested and shrubland were higher than that in cropped soil, but the RIN increased from the afforested to shrubland to cropped soil. The δ¹⁵N values of the organic soil was enriched from the afforested to shrubland to cropped soil, indicating an increased N loss from the cropped soil compared to afforested or shrubland soil. Changes in the δ¹³C ratio further revealed that the decay rate of C in the three land use types was the highest in the cropped soil.

Conclusions

Afforestation increased the SOC stocks resulted from a combination of large C input from belowground and low C losses because of decreasing soil C decomposition. Shifts in vegetation due to land use change could alter both the quantity and quality of the soil C and thus, have potential effects on ecosystem function and recovery.     

Afforestation alters community structure of soil fungi

Relatively little is known about the effect of afforestation on soil fungal communities. This study demonstrated that afforestation altered fungal community structure and that changes were correlated to pools of soil C. Pasture at three locations on the same soil type was afforested with Eucalyptus globulus or Pinus pinaster. The structure of fungal communities under the three land uses was measured after 13y using automated ribosomal intergenic spacer analysis (ARISA). Afforestation significantly altered the structure of fungal communities. The effect of location on the structure of fungal communities was limited to pasture soils; although these contained the same plant species, the relative composition of each species varied between locations. Differences in the structure of fungal communities between pasture, E. globulus and P. pinaster were significantly correlated with changes in the amount of total organic C and microbial biomass-C in soil. Afforestation of patches of agricultural land may contribute to conserving soil fungi in agricultural landscapes by supporting fungal communities with different composition to agricultural soils.     

The potential of cropping systems and soil amendments for carbon sequestration in soils under long-term experiments in subtropical India

An understanding of the dynamics of carbon (C) stock in soils, as impacted by management strategies, is necessary to identify the pathways of C sequestration in soils and for maintaining soil organic C (SOC) at a level critical for upkeeping soil health and also for restraining global warming. This is more important in tropical and subtropical region where soils are inherently low in organic C content and the production system is fragile. We evaluated the long‐term role of crop residue C inputs to soil in SOC sequestration and also the critical value of C inputs for maintenance of SOC across five different rice‐based cropping systems and four soil management practices including a fallow (no cultivation since initiation of the experiments) using five long‐term (7–36 years) fertility experiments in subtropical India. Cropping per se always caused a net depletion of SOC. Such depletion was inversely proportional to the amount of crop residue C incorporated into the soils (r=−0.92, P=0.001). Balanced fertilization with NPK, however, caused an enrichment (9.3–51.8% over the control) of SOC, its extent being influenced by the cropping systems. Long‐term application of organic amendments (5–10 Mg ha⁻¹ yr⁻¹) through farmyard manure (FYM) or compost could increase SOC hardly by 10.7% constituting only 18% of the applied C, the rest getting lost through oxidation. The total quantity of soil C sequestered varied from −11.5 to 14.5 Mg C ha⁻¹ and was linearly related (r²=0.40, P=0.005) with cumulative crop residue C inputs to the soils. On an average, the rate of its conversion to SOC came out to be 6.4%. This was more in presence of added organics (6.9%) than in its absence (4.2%). For sustenance of SOC level (zero change due to cropping) we found that a minimum quantity of 2.9 Mg C is required to be added per hectare per annum as inputs. The cropping systems and the management practices that could provide C input higher than the above critical level are likely to sustain the SOC level and maintain good soil health in the subtropical regions of the Indian subcontinent.