Isotope implications of groundwater recharge,residence time and hydrogeochemical evolution of the Longdong Loess Basin,Northwest China

Groundwater plays a dominant role in the eco-environmental protection of arid and semi-arid regions.Understanding the sources and mechanisms of groundwater recharge,the interactions between groundwater and surface water and the hydrogeochemical evolution and transport processes of groundwater in the Longdong Loess Basin,Northwest China,is of importance for water resources management in this ecologically sensitive area.In this study,71 groundwater samples(mainly distributed at the Dongzhi Tableland and along the Malian River)and 8 surface water samples from the Malian River were collected,and analysis of the aquifer system and hydrological conditions,together with hydrogeochemical and isotopic techniques were used to investigate groundwater sources,residence time and their associated recharge processes.Results show that the middle and lower reaches of the Malian River receive water mainly from groundwater discharge on both sides of valley,while the source of the Malian River mainly comes from local precipitation.Groundwater of the Dongzhi Tableland is of a HCO3-Ca-Na type with low salinity.The reverse hydrogeochemical simulation suggests that the dissolution of carbonate minerals and cation exchange between Ca²⁺,Mg²⁺and Na+are the main water-rock interactions in the groundwater system of the Dongzhi Tableland.Theδ¹⁸O(from-11.70‰to-8.52‰)andδ2H(from-86.15‰to-65.75‰)values of groundwater are lower than the annual weighted average value of precipitation but closer to summer-autumn precipitation and soil water in the unsaturated zone,suggesting that possible recharge comes from the summer-autumn monsoonal heavy precipitation in the recent past(≤220 a).The corrected 14C ages of groundwater range from 3,000 to 25,000 a old,indicating that groundwater was mainly from precipitation during the humid and cold Late Pleistocene and Holocene periods.Groundwater flows deeper from the groundwater table and from the center to the east,south and west of the Dongzhi Tableland with estimated migration rate of 1.29-1.43 m/a.The oldest groundwater in the Quaternary Loess Aquifer in the Dongzhi Tableland is approximately 32,000 a old with poor renewability.Based on theδ¹⁸O temperature indicator of groundwater,we speculate that temperature of the Last Glacial Maximum in the Longdong Loess Basin was 2.4℃-6.0℃ colder than the present.The results could provide us the valuable information on groundwater recharge and evolution under thick loess layer,which would be significative for the scientific water resources management in semi-arid regions.     

评《中国灾害通史·明代卷》

《中国灾害通史·明代卷》是系列丛书《中国灾害通史》的一卷.它主要运用史料学方法,结合灾害学、统计学等多学科的研究方法与手段,以有明一代的自然灾害为研究对象,揭示了灾害发生的普遍规律,并对灾害发生的原因和抗灾、救灾措施进行了总结,探讨了统治者的荒政思想.     

Differentiating sources of CO2 from organic soil under bioenergy crop cultivation: A field-based approach using C

We tested here a plant-soil system to separate recent, plant-derived and native, soil-derived carbon in soil respiration. The approach uses a perennial crop cultivated on an organic soil where upper soil layers have been removed as a result of peat extraction. There, the ¹⁴C signal from native organic matter is highly depleted compared to that in vegetation established at the site after peat extraction ceased. Radiocarbon was analyzed in carbon dioxide respired from soil over one growing season, and a two-pool isotope mixing model was applied to calculate the relative contribution of old vs. new carbon sources. The analysis showed that the approach is reliable for source partitioning with isotopes. After six years of cultivation, old peat decomposition contributed less to total soil respiration than respiration of recent plant material (30% vs. 70% on average, respectively), but the relative proportions were highly variable over the growing season. The approach offers a new possibility to follow the fate of old, native soil organic matter in highly organic soils.     

Soil carbon pools, plant biomarkers and mean carbon residence time after afforestation of grassland with three tree species

Afforestation of grassland has been globally identified as being an important means for creating a sink for atmospheric carbon (C). However, the impact of afforestation on soil C is still poorly understood, due to the paucity of well designed long-term experiments and the lack of investigation into the response of different soil C fractions to afforestation. In addition, little is known about the origins of soil C and soil organic matter (SOM) stability after afforestation. In a retrospective study, we measured C mass in the soil light and heavy fractions in the first 10 years after afforestation of grassland with Eucalyptus nitens, Pinus radiata and Cupressus macrocarpa. The results suggest that C mass in the soil heavy fraction remained stable, but the C mass in the light fraction decreased at year 5 under three species. Soil δ¹³C analysis showed that the decrease in the light fraction may be due to reduced C inputs from grassland species litter and low inputs from the still young trees. After the initial reduction, the recovery of soil C in the light fraction depended on tree species. At year 10, an increase of 33% in light fraction soil C was observed at the 0-30 cm depth under E. nitens, compared to that under the original grassland (year 0). Planting P. radiata restored light fraction soil C to the original level under grassland, whereas planting C. macrocarpa led to a decrease of 33%. We concluded that the increase of light fraction soil C between year 5 and 10 is most likely due to C input from tree residues. Most of the increased C was derived from root turnover under pine and from both root and leaf turnover under E. nitens, as indicated by plant C biomarkers such as lignin-derived phenols and suberin and cutin-derived compounds in the 0-5 cm soil layer. Modelling of soil ?¹⁴C‰ suggested that SOM had a greater mean residence time at year 10 than year 0 and 5 due to increased relative abundance of recalcitrant plant biopolymers.     

Identification of sand and dust storm source areas in Iran

Sand and dust storms(SDS) are common phenomena in arid and semi-arid areas. In recent years, SDS frequencies and intensities have increased significantly in Iran. A research on SDS sources is important for understanding the mechanisms of dust generation and assessing its socio-economic and environmental impacts. In this paper, we developed a new approach to identify SDS source areas in Iran using a combination of nine related datasets, namely drought events, temperature, precipitation, location of sandy soils, SDS frequency, human-induced soil degradation(HISD), human influence index(HII), rain use efficiency(RUE) and net primary productivity(NPP) loss. To identify SDS source areas, we firstly normalized these datasets under uniform criteria including layer reprojection using Lambert conformal conic projection, data conversion from shapefile to raster, Min-Max Normalization with data range from 0 to 1, and data interpolation by Kriging and images resampling(resolution of 1 km). After that, a score map for the possibility of SDS sources was generated through overlaying multiple datasets under average weight allocation criterion, in which each item obtained weight equally. In the score map, the higher the score, the more possible a specific area could be regarded as SDS source area. Exceptions mostly came from large cities, like Tehran and Isfahan. As a result, final SDS source areas were mapped out, and Al-Howizeh/Al-Azim marshes and Sistan Basin were identified as main SDS source areas in Iran. The SDS source area in Al-Howizeh/Al-Azim marshes still keeps expanding. In addition, Al-Howizeh/Al-Azim marshes are now suffering rapid land degradation due to natural and human-induced factors and might totally vanish in the near future. Sistan Basin also demonstrates the impacts of soil degradation and wind erosion. With appropriate intensity, duration, wind speed and altitude of the dust storms, sand particles uplifting from this area might have developed into extreme dust storms, especially during the summer.     

Soil organic carbon stocks,distribution, and composition affected by historic land use changes on adjacent sites

Historic alterations in land use from forest to grassland and cropland to forest were used to determine impacts on carbon (C) stocks and distribution and soil organic matter (SOM) characteristics on adjacent Cambisols in Eastern Germany. We investigated a continuous Norway spruce forest (F-F), a former cropland afforested in 1930 (C-F), and a grassland deforested in 1953 (F-G). For C and N stocks, we sampled the A and B horizons of nine soil pits per site. Additionally, we separated SOM fractions of A and B horizons by physical means from one central soil pit per pedon. To unravel differences of SOM composition, we analyzed SOM fractions by ¹³C-CPMAS NMR spectroscopy and radiocarbon analysis. For the mineral soils, differences in total C stocks between the sites were low (F-F = 8.3 kg m⁻²; C-F = 7.3 kg m⁻²; F-G = 8.2 kg m⁻²). Larger total C stocks (+25%) were found under continuous forest compared with grassland, due to the C stored within the organic horizons. Due to a faster turnover, the contents of free particulate organic matter (POM) were lower under grassland. High alkyl C/O/N-alkyl C ratios of free POM fractions indicated higher decomposition stages under forest (1.16) in relation to former cropland (0.48) and grassland (0.33). Historic management, such as burning of tree residues, was still identifiable in the subsoils by the composition and ¹⁴C activity of occluded POM fractions. The high potential of longer lasting C sequestration within fractions of slower turnover was indicated by the larger amounts of claybound C per square meter found under continuous forest in contrast to grassland.     

土壤绝对年龄测定方法研究进展

土壤年龄与各种成土因素密切相关,对于土壤生物地球化学循环、土壤环境变化、地质学评估、考古研究均有着重要意义。由于土壤样品所处环境复杂,干扰因素较多,因此在土壤年龄测定方面有很多困难。目前,土壤绝对年龄的主要研究方法涉及土壤学、地质学、考古学等多种学科,主要包括放射性同位素测年法、释光测年法、电子自旋共振测年法、古地磁法以及氨基酸外消旋测年法等。对常用测年方法的原理和研究进展进行了总结与分析,从全局的角度对各种方法的优劣进行了比较：放射性同位素测年所用元素广泛,可选土壤材料与时间尺度也各不相同;释光定年和电子自旋共振法以土壤中石英矿物为测定对象,时间尺度均为中等至长时间;氨基酸外消旋定年适用于有机质含量较高的土壤;富含磁性矿物如磁铁矿的土壤定年可以采用古地磁法,该方法时间尺度较长。认为土壤测年应重点从土壤粘土矿物和有机物着手,根据土壤样品理化特性进行对症分析,并通过多种方法综合评价,从而对土壤年龄进行更准确评价,并为土壤成因、环境气候变化以及长期碳氮循环等相关研究提供有力证据。     

绰墩遗址新石器时期水稻田、古水稻土剖面、植硅体和炭化稻形态特征的研究

绰墩遗址第六次发掘的300 m2范围内出土22块古田块,通过对其田埂、灌排系统(水口、水沟、水井)等结构特征的观察研究,对古田块和水沟中出土的灌溉用具——陶罐和陶盆碎片的鉴定是属于马家滨文化时期的器物;从田块表土层中淘洗出大量的炭化稻粒,每克土壤中检测到有上万至十余万颗水稻植硅体,这些结果证明所发现的田块是马家滨时期的灌溉稻田。对出土的炭化稻粒和土壤有机质的14C定年结果表明其年龄分别为5 907 a BP和6 280 a BP,是新石器即晚马家滨文化时期的水稻文明。对在遗址挖掘的两个整段剖面(P-01和P-03)的研究发现,P-01剖面上42~103 cm层位埋藏有马桥文化-商朝初期(3 320 a BP)和在103~200 cm层位埋藏有马家滨时期(6 280 a BP)的两个古水稻土剖面,这一结果佐证了前人关于8 000年来长江三角洲地区曾有过几次大的洪水泛滥而使人类耕作活动中断的气候变化事件的结论。然而距P-01仅15 m的P-03整段剖面上仅出现了马桥-商朝初期的古水稻土剖面(40~100 cm),未发现马家滨时期古水稻土剖面(100~200 cm),说明当时的生产力非常低下,仅能利用低洼平坦的地方种稻,稍高一点的居住点附近的坡地就无法改成梯田种植水稻了。这两个埋藏的古水稻土剖面均已经发育了水稻土的明显特征。对出土炭化稻粒的形态学研究表明,外型多为椭圆形,长宽比的变异较大,稃面有方格凹陷结构、周边有稃毛,顶端残留有稻芒的基部,基部有护颖而没有小梗,"钝形"的双峰乳突、浅平的凹陷乳沟等均显示其为人工栽培的粳型水稻。结果与丁颖20世纪50年代在黄河流域发现的汉代的炭化稻粒的特征是吻合的,从而支持了关于"我国长江流域与黄河流域古代最早栽培的水稻都是粳稻品种"的论点。     

宁夏长城塬黄土土壤剖面光释光测年研究

通过光释光断代仪对宁夏长城塬黄土土壤剖面7个样品进行年代测定,获得7个确切的地层年代值。结果表明：约12 580 a BP,剖面处于气候干旱时期,形成马兰黄土（L₁）;约11 498 a BP,剖面处于全新世升温过渡期,形成过渡层（L_t）;6 731～2 827 a BP,剖面处于全新世大暖期,形成古土壤(S₁);约1 394 a BP,剖面处于相对干旱时期,形成全新世黄土（L₀）。