土壤有机碳和氮分解对温度变化的响应趋势与研究方法

总结了土壤中碳和氮贮量与温度的关系、土壤碳和氮分解对温度时空差异和直接加热升温的响应,以及土壤碳和氮分解对低温冻结及冻融循环的响应趋势,讨论了其研究方法的误差和不确定性,并对今后的研究提出了一些建议．气候变暖在短期内将使土壤碳和氮分解加速并引起CO2释放量增加,而长期过程中却并不一定会引起土壤碳和氮分解加速．合理解释不同研究结果的差异,除了需要系统分析土壤碳和氮分解对温度变化响应的机制外,还需要充分认识土壤碳和氮分解对温度变化响应的长期过程和短期过程的差异,以及研究方法、植被、土壤和气候等因素的影响．     

大气CO2浓度升高对土壤碳库稳定性的影响

工业革命以来,大气CO₂浓度持续上升,升高的CO₂浓度会改变植物光合产物积累、土壤碳库的碳输入和碳输出过程,进而通过影响有机碳组成和周转特征来调控土壤碳库动态变化。土壤碳库是陆地生态系统碳库的重要组成部分,其碳储量的微小变化都会对大气CO₂浓度和气候变化产生巨大影响。但目前关于CO₂浓度升高对土壤碳库动态和稳定性的影响还不清楚,很大程度上限制了预测陆地生态系统碳循环对气候变化的反馈。系统综述国内外大气CO₂浓度升高对植被生产力、植被碳输入和土壤碳库影响的研究进展,旨在揭示土壤碳库物理、化学组成以及周转特征对CO₂浓度升高的响应过程和机理,探讨CO₂升高情境下土壤微生物特征对土壤碳库稳定性的影响和驱动机制,为深入理解全球变化下的土壤碳循环特征提供理论支撑。     

气候变化对陆地生态系统土壤有机碳储量变化的影响

通过研究气候变化对土壤有机碳储藏的影响,对预测未来气候变化下土壤有机碳动态变化与深入理解陆地生态系统变化和气候变化之间的相互作用有着极其重要的意义。本文归纳了土壤类型法、模型模拟法等途径对土壤有机碳储量估算的结果并分析它们各自的不确定性,综述了气候变化对土壤碳贮藏影响机理的研究与相应过程模拟的模型研究进展,并综合分析了当前研究中还存在的问题与不足。     

Climate change cannot be entirely responsible for soil carbon loss observed in England and Wales, 1978–2003

We present results from modelling studies, which suggest that, at most, only about 10–20% of recently observed soil carbon losses in England and Wales could possibly be attributable to climate warming. Further, we present reasons why the actual losses of SOC from organic soils in England and Wales might be lower than those reported.     

Soil carbon stocks and bulk density: spatial or cumulative mass coordinates as a basis of expression?

Accounting for CO₂ fluxes by determining changes in stocks of soil carbon (C) as a result of land use change is an option for complying nations under the Kyoto Protocol. The 1996 IPCC guidelines for C accounting recommend that soil C stocks to a depth of 30 cm be used in such accounting. However, the soil bulk density often changes with land use and the soil C per unit ground area to a fixed depth will also change even without any change in the mass fraction of C in dry soil. This problem will generally arise when soil C accounting is taken to a fixed depth (i.e. uses ‘spatial coordinates’). For accuracy in determining the land use change effects on soil C, soil sampling should be referred to a fixed dry soil mass per unit ground area (i.e. use ‘cumulative mass coordinates’). There has been intermittent literature‐discussion about this issue over several decades. Methods to accomplish C accounting on a mass coordinate basis, none of them accurate or efficient, have been suggested. Here, we propose a simple, accurate methodology for determining soil C stocks using cumulative mass coordinates, which does not involve repeat sampling trips, nominal specification of the location of boundaries between soil horizons, or independent sampling for determining soil bulk densities. Each core is taken a little (say 10 cm) below the nominal mass/depth required and the retrieved core is sliced into two at a point a little above the nominal mass/depth (say 10 cm above). An accurate determination of the depth of the core or slice is not needed, but an accurate determination of the dry mass of soil above and below the slice‐point is required. Linear interpolation between these two measurements is then used to estimate the cumulative soil C per unit ground area to the target dry soil mass per unit ground area. Even though this method eliminates the need for reporting soil bulk densities for C accounting, it is urged that the bulk densities and density changes still be routinely reported. This is because such information is of fundamental importance for understanding and predicting the movement of fluids and substances carried in them within the soil and between the soil and the environment. Hence, these data are likely to be of fundamental importance in developing our future understanding and predictive capacity of soil C changes with land use change.     

Soil carbon stabilization in jack pine stands along the Boreal Forest Transect Case Study

Boreal forests, containing >20% of the total organic carbon (OC) present at the surface of the Earth, are expected to be highly vulnerable to global warming. The objective of this study was to compare soil OC stocks and chemistry in jack pine stands located along a latitudinal climatic transect in central Canada. Total OC stocks (0–1 m) increased with decreasing mean annual temperature (MAT). We used a combination of physical fractionation of soil OC pools, ¹³C isotopic determination and cross‐polarization, magic‐angle spinning ¹³C nuclear magnetic resonance (NMR) spectroscopy to further characterize OC composition at all sites. Soil OC was dominated by labile pools. As illustrated by the C/N ratios, δ¹³C data and results from the ¹³C NMR analysis, the light fraction showed little alteration within the soil profiles. Instead, this fraction reflected the importance of fresh litter inputs and showed an increase in root contribution with depth. As opposed to the light fraction, the clay‐ and silt‐stabilized OC exhibited an increase in δ¹³C and a decrease in C/N with depth, indicating an increase in its degree of decomposition. These changes with depth were more marked at the southern than the northern sites. Results hence suggest that if the MAT were to increase in the northern boreal forest the overall jack pine soil OC stocks would decrease but the remaining OC would become more decomposed, and likely more stabilized than what is currently present within the soils.     

Alternative explanations for rising dissolved organic carbon export from organic soils

Since 1988, there has been, on average, a 91% increase in dissolved organic carbon (DOC) concentrations of UK lakes and streams in the Acid Waters Monitoring Network (AWMN). Similar DOC increases have been observed in surface waters across much of Europe and North America. Much of the debate about the causes of rising DOC has, as in other studies relating to the carbon cycle, focused on factors related to climate change. Data from our peat‐core experiments support an influence of climate on DOC, notably an increase in production with temperature under aerobic, and to a lesser extent anaerobic, conditions. However, we argue that climatic factors may not be the dominant drivers of DOC change. DOC solubility is suppressed by high soil water acidity and ionic strength, both of which have decreased as a result of declining sulphur deposition since the 1980s, augmented during the 1990s in the United Kingdom by a cyclical decline in sea‐salt deposition. Our observational and experimental data demonstrate a clear, inverse and quantitatively important link between DOC and sulphate concentrations in soil solution. Statistical analysis of 11 AWMN lakes suggests that rising temperature, declining sulphur deposition and changing sea‐salt loading can account for the majority of the observed DOC trend. This combination of evidence points to the changing chemical composition of atmospheric deposition, particularly the substantial reduction in anthropogenic sulphur emissions during the last 20 years, as a key cause of rising DOC. The implications of rising DOC export for the carbon cycle will be very different if linked primarily to decreasing acid deposition, rather than to changes in climate, suggesting that these systems may be recovering rather than destabilising.     

Direct measurement of soil organic carbon content change in the croplands of China

Agricultural soils in China have been estimated to have a large potential for carbon sequestration, and modelling and literature survey studies have yielded contrasting results of soil organic carbon (SOC) stock change, ranging from ?2.0 to +0.6% yr^?1. To assess the validity of earlier estimates, we collected 1394 cropland soil profiles from all over the country and measured SOC contents in 2007–2008, and compared them with those of a previous national soil survey conducted in 1979–1982. The results showed that average SOC content in the 0–20 cm soil increased from 11.95 g kg^?1 in 1979–1982 to 12.67 g kg^?1 in 2007–2008, averaging 0.22% yr^?1. The standard deviation of SOC contents decreased. Four major soil types had statistically significant changes in their mean SOC contents for 0–20 cm. These were: +7.5% for Anthrosols (paddy soils), +18.3% for Eutric Cambisols, +30.5% for Fluvisols, and ?22.3% for Chernozems. The change of SOC contents showed a negative relationship with the average SOC contents of the two sampling campaigns only when soils in the region south of Yangtse River were excluded. SOC contents of the two major soil types in the region south of Yangtse River, i.e., Haplic Alisols/Haplic Acrisols and Anthrosols (paddy soils), changed little or significantly increased, though with a high SOC content. We suggest that the increase of SOC content is mainly attributed to the large increase in crop yields since the 1980s, and the short history as cropland establishment is mainly responsible for the decrease in SOC content for some soil types and regions showing a SOC decline.     

Meeting Europe's climate change commitments: quantitative estimates of the potential for carbon mitigation by agriculture

Under the Kyoto Protocol, the European Union is committed to a reduction in CO₂ emissions to 92% of baseline (1990) levels during the first commitment period (2008–2012). The Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere. Thus, land‐use/land‐management change and forestry activities that are shown to reduce atmospheric CO₂ levels can be included in the Kyoto targets. These activities include afforestation, reforestation and deforestation (article 3.3 of the Kyoto Protocol) and the improved management of agricultural soils (article 3.4). In this paper, we estimate the carbon mitigation potential of various agricultural land‐management strategies and examine the consequences of European policy options on carbon mitigation potential, by examining combinations of changes in agricultural land‐use/land‐management. We show that no single land‐management change in isolation can mitigate all of the carbon needed to meet Europe's climate change commitments, but integrated combinations of land‐management strategies show considerable potential for carbon mitigation. Three of the combined scenarios, one of which is an optimal realistic scenario, are by themselves able to meet Europe's emission limitation or reduction commitments. Through combined land‐management scenarios, we show that the most important resource for carbon mitigation in agriculture is the surplus arable land. We conclude that in order to fully exploit the potential of arable land for carbon mitigation, policies will need to be implemented to allow surplus arable land to be put into alternative long‐term land‐use. Of all options examined, bioenergy crops show the greatest potential for carbon mitigation. Bioenergy crop production also shows an indefinite mitigation potential compared to other options where the mitigation potential is finite. We suggest that in order to exploit fully the bioenergy option, the infrastructure for bioenergy production needs to be significantly enhanced before the beginning of the first Kyoto commitment period in 2008. It is not expected that Europe will attempt to meet its climate change commitments solely through changes in agricultural land‐use. A reduction in CO₂‐carbon emissions will be key to meeting Europe's Kyoto targets, and forestry activities (Kyoto Article 3.3) will play a major role. In this study, however, we demonstrate the considerable potential of changes in agricultural land‐use and ‐management (Kyoto Article 3.4) for carbon mitigation and highlight the policies needed to promote these agricultural activities. As all sources of carbon mitigation will be important in meeting Europe's climate change commitments, agricultural carbon mitigation options should be taken very seriously.