Relative contribution of trees and crops to soil carbon content in a parkland system in Burkina Faso using variations in natural 13C abundance

The origin of organic matter was studied in the soils of a parkland of karité (Vitallaria paradoxa C.F. Gaertn) and néré (Parkia biglobosa (Jacq.) Benth.), which is extensively cultivated without the use of fertilisers. In such systems, fertility (physical, chemical and biological) gradients around trees have been attributed by some authors to a priori differences in fertility, allowing for better tree establishment on richer sites. In reverse, other workers believed that these gradients are due to the contribution of trees to the formation of soil organic matter through litter and decay of roots. Measurements of the variations in the ¹³C isotopic composition allowed for a distinction between tree (C₃) derived C and crop and grass (C₄) derived C in the total soil organic C content. The organic carbon contents of the soils were recorded under the two species at two soil depths and at five distances going from tree trunk to the open area and their C isotopic signatures were analysed. The results showed that soil carbon contents under karité (6.43 ± 0.45 g kg⁻¹) and néré (5.65 ± 0.27 g kg⁻¹) were significantly higher (p<0.01) than in the open area (4.09 ± 0.26 g kg⁻¹). The δ¹³C of soil C was significantly higher (p<0.001) in the open area (−17.5 ± 0.3‰) compared with the values obtained on average with depth and distance from tree under karité (−20.2 ± 0.4‰) and néré (−20.1 ± 0.4‰). The C₄-derived soil C was approximately constant, and the differences in total soil C were fully explained by the C₃ (tree) contributions to soil carbon of 4.01 ± 0.71, 3.02 ± 0.53, 1.53 ± 0.10 g kg⁻¹, respectively under karité, néré and in the open area. These results show that trees in parklands have a directly positive contribution to soil carbon content, justifying the need to encourage the maintenance of trees in these systems in semi-arid environments where the carbon content of soil appears to be the first limiting factor for crop growth.     

Temporal and spatial trends in soil organic carbon stocks following maize cultivation in semi-arid Tanzania,East Africa

Conversion of native ecosystems to agro-ecosystems influences the amount, quality and turnover of soil organic carbon (SOC). As most agro-ecosystems are not in a steady state in terms of the content of SOC, the time scale and feedback mechanisms of changes in SOC are highly relevant for predicting future soil fertility and potential rates of soil carbon losses or sequestration. This paper focuses on changes in land use linked to measured changes in the distribution of total stocks of SOC and the δ¹³C signature in the upper 0.5 m of cultivated soils in the semi-arid parts of Tanzania. Based on documented land use changes since 1950s using remote sensing data, 12 sampling sites along two transects were selected to represent semi-natural/natural savannah and maize fields cultivated for up to five decades. Comparisons between sites representing a chronosequence of well-drained soils showed that soils cultivated the last 50 years have in average less than 50% SOC compared to soils which have never been cultivated. Variations between sites were significant and a reduction in SOC could not be established at sites near present or former villages which have received substantial manure despite a long cultivation history or along a chronosequence representing wetter and more fine-grained soils. Spatial variations in land use changes were parameterized based on remote sensing data and successfully validated against sampling sites. Site-specific rates of soil element loss following cultivation were extrapolated to the study area and uncertainties related to scaling up were discussed.     

Soil organic carbon stocks in soil aggregates under different land use systems in Nepal

We investigated the soil organic carbon (SOC) associated with various aggregate size fractions in soil profiles under different land uses. Bulk soil samples were collected from incremental soil depths (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) from sites with the four dominant land use types [forest, grazing land, irrigated rice in level terraces (Khet) and upland maize-millet in sloping terraces (Bari)] of the Mardi watershed (area 144 km²), Nepal. The bulk soil was separated into five aggregate size fractions and the associated SOC contents were determined. Soil physical parameters necessary for estimating the soil SOC stock such as bulk density, stone and gravel content, and SOC content, were also measured for each soil depth. The SOC stock (mean ± SE, kg C m^–2) in the topsoil (0–10 cm) was higher in grazing land soil (3.4 ± 0.1) compared to forest soil (1.4 ± 0.2) and cultivated soil [Bari (2.0 ± 0.2) and Khet (1.2 ± 0.2)]. Forest and grazing lands had similar SOC contents, but the higher content of gravel and stone in forest soil resulted in a lower estimate of the SOC stock per unit area. The total SOC stock in the soil profile (to 1 m depth) over the entire watershed was estimated to be 721470 TC (tonnes of carbon). Its distribution was 52, 30, 11 and 7% in forestland, Bari, grazing land and Khet, respectively. The estimated depth wise distribution of SOC stock for 1 m soil depth in the entire watershed was 28, 22, 28 and 22% in the 0–10, 10–20, 20–40, and > 40 cm soil depths, respectively. There was a net loss of SOC stock (0–40 cm soil depth) of 29%, due to internal trading of land uses in the period from 1978 to 1996. Macro aggregates (> 1 mm) were found to be the dominant size in Bari and grazing land, whereas in forest and Khet soil micro aggregates (< 1 mm) dominated. Micro aggregates of size < 0.25 mm had a higher SOC concentration than aggregates of 0.25–0.5 mm, regardless of the depth or land uses and they may therefore contribute to soil C sequestration.     

Land use change and soil organic carbon dynamics

Historically, soils have lost 40–90 Pg carbon (C) globally through cultivation and disturbance with current rates of C loss due to land use change of about 1.6 ± 0.8 Pg C y⁻¹, mainly in the tropics. Since soils contain more than twice the C found in the atmosphere, loss of C from soils can have a significant effect of atmospheric CO₂ concentration, and thereby on climate. Halting land-use conversion would be an effective mechanism to reduce soil C losses, but with a growing population and changing dietary preferences in the developing world, more land is likely to be required for agriculture. Maximizing the productivity of existing agricultural land and applying best management practices to that land would slow the loss of, or is some cases restore, soil C. There are, however, many barriers to implementing best management practices, the most significant of which in developing countries are driven by poverty. Management practices that also improve food security and profitability are most likely to be adopted. Soil C management needs to considered within a broader framework of sustainable development. Policies to encourage fair trade, reduced subsidies for agriculture in developed countries and less onerous interest on loans and foreign debt would encourage sustainable development, which in turn would encourage the adoption of successful soil C management in developing countries. If soil management is to be used to help address the problem of global warming, priority needs to be given to implementing such policies.     

Effect of long-term application of inorganic fertilizer and organic amendments on soil organic matter and microbial biomass in three subtropical paddy soils

Soil organic matter (SOM), microbial carbon (C_mic), and microbial nitrogen (N_mic) status affected by the application of inorganic fertilizer and organic amendments in subtropical paddy soils were investigated. Soil samples were collected from the plow layer of three long-term (17 years) field experiments at Xinhua, Ningxiang, and Taojiang counties in Hunan Province, China. Results showed that, compared to the control, application of inorganic fertilizer alone showed no significant effect on soil organic C (SOC), total N (N_tot), C_mic and N_mic. The application of inorganic fertilizer along with manure or straw significantly increased SOC and N_tot and soil C_mic and N_mic contents for all three sites, while following an application of inorganic fertilizer along with straw only for two sites. C_mic and N_mic were closely correlated with SOC and N_tot, respectively. In conclusion, application of inorganic fertilizer along with manure or straw is an effective way of enhancing SOM and microbial biomass in subtropical paddy soils.     

高锰酸根褪色光度法测定土壤中的有机碳

建立了以KMnO。测定土壤中有机碳的分光光度法。在硫酸介质中,有机碳能快速、定量的还原高锰酸根,在加入有机碳前后,高锰酸钾溶液在波长525nm处的吸光度发生明显变化,且吸光度之差△A与加入的有机碳的浓度成正比。结果表明,有机碳浓度在0．40—28．00mg／L范围内与溶液吸光度差值呈良好线性关系,线性回归方程为△A=0．0466p＋0．058（mg／L）,相关系数r=0．9987,检出限为0．170mg／a,相对标准偏差（RSD）为0．390％,表观摩尔吸光系数ε=8．39X10^3L／（mol·em）。用于测定国家标准土壤样品中有机碳的含量,相对标准偏差小于2．90％。     

Soil organic carbon,total nitrogen and grain yields under long-term fertilizations in the upland red soil of southern China

A long-term experiment with various fertilizations was carried out during 1990–2006 in a double cropping system rotated with wheat (Triticum Aestivium L.) and corn (Zea mays L.) in the red soil of southern China. The experiment consisted of eight treatments: non-fertilization (CK), nitrogen–phosphorus fertilization (NP), phosphorus–potassium fertilization (PK), nitrogen–phosphorus–potassium fertilization (NPK), pig manure (M), pig manure and NPK fertilization (NPKM), high rates of NPKM (hNPKM), and straw returned with inorganic fertilizers (NPKS). Applications of manure (i.e., M, NPKM and hNPKM) significantly increased soil organic carbon (SOC) and total nitrogen contents. Applications of inorganic fertilizers without manure showed small influences on SOC, but resulted in declines of soil total nitrogen over the long-term experiment. Grain yields were more than doubled under fertilizations for both wheat and corn, with the highest under the NPKM and hNPKM treatments and the lowest under non-fertilization. Long-term cropping practices without fertilization or with unbalanced fertilizations (e.g., NP and PK) caused low grain yields. The balanced fertilization of NPK increased grain yields. However, such practice was not able to maintain high grain yields during the last few years of experiment. Our analyses indicate that both wheat and corn grain yields are significantly correlated with SOC, total and available nitrogen and phosphorus. However, the relationships are stronger with total nitrogen (r = 0.5–0.6) than with available nitrogen (r = 0.26–0.3), indicating the importance of maintaining soil total nitrogen in agricultural practice.     

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg^–1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content <0.2% did not affect yield significantly. At values >0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha^–1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.     

Characterization of potentially bioreactive soil organic carbon and nitrogen by acid hydrolysis

Studies determined the potential of acid hydrolysis for estimating the bioreactive fraction of organic carbon in soils (SOC). Three soils (clay loam, silt loam, and sandy loam) were hydrolyzed with 1 M or 6 M HCl under reflux for up to 24 h. Results showed that 1.7 to 3.2 % of SOC could be liberated as CO₂ from the acid hydrolysis of soil. This readily hydrolyzed fraction should be a part of the bioreactive SOC. Higher amounts of soluble SOC and N as well as CO₂ were released from all soils by 6 M HCl than by 1 M HCl. Soluble SOC and N contents in both 1 M HCl and 6 M HCl hydrolysates of all soils increased rapidly during the initial 2 hours of hydrolysis, and then increased very gradually. The amounts of CO₂-C evolved correlated with the amounts of NH₄-N released during the acid hydrolysis (r = > 0.88). The ratio of SOC to soluble N was lower in 6 M HCl hydrolysate than in 1 M HCl. Hydrolysis of soil by 1 M HCl for 4 h appeared to be a promising approach for estimating the more bioreactive pools of SOC and N. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fractions of organic carbon in soils under different crop rotations, cover crops and fertilization practices

We investigated the long-term effects (13–48 years) of crop rotations, cover crops and fertilization practices on soil organic carbon fractions. Two long-term experiments conducted on a clay loam soil in southeastern Norway were used. From the crop rotation experiment, two rotations, one with two years grain + four years grass and the second with grain alone (both for 6 years), were selected. Each rotation was divided into moderate fertilizer rate (30–40 kg N ha^–1), normal fertilizer rate (80–120 kg N ha^–1) and farmyard manure (FYM 60 Mg ha^–1 + inorganic N at normal rate). Farmyard manure was applied only once in a 6-year rotation, while NPK was applied to every crop. The cover crop experiment with principal cereal crops consisted of three treatments: no cover, rye grass and clover as cover crops. Each cover crop was fertilized with 0 and 120 kg ha^–1 N rates. Soil samples from both experiments were taken from 0–10 cm and 10–25 cm depths in the autumn of 2001. The classical extraction procedure with alkali and acid solution was used to separate humic acid (HA), fulvic acid (FA) and humin fractions, while H₂O₂ was used to separate black carbon (BC) from the humin fraction. The rotation of grain + grass showed a significantly higher soil organic carbon (SOC) compared with grain alone at both depths. Farmyard manure application resulted in significantly higher SOC than that of mineral fertilizer only. However, cover crops and N rates did not affect SOC significantly. Organic carbon content of FA, HA and humin fractions accounted for about 29%, 25% and 44% of SOC, respectively. The rotation of grain+grass gave a higher C content in HA and humin fractions, and a lower C in the FA fraction as compared with the rotation with grain alone. Farmyard manure increased HA and humin fractions more than did chemical fertilizers. Clover cover crop increased the C proportion of humin more than rye grass and no cover crop. No significant differences in C contents of FA, HA and humin fractions were observed between N rates. Effects of cover crop and N rates as well as fertilization with NPK on black carbon (BC) content were significant only at 10–25 cm depths. Farmyard manure increased the BC fraction compared with chemical fertilizers. Clover crop also enhanced the accumulation of the BC fraction. Application of 120 kg N ha^–1 resulted in a significant increase of the BC fraction.     

Total and organic soil carbon in cropping systems of Nepal

The significance of soil organic matter (SOM) in sustaining agriculture has long been recognized. The rate of change depends on climate, cropping system, cropping practice, and soil moisture. A 3-yr on-farm study was conducted in two major agro-ecologies (hills with warm-temperate climate and plains with subtropical climate) of Nepal. The soils in warm-temperate climate are Lithic subgroups of Ustorthents with well-drained loamy texture, and in subtropical climate are Haplaquepts with imperfectly drained loamy texture. Farmers’ predominant cropping systems were selected from different cultivation length in addition to a reference sample collected from adjacent virgin forest. The objectives were to examine the effect of cultivation length and cropping system on total carbon, KMnO₄-oxidizable soil C, C storage, and C/N ratio in two climatic scenarios: warm-temperate and subtropical. A large difference in KMnO₄-oxidizable soil organic C was observed due to the effect of cultivation length and cropping system. However, TC remained similar during the 3-year study. The decrease in KMnO₄-oxidizable C due to cultivation was more in the surface layer (43–56%) than in the subsurface layer (20–30%). Total C in uncultivated, < 10-year cultivated, and > 50-year cultivated soil was 22, 13, and 10 g kg⁻¹ in warm-temperate climate and 10, 6, and 5 g kg⁻¹ in subtropical climate, respectively. During the 3-year study period in both climates, large changes in soil C were observed for KMnO₄-oxidizable C but not for TC, confirming our earlier work on the usefulness of the KMnO₄ oxidized fraction for detecting a relatively short-term increase or decrease in soil C pool. The TC storage in uncultivated, < 10-year cultivated, and > 50-year cultivated soil was 38, 25, and 19 Mg ha⁻¹ in warm-temperate climate and 22, 15, and 12 Mg ha⁻¹ in subtropical climate, respectively. The rice–wheat and maize–potato cropping systems were good in storing soil C of 30 and 20 Mg ha⁻¹ for 0–15-cm soil depth in warm-temperate climate. The rice–wheat cropping system was also good in storing soil C in subtropical climate (19 Mg ha⁻¹) compared with other cropping systems studied.     

Assessing causes of recent organic carbon losses from cropland soils by means of regional-scaled input balances for the case of Flanders (Belgium)

Several recent reports on cropland soil organic carbon (SOC) stock changes throughout Europe indicate a general continuing loss of SOC from these soils. As most arable soils in Europe are not in an equilibrium situation because of past changes in land-use and management practices, shifts in both have been suggested to drive this decline of SOC stocks. A lack of data has prevented the unambiguous verification of the contribution of these factors to SOC loss. First, this study focused on recent evolutions in management options for SOC sequestration in Flanders and showed that despite such practices have increased since 1990, their current contribution is still limited. Strikingly, their expansion is at odds with the reported general losses of SOC (−0.48 t OC ha⁻¹ year⁻¹ on average). We used very detailed datasets of livestock numbers, N-application rates and cropping surfaces to calculate regional shifts in input of effective OC from animal manure application, cereal straw incorporation and crop residue incorporation which amounted to −0.094, −0.045 and −0.017 t OC ha⁻¹ year⁻¹, respectively. Shifts in management were identified to have potentially brought about but a third of the recent loss of SOC in the study area, although for central West-Flanders and the Eastern border of Flanders larger impacts of management were observed. This study suggests other influences such as land-use change and climate change to be involved as well. We estimated that another 10%–45% of the loss of SOC could potentially be attributed to land-use changes from grassland to cropland during the 1970–1990 period and about 10% to the observed temperature increase. While being a regional-scaled case study, these findings may be relevant to other European regions in particular (Denmark, The Netherlands, North-West Germany, Brittany and the North-West of France, the Po-valley in Italy and parts of England), with similar climate and intensity of agriculture, and where comparable trends in farming management may well have taken place.     

Long-term effects of fallow systems and lengths on crop production and soil fertility maintenance in West Africa

In the development of short fallow systems as alternatives to shifting cultivation in West Africa, a long-term trial was established at the International Institute of Tropical Agriculture (IITA) on an Alfisol in the forest-savanna transitional zone of southwestern Nigeria, comparing three fallow systems; natural regrowth fallow, cover crop fallow and alley cropping on soil productivity and crop yield sustainability. The natural fallow system consisted of natural regrowth of mainly Chromolaena odorata shrub as fallow vegetation. The cover crop fallow system consisted of Pueraria phaseoloides, a herbaceous legume as fallow vegetation. The alley cropping system consisted of woody hedgerows of Leucaena leucocephala as fallow vegetation. The fallow lengths were 0 (continuous cropping), 1, 2 and 3 years after 1 year of maize/cassava intercropping. Biomass produced from natural fallow and cover crop fallow was burnt during the land preparation. Fertilizer was not applied throughout the study. Without fertilizer application, maize yield declined from above 3.0 t ha^–1 to below 0.5 t ha^–1 during 12 years of cultivation (1989–2000) on a land cleared from a 23-year old secondary forest. Temporal change in cassava tuber yield was erratic. Mean maize grain yields from 1993–2000 except for 1999 were higher in cover crop fallow system (1.89 t ha^–1) than in natural fallow system (1.73 t ha^–1), while natural fallow system outperformed alley cropping system (1.46 t ha^–1). During the above 7 years, mean cassava tuber yield in cover crop system (7.7 t ha^–1) did not differ from natural fallow system (8.2 t ha^–1), and both systems showed higher cassava tuber than the alley cropping system (5.7 t ha^–1). The positive effect of fallowing on crop yields was observed for both crops in the three systems, however, insignificant effects were seen when fallow length exceeded 1 year for cover crop and alley cropping, and 2 years for natural fallow. Soil pH, organic carbon, available P and exchangeable Ca, Mg and K decreased considerably after 12 years of cultivation, even in a 3-year fallow subplot. After 12 years, soil organic carbon (SOC) within 0–5 cm depth in alley cropping (13.9 g kg^–1) and natural fallow (13.7 g kg^–1) was higher than in cover crop fallow (11.6 g kg^–1). Whereas significant increase in SOC with either natural fallow or alley cropping was observed only after 2 or 3 years of fallow, the SOC in the 1-year fallow alley cropping subplot was higher than that in continuous cropping natural fallow subplot. It can be concluded from our study that in transforming shifting cultivation to a permanent cropping, fallow with natural vegetation (natural fallow), herbaceous legumes (cover crop fallow) and woody legumes (alley cropping) can contribute to the maintenance of crop production and soil fertility, however, length of fallow period does not need to exceed 2 years. When the fallow length is reduced to 1 year, a better alternative to natural regrowth fallow would be the cover crop for higher maize yield and alley cropping for higher soil organic matter. For fallow length of 2 years, West African farmers would be better off with the natural fallow system.     

The role of soil microorganisms in soil organic matter conservation in the tropics

Soil is a large sink for organic carbon within the terrestrial biosphere. Practices which cause a decline in soil organic matter cause CO₂ release, in addition to damaging soil resilience and, often, agricultural productivity. The soil micro-organisms (collectively the soil microbial biomass) are the agents of transformation of soil organic matter, nutrients and of most key soil processes. Their activities are much influenced by soil physico-chemical and ecological interactions. This paper addresses two key issues. Firstly, ways of managing, and the extent to which it is possible to manage, soil biological functions. Secondly, the methodologies currently available for studying soil micro-organisms, and the functions they mediate, are discussed. It is concluded that, as the world population develops in this new millennium, there will be an increased dependence upon biological processes in soil to provide adequate crop nutrition for the majority of the world's farmers. Although a major increase in the use of artificial fertilisers will be necessary on a global scale, this will not be an option for large numbers of farmers due to their poverty. Instead they will rely on recycling of nutrients from animal and vegetable composts and urban wastes, and biological cycling from nitrogen fixation and mycorrhizae. The challenge is to select the most appropriate topics for further research. Not all aspects are likely to lead to significantly improved agricultural productivity, or sustainability within the foreseeable future. This revised version was published online in June 2006 with corrections to the Cover Date.     

活性炭纤维对有机废水的吸附研究

以碱性木质素为原料通过静电纺丝法制备得到活性炭纤维。采用比表面积及孔径分析仪对活性炭纤维进行表征分析,同时以该活性炭纤维为吸附剂对甲苯、甲醇和丙酮3种有机废水进行吸附法净化处理,结果表明该活性炭纤维的比表面积达到807.77 m~2/g,孔容为0.484 cm~3/g,中值孔径为2.11 nm;活性炭纤维对3种有机废水具有一定的吸附净化效果,3种有机物中甲苯的吸附最快,吸附量最大;对甲苯、甲醇和丙酮的最大吸附量分别是229.12、156.68和103.34 mg/g。3种有机废水的吸附动力学分析结果表明:活性炭纤维对甲苯、甲醇和丙酮的吸附数据分别与准二阶模型、Werber-Morris模型和准一阶模型具有较好的拟合相关性。     

缓释碳源材料的选择与制备探究

固体缓释碳材料可在地下水异氧反硝化过程中作为电子供体。目前,所研究的缓释碳源主要分为两大类:一类为天然释碳材料;另一类为人工合成释碳材料。复合释碳材料的制备工艺主要为挤出成型法、注塑成型法和压膜成型法3类。     

An enzymatic approach to the determination of the degree of stabilization of organic carbon in fertilizers

An enzymatic approach to assess the stability of organic matter extracted from organic fertilizers and amendments is proposed. The use of 0.1M NaOH plus 0.1M Na₄P₂O₇ previously suggested as a suitable extractant solution for soil organic matter was also found satisfactory for the extraction of organic matter from organic fertilizers and amendments, especially when the temperature was raised to 65°C. The presence of nonhumified compounds in the extracts from organic fertilizers may interfere considerably during fractionation of organic carbon. An enzymatic hydrolysis with lipase, lysozyme and pronase, added sequentially to the extracts, led to an appreciable reduction in the interference. The interference was further reduced by carrying out a successive acid hydrolysis with 3N H₂SO₄; in this case the DH values (percentage of humified fractions with respect to total extractable carbon) were reduced to less than 10% in all organic fertilizers, but remained higher than about 70% in organic amendments.     

二次盐水中总有机碳的测定

介绍采用Multi n/c-2100检测仪,利用高温催化燃烧氧化-非色散红外探测原理测定盐水中的总有机碳的方法以及测定中的注意事项。