Soil degradation in the tropical lowlands of Santa Cruz,Eastern Bolivia

This paper examines the soil degradation processes that are threatening the productivity of the mechanized annual cropping areas in the Santa Cruz lowlands of tropical eastern Bolivia. The dominant process is that of machinery-induced and natural compaction, which has resulted in an estimated 50 per cent of the soils in the Central Zone being moderately to severely compacted, causing serious root restriction and the loss of both transmission and water-storage pores. Degradation has made the soils increasingly susceptible to moisture stress due to the combined effect of (i) restricted rooting as a result of compaction and the hardsetting characteristics of the soils, (ii) reduced rainfall infiltration due to the loss of transmission pores and surface crusting, and (iii) a decrease in available soil moisture caused by the loss of storage pores, the incorporation of wind-blown fine sand deposits, and soil organic matter losses due to accelerated decomposition rates. The loss of transmission pores has also made the soils more prone to waterlogging in periods of high rainfall. The degradation tendencies of these soils are exacerbated by the greater variability of seasonal rainfall during the last 20 years that has led to a greater frequency or extremely high or extremely low rainfall events than hitherto.     

Reclaiming the erosion susceptible landscape of the Italian badlands for arable cultivation

During the last 50 years substantial tracts of the Italian badlands have been reclaimed for arable cultivation. It is a process that involves modelling the degraded clay landscape with bulldozers, often resulting in steep, unstable slopes. This work investigates the stability of reclaimed land in these erosion susceptible environments. Aggregate stability is used to assess the soil's erodibility with the aim of determining those physicochemical properties that govern the potential for erosion. Regression analysis demonstrates that the most significant variables in determining soil erodibility are the percentage of organic matter and the exchangeable sodium percentage (ESP). Threshold values of 1–2 per cent organic matter and 15 per cent ESP are presented above and below which, respectively, the soils attain a degree of stability. Reclaimed land is on the borderline of these thresholds and therefore represents a potential erosion hazard. However, the stability of reclaimed land was significantly higher than that of the badland parent material, ascribable to a decline in the ESP. The results are supported by a series of soil crust experiments using simulated rainfall in the laboratory. This investigation has implications for land management and landscape conservation. If the organic matter content of reclaimed land is maintained and the ESP reduced, then, when used in combination with other soil conservation techniques, erosion will be minimized. However, this implies that the badland landscape, and its diverse ecology and morphology, may have been permanently lost to agriculture and, as such, should be recognized as a threatened Mediterranean landscape. © 1998 John Wiley & Sons, Ltd.     

Integrating biophysical and socio‐economic aspects of soil conservation on the Loess Plateau,China. Part II. Productivity impact and economic costs of erosion

Using the simulation model described in Part I, this paper examines the impact of erosion on soil productivity, how the impact varies according to initial soil conditions and organic matter management and the economic cost of erosion measured as net present value. The reference crop is winter wheat grown in Chunhua in the southern Loess Plateau. Biomass yield is plotted over 100 years for four erosion scenarios represented by 0, 9, 27 and 47 per cent slopes, three initial soil conditions indicated by 0·5, 1 and 2 per cent organic matter, and two management levels determined by high or low levels of reincorporation of organic residues. Calculations of soil productive half‐life (time to half initial yield) and whole‐life (to equilibrial yield) are presented. The principal findings are that decline in soil productivity is caused by both erosion and insufficient return of organic matter. By increasing organic matter input, erosion damage is offset and soil productive life prolonged, but this is a costly strategy. If yield decline caused by erosion is isolated, erosion control is more important on a high organic matter input system. A maximum soil productive half‐life of 600 years is achieved with no erosion, high initial organic matter and return of organic residues; minimum half‐life of 10 years is with high erosion, low initial organic matter and little return of residues. In between, there are complicated interactions that significantly affect the economic cost of erosion and hence the decisions farmers make in investing in conservation practices. Copyright © 2000 John Wiley & Sons, Ltd.     

Modelling slaking sensitivity to assess the degradation potential of humid tropic soils under intense rainfall

In the humid tropics, soil erosion due to the impact of high‐intensity tropical rainfall is one of the important environmental problems. A quick assessment of slaking sensitivity of soils that are frequently subjected to the fast wetting of intense rainfall of the humid tropics is necessary for the selection of appropriate soil management practices to avoid soil structure deterioration that results in runoff, seal formation, erosion and eventual degradation. Unfortunately, field and laboratory measurements of slaking sensitivity are tedious, time consuming and expensive. Therefore, a slaking sensitivity ranking framework using readily available soil data, namely, clay content, organic matter content, exchangeable sodium percentage (ESP) and cation exchange capacity (CEC) determined to be important in slaking sensitivity and structural degradation under intense rainfall was developed. The ranking framework was subsequently used to classify 23 agriculturally important Trinidadian soils into slaking sensitivity classes for management recommendation. A simple mathematical model that provides a rapid assessment of slaking sensitivity was also developed using the soil data of 14 out of the 23 soils and subsequently tested on the remaining nine soils. Our results suggest that about 80 per cent or more of the soils are highly sensitive to slaking pressures, highly vulnerable to degradation and require management practices that reduce the rate of wetting and thus degradation of aggregates under intense rainfall. The developed model performed with a high degree of accuracy as the predicted values were in close agreement with measured values (r = 0·93). This suggests that the model gives a good indication of the structural degradation vulnerability of the soils studied under the conditions applied and criteria used. The model is, therefore, recommended for use in the tested humid tropical soils. However, more comprehensive testing is required on a broader range of soils prior to its more widespread application in other climatic conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

THE ORGANIC MATTER CONTENT OF THE SAVANNA SOILS OF WEST AFRICA

Published and unpublished data on the amounts of organic matter and nitrogen in the surface soils of the West African savanna are reviewed. In general, amounts are small; the mean carbon content of soils from 605 well-drained sites was 0.68 per cent. Two important factors governing amounts of organic matter in well-drained soils appear to be the clay content and a moisture factor related to the length of the wet season and represented here by mean annual rainfall. Multiple linear regression on soil clay content and rainfall accounted for 46.5 per cent and 57.2 per cent, respectively, of the observed variability of soil carbon and nitrogen contents. These findings suggest that the low levels of organic matter in savanna soils arise from their predominantly sandy nature and from the relatively low rainfall. In poorly drained soils organic matter levels are higher but are less significantly related to clay content and rainfall. The influence of human interference and of parent material and altitude on organic matter is demonstrated in the context of geographically limited areas within the savanna for which more detailed information was available.     

Temporal dynamics of iron-rich, tropical soil organic carbon pools after land-use change from forest to sugarcane

Background, aim, and scope  

Land-use change can significantly influence carbon (C) storage and fluxes in terrestrial ecosystems. Soil–plant systems can act as sinks or sources of atmospheric CO₂ depending on formation and decomposition rates of soil organic matter. Therefore, changes in tropical soil C pools could have significant impacts on the global C cycle. This study aims to evaluate the impacts of long-term sugarcane cultivation on soil aggregation and organic matter, and to quantify temporal dynamics of soil organic matter in cultivated sugarcane plantation soils previously under a tropical natural secondary forest.     

Effects of climatic and soil properties on cellulose decomposition rates in temperate and tropical forests

Cellulose decomposition experiments were conducted under field conditions to analyze the effects of climatic and soil properties on rates of organic matter decomposition in temperate and tropical forests. The mass loss rates of cellulose filter papers buried in the soil surface were measured to estimate the respiratory C fluxes caused by cellulose decomposition and mean residence time (MRT) of cellulose. The rates of cellulose decomposition increased with soil temperature, except for during the dry season, while rate constants of decomposition (normalized for temperature) decreased with decreasing pH because of lower cellulase activity. The estimated MRTs of soil cellulosic carbohydrates varied from 81 to 495 days for the temperate forests and from 31 to 61 days for the tropical forests. As a major organic substrate, the C fluxes from cellulose decomposition can account for a substantial fraction of heterotrophic (basal) soil respiration. However, the respiratory C fluxes can be limited by the low substrate availability and low pH in tropical soils, despite high microbial activity. The rate-regulating factors of cellulose decomposition, i.e., temperature, soil pH, and substrate availability, can accordingly influence the rates of heterotrophic soil respiration.     

Testing the Rothamsted Carbon Model against data from long-term experiments on upland soils in Thailand

We tested the Rothamsted Carbon Model (RothC) against three long‐term (27–28 years) experimental sites on Thai upland soils in order to see how this widely used ‘temperate’ soil carbon turnover model performed in a typical farming region in the tropics. We were able to verify – over a much longer period than had been examined in previous studies – that RothC performs well in a tropical region in plots used for continuous cropping experiments of maize and cassava without organic matter application. However, the model overestimated soil organic carbon (SOC) in some plots to which large amounts of organic matter (rice straw or cassava stalks) were applied. This overestimate could not be attributed to errors in estimating either the amount of C input to the soil or the ratio of decomposable plant materials to resistant plant materials entering the soil. Among many factors affecting SOC dynamics (e.g. weather conditions, soil characteristics, etc.), which are different in tropical regions from temperate regions, we conclude that the activity of soil fauna might be a major factor which makes the performance of RothC worse where much organic matter was applied. We suggest that care should be taken when applying RothC to tropical soils with large amounts of added organic matter.     

Organic matter decomposition and carbon content in soil fractions as affected by a gradient of labile carbon input to a temperate forest soil

Labile carbon (C) input to soils is expected to affect soil organic matter (SOM) decomposition and soil organic C (SOC) stocks in temperate coniferous forests. We hypothesized that the SOM...     

An experimental study of crust development on chalk downland soils and their impact on runoff and erosion

The sealing of soil surfaces by rainfall, the development of soil crusts and their impact on runoff and erosion was investigated in the laboratory by means of simulated rainfall. The soils investigated were stone-free samples of chalk soils from southeast England, and soils with a 25% cover of stones. Vertical change to the surface and immediate subsurface of the soils was assessed through the examination of thin sections scanned into a computer and analysed with image processing software. Changes in roughness and microtopography of the soil surface were measured by use of a laser micromapper. Crusting occurred both in the presence and absence of stones and was inversely related to the organic matter content and aggregate stability of the soils. Crusting of stone-free soils was accompanied by a reduction in roughness of the soil surface, but roughness of the stone-covered surfaces increased as crusting developed. Increases in the particle density of the crust were related to silt content. Organic-rich soil from under permanent grass and from a soil recently brought into arable cultivation crusted less than soils used for arable cultivation for longer periods with lower organic content. The inwashing of silt into the pores of the soil during crusting reduces infiltration, and increases runoff and erosion. In the presence of a 25% cover of surface stones, the reduction in infiltration was 25% less than for stone-free soils and erosion ∼50% less.     

CHANGES IN THE CHEMICAL NATURE OF SOIL ORGANIC PHOSPHATE DURING PEDOGENESIS

In two chronosequences of soils, total organic phosphate (P₀) accumulated rapidly during the first 50 years of soil development, when organic matter increased and pH decreased. The rate of P₀ accumulation then declined with age of soil until a ‘steady state’ was reached. The amounts of phospholipid, inositol phosphates and humic acid—P₀ followed the same trends as the total P₀. Parent materials and very young soils contained largely citric acid-soluble P₀, but after less than 50 years, surface horizons accumulated sufficient organic matter to complex a considerable proportion of the P₀. An upper limit of 20–5 per cent citric acid-soluble P₀ and 70–80 per cent NaOH-soluble P₀ was attained within 50 years of soil formation in the surface layers, and this slowly extended down the profile so that, after 10000 years, the soil had 20 per cent citric acid soluble-P₀ and 70 per cent NaOH-soluble P₀ to a depth of 1 m.     

Losing ground? soil loss and erosion in the highlands of papua new guinea

First impressions suggest that the risk of soil loss through fluvial erosion from land under cultivation is considerable in the Southern Highlands Province of Papua New Guinea. the climate is very wet all year round, The terrain precipitous, and people regularly farm on steep slopes. the Wola-speaking people, who occupy a series of valleys in the centre of the province, and who practice a semi-shifting form of cultivation, are nonetheless off-hand about soil conservation and declare that erosion is not a serious problem. This paper assesses the status of their assertions by calculating potential soil loss rates. It applies the universal soil loss equation to data on rainfall erosivity, soil erodibility, slope length and steepness, vegetation cover and conservation measures, to compute likely runoff losses. the calculations suggest that, The steep slopes cultivated and wet climate notwithstanding, The local population's assessment of the dangers of erosion is realistic and not reckless. Although rainfall is high, it is rarely of an intensity sufficient to threaten serious soil erosion losses. the physical properties of the soils, which feature volcanic ash components and high organic matter levels, are such that they are particularly resistant to erosion. the staple crop of the region, sweet potato (Ipomoea batatas), also gives particularly good ground cover and protection when established, effectively shielding the soil from erosive rainfall.     

Effect of grinding on the decomposition of soil organic matter—II. Oxygen uptake and nitrogen mineralization in virgin and cultivated cracking clay soils

Grinding caused large increases in both aerobic nitrogen mineralization and oxygen uptake in virgin and cultivated cracking clay soils. The large increases in organic matter decomposition in the cultivated soils, which had been depleted by 50 per cent in total organic matter, is discussed in relation to the rate of exposure of organic matter by field tillage. Grinding caused a large increase in the rate of decomposition of organic matter soon after incubation started. With time, however, the rates of decomposition in the ground and control samples were approximately equal. The similarity between this pattern and that obtained when artificial substrate is added to soil suggests that grinding increased the quantity of organic substrate available to the soil microflora.     

Nature,extent and severity of soil erosion in upland Scotland

This paper examines the nature, extent and severity of soil erosion in upland Scotland, an area subject to land management pressures typical of temperate maritime uplands. Erosion features were classified and their area measured from aerial photographs. Some 12 per cent of the upland area sampled was subject to some form of erosion, which is very similar to the percentage for Europe as a whole. The most significant erosion category was peat erosion, with 6 per cent of the area being affected; a figure which increased to 20 per cent in one sub-region. Peat erosion varied in severity, with the most severe erosion being in areas of eastern Scotland with the greatest land management pressures. Gully erosion of slopes on mineral soils was found in almost 5 per cent of the area sampled, particularly in sub-regions with large amplitude of relief. Debris flow/cone features and screes were less extensive and largely found at higher altitudes. Footpath erosion was mapped in popular mountain areas, but overall the mean length of eroded footpaths was less than that of large gullies. There was little evidence of spatial linkages between erosion of mineral soils and land management at the scale of the survey. Separating the roles of upland management and extreme rainfall events in the inception of erosion remains a key issue in the study of accelerated erosion in temperate maritime upland areas. Some guidance for management is presented.     

海南岛三大流域农业土壤源污染物流失特征研究

采用径流场结合人工模拟降雨方式,研究了海南岛万泉河、南渡江和昌化江三大流域土壤中氮、磷、有机质等营养物质的流失特征。结果表明,三大流域土壤径流系数和泥沙流失速率的大小顺序为：暴雨〉大雨〉中雨;相同雨强条件下,万泉河的径流系数与南渡江相近,昌化江最小;泥沙流失速率大小顺序为：万泉河〉南渡江〉昌化江;雨强对总磷（TP）流失速率的影响达到极显著水平,磷随径流流失以颗粒磷（PP）为主;氮在雨强较小时以可溶氮（DN）流失为主,当达到暴雨时则以颗粒氮（PN）流失为主;雨强越大,地表径流中COD、TN、DN和PN流失速率越高。三大流域区土壤养分随泥沙流失特征相似,不同雨强条件下,三大流域的总氮、总磷和有机质流失速率的规律一致,雨强越大,流失速率越高;在同一雨强条件下,三流域区总氮、总磷和有机质随泥沙流失速率为：昌化江〉万泉河〉南渡江。影响面源流失的主要因素为坡度、雨强、土质等。     

Rate of decomposition of organic matter in soil as influenced by repeated air drying-rewetting and repeated additions of organic material

Repeated air drying and rewetting of three soils followed by incubation at 20°C resulted in an increase in the rate of decomposition of a fraction of ¹⁴C labeled organic matter in the soils. The labeled organic matter originated from labeled glucose, cellulose and straw, respectively, metabolized in the soils during previous incubation periods ranging from 1.5 to 8 years.Air drying and rewetting every 30th day over an incubation period of 260–500 days caused an increase in the evolution of labeled CO₂ ranging from 16 to 121 per cent as compared to controls kept moist continuously. The effect of the treatment was least in the soil which had been incubated with the labeled material for the longest time.Additions of unlabeled, decomposable organic material also increased the rate of decomposition of the labeled organic matter. The evolution of labeled CO₂ during the 1st month of incubation after addition was in some cases 4–10 times larger than the evolution from the controls. During the continued incubation the evolution decreased almost to the level of the controls, indicating that the effect was related to the increased biological activity in the soils during decomposition of the added material.Three additions of organic material during the period of incubation resulted totally in an increase over the controls ranging from 36 to 146 per cent.     

Soil loss and runoff on semiarid land as amended with urban solid refuse

Organic amendment is a proved method of improving soil physical properties thus affecting runoff and soil erosion. Urban wastes are a potential source of organic matter and their use would also be a convenient way of disposing of them. A field experiment was conducted from October 1988 to September 1993 in a semiarid Mediterranean site to determine the effect of applying several rates (65, 130, 195 and 260 Mg ha⁻¹) of organic urban solid refuse (USR) on total runoff and soil loss. At the lowest rate, total runoff decreased by 67 per cent compared to the control plot. The decrease was 98 per cent when the highest rate was used. The lowest rate reduced total sediment loss by 81 per cent and the highest rate of 99 per cent. The decrease in soil erodibility at the different USR rates varied from 76 to 95 per cent depending on the year for the lowest rate and between 90 and 99 per cent for the other rates. Clear differences in the hydrologic and erosion responses were found between the eight initial rainfall events (during the first 10 months of the experiment) and the remainder of the events. The causes of such differences were due to the initial tillage of the soil by rotovation and the growth of natural vegetation in the treated plots. The mechanical effect of tillage reduced runoff and increased soil erodibility, although the effect was short‐lived. The addition of USR reduced runoff but lasted longer. An applied rate of 90–100 Mg ha⁻¹ could be considered suitable for application in semiarid zones. Copyright © 2000 John Wiley & Sons, Ltd.     

Microbial activities in a histosol: Effects of wood ash and NPK fertilizers

Mineralization of organic matter and microbial activities in an intensively cultivated acid, N-rich peat soil planted with Salix sp. cv. aquatica were examined for 3 yr. The soil was amended with wood ash or NPK fertilizers providing N as ammonium nitrate or urea. The wood ash amendment (10 tons ha^?1) increased soil pH from 4.6 to 5.5 and increased markedly all microbial activities measured, resulting in increased mineralization and N availability, and in loss of 9% total soil N during the first year. The addition of ammonium nitrate caused a corresponding though less pronounced increase in N mineralization. Cellulose decomposition increased in all amended soils, reaching rates 53–86% higher than in non-amended soil. Potential N₂ fixation (C₂H₂ reduction) by free-living organisms was increased by the ash-amendment. Potential denitrification rates were positively correlated (r = 0.98) with the presence of water-soluble organic-C, which was more abundant in ash-amended and non-amended soils than in the soils fertilized with N.     

有机质影响溅蚀破坏土壤团聚体的主要作用机制

土壤有机质是团聚体的重要组成部分,其在团聚体形成过程中作用机制已有大量研究,但有机质在团聚体破坏过程中起何种作用尚未明晰,其对团聚体破坏过程中雨滴机械打击和消散作用贡献的影响也有待深入研究.为研究不同有机质含量对土壤团聚体的影响,选取5种不同退耕还林年限的土壤为研究对象,利用95％酒精和超纯水作为降雨液体,分别在4个高...     

SOIL QUALITY AND SPATIAL VARIABILITY ASSESSMENT OF LAND USE EFFECTS IN A TYPIC HAPLUSTOLL

Soil management practices can have negative or positive effects on soil quality. Our objective was to assess the effect of long‐term agricultural practices by evaluating selected soil physical and chemical properties. Soil samples were collected from two depths (0 to 15 and 15 to 30 cm) within a native pasture and an adjacent agricultural field that was being used for three different crop rotations. Soil quality was quantified using aggregate stability, bulk density, soil texture and available water content as physical properties and pH, electrical conductivity, organic matter and available phosphorus as chemical properties. The farmland soils were functioning at 71 and 70 per cent of their full potential at the 0‐ to 15‐ and 15 to 30‐cm‐depth increments, respectively, whereas those from the pasture were functioning at 73 and 69 per cent, respectively. The assessment showed substantial loss in soil organic carbon following 50 years of farmland cultivation. Tillage and fertilizer applications were presumably the primary reasons for weaker spatial dependence within farmland at the 0‐ to 15‐cm depth. Grazing was postulated as the main reason for weaker spatial dependence within the pasture soils at the 15‐ to 30‐cm depth. Overall, we conclude that 50 years of cultivation has not caused soil quality to decline to a point that threatens sustainability of the agricultural fields. Copyright © 2011 John Wiley & Sons, Ltd.