三峡库区土壤侵蚀空间分布特征

结合三峡库区的区位特征和数据获取条件,对RUSLE模型的因子算法进行相应修正,最后基于GIS评估库区土壤侵蚀风险及其空间分布特征。结果表明:1)库区年均土壤侵蚀量为1亿8 359.43万t/a,平均土壤侵蚀模数为31.85 t/(hm2.a),水土流失面积占库区土地总面积的66.97%;2)库区土壤侵蚀强度与高程梯度间无直接线性关系,中度以上侵蚀主要分布在500～1 500 m高程带上,其中极强烈、剧烈侵蚀在500～1 000 m间出现频率最高,这应该与该区域强烈的人类活动有关;3)库区土壤侵蚀强度与坡度呈显著正相关,中度以上侵蚀在15°以上坡度带的发生频率急剧增加,其53.74%的面积比例贡献了89.06%土壤侵蚀量;4)库区土壤侵蚀强度在不同坡向上表现为半阴坡>正阴坡>半阳坡>正阳坡,阴坡的侵蚀量稍大于阳坡,占库区总侵蚀量的56.63%。说明500～1 500 m高程带、>15°坡度带以及阴坡是库区发生土壤侵蚀的主要区域,也是水土流失防治及治理的重点区域。     

龙门山地震带坡耕地土壤侵蚀对有机碳迁移的影响

坡耕地土壤再分布对土壤有机碳(SOC,soil organic carbon)迁移的作用机制研究已成为土壤侵蚀学研究的热点,然而目前极少有研究关注地震后生态脆弱的龙门山地震带坡耕地土壤侵蚀机理及其导致的土壤有机碳再分布规律。该研究选择龙门山地震带内(都江堰市)一块陡坡耕地和一个梯田系列,采用137Cs法和野外调查,对比分析强震导致田埂垮塌和未受损情况下坡耕地土壤侵蚀空间变化特征和有机碳运移变化机理。结果表明,该区黄棕壤有效137Cs背景值为1 473 Bq/m2;坡度较小的坡式梯田内部上坡表现为侵蚀,下坡表现为沉积,同时,上部梯田的侵蚀速率高于下部梯田,但整个梯田系列净侵蚀量非常小,这表明梯田之间由于缺乏田埂的保护,水力也起着侵蚀、搬运上坡梯田土壤的作用,但是整个坡式梯田系列可以起到较好的保土作用,同时,坡式梯田内部主要以耕作侵蚀为主,是造成梯田上部坡位土壤流失严重的主要原因;陡坡耕地的地形为复合坡,由于田埂垮塌导致其土壤侵蚀速率显著高于坡式梯田系列,在整个坡面上,除了坡顶土壤侵蚀速率高之外,下坡坡度变大(曲率较大)的部位土壤侵蚀速率也非常高,同时,土壤沉积也发生在2个坡位(中下坡坡度较缓的部位和坡脚部位);在梯田系列和陡坡耕地上,SOC与土壤137Cs的空间变化规律较为一致。研究结果表明,在龙门山地震带,质量较好的石埂梯田仍然发挥着较好的土壤保持效果,同时,耕作侵蚀是该区坡耕地上一种重要的土壤侵蚀形式,在制定相应的土壤保持措施时,必须充分考虑耕作侵蚀的作用,才能有效地控制土壤侵蚀,此外,该研究结果还表明采用137Cs核素示踪技术可以比较科学地解释该区域的土壤侵蚀速率和SOC的空间变异规律。     

基于RUSLE模型的淮河流域土壤侵蚀定量评价

为探究淮河流域的土壤侵蚀状况及其空间分布特征,基于GIS技术和RUSLE模型,通过运用降雨、土壤类型、DEM、遥感影像和土地利用类型等数据确定模型中的参数因子,对2000年至2015年间淮河流域涉及的189个县（市、区）的土壤侵蚀强度及其空间分异特征进行了定量分析。结果表明:淮河流域土壤侵蚀区域主要分布在上游河南和湖北的丘陵地区;流域2000至2015年间土壤微度和轻度侵蚀占总土壤侵蚀量的84.51% ~ 96.10%,土壤侵蚀状况较轻;流域的土壤侵蚀量由2000年的8647.00 × 104 t a?1减少至2015年的5045.14 × 104 t a?1,2000年到2015年轻度等级以上的土壤侵蚀强度大部分都向低一级强度转移,流域土壤侵蚀状况得到明显改善;其中,2000年到2005年,流域的土壤侵蚀表现为极强度等级及以下,2005年到2015年流域土壤侵蚀强度等级减弱,均表现为中度等级及以下;强度以上的土壤侵蚀面积由2000年的300.94 km2下降为2015年的27.60 km2,土壤侵蚀强度逐年减弱。说明淮河流域长期的土壤侵蚀治理措施是卓有成效的,未来需进一步重点关注的区域是在林地和草地集中、坡度大、海拔高和坡向偏北的区域,需要确定重点区域的开发边界,严格控制过度开发。     

Soil carbonation processes as evidence of tillage-induced erosion

Tillage-induced soil erosion or redistribution increases spatial variation of several soil properties and often reduces the productive capacity of soil resources. Our objectives were to identify the extent of this type of erosion by observing the changes in soil morphological properties in the field and analysing its possible effects on soil productivity. The study was initiated in 2001 and conducted at two irrigated sites located approximately at Terena, Alandrol, 80 km east of Évora, Portugal. They were planted to corn (Zea mays L.) during this study, but have a long history of agricultural use with a trend toward increasing intensity in recent years. Soils in the field studies are classified mainly as Calcaric Regosols, Calcaric Cambisols, Luvisols and small areas of Fluvisols. The amount of erosion was estimated by simulation and verified by describing the lithology and measuring soil carbonates. The presence of carbonates in the superficial Ap horizons of soils that were previously devoid of this compound, provide evidence of soil redistribution: (1) in soils derived from calcareous parent material, this is the result of a re-carbonation process; (2) in soils derived from non-calcareous parent material the presence of carbonates in the superficial Ap horizons results from a carbonation process. On both sites, A and B, approximately 17% of the soils sampled were either carbonated or re-carbonated. Carbonation and re-carbonation of soil profiles confirmed that tillage had redistributed the soil-ploughing layer over time. Decreased corn yield was also observed as slope increase. If current agricultural practices are continued in this area, a decrease in soil quality and maximum yield on higher slopes can be expected.     

基于RUSLE的北洛河上游流域侵蚀产沙模拟研究

以RS、GIS和RUSLE模型结合SEDD模型,分析了退耕还林前后北洛河上游流域1990年、2000年和2010年土壤侵蚀强度和产沙量的时空变化特征。结果表明:3个时期年平均土壤侵蚀模数分别为18189.72,7 408.93,2 857.76t/(km~2·a),年均输沙模数分别为14 093.31,5 997.65,2 394.37t/(km~2·a),均呈减小趋势。3个时期的土壤侵蚀量在地形上的分布表现出趋同性,即高程上均在1 475~1 575m内平均侵蚀模数和侵蚀量表现出最大值。随着坡度增加,平均侵蚀模数增加,流域内75%以上的侵蚀量均来自于坡度15°区域。3个时期平均侵蚀模数均遵循阳坡半阳坡半阴坡阴坡的规律。研究为该区域生态环境建设效益评价及水土资源合理利用提供有益信息。     

Soil fertility differences across a land-use intensification gradient in the highlands of Chiapas,Mexico

In the highlands of Chiapas, southern Mexico, soil texture and soil chemical properties were measured in 70 agricultural fields covering a range of slope positions and managements. Fields represented four corn cropping systems: long fallow, short fallow, pasture–cultivation rotation, and annual continuous cultivation, in addition to fallow at rest (forest, shrubland, and pastures). Fields were located in four slope positions (upper, middle, and lower slopes, and doline floor) in a karst landscape developed on limestone with additions of acid volcanic ashes. Distribution of clays and sands were related to the toposequence and the percentage of clay fraction increased from upper slope to doline floor. Sand presented a reverse pattern. Some soil chemical properties also vary in a characteristic way along the toposequence. Exchangeable Ca²⁺ and Mg²⁺ effective cation exchange capacity (ECEC) and pH were higher in the doline floor than in the other slope positions. Soil organic matter and total N contents were 30.4 and 35.2% higher under long fallow than under annual continuous cultivation. Soil Olsen P was greater under cultivation than under fallow (irrespective of the cropping system), with the highest values under annual continuous cultivation (16.2±8.3 mg kg⁻¹) and the lowest in the forest (5.3±2.5 mg kg⁻¹). Exchangeable K⁺ was 74 and 51% higher in cultivated plots of the long and the short fallow than under forest and shrubland, respectively. Exchangeable Ca²⁺ and Mg²⁺ and ECEC did not vary significantly among the cropping systems. Inputs of basic cations through burning of tree and shrub vegetation increased the soil pH. These results indicate that land use intensification is leading to a decline in soil organic matter and total N, whereas Olsen P increased with intensification due to the continuous application of P fertilizers and sheep manure as soil conditioners in intensively cultivated fields.     

Chemical,physical and micromorphological properties of termite mounds and adjacent soils along a toposequence in Zona da Mata,Minas Gerais State,Brazil

Little is known about the effects of neotropical mound-building termites in soil chemical and physical properties. The influence of soil termite activity on soil characteristics was studied by assessing chemical, physical and micromorphological properties of a toposequence of Latosols (Oxisols). Soil samples were collected from the walls and inner parts of termite mounds and also from adjacent soil. A high diversity of termite genera was found in the mounds along the toposequence, together with the inquiline termites and other soil-dwelling arthropods. Chemical analyses showed that pH and the contents of organic C and N, P, Ca and Mg were significantly higher in termite mounds compared with adjacent areas, with an inverse trend for Al content. Significant differences in pH and exchangeable Al were observed between soil and mound across the slopes. The mound density across the landscape was higher at the upper slope segment, followed by the hill top, middle slope and lower slope segments. Considering a lifespan of 30 years and dimensions of termite mounds found in the toposequence we conclude that the textural and chemical uniformity of Latosols may be increased, following the pedobiological turnover during mound building, with local rates varying from 2.1 to 7.5 m³ ha^− 1.     

Evaluation of soil erosion risk and identification of soil cover and management factor (C) for RUSLE in European vineyards with different soil management

Vineyards show some of the largest erosion rates reported in agricultural areas in Europe. Reported rates vary considerably under the same land use, since erosion processes are highly affected by climate, soil, topography and by the adopted soil management practices. Literature also shows differences in the effect of same conservation practices on reducing soil erosion from conventional, bare soil based, management. The Revised Universal Soil Loss Equation (RUSLE) is commonly adopted to estimate rates of water erosion on cropland under different forms of land use and management, but it requires proper value of soil cover and management (C) factors in order to obtain a reliable evaluation of local soil erosion rates. In this study the ORUSCAL (Orchard RUSle CALibration) is used to identify the best calibration strategy against long-term experimental data. Afterwards, ORUSCAL is used in order to apply the RUSLE technology from farm based information across different European wine-growing regions. The results suggest that the best strategy for calibration should incorporate the soil moisture sub-factor (S_m) to provide better soil loss predictions. The C factor, whose average values ranged from 0.012 to 0.597, presented a large spatial variability due to coupling with local climate and specific local management. The comparison across the five wine-growing regions indicates that for the soil protection management, permanent cover crop is the best measure for accomplishing sustainable erosion rates across the studied areas. Alternate and temporary cover crops, that are used in areas of limited water resources to prevent competition with vines, failed to achieve sustainable erosion rates, that still need to be addressed. This raises the need for a careful use of C values developed under different environmental conditions.     

Recent land-use and soil-erosion history within a small catchment in Connemara, western Ireland: evidence from lake sediments and documentary sources

Environmental magnetism, elemental chemistry, pollen, macrofossil, and radioisotopes were studied on top-metre cores from Ballydoo Lough, Connemara, western Ireland, to reconstruct the impact of changing farming practices on soil erosion in a lake catchment. Documentary evidence, including detailed agricultural statistics, gave an independent land-use history over the period represented in the sediment record, i.e. most of the last two centuries. Maximum soil erosion, which involved unweathered coarse material as well as topsoil, was associated with a rapidly rising rural population and increased arable activity during the early 19th century. The Great Famine (1845–1847) halved the population, but arable and especially pastoral farming quickly recovered and were maintained at a high level from the late 1850s until the late 1890s. During this time, the rate of soil erosion slowed down considerably and the contribution of coarse mineral material relative to topsoil declined. Erosion of peaty soils was less important as potato cultivation declined and the middle slopes of the catchment were no longer cultivated. From the end of the 18th century to the 1970s, sediment accumulation and erosion rates were less and progressive podzolisation occurred as pastoral and particularly arable farming declined. The uppermost sediments record a sharp increase in soil erosion, involving mainly organic-rich soils from the upper slopes. This increase is attributed to a sharp rise in livestock numbers, especially sheep. A feature of the pollen record is the high arboreal pollen representation (43%–66% of total terrestrial pollen), which consists mainly of secondary pollen. It indicates a substantial input of pollen-bearing organic detritus derived from peaty soils and peat cuttings in the catchment.     

Acid sulphate soil characterization in Negara Brunei Darussalam: a case study to inform management decisions

A diverse range of acid sulphate soils occur in Negara Brunei Darussalam on the inland flat areas that are important agricultural lands. Prior to this study, there was no information on their occurrence. Information about these soils is critical because they present significant management challenges for both agriculture and protection of the environment. Field surveys and laboratory analysis conducted in eight areas of the Brunei‐Muara district and four areas of the Belait district identified, characterized and classified using Soil Taxonomy, a wide range of 10 acid sulphate soil types in four soil orders: Histosols, Vertisols, Inceptisols and Entisols. A user‐friendly soil identification key using easily observed soil characteristics was developed to assist users with the recognition of the range of acid sulphate soils. Conceptual soil hydro‐toposequence models in the form of cross‐sections were constructed to explain the spatial heterogeneity of (i) acid sulphate soil properties comprising a range of features (e.g. organic‐rich materials/peats, clays, sands, cracks and jarosite‐rich mottles), sulphidic material and sulphuric horizons, (ii) pyrite shale outcrops and (iii) soil types using both the soil identification key and Soil Taxonomy. The soil hydro‐toposequence models together with the soil identification key helped to easily visualize and illustrate the complexities and importance of understanding specific sites to assess the detailed behaviour and implications of various soil, regolith and topographic features.     

Soil loss estimation using rusle model to prioritize erosion control in KELANI river basin in Sri Lanka

Soil erosion contributes negatively to agricultural production, quality of source water for drinking, ecosystem health in land and aquatic environments, and aesthetic value of landscapes. Approaches to understand the spatial variability of erosion severity are important for improving landuse management. This study uses the Kelani river basin in Sri Lanka as the study area to assess erosion severity using the Revised Universal Soil Loss Equation (RUSLE) model supported by a GIS system. Erosion severity across the river basin was estimated using RUSLE, a Digital Elevation Model (15 × 15 m), twenty years rainfall data at 14 rain gauge stations across the basin, landuse and land cover, and soil maps and cropping factors. The estimated average annual soil loss in Kelani river basin varied from zero to 103.7 t ha-1 yr⁻¹, with a mean annual soil loss estimated at 10.9 t ha⁻¹ yr⁻¹. About 70% of the river basin area was identified with low to moderate erosion severity (<12 t ha−1 yr⁻¹) indicating that erosion control measures are urgently needed to ensure a sustainable ecosystem in the Kelani river basin, which in turn, is connected with the quality of life of over 5 million people. Use of this severity information developed with RUSLE along with its individual parameters can help to design landuse management practices. This effort can be further refined by analyzing RUSLE results along with Kelani river sub-basins level real time erosion estimations as a monitoring measure for conservation practices.     

紫色土区小流域土壤保持服务功能的空间分布特征

[目的]分析不同坡度、土壤类型、土地利用下的土壤保持服务功能分布,为减少土壤流失和改进土地利用规划提供科学依据。[方法]结合修正通用土壤流失方程(RUSLE)和GIS技术,以曲水河小流域为典型研究区,基于DEM、土壤、土地利用等基础数据,分析了研究区土壤保持服务功能的分布现状及影响因素。[结果](1)该流域土壤保持总量为1.10×107 t/a,单位面积土壤保持量为384.74t/(hm2·a),具有较高的生态系统服务功能重要性;(2)土壤保持服务功能总体呈现从河谷阶地逐渐向周边丘陵坡地递增的空间分布特征;(3)强烈及以上侵蚀强度区、8°~25°的坡度区、耕地、钙质紫泥田区和红棕紫泥土区应为土壤保持工作的热点区域。[结论]不同坡度、土壤类型、土地利用类型下土壤保持服务功能差异显著,可以通过改良土壤、提高植被覆盖和采取土壤侵蚀防治措施提高土壤保持服务功能水平。     

Assessment of soil degradation in the Canary Islands (Spain)

In the Canary Islands a number of factors, both natural and induced by human activity, act on the fragile ecosystems and agricultural land to cause increasing problems with desertification and progressive degradation of soil productivity. the results of an assessment of soil degradation in the Canary Islands, The processes, causes and impacts, are presented in this paper. Although several processes and factors contribute to soil degradation in the Canary Islands, two have been found to exert a greater qualitative influence: (a) accelerated erosion (water and aeolian); (b) salinization-sodification (natural and induced by agricultural use). Approximately 40 per cent of the Canary Islands' land is undergoing rapid erosion. the factors involved May, be grouped into: natural erosion—torrential rainfall, sparse vegetation, high soil erodibility, rugged relief; and erosion due to human activities—unsuitable management of arable soils on the steep slopes, overgrazing and deforestation. About 60 per cent of the surface of the archipelago, including areas given over to intensive agriculture, is affected by salinization.The main factors responsible are: natural—an arid climate and a regime of oceanic winds; and human activities—overexploitation of the aquifers, irrigation with water having a high salt and/or sodium content, intensive monoculture, and excessive and indiscriminate use of chemical fertilizers and other agrochemicals.     

Spatial Estimation of Soil Erosion Risk by Land‐cover Change in the Andes OF Southern Ecuador

Ecuador has the highest deforestation rate in South America, causing large‐scale soil erosion. Inter‐Andean watersheds are especially affected by a rapid increase of the population leading to the conversion of large areas of montane forest into pasture and cropland. In this study, we estimate soil erosion risk in a small mixed land‐use watershed in the southern Andes of Ecuador. Soil loss was estimated at a spatial resolution of 30 m, using the Revised Universal Soil Loss Equation (RUSLE) where the RUSLE factors were estimated on the basis of limited public available data. Land‐cover maps for 1976, 2008 and 2040 were created assuming increasing deforestation rates over the ensuing decades. Greater erosion rates are estimated for succession areas with agricultural cropland and pasture with maximum values of 936 Mg ha⁻¹ y⁻¹, where slopes and precipitation amounts are the greatest. Under natural forest vegetation, the estimated soil erosion rates are negligible (1·5 to 40 Mg ha⁻¹ y⁻¹) even at steep slopes and higher elevations where rainfall amounts and intensities are generally higher. When the entire watershed has undergone substantial deforestation in 2040, erosion values may reach 2,021 Mg ha⁻¹ y⁻¹. Vegetation cover is the most important factor for potential soil erosion. Secondary factors are related to rainfall (R‐factor) and topography (LS factors). Although the spatial predictions of potential soil erosion have only limited meaning for erosion risk, this method provides an important screening tool for land management and assessment of land‐cover change. Copyright © 2013 John Wiley & Sons, Ltd.     

Erodibility of agricultural soils on the Loess Plateau of China

Soil erodibility is thought of as the ease with which soil is detached by splash during rainfall or by surface flow. Soil erodibility is an important factor in determining the rate of soil loss. In the universal soil loss equation (USLE) and the revised universal soil loss equation (RUSLE), soil erodibility is represented by an erodibility factor (K). The K factor was defined as the mean rate of soil loss per unit rainfall erosivity index from unit runoff plots. Although high rate of soil loss from the Loess Plateau in China is well known and widely documented, it is remarkable that there is little systematic attempt to develop and validate an erodibility index for soils on the Loess Plateu for erosion prediction. Field experimental data from four sites on the Loess Plateau were analyzed to determine the K factor for USLE/RUSLE and to compare with another erodibility index based on soil loss and runoff commonly used for the region. The data set consists of event erosivity index, runoff, and soil loss for 17 runoff plots with slope ranging from 8.7 to 60.1%. Results indicate that the K factor for USLE/RULSE is more appropriate for agricultural soils on the Loess Plateau than the erodibility index developed locally. Values of the K factor for loessial soils range from 0.0096 to 0.0269 t h/(MJ mm). The spatial distribution of the K value in the study area follows a simple pattern showing high values in areas with low clay content. For the four sites investigated, the K factor was significantly related to the clay content, (K=0.031−0.0013 Cl, r²=0.75), where Cl is the clay content in percent. The measured values of the K factor are systematically lower than the nomograph-based estimates by a factor of 3.3–8.4. This implies that use of the nomograph method to estimate soil erodibility would considerably over-predict the rate of soil loss, and local relationship between soil property and the K factor is required for soil erosion prediction for the region.     

Quantitative assessment of effectiveness of soil conservation measures using a combination of Cs radioactive tracer and conventional techniques

Quantitative assessment of soil redistribution in landscapes remains a challenging task. In this study we used radioactive soil redistribution tracer ¹³⁷Cs together with soil morphological characteristics and empirically-based modeling for quantitative assessment of long-term soil conservation effectiveness. Three pairs of arable slopes were selected, all located within the territory of the Novosil experimental station (the Orel Region, central European Russia). One slope in each pair undergone creation of artificial terraces with forest shelter belts located parallel to topography contour lines and spaced at approximately 100 m from each other.Preliminary results have shown that slopes with soil-protective measures are characterized by a 11–80% reduction of average soil redistribution rates, as shown by soil profile morphology and ¹³⁷Cs methods. Discrepancy in values obtained can be attributed to differences in temporal resolution of methods as well as possible influence of individual extreme events on results yielded by the ¹³⁷Cs method. On the other hand, more significant decrease in average soil degradation rates on slopes with soil conservation (62–75% for each pair of slopes) was predicted by the model.The ¹³⁷Cs method overestimates gross and net soil redistribution rates, as a result of the influence of extreme erosion prior to tillage mixing of a fresh fallout isotope, not accounted for by calibration models used. Another shortcoming of the estimations obtained is that sediment redeposition directly within forest belts was not taken into account. Therefore, net erosion rates obtained for slopes with forest belts should be regarded as overestimation. Nevertheless, it can be generally concluded that the multi-technical approach has allowed acquiring much more detailed information on temporal and spatial variability of soil redistribution rates than single method-based studies.     

Applications and limitations of chernobyl radiocaesium measurements in a carpathian erosion investigation,Poland

The bomb-test fallout radionuclide caesium-137, has found increasing application in geomorphological investigations of soil erosion. Comparatively little work has investigated the potential for using ¹³⁴Cs and ¹³⁷Cs derived from the 1986 Chernobyl accident. Results are reported from an agricultural foothill environment in the Beskidy Mountains of southern Poland. The high degree of spatial variability associated with Chernobyl fallout deposition poses considerable limitations on the potential for using radiocaesium measurements to elucidate detailed patterns of soil loss. Despite this problem, the redistribution of radiocaesium from field plots to terrace edges suggests a means for estimating the overall budgets for sediment transfer on cultivated slopes.     

基于RUSLE模型的湟水流域土壤侵蚀时空变化

研究黄河上游土壤侵蚀的时空变化对于维持黄河上游生态系统服务功能、保护黄河上游水塔具有重要意义。以黄河上游典型区域湟水流域为研究区,采用RUSLE模型定量评估了该流域2000—2015年土壤侵蚀的时空变化特征,并分析了有无梯田措施下土壤侵蚀的空间变化,从而量化了梯田建设对防治坡面土壤侵蚀的影响。结果表明:2000—2015年,湟水流域的土壤侵蚀强度整体呈现减小趋势,侵蚀模数由1 183 t/(km~2·a)降低至940 t/(km~2·a),减少幅度为20.54%。不同土地利用类型以及不同坡度下的土壤侵蚀强度均有所降低,其中耕地上的减幅最大为20.58%。15°～20°坡度区间的侵蚀模数减幅最显著,为23.11%。通过有无梯田措施情景模拟发现,湟水流域2015年土壤侵蚀模数由940 t/(km~2·a)降低至有梯田的837 t/(km~2·a),减少11.00%。研究结果可为流域的水土流失防治和生态环境保护提供科学依据。     

Hydrolytic enzyme activities in agricultural and forest soils. Some implications for their use as indicators of soil quality

Although a great deal of information exists about the effect of land use on soil enzyme activities, much of this is contradictory and brings into question the suitability of soil enzyme activities as indicators of how land use affects soil quality. The purpose of this study was to investigate the effect of land use on different soil biochemical properties, especially hydrolytic enzyme activities, with the aim of providing knowledge about the problems related to the use of enzymes as indicators of soil quality. The data presented derive from various studies in which a large number of soils under different types of forest or agricultural management were analysed by the same methods. All of the soil samples were characterized in terms of their main physical and chemical properties, the activity of several hydrolases, microbial biomass C and soil basal respiration. The results indicate that soil use causes a large reduction in organic matter content and that the effect on enzyme activity varies depending on the type of land use or management and the type of enzyme. Furthermore, the enzyme activities per carbon unit (specific activities) in soils affected by land use are almost always higher than in maximum quality soils (climax soils under oak vegetation or oak soils), and land use also generates greater increases in the specific activity as the C content decreases. The mechanism responsible for these increases probably involves loss of the most labile organic matter. Enzyme enrichment is not always produced to the same degree, as it varies as a function of the enzyme and the type of land use under consideration. It is concluded that the complexity of the behaviour of the soil enzymes raises doubts about the use of enzyme activities as indicators of soil degradation brought about by land use.     

Modeling spatial variation in productivity due to tillage and water erosion

The advent of precision farming practices has heightened interest in managing field variability to optimize profitability. The large variation in yields across many producer fields demonstrated by yield–monitor–equipped combines has generated concern about management-induced causes of spatial variation in soil productivity. Soil translocation from erosion processes may result in variation in soil properties across field landscape positions that produce long-term changes in soil productivity. The objective of this study was to examine the relationships between soil redistribution caused by tillage and water erosion and the resulting spatial variability of soil productivity in a soil catena in eastern South Dakota. An empirical model developed to estimate tillage erosion was used to evaluate changes expected in the soil profile over a 50-year period on a typical toposequence found in eastern South Dakota and western Minnesota. Changes in the soil profile due to water erosion over a 50-year period were evaluated using the WEPP hillslope model. The tillage erosion model and the WEPP hillslope model were run concurrently for a 50-year period to evaluate the combined effect of the two processes. The resulting changes in soil properties of the root zone were evaluated for changes in productivity using a productivity index model. Tillage erosion resulted in soil loss in the shoulder position, while soil loss from water erosion occurred primarily in the mid to lower backslope position. The decline in soil productivity was greater when both processes were combined compared to either process acting alone. Water erosion contributed to nearly all the decline in soil productivity in the backslope position when both tillage and water erosion processes were combined. The net effect of soil translocation from the combined effects of tillage and water erosion is an increase in spatial variability of crop yields and a likely decline in overall soil productivity.