Microsite differences in fungal hyphal length, glomalin, and soil aggregate stability in semiarid Mediterranean steppes

Glomalin is a recently discovered glypoproteinaceous substance produced by arbuscular mycorrhizal fungi (AMF) that plays an important role in structuring soil. We quantified soil fungal hyphal length and glomalin content at vegetated and open microsites in Stipa tenacissima steppes of SE Spain. Soils underneath the canopy of S. tenacissima had higher glomalin pools compared to open microsites. We also found significant differences between sites, suggesting the presence of landscape level heterogeneity in glomalin concentration. Soil fungal hyphal length also differed significantly among the sites, but there was no significant effect of microsite. Water-stable aggregates (1-2 mm diameter; WSA_1-2 mm), however, while differing among sites, did not vary as a function of microsite. Furthermore, WSA_1-2 mm was negatively correlated with glomalin fractions, as well as soil organic C. Carbonates were likely the major binding agents in these carbonate-rich (average carbonate content: 71%) soils, and not organic C (including glomalin). AMF-mediated stabilization of soil aggregates did not contribute to the formation and maintenance of fertile islands underneath the canopy of S. tenacissima.     

Soil management options for Alfisols in the semi-arid tropics: annual and perennial crop production

A field experiment was conducted in the semi arid tropics to study the effects of soil structural modification on cropping systems. The aim was to improve crop production and land resource protection using innovative soil management practices. Tillage, mulch and perennial/annual rotational based systems were compared for 5 years in an Alfisol at ICRISAT in India. Crop yield parameters, including grain and biomass yield, leaf area index, crop cover, and plant height were measured. Results indicate significant benefits to annual crop yield (maize, sorghum) from improved water supply due to mulching with farmyard manure or and rice straw, and due to rotation with prior-perennial crops. Grain yields were 16 to 59% higher in mulched treatments compared to unmulched treatments, with similar increases for fodder yields. Annual crop yields after 4 years of perennials were 14 to 81% higher than unmulched treatments, except for low fertility maize grown after buffel grass. The interaction with chemical fertility was less clear than for water supply. The results have implications for soil management throughout the semi-arid tropics.     

Interaction between AM fungi and a liquid organic amendment with respect to enhancement of the performance of the leguminous shrub Retama sphaerocarpa

This study examined the interactions between the inoculation with three arbuscular mycorrhizal fungi, namely, Glomus intraradices, Glomus deserticola and Glomus mosseae, and the addition of a liquid organic amendment at different rates (0, 50, 100 or 300 mg C of liquid amendment per kilogram soil) obtained by alkaline extraction of composted dry olive residue with respect to their effects on growth of Retama sphaerocarpa seedlings and on some microbiological and physical properties of soil. One year after planting, both mycorrhizal inoculation treatments and the addition of amendment had increased plant growth and dehydrogenase, urease and benzoyl argininamide hydrolysing activities. The inoculation with G. mosseae increased plant growth to a greater extent than the addition of the amendment (about 35% greater than plants grown in the amended soil and about 79% greater than control plants) and both treatments produced similar increases in soil aggregate stability (about 31% higher than control soil). The organic amendment produced a very significant decrease in the levels of microbial biomass C and a strong increase in soil dehydrogenase and urease activities, which were proportional to the amendment rate. Only the combined treatment involving the addition of a medium dose of amendment (100 mg C kg⁻¹ soil) and the mycorrhizal inoculation with G. intraradices or G. deserticola produced an additive effect on the plant growth with respect to the treatments applied individually (about 77% greater than plants grown in the amended soil and about 63% greater than inoculated plants).     

Spatial variability of infiltration patterns related to site characteristics in a semi-arid watershed

Preferential flow may strongly affect hydrology at different scales. Measurement of preferential flow however remains very difficult. Tracer-infiltration profiles are often used to measure the degree of preferential flow at plot scale. These experiments are time-consuming, costly and destructive. As a result existing dye-tracer studies are often based on a limited number of profiles. The aim of this study is therefore to select a limited number of soil and landscape characteristics with high predictive value for the degree of preferential flow. 18 rainfall simulations with a dye-tracer and additional site measurements such as soil type, vegetation type and soil physical parameters were performed in a 1 km² catchment in the Dehesa (Extremadura, Spain). A stepwise multiple regression procedure was used to select variables with a high predictive value for the degree of preferential flow.     

Variations in landscape patterns and vegetation cover between 1957 and 1994 in a semiarid Mediterranean ecosystem

The aim of this study was to analyze the main processes that determine changes in landscape patterns and vegetation cover from 1957-1994 to develop a model for land cover dynamics. Land cover and landscape patterns were assessed and compared using aerial photographs taken in 1957, 1985, and 1994. Over this period, tall grass steppe and arid garrigues increased by 6% and 4%, respectively, while crop fields decreased by 15% and tall arid brush remained the same. Over the same period, tall grass steppes and arid garrigues became less fragmented.Changes in land use were triggered by socioeconomic forces, which were constrained by the underlying structure of the physical landscape. The best preserved vegetation (tall arid brushes) was concentrated at higher elevations, with a pronounced slope, not oriented towards the sea, and in volcanic substrate. Communities tended to be better preserved further away from towns and at lower house densities. Tall grass steppe was present on more gradual sea-oriented slope and in calcareous substrate, and increased at higher elevations, although not far from the town but away from high anthropogenic influence. Previous studies have revealed that traditional land uses of this landscape, particularly grazing, favoured the transition from tall arid brush to tall grass steppe. In this study, we analyzed to what extent the underlying structure of the physical landscape imposes limitations to the vulnerability to human activity of the main vegetation types. According to the data on the probability of vegetation transition over the 37-year period, the shift from tall arid brush to tall grass steppe appeared to be favoured by gradual slopes. Tall arid brush recovered from either arid garrigues or tall grass steppes at steeper slopes. Thus, steep terrain had a favourable effect on the formation of brushwood and more gradual terrain favoured tall grass steppe. The prevalent trends were confirmed by a projection of a transition matrix over 100 years.This revised version was published online in May 2005 with corrections to the Cover Date.     

半干旱农牧交错区人工林退化:概化模型和现状评价

在干旱半干旱地区,造林通常被作为荒漠化防治最为有效的措施之一,然而这一区域人工林退化也是不可回避的事实。本文以半干旱农牧交错区的内蒙古伊金霍洛旗为例,对其林地退化状况进行评价。根据野外调查结果我们建立了一个林地退化概化模型,据此筛选出湿润指数、植被指数、土壤类型和土壤侵蚀模数4个评价指标,并采用GIS对该区人工林退化状况进行评价。结果表明,在336.09km2人工林中,退化面积达311.35km2,其中轻度、中度和重度退化面积分别为78.97,119.73和112.65km2。退化人工林的防治重点应放在现有林的改良和造林植物种特别是灌木种的选择上。图4参15。     

定西半干旱区小流域暴雨资源无害化利用的探讨

在分析定西半干旱区小流域暴雨径流量与土壤流失量的时间分布、空间分布规律的基础上,从暴雨资源时间调控和空间调控利用两方面提出了暴雨资源无害化利用的途径和方法。     

Runoff and sediment yield in a semi-arid environment: the effect of land management after farmland abandonment

This paper deals with the evolution of abandoned fields in semi-arid environments. The study has been carried out in the Central Ebro Depression, NE Spain, where large areas traditionally cropped with cereals alternating with fallow have been recently set-aside, following the agrarian policy of the European Union. Most of the information has been obtained using a rainfall simulator on 220 m², open plots, reproducing six possible land-uses: control plot 1 (abandoned), control plot 2 (abandoned, adding chemical fertilizer), cereal, fallow land, fallow land with organic fertilizer and fallow land with chemical fertilizer. The results obtained show that abandoned land produces a very quick response to precipitation, high peak flows and runoff coefficients, and a shallow wetting front, confirming the effects of the low density of plant cover and the development of a microcrust. Fertilizing fallow land plots increases the penetration of the wetting front, with moderate impact on the other hydrologic parameters. The study demonstrates that (i) surface runoff is controlled by the addition (or not) of fertilizer; and (ii) sediment concentration is mostly controlled by ploughing.     

Tree phenology and water availability in semi-arid agroforestry systems

Four common agroforestry trees, including both exotic and native species, were used to provide a range of leafing phenologies to test the hypothesis that temporal complementarity between trees and crops reduces competition for water in agroforestry systems during the cropping period and improves utilisation of annual rainfall. Species examined included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves once during the long dry season, and the evergreen Croton megalocarpus. Phenological patterns were examined in relation to climatic conditions in the bimodal rainfall regions of Kenya to identify factors which dictate the intensity of competition between trees and crops.

The main differences in leaf cover patterns were between indigenous and exotic tree species. The Central American species, S. spectabilis and G. sepium, shed their foliage during the dry season before the short rains, whereas the native species, M. volkensii and C. megalocarpus, exhibited reduced leaf cover during both dry seasons. C. megalocarpus was the only species to maintain leaf cover throughout the 2-year experimental period. M. volkensii and S. spectabilis exhibited similar leafing phenology, losing almost all leaf cover during the long dry season (July–October) and flushing before the onset of the ensuing rains. S. spectabilis lost few leaves during the short dry season, whereas M. volkensii shed a greater proportion of its foliage before flushing prior to the long rains (March–July). M. volkensii lost much of its leaf cover during the 1997/1998 short rains (October–February), when soil water content was unusually high. Although essentially evergreen, leaf cover in C. megalocarpus decreased during the dry season and increased rapidly during periods of high rainfall. G. sepium exhibited a period of low leaf cover during the long dry season and did not regain full leaf cover until mid-way through the short rains. The mechanisms responsible for these phenological changes and the implications of tree phenology for resource utilisation and competition with crops are discussed.     

中国半干旱区公众环境意识历史演变的研究

本文通过对半干旱地区公众环境意识历史演变的分析 ,总结出了环境意识演变的特点 ,指出了半干旱区公众环境意识历史演变形成的误区 ,并在此基础上提出了其变革的对策。