A. Roldán

Plant-growth-promoting rhizobacteria and arbuscular mycorrhizal fungi modify alleviation biochemical mechanisms in water-stressed plants

This study examined the effect of inoculation with the plant-growth-promoting rhizobacterium (PGPR) Pseudomonas mendocina Palleroni, alone or in combination with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith) or Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe, on antioxidant enzyme activities (superoxide dismutase, catalase and total peroxidase activities), phosphatase and nitrate reductase activities and solute accumulation in leaves of Lactuca sativa L. cv. Tafalla affected by three different levels of water stress. At moderate drought, bacterial inoculation and mycorrhizal inoculation with G. intraradices, alone or in combination, stimulated significantly nitrate reductase activity. At severe drought, fertilisation and P. mendocina inoculation, alone or in combination with either of the selected AM fungi, increased significantly phosphatase activity in lettuce roots and proline accumulation in leaves. Total peroxidase (POX) and catalase (CAT) activities increased in response to drought, whereas superoxide dismutase activity decreased. Inorganic fertilisation and both combined treatments of PGPR and AM fungus showed the highest values of leaf POX activity under severe drought. The highest CAT activity was recorded in the fertilised plants followed by the P. mendocina-inoculated plants grown under severe stress conditions. These results support the potential use of a PGPR as an inoculant to alleviate the oxidative damage produced under water stress.

Analysis of the mycorrhizal potential in the rhizosphere of representative plant species from desertification-threatened Mediterranean shrublands

An evaluation of the mycorrhizal status of desertification-threatened ecosystems has been recommended as a first step in rehabilitation/restoration approaches based on revegetation strategies using arbuscular mycorrhizal (AM) technology. Representative desertified semiarid areas were selected from southeast Spain where the vegetation is dominated by grasses, with Stipa tenacissima usually present, and with some patches of the shrubs Pistacia lentiscus, Rhamnus lycioides, Olea europaea subsp. sylvestris and Retama sphaerocarpa. The objective of this study was to evaluate the mycorrhizal potential in these soils, the contribution of the different species established to the mycorrhizal potential of the soils and to assess the main mycorrhizal propagules involved. There were more AM fungal propagules in the rhizospheres of all the shrub species studied compared with adjacent fallow soils, suggesting that AM propagules can be considered as a functional component of the resource islands developing around plant roots. R. sphaerocarpa and O. europaea had a higher capacity to enhance the development of mycorrhizal propagules in their rhizospheres than R. lycioides and P. lentiscus. Correlation analyses showed that the number of spores of the most representative AM fungal species, i.e. Glomus constrictum, and the total length of extraradical AM mycelium are the propagule sources which were best correlated with the mycorrhizal potential in terms of the number of “infective” AM propagules in the rhizosphere of the target plant species. The contribution of AM symbiosis to the potentiality of S. tenacissima as nurse plant was site dependent. Diversity of AM fungi present in the test area is rather low, indicating the high degree of degradation of the ecosystem. At most, only four AM fungal spore morphoecotypes were consistently detected in the rhizosphere of the target plant species.

An incubation experiment to determine factors involving aggregation changes in an arid soil receiving urban refuse

The effect of the addition of urban refuse (UR) on the percentage of stable aggregates in a semi-arid structureless soil was studied in a 67-day incubation experiment. Twelve systems were established combining amendments with two doses of UR (2.4 and 4.8%) and treatments with cycloheximide, chloramphenicol or formaldehyde to evaluate the relative efficiency of carbohydrates and microbial (fungi and bacteria) communities in promoting changes in aggregate stability. The addition of UR to the soil increased the abundance of stable aggregates and this increase was proportional to the quantity of UR added. The degree of aggregate stability was related to all the factors considered, although this relationship was more evident for the fungal communities (R = 0.81, P < 0.001) and, to a lesser extent, for the bacterial communities and carbohydrate content. Carbohydrates were significantly correlated with stable aggregates during the first stages of incubation (R = 0.83, P < 0.001, at day 4), but this effect was short-lived and the maintenance and increase of the aggregate stability in the subsequent phases was attributable to the increases in microbial populations, and particularly to the proliferation of fungal mycelium.

No-tillage, crop residue additions, and legume cover cropping effects on soil quality characteristics under maize in Patzcuaro watershed (Mexico)

Abstract Intensive maize ( Zea mays L.) cropping based on conventional tillage practices has resulted in soil quality degradation in the Patzcuaro Watershed in central Mexico. A field experiment with seven soil management treatments was implemented on a sandy loam Andisol to evaluate the impact on soil quality of maize cropping with conventional tillage, no-tillage with varying percentages of surface residue coverage (0, 33, 66 and 100%), and no-tillage with 33% residue coverage together with cover crops of either Vicia sp. or Phaseolus vulgaris L. The treatments of no-tillage under crop residue coverage were established in 1995 and the leguminous species were planted in 1998. By 2000, the alternative management treatments had increased soil enzymes, soil organic C, biodegradable C fractions such as water soluble C, water soluble carbohydrates, and microbial biomass C, and soil wet aggregate stability, compared to the CT treatment. Wet aggregate stability was increased by adopting no-tillage and even further by additional residue. Most soil quality characteristics improved in direct proportion to residue inputs. The use of no-tillage management together with a moderate amount of crop residue (33%) and planted to leguminous species rapidly improved some soil quality characteristics. We conclude that conservation tillage practices can provide an alternative technology contributing to sustainable agriculture in the Patzcuaro watershed of Mexico, which can be extrapolated to similar areas elsewhere in Latin America.

Establishment of shrub species in a degraded semiarid site after inoculation with native or allochthonous arbuscular mycorrhizal fungi

The re-establishment of native shrub species in the Mediterranean basin serves to restore the characteristic biodiversity and to prevent the processes of erosion and desertification in semiarid areas. A field experiment was carried out in an abandoned semiarid agricultural Mediterranean area to assess the effectiveness of mycorrhizal inoculation, with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L., Pistacia lentiscus L., Retama sphaerocarpa (L.) Boissier and Rhamnus lycioides L. seedlings in this area. One year after planting, shoot biomass of inoculated O. europaea and P. lentiscus seedlings was greater, by about 630% and 300%, respectively, than that of non-inoculated plants. Shoot biomass of G. claroideum-colonised R. sphaerocarpa plants was significantly greater than that of seedlings inoculated with the mixed native AM fungi after 12 months. The increase of R. lycioides growth due to inoculation with native AM fungi was significantly greater than that of G. claroideum-colonised seedlings during the same growth period. Inoculation with a mix of native AM fungi was the most effective treatment for increasing shoot biomass and N, P and K contents in shoot tissues of R. lycioides seedlings. The mixture of native AM fungi was the most effective with respect to colonisation of the roots of O. europaea and R. lycioides, but the native AM fungi and G. claroideum achieved similar levels of colonisation in P. lentiscus and R. sphaerocarpa. The use of native mycorrhizal potential as a source of AM inoculum may be considered a preferential inoculation strategy to guarantee the successful re-establishment of native shrub species in a semiarid degraded soil.

Aggregate stability changes after organic amendment and mycorrhizal inoculation in the afforestation of a semiarid site with Pinus halepensis

Abstract The recovery of soil structural stability is a precondition for successful afforestation programmes in semiarid environments. A multifactorial field experiment was carried out in a semiarid rangeland in south-eastern Spain to evaluate the influence of a fresh organic residue addition (first factor), mycorrhizal inoculation with Pisolithus arhizus (second factor), and the rhizosphere of Pinus halepensis (third factor) on soil aggregate stability. A total of 6 years after planting, the addition of residue was seen to increase the levels of stable aggregates to a greater extent than the mycorrhizal inoculation. Both reafforestation methods increased C-fractions and enzyme activities measured (dehydrogenase and phosphatase). The rhizosphere also affected aggregate stability, particularly when P. halepensis was inoculated with P. arhizus. Aggregate stability in the rhizosphere of P. halepensis was strongly correlated (P

Re-establishment of Retama sphaerocarpa as a target species for reclamation of soil physical and biological properties in a semi-arid Mediterranean area

Abstract Re-establishment of the indigenous shrub vegetation is a key step in the restoration of abandoned agricultural semi-arid lands. A field experiment was carried out to evaluate the effect of a composted residue and mycorrhizal inoculation with Glomus intraradices on the viability of Retama sphaerocarpa (L.) Boissier. The influence of the rhizosphere of this shrub legume on the physical–chemical and biological properties of an abandoned semi-arid agricultural area in south-eastern Spain was also assessed. Eighteen months after planting, the combined treatment of mycorrhizal inoculation and composted residue was the most effective for increasing the growth and the N and P contents in shoot tissues of R. sphaerocarpa. There was a highly positive significant correlation between shoot dry weight and the nutrient contents in shoot tissues. Water-soluble C, water-soluble carbohydrates, biomass C contents, and enzyme activities (dehydrogenase, urease, protease–N-α-benzoyl- l -argininamide (BAA), and acid phosphatase) measured in the rhizosphere of R. sphaerocarpa were higher than in the bare soil. Rhizosphere aggregate stability of R. sphaerocarpa was about 47% higher than that of bare soil. These improvements in the physical–chemical and biological properties of the rhizosphere soil of R. sphaerocarpa could facilitate the establishment and development of new plants in the surrounding area, which would aid the revegetation of semi-arid ecosystems.

Improvement of rhizosphere aggregate stability of afforested semiarid plant species subjected to mycorrhizal inoculation and compost addition

Soil aggregate stability is one of the most important properties controlling plant growth in semiarid Mediterranean environments. A field study was carried out to evaluate the effect of the rhizosphere of Olea europaea subsp. sylvestris and Rhamnus lycioides, the addition of a composted residue and mycorrhizal inoculation with Glomus intraradices on rhizosphere aggregate stability and on the viability of both plant species in a semiarid structureless soil. For both plant species, water-soluble carbon (WSC) content and enzyme activities (urease, acid phosphatase and β-glucosidase) measured in the rhizosphere aggregate were higher than in the non-rhizosphere soil. Rhizosphere aggregate stability of both plant species was on average 1.8-fold higher than that of non-rhizosphere aggregate. The addition of composted residue was the most effective treatment for increasing rhizosphere aggregate stability. The water-soluble carbon content was correlated positively with aggregate stability of the O. europaea rhizosphere. The mycorrhizal component was increasingly important for improving the growth of both seedlings following the addition of composted residue to soil under the severe climatological conditions of the area.

Assessing the effectiveness of mycorrhizal inoculation and soil compost addition for enhancing reafforestation with Olea europaea subsp. sylvestris through changes in soil biological and physical parameters

A field experiment was carried out in a semiarid area to assess the influence of mycorrhizal inoculation and soil compost addition on establishment of Olea europaea seedlings. Associated changes in soil biological and physical parameters were observed. One year after planting, both reafforestation methods had significantly improved the soil structure. Thus, mycorrhizal inoculation increased soil aggregate stability (AS) and composted residue addition decreased soil bulk density (BD). A significant correlation ( P< 0.05) was found between BD and several biochemical parameters (dehydrogenase, protease and -glucosidase activities), indicating that soil biological agents play an important role in improving soil structure. The growth of O. europaeawas significantly enhanced by both composted organic residue addition and mycorrhizal inoculation treatment. The increase in mycorrhizal O. europaeaseedling growth may be due to the positive influence of mycorrhiza on soil AS. The combination of high fertility levels and low BD also favoured the growth of O. europaea in compost-amended soils. Finally, the positive interaction between the two methods in relation to seedling height growth could be related to the capacity of the fungus to increase nutrient uptake from the composted residue.

Aggregate stability changes in a semiarid soil after treatment with different organic amendments

Structural stability of soils is one of the most important characteristics related to soil degradation in semiarid areas. Organic additions can considerably improve soil structure and prevent soil degradation. In this experiment the effectiveness was compared of four types of organic amendments on the improvement of soil structure. The four treatments are sewage sludge, fresh uncomposted urban refuse, composted urban refuse, and horse manure. The uncomposted urban refuse was the most effective in increasing soil stable aggregates. The higher doses of sewage sludge and compost also improved aggregation of soil particles. Horse manure did not significantly improve soil aggregation. A significant correlation (r = 0.955, ρ < 0.01) was found between fungal populations and stable aggregates. No significant correlations were found between stable aggregates in soil and the other parameters assessed. The organic treatments increasing soil microbial populations and promoting their activity appear to be very effecti...