Clover Species Specific Influence on Microbial Abundance and Associated Enzyme Activities in Rhizosphere and Non-Rhizosphere Soils

Legume cultivation, especially the clover species, has shown promoting effects on soil biological properties. However, the ways in which various clover species contribute to beneficial plant-rhizosphere soil interactions have remained neglected in the past. Therefore, we performed a field experiment to assess and compare the species-specific influence of five different clover species on plant traits, microbial soil health indicators, namely soil enzymes, microbial biomass and abundance and their potential nutrient cycling abilities under rhizosphere and non-rhizosphere soils. For this, soil samples from bulk soil and rhizosphere of each clover species were collected and analyzed for soil enzymes including β-glucosidase, arylsulfatase, phosphatase, N-acetyl-β-D-glucosaminidase, and urease and microbial communities’ abundance. Results revealed that the soil biological properties were more affected in the rhizosoil than in the bulk soil, although the individual legume crop variants differed in the rate and extent of the differential impact on either rhizosoil or bulk soil. The most significantly affected species-specific properties were ammonium oxidizing bacteria and phosphorus-solubilizing microbiota in the rhizosoil of white clover and alsike clover variants, whereas the least impact was exerted by sweet clover. The biological properties of rhizosoil showed a significant effect on the plant qualitative and quantitative properties. We further detected antagonism among N and P + K transfer from the rhizosoil to plants, which influenced above ground and root biomass. Overall, these results suggest that the positive effects of clover species cultivation on rhizosphere soil properties are species specific.

Optimal Concentrations of Silicon Enhance the Growth of Soybean (Glycine max L.) Cultivars by Improving Nodulation, Root System Architecture, and Soil Biological Properties

Today, the beneficial role of silicon (Si) in increasing the growth and yield of monocotyledons has been proven. But the effect of this useful element on dicotyledonous plants such as legumes has been less studied. In addition, the effect of Si on the development of roots and on nodulation in soybeans is still an unexplored research area. In this study, the effect of different levels of Si (0, 100, 200, 400, 600 and 800 mg Si kg-1 soil) from potassium silicate source on some soil biological properties, root morphological characteristics and nutritional responses of four soybean cultivars (cv., Katool, Sari, Saland and Saman with specific growth groups and identity cards) was studied under greenhouse conditions. The results showed that Si application in all cultivars caused a significant increase in shoot dry weight, root length and increased Si and nitrogen uptake in soybean shoots. Also, the application of Si increased nodulation in four soybean cultivars compared to the control treatment. The observed different responses to Si addition were cultivar-specific, probably related with the various Si efficiency strategies developed by these four soybean cultivars. The responses of soybeans to the application of Si levels were finally positive up to the level of 600 mg Si kg-1 and at higher levels there were no any increase (or an inhibitory effect) in nutritional responses and other growth characteristics compared to control. Silicon also caused a significant increase in total bacterial population, silicate-solubilizing bacteria population, microbial biomass, and microbial respiration rate of the soil under cultivation of different soybean cultivars. In this study, the improved growth (shoot dry weight) of soybean cultivars associated with Si treatment was highly correlated with nodulation, root morphological traits, and soil biological properties. In general, our findings suggest that optimal concentrations of Si can be a promising way to improve the production of soybean cultivars.

Earthworms as an Ecological Indicator of Soil Recovery after Mechanized Logging Operations in Mixed Beech Forests

Soil damage caused by logging operations conducted to obtain and maximize economic benefits has been established as having long-term effects on forest soil quality and productivity. However, a comprehensive study of the impact of logging operations on earthworms as a criterion for soil recovery has never been conducted in the Hyrcanian forests of Iran. The aim of this study was to determine the changes in soil biological properties (earthworm density and biomass) and its recovery process under the influence of traffic intensity, slope and soil depth in various intervals according to age after logging operations. Soil properties were compared among abandoned skid trails with different ages (i.e., 3, 10, 20, and 25 years) and an undisturbed area. The results showed that earthworm density and biomass in the high traffic intensity and slope class of 20–30% at the 10–20 cm depth of the soil had the lowest value compared to the other treatments. Twenty-five years after the logging operations, the earthworm density at soil depth of 0–10 and 10–20 cm was 28.4% (0.48 ind. m−2) and 38.6% (0.35 ind. m−2), which were less than those of the undisturbed area, respectively. Meanwhile, the earthworm biomass at a soil depth of 0–10 and 10–20 cm was 30.5% (2.05 mg m−2) and 40.5% (1.54 mg m−2) less than the values of the undisturbed area, respectively. The earthworm density and biomass were positively correlated with total porosity, organic carbon and nitrogen content, while negatively correlated with soil bulk density and C/N ratio. According to the results, 25 years after logging operations, the earthworm density and biomass on the skid trails were recovered, but they were significantly different with the undisturbed area. Therefore, full recovery of soil biological properties (i.e., earthworm density and biomass) takes more than 25 years. The conclusions of our study reveal that the effects of logging operations on soil properties are of great significance, and our understanding of the mechanism of soil change and recovery demand that harvesting operations be extensively and properly implemented.