Biology and Fertility of Soils

Differentiated interactions between humic substances and isoelectric focusing-carrier ampholytes were demonstrated by comparing specific absorption ratios from the supernatant after precipitation with 2 M HCl and with acid ampholytes (pH 2–4) containing both carboxylic and sulphate groups. An increasingly stronger interaction was observed when comparing soil humic substances to those of synthetic origin or those extracted from young reclaimed soil material. Electrofocusing of the humic substances under study revealed a fingerprint-like quality; the band patterns observed were consistent with the concept of three main fractions.

Proteins are the main N compounds and are important sources of both C and N for plants and microbes. Triple-labeled dissolved organic N obtained from white clover shoots was separated into five different molecular weight (Mw) size fractions (> 1 kDa). In this study, we investigated the mineralization of and the presence of C and N remaining in soil solution over 14 days across different soil pH values. The evolved CO2 decreased by increasing Mw, suggesting an apparent bottleneck in the decomposition of organic N above Mw 30 kDa. The initial parallel loss of 13C and 15 N from soil was followed by an increase in dissolved 15 N describing a two-phase process of organic N turnover. The first-phase concerns the removal of organic N whereas the second-phase concerns the release of inorganic N. Microbial N and C use efficiency was increased with increasing Mw size suggesting the importance of organic N quality in predicting the impact on soil C and N cycling.

Yak and Tibetan sheep grazing is a common phenomenon on natural grasslands in the Qinghai-Tibetan Plateau, and large amounts of excrement are directly deposited onto alpine grasslands. However, little is known about the effects of excrement return on soil N supply and N retention capacity. This study investigated the short-term effects of yak and Tibetan sheep dung on gross N transformation rates determined by 15N tracing technique of alpine steppe (AS) and meadow (AM) soils at 60 % water holding capacity (WHC) under laboratory conditions. Cumulative gross N mineralization and NH4 + immobilization over the 21-day incubation period in AM soil were around 2.8 and 2.0 times as high as that in AS soil, respectively. Cumulative gross nitrification in AM soil was 0.96 times higher, while the value of gross NO3 − immobilization rate was 0.65 times lower than in AS soil, resulting in higher NO3 − accumulation in AM soil. Dung addition increased soil gross N mineralization and NH4 + immobilization rates by 12–35 % and 17–59 %, respectively. Amending yak and sheep dung decreased gross nitrification rates by 3–23 % but increased gross NO3 − immobilization rates by 25–190 %, which led to a decreased net NO3 − accumulation and NO3 − losses risk through leaching. The cumulative CO2 emissions over the 21 days of incubation period were enhanced by 65 and 120 % for AS and AM soil, respectively. The application of dung stimulated cumulative N2O emissions, but the stimulation was only significant in AM soil. In general, yak and sheep dung return has a positive effect on soil N supply and retention owing to the increase in NH4 + availability for plant with simultaneously decreasing NO3 − accumulation in soils.

We investigated whether tree species identity has a significant impact on the structure of soil bacterial communities in a tropical tree plantation (Sardinilla, Panama). The experimental site contains tree species native to Panama, planted in both monoculture and mixed-species plots. Using a DNA fingerprinting approach (automated ribosomal intergenic spacer analysis [ARISA]), we identified significant differences in the community structure of abundant bacterial taxa in the bulk soil among all monoculture plots. We similarly found differences among plots containing five, three, one or no tree species. While distance-based gradients in bacterial community structure were detected across the plantation, further investigation revealed that the observed heterogeneity was, in fact, poorly related to the tree species in a given plot. We provide evidence that site related features (e.g., variability in soil pH) play a more important role in regulating the structure of bacterial communities within the bulk soil than tree species identity or richness.

We have studied the effect of soil P-related properties and inoculum concentration on plant growth promotion by the phytase-producing strain Bacillus amyloliquefaciens FZB45. The response (shoot fresh weight/Pi content) of a mycorrhizal-independent plant, Chinese cabbage, was evaluated in a soil with well-defined P-related properties. Two inoculum concentrations were evaluated under four P regimes: no P addition, inorganic P, and two levels of phytate. Significant interaction between P regime and bacterial inoculation was found. FZB45 only promoted plant growth and P uptake at the higher rate of phytate, confirming that phytase activity is its major mechanism and that is limited by soil phytate availability. The effect caused by the lower inoculum concentration was superior than that by the higher, suggesting the simultaneous involvement of a direct effect. This effect was confirmed by a soilless test, which showed a hormone-like response. FZB45 produced IAA in vitro, but its role is to be determined. These results demonstrate that soil modulates the performance of plant growth-promoting rhizobacteria in a specific manner, consistent to the mechanisms of action involved. Determination of those mechanisms and their modulating factors helps predict conditions where plant growth promotion will result, an important step in increasing the consistency of PGPR.

Azospirillum is one of the most studied plant growth-promoting bacteria (PGPB); it represents a common model for plant-bacterial interactions. While Azospirillum brasilense is the species that is most widely known, at least 22 species, including 17 firmly validated species, have been identified, isolated from agricultural soils as well as habitats as diverse as contaminated soils, fermented products, sulfide springs, and microbial fuel cells. Over the last 40 years, studies on Azospirillum-plant interactions have introduced a wide array of mechanisms to demonstrate the beneficial impacts of this bacterium on plant growth. Multiple phytohormones, plant regulators, nitrogen fixation, phosphate solubilization, a variety of small-sized molecules and enzymes, enhanced membrane activity, proliferation of the root system, enhanced water and mineral uptake, mitigation of environmental stressors, and competition against pathogens have been studied, leading to the concept of the Multiple Mechanisms Hypothesis. This hypothesis is based on the assumption that no single mechanism is involved in the promotion of plant growth; it posits that each case of inoculation entails a combination of a few or many mechanisms. Looking specifically at the vast amount of information about the stimulatory effect of phytohormones on root development and biological nitrogen fixation, the Efficient Nutrients Acquisition Hypothesis model is proposed. Due to the existence of extensive agriculture that covers an area of more than 60 million hectares of crops, such as soybeans, corn, and wheat, for which the bacterium has proven to have some agronomic efficiency, the commercial use of Azospirillum is widespread in South America, with over 100 products already in the market in Argentina, Brazil, and Uruguay. Studies on Azospirillum inoculation in several crops have shown positive and variable results, due in part to crop management practices and environmental conditions. The combined inoculation of legumes with rhizobia and Azospirillum (co-inoculation) has become an emerging agriculture practice in the last several years, mainly for soybeans, showing high reproducibility and efficiency under field conditions. This review also addresses the use of Azospirillum for purposes other than agriculture, such as the recovery of eroded soils or the bioremediation of contaminated soils. Furthermore, the synthetic mutualistic interaction of Azospirillum with green microalgae has been developed as a new and promising biotechnological application, extending its use beyond agriculture.