AbstractIntensive fertilization of grasslands with cattle slurry can cause high environmental nitrogen (N) losses in form of ammonia (NH3), nitrous oxide (N2O), and nitrate (NO3−) leaching. Still, knowledge on short-term fertilizer N partitioning between plants and dinitrogen (N2) emissions is lacking. Therefore, we applied highly 15N-enriched cattle slurry (97 kg N ha−1) to pre-alpine grassland field mesocosms. We traced the slurry 15N in the plant-soil system and to denitrification losses (N2, N2O) over 29 days in high temporal resolution. Gaseous ammonia (NH3), N2 as well N2O losses at about 20 kg N ha−1 were observed only within the first 3 days after fertilization and were dominated by NH3. Nitrous oxide emissions (0.1 kg N ha−1) were negligible, while N2 emissions accounted for 3 kg of fertilizer N ha−1. The relatively low denitrification losses can be explained by the rapid plant uptake of fertilizer N, particularly from 0–4 cm depth, with plant N uptake exceeding denitrification N losses by an order of magnitude already after 3 days. After 17 days, total aboveground plant N uptake reached 100 kg N ha−1, with 33% of N derived from the applied N fertilizer. Half of the fertilizer N was found in above and belowground biomass, while at about 25% was recovered in the soil and 25% was lost, mainly in form of gaseous emissions, with minor N leaching. Overall, this study shows that plant N uptake plays a dominant role in controlling denitrification losses at high N application rates in pre-alpine grassland soils.

Cobalamin (Vitamin B12) is a cofactor for many enzymes, including those in bacteria, archaea, algae, and mammals. In humans, cobalamin deficiency can lead to pernicious anaemia as well as gastrointestinal and neurological disorders. In contrast to marine ecosystems, there is a great paucity of information on the role of soils and terrestrial plants in the supply of cobalt and cobalamin to microorganisms and animals. The content of cobalt cations in most soils is usually sufficient to maintain growth, and the density of cobalamin-producing soil prokaryotes is high in comparison to water bodies. The cobalt content of most soils is usually sufficient in comparison with water, and the density of cobalamin-producing soil prokaryotes is high. Therefore, terrestrial plants are an important cobalt source for cobalamin-producing rumen and gut prokaryotes. The major source of cobalamin for most other animals is the meat of ruminants as well as other animal-derived products, bacteria in insects, and coprophagy, e.g., by rodents. In addition, faecal deposits, and fertilizers as well as soil bacteria add to the cobalamin supply. However, those archaea and bacteria that do not produce cobalamin obtain this coenzyme or its analogues from the environment. Therefore, presence or absence of cobalamin-producing species in soil affects the whole soil microbiome. However, our knowledge concerning microbial producers and consumers of cobalamin in soils is still limited, despite some recent advances. The main reasons are a low cobalamin content in soils and challenging methods of determination. In this regard, advanced analytical knowledge and technical equipment are required, which are usually unavailable in soil laboratories. This review provides relevant methodological information on sample homogenization, extraction, concentration, and purification as well as analysis of cobalamin.

Polyphosphates (Poly-P) are known to fulfil several important physiological functions. Many microorganisms can accumulate large amounts of Poly-P in their biomass. Regardless of these facts, systematic research on Poly-P in soil is missing, probably due to the absence of any method of direct Poly-P quantification. In this study, we attempted to unequivocally prove the presence of Poly-P in the biomass of soil microorganisms and quantify their extractability and contribution to microbial biomass phosphorus. To do so, we combined several approaches that can indicate Poly-P presence in soil microbial biomass indirectly, i.e. growth of soil inoculum on media without phosphorus, associated with measurement of changes in the microbial biomass stoichiometry, and the colour of the microbial suspension stained by the Neisser method. All soil microbial communities exhibited growth on media without phosphorus. As the growth on this media depleted Poly-P content, the biomass carbon to phosphorus and nitrogen to phosphorus ratio increased and the colour of the microbial suspension stained by the Neisser method changed predictively. The associated Poly-P addition experiment indicated that the recovery of added Poly-P from soil in form of soluble reactive phosphorus in sodium bicarbonate extract may reach up to 93% mainly due to abiotic depolymerization. Using a simple stoichiometric model applied to measured data, we calculated that the Poly-P content of microbial biomass in our soils may be up to 45 or 70% of total microbial biomass phosphorus depending on the assumptions applied regarding parameter values. We discuss the magnitude of error associated with the measurement of soil microbial phosphorus due to the high extractability of Poly-P.

Nitrogen (N) is a crucial nutrient for the growth and activity of rhizosphere microorganisms, particularly during drought conditions. Plant root-secreted mucilage contains N that could potentially nourish rhizosphere microbial communities. However, there remains a significant gap in understanding mucilage N content, its source, and its utilization by microorganisms under drought stress. In this study, we investigated the impact of four maize varieties (DH02 and DH04 from Kenya, and Kentos and Keops from Germany) on the secretion rates of mucilage from aerial roots and explored the origin of mucilage N supporting microbial life in the rhizosphere. We found that DH02 exhibited a 96% higher mucilage secretion rate compared to Kentos, while Keops showed 114% and 89% higher secretion rates compared to Kentos and DH04, respectively. On average, the four maize varieties released 4 μg N per root tip per day, representing 2% of total mucilage secretion. Notably, the natural abundance of 15N isotopes increased (higher δ15N signature) with mucilage N release. This indicates a potential dilution of the isotopic signal from biological fixation of atmospheric N by mucilage-inhabiting bacteria as mucilage secretion rates increase. We proposed a model linking mucilage secretion to a mixture of isotopic signatures and estimated that biological N fixation may contribute to 45 - 75% of mucilage N per root tip. The N content of mucilage from a single maize root tip can support a bacterial population ranging from 107 to 1010 cells per day. In conclusion, mucilage serves as a significant N-rich resource for microbial communities in the rhizosphere during drought conditions.

In a three-year field study, we inoculated two potato varieties with a selection of four beneficial microbial strains (i.e. Rhizophagus irregularis MUCL41833, Trichoderma asperelloides A, Pseudomonas brassicacearum 3Re2-7 and Paraburkholderia phytofirmans PsJN), alone or in combination. Plants were grown under rainfed or irrigated conditions, and potato yield and development of several diseases were evaluated. The microbial inoculants were traced in the root system at different stages of crop development via molecular markers. Whatever the water supply, the inoculants had no effect on yield. Conversely, some of the inoculants were able to lower the incidence and/or severity of several blemish diseases, namely common scab-associated symptoms (CSAS) and silver scurf/black dot-associated symptoms (SSAS). Microbial consortia were more efficient in decreasing symptoms compared to single strain inoculations. The best control was obtained with the combination of R. irregularis and P. brassicacearum, which reduced the incidence of CSAS by 22% and severity of SSAS by 21%. Root tracking revealed that P. brassicacearum and P. phytofirmans PsJN were able to establish in the root system of the potato, while only P. brassicacearum was detected from emergence until flowering of the plants.