Publications

<p>Greater plant diversity facilitates soil C gains, yet the exact mechanisms of this effect are still under intensive discussion. Whether a plant grows in monoculture or in an inter-cropped mixture can affect allocation of plant assimilates, belowground exudation, and microbial stimulation. The goal of this study was to examine the effects of inter-cropping on a previously overlooked aspect of plant-soil interactions, namely, on locations where plant assimilated C is allocated within the soil pore system and its subsequent fate in relation to soil pores. The soil for the study originated from a greenhouse experiment with switchgrass (Panicum virgatum L.) (var. Cave'n'Rock) (SW), big bluestem (Andropogon gerardii Vitman) (BB), and wild  bergamot (Monarda fistulosa L.) (WB) grown in monocultures and in inter-cropped pairs and subjected to species specific C pulse labeling (Kravchenko et al., 2021). Intact soil cores (8 mm Ø) were collected from the experimental pots, subjected to a short-term (10 day) incubation, X-ray computed micro-tomography (μCT) scanning, and soil C micro-sampling "geo-referenced" to μCT images. Results indicated that in the plant systems with demonstrated interplant C transfer soil C was positively correlated with <10 μm Ø pores immediately after plant termination and with 20-80 μmØ pores after the incubation. In the systems without marked interplant C transfer, soil C was positively correlated with 20-30 μm Ø pores, however, the correlations disappeared after the incubation. Soils from the systems with demonstrated belowground C transfer displayed lower losses of root-derived C during incubation than the systems where interplant C transfer was negligible. These differences suggest dissimilarities in the possible mechanisms of adding photoassimilated C to the soil: via mycorrhizal hyphae into small sized pores vs. via roots into mediumsized pores. In the latter case the plant-derived C was quickly lost during subsequent incubation. Our findings indicate that greater losses of plant assimilated C from the soil often reported during comparisons of monocultures with inter-cropped plant mixtures are related not only to monoculture vs. polyculture dichotomy, but to the route of plant C additions to the soil and its localization within the soil pores.</p>

Direct 33P labeling approach is a very powerful technique that has high sensitivity in tracing the fate of added phosphorus (P) fertilizers across various P pools. Nonetheless, only a few studies have used this approach. This study traced the fate of applied P fertilizer in acidic P-limited soil using the 33P labeling approach.The incorporation of 33P-labeled KH2PO4 in available P (PAEM), microbial biomass P (Pmic) and Fe/Al-bound P (PNaOH) pools was followed in Cambisol as influenced by C and N sources applied as glucose and ammonium sulfate, respectively.Results showed that not all of the added P fertilizer remains in available pool; instead, it was distributed to poorly-available pools. Fast, almost instantaneous P fixation by the Fe and Al oxides and immobilization by microbial uptake were recorded.Applying glucose boosts microbial growth and demand for P, resulting in increased 33P recovery. High 33P recovery in Pmic (20% of the applied 33P) and in PNaOH (45% of applied 33P) showed the dominance of P immobilization by microorganisms and adsorption by Fe and Al oxides on the fate of P in an acidic soil. Nevertheless, these can contribute to long-term P availability after the turnover of microbial biomass and desorption of fixed P.

The coupling of photosynthesis with belowground processes appears to be much faster than the time needed for assimilate translocation with the phloem flow.

Plants allocate carbon (C) to sink tissues depending on phenological, physiological or environmental factors. We still have little knowledge on C partitioning into various cellular compounds and metabolic pathways at various ecophysiological stages. We used compound-specific stable isotope analysis to investigate C partitioning of freshly assimilated C into tree compartments (needles, branches and stem) as well as into needle water-soluble organic C (WSOC), non-hydrolysable structural organic C (stOC) and individual chemical compound classes (amino acids, hemicellulose sugars, fatty acids and alkanes) of Norway spruce (Picea abies) following in situ 13C pulse labelling 15 days after bud break. The 13C allocation within the above-ground tree biomass demonstrated needles as a major C sink, accounting for 86% of the freshly assimilated C 6 h after labelling. In needles, the highest allocation occurred not only into the WSOC pool (44.1% of recovered needle 13C) but also into stOC (33.9%). Needle growth, however, also caused high 13C allocation into pathways not involved in the formation of structural compounds: (i) pathways in secondary metabolism, (ii) C-1 metabolism and (iii) amino acid synthesis from photorespiration. These pathways could be identified by a high 13C enrichment of their key amino acids. In addition, 13C was strongly allocated into the n-alkyl lipid fraction (0.3% of recovered 13C), whereby 13C allocation into cellular and cuticular exceeded that of epicuticular fatty acids. 13C allocation decreased along the lipid transformation and translocation pathways: the allocation was highest for precursor fatty acids, lower for elongated fatty acids and lowest for the decarbonylated n-alkanes. The combination of 13C pulse labelling with compound-specific 13C analysis of key metabolites enabled tracing relevant C allocation pathways under field conditions. Besides the primary metabolism synthesizing structural cell compounds, a complex network of pathways consumed the assimilated 13C and kept most of the assimilated C in the growing needles.

No abstract is provided for this article.

Bacterial communities in soil play a key role in carbon (C) and nutrient cycling. Unravelling how bacterial community assemble and distribute with soil depth is a prerequisite for understanding microbial functions, nutrient cycling and management. Twenty-six rice fields in a typical red soil area in a wet subtropical climate were sampled in the topsoil (0–10 and 10–20 cm) and subsoil (20–40 cm). Physico-chemical soil properties, quantitative fluorescence PCR and high-throughput sequencing were used to analyse the V4 region of 16S rDNA. The rRNA operon copy number and alpha diversity decreased continuously with soil depth because of reduced access to carbon, energy, oxygen and nutrients. The relative abundance of the dominant phyla Proteobacteria and Actinobacteria decreased with increasing soil depth, whereas the opposite trend was observed for the phylum Nitrospirae. The interaction intensity between taxa increased with depth, as limited carbon and nutrients in the undisturbed subsoil lead to the cooccurrence of taxa with similar ecological niches that cooperated to reduce functional redundancy. The higher modularity of the bacterial network in the topsoil is associated with greater environmental perturbations (flooding, fertilization, etc.) to maintain the robustness of the microbial community. Bacterial community assembly processes were stochastic up to 40 cm, but ecological drift was the predominant process in the topsoil, whereas dispersal limitation was dominant in the subsoil. The contribution of abiotic factors (e.g. nutrient and iron contents) and biotic factors (taxa-taxa interactions) as well as dispersal limitations to bacterial community assembly was depth specific. Concluding, the basic principles of bacterial community assembly were evaluated for the first time for a broad range of paddy soils.

No abstract is provided for this article.

The residence time of carbon (C) and nitrogen (N) in soil is a fundamental parameter reflecting the rates of soil organic matter (SOM) transformation and the contribution of soils to greenhouse gases fluxes. Based on the global database of the stable isotope composition of C (δ13C) and N (δ15N) depending on soil depth (171 profiles), we assessed С and N turnover and related them to climate, biome types and soil properties. The 13C and 15N discrimination between the litter horizon and mineral soil was evaluated to explain the key litter transformation processes. The 13C and 15N discrimination by microbial utilization of litter and SOM, as well as the continuous increase of δ13C and δ15N with depth, enabled to assess C and N turnover within SOM. N turnover was two times faster than that of C, which reflects i) repeated N recycling by microorganisms accelerating the N turnover, ii) C loss as CO2 and input of new C atoms to cycling, which reduces the C turnover, and iii) generally slower turnover of N free persistent organic compounds (e.g. lignin, suberin, cellulose) compared to the N containing compounds (e.g. amino acids, ribonucleic acids). An increase in temperature and precipitation accelerated C and N turnover because: i) higher microbial activity and SOM decomposition rate, ii) larger soil moisture and fast diffusion of dissolved organics towards exoenzymes, iii) downward transport of 13C-enriched organic matter (e.g. sugars, amino acids), and iii) leaching of 15N-depleted nitrates from the topsoil and losses from the whole soil profile. Temperature accelerates SOM turnover stronger than precipitation. The temperature increase by 10 °C accelerates the C and N turnover for 40%. SOM turnover is boosted by decreasing C/N ratio because: i) SOM with a high C/N ratio originated from litter is converted to microbially-derived SOM in mineral soil characterized by a low C/N ratio; ii) litter with a low C/N ratio is decomposed faster than litter with a high C/N; iii) microbial carbon-use efficiency increases with N availability. The biome type affects SOM decomposition by i) climate: slower turnover under wetter and colder conditions, and ii) by litter quality: faster utilization of leaves than needles. Thus, the fastest C turnover is common under evergreen forests and the lowest under mixed and coniferous ones, whereas temperature and C/N ratio are the main factors controlling SOM turnover. Concluding, the assessment of SOM turnover by δ13C and δ15N approach showed two times faster N turnover compared to C, and specifics of SOM turnover depending on the biomes as well as climate conditions. Soldatova E, Krasilnikov S, Kuzyakov Y 2024. Soil organic matter turnover: global implications from δ13C and δ15N signatures. Science of the Total Environment 912, 169423. https://doi.org/10.1016/j.scitotenv.2023.169423