Following the developed concept of carbon (C) flows during soil organic matter (SOM) formation, from which the probable C pathways between the aggregates and SOM fractions can be suggested based on the natural changes of the 13C/12C ratios, we have prepared the review based on 42 publications. The data were collected from the existing databases using the following keywords: &#8220;soil organic matter fractions and 13C&#8221;, &#8220;density fractionation and 13C&#8221;, and &#8220;soil aggregates and 13C&#8221;; publications contained the data from forest, shrubland, grassland, and cropland ecosystems that were located in the Temperate, Mediterranean, subtropical and tropical climatic zones were chosen; only the top 20 cm were considered. Besides the &#948;13C data, the main soil properties, including pH, total C and nitrogen contents, texture, and the dominant type of soil minerals, were collected. All data for the isotopic composition of aggregates (>2000, 250-2000, 52-250, and <53 &#181;m) and density fractions (<1.4, 1.4-1.6, 1.8-2.0, and >2.2 g cm-3) were normalized to the &#948;13C values of bulk soils. The preliminary analyses have shown that the isotopic composition of density fractions separated from the soils allocated in temperate and Mediterranean climates followed the previously established order, namely was getting heavier with the increase of particle densities. In contrast, density fractions separated from the soils of subtropical and tropical zones did not show prominent trends, or isotopic composition showed the enrichment in 12C with increased particle density. The isotopic composition of fractions separated from forest soils was also found with more minor variations compared to cropland and grassland. The data related to the probability of C flow between the density fractions and aggregates during SOM formation were also calculated and will be presented, as well as the concept explaining the effect of land use and climatic variables on the changes of the isotopic composition of density fractions and aggregates.
A novel method of separating exudates from root respiration in non-sterilized soils has been developed. The method is based on a simultaneous elution of exudates from rhizosphere and the blowout of CO2 originating from root respiration. The innovation of the method lies in the function of a membrane pump to drive the movement of air and simultaneously the circulation of water according to the Siphon principle. The separation method was tested by means of 14C pulse labeling of Lolium perenne to track the C dynamics in the production of rhizosphere CO2 and of exudates, which were eluted. The total 14C activity of rhizosphere CO2 and of eluted exudates was found to be 8.5 % and 2.3 % of total assimilated 14C, respectively. Thus, at least 19 % of root-derived C can be accounted to root exudation. However, the suggested Siphon method underestimates the amount of exudates and shows only a minimum of organic substances exuded by roots. The diurnal dynamics of exudation was detected, but no significant day-night changes were measured in root and microbial respiration. Tight coupling of assimilation with exudation, but not with root and microbial respiration, was observed. The advantages, shortcomings, and possible applications of the Siphon method are discussed. Eine neue Methode zur Trennung der wurzelbürtigen organischen Substanzen von der Wurzelatmung im nicht-sterilen Boden Es wurde eine neue Methode zur Trennung der Exsudate von der Wurzelatmung und separaten Quantifizierung des wurzelbürtigen C in einem nicht sterilen Boden erarbeitet. Die Methode basiert auf der Auswaschung der Exsudate aus der Rhizosphäre beim gleichzeitigen Ausblasen von CO2 aus der Wurzelatmung. Ein wesentlicher Teil der Methode besteht darin, dass die Membranpumpe, die den Luftstrom fördert, auch für die Wasserzirkulation nach dem Siphon-Prinzip verantwortlich ist. Die Trennungsmethode wurde durch 14C-Pulsmarkierung von Lolium perenne überprüft, um die 14C-Dynamik in Exsudaten und im CO2-Efflux aus dem Boden verfolgen zu können. Im CO2-Efflux aus dem Boden wurden 8,5 % des gesamten von der Pflanze assimilierten 14C gefunden und in den ausgewaschenen Exsudaten hingegen nur 2,3 %. Die Siphon-Methode unterschätzt die Exsudatmenge und zeigt nur eine minimale Menge an wasserlöslichen Exsudaten. Folglich entsprechen die 81 % des im CO2-Efflux aus dem Boden gemessen 14C nicht allein der Wurzelatmung, sondern zum Teil auch der mikrobiellen Veratmung der Exsudate. Die 14C-Dynamik der ausgewaschenen Exsudate und des CO2-Effluxes aus dem Boden zeigte, dass die Exsudation einem Tag-Nacht-Zyklus folgt, dagegen wurde bei der Wurzelatmung keine diurnale Dynamik festgestellt. Eine enge Kopplung der Exsudation mit der Assimilation, jedoch nicht mit der Wurzelatmung, wurde beobachtet. Die Vor- und Nachteile und mögliche Anwendungsbereiche der Siphon-Methode werden diskutiert.
No abstract is provided for this article.
The carbon use efficiency (CUE) of soil microorganisms is a critical parameter for the first step of organic carbon (C) transformation by and incorporation into microbial biomass and shapes C cycling in terrestrial ecosystems. As C and nitrogen (N) cycles interact closely and N availability affects microbial metabolism, N addition to soil may shift the microbial CUE. We conducted a meta-analysis (100 data pairs) to generalize information about the microbial CUE response to N addition in soil based on the two most common CUE estimation approaches: (i) 13C-labelled substrate addition (13C-substrate) and (ii) 18O-labelled water addition (18O-H2O).The mean microbial CUE in soils across all biomes and approaches was 0.37. The effects of N addition on CUE, however, were depended on the approach: CUE decreased by 12% if measured by the 13C-substrate approach, while CUE increased by 11% if measured by the 18O-H2O approach. These differences in the microbial CUE response depending on the estimation approach are explained by the divergent reactions of microbial growth to N addition: N addition decreases the 13C incorporation into microbial biomass (this parameter is in the numerator by CUE calculation based on the 13C-substrate approach). In contrast, N addition slightly increases (although statistically insignificant) the microbial growth rate (in the numerator of the CUE calculation when assessed by the 18O-H2O approach), significantly raising the CUE. We explained these N addition effects based on CUE regulation mechanisms at the metabolic, cell, community, and ecosystem levels. Consequently, the differences in the microbial responses (microbial growth, respiration, C incorporation, community composition, and dormant or active states) between the 13C-substrate and 18O-H2O approaches need to be considered. Thus, these two CUE estimation approaches should be compared to understand microbially mediated C and nutrient dynamics under increasing anthropogenic N input and other global change effects.ReferenceHu J, Huang C, Zhou S, Kuzyakov Y 2022. Nitrogen deposition affects microbial carbon use efficiency: Meta-analysis of similarities and differences in 18O and 13C approaches. Global Change Biology 28 (16), 4977-4988. http://doi.org/10.1111/gcb.1622
No abstract is provided for this article.
. More dry-season droughts and wet-season storms have been predicted in subtropical areas. Since subtropical forest soils are significant sources of N2O and NO3−, it is important to understand the features and determinants of N transformation responses to the predicted precipitation changes. A precipitation manipulation field experiment was conducted to reduce dry-season precipitation and increase wet-season precipitation, while keeping the annual precipitation unchanged in a subtropical forest. Net N mineralization, net nitrification, N2O emission, nitrifying (bacterial and archaeal amoA) and denitrifying (nirK, nirS and nosZ) genes abundance, microbial biomass carbon (MBC) and soil physicochemical properties were monitored to characterize and explain soil N transformation responses. Dry-season precipitation reduction decreased net nitrification and N mineralization rates by 13–20 %, while wet-season precipitation addition increased both rates by 50 %. More than 20 % of the total variation of net nitrification and N mineralization could be explained by microbial abundance and soil water content (SWC), but archaeal amoA abundance was the main factor. Increased net nitrification in wet season together with large precipitation events caused substantial NO3− losses via leaching. However, N2O emission decreased moderately either in dry or wet seasons due to changes in nosZ gene abundance, MBC, net nitrification and SWC (decreased by 10–21 %). We conclude that reducing dry-season precipitation and increasing wet-season precipitation affect N transformation mainly through altering functional microbial abundance and MBC, which are further determined by changes in DOC and NH4+ availabilities. Such contrasting precipitation pattern will increase droughts and NO3− leaching in subtropical forests.
Understanding photoassimilate allocation into the roots and the release of organic substances from the roots into the rhizosphere is an important prerequisite for characterizing the belowground C input, the spatial and temporal distribution of C, and the interactions between plants and soil microorganisms. Based on 14 C phosphor imaging, we visualized the allocation of assimilates into Lolium perenne roots and estimated the life time of hotspots at the root tips. Lolium shoots were labeled in a 14 CO 2 atmosphere, and herbariums of roots and shoots were prepared 6 h, 2 d, and 11 d after the 14 C pulse. The 14 C distribution in roots and leaves revealed that pulse labeling does not yield homogeneously labeled plant material. The spatial distribution of assimilate allocation was evaluated based on the 14 C specific activity expressed as digital light units (DLU mm –2 ) of the imaging plates. Areas with high relative 14 C activity were classified as hotspots. Strong 14 C hotspots were detected mainly at the root tips already 6 h after the 14 C assimilation, and they remained active for at least 2 d. Eleven days after the 14 C assimilation, the hotspots at the root tips disappeared and the 14 C distribution was much more even than after 6 h or after 2 d. 14 C phosphor imaging proved to be a promising tool to visualize the allocation of photoassimilates into the roots and the rhizosphere and can be used to identify hotspots and their dynamics.
Transformation of sugars by microorganisms is a key process influencing carbon (C) stabilization in soil. To reveal the mechanisms responsible for the persistence of labile C in soil, the fates of position-specific and uniformly 13C labeled glucose and ribose were studied under field conditions for 800 days. We hypothesized a convergence of the fate of individual C positions and substances because of the long-term C recycling by microorganisms. Position-specific data revealed that both sugars were simultaneously metabolized via glycolysis and the pentose phosphate pathway (PPP). The position-specific 13C recovery pattern in soil and in microbial biomass was similar. This similarity demonstrated significant contribution of microbial products and necromass to soil organic matter (SOM) formation. Based on a biexponential model, the mean residence times (MRTs) of glucose C-6 and ribose C-5 in the soil were longer than the other C positions. However, the MRT of uniformly labeled 13C from ribose in the soil was 3 times longer than that from glucose. Consequently, ribose and glucose were incorporated into different cellular components, defining their long-term fate in soil. The convergence of glucose C positions in soil and microbial biomass revealed that recycling and modification of recycled components dominated glucose transformation. In contrast, divergence of ribose C positions in soil revealed that intact ribose-derived cell components are reused or preserved in SOM. Thus, convergence versus divergence of individual C positions distinguished the two key stabilization mechanisms explaining the long persistence of C from easily available sources in the soil: sustained microbial recycling (convergence) versus preservation (divergence) in long-term stabilized compound classes.
