Maxim Dorodnikov

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size.

Increased root exudation under elevated atmospheric CO(2) and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO(2) on the contribution of fast- (r-strategists) and slow-growing (K-strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO(2) treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25-2.00 mm), and microaggregates (<0.25 mm) using optimal moist sieving. Microbial biomass (C(mic)), the maximum specific growth rate (mu), growing microbial biomass (GMB) and lag-period (t(lag)) were estimated by the kinetics of CO(2) emission from bulk soil and aggregates amended with glucose and nutrients. Although C(org) and C(mic) were unaffected by elevated CO(2), mu values were significantly higher under elevated than ambient CO(2) for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO(2) treatments. Based on changes in mu, GMB and lag period, we conclude that elevated atmospheric CO(2) stimulated the r-selected microorganisms, especially in soil microaggregates. Such an increase in r-selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO(2) environment.

Rice rhizodeposition and its utilization by microbial groups depends on N fertilization

Rhizodeposits have received considerable attention, as they play an important role in the regulation of soil carbon (C) sequestration and global C cycling and represent an important C and energy source for soil microorganisms. However, the utilization of rhizodeposits by microbial groups, their role in the turnover of soil organic matter (SOM) pools in rice paddies, and the effects of nitrogen (N) fertilization on rhizodeposition are nearly unknown. Rice (Oryza sativa L.) plants were grown in soil at five N fertilization rates (0, 10, 20, 40, or 60xa0mg N kg−1 soil) and continuously labeled in a 13CO2 atmosphere for 18xa0days during tillering. The utilization of root-derived C by microbial groups was assessed by 13C incorporation into phospholipid fatty acids. Rice shoot and root biomass strongly increased with N fertilization. Rhizodeposition increased with N fertilization, whereas the total 13C incorporation into microorganisms, as indicated by the percentage of 13C recovered in microbial biomass, decreased. The contribution of root-derived 13C to SOM formation increased with root biomass. The ratio of 13C in soil pools (SOM and microbial biomass) to 13C in roots decreased with N fertilization showing less incorporation and faster turnover with N. The 13C incorporation into fungi (18:2ω6,9c and 18:1ω9c), arbuscular mycorrhizal fungi (16:1ω5c), and actinomycetes (10Me 16:0 and 10Me 18:0) increased with N fertilization, whereas the 13C incorporation into gram-positive (i14:0, i15:0, a15:0, i16:0, i17:0, and a17:0) and gram-negative (16:1ω7c, 18:1ω7c, cy17:0, and cy19:0) bacteria decreased with N fertilization. Thus, the uptake and microbial processing of root-derived C was affected by N availability in soil. Compared with the unfertilized soil, the contribution of rhizodeposits to SOM and microorganisms increased at low to intermediate N fertilization rates but decreased at the maximum N input. We conclude that belowground C allocation and rhizodeposition by rice, microbial utilization of rhizodeposited C, and its stabilization within SOM pools are strongly affected by N availability: N fertilization adequate to the plant demand increases C incorporation in all these polls, but excessive N fertilization has negative effects not only on environmental pollution but also on C sequestration in soil.

Sensitivities to nitrogen and water addition vary among microbial groups within soil aggregates in a semiarid grassland

We investigated whether enhanced nitrogen (N) and water inputs would redistribute the microbial community within different soil aggregate size classes in a field manipulation experiment initiated in 2005. Distribution of microbial groups was monitored in large macroaggregates (>2000xa0μm), small macroaggregates (250–2000xa0μm), and microaggregates (<250xa0μm) in a semiarid grassland. Both arbuscular mycorrhizal (AM) fungi and saprophytic fungi were the most abundant in soil macroaggregates. The gram-negative bacteria were more abundant in soil microaggregates. Total phospholipid fatty acid (PLFA) concentration in general and actinomycetes in particular decreased with N addition under ambient precipitation but was unaffected by combined additions of N and water within the three soil aggregate fractions as compared to control plots. In contrast, the abundance of saprophytic fungi decreased with combined N and water addition, but it was not affected by N addition under ambient precipitation. The abundance of gram-positive bacteria increased with N addition under both ambient and elevated water conditions for all soil aggregate fractions. In summary, the higher short-term nutrient and water availabilities provoked a shift in soil microbial community composition and increased total PLFA abundance irrespectively of the level of soil aggregation. In the long term, this could destabilize soil carbon pools and influence the nutrient limitation of soil biota within different soil aggregate size classes under future global change scenarios.

Effect of plant communities on aggregate composition and organic matter stabilisation in young soils

ObjectivesCarbon (C) content in pools of very young soils that developed during 45xa0years from loess was analysed in relation to vegetation: deciduous and coniferous forests and cropland. We hypothesised that variations in the amount of particulate organic matter (POM) can explain the C accumulation and also affects the C bound to mineral surfaces in soil under various vegetation.MethodsSoil samples were collected under three vegetation types of a 45-year-old experiment focused on initial soil development. Aggregate and density fractionations were combined to analyse C accumulation in large and small macro- and microaggregates as well as in free and occluded POM and mineral factions.ResultsDeciduous forest soil accumulated the highest C content in the 0–5xa0cm layer (43xa0g C kg−1), whereas values in coniferous forest and arable soils were lower (30 and 12xa0g C kg−1, respectively). The highest portion of C in arable soil was accumulated in the mineral fraction (80xa0%), whereas 50–60xa0% of the C in forest soils were in POM. More C was associated with minerals in deciduous forest soil (16xa0g C kg−1 soil) than under coniferous forest and arable land (8–10xa0g C kg−1 soil).ConclusionsParticulate organic matter explains most of the differences in organic C accumulation in soils developed during 45xa0years under the three vegetation types on identical parent material. The C content of the mineral soil fraction was controlled by plant cover and contributed the most to differences in C accumulation in soils developed under similar vegetation type (forest).

Effects of atmospheric CO2 enrichment on δ13C, δ15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO2 applied for Free-Air CO2 Enrichment (FACE) experiments is strongly depleted in 13C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ13C value. Simultaneous use of 15N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380xa0ppm) and elevated CO2 (540xa0ppm; for 3xa0years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ13C and δ15N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO2, respectively. δ13C values of SOM pools under elevated CO2 treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92xa0years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12xa0years. The amount of fertilizer-derived N after 2xa0years of application in ambient and elevated CO2 treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60xa0years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400xa0years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N.

Fertilizing effect of the increasing CO2 concentration in the atmosphere

The effect of three atmospheric CO2 concentrations (ambient, 400 ppm; double, 800 ppm; and triple, 1200 ppm) on the productivity of cottonwood (Populus deltoides Barr.) and the activity of the soil microbial biomass in the root-inhabited zone was studied. The total biomass of the cottonwood increased with increasing CO2 concentration (2.28, 5.28, and 3.78 kg/tree for 400, 800, and 1200 ppm, respectively). The strongest responses were observed for the trunk and coarse roots (three and two times higher as compared to the ambient CO2 concentration). The triple concentration of CO2 had a greater effect on the roots of the trees, but the growth of leaves and branches was insignificant or absent. The shoot-to-root ratio changed as follows: 2.86, 2.80, and 1.57 at 400, 800, and 1200 ppm, respectively. The rate of C-CO2 release from the soil samples incubated for 70 days increased in the following order: 400, 800, and 1200 ppm CO2; the average values of the CO2 emission were 2.02, 2.33, and 2.76 mg/100 g per day, respectively. The greatest content of Cmb (75.1 mg/100 g) was observed in the treatment with the triple CO2 concentration, and the lowest content (53.7 mg/100 g) was found for the ambient CO2 concentration. This study suggests that the responses of the cottonwood biomass and soil microbial activity vary depending on the CO2 concentration. The microbial biomass gradually increased with increasing CO2 concentration; the total plant biomass showed the highest response at the double CO2 concentration, and the triple CO2 concentration was likely to suppress the growth of the above-ground plant biomass.

Thermal stability of soil organic matter pools and their turnover times calculated by δ13C under elevated CO2 and two levels of N fertilisation

Soil from Free-Air Carbon dioxide Enrichment (FACE) plots (FAL, Braunschweig) under ambient air (375 ppm; δ13C–CO2−9.8‰) and elevated CO2 (550 ppm; for six years; δ13C–CO2−23‰), either under 100% nitrogen (N) (180 kg ha−1) or 50% N (90 kg ha−1) fertilisation treatments, was analysed by thermogravimetry. Soil samples were heated up to the respective temperatures and the remaining soil was analysed for δ13C and δ15N by Isotope Ratio Mass Spectrometry (IRMS). Based on differential weight losses, four temperature intervals were distinguished. Weight losses in the temperature range 20–200 °C were connected mostly with water volatilisation. The maximum weight losses and carbon (C) content were measured in the soil organic matter (SOM) pool decomposed at 200–360 °C. The largest amount of N was detected in SOM pools decomposed at 200–360 °C and 360–500 °C. In all temperature ranges, the δ13C values of SOM pools were significantly more negative under elevated CO2 versus ambient CO2. The incorporation of new C into SOM pools was not inversely proportional to its thermal stability. 50% N fertilisation treatment gained higher C exchange under elevated CO2 in the thermally labile SOM pool (200–360 °C), whereas 100% N treatment induced higher C turnover in the thermally stable SOM pools (360–500 °C, 500–1000 °C). Mean Residence Time of SOM under 100% N and 50% N fertilisation showed no dependence between SOM pools isolated by increasing temperature of heating and the renovation of organic C in those SOM pools. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C. Revised version of a paper presented at the 30th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 8–10 October 2007, Bayreuth, Germany.

CH4 and CO2 production below two contrasting peatland micro-relief forms: An inhibitor and δ13C study

Two peatland micro-relief forms (microforms) - hummocks and hollows - differ by their hydrological characteristics (water table level, i.e. oxic-anoxic conditions) and vegetation communities. We studied the CH4 and CO2 production potential and the localization of methanogenic pathways in both hummocks and hollows at depths of 15, 50, 100, 150 and 200cm in a laboratory incubation experiment. For this purpose, we measured CH4 and CO2 production rates, peat elemental composition, as well as δ13C values of gases and solids; the specific inhibitor of methanogenesis BES (2-bromo-ethane sulfonate, 1mM) was aimed to preferentially block the acetoclastic pathway. The cumulative CH4 production of all depths was almost one fold higher in hollows than in hummocks, with no differences in CO2. With depth, CO2 and CH4 production decreased, and the relative contribution of the hydrogenotrophic pathway of methanogenesis increased. The highest methanogenic activity among all depths and both microforms was measured at 15cm of hollows (91%) at which the highest relative contribution of acetoclastic vs. hydrogenotrophic pathway (92 and 8%, respectively) was detected. For hummocks, the CH4 production was the highest at 50cm (82%), where relative contribution of acetoclastic methanogenesis comprised 89%. The addition of 1mM BES was not selective and inhibited both methanogenic pathways in the soil. Thus, BES was less efficient in partitioning the pathways compared with the δ13C signature. We conclude that the peat microforms - dry hummocks and wet hollows - play an important role for CH4 but not for CO2 production when the effects of living vegetation are excluded.

Changes in the carbon and nitrogen isotopic composition of organic matter in soils of different thermal stability after free-air CO2 enrichment for three years

The hypothesis that the biological availability of soil organic matter (SOM) pools is inversely proportional to their thermal stability was tested using the isotopic difference between the atmospheric CO2 (δ13C = −8.0‰) and 13C-enriched CO2 (δ13C = −47‰) fertilizers, as well as 15N-labeled fertilizers. The soil samples from spring wheat plots subjected to treatment with ambient (370 ppm) and elevated (540 ppm) CO2 concentrations for three years were analyzed by the thermogravimetric method. Based on the weight loss, five SOM pools were distinguished where the total C and N contents and isotopic compositions (δ13C and (δ15N) were determined. The contents of new C and N and their mean residence times in pools were calculated. The incorporation of 13C and 15N and their turnover rates did not depend on the thermal stability of the SOM pools, which disproved the hypothesis being tested.