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The ammonia-oxidizing microbial community colonizing clay tiles in flow channels changed in favor of ammonia-oxidizing bacteria during a 12-week incubation period even at originally high ratios of ammoniaoxidizing archaea to ammonia-oxidizing bacteria (AOB). AOB predominance was established more rapidly in flow channels incubated at 350 M NH 4 ؉ than in those incubated at 50 or 20 M NH 4 ؉ . Biofilm-associated potential nitrification activity was first detected after 28 days and was positively correlated with bacterial but not archaeal amoA gene copy numbers.Nitrogen transformation processes in small-river ecosystems are primarily associated with the sediment or epilithic biofilms (4,5,18). Here, nitrification as the first step in coupled nitrification-denitrification is a key step in the removal of the allochthonous nitrogen load, preventing the eutrophication of downstream ecosystems (1,5,29,39). The activity and community composition of biofilm-associated nitrifying microorganisms have been investigated extensively in wastewater treatment systems (27,36), in drinking water distribution systems (17, 41), and in lakes, large rivers, and estuaries (7,19,21,39) but only rarely in small-creek ecosystems. The first and ratelimiting step in nitrification, oxidation of ammonia, is carried out by ammonia-oxidizing archaea (AOA) (14) and ammoniaoxidizing bacteria (AOB) (16). Molecular surveys targeting the amoA gene, which encodes catalytic subunit A of ammonia monooxygenase, the key enzyme of ammonia oxidation (33), as well as cultivation-based studies, indicated that pH (26), salinity (25, 34), and especially ammonium availability (6, 22) could play important roles in the ecological niche differentiation of AOA and AOB. However, only little is known about archaeal versus bacterial ammonia oxidizers in creek or river ecosystems or about the association of AOA with biofilms (37,43). In this study, we used flow channels (FC) as simulated creek ecosystems to investigate the influence of ammonium availability (i) on the establishment of biofilm-associated nitrification activity, (ii) on the community composition of biofilm-associated AOA and AOB, and (iii) on temporal patterns of the abundance of AOA and AOB during a 12-week incubation period.FC experiment. Water for FC was obtained from three shell limestone creeks (Ammerbach, Leutra South, and Leutra North) located near the city of Jena (Thuringia, Germany). The creeks differed in their nutrient concentrations, with the highest nutrient load at Ammerbach resulting from diffuse wastewater inputs (Table 1). Flow velocities ranged from 0.105 Ϯ 0.092 to 0.230 Ϯ 0.224 m s Ϫ1 . Plexiglas FC measuring 160 by 10 by 18 cm (see Fig. S1 in the supplemental material) were set up in triplicate for each creek. Concentrations of NH 4 ϩ in the water phase of the FC were adjusted to 350 mol liter Ϫ1 (Ammerbach; FC 1 to 3), 50 mol liter Ϫ1 (Leutra South, FC 4 to 6), and 20 mol liter Ϫ1 (Leutra North, FC 7 to 9) to run the experiment at three distinct levels of NH 4 ϩ . pHs and concentrat...

We assessed the accuracy and utility of a modified high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) system for measuring the amount and stable carbon isotope signature of dissolved organic matter (DOM) <1 µm. Using a range of standard compounds as well as soil solutions sampled in the field, we compared the results of the HPLC/IRMS analysis with those from other methods for determining carbon and (13)C content. The conversion efficiency of the in-line wet oxidation of the HPLC/IRMS averaged 99.3% for a range of standard compounds. The agreement between HPLC/IRMS and other methods in the amount and isotopic signature of both standard compounds and soil water samples was excellent. For DOM concentrations below 10 mg C L(-1) (250 ng C total) pre-concentration or large volume injections are recommended in order to prevent background interferences. We were able to detect large differences in the (13)C signatures of soil solution DOM sampled in 10 cm depth of plots with either C3 or C4 vegetation and in two different parent materials. These measurements also demonstrated changes in the (13)C signature that demonstrate rapid loss of plant-derived C with depth. Overall the modified HLPC/IRMS system has the advantages of rapid sample preparation, small required sample volume and high sample throughput, while showing comparable performance with other methods for measuring the amount and isotopic signature of DOM.

Stable isotopic content of dissolved organic carbon (δ(13)C-DOC) provides valuable information on its origin and fate. In an attempt to get additional insights into DOC cycling, we developed a method for δ(13)C measurement of DOC size classes by coupling high-performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) to online isotope ratio mass spectrometry (IRMS). This represents a significant methodological contribution to DOC research. The interface was evaluated using various organic compounds, thoroughly tested with soil-water from a C3-C4 vegetation change experiment, and also applied to riverine and marine DOC. δ(13)C analysis of standard compounds resulted in excellent analytical precision (≤0.3‰). Chromatography resolved soil DOC into 3 fractions: high molecular weight (HMW; 0.4-10 kDa), low molecular weight (LMW; 50-400 Da), and retained (R) fraction. Sample reproducibility for measurement of δ(13)C-DOC size classes was ±0.25‰ for HMW fraction, ± 0.54‰ for LMW fraction, and ±1.3‰ for R fraction. The greater variance in δ(13)C values of the latter fractions was due to their lower concentrations. The limit of quantification (SD ≤0.6‰) for each size fraction measured as a peak is 200 ng C (2 mg C/L). δ(13)C-DOC values obtained in SEC mode correlated significantly with those obtained without column in the μEA mode (p < 0.001, intercept 0.17‰), which rules out SEC-associated isotopic effects or DOC loss. In the vegetation change experiment, fractions revealed a clear trend in plant contribution to DOC; those in deeper soils and smaller size fractions had less plant material. It was also demonstrated that the technique can be successfully applied to marine and riverine DOC without further sample pretreatment.

We investigated the effect of leaf litter on below ground carbon export and soil carbon formation in order to understand how litter diversity affects carbon cycling in forest ecosystems. 13C labeled and unlabeled leaf litter of beech (Fagus sylvatica) and ash (Fraxinus excelsior), characterized by low and high decomposability, were used in a litter exchange experiment in the Hainich National Park (Thuringia, Germany). Litter was added in pure and mixed treatments with either beech or ash labeled with 13C. We collected soil water in 5 cm mineral soil depth below each treatment biweekly and determined dissolved organic carbon (DOC), δ13C values and anion contents. In addition, we measured carbon concentrations and δ13C values in the organic and mineral soil (collected in 1 cm increments) up to 5 cm soil depth at the end of the experiment. Litter-derived C contributes less than 1% to dissolved organic matter (DOM) collected in 5 cm mineral soil depth. Better decomposable ash litter released significantly more (0.50±0.17%) litter carbon than beech litter (0.17±0.07%). All soil layers held in total around 30% of litter-derived carbon, indicating the large retention potential of litter-derived C in the top soil. Interestingly, in mixed (ash and beech litter) treatments we did not find a higher contribution of better decomposable ash-derived carbon in DOM, O horizon or mineral soil. This suggest that the known selective decomposition of better decomposable litter by soil fauna has no or only minor effects on the release and formation of litter-derived DOM and soil organic matter. Overall our experiment showed that 1) litter-derived carbon is of low importance for dissolved organic carbon release and 2) litter of higher decomposability is faster decomposed, but litter diversity does not influence the carbon flow.

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Abstract. The deep soil, >1 m, harbors a substantial share of the global microbial biomass. Currently, it is not known whether microbial activity several meters below the surface is fueled by recently fixed carbon or by old carbon that persisted in soil for several hundred years. Understanding the carbon source of microbial activity in deep soil is important to identify the drivers of biotic processes in the critical zone. Therefore, we explored carbon cycling in soils in three climate zones (arid, mediterranean, and humid) of the Coastal Cordillera of Chile down to a depth of 6 m, using carbon isotopes. Specifically, we determined the 13C : 12C ratio (δ13C) of soil and roots and the 14C : 12C ratio (Δ14C) of soil organic carbon and CO2–C respired by microorganisms. We found that the Δ14C of the respired CO2–C was significantly higher than that of the soil organic carbon in all soils. Further, we found that the δ13C of the soil organic carbon changed only in the upper decimeters (by less than 6 ‰). Our results show that microbial activity several meters below the soil surface is mostly fueled by recently fixed carbon that is on average much younger than the total soil organic carbon present in the respective soil depth increments, in all three climate zones. Further, our results indicate that most decomposition that leads to enrichment of 13C occurs in the upper decimeters of the soils, which is possibly due to stabilization of organic carbon in the deep soil. In conclusion, our study demonstrates that microbial processes in the deep soil several meters below the surface are closely tied to input of recently fixed carbon.