To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

A molecular photocatalyst consisting of a Ru(II) photocenter, a tetrapyridophenazine bridging ligand, and a PtX2 (X=Cl or I) moiety as the catalytic center functions as a stable system for light-driven hydrogen production. The catalytic activity of this photochemical molecular device (PMD) is significantly enhanced by exchanging the terminal chlorides at the Pt center for iodide ligands. Ultrafast transient absorption spectroscopy shows that the intramolecular photophysics are not affected by this change. Additionally, the general catalytic behavior, that is, instant hydrogen formation, a constant turnover frequency, and stability are maintained. Unlike as observed for the Pd analogue, the presence of excess halide does not affect the hydrogen generation capacity of the PMD. The highly improved catalytic efficiency is explained by an increased electron density at the Pt catalytic center, this is confirmed by DFT studies.

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Nitrifier denitrification (i.e. nitrite reduction by ammonia oxidizers) is one of the biochemical pathways of nitrous oxide (N 2 O) production. It is increasingly suggested that this pathway may contribute substantially to N 2 O production in soil, the major source of this greenhouse gas. However, although monoculture studies recognize its potential, methodological drawbacks prohibit conclusive proof that nitrifier denitrification occurs in actual soils. Here we suggest and apply a new isotopic approach to identify its presence in soil. In incubation experiments with 12 soils, N 2 O production was studied using oxygen (O) and nitrogen (N) isotope tracing, accounting for O exchange. Microbial biomass C and N and phospholipid fatty acid (PLFA) patterns were analysed to explain potential differences in N 2 O production pathways. We found that in at least five of the soils nitrifier denitrification must have contributed to N 2 O production. Moreover, it may even have been responsible for all NH 4 + -derived N 2 O in most soils. In contrast, N 2 O as a by-product of ammonia oxidation contributed very little to total production. Microbial biomass C and N and PLFA-distinguished microbial community composition were not indicative of differences in N 2 O production pathways. Overall, we show that combined O and N isotope tracing may still provide a powerful tool to understand N 2 O production pathways, provided that O exchange is accounted for. We conclude that nitrifier denitrification can indeed occur in soils, and may in fact be responsible for the greater proportion of total nitrifier-induced N 2 O production.

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We conducted a trenching experiment in a mountain forest in order to assess the contribution of the autotrophic respiration to total soil respiration and evaluate trenching as a technique to achieve it. We hypothesised that the trenching experiment would alter both microbial biomass and microbial community structure and that fine roots (less than 2 mm diameter) would be decomposed within one growing season. Soil C0 2 efflux was measured roughly biweekly over two growing seasons. Root presence and morphology parameters, as well as the soil microbial community were measured prior to trenching, 5 and 15 months after trenching. The trenched plots emitted about 20 and 30% less C0 2 than the control plots in the first and second growing season, respectively. Roots died in trenched plots, but root decay was slow. After 5 and 15 months, fine root biomass was decreased by 9% (not statistically different) and 30%, (statistically different) respectively. When we corrected for the additional trenched-plot C0 2 efflux due to fine root decomposition, the autotrophic soil respiration rose to ~26% of the total soil respiration for the first growing season, and to ~44% for the second growing season. Soil microbial biomass and community structure was not altered by the end of the second growing season. We conclude that trenching can give accurate estimates of the autotrophic and heterotrophic components of soil respiration, if methodological side effects are accounted for, only.

Für die Entwicklung hocheffizienter molekularer Photokatalysatoren zur Wasserstoffproduktion mit sichtbarem Licht ist ein Verständnis der elektronischen und chemischen Prozesse im Photokatalysator von wesentlicher Bedeutung. In‐situ‐Röntgenabsorptionsspektroskopie (XAS) zeigte hierbei, dass die Art des katalytisch aktiven Metallzentrums in einer (N^N)MCl2‐Koordinationssphäre (M=Pd oder Pt) wesentlichen Einfluss auf den Mechanismus der Wasserstoffproduktion hat. Während Pd als Metallzentrum eine signifikante Veränderung der chemischen Umgebung und die Ausbildung von metallischen Kolloiden aufweist, zeigt Pt als Katalysezentrum keinerlei Veränderung der Koordinationssphäre unter katalytischen Bedingungen. Dieser Befund wird durch die Unabhängigkeit der Katalyse von Chloridionenzusatz und dem Quecksilbertest gestützt. Hieraus kann ein vollständig anderer Katalysemechanismus ohne Beteiligung von Kolloiden abgeleitet werden.

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