Publications

The rhizosphere is a hot-spot for biogeochemical cycles, including production of greenhouse gases, as microbial activity is stimulated by rhizodeposits rel

The stability and turnover of soil organic matter ( SOM ) are a very important but poorly understood part of carbon (C) cycling. Conversion of C 3 grassland to the C 4 energy crop Miscanthus provides an ideal opportunity to quantify medium‐term SOM dynamics without disturbance (e.g., plowing), due to the natural shift in the δ 13 C signature of soil C. For the first time, we used a repeated 13 C natural abundance approach to measure C turnover in a loamy Gleyic Cambisol after 9 and 21 years of Miscanthus cultivation. This is the longest C 3 –C 4 vegetation change study on C turnover in soil under energy crops. SOM stocks under Miscanthus and reference grassland were similar down to 1 m depth. However, both increased between 9 and 21 years from 105 to 140 mg C ha −1 ( P < 0.05), indicating nonsteady state of SOM . This calls for caution when estimating SOM turnover based on a single sampling. The mean residence time ( MRT ) of old C (>9 years) increased with depth from 19 years (0–10 cm) to 30–152 years (10–50 cm), and remained stable below 50 cm. From 41 literature observations, the average SOM increase after conversion from cropland or grassland to Miscanthus was 6.4 and 0.4 mg C ha −1 , respectively. The MRT of total C in topsoil under Miscanthus remained stable at ~60 years, independent of plantation age, corroborating the idea that C dynamics are dominated by recycling processes rather than by C stabilization. In conclusion, growing Miscanthus on C‐poor arable soils caused immediate C sequestration because of higher C input and decreased SOM decomposition. However, after replacing grasslands with Miscanthus , SOM stocks remained stable and the MRT of old C 3 ‐C increased strongly with depth.

. Tidal wetlands, such as tidal marshes and mangroves, are hotspots for carbon sequestration. The preservation of organic matter (OM) is a critical process by which tidal wetlands exert influence over the global carbon cycle and at the same time gain elevation to keep pace with sea-level rise (SLR). The present study provides the first global-scale field-based experimental evidence of temperature and relative sea level effects on the decomposition rate and stabilization of OM in tidal wetlands. The study was conducted in 26 marsh and mangrove sites across four continents, utilizing commercially available standardized OM. While effects on decomposition rate per se were minor, we show unanticipated and combined negative effects of temperature and relative sea level on OM stabilization. Across study sites, OM stabilization was 29 % lower in low, more frequently flooded vs. high, less frequently flooded zones. OM stabilization declined by ~ 90 % over the studied temperature gradient from 10.9 to 28.5 °C, corresponding to a decline of ~ 5 % over a 1 °C temperature increase. Additionally, data from the long-term ecological research site in Massachusetts, US show a pronounced reduction in OM stabilization by > 70 % in response to simulated coastal eutrophication, confirming the high sensitivity of OM stabilization to global change. We therefore provide evidence that rising temperature, accelerated SLR, and coastal eutrophication may decrease the future capacity of tidal wetlands to sequester carbon by affecting the initial transformations of recent OM inputs to soil organic matter.

Microbial biomass is increasingly considered to be the main source of organic carbon (C) sequestration in soils. Quantitative information on the contribution of microbial necromass to soil organic carbon (SOC) formation and the factors driving necromass accumulation, decomposition and stabilization during the initial soil formation in biological crusts (biocrusts) is absent. To address this knowledge gap, we investigated the composition of microbial necromass and its contributions to SOC sequestration in a biocrust formation sequence consisting of five stages: bare sand, cyanobacteria stage, cyanobacteria-moss stage, moss-cyanobacteria stage, and moss stage on sandy parent material on the Loess Plateau. The fungal and bacterial necromass C content in soil was analyzed based on amino sugars - the cell wall biomarker. Microbial necromass was an important source of SOC, and was incorporated into the particulate and mineral-associated organic C (MAOC). Because bacteria have smaller and thinner cell wall fragments as well as more proteins than fungi, bacterial necromass mainly contributed to the MAOC pool, while fungal residues remained more in the particulate organic C (POC). MAOC pool was saturated fast with the increase of microbial necromass, and POC more rapid accumulation than MAOC suggests that the clay content was the limiting factor for stable C accumulation in this sandy soil. The necromass exceeding the MAOC stabilization level was stored in the labile POC pool (especially necromass from fungi). Activities of four enzymes (i.e., β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase, and alkaline phosphatase) increasing with fungal and bacterial necromass suggest that the raised activity of living microorganisms accelerated the turnover and formation of necromass. Microbial N limitation raised the production of N acquisition enzymes (e.g., β-1,4-N-acetyl-glucosaminidase and leucine aminopeptidase) to break down necromass compounds, leading to further increase of the nutrient pool in soil solution. The decrease of microbial N limitation along the biocrusts formation chronosequence is an important factor for the necromass accumulation during initial soil development. High microbial N demands and insufficient clay protection lead to fast necromass reutilization by microorganisms and thus, result in a low necromass accumulation coefficient, that is, the ratio of microbial necromass to living microbial biomass (on average, 9.6). Consequently, microbial necromass contribution to SOC during initial soil formation by biocrust is lower (12–25%) than in fully developed soils (33%–60%, literature data). Nitrogen (N) limitation of microorganisms and an increased ratio between N-acquiring enzyme activities and microbial N, as well as limited clay protection, resulted in a low contribution of microbial necromass to SOC by initial formation of biocrust-covered sandy soil. Summarizing, soil development leads not only to SOC accumulation, but also to increased contribution of microbial necromass to SOC, whereas the plant litter contribution decreases.

Short-term acceleration of soil organic matter decomposition by increasing temperature conflicts with the thermal adaptation observed in long-term studies. Here we used the altitudinal gradient on Mt. Kilimanjaro to demonstrate the mechanisms of thermal adaptation of extra- and intracellular enzymes that hydrolyze cellulose, chitin and phytate and oxidize monomers ( 14 C-glucose) in warm- and cold-climate soils. We revealed that no response of decomposition rate to temperature occurs because of a cancelling effect consisting in an increase in half-saturation constants (K m ), which counteracts the increase in maximal reaction rates (V max with temperature). We used the parameters of enzyme kinetics to predict thresholds of substrate concentration (S crit ) below which decomposition rates will be insensitive to global warming. Increasing values of S crit, and hence stronger canceling effects with increasing altitude on Mt. Kilimanjaro, explained the thermal adaptation of polymer decomposition. The reduction of the temperature sensitivity of V max along the altitudinal gradient contributed to thermal adaptation of both polymer and monomer degradation. Extrapolating the altitudinal gradient to the large-scale latitudinal gradient, these results show that the soils of cold climates with stronger and more frequent temperature variation are less sensitive to global warming than soils adapted to high temperatures.

Das Tibetische Plateau, mit seinen Kobresia pygmea Weiden, stellt eine globale Senke fur organischen Kohlenstoff (OC) dar. Uberweidung fuhrt zur Zerstorung der Kobresia Wurzelmatten und damit zur Graslanddegradation. Dies hat massive OC Verluste und die Destabilisierung des Okosystems zur Folge. Diese Arbeit analysiert den Effekt der degradationsbedingten veranderten Zusammensetzung der organischen Bodensubstanz (SOM) auf die Zusammensetzung der mikrobiellen Gemeinschaft und deren Funktion entlang einer Sequenz von sechs Degradationsstufen. Dazu wurden die Aktivitaten von sechs extrazellularen Enzyme gemessen sowie die Zusammensetzung der bakteriellen und pilzlichen Gemeinschaft (mittels t-RFLP Analyse gefolgt von Illumina MiSeq Sequenzierung) analysiert. So konnte die Zusammensetzung der mikrobiellen Gemeinschaft mit den vertikalen Gradienten der δ13C und δ15N-Werte, sowie den Neutralzucker-, Cutin-, Suberin- und Ligningehalten in Beziehung gesetzt werden. Mit zunehmender Degradation deuten negativere δ13C-Werte auf eine relative Anreicherung an 13C-armen Makromolekulen wie bspw. Lignin hin. Zusammen mit ebenfalls verringerten δ15N-Werten und veranderten C/N-Verhaltnissen belegt dies einen verstarkten SOM-Abbau. Gleichzeitig nimmt die Aktivitat von hydrolytischen Enzymen, mit fortschreitender Degradation zu. Lediglich in der letzten Degradationsstufe ist ein drastischer Ruckgang der Aktivitat dieser Enzymgruppe zu beobachten. Im Gegensatz dazu zeigen Phenoloxidasen (Mineralisation komplexer SOM) zunehmende Aktivitaten mit steigender Degradation die mit einer Anreicherung dieser Substrate (z.B. Ligin) einhergeht. Die beobachteten Anderungen der Enzymaktivitaten sind mit einer veranderten Zusammensetzung der mikrobiellen Gemeinschaft, besonders des Unterbodens, begleitet. So nehmen Actinobacteria mit der Degradation ab wahrend Nitrosomonas zunehmenden, was einen steigenden Verlust von mineralisiertem N aus dem Boden erklart. Bis jetzt ist unklar ob die degradationsbedingten funktionellen und strukturellen Veranderungen der mikrobiellen Gemeinschaft der Kobresia Graslander reversibel sind, oder ob mit steigendem Verlust an Nahrstoffen und Boden ein Punkt erreicht wird, der eine Erholung der Kobresia-Matten nicht mehr erlaubt.

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.

Aims Plastic film mulching induces significant shifts in soil temperature and water balance, thereby influencing microbial activities, particularly those associated with nitrogen (N) transformations. However, its effects on interactions between N fate and soil N transformations remain unclear. Methods We investigated the complex interplay of soil N transformation processes using a 15 N tracing method, N availability, and N fate under plastic film mulched ridges (PFM), in contrast to a non-mulched flat system (control). Results PFM resulted in 20–28% reduction in gross N mineralization and nitrification rates and increased rates of gross microbial N immobilization. Maize showed a 19% increase in N uptake and a 127% increase in N accumulation in the PFM-treated soil (up to 80 cm depth) compared to the control. PFM effectively inhibited N leaching, while also reducing N 2 O and NH 3 gas emissions (by 32 kg N ha -1 ). In the early stages of maize growth, PFM-treated soil showed increased N availability due to accelerated rates of gross N mineralization and nitrification, which in turn bolstered N uptake by both maize and microorganisms. Furthermore, PFM effectively mitigated gaseous N emissions and N leaching, contributing to increased soil N retention and N use efficiency. As the rates of gross N mineralization and nitrification declined in the later stages of maize growth, PFM maintained substantial N availability. This was achieved by limiting NO 3 - leaching and microbial N immobilization, resulting in heightened N uptake and increased maize yield. Conclusion Plastic film mulching produced changes in soil N transformation processes that included gross N mineralization, nitrification, and immobilization rates. These changes manifested in increased N availability, maize N uptake, soil N retention, and reduced N losses.