"Non-metabolizable" glucose analogue shines new light on primingmechanisms: Triggering of microbial metabolism

The rhizosphere and detritusphere are characterized by increased carbon availability, including low-molecular weight organic substances. Such easily biodegradable organic substances can change the mineralization rates of pre-existing soil organic matter, a phenomenon termed priming. Priming of soil organic matter decomposition has attracted much research interest, yet a conclusive mechanistic explanation remains elusive. One proposal is that low molecular weight organic substances might “trigger” an acceleration of microbial metabolism. For the first time, we applied a glucose analogue to soil to demonstrate triggering of microbial metabolism, and to estimate its relative contribution to priming. “Non-metabolizable” glucose analogues have been widely used in pure culture studies to mimic glucose, but never in soil biochemistry. We hypothesized that analogue molecules will elicit a metabolic response in microorganisms despite limited catabolism, and thereby confirm the proposed triggering. The effect of 14C-labeled 3-O-methyl-D-glucose (OMG) – a common “non-metabolizable” glucose analogue – on soil organic matter mineralization was compared to that of 14C-labeled D-glucose. OMG was mineralized, but its mineralization was initially impeded and substantially delayed, relative to glucose. OMG caused brief but strong priming in the first 24 h, increasing unlabeled CO2 efflux by 173%, 89% and 36% above control for additions of 0.49, 2.4 and 4.9 mmol OMG g-1 soil, respectively. In contrast, glucose caused low or negative priming on the first day. On the first day after OMG addition, a negative correlation between priming and OMG mineralization indicated that triggering is a valid mechanism of microbial activation during a famine-feast transition, but is short-lived. Glucose mineralization peaked on the second day for medium and high additions, coinciding with peaks in positive priming. Maximum substrate mineralization also coincided with peaks in priming for medium and high OMG levels, but these occurred 9 and 11 days after addition, respectively. This revealed non-triggering priming mechanisms, which contributed most to priming and were closely coupled to substrate mineralization. By separating energy- and substrate-dependent metabolic processes from triggering processes, the glucose analogue 3-O-methyl-D-glucose enabled triggering to be demonstrated, but triggering by glucose occurs without contributing greatly to priming

Distinct responses of soil respiration to experimental litter manipulation in temperate woodland and tropical forest

Global change is affecting primary productivity in forests worldwide, and this, in turn, will alter long‐term carbon (C) sequestration in wooded ecosystems. On one hand, increased primary productivity, for example, in response to elevated atmospheric carbon dioxide (CO2), can result in greater inputs of organic matter to the soil, which could increase C sequestration belowground. On other hand, many of the interactions between plants and microorganisms that determine soil C dynamics are poorly characterized, and additional inputs of plant material, such as leaf litter, can result in the mineralization of soil organic matter, and the release of soil C as CO2 during so‐called “priming effects”. Until now, very few studies made direct comparison of changes in soil C dynamics in response to altered plant inputs in different wooded ecosystems. We addressed this with a cross‐continental study with litter removal and addition treatments in a temperate woodland (Wytham Woods) and lowland tropical forest (Gigante forest) to compare the consequences of increased litterfall on soil respiration in two distinct wooded ecosystems. Mean soil respiration was almost twice as high at Gigante (5.0 μmol CO2 m−2 s−1) than at Wytham (2.7 μmol CO2 m−2 s−1) but surprisingly, litter manipulation treatments had a greater and more immediate effect on soil respiration at Wytham. We measured a 30% increase in soil respiration in response to litter addition treatments at Wytham, compared to a 10% increase at Gigante. Importantly, despite higher soil respiration rates at Gigante, priming effects were stronger and more consistent at Wytham. Our results suggest that in situ priming effects in wooded ecosystems track seasonality in litterfall and soil respiration but the amount of soil C released by priming is not proportional to rates of soil respiration. Instead, priming effects may be promoted by larger inputs of organic matter combined with slower turnover rates

Peat decomposition indicators of two contrasting bogs in the Eastern Alps, Austria

Since carbon (C) in peatlands is labile and sensitive to disturbances, peatlands have the potential to release high C amounts by land use changes and to accelerate global warming. Therefore, adequate peat decomposition indicators (PDI) are necessary to assess the peatland degradation status and potential for CO2 release. In order to assess the peat degradation status of nine sites in Alpine bogs (Enns valley, Austria), we compared PDI of two peat bogs with contrasting land-use histories. The conventional PDI: loss on ignition, bulk density, C:N ratios, water table depths (WTD) were compared with the recently introduced PDI: stable carbon isotope signature (d13C) and nitrogen isotope signature (d15N). The most PDI were different between the two bogs and the study sites with contrasting WTD and degree of peat decomposition. We demonstrated strong relationships and similar depth profiles of variables: Loss of ignition of strongly degraded peat decreases from the acrotelm to the catotelm, but remains stable at less degraded peat. Bulk density generally increases with depth, and was lowest in the acrotelm of the central bog area and highest in the catotelm of the former peat cutting areas. C:N ratios increased slightly with the degree of peat decomposition. d13C and d15N increased from the top to the depths of -24 to -42 cm at all study sites. In the catotelm, dC13 were significantly lower in strongly decomposed peat compared to the less degraded sites. Higher d15N values in acrotelm and catotelm of strongly degraded peat may be evidence for more pronounced N fractionation during decomposition compared to less degraded sites. Decomposers tend to preferably use substances with 12C for respiration, resulting in a relative enrichment of 13C in the residual organic matter. Accordingly, the increase of d13C with depth in the acrotelm in strongly decayed peat may be assigned to 12C loss by respiration

Effect of land-use and elevation on microbial biomass and water-extractable carbon in soils from Mt. Kilimanjaro ecosystems

Microbial biomass carbon (MBC) and waterextractable organic carbon (WOC) – as sensitive and important parameters for soil fertility and C turnover – are strongly affected by land-use changes all over the world. These effects are particularly distinct upon conversion of natural to agricultural ecosystems due to very fast carbon (C) and nutrient cycles and high vulnerability, especially in the tropics. The objective of this study was to use the unique advantage of Mt. Kilimanjaro – altitudinal gradient leading to different tropical ecosystems but developed all on the same soil parent material – to investigate the effects of and-use change and elevation on MBC and WOC contents during a transition phase from dry to wet season. Down to a soil depth of 50 cm, we compared MBC and WOC contents of 2 natural (Ocotea and Podocarpus forest), 3 seminatural (lower montane forest, grassland, savannah), 1 sustainably used (homegarden) and 2 intensively used (maize field, coffee plantation) ecosystems on an elevation gradient from 950 to 2850 m a.s.l. Independent of land-use, both MBC and WOC strongly increased with elevation on Mt. Kilimanjaro corresponding to ecosystem productivity and biodiversity. Through the agricultural use of ecosystems MBC and WOC contents decreased – especially in surface layers – on average by 765 mg kg-1 for MBC and 916 mg kg-1 for WOC, compared to the respective natural ecosystems. The decrease with depth was highest for forests > grasslands > agroecosystems and also was positively correlated with elevation. We conclude that MBC and WOC contents in soils of Mt. Kilimanjaro ecosystems are highly sensitive to landuse changes, especially in topsoil. The MBC and WOC contents were considerably reduced even in sustainable agricultural systems. Since MBC and WOC are very fast reacting and sensitive C pools, we expect a decrease in other soil C pools accompanied by a strong decrease in fertility and productivity due to changes in land use from natural to agricultural ecosystems

Soil cation exchange capacity: main factor of shell carbonate diagenesis

Shell carbonate diagenesis occurs in interaction with soil solution, where the concentration of Ca2+ is in equilibrium with exchangeable Ca2+ and weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium with Ca-bearing minerals achieves after months. We hypothesized that shell carbonate diagenesis proceeds slower in soils with high cation exchange capacity (CEC) than those with low CEC. The goal of this study was to determine the effects of soil CEC and exchangeable cations on shell carbonate diagenesis. Shells of Protothaca staminea were mixed in glass bottles with 1) carbonate-free sand (CEC = 0.37 cmol+kg-1) (S), 2) a native loamy soil (CEC = 16 cmol+kg-1) (LS) and 3) the same loamy soil saturated with KCl (replacing exchangeable cations with K+) (LSK). Samples were incubated at room temperature for 5, 20, 60 and 120 days. Bottles air was labeled with 14CO2 at the beginning and day 55. 14C was measured at sampling dates in bottles air, soil solutions, bulk soils and shells. Dissolved and exchangeable cations were measured. Shell carbonate diagenesis in LS and LSK (0.016 and 0.024 mg CaCO3, respectively) was one order of magnitude lower than in S (0.13 mg CaCO3). Shell carbonate dissolution and consequently recrystallization decreased at high amounts of exchangeable Ca2+ because exchange is faster than dissolution. Therefore, soil CEC and composition of exchangeable cations is a determinant factor of shell carbonate diagenesis and it should be considered by radiocarbon dating. Because shells in soils with lower CEC undergo more intensive diagenesis, they need further pretreatments before dating. Soil CEC should be also included in shell carbonate diagenesis models. Furthermore, 14C labeling can be used to investigate the rates of minerals weathering - at least for Ca-bearing minerals - and soil acidification

Root development controls hotspots localization and temperature sensitivity of enzyme activity in the rhizosphere

The rhizosphere is a very important and dynamic hotspot of microbial activity in soil. Consequently, the enzyme activities in the rhizosphere are a footprint of complex plant-microbial interactions and may reflect functional response to climate changes.The temperature sensitivity of enzymes responsible for organic matter decomposition in soil is crucial for predicting the effects of global warming on the carbon cycle and sequestration. For the first time, we applied the in situ soil zymography for identification and localization of hotspots of phosphatase and chitinase activity in the rhizosphere of rice (Oryza sativa L.) under warming effect - (18 and 25 °C) after 14 and 30 days. Thus, we test the hypotheses that due to high inputs of easily degradable organic compounds from the roots canceling effect: strong reduction of temperature sensitvity (Q10~1) of catalytic reactions will not accoure in the rhizosphere. Correspondingly, the Q10 values for reaction rates were always >1, at root-soil interface, with the average range of 1.3 –1.4 Independent of enzymes, canceling was never observed at vicinity of root. Thus, canceling effect is a substrate concentration dependence phenomenon. To our knowledge, this is the first study explored the canceling effect in the rhizosphere. Absence of canceling at root-soil interface for phosphates and chitinase revealed that warming will accelerate P and N mobilization in the rhizosphere. Altogether, for the first time we showed that extent of enzyme activity’s rhizosphere is constant, temporally however, there is a temporal heterogeneity of enzymatic hotspots localization in soil. Thus, increasing in temperature had a positive impact on overall enzyme activities, Rice growth and root development, conducted an enzyme specific impact on hotspots percentage and localization patterns. We conclude that absence of canceling at root-soil interface for tested enzymes revealed that warming will accelerate nutrient mobilization in the rhizosphere more than root free soil

Soil carbon losses and estimation of erosion and decomposition by δ Carbon-13 in riparian soils under lowland rainforest transformation systems on Sumatra, Indonesia

Indonesia´s forest is ranked among the Amazonian and the Kongo Basin as the largest tropical rainforest area worldwide. However, the country experiences a severe forest loss since the 1970s. Besides a growing population, the primary pressures are export-orientated timber production and a global commodity demand that lead to a permanent conversion from forest to agricultural areas. The roles of resulting transformation systems of tropical riparian rainforests for ecological functions have yet received little attention in scientific research. Especially C stocks in riparian zones are strongly affected by climate and land use changes that lead to changes in water regime and ground water level drops. We investigated the effects of land transformations in riparian ecosystems of Sumatra, on soil C content, stocks and decomposability. C losses in rubber and oil palm plantations and rainforests were compared and the contribution of soil erosion and organic matter mineralization was estimated. Based on δ Carbon-13 along soil depth, two processes decreasing C stocks were distinguished: erosion and mineralization of soil organic matter (SOM). Depending on the shift of the δ Carbon-13 value of SOC in the topsoil from the linear regression calculated by δ Carbon-13 value with log(SOC) in the topsoil, modification of C turnover rate in the top soil was evaluated. Erosion was estimated by the shift of the δCarbon-13 value of SOC in the subsoil under plantations. The Ah-horizons in non-riparian soils under oil palm and rubber plantations showed with 70% and 62 % a strong reduction in C content and a strong erosion: 35 ±8 cm in oil palm and 33 ±10 cm in rubber plantations. Within the riparian zones an inhomogenous spatial distribution of C content is expected, due to the trend of increasing C stocks from terrestrial through semi –terrestrial to wet conditions. By comparing decreasing δ Carbon-13 values of SOC in the topsoil to those in subsoil, a lower erosion in all transformation systems in riparian zones could be detected

Towards to physiological status of soil microorganisms determined by RNA:dsDNA ratio

Despite soil microorganisms spend most of their lifetime in a state of dormancy, they are quickly activated by substrate input and easily switch to growth. As both the dsDNA- and RNA- contents increase during microbial growth, the RNA:dsDNA ratio reflects a promising predictor, whether the response of a microbial community to environmental changes is due to an increase in population (by dsDNA) or due to an increase in activity (by RNA). This prediction of the RNA:dsDNA ratio can be accomplished by the comparison of microbial incubation approaches with and without addition of easily available substrates. We exhibited the RNA:dsDNA ratios for four contrasting soil types during substrate-induced growth. Overall, after glucose addition, a strong increase of dsDNA and RNA contents were determined in most of the soil types during 72 h of incubation. Furthermore, we identified distinct temporal soil-specific RNA:dsDNA patterns. The dsDNA- and RNA-contents yielded 26–174 and 0.3–30 µg g-1 soil, respectively. The soil texture was strongly associated with the reduction of RNA recovery, by means of an exponential decrease of RNA-content with increasing clay content. The lower RNA recovery in virgin and arable Chernozem (>30%) compared to soil types with lower clay contents (<17% for Retisol, Luvisol and Calcisol) suggests, that the undercount of RNA yields in clayey soils biased the RNA:dsDNA ratio, and subsequently the physiological state of the microbial community is not adequately represented in soils with clay contents exceeding 30%