Soil macrofauna and nitrogen on a sub-Antarctic island

Summary The densities, diets and habitat preferences of the soil macrofaunal species on sub-Antarctic Marion Island (47°S, 38°E) are described. Their role in N cycling on the island is assessed, using a mire-grassland community as an example. Primary production on the island is high and this leads to a substantial annual requirement of nutrients by the vegetation. This requirement must almost wholly be met by mineralization of nutrient reserves in the organic matter. Rates of peat nitrogen mineralization mediated by microorganisms alone are much too low to account for rates of N uptake by the vegetation. Although soil macroinvertebrates, and bacteria represent a very small fraction of the total N pool, their interaction accounts for most of the peat N mineralization, as indicated by the amounts of inorganic N released into solution in microcosms. Extrapolation of the microcosm results shows that the soil macrofauna (mainly earthworms) stimulate the release of enough N from the mire-grassland peat to account for maximum N mineralization rates calculated from temporal changes in peat inorganic N levels and plant uptake during the most active part of the growing season. Considering that large numbers of mesoand microinvertebrates occur and must also contribute to nutrient mineralization, the soil faunal component is clearly of crucial importance to nutrient cycling on Marion Island. This is probably true of all sub-Antarctic islands.     

Rapid degradation of pyrogenic carbon

Pyrogenic carbon (PC‐ charcoal, biochar or black carbon) represents a poorly understood component of the global carbon (C) cycle, but one that has considerable potential to mitigate climate change through provision of long‐term soil C sequestration. Mass balance calculations suggest global PC production and stocks are not in balance, indicating a major gap in our understanding of the processes by which PC is re‐mineralized. We collected PC samples derived from the same wood material and exposed to natural environmental conditions for 1 and 11 years. We subjected these materials to repeated laboratory incubation studies at temperatures of up to 60 °C, as ground surface temperatures above 30 °C and up to 60 °C occur regularly over a significant area of the tropics and sub‐tropics. Mineralization rates were not different for the two samples and followed an exponential Arrhenius function that suggest an average turnover time of 67 years for conditions typical of a tropical savannah environment. Microbial biomass as measured by chloroform fumigation and DNA extractions was the same for the two samples, but abiotic CO₂ production was lower for the fresh PC sample than that for the aged sample. Nuclear magnetic resonance spectroscopy, hydrogen pyrolysis and scanning electron microscopy demonstrate that the measured CO₂ production originates dominantly from polycyclic aromatic compounds rather than any minor labile components. Therefore, rapid, sub‐centennial rates of re‐mineralization of PC on the soil surface in tropical and sub‐tropical environments may represent a major and hitherto unidentified mechanism for balancing the PC production at the global scale.     

Populations and activity of carbofuran-degrading microorganisms in soil

Kendaia clay loam contained more than 10⁵ microbial cells per g able to convert ¹⁴C-carbonyl-labelled carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) to ¹⁴CO₂ but never more than 130 cells per g transforming ¹⁴C-ring-labelled carbofuran to CO₂. The sizes of the population rarely increased as a result of addition of the insecticide to soil. Mineralization of these compounds proceeded with little or no acclimation phase, and subsequent additions were usually etabolized more readily, except at 10 mg of carbofuran per kg or if subsequent additions of the pesticide were made long after the first. More than 60% of the ¹⁴C in the carbonyl but less of the ¹⁴C in the ring was microbiologically converted to ¹⁴CO₂ in this soil. Streptomycin and cycloheximide each inhibited conversion of the carbonyl or ring carbon to CO₂. Urea but not NH₄NO₃ markedly inhibited the conversion of the carbonyl-labelled insecticide to ¹⁴CO₂. The addition of glucose and succinate together with the insecticide did not enhance mineralization of ring- or carbonyl-labelled carbofuran. The data suggest that soils containing a large population of microorganisms able to convert the carbonyl carbon to CO₂ will not show a marked effect of prior treatment with the insecticide and that few organisms individually are able to mineralize the ring.     

Humic substance mineralization in a tropical oxbow lake (São Paulo,Brazil)

We evaluated the mineralization rates of humic substances in Infernão oxbow lake (State of São Paulo, Brazil). Experiments were conducted under aerobic and anaerobic conditions using fulvic acid and humic acid from four sources: Scirpus cubensis and Cabomba piauhyensis leachate submitted to a 120-day degradation process, sediment, and dissolved organic matter from the lake water. A fixed amount of substrate was added to 450 ml of water from Infernão lake, filtered over glass wool. After adding substrate, the flasks were incubated at 21.0°C under aerobic and anaerobic conditions. The dissolved organic carbon was monitored during 95 days. The results were fitted to first-order kinetics model, which pointed to one labile and one refractory fraction. The refractory fractions predominated, ranging from 71.4 to 84.3% for fulvic acid and from 73.4 to 85.0% for humic acid. Mineralization rates of the labile fractions of dissolved organic carbon were higher under aerobic than anaerobic conditions, while the converse was true for the refractory fractions.     

Ammonium uptake and mineralization in prairie streams: chamber incubation and short-term nutrient addition experiments

1. We used two separate approaches to estimate ambient ammonium cycling in the north and south branches of Kings Creek, a prairie stream. Chamber experiments were conducted to determine ammonium uptake and mineralization rates associated with epilithic biofilms and filamentous algae collected from the streams. A series of short-term whole-stream ammonium addition experiments were also conducted to estimate the rate of uptake at ambient stream concentrations, based on the relationship between ammonium concentrations and uptake rates.
2. Chamber experiments were scaled up to whole-stream levels, resulting in ambient gross uptake estimates of 0.08  μ g⁻² s⁻¹ for the north branch and 0.16  μ g⁻² s⁻¹ for the south branch. The substrata-specific estimates of mineralization were higher than uptake in both streams.
3. Substrata-specific measurements indicated that ammonium uptake is higher in riffle habitats than in pools habitats. The results of the short-term ammonium addition experiments support these findings.
4. Short-term ammonium addition experiments show that uptake rates saturate with increasing ammonium concentrations. The observed saturation of uptake rates is consistent with a Michaelis–Menten relationship.
5. Scaled estimates of uptake from the chamber experiments were similar to estimates of ambient ammonium uptake based on the whole-stream experiments, and were comparable with previous estimates of ammonium uptake and mineralization made by using stable isotope tracer methods in Kings Creek.     

Climate change amplifies gross nitrogen turnover in montane grasslands of Central Europe in both summer and winter seasons

The carbon‐ and nitrogen‐rich soils of montane grasslands are exposed to above‐average warming and to altered precipitation patterns as a result of global change. To investigate the consequences of climatic change for soil nitrogen turnover, we translocated intact plant–soil mesocosms along an elevational gradient, resulting in an increase of the mean annual temperature by approx. 2 °C while decreasing precipitation from approx. 1500 to 1000 mm. Following three years of equilibration, we monitored the dynamics of gross nitrogen turnover and ammonia‐oxidizing bacteria (AOB) and archaea (AOA) in soils over an entire year. Gross nitrogen turnover and gene levels of AOB and AOA showed pronounced seasonal dynamics. Both summer and winter periods equally contributed to cumulative annual N turnover. However, highest gross N turnover and abundance of ammonia oxidizers were observed in frozen soil of the climate change site, likely due to physical liberation of organic substrates and their rapid turnover in the unfrozen soil water film. This effect was not observed at the control site, where soil freezing did not occur due to a significant insulating snowpack. Climate change conditions accelerated gross nitrogen mineralization by 250% on average. Increased N mineralization significantly stimulated gross nitrification by AOB rather than by AOA. However, climate change impacts were restricted to the 2–6 cm topsoil and rarely occurred at 12–16 cm depth, where generally much lower N turnover was observed. Our study shows that significant mineralization pulses occur under changing climate, which is likely to result in soil organic matter losses with their associated negative impacts on key soil functions. We also show that N cycling processes in frozen soil can be hot moments for N turnover and thus are of paramount importance for understanding seasonal patterns, annual sum of N turnover and possible climate change feedbacks.     

Tree physiological responses to above-ground herbivory directly modify below-ground processes of soil carbon and nitrogen cycling

Above‐ground herbivory is ubiquitous in terrestrial ecosystems, yet its impacts on below‐ground processes and consequences for plants remain ambiguous. To examine whether physiological responses of individual trees may potentially modify soil nutrient availability, we subjected Fagus sylvatica L. (European beech) and Abies alba Mill. (silver fir) to simulated foliar herbivory over two growing seasons. Above‐ground herbivory enhanced N mineralization and inorganic N availability in the soil. The total input of C from the plant roots to the soil is not known; however, carbon sequestration in the soil, measured using stable isotopic techniques, was unaffected by herbivory. Fagus responded to herbivory by producing larger leaves, with increased photosynthetic capacity and N content, which largely compensated for the loss of biomass; Abies exhibited no such response. We conclude that despite large interspecific differences in the growth response, tree physiological responses to foliar herbivory are capable of directly modifying soil biological processes.     

Quantitative electron microscopic investigations of mineral nucleation in collagen

Summary It has been previously shown that the distances between the nuclei within the collagen bundles of mineralizing tissues were in good agreement with the repeat distances of the cross-banding pattern of collagen, which supports the assumption that the distances between the mineral deposits reflect to a good approximation the distances between nucleation centres on the collagen macromolecule. However, the lateral separation of the nuclei were significantly higher than the distances between close-packed triple helices.Recently a new model of collagen aggregation has been proposed in which the smallest morphological units are subfibrils (Ø approx. 39 Å) packed in tetragonal array. This led us to measure once again the lateral separation between a) close-packed calcium phosphate needles lying in bundles at (1) the mineralizing front of mantle dentine and (2) at the mineralizing front of rat tail bone, and b) between the uranyl-lead nuclei produced in the staining of rat tail tendon.The mean lateral distances separating these nuclei fell within the range of 39–47 Å, which is a little higher than the distances of 39 Å which separate the microholes between the subfibrils in the tetragonal packing model, which are regarded as the likely sites of nucleation. If, however, it is assumed that the forces generated during mineralization can cause the collagen fibres to swell, then the lateral separation of the nuclei and the distances between the microholes would correspond very closely.We thank the Deutsche Forschungsgemeinschaft for financial support. We thank Prof. Dr. K. Heckmann and Dr. U. Mays, Dept. of Zoology, Münster, for allowing us to use their Siemens-Elmiskop 101 sponsored by Stiftung Volkswagenwerk, and Frau Dr. Weichan, Applikationslabor Siemens, Berlin, for performing the tilting experiments at their Siemens-Elmiskop 102. We thank Fräulein Ute Sporman for valuable technical help.