Sources of spatial and temporal variability of inorganic nitrogen in pure and mixed deciduous temperate forests

We investigated the influence of tree canopy composition and structure on the spatial and temporal variability of (i) concentrations of inorganic N (NH₄⁺ and NO₃⁻) and (ii) net N-mineralization and net nitrification, within the temperate forest floor. We compared a pure European beech stand (PS) with a mixed beech-hornbeam one (MS). Three sampling areas were chosen in each stand. Within the PS, the tree locations represented a decreasing gradient of light intensity reaching the forest floor. Within the MS they represented a gradient in the amount of hornbeam leaves present in the litter. In the field NH₄⁺ and NO₃⁻ concentrations were measured in the upper mineral soil (UMS) and the overlying organic layers (OL and OF+OH). Field exposures using buried bags were carried out on UMS over 1 year to measure in situ net N-mineralization and net nitrification. Potential net N-mineralization and net nitrification were investigated in summer with UMS, OL and OF+OH incubated at 28 °C for 28 days in the laboratory. We hypothesize that with the presence of a mull-forming species (hornbeam) within a stand dominated by a moder-forming one (European beech), (i) the spatial and (ii) temporal patterns of soil inorganic N concentrations, net N-mineralization and net nitrification would be different in the two stands. Our main results show that tree species composition has an influence on both spatial and temporal patterns of nitrification. The PS exhibited its highest peaks of UMS NO₃⁻ concentration and net nitrification in spring and early summer while they were highest in the MS in winter. Furthermore, PS exhibited a higher rate of net nitrification than MS. We discuss this unexpected result and suggest that dissolved organic C may be the controlling factor for net nitrification in the MS.     

Effects of seasonal variations and pond age on trace elements and their correlations with plankton productivity in commercial freshwater crayfish (cherax cainii austin, 2002) earthen ponds

Seasonal variations can affect the concentration of trace elements, and the change in their concentrations can affect the natural productivity of freshwater aquaculture ponds. Hence, we studied the seasonal variations of the 12 pre‐selected trace elements (Co, Cu, Fe, Mn, Zn, Se, Ca, Mg, P, S, Al, Si) and their relationships with primary and secondary productivity in two aged ponds, stocked with three different life stages of marron (Cherax cainii), for a period of 1 year. Trace element analysis was performed by using (Agilent, ICP‐OES). Except Co and Se, all trace elements, and primary and secondary productivity were influenced by seasonal variation. The pond age significantly influenced the concentrations of some trace elements. On a seasonal basis, trace elements were positively correlated with the plankton abundance, species diversity, and wet and dry plankton weights. Seasonal variations and pond age affected the dissolved concentrations of trace elements and plankton productivity.     

Evaluation of parsley for resistance to the pathogens Alternaria radicina, Erysiphe heraclei, Fusarium oxysporum, and celery mosaic virus (CeMV)

Resistance tests of 127 parsley accessions (Petroselinum crispum) were carried out during 2 years of field investigations involving natural infection for black leaf spot (Alternaria radicina), mildew (Erysiphe heraclei), leaf chlorosis (Fusarium oxysporum) and viruses, particularly celery mosaic virus (CeMV). In this first extensive evaluation of parsley for all tested pathogens accessions were found to be free of symptoms. Varieties of vars. crispum and tuberosum contained more accessions weakly infected or immune to black leaf spot than vars. vulgare and neapolitanicum. In a separate test five var. crispum accessions showed significantly reduced disease ratings, indicating their potential as sources of resistance. Mildew attacked almost the same number of accessions in both years: 29 accessions of var. crispum showed no symptoms, but 16 of the var. tuberosum accessions were highly susceptible. Whereas in the var. vulgare one half of the introductions were moderately or strongly infected by viruses, mainly CeMV, a high number of plants remained uninfected in the vars. crispum, neapolitanicum and tuberosum. In the case of F. oxysporum, the greatest amount of infection was in the var. crispum during both years. The sources of resistance described including multiple resistance would be of interest in future breeding programmes for resistance to the parsley pathogens discussed.     

Variability in parsley (Petroselinum crispum [Mill.] Nyman) for reaction to Septoria petroselini Desm., Plasmopara petroselini Săvul. et O. Săvul. and Erysiphe heraclei DC. ex Saint-Aman causing Septoria blight, downy mildew and powdery mildew

A collection of 220 accessions of parsley (Petroselinum crispum [Mill.] Nyman) was evaluated at two experimental stations (Gatersleben and Quedlinburg, Germany) under natural infection. Widespread origins of tested accessions provide the opportunity for first-time characterization of existence, spreading and level of resistance/susceptibility to the economically important pathogens Septoria petroselini Desm., Plasmopara petroselini S?vul. et O. S?vul. and Erysiphe heraclei DC. ex Saint-Aman causing Septoria blight, downy mildew and powdery mildew respectively throughout the species P. crispum. For each pathogen, accessions free or nearly free of symptoms were found: S. petroselini: free 1, nearly free 25, P. petroselini: free 51, nearly free 22 and E. heraclei: free 166. Eight accessions are free or nearly free of symptoms for all three pathogens: PET36, PET169, PET172, PET177, PET178, PET192, PET212 and PET214. The accession PET16 is free of symptoms for both S. petroselini and P. petroselini. Most of the resistant material comes from Southern and Eastern Europe. There could be a centre for resistances of parsley.     

Fungal, bacterial and plant dsDNA contributions to soil total DNA extracted from silty soils under different farming practices: Relationships with chloroform-labile carbon

Twelve differently-managed silty soils from North-Western France were chosen to compare two common methods of quantifying soil microbial biomass: Chloroform fumigation and extraction-labile carbon (CL_C) and microbial double stranded DNA (dsDNA). We also determined the contributions of each of the fungal, bacterial, and plant kingdoms to the total community dsDNA using real-time Polymerase Chain Reaction with kingdom-specific ribosomal primer sets. Regardless of the method, the highest microbial biomasses were associated with long-term untilled plots. Site (locations) specificities could also be detected, especially in conventionally cultivated lands. Regardless of site, a strong linear relationship could be drawn between CL_C and dsDNA in tilled lands (r = 0.91, n = 15, P = 0.01) and in grasslands (r = 0.78, n = 21, P = 0.01). Moreover, we propose a logarithmic model describing all of our silty soils, irrespective of management. In order to explain the non-linearity (log) of this relationship, we tested the hypothesis of a weak plant dsDNA contribution in total dsDNA in comparison with the well-documented root cell contribution to CL_C quantifications. Plant dsDNA never exceeded 2.6% of total dsDNA content for all of the soils studied. Among groups examined, the bacterial dsDNA contribution to the community dsDNA pool was the most site- and/or pedoclimatic-dependent. Fungi constituted a major component of total microbial biomass in grassland or in land with permanent plant cover where their proportion reached almost 50% of total dsDNA. More precisely, fungal dsDNA concentration was highly related to tillage. Our study demonstrated the expediency of the total microbial dsDNA quantification in agricultural silty soils rather than the time-consuming quantification of CL_C. Quantifying the relative contribution of bacterial or fungal biomass in total dsDNA by real-time PCR allows to access to a new level of knowledge of the soil microbial biomass and to reveal the balances between those two kingdoms according to soils or farming practices.     

Response of collembolan assemblages to plant species successional gradient

It has been acknowledged that soil organisms play a significant role in nutrient cycling and thus may affect productivity and competition within plant assemblages with potential effects on vegetation trajectories. However, few studies have considered a single conceptual framework referring to both above–belowground linkages and plant succession. Consequently, we lack knowledge on the effects of plant successional processes on the dynamics of soil biota. Given this dearth of information, our study aimed to describe the dynamics of a major group of soil fauna, namely collembola, during the secondary succession on chalky slopes edging the Seine River. We selected five different plant assemblages as representative of a chronosequence: short grassland, tall grassland, encroached grassland, shrubland and forest. Our results clearly highlighted a strong response of collembolan assemblages to vegetation gradient. The changes in collembolan diversity were due to a high turnover rate in early successional stages and to nestedness in late successional stages. In addition, each collembolan life form had a specific response to vegetal succession. Euedaphic assemblages increased progressively during the succession while epedaphic did not follow a clear pattern. Our results also indicated a link between the dynamics of collembolan assemblages and plant life forms (e.g. trees and graminoids) rather than plant diversity. A focus on life forms, for plants and soil biota, seems to provide a good framework to study linkages between above and belowground biota.     

Changes in soil N mineralization and nitrification pathways along a mixed forest chronosequence

Changes in soil N mineralization pathways occurring along a full rotation cycle have received little attention to date, while tree uptake for N may change during forest ageing. The aims of this study were (i) to characterize changes in potential net N mineralization and potential net nitrification within organic layers and the topsoil (organo-mineral horizon) along a 100-year chronosequence for a temperate oak–hornbeam forest and (ii) to reveal covariances between potential net N mineralization pathways and the properties of the humic epipedon (defined as the sum of organic layers and topsoil). For that purpose, a space-for-time substitution procedure and aerobic laboratory incubation method for 28 days at 28 °C in the dark were used. In addition, acetylene and captan were used to discriminate between autotrophic and heterotrophic (bacterial and/or fungal) nitrification. Several humic epipedon properties were determined, e.g. pH, exchangeable cation concentrations, effective cation exchange capacity, total C and N, dissolved organic C and N, fungal and microbial biomass N. Potential net N mineralization and nitrification pathways changed greatly along the mixed forest chronosequence. Potential net N mineralization in the organic layers increased with stand maturation whereas potential net nitrification in the topsoil decreased significantly. Selective inhibitors revealed changes in nitrification pathways along the chronosequence, i.e. potential net nitrification was autotrophic in the topsoil while it was mainly heterotrophic within the organic layers. In the organic layer, potential net nitrification was autotrophic at the onset of the chronosequence while it appeared heterotrophic during the aggradation phase and finally fungal in mature stands. A Co-Inertia Analysis was used to reveal covariances between N mineralization pathways and humic epipedon properties. The analysis showed two functional temporal shifts within N cycling along the chronosequence, one probably controlled by organic matter quality and high competition for available N resulting in the autotrophic versus heterotrophic nitrification shift in the organic layers and one mainly controlled by (i) fine organic matter abundance, allowing high N mineralization in the organic layers and (ii) acidity inhibited autotrophic nitrification in the topsoil.