δ15N of forest soil and understorey vegetation reflect the former agricultural land use

Since the middle of the 19th century, the area covered by forests in France has doubled. These new forests grow on previous agricultural lands. We have studied the influence of this agricultural history on the ¹⁵N abundance of present-day forests planted on farmlands in the Vosges mountains (north-eastern France) between 1898 and 1930. Different types of land use were identified from old cadastres (1814–1836) of 16 farms. Ancient forests adjacent to farmlands were used as controls. Former pastures, meadows, croplands, gardens and ancient forests were compared for soil δ¹⁵N (fraction <50 μm and total soil), C/N, P and N content and fern (Dryopteris carthusiana) δ¹⁵N. The mean δ¹⁵N of soil increased in the order ancient forests (+0.0‰)<pastures (+1.4‰)<croplands (+1.6‰)<meadows (+2.5‰)<gardens (+3.8‰). This increase in soil δ¹⁵N with the intensity of former land use was related to the former input of ¹⁵N-enriched manure, and to an activation of soil nitrification leading to ¹⁵N-depleted nitrate export on previously manured parcels. Fern δ¹⁵N increased in the same order as soil δ¹⁵N in relation to past land use. The mean δ¹⁵N of fern in ancient forests (–4.4‰) and former pastures (–3.4‰) was 5‰ lower than soil δ¹⁵N and the two variables were strongly correlated. The δ¹⁵N of fern in formerly manured parcels varied little (cropland: –2.7‰, meadows: –2.6‰ and gardens: –2.2‰) and independently of soil δ¹⁵N, suggesting that the soil sources of fern N differed between unmanured and manured parcels. Understorey plant δ¹⁵N and soil δ¹⁵N appear to be excellent tracers of previous land use in forests, and could be used in historical studies. The persistence of high isotopic ratios in previously manured parcels, almost a century after afforestation, suggests a long-term influence of former land use on the N cycle in forest soils. Received: 22 January 1999 / Accepted: 22 July 1999     

Rous sarcoma virus mutant dlPA105 induces different transformed phenotypes in quail embryonic fibroblasts and neuroretina cells.

dlPA105 is a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion in the N-terminal portion of the v-src gene coding sequence. This virus was isolated on the basis of its ability to induce proliferation of quiescent quail neuroretina cells. The altered v-src gene encodes a phosphoprotein of 45,000 daltons which possesses tyrosine kinase activity. DNA sequencing of the mutant v-src gene has shown that deletion extends from amino acid 33 to 126 of wild-type p60v-src. We investigated the tumorigenic and transforming properties of this mutant virus. dlPA105 induced fibrosarcomas in quails with an incidence identical to that induced by wild-type virus. Quail neuroretina cells infected with the mutant virus were morphologically transformed and formed colonies in soft agar. In contrast, dlPA105 induced only limited morphological alterations in quail fibroblasts and was defective in promoting anchorage-independent growth of these cells. Synthesis and tyrosine kinase activity of the mutant p45v-src were similar in both cell types. These data indicate that the portion of the v-src protein deleted in p45v-src is dispensable for the mitogenic and tumorigenic properties of wild-type p60v-src, whereas it is required for in vitro transformation of fibroblasts. The ability of dlPA105 to induce different transformation phenotypes in quail fibroblasts and quail neuroretina cells is a property unique to this Rous sarcoma virus mutant and provides evidence for the existence of cell-type-specific response to v-src proteins.     

Contribution of Flooded Soils to Sediment and Nutrient Fluxes in a Hydropower Reservoir (Sarrans,Central France)

Ecosystems - When a water reservoir is created, the pre-existing soils and vegetation are flooded. Here, we took advantage of the complete emptying of the Sarrans Reservoir, which was flooded...     

Mg and Ca root uptake and vertical transfer in soils assessed by an in situ ecosystem-scale multi-isotopic (26Mg & 44Ca) tracing experiment in a beech stand (Breuil-Chenue, France)

Background and aims

The sustainability of forest ecosystems may be at stake especially in forests on base-poor soils due to reduced nutrient deposition and intensified silvicultural practices. Understanding nutrient availability and cycling is therefore essential to manage forest soil fertility. This study aims to assess in a beech plot Mg and Ca vertical transfer in soil and root uptake using an isotopic tracing experiment.

Methods

A simulated rainfall containing a small amount (960 g?Mg.ha^?1; 530 g Ca.ha^?1) of highly enriched ²⁶Mg and ⁴⁴Ca was sprayed on the forest floor of a 35-yr-old beech plot. The isotopic composition of fine roots and of the soil exchangeable Mg and Ca pool was monitored during 1 year. A pool and flux model (IsoMod) was developed to predict the labeling of the soil and vertical transfer of tracers.

Results

Tracers (⁴⁴Ca and ²⁶Mg) were immediately retained in the thin litter layer. During the following year, Mg and to a lesser extent Ca were progressively released. After 1 year, the exchangeable Mg and Ca pools of the upper mineral layer (0–5 cm) were strongly labeled (~660?‰, representing ~55 % of the tracer input and ~370?‰, ~41 % of the tracer input respectively). A significant proportion (~8 % ²⁶Mg, ~2 % ⁴⁴Ca) of tracer was leached through the soil, below 10 cm. This amount was much larger than what was predicted using a simple mixing model. The Ca and Mg isotopic composition of fine roots at all depths was close or lower than that of exchangeable Ca and Mg respectively.

Conclusions

An in situ ecosystem-scale ²⁶Mg and ⁴⁴Ca isotopic tracing experiment was successfully carried out. Tracers were at first strongly retained in the litter layer, then progressively transferred to soil horizons below. Nutrient cycling of Mg and Ca were proven to be very different. Mg had a higher mobility in the soil than Ca, and nutrient uptake sources were proven to be different.     

Variations of bioavailable Sr concentration and 87Sr/86Sr ratio in boreal forest ecosystems

The mean depth of Sr and water uptake in mixed Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) stands was investigated, using natural variations of ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O in soils in relation to depth. Three spruce-pine pairs were studied on a podzol and a peat site in Northern Sweden. Tree leaf and wood, as well as soils, soil solutions and roots below each tree were analysed for Sr and Ca concentrations and ⁸⁷Sr/⁸⁶Sr ratio. The ¹⁸O/¹⁶O ratio was also determined in xylem sap and soil solutions in relation to depth. Soil solution ¹⁸O/¹⁶O decreased in relation to depth. Comparing with xylem sap ¹⁸O/¹⁶O data indicated a deeper uptake of soil water by pine than spruce on the podzol site and a superficial uptake by both species on the peat. The ⁸⁷Sr/⁸⁶Sr ratio of bioavailable Sr generally increased in soils in relation to depth. Contrastingly, the ⁸⁷Sr/⁸⁶Sr ratio in spruce wood was generally higher than in pine wood suggesting a deeper uptake of Sr by spruce. But the ⁸⁷Sr/⁸⁶Sr ratio and concentrations of bioavailable Sr were systematically higher below spruce than below pine. In order to explain these unexpected results, we built a simple flux model to investigate the possible effects of interspecific variations in Sr cycling, soil mineral weathering and depth of Sr uptake on soil and tree ⁸⁷Sr/⁸⁶Sr ratio. At the study sites, spruce cycled in litterfall up to 12 times more strontium than pine. The use of the model showed that this difference in Sr cycling could alone explain higher isotopic signatures of trees and topsoils below spruce. Besides, high isotopic signatures of roots in the A/E horizons below spruce led us to hypothesise a species-specific weathering process. Finally, the comparison between the ⁸⁷Sr/⁸⁶Sr ratios in wood and root or soil solutions below each species suggested that the average depth of Sr and water uptake were close, but irregular variations of the Sr isotopic ratio with depth reduce the accuracy of the results. Tree species strongly influence Sr isotopic ratios in boreal forest soils through differences in Sr cycling, and possibly through specific mineral weathering.     

Induction of nephroblastoma by myeloblastosis-associated virus type 1: state of proviral DNAs in tumor cells

Myeloblastosis-associated virus type 1 (MAV1) derived from a molecular clone of infectious proviral DNA (B. Perbal, J. S. Lipsick, J. Svoboda, R. F. Silva, and M. A. Baluda, J. Virol. 56:240-244, 1985) was shown to specifically induce nephroblastoma in chickens and therefore belongs to the MAV-N class. We show that nephroblastomas are polyclonal tumors containing rearranged proviral genomes. Rearrangements occur preferentially in the gag-pol region of the MAV1 proviral genome, and similar rearrangements can be detected in well-developed independent tumors.     

Germline transmission of exogenous genes in chickens using helper-free ecotropic avian leukosis virus-based vectors

We have used vectors derived from avian leukosis viruses to transduce exogenous genes into early somatic stem cells of chicken embryos. The ecotropic helper cell line, Isolde, was used to generate stocks of NL-B vector carrying theNeo^r selectable marker and theEscherichia coli lacZ gene. Microinjection of the NL-B vector directly beneath unincubated chicken embryo blastoderms resulted in infection of germline stem cells. One of the 16 male birds hatched (6.25%) from the injected embryos contained vector DNA sequences in its semen. Vector sequences were transmitted to G₁ progeny at a frequency of 2.7%.Neo^r andlacZ genes were transcribedin vitro in chicken embryo fibroblast cultures from transgenic embryos of the G₂ progeny.     

The noncoding and surface envelope coding sequences of myeloblastosis-associated virus are respectively responsible for nephroblastoma development and renal hyperplasia.

The use of chimeric viruses allowed us to establish that myeloblastosis-associated virus long terminal repeat sequences are necessary and sufficient for induction of nephroblastoma in chickens and that the blastemal hyperplasia induced by env SU is not a prerequisite for tumor development but rather constitutes a predisposing stage.