how species diversity is maintained in tropical rainforests is a critical issue in contemporary ecology . according to the niche complementarity hypothesis ,
consequently , resource partitioning and functional complementarity could be important in maintaining species diversity in plant communities .
the differential use of soil n mineral resources ( nitrate , ammonium ) may , among other mechanisms , reflect tree functional complementarity [ 13 ] .
the spatial variability of soil nitrate - to - ammonium ratio may partly result from the spatially heterogeneous occurrence of two of the major soil microbial processes , nitrification and denitrification .
a number of studies have linked the n uptake strategies of tropical trees to their successional status ( see   for review ) .
.   showed that coexistence among functionally diverse species was not linked to the particular form of n available . in french guiana ,
roggy et al .   reported that among non - n2-fixing trees two groups of late - successional species with contrasting natural leaf n abundance coexisted .
this was interpreted as different abilities of species to utilize different soil mineral n pools . based on these results
, we carried out a preliminary study in french guiana with two abundant cooccurring late - successional species , eperua falcata and dicorynia guianensis .
these species were chosen as representative of the two groups of non - n2-fixing species with contrasting leaf n signatures .
we demonstrated that both species exhibited complementary n uptake strategies and that their leaf n were related to the n signatures of their soil n source ( no3 or nh4 ) . in this study ,
the objectives were to investigate if complementary n uptake strategies of these two tree species occurred under contrasted soil n resources in sites with distinct geological substrates representative of the coastal rainforest in french guiana .
specifically , we tested the occurrence of different leaf n signatures associated with the two species on volcanic - plutonic rocks of the paramaca series ( high total soil nitrogen concentration ) and schists of the orapu series ( low total soil nitrogen concentration ) .
we also estimated variations in nitrification and denitrification potentials in soils and litters under the two species .
the study was conducted in crique plomb , an experimental site of natural forest in the northern part of french guiana ( 50-52n , 5256-5257w ) .
this site is characterized by a clear segregation , by a small river ( crique plomb ) running along a geological boundary , between two geomorphologic domains representative of the contrast between metamorphosed sediments and a volcanic formation ( figure 1 ) .
the northern part lies in a hilly area where the geologic substrate is pelitic schist of the precambrian 
( orapu schist of armina series [ 9 , 10 ] ) . physical and chemical erosion of a preexisting deep oxisol have resulted in a deep oxisol with a deep vertical drainage .
the southern part is a single topographic unit where the geologic substrates are of volcanic - plutonic origins ( paramaca series ) .
the soil cover units consist in numerous zones of ferricrete interspersed with deep nonhydromorphic soils .
six plots of one hectare were selected in the two main geomorphologic domains , that is , three replicates per domain : plots cp3 , cp2 , and cp4 in the schist domain and plots cp1 , cp7 , and cp6 in the volcanic - plutonic domain . on each domain
, plots were separated from at least 1000  m in order to integrate potential spatial variations in nutrient uptake by trees ( figure 1 ) . although the two soil types had similar ph levels , chemical analyses showed significant differences ( table 1 ) .
the soils of the volcanic - plutonic domain had higher levels of nitrogen ( 0.83% versus 0.51% ) and carbon ( 13.58% versus 8.61% ) but identical c - to - n ratios ( 16 ) .
three angiosperm families represent around 60% of individuals in the crique plomb area : the lecythidaceae , caesalpinioideae , and euphorbiaceae .
more than 700 tree species with stems over 10  cm dbh ( diameter at breast height ~ 1.30  m ) belonging to 270 genera and 68 families were recorded for 12,000 inventoried individuals .
there are around 150 tree species per hectare in the plots and rare species ( species with few individuals ) dominate .
the climate is characterized by a clear seasonal pattern : a wet season from december to july , which is normally interrupted in february or march by a short dry period , and a long dry season from august to november with monthly precipitation of less than 100  mm .
( tribe cassieae ) and eperua falcata aublet ( tribe detarieae ) are two cooccurring , shade hemitolerant caesalpinioideae species belonging to the late - successional species group .
bothare known as non - n2-fixing species with high and low leaf n values , respectively [ 1 , 2 ] . both exhibit a paris - type mycorrhizal association [ 13 , 14 ] . in each of the six plots ,
at least five trees per species ( representing up to 25% to 100% of the individuals per species per plot ) with a dbh of 10  cm or more were randomly selected by pairs of the two species .
leaves from the upper canopy were collected using a shotgun . only young , fully expanded leaves were selected for analysis since n might be sensitive to leaf age .
three samples of litter and soil were collected underneath the canopy of each individual tree .
soil and litter samples were analyzed for their capacities ( i.e. , potential rates ) to express nitrification and denitrification ( both processes being directly involved in the balance between nh4 and no3 ) .
we will focus on potential activities rather than on actual activities because they are less fluctuating and less sensitive to short spatiotemporal variations , which allow between sites comparisons .
moreover , potential activities are not under the control of limitation or inhibition factors of the environment and represent the maximal capacity of the enzyme pool in soil to realize a biotransformation .
the leaf laminae ( i.e. , without petioles or rachis ) were oven - dried at 60c for 48  h. leaf laminae were then milled to a fine powder and packed in airtight containers .
n / n ) and total nitrogen concentrations were measured as described by casabianca , using an elemental analyzer ( sca , cnrs , solaize , france ) coupled with a mass spectrometer ( finnigan mat , ds , bremen , germany ) .
the precision of measurement was 0.2 ( standard error of the mean , n = 1000 ) .
results are reported as percent n ( leaf n ( % ) ) and n (  ) where 
 ( 1)15n()=(rsamplerstandard)rstandard1000 , 
 where the standard is atmospheric n2 ( atom%n = 0.3663 , ) .
for each sample , 10  g of sieved and air - dried soil or 2  g of air - dried litter was placed in a 150  ml plasma - flask and sealed with rubber stoppers . in each flask
c2h2 mixture ( 90/10 , vol / vol ) to inhibit the n2o - reductase .
a solution containing 100  g of n - kno3 , 1  mg of c - glucose , and 1  mg of c - glutamic acid / g of sample ( soil or litter ) was added to ensure saturation .
the flasks were incubated at 28c for 5  h and gas samples of 200  l were analyzed for n2o on a gas chromatograph with an electron capture detector ( varian 3800-cp , les ulis , france ) .
the reason for these short incubations was to avoid de novo enzymatic synthesis and cell growth .
each sample was divided into 20  g ( soil ) or 2  g ( litter ) subsamples and placed in 150  ml plasma flasks .
the first subsamples were used to estimate the initial no3 content expressed in g n - no3/g soil or litter .
they were supplied with 6  cm of a suspension of a denitrifying organism ( pseudomonas fluorescens , o.d.580 = 2 ) in a solution containing glucose and glutamic acid ( 1  mg c / g dry soil or litter of final c content for each compound ) .
soils and litter were incubated for 24  h in anaerobiosis to ensure total transformation of the nitrate into n2o .
anaerobic conditions and n2o - reductase inhibition were ensured by replacing the atmosphere of each flask with a he 
n2o accumulation was monitored until total conversion of no3 into n2o ( i.e. , a constant value ) .
the second subsamples were used to determine the kinetics of no3 accumulation in soil and litter .
after enrichment with 2  cm of a ( nh4)2so4 solution ( final n content 0.2  mg / g dry soil or litter and 80% w.h.c ) , the flasks were sealed with parafilm and incubated at 28c for 48  h in a horizontal position to ensure optimal , homogeneous aeration of the sample .
samples were then enriched with 4  cm of a p. fluorescens suspension ( o.d.580 = 2 ) in a glucose / glutamic acid solution ( with concentration values adjusted to achieve 1  mg c / g ) .
then anaerobiosis and n2o inhibition were obtained in the flasks as described above and the n2o accumulation was monitored until a constant value was reached .
the enzymatic potential of nitrification was computed by subtracting the nitrate initially present in the sample ( soil or litter ) from the nitrate accumulated after aerobic incubation and expressed in ng of n - no3/g / h . all analyses were made using the statistica 7.1 software ( statsoft inc .
data were ranked to avoid the assumptions of normality , and the differences between the means were analysed with the mann - whitney u - test ( p < 0.05 ) after a one - way anova of kruskal - wallis .
leaf samples of d. guianensis and e. falcata had consistently different n isotopic signatures ( average difference between the two species around 3.5 , figure 2 ) , with lower values on schist than on the volcanic - plutonic domain .
this variability of the n between species may be due to spatial variability of soil n , which essentially reflect the isotopic signatures of organic pools . in french guiana ,
the n of the organic matter in soils and litters vary among forests on different substrates but remain spatially uniform for a given type of soil [ 12 , 23 ] and in litters and soils collected underneath e. falcata and d. guianensis .
these studies showed that the inorganic n assimilated by these trees came from an organic n source with a homogeneous isotopic composition .
a second source of variability could come from different mycorrhizal status of trees , mycorrhized trees having access to organic soil n with a particular n abundance . in french guiana , both species exhibit a paris - type mycorrhizal association , which exclude differentiation of leaf n due to different mycorrhizal status .
a third source of variability could lie in abilities of roots to use various pools of soil inorganic n with different n abundances .
thus , the foliar n observed in the two species would be linked to the n of the inorganic n sources they used , knowing that uptake by and translocation of n within the plants are not considered to cause appreciable isotopic fractionation . in this study , the differences of n observed in leaves of e. falcata and d. guianensis showed clearly that both species used different soil inorganic n sources with different n signatures on both substrates .
differences of n between the two soil n sources are due to fractionation of n during enzyme - driven transformation process ( e.g. , nitrification ) .
soil microbial activities were spatially partitioned between the soil ( denitrification ) and litter ( nitrification , table 2 ) , confirming the results found by schimann et al .   on a different type of substrate . despite different total soil c and n concentrations in the two substrates ( table 1 ) ,
pers com ) have showed that n2 fixation by trees is twice higher on schist than on volcanic - plutonic soil ( 6.5 versus 3.5  kg  n / ha , resp . ) .
additional inputs of labile organic matter by litters of n2-fixing trees are known to enhance microbial growth and activities [ 25 , 26 ] and particularly nitrification rates [ 4 , 27 ] .
. nevertheless , these processes are not sufficient to allow accumulating organic matter in soils as shown by the lower contents of c and n in schist soil than in volcanic - plutonic soil .
as indicated , the two species displayed different isotopic signatures whatever the geological substrates but the n values were lower in the schist soils ( figure 2 ) . in these soils , there is probably a global decrease of n in litters and soils due to the higher proportion of n2-fixing species in these forests which supply litters with low n values [ 1 , 23 , 28 ] . on this type of substrate , nh4 and no3 are supposed to be produced from a more n - depleted organic matter as compared to the organic matter of the volcanic - plutonic soil .
in conclusion , our results together with those of schimann et al .   showed that the two tree species exhibit complementary n uptake strategies on all substrates representatives of coastal rainforests of french guiana with contrasted soil n resources .
consequently , their leaf n can be used to trace the presence of inorganic n - forms in soils ( nh4 and no3 ) and thus to indicate the capacity of soils to provide each of these two n sources to the plant community .
 within tree communities , the differential use of soil n mineral resources , a key factor in ecosystem functioning , may reflect functional complementarity , a major mechanism that could explain species coexistence in tropical rainforests . 
 eperua falcata and dicorynia guianensis , two abundant species cooccurring in rainforests of french guiana , were chosen as representative of two functional groups with complementary n uptake strategies ( contrasting leaf 15n signatures related to the 15n of their soil n source , no3 or nh4 + ) . 
 the objectives were to investigate if these strategies occurred under contrasted soil n resources in sites with distinct geological substrates representative of the coastal rainforests . 
 results showed that species displayed contrasting leaf 15n signatures on both substrates , confirming their complementary n uptake strategy . 
 consequently , their leaf 15n can be used to trace the presence of inorganic n - forms in soils ( nh4 + and no3 ) and thus to indicate the capacity of soils to provide each of these two n sources to the plant community . 
1. Introduction
2. Materials and Methods
3. Results and Discussion
4. Conclusions