Micheline Colin-Belgrand

Decomposition of 15N-labelled beech litter and fate of nitrogen derived from litter in a beech forest.

Abstract The decomposition and the fate of 15N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, 15N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and 15N concentration. Mass loss of the 15N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released 15N accumulated essentially at the soil surface. 15N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. 15N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of 15N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales.

The dynamics of biomass and nutrient accumulation in a Douglas-fir (Pseudotsuga menziesii Franco) stand studied using a chronosequence approach

Abstract Douglas-fir is presently one of the dominant species used in France for afforestation. It was widely introduced without knowing exactly its effects on soils and superficial waters. The aim of this research was to obtain quantitative data on the biogeochemical functioning of a Douglas-fir ecosystem, and to provide relevant information to forest managers. The results presented here concern nutrient accumulation on a stand according to its stage of development. A chronosequence of stands was used to rapidly obtain information on ecosystem dynamics in particular because it is possible to assimilate stands of different ages to the different stage of development of a single stand. The present results make it possible to evaluate with sufficient accuracy nutrient losses at crop harvest. Several scenarios could be drawn using various rotation lengths and harvesting intensities. These results could be introduced in management models. Nevertheless, a direct relationship between nutrient losses and the immediate or delayed decrease of forest productivity is not easy to predict. It depends more on soil nutrient dynamics than on the present available nutrient pool. Comparison between the two main species used in forestry showed that the nutrient efficiency of Douglas-fir for biomass production was at least the same as for Norway spruce. This important conclusion needs to be confirmed by other measurements.

N deposition, N transformation and N leaching in acid forest soils

Nitrogen deposition, mineralisation, uptake and leaching were measured on a monthly basis in the field during 2 years in six forested stands on acidic soils under mountainous climate. Studies were conducted in three Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantations (D20: 20 year; D40: 40 yr; D60: 60 yr) on abandoned croplands in the Beaujolais Mounts; and two spruce (Picea abies Karst.) plantations (S45: 45 yr; S90: 90 yr) and an old beech (Fagus sylvatica L.) stand (B150: 150 yr) on ancient forest soils in a small catchment in the Vosges Mountains. N deposition in throughfall varied between 7–8 kg ha−1 year−1 (D20, B150, S45) and 15–21 kg ha−1 yr−1 (S90, D40, D60). N in annual litterfall varied between 20–29 kg ha−1 (D40, D60, S90), and 36–43 kg ha−1 (D20, S45, B150). N leaching below root depth varied among stands within a much larger range, between 1–9 kg ha−1 yr−1 (B150, S45, D60) and 28–66 kg ha−1 yr−1 (D40, S90, D20), with no simple relationship with N deposition, or N deposition minus N storage in stand biomass. N mineralisation was between 57–121 kg ha−1 yr−1 (S45, D40, S90) and between 176–209 kg ha−1 yr−1 in (B150, D60 and D20). The amounts of nitrogen annually mineralised and nitrified were positively related. Neither general soil parameters, such as pH, soil type, base saturation and C:N ratio, nor deposition in throughfall or litterfall were simply related to the intensity of mineralisation and/or nitrification. When root uptake was not allowed, nitrate leaching increased by 11 kg ha−1 yr−1 at S45, 36 kg ha−1 yr−1 at S90 and between 69 and 91 kg ha−1 yr−1 at D20, D40, B150 and D60, in relation to the nitrification rates of each plot. From this data set and recent data from the literature, we suggest that: high nitrification and nitrate leaching in Douglas-fir soils was likely related to the former agricultural land use. High nitrification rate but very low nitrate leaching in the old beech soil was related to intense recycling of mineralised N by beech roots. Medium nitrification and nitrate leaching in the old spruce stand was related to the average level of N deposition and to the deposition and declining health of the stand. Very low nitrification and N leaching in the young spruce stand were considered representative of fast growing spruce plantations receiving low N deposition on acidic soils of ancient coniferous forests. Consequently, we suggest that past land use and fine root cycling (which is dependent on to tree species and health) should be taken into account to explain the variability in the relation between N deposition and leaching in forests.

Influence of tree species on gross and net N transformations in forest soils

We compared N fluxes in a 150-year-old Fagus sylvatica coppice and five adjacent 25-year-old plantations of Fagus sylvatica, Picea abies, Quercus petraea, Pinus laricio and Pseudotsuga menziesii. We measured net N mineralization fluxes in the upper mineral horizon (A1, 0–5 cm) for 4 weeks and gross N mineralization fluxes for two days. Gross rates were measured during the 48-h period after addition of 15NH4 and 15NO3. Mineralization was measured by the 15NH4 dilution technique and gross nitrification by 15NO3 production from the addition of 15NH4, and by 15NO3 dilution. Net and gross N mineralization was lower in the soil of the old coppice, than in the plantations, both on a soil weight and organic nitrogen basis. Gross nitrification was also very low. Gross nitrification measured by NO3 dilution was slightly higher than measured by 15NO3 production from the addition of 15NH4. In the plantations, gross and net mineralization and nitrification from pool dilution were lowest in the spruce stand and highest in the beech and Corsican pine stands. We concluded that: (1) the low net mineralization in the soil of the old coppice was related to low gross rate of mineralization rather than to the concurrent effect of microbial immobilisation of mineral N; (2) the absence of nitrate in the old coppice was not related to the low rate of mineralization nor to the absence of nitrifyers, but most probably to the inhibition of nitrifyers in the moder humus; (3) substituting the old coppice by young stands favours nitrifyer communities; and (4) heterotrophic nitrifyers may bypass the ammonification step in these acid soils, but further research is needed to check this process and to characterize the microbial communities.RésuméNous avons mesuré les flux de minéralisation nette d’azote au cours d’une incubation de quatre semaines et les flux bruts d’azote au cours d’une incubation de deux jours dans 6 sols prélevés dans une comparaison d’espèces forestières. Nous avons comparé les horizons A1 d’un taillis sous futaie (TSF) de Fagus sylvatica et de cinq plantations adjacentes de 25 ans de Fagus sylvatica, Picea abies, Quercus petraea, Pinus laricio et Pseudotsuga menziesii. Les taux bruts ont été mesurés 48 h après l’addition de 15NH4, et 15NO3. La minéralisation brute a été calculée à partir de la dilution de 15NH4 et la nitrification brute à partir de la dilution de 15NO3 mais aussi de la production de 15NO3 à partir de l’apport de 15NH4. La minéralisation brute et nette est la plus basse dans le TSF, exprimée par gramme de sol ou d’azote organique. La nitrification nette et brute mesurée par enrichissement en 15NO3 est très faible, mais la nitrification brute est sensiblement plus élevée lorsqu’on l’évalue par dilution isotopique du 15NO3. Dans les plantations, la minéralisation et la nitrification brute et nette sont plus faibles sous épicéa et plus élevées sous hêtre et pin Laricio. Nous en concluons que (1) la faible minéralisation d’azote dans le TSF est directement liée à une faible minéralisation brute et non à l’expression d’une immobilisation microbienne de l’azote minéral formé; (2) l’absence de nitrate dans le TSF n’est pas liée à l’absence de nitrifiants mais plutôt à l’inhibition de leur activité sous le moder; (3) la coupe rase du TSF et sa plantation entraîne une levée partielle ou totale de cette inhibition; et (4) l’activité de nitrifiants hétérotrophes sans libération intermédiaire de NH4 est possible dans ces sols acides. Des études plus approfondies devraient permettre de vérifier ce point et d’identifier ces populations.

The Fate of 15N-Labelled Nitrogen Inputs to Coniferous and Broadleaf Forests

Nitrogen in forest soils is mainly composed of organic N compounds originating from litterfall. During leaf senescence of the forest vegetation, N compounds are either allocated to perennial tissues or remain in the leaf litter, mainly as polyphenol-protein condensates. For example, senescent beech leaves are composed of 45% cellulose and hemicellulose, 5 to 10% lignin and 25 to 35% brown polyphenol condensates which contain about 70% of the litter N (Berthelin et al. 1994). Beech litter has a C/N mass ratio of 50–70 and evolves into soil organic matter with a C/N ratio ranging from 10 to 30 depending on the humus type. These organic N compounds in forest soils are highly protected from major N losses due to their high chemical stability and low mobility.

Production and root uptake of mineral nitrogen in a chronosequence of Douglas-fir (Pseudotsuga menziesii) in the Beaujolais Mounts

Nitrogen cycle dynamics were studied in a chronosequence of Douglas-fir (Pseudotsuga menziesii) plantations, aged 20, 40 and 60 years, respectively. Mineralizing capacity was high in the three stands. After mineralization, nitrogen was mainly nitrified. Mineralization and nitrification were independent of stand age. Root uptake of nitrogen was high but lower than mineralization. Consequently, losses by leaching were high, especially in the younger stand. Nitrate leaching was accompanied by cation leaching, resulting from soil acidification. The destabilization of organic matter following the introduction of a non-native species may have been reinforced by former land use. Soils were in the process of losing nutrients, especially in the young stand. As nutrient losses decrease with increasing stand age, forest rotations of over 60 years are recommended, as fertility budgets are closer to equilibrium.

Influence of Tree Roots on Nitrogen Mineralization

This study compared the total carbon (C), mineral nitrogen (N) contents and N mineralization potentials of the rhizospheric and bulk soils, collected at two depths in three forest sites in France. The site at Breuil is a comparative plantation of different species with or without fertilization, the Fougeres site is a time sequence of four Fagus sylvatica L . stands including a limed plot, and the Aubure site is a comparison between adjacent young and old Picea abies. (L.) Karst stands with different nitrifying activity. Mineral N was extracted from fresh soil with K 2 SO 4 and after laboratory incubation at 15°;C for 2 days or 1 week. The moisture, C and N contents of the rhizospheric soil were higher than in the bulk soil in the A 1 horizon, but only slightly higher or similar in A 1 B horizons. Soil-extractable NH 4 and net mineralization were much larger in the rhizospheric soil than in the bulk soil. Soil-extractable NO 3 and net nitrification were not significantly different. Soil-extractable NH 4 an...

Nutrient dynamics during the development of a Douglas-fir (Pseudotsuga menziesii Mirb.) stand

Abstract The general process of nutrient cycling in forest stands is relatively well-known. Nevertheless, fluxes in representative ecosystems have not yet been quantified, certainly due to the amount of measurements required. The unavailability of such information is delaying the validation of models describing the ecosystem function. Numerous observations of nutrient stocks and fluxes carried out on a chronosequence of Douglas-fir stands in the Beaujolais mountains (France) were used: i) to quantify the main fluxes of the biological cycle of nutrients for the main stages of development of a single stand, ii) to characterize their dynamics during forest development and iii) to quantify the real amount of nutrients involved in forest growth and to identify their origins. The present soil nutrient reserves of the ecosystem studied were relatively low if compared to stand nutrient uptake. The biological cycle represents a key process in the maintenance of forest productivity under such conditions. Cumulation of fluxes during rotation was very useful in quantifying the real amounts of nutrients involved in tree nutrition, in demonstrating the efficiency of the systems use of the limited pool of nutrients needed to produce biomass and to show the relevance of the biological cycle in the soil function. These results will be helpful in understanding the nutrient behaviour in a highly productive forest plantation and thereby providing decisive information for their sustainable management.

Nutrient dynamics of chestnut tree (Castanea sativa Mill.) coppice stands.

Abstract A chronosequence of chestnut coppice stands was used to study nutrient dynamics in an ecosystem during stand development. The strategy of the ecosystem was demonstrated in this study: to achieve a large biomass production under conditions of limited nutrient availability. Contrary to what is generally expected, forest production has high nutrient requirements. Perennial vegetation fulfilled half its requirements of N, P and K from internal translocation of nutrients that were reused when physiological activity ceased; the remaining part was taken up from available soil reserves. Half of the uptake returned to the soil, mainly as litterfall. As a result, overal immobilization was low. As a consequence, decreasing the rotation length will enhance the depletion of forest soil nutrients, as will harvesting a larger part of the forest biomass and hence the smaller biomass compartments, which contain most of the nutrients in the vegetation. Dynamics of biological fluxes clearly show the importance of tree vegetation on soil mineral functions, especially by continuously returning organic matter and nutrients back to the soil surface.