Throughfall Solution Research Articles

Grasshopper herbivory immediately affects element cycling but not export rates in an N‐limited grassland system

AbstractAs a cause of ecosystem disturbances, phytophagous insects are known to directly influence the element and organic matter (OM) cycling in ecosystems by their defoliation and excretion activity. This study focuses on the interplay between short‐term, insect herbivory, plant responses to feeding activity, rhizosphere processes, and belowground nutrient availability under nutrient‐poor soil conditions. To test the effects of insect herbivory on OM and nutrient cycling in an N‐limited pasture system, mesocosm laboratory experiments were conducted using Dactylis glomerata as common grass species and Chorthippus dorsatus, a widespread grasshopper species, to induce strong defoliating herbivory. 13CO2 pulse labeling was used together with labeled 15N feces to trace the fate of C in soil respiration at the beginning of herbivory, and of C and N in above‐ and belowground plant biomass, grasshopper, feces, bulk soil, soil microbial biomass, throughfall solutions, and soil solutions. Within five days, herbivory caused a reduction in aboveground grass biomass by about 34%. A linear mixed‐effects model revealed that herbivory significantly increased total dissolved C and N amounts in throughfall solutions by a factor of 4–10 (P < 0.05) compared with the control. In total, 27.6% of the initially applied feces 15N were translocated from the aboveground to the belowground system. A significant enrichment of 15N in roots led to the assumption that feces‐derived 15N was rapidly taken up to compensate for the frass‐related foliar N losses in light of N shortage. Soil microorganisms incorporated newly available 13C; however, the total amount of soil microbial biomass remained unaffected, while the exploitative grass species rapidly sequestered resources to facilitate its regrowth after herbivory attack. Heavy herbivory by insects infesting D. glomerata‐dominated, N‐deficient grasslands remarkably impacted belowground nutrient cycling by an instant amplification of available nutrients, which led to an intensified nutrient competition between plants and soil microorganisms. Consequently, these competitive plant–soil microbe interactions accelerated N cycling and effectively retained herbivory‐mediated C and N surplus release resulting in diminished N losses from the system. The study highlighted the overarching role of plant adaptations to in situ soil fertility in short‐term ecosystem disturbances.

Properties of dissolved and total organic matter in throughfall, stemflow and forest floor leachate of central European forests

Abstract. We present the first investigation of the composition of dissolved organic matter (DOM) compared to total organic matter (TOM, consisting of DOM, < 0.45 μm and particulate organic matter 0.45 μm < POM < 500 μm) in throughfall, stemflow and forest floor leachate of common beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H. Karst.) forests using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. We hypothesized that the composition and properties of organic matter (OM) in forest ecosystem water samples differ between DOM and TOM and between the two tree species. The 13C NMR results, derived from 21 samples, point to pronounced differences in the composition of DOM and TOM in throughfall solution at the beech sites, with TOM exhibiting higher relative intensities for the alkyl C region, which represents aliphatic C from less decomposed organic material compared to DOM. Furthermore, TOM shows lower intensities for lignin-derived and aromatic C of the aryl C region resulting in lower aromaticity indices and a diminished degree of humification. Across the ecosystem compartments, differences in the structural composition of DOM and TOM under beech lessened in the following order: throughfall > stemflow ≈ forest floor leachate. In contrast to the broadleaved sites, differences between DOM and TOM in throughfall solution under spruce were less pronounced and spectra were, overall, dominated by the alkyl C region, representing aliphatic C. Explanations of the reported results might be substantiated in differences in tree species-specific structural effects, leaching characteristics or differences in the microbial community of the tree species' phyllosphere and cortisphere. However, the fact that throughfall DOM under beech showed the highest intensities of recalcitrant aromatic and phenolic C among all samples analysed likely points to a high allelopathic potential of beech trees negatively affecting other organisms and hence ecosystem processes and functions.

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

Apart from the forest floor, the canopy of forested ecosystems functions as the second most important source for dissolved and particulate fractions of organic and inorganic C and N compounds. However, under mass outbreak situations of insect herbivores this flux path of organic matter is considerably intensified clearly exceeding C and N fluxes from the forest floor. In this paper we report on herbivore-altered C and N fluxes from the canopy to the forest floor and effects on forest floor nutrient fluxes during severe defoliating herbivory of the winter moth (Operophtera brumata) and the mottled umber moth (Eranis defoliaria) in an oak forest in Germany. Over the course of 6.5 months we followed the C and N fluxes with bulk deposition, throughfall solution, insect frass deposits (green-fall together with insect faeces) and with forest floor solution in an 117-yr-old oak (Quercus petraea) forest. Compared to the control, herbivore defoliation significantly enhanced throughfall inputs of total and dissolved organic carbon and nitrogen by a factor of 3 and 2.5 (for TOC and DOC), and by 1.4 and 1.3 times (for TNb and DNb), respectively. Frass plus green-fall C and N fluxes peaked in May with 592 kg C ha−1 and 33.5 kg N ha−1 representing 79.6% (for C) and 78.3% (for N) of the total C and N input over 2.5 months. The quantitative and qualitative C and N input via faeces and litter deposition significantly differ between the insect affected and non-affected site. However, the C and N fluxes with throughfall did not significantly correlate with forest floor leachates. In this context, forest floor fluxes of TOC, DOC and NO3-N were significantly lower at the infested site compared to the control, whereas fluxes of NH4-N together with DON were significantly higher. The study demonstrates the importance of linking the population and associated frass dynamics of herbivorous insects with the cycling of nutrients and organic matter in forest ecosystems, highlighting the remarkable alterations in the timing, amounts and nature of organic matter dynamics on the ecosystem level. Consequently, the ecology of phytophagous insects allows partly to explain temporal-spatial alterations in nutrient cycling and thus ecosystem functioning.

Effects of Insect Mass Outbreaks on Throughfall Composition in Even Aged European Pine Stands - Implications for the C and N Cycling

In this paper we report on the herbivore-affected C and N concentrations in the throughfall and altered canopy to soil transfer during a Pine Lappet (Dendrolimus pini L) mass infestation in 60-year-old Scots Pine (Pinus sylvestris L) forests. Our investigations covered a period of 7 months and could show, that herbivore defoliation significantly altered C and N concentrations in the throughfall solution and enhanced organic matter input situation to the forest floor. Compared to the uninfested site mean concentrations of the throughfall solution at the infested site under strong frass activity contained 80% more C and 61% more N. Additionally, C and N inputs were with 131 kg C ha-1 time-1 and 9.6 kg N ha-1 time-1 higher under strong frass activity during 3 months compared to the input at the uninfested site. We assume that outbreaks of phytophagous insects play an important key role in monoculture forest by influencing the nutrient turnover.

L’importance des dérivés dissous et des particules de matière organique (DOM et POM) de la canopée — comparaison des solutions des précipitations par égouttement sous les couverts des forêts feuillues et des conifères

• Dissolved organic matter (DOM) and its main constituents carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Although many investigations have been addressing this issue, the knowledge on particulate organic matter (0.45 μm Fagus sylvatica L.) and a Norway spruce ( Picea abies L.) forest site. Bulk and throughfall precipitation were sampled in weekly (2005) and fortnightly (2006–2007) intervals and analyzed for dissolved ( 3 -N) species. Proportions of particulate organic C and N were determined by difference between total and dissolved fractions. • Under spruce, throughfall concentrations of most C and N fractions were twice as high as under beech. At both sites, concentrations and fluxes were significantly higher during the growing than the dormant season. At the broadleaved site, 80% of the annual fluxes of the DOC and TOC and 70% of the DN and TN were released during the growing season, compared to 60% for C and N at the coniferous site. POC under beech contributes with up to 30% to TOC compared to less than 20% at the spruce site.• We suggest that pollen deposition, insect excretions and accumulated organic matter mobilised by dry/wet precipitation patterns play a supreme role for the formation of DOM and POM in forest canopies. The study demonstrates that the canopy is an important source for POM. Dynamics of DOM and POM are mainly driven by tree species effects and seasonality as well as by biotic agents.

Canopy herbivory altering C to N ratios and soil input patterns of different organic matter fractions in a Scots pine forest

Herbivorous insects can affect ecological processes in forested ecosystems such as nutrient and matter cycling especially during outbreak situations. However, the knowledge about their contribution to the quality and flows of energy and matter in forests is still imperfect. In this paper we report on the herbivore-affected C to N ratios in different fractions of organic matter cascading from the canopy to the forest floor during a pine lappet (Dendrolimus pini L.) mass infestation. Throughout a four months period we monitored the C and N fluxes with throughfall, and the C/N ratios of insect excrements (faeces) and pine needles in an 80-year-old Scots pine forest. Compared to the control, herbivore defoliation significantly magnifies C and N input fluxes by two to three times amounting to 95 kg TOC and 5.9 kg TN ha−1 in addition. Concurrently NO3-N fluxes diminished and the C/N ratios in throughfall solutions increased during peaking frass activity. Compared to fresh needle biomass, the C/N ratios in insect faeces triple during peaking frass activity resulting in values between 70 and 100. This study demonstrates the importance of herbivorous insect’s pests on element cycling as they act as a short-time phenomenon altering the nutrient quality and quantity reaching the forest floor and potentially affecting below-ground processes.

The source of calcium in wet atmospheric deposits: Ca-Sr isotope evidence

Tracing the origin of calcium in wet atmospheric deposits is important for environmental studies since it is a dominant acid neutralising cation and an essential nutrient. Here we present the first calcium isotope ratio measurements for rain and snow. Samples from northeastern and southern locations (France, Switzerland, Luxembourg, California and Japan) suggest a carbonate origin. Only at a local scale in a forested watershed (Vosges mountains, France) the Ca isotopic compositions of rainwater indicate that the rainwater becomes admixed with throughfall solution, which is a mixing product of Ca from dissolved atmospheric dust particles and leave excretions. If this proves to be widely true, the contribution of Ca from the canopy could be very important for the neutralisation of acid rain and of the corresponding soil system in forested watersheds. In such cases Ca and Sr cycles are decoupled in the lower atmosphere. In contrast to Sr isotopes, Ca isotopes can be applied to tracing processes occurring in the biosphere.

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce ( Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH 4NO 3-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day −1 g C −1 and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.

Importance of canopy herbivores to dissolved and particulate organic matter fluxes to the forest floor

Few studies deal with the origin and contribution of organic matter (OM) fluxes from forest canopies to the variability of DOM and nutrient dynamics in the forest floor. Recent studies indicate a microbial and phytophages origin of OM in the throughfall of temperate forest ecosystems and reported inputs of dissolved (<0.45 μm) organic carbon (DOC) with throughfall range between 40 and 160 kg C ha −1 year −1. Data on particulate organic matter (0.45 μm<POM<2 mm) associated with throughfall fluxes are rare and not often considered in terrestrial ecosystem element budgets. Using field and laboratory experiments, we investigated the direct effects of canopy infestation by two functional groups of herbivores (sap and leaf feeders) on dissolved and particulate organic matter fluxes in throughfall and their impact on forest floor processes. Throughfall fluxes of particulate amino-nitrogen beneath infested spruce were about 60% and those of hexose-carbon beneath infested hardwood trees up to 70% higher compared to uninfested control trees. Corresponding fluxes to filtered samples showed no statistically significant differences. In column irrigation experiments, different levels of aphid infestation (uninfested, moderately, heavy infested) were simulated by adding honeydew to the irrigation solution. Forest floor solution chemistry from forest floor showed a significant reduction in NH 4–N and NO 3–N fluxes, slightly reduced DON fluxes and significantly increased CO 2 effluxes following honeydew application. We suggest that the amount of POM compounds transported with throughfall solution and its subsequent effects on forest floor processes depend on the herbivore functional group and on the level of infestation. From both a quantitative and qualitative point of view, our data indicate that herbivory-mediated organic matter in particulate and dissolved forms contribute considerably to the overall throughfall input of organic substances into the forest floor.

Monitoring Nitrogen Deposition in Throughfall Using Ion Exchange Resin Columns: A Field Test in the San Bernardino Mountains

Conventional throughfall collection methods are labor intensive and analytically expensive to implement at broad scales. This study was conducted to test an alternative approach requiring infrequent sample collection and a greatly reduced number of chemical analyses. The major objective of the study was to determine the feasibility of using ion exchange resin (IER) to measure N deposition in throughfall with field deployment periods of 3 to 12 mo. Nitrogen deposition measurements in bulk throughfall collected under pine (Pinus sp.) canopies and in forest clearings were compared between co-located conventional throughfall solution collectors and IER throughfall collectors using mixed bed IER columns. Deposition data were collected for 1 yr at a high deposition site (Camp Paivika, CP) and a relatively low one (Barton Flats, BF) in the San Bernardino Mountains in southern California: Annual throughfall deposition values (kg ha(-1) of NH(4)-N + NO(3)-N) under large ponderosa pine trees (Pinus ponderosa Laws.) were 145.8 and 143.9 at CP and 17.0 and 15.0 at BF according to the IER and conventional methods, respectively. Analogous values for bulk deposition in forest clearings were 15.6 and 12.3 at CP and 4.0 and 3.3 at BF. It was concluded that the IER collectors can be used for routine monitoring of deposition in throughfall and bulk deposition, provided that field blanks are used to account for background levels of N in the IER columns, which at times are slightly elevated, possibly from slow release of amine groups from the anion exchange resin during field exposures.

Assessment of the Water Balance of European Forests: A Model Study

As part of the UN-ECE Intensive Monitoring Program, data on precipitation, throughfall and soil solution concentrations are measured on a regular basis in approximately 300 forest stands. These data were used to construct element budgets for European forests. To construct such budgets drainage fluxes have to be modeled. In this paper, the research chain from model selection to data derivation and application of the selected model to 245 of the 300 sites is described. To select a suitable hydrological model the Cl- balance method, two capacity models (a multi and a single layer version) and a Darcy model have been applied to two forest sites. The results indicate that drainage fluxes calculated with the Darcy model are more accurate than fluxes derived with the capacity model, in particular in situations where water availability is limited. The Darcy model was applied to the sites using a mixture of generic data and site data. Despite the use of generic data, the calculated drainage fluxes appear feasible. Median transpiration fluxes were 350 mm and the lowest values are found in northern Europe and highest values are found in central Europe. Median drainage fluxes were 150 mm yr-1 with the highest values in areas with high rainfall. Uncertainty analyses indicate that the use of local instead of interpolated meteorological data leads to lower drainage fluxes at 70% of the sites. The median deviation in calculated drainage fluxes is 20 mm yr-1. The use of local soil data had little impact on the calculated fluxes.

The function of the superficial root mat in the biogeochemical cycles of nutrients in congolese eucalyptus plantations.

The importance of superficial root mats inside the forest floor for the nutrition of Amazonian rain forests has been extensively investigated. The present study was aimed at assessing the function of a root mat adherent to decomposing organic material observed in Eucalyptus plantations. The development of the root mat was studied through micromorphological observations of thin litter sections, and the influence of soil microtopography and soil water repellency on root mat biomass was assessed in situ on an area of 5 m2. In addition, input-output budgets of nutrients within the forest floor were established from measurements of litterfall, dissolved nutrients in gravitational solutions, and forest floor nutrient contents. The amounts of nutrients released during litter decay in this ecosystem during the period of study were, on average, 46, 3, 4, 19 and 17 kg ha-1 year-1 for N, P, K, Ca and Mg, respectively. The simultaneous measurements of the chemical composition of throughfall solutions and leachates beneath the forest floor showed a very quick uptake of nutrients by the root mat during the decomposition processes. Indeed, the solutions did not become noticeably enriched in nutrients during their passage through the holorganic layer, despite large amounts of elements being released during litter decay. The root mat biomass decreased significantly during the dry season, and a preferential development in microdepressions at the soil surface was observed. A strong water repellency observed in these depressions might enhance the ability of the roots to take up water and nutrients during the dry periods. The root mat was active throughout the year to catch the flux of nutrients from the biodegradation of the forest floor, preventing the transfer of dissolved nutrients toward deeper soil horizons. This mechanism is involved in the successful adaptation of this Eucalyptus hybrid in areas covered by 'climacic' savannas in Congo.

Long-term N addition effects on the C mineralization and DOC production in mor humus under spruce

This study was based on laboratory incubations of mor humus from two N fertilized stands of Norway spruce in Sweden (Skogaby and Stråsan), which had received repeated N additions (100 kg N ha −1 yr −1 as (NH 4) 2SO 4 at Skogaby and 35, 73 and 108 kg ha −1 yr −1 as NH 4NO 3 at Stråsan) during 8 and 24–29 years, respectively. The aim was to investigate long-term N effects on the mineralization of C and production of DOC. Mor humus (Oe and Oa) was incubated in columns at 20 °C for 49 days. Columns were leached once a week with artificial throughfall solution, which was analyzed for DOC, total N, NH 4 +-N and NO 3 −-N. Prior to each leaching event, CO 2 evolution from the columns was determined. C-to-N ratios in the N-treated Oe layers at Stråsan (21–24) and Skogaby (24) were significantly lower than those of the controls (Stråsan, 32; Skogaby, 28). The cumulative amount of CO 2-C showed a significant treatment effect in the Oe layer at Skogaby, i.e. 18 and 29 mg C g −1 C in the N treatment and control, respectively. At Stråsan, the cumulative CO 2-C was significantly lower in the N3 treatment compared to the control in both layers (33 compared to 74 mg C g −1 C in the Oe layer and 16 compared to 35 mg C g −1 C in the Oa layer). Neither the DOC nor the DON production showed any significant treatment effects at the two sites. However, DOC was lower in the fertilized Oe layer at Skogaby throughout the incubation. The leaching of DON was highest in the Oe layers at both sites, and DON increased with time at Skogaby while there was a decreasing trend at Stråsan. The DOC-to-DON ratio tended to be lower in the fertilized Oe layers at both sites. The NH 4 + leaching at Skogaby decreased in the N-treated Oe and Oa layers. At Stråsan, NH 4 + from the Oe layer increased in N2 and control. The NO 3 − leaching was low and constant in both Skogaby layers. At Stråsan, NO 3 − increased in the Oe layer of N1. Cumulative CO 2 was positively correlated to C-to-N ratio (r 2=0.71, p<0.01) and to cumulative DOC (r 2=0.63, p<0.05) in the Oe layer at Stråsan. Our conclusion was that long-term N additions caused decreased C-to-N ratios and decreased CO 2 evolution rates. The correlation between CO 2 and C-to-N ratio in the Oe layers at Stråsan may be due to a changed quality of the fertilized forest floor material and presence of more N efficient microorganisms.

Characterization and migration of atmospheric REE in soils and surface waters

Rainwater and snow collected from three different sites in France (Vosges Mountains, French Alps and Strasbourg) show more or less similar shapes of their REE distribution patterns. Rainwater from Strasbourg is the most REE enriched sample, whereas precipitations from the two mountainous, less polluted catchments are less REE enriched and have concentrations close to seawater. They are all strongly LREE depleted. Different water samples from an Alpine watershed comprising snow, interstitial, puddle and streamwater show similar REE distributions with LREE enrichment (rainwater normalized) but MREE and HREE depletion. In this environment, where water transfer from the soil to the river is very quick due to the low thickness of the soils, it appears that REE in streamwater mainly originate from atmospheric inputs. Different is the behaviour of the REE in the spring- and streamwaters from the Vosges Mountains. These waters of long residence time in the deep soil horizons react with soil and bedrock REE carrying minerals and show especially significant negative Eu anomalies compared to atmospheric inputs. Their Sr and Nd isotopic data suggest that most of the Sr and Nd originate from apatite leaching or dissolution. Soil solutions and soil leachates from the upper soil horizons due to alteration processes strongly depleted in REE carrying minerals, have REE distribution patterns close to those of lichens and throughfall. Throughfall is slightly more enriched especially in light REE than filtered rainwater probably due to leaching of atmospheric particles deposited on the foliage and also to leaf excretion. Data suggest that Sr and Nd isotopes of the soil solutions in the upper soil horizons originate from two different sources: 1) An atmospheric source with fertilizer, dust and seawater components and 2) A source mainly determined by mineral dissolution in the soil. These two different sources are also recognizable in the Sr and Nd isotopic composition of the tree’s throughfall solution. The atmospheric contributions of Sr and Nd to throughfall and soil solution are of 20 to 70 and 20%, respectively. In springwater, however, the atmospheric Sr and REE contribution is not detectable.

Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo : 1. Chemical composition of rainfall, throughfall and stemflow solutions

The dynamics of nutrients was compared in a clonal Eucalyptus plantation and in a native savanna in Congo. This paper focuses on changes in the precipitation chemistry during the transfer of solutions aboveground in both ecosystems. Average rainfall during the rainy and the dry seasons was 151 and 7 mm per month, respectively. Chemical analyses performed monthly over a 3-year period showed that the concentration of all the elements in rainfall increased sharply during the dry season. Wet depositions were dominated by the cations Na +, N-NH 4 +, H + and the anions Cl −, S-SO 4 2−, N-NO 3 − and amounts of dry depositions of Ca 2+, Cl − and H + were highly variable depending on the characteristics of the collector. Throughfall and stemflow were enriched in most of the elements, but N foliar uptake was observed in both stands. Clear differences in behaviour between the savanna and the Eucalyptus stand were evidenced. Whereas the enrichment of solutions through the foliage mainly made up of dry depositions in the plantation, foliar leaching processes were much greater in the savanna, particularly for K +, Mg 2+ and P-H 2PO 4 −. Stemflow solutions followed the same pattern as throughfall solutions. They were greatly enriched for most elements but N uptake also occurred. The low availability of K + in the soil might account for the particularly low levels of foliar leaching of this element in the Eucalyptus stand compared to values reported for most forest ecosystems. The time course of concentrations of elements in rainfall, throughfall and stemflow provided relevant data to distinguish external inputs from foliar leaching in the enrichment of solutions through both stands. This qualitative information will be helpful to establish input–output budgets of nutrients in these ecosystems.

Annual Element Budget of Soil in Snow-Dominated Forested Ecosystem

Seasonal fluctuation of concentration and flux of major inorganic ions in throughfall, stem flow, snowpack and soil solution was investigated at a natural cool temperature mixed forest in Hokkaido, northern Japan, in order to clarify the effect of snowmelt on the solute dynamics in the forest soil in snow-dominated region. Na+, Ca2+, Mg2+, Cl− and SO42− concentrations in soil solution showed a large fluctuation in the snowmelt period. The percentage of output of these elements from soil during the snowmelt period in the annual output was as follows. Mg2+: 51%, Na+ and Cl−: 59 and 60%, SO42−: 65%, Ca2+: 77%. Our results indicated that the snowmelt event was very important to quantify the annual elemental budgets in this region. Although the leaching of base cation from the soil was larger than that of inputs and accumulation into the vegetation, annual decreasing rate of acid neutralization capacity (ANC(s)) from the soil was mostly affected by the base cation accumulation into the vegetation, related that the base cations weathering accompanied with bicarbonate was slow due to the acidic and weathered soil in the studied site. It is suggested that the weakly acidic soil which has low ANC(s) in snow-dominated region will be relatively sensitive to the future increase of acidic deposition.

Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest

Despite growing attention concerning therole of dissolved organic matter (DOM) inelement cycling of forest ecosystems, thecontrols of concentrations and fluxes of bothdissolved organic carbon (DOC) and nitrogen(DON) under field conditions in forest soilsremain only poorly understood. The goal ofthis project is to measure the concentrations and fluxes of DON, NH4 +, NO3 −and DOC in bulkprecipitation, throughfall, forest floorleachates and soil solutions of a deciduousstand in the Steigerwald region (northernBavaria, Germany). The DOC and DONconcentrations and fluxes were highest inleachates originating from the Oa layer of theforest floor (73 mg C L−1, 2.3 mg NL−1 and about 200–350 kg C, 8–10 kg Nha−1 yr−1). They were observed to behighly variable over time and decreased in themineral topsoil (17 mg C L−1, 0.6 mg NL−1 and about 50–90 kg C, 2.0 to 2.4 kg Nha−1 yr−1). The annual variability ofDOC and DON concentrations and subsequentialDOC/DON ratios was substantial in allsolutions. The DOC and DON concentrations inthroughfall were positively correlated withtemperature. The DOC and DON concentrationsdid not show seasonality in the forest floorand mineral soil. Concentrations were notrelated to litterfall dynamics but didcorrespond in part to the input of DOC and DONfrom throughfall. The throughfall contributionto the overall element fluxes was higher forDON than for DOC. Concentrations and fluxes ofDON were significantly correlated to DOC inthroughfall and the Oi layer. However, thecorrelation was weak in Oa leachates. Inaddition, seasonal and annual variation ofDOC/DON ratios indicated different mechanismsand release rates from the forest floor forboth components. The concentrations of DOC andDON in forest floor leachates were in mostcases dependent neither on the pH value orionic strength of the solution, nor on thewater flux or temperature changes. As aconsequence, the DOC and DON fluxes from theforest floor into the mineral soil werelargely dependent on the water flux if annualand biweekly time scales are considered.

Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15-20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10 degreesC in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d(-1) with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH4, NO3, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.

Environmental factors controlling NO 3− leaching, N 2O emissions and numbers of NH 4+ oxidisers in a coniferous forest soil

Main and interactive effects of temperature, throughfall volume and NH 4 + deposition on soil solution NO 3 − concentrations, N 2O emissions and numbers of NH 4 + oxidisers were investigated in a controlled laboratory experiment. Large intact soil cores from a Picea abies (L.) Karst. stand were incubated according to an `incomplete factorial design' at 4, 12 or 20°C and watered every 2 weeks with 300, 500 or 700 ml (442, 737 and 1032 mm yr −1) of a natural throughfall solution enriched with 0, 37.5 or 75 kg NH 4 +–N ha −1 yr −1. Watering and sampling were performed every 2 weeks, during a 112 d period. At d 112, a temperature optimum for NO 3 −–N concentrations in the leachate, NO 3 −–N fluxes and numbers of NH 4 + oxidisers in the mineral soil layer was determined at ca. 11°C. NO 3 −–N concentrations also decreased with throughfall volume, towards a minimum at 590 ml, with temperature however contributing most to modelling NO 3 −–N concentrations and the two factors acting independently. The model explained 59% of the variability in the data, and the regression between observed and predicted concentrations was highly significant ( P<0.0001, r 2=0.93). NO 3 −–N fluxes increased quadratically with throughfall volume, and throughfall volume and NH 4 + deposition interacted significantly in determining the numbers of NH 4 + oxidisers in the mineral soil layer. Numbers of NH 4 + oxidisers were higher in the humus layer and decreased with increasing temperatures. N 2O fluxes increased quadratically with temperature, and the linear and quadratic effects of throughfall volume (maximum at 500 ml). Results suggest that optimum temperatures for net nitrification may have been overestimated in previous studies by the use of disturbed soils.

Leaching of nutrient cations from the forest floor: effects of nitrogen saturation in two long-term manipulations

Nitrogen saturation results in greater mobility of nitrate, which in turn is often correlated with concentrations of nutrient cations in soil solution and streamwater. At the Harvard Forest, U.S.A., under long-term NH4NO3 inputs, a Pinus resinosa Ait. forest has exhibited signs of N saturation more rapidly than a mixed-Quercus forest. We test the hypothesis that increased nitrate leaching causes increased concentrations of nutrient cations in soil solution. Over 2 years (years 6 and 7 of treatment) we measured SO42-, NO3-, Cl-, Ca2+, K+, Mg2+, Na+, H+, and NH4+ in throughfall solution and in forest-floor (Oa) leachate. Concentrations of NO3- in forest-floor leachate increased with rates of N amendment and correlated positively with cation concentrations, with stronger overall correlations in the pine forest: r2 values were 0.51 (pine forest) and 0.39 (oak forest) for Ca2+, 0.45 (pine) and 0.16 (oak) for K+, and 0.62 (pine) and 0.50 (oak) for Mg2+. In summer and fall, the oak forest showed some negative relationships between nutrient cation leaching and rate of N amendment. These contrasts showed retention of cations and N to occur together in an N-limited system, whereas increased nitrate mobility occurred with increased cation losses in an N-saturated system.