Fluxes and concentrations of dissolved organic carbon and nitrogen – a synthesis for temperate forests

Dissolved organic carbon (DOC) and nitrogen (DON)represent an important part of the C and N cycles inforest ecosystems. Little is known about the controlson fluxes and concentrations of these compounds insoils under field conditions. Here we compiledpublished data on concentrations and fluxes of DOC andDON from 42 case studies in forest ecosystems of thetemperate zone in order to evaluate controls on alarger temporal and spatial scale. The focus was onannual fluxes and concentrations in throughfall,forest floor leachates and soil solutions. In allcompartments considered, concentrations and fluxesdiffered widely between the sites. Highestconcentrations of DOC and DON were generally observedin forest floor leachates and in A horizons. Highestfluxes occurred in forest floor leachates. The fluxesof DOC and DON in forest floor leachates increasedwith increasing annual precipitation and were alsopositively related to DOC and DON fluxes withthroughfall. Variation in throughfall fluxes couldexplain 46% and 65% of the variation in DOC and DONfluxes from the forest floor, respectively. No generaldifference in DOC and DON concentrations and fluxes inforest floor leachates was found when comparingconiferous and hardwood sites. Concentrations of DOCin forest floor leachates were positively correlatedto the pH of the forest floor. Furthermore, there wasno relationship between organic C and N stocks, soilC/N, litterfall or mineral N inputs and concentrationsand fluxes of DOC and DON in forest floor leachates.Including all compartments, fluxes of DOC and DON werehighly correlated. Ratios of DOC to DON calculatedfrom fluxes from the forest floor were independent ofthe amount of annual precipitation, pointing to asimilar response of DOC and DON to precipitationconditions. A decrease in the ratio of DOC to DON withsoil depth as observed on a plot-scale, was notconfirmed by data analysis on a large scale. Thecontrols observed on annual fluxes and concentrationsof DON and DOC at regional scale differed from thosereported for smaller time and space scales.     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.     

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.     

Controls on the release of dissolved organic carbon and nitrogen from a deciduous forest floor investigated by manipulations of aboveground litter inputs and water flux

Although dissolved organic matter (DOM) released from the forest floor plays a crucial role in transporting carbon and major nutrients through the soil profile, its formation and responses to changing litter inputs are only partially understood. To gain insights into the controlling mechanisms of DOM release from the forest floor, we investigated responses of the concentrations and fluxes of dissolved organic carbon (DOC) and nitrogen (DON) in forest floor leachates to manipulations of throughfall (TF) flow and aboveground litter inputs (litter removal, litter addition, and glucose addition) at a hardwood stand in Bavaria, Germany. Over the two-year study period, litter manipulations resulted in significant changes in C and N stocks of the uppermost organic horizon (Oi). DOC and DON losses via forest floor leaching represented 8 and 11% of annual litterfall C and N inputs at the control, respectively. The exclusion of aboveground litter inputs caused a slight decrease in DOC release from the Oi horizon but no change in the overall leaching losses of DOC and DON in forest floor leachates. In contrast, the addition of litter or glucose increased the release of DOC and DON either from the Oi or from the lower horizons (Oe + Oa). Net releases of DOC from the Oe + Oa horizons over the entire manipulation period were not related to changes in microbial activity (measured as rates of basal and substrate-induced respiration) but to the original forest floor depths prior to manipulation, pointing to the flux control by the size of source pools rather than a straightforward relationship between microbial activity and DOM production. In response to doubled TF fluxes, net increases in DOM fluxes occurred in the lower forest floor, indicating the presence of substantial pools of potentially soluble organic matter in the Oe + Oa horizons. In contrast to the general assumption of DOM as a leaching product from recent litter, our results suggest that DOM in forest floor leachates is derived from both newly added litter and older organic horizons through complex interactions between microbial production and consumption and hydrologic transport.     

Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire

Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N deposition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old-growth and historically (80–110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean (± SD) nitrate export from old-growth watersheds (1.4 ± 0.6 kg N ha⁻¹ y⁻¹) was four times greater than from disturbed watersheds (0.3 ± 0.3 kg N ha⁻¹ y⁻¹), suggesting that biotic control over nitrate loss can persist for a century. DON loss averaged 0.7 (± 0.2) kg N ha⁻¹ y⁻¹ and accounted for 28–87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. Received 27 April 1999; accepted 30 May 2000.     

Response of dissolved organic matter in the forest floor to long-term manipulation of litter and throughfall inputs

Dissolved organic matter (DOM) contributes to organic carbon either stored in mineral soil horizons or exported to the hydrosphere. However, the main controls of DOM dynamics are still under debate. We studied fresh leaf litter and more decomposed organic material as the main sources of DOM exported from the forest floor of a mixed beech/oak forest in Germany. In the field we doubled and excluded aboveground litter input and doubled the input of throughfall. From 1999 to 2005 we measured concentrations and fluxes of dissolved organic C and N (DOC, DON) beneath the Oi and Oe/Oa horizon. DOM composition was traced by UV and fluorescence spectroscopy. In selected DOM samples we analyzed the concentrations of phenols, pentoses and hexoses, and lignin-derived phenols by CuO oxidation. DOC and DON concentrations and fluxes almost doubled instantaneously in both horizons of the forest floor by doubling the litter input and DOC concentrations averaged 82 mg C l⁻¹ in the Oe/Oa horizon. Properties of DOM did not suggest a change of the main DOM source towards fresh litter. In turn, increasing ratios of hexoses to pentoses and a larger content of lignin-derived phenols in the Oe/Oa horizon of the Double litter plots in comparison to the Control plots indicated a priming effect: Addition of fresh litter stimulated microbial activity resulting in increased microbial production of DOM from organic material already stored in Oe/Oa horizons. Exclusion of litter input resulted in an immediate decrease in DOC concentrations and fluxes in the thin Oi horizon. In the Oe/Oa horizon DOC concentrations started to decline in the third year and were significantly smaller than those in the Control after 5 years. Properties of DOM indicated an increased proportion of microbially and throughfall derived compounds after exclusion of litter inputs. Dissolved organic N did not decrease upon litter exclusion. We assume a microbial transformation of mineral N from throughfall and N mineralization to DON. Increased amounts of throughfall resulted in almost equivalently increased DOC fluxes in the Oe/Oa horizon. However, long-term additional throughfall inputs resulted in significantly declining DOC concentrations over time. We conclude that DOM leaving the forest floor derives mainly from decomposed organic material stored in Oe/Oa horizons. Leaching of organic matter from fresh litter is of less importance. Observed effects of litter manipulations strongly depend on time and the stocks of organic matter in forest floor horizons. Long-term experiments are particularly necessary in soils/horizons with large stocks of organic matter and in studies focusing on effects of declined substrate availability. The expected increased primary production upon climate change with subsequently enhanced litter input may result in an increased production of DOM from organic soil horizons.     

Linking Foliar Chemistry to Forest Floor Solid and Solution Phase Organic C and N in Picea abies [L.] Karst Stands in Northern Bohemia

Dissolved organic carbon and nitrogen (DOC and DON) produced in the forest floor are important for ecosystem functions such as microbial metabolism, pedogenesis and pollutant transport. Past work has shown that both DOC and DON production are related to litterfall and standing stocks of C and N in the forest floor. This study, conducted in spring, 2003, investigated variation in forest floor water extractable DOC (WEDOC) and DON (WEDON) and forest floor C and N as a function of lignin, cellulose and N contained in live canopy foliage across eight Picea abies [L.] Karst stands in northern Bohemia. Based on Near Infrared Spectroscopy (NIR) analysis of foliar materials, lignin:N and cellulose:N content of the youngest needles (those produced in 2002) were positively and significantly related to WEDOC (R² = 0.82–0.97; P<0.01) and to forest floor C:N ratio (R = 0.72–0.78; P<0.01). Foliar N was strongly and negatively related to WEDOC and C:N ratio (R = −0.91 and 0.72; P<0.05) among our study sites. WEDON was positively correlated to foliar lignin:N (R = 0.48; P<0.05; n=40). Forest floor C pools were not positively correlated with foliar lignin and cellulose and forest floor N pools were not positively correlated with foliar N. Instead, a significant negative correlation was found between forest floor N pools and foliar cellulose (R=−0.41; P<0.05), and between forest floor C pools and foliar N (R = −0.44; P<0.05). From a remote sensing standpoint, our results are important because canopy reflectance properties are primarily influenced by the most recent foliage, and it was the chemistry of the most recently produced needles that showed a stronger relationship with forest floor WEDOC and C:N ratio suggesting forest floor production of WEDOC can be calculated regionally with remote sensing.     

Does Anthropogenic Nitrogen Enrichment Increase Organic Nitrogen Concentrations in Runoff from Forested and Human-dominated Watersheds?

Although the effects of anthropogenic nitrogen (N) inputs on the dynamics of inorganic N in watersheds have been studied extensively, “the influence of N enrichment on organic N loss” is not as well understood. We compiled and synthesized data on surface water N concentrations from 348 forested and human-dominated watersheds with a range of N loads (from less than 100 to 7,100 kg N km⁻² y⁻¹) to evaluate the effects of N loading via atmospheric deposition, fertilization, and wastewater on dissolved organic N (DON) concentrations. Our results indicate that, on average, DON accounts for half of the total dissolved N (TDN) concentrations from forested watersheds, but it accounts for a smaller fraction of TDN in runoff from urban and agricultural watersheds with higher N loading. A significant but weak correlation (r ² = 0.06) suggests that N loading has little influence on DON concentrations in forested watersheds. This result contrasts with observations from some plot-scale N fertilization studies and suggests that variability in watershed characteristics and climate among forested watersheds may be a more important control on DON losses than N loading from atmospheric sources. Mean DON concentrations were positively correlated, however, with N load across the entire land-use gradient (r ² = 0.37, P < 0.01), with the highest concentrations found in agricultural and urban watersheds. We hypothesize that both direct contributions of DON from wastewater and agricultural amendments and indirect transformations of inorganic N to organic N represent important sources of DON to surface waters in human-dominated watersheds. We conclude that DON is an important component of N loss in surface waters draining forested and human-dominated watersheds and suggest several research priorities that may be useful in elucidating the role of N enrichment in watershed DON dynamics.     

High precipitation causes large fluxes of dissolved organic carbon and nitrogen in a subtropical montane <Emphasis Type="Italic">Chamaecyparis</Emphasis> forest in Taiwan

Fluxes of dissolved organic carbon (DOC) and nitrogen (DON) may play an important role for losses of C and N from the soils of forest ecosystems, especially under conditions of high precipitation. We studied DOC and DON fluxes and concentrations in relation to precipitation intensity in a subtropical montane Chamaecyparis obtusa var. formosana forest in Taiwan. Our objective was, to quantify DOC and DON fluxes and to understand the role of high precipitation for DOC and DON export in this ecosystem. From 2005 to 2008 we sampled bulk precipitation, throughfall, forest floor percolates and seepage (60 cm) and analyzed DOC, DON and mineral N concentrations. Average DOC fluxes in the soil were extremely high (962 and 478 kg C ha^?1 year^?1 in forest floor percolates and seepage, respectively) while DON fluxes were similar to other (sub)tropical ecosystems (16 and 8 kg N ha^?1 year^?1, respectively). Total N fluxes in the soil were dominated by DON. Dissolved organic C and N concentrations in forest floor percolates were independent of the water flux. No dilution effect was visible. Instead, the pool size of potentially soluble DOC and DON was variable as indicated by different DOC and DON concentrations in forest floor percolates at similar precipitation amounts. Therefore, we hypothesized, that these pools are not likely to be depleted in the long term. The relationship between water fluxes in bulk precipitation and DOC and DON fluxes in forest floor percolates was positive (DOC r = 0.908, DON r = 0.842, respectively, Spearman rank correlation). We concluded, that precipitation is an important driver for DOC and DON losses from this subtropical montane forest and that these DOC losses play an important role in the soil C cycle of this ecosystem. Moreover, we found that the linear relationship between bulk precipitation and DOC and DON fluxes in forest floor percolates of temperate ecosystems does not hold when incorporating additional data on these fluxes from (subtropical) ecosystems.     

Ammonium,nitrate and dissolved organic nitrogen in seepage water as affected by compost amendment to European beech,Norway spruce,and Scots pine forests

Fertilization of nutrient-depleted and degraded forest soils may be required to sustain utilization of forests. In some European countries, the application of composts may now be an alternative to the application of inorganic fertilizers because commercial compost production has increased and compost quality has been improved. There is, however, concern that compost amendments may cause increased leaching of nitrogen, trace metals and toxic organic compounds to groundwater. The objective of this study was to assess the risk of ammonium (NH₄ ⁺), nitrate (NO₃ ^–) and dissolved organic nitrogen (DON) leaching following a single compost application to silty and sandy soils in mature beech (Fagus sylvatica L.), pine (Pinus silvestris L.) and spruce (Picea abies Karst.) forests at Solling and Unterlüß in Lower Saxony, Germany. Mature compost from separately collected organic household waste was applied to the soil surface at a rate of 6.3 kg m^–2 in the summer of 1997 and changes in NH₄ ⁺, NO₃ ^– and DON concentrations in throughfall and soil water at 10 and 100 cm soil depths were determined for 32 months. The spruce forests had the highest N inputs by throughfall water and the highest N outputs in both the control and compost plots compared with the pine and beech forests. Overall, the differences in total N outputs at 100 cm soil depth between the control and compost plots ranged between 0.3 and 11.2 g N m^–2 for the entire 32-month period. The major leaching of these amounts occurred during the first 17 months after compost amendments, but there was no significant difference in total N outputs (–0.2 to 1.8 g N m^–2) between the control and compost plots during the remaining 15 months. Most of the mineral soils acted as a significant sink for NO₃ ^– and DON as shown by a reduction of their outputs from 10 to 100 cm depth. Based on these results, we conclude that application of mature compost with high inorganic N contents could diminish the groundwater quality in the first months after the amendments. A partial, moderate application of mature compost with low inorganic N content to nutrient depleted forest soils can minimize the risk of NO₃ ^– leaching.     

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH₄NO₃) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m^-2 y^-1), high N (15 g N m^-2 y^-1), and low N + sulfur (5 g N m^-2 y^-1 plus 7.4 g S m^-2 y^-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of ¹⁵N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. Received 2 April 1999; Accepted 29 October 1999.     

Soil–Nitrogen Cycling in a Pine Forest Exposed to 5 Years of Elevated Carbon Dioxide

Empirical and modeling studies have shown that the magnitude and duration of the primary production response to elevated carbon dioxide (CO₂) can be constrained by limiting supplies of soil nitrogen (N). We have studied the response of a southern US pine forest to elevated CO₂ for 5 years (1997–2001). Net primary production has increased significantly under elevated CO₂. We hypothesized that the increase in carbon (C) fluxes to the microbial community under elevated CO₂ would increase the rate of N immobilization over mineralization. We tested this hypothesis by quantifying the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first 5 years of CO₂ fumigation. We observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO₂. Similarly, elevated CO₂ had no statistically significant effect on the concentration or flux of organic N, including amino acids. Microbial biomass N was not significantly different between CO₂ treatments. Thus, we reject our hypothesis that elevated CO₂ increases the rate of N immobilization. The quantity and chemistry of the litter inputs to the forest floor and mineral soil horizons can explain the limited range of microbially mediated soil–N cycling responses observed in this ecosystem. Nevertheless a comparative analysis of ecosystem development at this site and other loblolly pine forests suggests that rapid stand development and C sequestration under elevated CO₂ may be possible only in the early stages of stand development, prior to the onset of acute N limitation.     

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO₃⁻) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha⁻¹ y⁻¹ as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha⁻¹ y⁻¹). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha⁻¹ y⁻¹. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO₃⁻ leaching in all three forests. We hypothesize that abiotic incorporation of excess NO₃⁻ (through chemically reactive NO₂⁻) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing; Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of DON model.     

Concentrations and fluxes of dissolved organic carbon in an age-sequence of white pine forests in Southern Ontario,Canada

We determined concentrations and fluxes of dissolved organic carbon (DOC) in precipitation, throughfall, forest floor and mineral soil leachates from June 2004 to May 2006 across an age-sequence (2-, 15-, 30-, and 65-year-old) of white pine (Pinus strobus L.) forests in southern Ontario, Canada. Mean DOC concentration in precipitation, throughfall, leachates of forest floor, Ah-horizon, and of mineral soil at 1 m depth ranged from ∼2 to 7, 9 to 18, 32 to 88, 20 to 66, and 2 to 3 mg DOC L⁻¹, respectively, for all four stands from April (after snowmelt) through December. DOC concentration in forest floor leachates was highest in early summer and positively correlated to stand age, aboveground biomass and forest floor carbon pools. DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon between were in the range of ∼1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m⁻², respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge was estimated as ∼7, 4, 3, and 2 g DOC m⁻² for the 2-, 15-, 30-, and 65-year-old sites, respectively, indicating a decrease with increasing stand age. Laboratory DOC sorption studies showed that the null-point DOC concentration fell from values of 15 to 60 mg DOC L⁻¹ at 0 to 5 cm to <15 mg DOC L⁻¹ at 50 cm. Specific ultraviolet light absorption at 254 nm (SUVA₂₅₄) increased from precipitation and throughfall to a maximum in forest floor and decreased with mineral soil depth. No age-related pattern was observed for SUVA₂₅₄ values. DOC concentration in forest floor soil solutions showed a positive exponential relationship with soil temperature, and a negative exponential relationship with soil moisture at all four sites. Understanding the changes and controls of DOC concentrations, chemistry, and fluxes at various stages of forest stand development is necessary to estimate and predict DOC dynamics on a regional landscape level and to evaluate the effect of land-use change.     

An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH₄ ⁺–N?+?NO₃ ^?–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25?cm depth. Fluxes of DIN were generally dominated by NO₃ ^?–N; but NH₄ ⁺–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH₄ ⁺–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH₄ ⁺–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p?<?0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p?<?0.01) and surface litter (p?<?0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.     

耕作扰动对喀斯特土壤可溶性有机质及其组分迁移淋失的影响

土壤可溶性有机质(DOM)及其组分淋失特征研究对深入理解干扰作用下土壤碳氮养分损失机制具有重要意义,本研究基于翻耕模拟试验,分析喀斯特石灰土可溶性有机碳(DOC)、可溶性有机氮(DON)、及DOM官能团组分的淋失动态特征及其对不同耕作频率的响应,并探讨其影响因子。结果表明:(1)土壤DOC与DON的淋失量均随翻耕频率的增加而增加,但4个官能团特征参数对翻耕频率响应均不显著;DOC/DON淋失比随翻耕频率的增大而减少,DON占淋溶水可溶性总氮(TDN)比例随翻耕频率增加而增加。(2)DOC和DON月淋失量同时受翻耕处理与季节变化及其交互作用的影响,4个官能团特征参数仅受季节变化的影响;翻耕处理实施后,DOC月淋失量表现为初期大、后期小,各处理间差异性逐渐降低;但DON月淋失量初期小、后期大,各处理间差异性逐渐增大。(3)DOC淋失量与4个官能团特征参数呈显著负相关(P0.05),与Ca~(2+)、NH_4~+-N的淋失量呈显著正相关;DON淋失量与4个官能团参数无显著相关关系,与Mg~(2+)淋失量呈显著正相关关系。以上结果表明耕作扰动会加剧土壤DOM淋失,但淋失组分中稳定性组分没有变化,意味着耕作干扰将导致土壤有机质的持续损失,且由于其淋失组分碳氮比(DOC/DON)随扰动频率增加而降低,DON/TDN比随扰动频率增加而增加,持续的耕作干扰将大大增加水体氮素污染风险。     

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m² delineated by trenches, and a headwater catchment of 0.7km².The average annual DOC export from the sub-catchments was 185 kg DOCha^–1 y^–1 for the forest, 108 kg DOCha^–1 y^–1 for the grassland and 84 kgDOC ha^–1 y^–1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha^–1 y^–1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l^–1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.     

Nutrient and mineralogical control on dissolved organic C, N and P fluxes and stoichiometry in Hawaiian soils

We measured DOM fluxes from the O horizon of Hawaiiansoils that varied in nutrient availability and mineralcontent to examine what regulates the flux ofdissolved organic carbon (DOC), nitrogen (DON) andphosphorus (DOP) from the surface layer of tropicalsoils. We examined DOM fluxes in a laboratory study from N, P and N+Pfertilized and unfertilized sites on soils that rangedin age from 300 to 4 million years old. The fluxesof DOC and DON were generally related to the % Cand % N content of the soils across the sites. Ingeneral, CO₂ and DOC fluxes were not correlatedsuggesting that physical desorption, dissolution andsorption reactions primarily control DOM release fromthese surface horizons. The one exception to thispattern was at the oldest site where there was asignificant relationship between DOC and CO₂flux. The oldest site also contained the lowestmineral and allophane content of the three sites andthe DOC-respiration correlation indicates arelationship between microbial activity and DOC fluxat this site. N Fertilization increased DON fluxes by50% and decreased DOC:DON ratios in the youngest,most N poor site. In the older, more N rich sites, Nfertilization neither increased DON fluxes nordecreased DOM C:N ratios. Similarly, short termchanges in N availability in laboratory-based soil Nand P fertilization experiments did not affect the DOMC:N ratios of leachate. DOM C:N ratios were similar tosoil organic matter C:N ratios, and changes in DOM C:Nratios with fertilization appeared to have beenmediated through long term effects on SOM C:N ratiosrather than through changes in microbial demand for Cand N. There was no relationship between DON andinorganic N flux during these incubations suggestingthat the organic and inorganic components of N fluxfrom soils are regulated by different factors and thatDON fluxes are not coupled to immediate microbialdemand for N. In contrast to the behavior of DON, thenet flux of dissolved organic phosphorus (DOP) and DOMC:P ratios responded to both long-term P fertilizationand natural variation in reactive P availability. There was lower DOP flux and higher DOM C:P ratiosfrom soils characterized by low P availability andhigh DOP flux and narrow DOM C:P ratios in sites withhigh P availability. DOP fluxes were also closelycorrelated with dissolved inorganic P fluxes. PFertilization increased DOP fluxes by 73% in theyoungest site, 31% in the P rich intermediate agesite and 444% in the old, P poor site indicating thatDOP fluxes closely track P availability in soils.