scholarly journals Subsurface flow and phosphorus dynamics in beech forest hillslopes during sprinkling experiments: how fast is phosphorus replenished?

2021 ◽  
Vol 18 (3) ◽  
pp. 1009-1027 ◽  
Author(s):  
Michael Rinderer ◽  
Jaane Krüger ◽  
Friederike Lang ◽  
Heike Puhlmann ◽  
Markus Weiler
Keyword(s):  
Soil Solution ◽  
Forest Floor ◽  
Soil Depth ◽  
Mineral Soil ◽  
Mineral Weathering ◽  
Field Conditions ◽  
Strong Increase ◽  
Forest Plot ◽  
P Loss ◽  
P Concentration

Abstract. The phosphorus (P) concentration of soil solution is of key importance for plant nutrition. During large rainfall events, the P concentration is altered by lateral and vertical subsurface storm flow (SSF) that facilitates P mobilization, redistribution within the soil profile and potential P export from the ecosystem. These processes are not well studied under field conditions. Important factors of the replenishment of P concentrations in soil solutions are the rate of P replenishment (by biotic and abiotic processes) and the P buffering capacity of soils. Lab experiments have shown that replenishment times can vary between minutes and months. The question remains of how P concentrations in lateral and vertical SSF vary under natural field conditions. We present results of large-scale sprinkling experiments simulating 150 mm throughfall at 200 m2 plots on hillslopes at three beech forests in Germany. We aimed at quantifying lateral and vertical SSF and associated P concentrations on the forest floor, in the mineral soil and in the saprolite during sprinkling experiments in spring and summer. The sites differed mainly in terms of soil depth, skeleton content and soil P stock (between 189 and 624 g/m2 in the top 1 m soil depth). Vertical SSF in the mineral soil and in the saprolite was at least 2 orders of magnitude larger than lateral SSF at the same depth. Vertical and lateral SSF consisted mainly of pre-event water that was replaced by sprinkling water. Higher P concentrations in SSF in the first 1 to 2 h after the onset of SSF indicated nutrient flushing, but P concentrations in the mineral soil and saprolite were nearly constant thereafter for most of the experiment despite a strong increase in SSF. This suggests that P in the soil solution at all three sites was replenished fast by mineral or organic sources. If chemostatic transport conditions would dominate in SSF, annual P losses at the lateral and vertical boundary of a forest plot could be approximated by knowing the average P concentration and the water fluxes in forest soils. A rough estimation of the annual P loss based on this simplified assumption for one of our sites with longer SSF data resulted in an annual P loss of 3.16 mg/m2/a. This P loss is similar to estimates from a previous study at the same site using bi-weekly groundwater samples. Our approximated annual P loss in SSF was in a similar order of magnitude as P input by dry and wet deposition and by mineral weathering. Despite the fact that P losses from the ecosystem seem to be small, the translocation of P from the forest floor to the mineral soil might be of high relevance at sites with low P stocks where the forest floor is the dominant source for the P nutrition of trees.

scholarly journals Dynamic of nitrogen and dissolved organic carbon in an alpine forested catchment: atmospheric deposition and soil solution trends

2019 ◽  
Vol 34 ◽  
pp. 41-66 ◽  
Author(s):  
Raffaella Balestrini ◽  
Carlo Andrea Delconte ◽  
Andrea Buffagni ◽  
Alessio Fumagalli ◽  
Michele Freppaz ◽  
...  
Keyword(s):  
Soil Solution ◽  
Forest Ecosystem ◽  
Forest Floor ◽  
Mineral Soil ◽  
Chemical Species ◽  
Organic N ◽  
Seasonal Temperature ◽  
Inorganic N ◽  
Forest Floor Leachates ◽  
Throughfall Deposition

A number of studies have reported decreasing trends of acidifying and N deposition inputs to forest areas throughout Europe and the USA in recent decades. There is a need to assess the responses of the ecosystem to declining atmospheric pollution by monitoring the variations of chemical species in the various compartments of the forest ecosystem on a long temporal scale. In this study, we report on patterns and trends in throughfall deposition concentrations of inorganic N, dissolved organic N (DON) and C (DOC) over a 20-year (1995–2015) period in the LTER site -Val Masino (1190 m a.s.l.), a spruce forest, in the Central Italian Alps. The same chemical species were studied in the litter floor leachates and mineral soil solution, at three different depths (15, 40 and 70 cm), over a 10-year period (2005–2015). Inorganic N concentration was drastically reduced as throughfall and litter floor leachates percolated through the topsoil, where the measured mean values (2 µeq L-1) were much lower than the critical limits established for coniferous stands (14 µeq L-1). The seasonal temperature dependence of throughfall DOC and DON concentration suggests that the microbial community living on the needles was the main source of dissolved organic matter. Most of DOC and DON infiltrating from the litter floor were retained in the mineral soil. The rainfall amount was the only climatic factor exerting a control on DOC and N compounds in throughfall and forest floor leachates over a decadal period. Concentration of SO4 and NO3 declined by 50% and 26% respectively in throughfall deposition. Trends of NO3 and SO4 in forest floor leachates and mineral soil solution mirrored declining depositions. No trends in both DON and DOC concentration and in DOC/DON ratio in soil solutions were observed. These outcomes suggest that the declining NO3 and SO4 atmospheric inputs did not influence the dynamic of DON and DOC in the Val Masino forest. The results of this study are particularly relevant, as they are based on a comprehensive survey of all the main compartments of the forest ecosystem. Moreover, this kind of long-term research has rarely been carried out in the Alpine region.

scholarly journals Natural abundance of 15N of a spruce forest ecosystem under acid rain and manipulated clean rain field conditions

2008 ◽  
Vol 54 (No. 8) ◽  
pp. 377-387
Author(s):  
P. Sah S ◽  
R. Brumme ◽  
N. Lamersdorf
Keyword(s):  
Forest Floor ◽  
Forest Ecosystems ◽  
Soil Depth ◽  
Mineral Soil ◽  
Control Plot ◽  
Natural Abundance ◽  
Spruce Forest ◽  
Litter Fall ◽  
Bulk Precipitation ◽  
N Status

We analysed stable isotopes of N in a spruce forest under ambient rainfall (no further manipulation of the atmospheric input) and clean rain application (10 years of reduced inorganic N- and acid-constituent input). The objectives of the study were to assess whether or not the natural <sup>15</sup>N abundance would function as an indicator for the N-status of our forest ecosystems. For this purpose, natural <sup>15</sup>N abundance values were measured in needles, litter fall, roots, soil, bulk precipitation, throughfall and soil water of both plots. In the bulk precipitation and throughfall the &delta;<sup>15</sup>N values of NO<sub>3</sub>-N were in the range reported by other studies (–16 to + 23‰). In both plots, the throughfall was greatly depleted of <sup>15</sup>N compared to the bulk precipitation and this was attributed to nitrification in the canopy leaves, leading to &delta;<sup>15</sup>N-depleted nitrate production in the leaves that leaches down the soil surface. Nitrate in seepage water showed a general increase in &delta;<sup>15</sup>N values when it passes through the upper mineral soil (10 cm soil depth) and infiltrates into deeper mineral soil horizons (100 cm soil depth), similar to the &delta;<sup>15</sup>N enrichment of total nitrogen in the mineral soil. We observed <sup>15</sup>N depletion in both green needles and litter fall at the clean rain plot, compared to the N-saturated control plot. We assumed it to be due to increased mycorrhizal N-uptake under N limited, i.e. clean rain conditions which are indicated by relatively lower N concentrations of green needles. With respect to the vertical gradient of the <sup>15</sup>N abundance in the forest floor, both plots differ from each other, showing an untypical peak of &delta;<sup>15</sup>N depletion in the humus layer, which is more pronounced at the control plot. In contrast to the mineral soil where mineralisation is a dominant process for fractionation we attribute the &delta;<sup>15</sup>N pattern in the forest floor to additional processes like litter input and immobilisation. We conclude that the &delta;<sup>15</sup>N natural abundance analysis is helpful for interpreting the N-status of forest ecosystems but further research is needed especially with respect to the soil-root interface.

scholarly journals Patterns of dissolved organic carbon and nitrogen fluxes in deciduous and coniferous forests under historic high nitrogen deposition

2009 ◽  
Vol 6 (12) ◽  
pp. 2743-2758 ◽  
Author(s):  
S. Sleutel ◽  
J. Vandenbruwane ◽  
A. De Schrijver ◽  
K. Wuyts ◽  
B. Moeskops ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Dissolved Organic Carbon ◽  
Nitrogen Deposition ◽  
Soil Solution ◽  
Forest Floor ◽  
Inorganic Nitrogen ◽  
Mineral Soil ◽  
N Deposition ◽  
High Nitrogen ◽  
Leaching Losses

Abstract. Numerous recent studies have indicated that dissolved organic carbon (DOC) and nitrogen (DON) play an important role in C and N cycling in natural ecosystems, and have shown that N deposition alters the concentrations and fluxes of dissolved organic substances and may increase leaching losses from forests. Our study was set up to accurately quantify concentrations and flux patterns of DOC, DON and dissolved inorganic nitrogen (DIN) in deciduous and coniferous forest in Flanders, Belgium, under historical high nitrogen deposition. We measured DOC, DON and DIN concentrations at two weekly intervals in a silver birch (SB) stand, a corsican pine (CP) stand and a pine stand with higher N deposition (CPN), and used the SWAP model (calibrated with PEST) for generating accurate water and matter fluxes. The input with precipitation was an important source of DON, but not for DOC. Release of DOC from the forest floor was minimally affected by forest type, but higher N deposition (CPN stand) caused an 82% increase of DOC release from the forest floor. Adsorption to mineral soil material rich in iron and/or aluminum oxyhydroxides was suggested to be the most important process removing DOC from the soil solution, responsible for substantial retention (67–84%) of DOC entering the mineral soil profile with forest floor leachate. Generally, DON was less reactive (i.e. less removal from the soil solution) than DOC, resulting in decreasing DOC/DON ratios with soil depth. We found increased DOC retention in the mineral soil as a result of higher N deposition (84 kg ha−1 yr−1 additional DOC retention in CPN compared to CP). Overall DON leaching losses were 2.2, 3.3 and 5.0 kg N yr−1 for SB, CP and CPN, respectively, contributing between 9–28% to total dissolved N (TDN) leaching. The relative contribution to TDN leaching from DON loss from SB and CP was mainly determined by (large) differences in DIN leaching. The large TDN leaching losses are alarming, especially in the CPN stand that was N saturated.

scholarly journals Phosphorus Transport in Subsurface Flow at Beech Forest Stands: Does Phosphorus Mobilization Keep up with Transport?

2020 ◽  
Author(s):  
Michael Rinderer ◽  
Jaane Krüger ◽  
Friederike Lang ◽  
Heike Puhlmann ◽  
Markus Weiler
Keyword(s):  
Forest Ecosystems ◽  
Preferential Flow ◽  
Soil Depth ◽  
Mineral Soil ◽  
Subsurface Flow ◽  
Climatic Conditions ◽  
Strong Increase ◽  
Limiting Factor ◽  
Phosphorus Transport ◽  
Soil P

Abstract. Phosphorus (P) is a limiting factor of primary productivity in most forest ecosystems but little is known about retention within and losses of P from forests. Subsurface flow (SSF) is one of the important pathways of P export but few attempts exist to quantify it. We present results of sprinkling experiments with ca. 150 mm, 2H labelled, total rainfall conducted at 200 m2 plots on hillslopes with slopes between 14° and 28° at three beech forests in Germany in summer and spring. We aimed at quantifying vertical and lateral SSF and associated P transport in the forest floor, the mineral soil and the saprolite. The study sites differed regarding soil depth, skeleton content and soil P stocks (between 678 g/m2 and 209 g/m2, in the first 1 m soil depth). Vertical SSF in the mineral soil and in the saprolite was at least two orders of magnitude larger than lateral SSF in the same depth. Vertical and lateral SSF consisted mainly of pre-event water that was replaced by sprinkling water (piston flow mechanism). Short spikes of event water at the beginning of the experiment at two of the sites with high skeleton content indicate that preferential flow occurred in parallel to matrix flow. We observed a significant decrease in P concentrations in SSF with increasing soil depth suggesting effective retention of P by adsorption to soil particles in all three forest ecosystems. Higher P concentrations in SSF at the beginning of the experiments indicate nutrient flushing but P concentrations were nearly constant thereafter despite strong increase in SSF. P concentrations did also not change significantly with increasing share of event water in SSF. These chemostatic transport conditions suggests that P mobilization rates were similar to transport rates in both, P-rich and P-poor sites. The observed first flush effect implies that P export by SSF will increase as rainfall events with high transport capacity are predicted to occur more frequent under future climatic conditions.

scholarly journals Patterns of dissolved organic carbon (DOC) and nitrogen (DON) fluxes in deciduous and coniferous forests under historic high nitrogen deposition

2009 ◽  
Vol 6 (4) ◽  
pp. 7133-7173 ◽  
Author(s):  
S. Sleutel ◽  
J. Vandenbruwane ◽  
A. De Schrijver ◽  
K. Wuyts ◽  
B. Moeskops ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Dissolved Organic Carbon ◽  
Nitrogen Deposition ◽  
Soil Solution ◽  
Forest Floor ◽  
Inorganic Nitrogen ◽  
Mineral Soil ◽  
N Deposition ◽  
High Nitrogen ◽  
Leaching Losses

Abstract. Numerous recent studies have indicated that dissolved organic carbon (DOC) and nitrogen (DON) play an important role in C and N cycling in natural ecosystems, and have shown that N deposition alters the concentrations and fluxes of dissolved organic substances and may increase leaching losses from forests. Our study was set up to accurately quantify concentrations and flux patterns of DOC, DON and dissolved inorganic nitrogen (DIN) in deciduous and coniferous forest in Flanders under historical high nitrogen deposition. We measured DOC, DON and DIN concentrations at two weekly intervals in a silver birch (SB) stand, a corsican pine (CP) stand and a pine stand with higher N deposition (CPN), and used the SWAP model (calibrated with PEST) for generating accurate water and matter fluxes. The input with precipitation was an important source of DON, but not for DOC. Release of DOC from the forest floor was minimally affected by forest type, but higher N deposition (CPN stand) caused an 82% increase of DOC release from the forest floor. Adsorption to mineral soil material rich in iron and/or aluminum oxyhydroxides was suggested to be the most important process removing DOC from the soil solution, responsible for substantial retention (67–84%) of DOC entering the mineral soil profile with forest floor leachate. Generally, DON was less reactive (i.e. less removal from the soil solution) than DOC, resulting in decreasing DOC/DON ratios with soil depth. We found increased DOC retention in the mineral soil as a result of higher N deposition (84 kg N ha−1 yr−1 additional DOC retention in CPN compared to CP). Overall DON leaching losses were 2.2, 3.3 and 5.0 kg N ha−1 yr−1 for SB, CP and CPN, respectively, contributing between 9–28% to total dissolved N (TDN) leaching. DON loss from SB and CP was not much higher than from unpolluted forests, and its relative contribution to TDN leaching was mainly determined by (large) differences in DIN leaching. The large TDN leaching losses are alarming, especially in the CPN stand that was N saturated.

Distribution and cycling of elements in a Pinus resinosa plantation ecosystem, Wisconsin

1983 ◽  
Vol 13 (4) ◽  
pp. 609-619 ◽  
Author(s):  
J. G. Bockheim ◽  
S. W. Lee ◽  
J. E. Leide
Keyword(s):  
Forest Floor ◽  
Net Primary Production ◽  
Mineral Soil ◽  
Soil Surface ◽  
Belowground Biomass ◽  
Mineral Weathering ◽  
Nutrient Concentrations ◽  
Rooting Zone ◽  
Atmospheric Inputs ◽  
Turn Over

Biomass in 34-year-old planted red pine (Pinusresinosa Ait.) on a Typic Udipsamment soil was 127 t ha−1. Aboveground net primary production was 10.2 t ha−1 year−1. Nutrient concentrations in the trees varied by (i) tissue, (ii) position in tree, and (iii) age of foliage. In the aboveground tree concentrations of N, P, K, Mg, and S were greatest in foliage followed by bolebark, live branches, and bolewood. Concentrations of N and S in foliage, bolebark, and live branches and concentrations of K, Mg, and P in the bolebark and live branches increased toward the apex of the tree. Whereas concentrations of Ca, Mg, and S increased with foliar age, concentrations of K and P decreased with foliar age. Total elements in the aboveground and belowground biomass were ranked: N > Ca > K > Mg > S > P. The forest floor contained greater quantities of N (254 kg ha−1) than the vegetation (222 kg ha−1). The upper 100 cm of mineral soil contained from 8.2% (N) to 89% (Mg) of the readily available nutrients in the ecosystem: Atmospheric inputs were the major source of N and S. Output of a given element by leaching beyond the rooting zone (55 cm) was less than 7 kg ha−1 year−1, except for S which was 12 kg ha−1 year−1. Litterfall returned the greatest amounts of each of the elements to the soil surface, followed by throughfall and stemflow. Because of foliar leaching, the net loading of each of the elements exceeded that of the precipitation. With the exception of [Formula: see text], [Formula: see text], and H, loading of elements in the forest floor leachate exceeded that of the throughfall + stemflow. Nitrogen mineralization was estimated to be 44 kg ha−1 year−1. Whereas N, S, and P in the forest floor required approximately 10 years to turn over, Ca, Mg, and K turned over in about 5 years. Mineral weathering provided 22, 6.9, and 5.0 kg ha−1 year−1 of Ca, K, and Mg, respectively. These estimates, determined from the mass balance approach, are considered to be too high. Elements were taken up by the vegetation in the following order: Ca (56 kg ha−1 year−1), N (54), K (18), S (13), and Mg (10 kg ha−1 year−1). Elemental retention, which ranged from 30% for S to 50% for K, was lower than for comparable ecosystems.

Response of black spruce (Piceamariana) ecosystems to soil temperature modification in interior Alaska

1990 ◽  
Vol 20 (9) ◽  
pp. 1530-1535 ◽  
Author(s):  
Keith Van Cleve ◽  
Walter C. Oechel ◽  
John L. Hom
Keyword(s):  
Soil Temperature ◽  
Soil Solution ◽  
Forest Floor ◽  
Black Spruce ◽  
Mineral Soil ◽  
Nutrient Content ◽  
Experimental Period ◽  
Degree Days ◽  
Soil Heating ◽  
Interior Alaska

This paper reports results of a study designed to examine the control that soil temperature exerts on soil processes associated with nutrient flux, and in turn, on tree nutrition in interior Alaska black spruce ecosystems. Approximately 50 m2 of forest floor in a 140-year-old black spruce ecosystem, which had developed on permafrost, was heated to 8–10 °C above ambient temperature. This perturbation amounted to approximately a 1589 degree-day seasonal heat sum (above 0 °C), 1026 degree-days above the control total of 563 degree-days. The forest floor, surface 5 cm of mineral soil, and soil solution were compared with those of an adjacent control plot to evaluate the change in nutrient content and decomposition rate of the forest floor. The nutritional response to soil heating of current black spruce foliage also was evaluated. Soil heating significantly increased decomposition of the forest floor, principally because of an increase in biomass loss of the O21 layer. The increased decomposition resulted in greater extractable N and P concentrations in the forest floor, higher N concentrations in the soil solution, and elevated spruce needle N, P, and K concentrations for the experimental period. These results are discussed in light of the importance of soil temperature and other state factors that mediate ecosystem function.

scholarly journals Current, steady-state and historical weathering rates of base cations at two forest sites in northern and southern Sweden: a comparison of three methods

2020 ◽  
Vol 17 (2) ◽  
pp. 281-304 ◽  
Author(s):  
Sophie Casetou-Gustafson ◽  
Harald Grip ◽  
Stephen Hillier ◽  
Sune Linder ◽  
Bengt A. Olsson ◽  
...  
Keyword(s):  
Steady State ◽  
Soil Depth ◽  
Base Cations ◽  
Base Cation ◽  
Mineral Weathering ◽  
Soil Horizon ◽  
Soil Horizons ◽  
Weathering Rate ◽  
Weathering Rates ◽  
Forest Sites

Abstract. Reliable and accurate methods for estimating soil mineral weathering rates are required tools in evaluating the sustainability of increased harvesting of forest biomass and assessments of critical loads of acidity. A variety of methods that differ in concept, temporal and spatial scale, and data requirements are available for measuring weathering rates. In this study, causes of discrepancies in weathering rates between methods were analysed and were classified as being either conceptual (inevitable) or random. The release rates of base cations (BCs; Ca, Mg, K, Na) by weathering were estimated in podzolised glacial tills at two experimental forest sites, Asa and Flakaliden, in southern and northern Sweden, respectively. Three different methods were used: (i) historical weathering since deglaciation estimated by the depletion method, using Zr as the assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on quantitative analysis of soil mineralogy; and (iii) BC budget at stand scale, using measured deposition, leaching and changes in base cation stocks in biomass and soil over a period of 12 years. In the 0–50 cm soil horizon historical weathering of BCs was 10.6 and 34.1 mmolc m−2 yr−1, at Asa and Flakaliden, respectively. Corresponding values of PROFILE weathering rates were 37.1 and 42.7 mmolc m−2 yr−1. The PROFILE results indicated that steady-state weathering rate increased with soil depth as a function of exposed mineral surface area, reaching a maximum rate at 80 cm (Asa) and 60 cm (Flakaliden). In contrast, the depletion method indicated that the largest postglacial losses were in upper soil horizons, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil horizons, PROFILE produced higher weathering rates than the depletion method, particularly of K and Na in deeper soil horizons. The lower weathering rates of the depletion method were partly explained by natural and anthropogenic variability in Zr gradients. The base cation budget approach produced significantly higher weathering rates of BCs, 134.6 mmolc m−2 yr−1 at Asa and 73.2 mmolc m−2 yr−1 at Flakaliden, due to high rates estimated for the nutrient elements Ca, Mg and K, whereas weathering rates were lower and similar to those for the depletion method (6.6 and 2.2 mmolc m−2 yr−1 at Asa and Flakaliden). The large discrepancy in weathering rates for Ca, Mg and K between the base cation budget approach and the other methods suggests additional sources for tree uptake in the soil not captured by measurements.

Nitrogen storage and availability during stand development in a New Zealand Nothofagus forest

2002 ◽  
Vol 32 (2) ◽  
pp. 344-352 ◽  
Author(s):  
P W Clinton ◽  
R B Allen ◽  
M R Davis
Keyword(s):  
New Zealand ◽  
Forest Floor ◽  
Woody Debris ◽  
Mineral Soil ◽  
Stand Development ◽  
N Availability ◽  
Net N Mineralization ◽  
Nitrogen Storage ◽  
Nothofagus Forest ◽  
Mature Stands

Stemwood production, N pools, and N availability were determined in even-aged (10, 25, 120, and >150-year-old) stands of a monospecific mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in New Zealand recovering from catastrophic canopy disturbance brought about by windthrow. Nitrogen was redistributed among stemwood biomass, coarse woody debris (CWD), the forest floor, and mineral soil following disturbance. The quantity of N in stemwood biomass increased from less than 1 kg/ha in seedling stands (10 years old) to ca. 500 kg/ha in pole stands (120 years old), but decreased in mature stands (>150 years old). In contrast, the quantity of N stored in CWD declined rapidly with stand development. Although the mass of N stored in the forest floor was greatest in the pole stands and least in the mature stands, N availability in the forest floor did not vary greatly with stand development. The mass of N in the mineral soil (0–100 mm depth) was also similar for all stands. Foliar N concentrations, net N mineralization, and mineralizable N in the mineral soil (0–100 mm depth) showed similar patterns with stage of stand development, and indicated that N availability was greater in sapling (25 years old) and mature stands than in seedling and pole stands. We conclude that declining productivity in older stands is associated more with reductions in cation availability, especially calcium, than N availability.

scholarly journals The impact of four decades of annual nitrogen addition on dissolved organic matter in a boreal forest soil

2013 ◽  
Vol 10 (3) ◽  
pp. 1365-1377 ◽  
Author(s):  
M. O. Rappe-George ◽  
A. I. Gärdenäs ◽  
D. B. Kleja
Keyword(s):  
Organic Matter ◽  
Dissolved Organic Matter ◽  
Soil Solution ◽  
Mineral Soil ◽  
Solution Concentration ◽  
Oxidative Enzymes ◽  
N Saturation ◽  
N Addition ◽  
Mineral N ◽  
The Impact

Abstract. Addition of mineral nitrogen (N) can alter the concentration and quality of dissolved organic matter (DOM) in forest soils. The aim of this study was to assess the effect of long-term mineral N addition on soil solution concentration of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in Stråsan experimental forest (Norway spruce) in central Sweden. N was added yearly at two levels of intensity and duration: the N1 treatment represented a lower intensity but a longer duration (43 yr) of N addition than the shorter N2 treatment (24 yr). N additions were terminated in the N2 treatment in 1991. The N treatments began in 1967 when the spruce stands were 9 yr old. Soil solution in the forest floor O, and soil mineral B, horizons were sampled during the growing seasons of 1995 and 2009. Tension and non-tension lysimeters were installed in the O horizon (n = 6), and tension lysimeters were installed in the underlying B horizon (n = 4): soil solution was sampled at two-week intervals. Although tree growth and O horizon carbon (C) and N stock increased in treatments N1 and N2, the concentration of DOC in O horizon leachates was similar in both N treatments and control. This suggests an inhibitory direct effect of N addition on O horizon DOC. Elevated DON and nitrate in O horizon leachates in the ongoing N1 treatment indicated a move towards N saturation. In B horizon leachates, the N1 treatment approximately doubled leachate concentrations of DOC and DON. DON returned to control levels, but DOC remained elevated in B horizon leachates in N2 plots nineteen years after termination of N addition. We propose three possible explanations for the increased DOC in mineral soil: (i) the result of decomposition of a larger amount of root litter, either directly producing DOC or (ii) indirectly via priming of old SOM, and/or (iii) a suppression of extracellular oxidative enzymes.

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