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.     

Modelling and measuring the effect of nitrogen catch crops on the nitrogen supply for succeeding crops

Nitrogen catch crops are grown to absorb nitrogen from the rooting zone during autumn and winter. The uptake of N (N_upt) from the soil inorganic N pool (N_min) to a pool of catch crop nitrogen, will protect the nitrogen against leaching. After incorporation, a fraction (m) of the catch crop nitrogen is mineralized and becomes available again. However, not all available nitrogen present in the soil in the autumn is lost by leaching during winter. A fraction (r) of the nitrogen absorbed by the catch crop would, without a catch crop, have been retained within the rooting zone. The first year nitrogen beneficial effect (N_eff) of a catch crop may then be expressed b N _eff = m*N _upt - r* N _upt The soil-plant simulation model DAISY was evaluated for its ability to simulate the effects of catch crops on spring N_min and N_eff. Based on incubation studies, parameter values were assigned to a number of catch crop materials, and these parameter values were then used to simulate spring Nmin. The model was able to predict much of the vairiation in the measured spring Nmin (r² = 0.48***) and there was good agreement between the measured and the simulated effect of winter precipitation on spring N_min and Neff.Scenarios including variable soil and climate conditions, and variable root depth of the succeeding crop were simulated. It is illustrated that the effect of catch crops on nitrogen availability for the succeeding crop depends strongly on the rooting depth of the succeeding crop. If the succeeding crop is deep rooted and the leaching intensity is low, there is a high risk that a catch crop will have a negative effect on nitrogen availability. The simulations showed that the strategy for the growing of catch crops should be adapted to the actual situation, especially to the expected leaching intensity and to the rooting depth of the succeeding crop.     

Quantitative effects of soil nitrate,growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha^–1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha^–1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha^–1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m^–2 degree-day^–1, equivalent to 7 kg N ha^–1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.     

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.     

Simulation of field data by a basic three-dimensional model of interactive root growth

Published field data for lupins grown in a deep sandy soil in the wheatbelt of south-western Australia were used to test the predictive ability of a model of three-dimensional root growth. The model has the capacity to simulate the growth of individual root sections in response to the supply and demand for water and nitrate. N mineralisation was not modelled explicitly, but was accounted for through the use of a seasonally variable mineralisation input derived from the field data. Simulated nitrogen and water contents and root length densities in the soil profile agreed well with observed profiles, although all were slightly under-predicted. A sensitivity analysis revealed that model predictions were most sensitive to the drained upper limit values (v/v) and the mineralisation rates (gN m^–3 s^–1) incorporated as external inputs to the model, along with the unit rate of N₂ fixation (mol nodule^–1 s^–1) and unit root growth rates (m mol^–1 s^–1) which are physiological parameters previously calibrated for lupins. The amount of nitrate leached was predicted well. Spatial plots of nitrate leaching were a close inverse of the root length density plots, with the highest nitrate leaching below the inter-plant zones, and the least nitrate leaching directly below each plant. These results suggest that the root distribution of a legume species such as lupin can have an effect on the leaching of nitrate to depth. It may thus be possible to reduce the total amount of nitrate leached under lupin crops by investigating factors such as the spatial deployment of roots, planting densities and intercropping.     

A simple model for estimating the contribution of nitrogen mineralization to the nitrogen supply of crops from a stabilized pool of soil organic matter and recent organic input

A simple model was developed to estimate the contribution of nitrogen (N) mineralization to the N supply of crops. In this model the soil organic matter is divided into active and passive pools. Annual soil mineralization of N is derived from the active pool. The active pool comprises stabilized and labile soil organic N. The stabilized N is built up from accumulated inputs of fresh organic N during a crop rotation but the labile N is a fraction of total N added, which mineralizes faster than the stabilized N. The passive pool is considered to have no participation in the mineralization process. Mineralization rates of labile and stabilized soil organic N from different crop residues decomposing in soil were derived from the literature and were described by the first-order rate equation dN/dt =-K*N, where N is the mineralizable organic N from crop residues andK is a constant. The data were groupedK ₁ by short-term (0–1 year) andK ₂ by long-term (0–10 years) incubation. Because the range of variation inK ₂ was smaller than inK ₁ we felt justified in using an average value to derive N mineralization from the stabilized pool. The use of a constant rate ofK ₁ was avoided so net N mineralization during the first year after addition is derived directly from the labile N in the crop residues. The model was applied to four Chilean agro-ecosystems, using daily averages of soil temperature and moisture. The N losses by leaching were also calculated. The N mineralization varied between 30 and 130 kg N ha^–1 yr^–1 depending on organic N inputs. Nitrogen losses by leaching in a poorly structured soil were estimated to be about 10% of total N mineralized. The model could explain the large differences in N- mineralization as measured by the potential N mineralization at the four sites studied. However, when grassland was present in the crop rotation, the model underestimated the results obtained from potential mineralization.     

Throughfall Nitrogen Deposition Has Different Impacts on Soil Solution Nitrate Concentration in European Coniferous and Deciduous Forests

Increases in the deposition of atmospheric nitrogen (N) influence N cycling in forest ecosystems and can result in negative consequences due to the leaching of nitrate into groundwaters. From December 1995 to February 1998, the Pan-European Programme for the Intensive and Continuous Monitoring of Forest Ecosystems measured forest conditions at a plot scale for conifer and broadleaf forests, including the performance of time series of soil solution chemistry. The influence of various ecosystem conditions on soil solution nitrate concentrations at these forest plots (n = 104) was then analyzed with a statistical model. Soil solution nitrate concentrations varied by season, and summer concentrations were approximately 25% higher than winter ones. Soil solution nitrate concentrations increased dramatically with throughfall (and bulk precipitation) N input for both broadleaf and conifer forests. However, at elevated levels of throughfall N input (more than 10 kg N ha^–1 y^–1), nitrate concentrations were higher in broadleaf than coniferous stands. This tree-specific difference was not observed in response to increased bulk precipitation N input. In coniferous stands, throughfall N input, foliage N concentration, organic layer carbon–nitrogen (C:N) ratio, and nitrate concentrations covaried. Soil solution nitrate concentrations in conifer plots were best explained by a model with throughfall N and organic layer C:N as main factors, where C:N ratio could be replaced by foliage N. The organic layer C:N ratio classes of more than 30, 25–30, and less than 25, as well as the foliage N (mg N g^–1) classes of less than 13, 13–17, and more than 17, indicated low, intermediate, and high risks of nitrate leaching, respectively. In broadleaf forests, correlations between N characteristics were less pronounced, and soil solution nitrate concentrations were best explained by throughfall N and soil pH (0–10-cm depth). These results indicate that the responses of soil solution nitrate concentration to changes in N input are more pronounced in broadleaf than in coniferous forests, because in European forests broadleaf species grow on the more fertile soils.     

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content,and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d^–1 °C^–1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d^–1 °C^–1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m^–1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 g L^–1 without a catch crop to 61 g L^–1 under Italian ryegrass and to only 1.5 g L^–1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

Effect of millipede Parafontaria tonominea Attems (Diplopoda: Xystodesmidae) adults on soil biological activities: A microcosm experiment

Effects of high density adult millipede populations on soil ecosystem properties were investigated using laboratory and field microcosm methods in a deciduous broad-leaved forest in western Japan. The density of Parafontaria tonominea adults was 25.6–72.0 individuals m^–2 on 15 September 1996, then the density declined to 0–5.4 m^–2 on 22 October 1996. Addition of millipedes to the laboratory microcosm enhanced soil respiration and decreased soil microbial biomass. Soil microcosms with and without millipedes (one and two pairs of adults) were set on a forest floor, and soil respiration, dissolved ion concentration in leacheate water were observed for 8 weeks. The millipedes ingested both leaf litter and soil, which increased soil respiration, leaching of Ca²⁺, Mg²⁺, and nitrate from the soil, whereas the soil microbial biomass was not changed at 8 weeks after introduction of the animals. Millipede feeding on soil enhanced microbial activity and nutrient leaching from the forest soil.     

A catchment-oriented and cost-effective policy for water protection

Governmental programmes and international agreements to counteract eutrophication have largely not attained agreed objectives (e.g. reduction by half of the anthropogenic nitrogen load on Swedish coastal waters). Important components of such programmes are improved removal of nitrogen in municipal treatment plants and changed agricultural practices. In addition, increased N-removal during runoff, i.e. restoration of ponds and wetlands, is an important strategy. One explanation of the fact that the objectives have yet not been achieved might be that the most effective step to counteract diffuse pollution has not been fully implemented. It is therefore important to stress the potential of effective measures and find ways to fully implement them at the watershed level. It is important to avoid excessive applications of fertilizers because this leads to an exponential increase in leaching. Field experiments indicate that the use of winter crops or an undersown catch crop outside the main cropping season has reduced nitrate losses by up to 75% in single years, and by nearly 50% over successive years. In southern Sweden, the area of wetlands has been reduced considerably (more than 90%) by melioration activities. In a recent project, budget studies with restored ponds verified the importance of ponds and wetlands in nitrogen retention. Per unit area, increased nitrogen loading implied increased nitrogen retention, but often a decrease in the percent retained. Ponds with depths of 0.4–2.0 m and hydrological loads of 0.14–5.2 m³ m⁻² day⁻¹ were created. One hundred and fifty to seven thousand kg N ha⁻¹ year⁻¹ was removed in ponds loaded by streams dominated by agricultural run off. A pond receiving pre-treated municipal wastewater removed 8000 kg N ha⁻¹ year⁻¹. The upper limit for N-removal is set by the hydrological conditions. Sedimentation of organic material must be favoured in order to obtain adequate conditions for denitrification. To achieve the governmental objective in nitrogen load reduction changed cultivation practices within the agricultural sector must be combined with restoration of ponds/wetlands.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Simulated nitrogen dynamics and nitrate leaching in a perennial grass ley

A soil nitrogen model was used for a 4-year simulation of nitrogen dynamics and nitrate leaching, both during grass ley growth and after ploughing a grass ley. Model results were compared with field measurements of soil mineral-N status and leaching. A soil water and heat model provided daily values for abiotic conditions, which were used as driving variables in the nitrogen simulation. Simulated values for mineral-N levels in the soil agreed well with field data for the first 3 years of the simulation. During the final year the model predicted considerably higher levels of soil mineral-N content compared with measurements. To reach the mineral-N level measured at the time of ploughing the ley, the simulated N-uptake by plants had to be increased by 8 g N m⁻². Simulations of nitrate leaching suggested that estimates of leaching based on measurements in tile-drained plots can be considerably underestimated. Accurate quantification of leaching in tile-drained plots often requires additional information on water-flow paths. A substantial increase in simulated and measured values for the mineral-N content of the soil occurred after ploughing the ley. In the simulation, most of the increase was due to a high crop residue input and the absence of a growing crop after ploughing. Litter accumulations in the soil during the 4-year period contributed little to the increase in soil mineral-N.     

Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with ¹⁵N in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and ¹⁵N-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N ^-2) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with¹⁵ NH₄ ¹⁵NO³ corresponding to the inorganic N applied with the slurries (6 g N m^-2). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m^-2) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. ¹⁵N in the urine fraction was less available for crop uptake than mineral fertilizer ¹⁵N. The first barley crop on the sandy loam removed 49% of the ¹⁵N applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer ¹⁵N (36%) and urine¹⁵ N (32%) differed less on the coarse sand. Of the¹⁵ N added with the faeces fraction, 12–14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones.About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of¹⁵ N from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop.More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3–5% and 1–3%, respectively, of the ¹⁵N in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.     

Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m^{– 2}, respectively, while total N was 4.8, 10.9 and 85.7 g m^{– 2}.We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with ¹⁵N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha^{– 1} y^{– 1} – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued.We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.     

Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: −5 g N m⁻² y⁻¹, Medium-N: −10 g N m⁻² y⁻¹, and High-N: −15 g N m⁻² y⁻¹, were employed to evaluate decomposition_. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem.     

Influence of Soil Moisture on Litter Respiration in the Semiarid Loess Plateau

Understanding the response mechanisms of litter respiration to soil moisture in water-limited semi-arid regions is of vital importance to better understanding the interplay between ecological processes and the local carbon cycle. In situ soil respiration was monitored during 2010–2012 under various conditions (normal litter, no litter, and double litter treatments) in a 30-year-old artificial black locust plantation (Robinia pseudoacacia L.) on the Loess Plateau. Litter respiration with normal and double litter treatments exhibited similar seasonal variation, with the maximum value obtained in summer (0.57 and 1.51 μmol m⁻² s⁻¹ under normal and double litter conditions, respectively) and the minimum in spring (0.27 and 0.69 μmol m⁻² s⁻¹ under normal and double litter conditions, respectively). On average, annual cumulative litter respiration was 115 and 300 g C m⁻² y⁻¹ under normal and double litter conditions, respectively. Using a soil temperature of 17°C as the critical point, the relationship between litter respiration and soil moisture was found to follow quadratic functions well, whereas the determination coefficient was much greater at high soil temperature than at low soil temperature (33–35% vs. 22–24%). Litter respiration was significantly higher in 2010 and 2012 than in 2011 under both normal litter (132–165 g C m⁻² y⁻¹ vs. 48 g C m⁻² y⁻¹) and double litter (389–418 g C m⁻² y⁻¹ vs. 93 g C m⁻² y⁻¹) conditions. Such significant interannual variations were largely ascribed to the differences in summer rainfall. Our study demonstrates that, apart from soil temperature, moisture also has significant influence on litter respiration in semi-arid regions.     

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N<Subscript>2</Subscript>-fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N₂-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r ² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha^–1 y^–1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.     

Nitrate leaching from sandy loam soils under a double-cropping forage system estimated from suction-probe measurements

Nitrate leaching from a double-cropping forage system was measured over a 2-year period (June 1994–May 1996) in the Northwest region of Portugal using ceramic cup samplers. The crops were grown for silage making and include maize (from May to September) and a winter crop (rest of the year) consisting of a mixture of cereals and Italian ryegrass. The experiment was performed on two different sites with a history of many years under the same crop and fertiliser management, but differing in the amounts of N applied as fertiliser and by regular cattle slurry applications. The annual nitrate leaching losses measured ranged from 154 to 338 kg N ha^-1. These amounts lead to annual mean concentrations between 22 and 41 mg -N L^-1 in the drained water. The coarse textured soils (sandy loams) and the climatic conditions of the region with more than 600 mm of drainage concentrated between October and March, tended to promote the leaching of all the nitrate-N left in the soil after the maize crop plus the N released by mineralization during the winter period. On these soils, the minimum amount of drainage (necessary to provide the complete leaching of all the nitrate-N in the soil profile in the end of summer), seems to be between 300 and 400 mm. The winter crops removed important quantities of N (83–116 kg N ha^-1) but, due to their late establishment in autumn they did not succeed in taking up the nitrate-N left in the soil after the maize crop. Approaches for reducing the nitrate leaching losses in this particular system are discussed.     

Field observations on nitrogen catch crops. III. Transfer of nitrogen to the succeeding main crop

In temperate climates with a precipitation surplus during autumn and winter, nitrogen (N) catch crops can help to reduce nitrogen losses from cropping systems by absorbing nitrogen from the soil and transfer it to a following main crop. In two field experiments the catch crop species winter rye (Secale cereale) and forage rape (Brassica napus ssp. oleifera (Metzg.) Sinsk) or oil radish (Raphanus sativus spp. oleiferus (DC.) Metzg.) were planted end of August and 3 weeks later with a non-limiting supply of N and zero-N controls. In the next spring catch crops were incorporated into the soil. In Expt 1, N transfer was measured as (i) the N uptake of a potato test crop, grown with zero and 12.5 g m^–2 N applied, and (ii) the increase in soil mineral N (0–30 cm) in uncropped soil covered with polythene film. In Expt 2, N transfer was measured as the increase in soil mineral N in covered cylinders placed in uncropped soil (in situ incubation). Subsidiary laboratory incubations were performed in Expt 2. In Expt 1, the apparent recovery in potato of fertilizer N (R _f) was 0.56. The recovery in potato of N mineralized from 'native' N pools other than catch crop material (R _n) ranged from 0.43 to 0.51, depending on the value assumed for the depth of N extraction by potato roots. The average recovery in potato of incorporated catch crop N (R _c) was 0.34. Expressed as `fertilizer N replacement factor' (F _r) the latter was 0.61 (i.e. 1 kg of N in catch crop material counts for 0.61 kg fertilizer N). Under the film in Expt 1 the fraction net mineralization of incorporated catch crop N (M _n) was 0.36 on August 11 and 0.43 on October 18. In Expt 2, the average value of M _n was 0.31, which was lower than in Expt 1 and probably associated with the drier soil in Expt 2. In the laboratory incubations (20°C) M _n showed values up to 0.54 after 84 days with the largest rates of change in mineralization occuring early after the start of the incubation. In conjunction with literature data it is concluded that cultivation of nitrogen catch crops shows promise as a means to reduce N input and N losses in temperate climates with wet winters.