Fluxes of nitrogen and phosphorus in a gallery forest in the Cerrado of central Brazil

The Gallery forests of the Cerrado biome play a critical role in controlling stream chemistry but little information about biogeochemical processes in these ecosystems is available. This work describes the fluxes of N and P in solutions along a topographic gradient in a gallery forest. Three distinct floristic communities were identified along the gradient: a wet community nearest the stream, an upland dry community adjacent to the woodland savanna and an intermediate community between the two. Transects were marked in the three communities for sampling. Fluxes of N from bulk precipitation to these forests resulted in deposition of 12.6 kg ha^?1 y^?1 of total N of which 8.8 kg ha^?1 was as inorganic N. The throughfall flux of total N was generally <8.4 kg ha^?1 year^?1. Throughfall NO₃?CN fluxes were higher (7?C32%) while NH₄?CN and organic N fluxes were lower (54?C69% and 5?C46%) than those in bulk precipitation. The throughfall flux was slightly lower for the wet forest community compared to other communities. Litter leachate fluxes differed among floristic communities with higher NH₄?CN in the wet community. The total N flux was greater in the wet forest than in the dry forest (13.5 vs. 9.4 kg ha^?1 year^?1, respectively). The stream water had total N flux of 0.3 kg ha^?1 year^?1. The flux of total P through bulk precipitation was 0.7 kg ha^?1 year^?1 while the mean fluxes of total P in throughfall (0.6 kg ha^?1 year^?1) and litter leachate (0.5 kg ha^?1 year^?1) declined but did not differ between communities. The low concentrations presented in soil solution and low fluxes in stream water (0.3 and 0.1 kg ha^?1 year^?1 for N and P, respectively) relative to other flowpaths emphasize the conservative nutrient cycling of these forests and the importance of internal recycling processes for the maintenance and conservation of riparian and stream ecosystems in the Cerrado.     

Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

Nitrogen (N) biogeochemistry of a mature Scots pine (Pinus sylvestris L.) stand subjected to an average total atmospheric N deposition of 48 kg ha^?1 year^?1 was studied during the period 1992–2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha^?1 year^?1 over the 16-year monitoring period. The throughfall fluxes contained also considerable amounts of dissolved organic nitrogen (DON) (5–8.5 kg N ha^?1 year^?1), which should be incorporated in the estimate of N flux using throughfall collectors. Throughfall DIN fluxes declined at a rate of ?0.9 kg N ha^?1 year^?1, mainly due to the decreasing TF fluxes of ammonium (NH₄), which accounted for 70% to TF DIN. The decrease in TF DIN was accompanied by a decrease in DIN leaching in the seepage water (?1.6 kg N ha^?1 year^?1), which occurred exclusively as nitrate (NO₃ ^?). Nitrate losses in the leachate of the forest floor (LFH) equalled the TF NO₃ ^? delivered to the LFH-layer. On the contrary, about half of the TF NH₄ ⁺ was retained within the LFH-layer. Approximately 60% of the TF DIN fluxes were leached indicating that N inputs were far in excess of the N requirements of the forest. For DON, losses were only substantial from the LFH-layer, but no DON was leached in the seepage water. Despite the high N losses through nitrate leaching and NO _x emission, the forest was still accumulating N, especially in the aggrading LFH-layer. The forest stand, on the contrary, was found to be a poor N sink.     

Carbon cycling and net ecosystem production at an early stage of secondary succession in an abandoned coppice forest

Secondary mixed forests are one of the dominant forest cover types in human-dominated temperate regions. However, our understanding of how secondary succession affects carbon cycling and carbon sequestration in these ecosystems is limited. We studied carbon cycling and net ecosystem production (NEP) over 4 years (2004–2008) in a cool-temperate deciduous forest at an early stage of secondary succession (18 years after clear-cutting). Net primary production of the 18-year-old forest in this study was 5.2 tC ha^?1 year^?1, including below-ground coarse roots; this was partitioned into 2.5 tC ha^?1 year^?1 biomass increment, 1.6 tC ha^?1 year^?1 foliage litter, and 1.0 tC ha^?1 year^?1 other woody detritus. The total amount of annual soil surface CO₂ efflux was 6.8 tC ha^?1 year^?1, which included root respiration (1.9 tC ha^?1 year^?1) and heterotrophic respiration (RH) from soils (4.9 tC ha^?1 year^?1). The 18-year forest at this study site exhibited a great increase in biomass pool as a result of considerable total tree growth and low mortality of tree stems. In contrast, the soil organic matter (SOM) pool decreased markedly (?1.6 tC ha^?1 year^?1), although further study of below-ground detritus production and RH of SOM decomposition is needed. This young 18-year forest was a weak carbon sink (0.9 tC ha^?1 year^?1) at this stage of secondary succession. The NEP of this 18-year forest is likely to increase gradually because biomass increases with tree growth and with the improvement of the SOM pool through increasing litter and dead wood production with stand development.     

Effects of nitrogen deposition and management practices on leaf litterfall and N and P return in a Moso bamboo forest

The effects of nitrogen (N) deposition and management practices on leaf litterfall and N and P return in Moso bamboo forest are not yet known. In this study, we investigated the effects of four levels of simulated N deposition, including low-N (N30, 30 kg ha^?1 year^?1), medium-N (N60, 60 kg ha^?1 year^?1), and high-N (N90, 90 kg ha^?1 year^?1), and a control with no N (N-free addition). The experiment was performed in a Moso bamboo forest under conventional management (CM) and intensive management (IM). The results showed that leaf litterfall and N and P return occurred mainly from March to June and accounted for 78.2–82.2, 78.5–82.1, and 85.6–94.6% of annual leaf litterfall, N return, and P return, respectively. Unlike CM, IM significantly increased leaf litterfall and N and P return. The positive effects were further amplified by low- and medium-N deposition, but not high-N deposition. The combination of low- and medium-N deposition and IM significantly increased N and P return, but not litterfall. Our results indicated that the interaction of anthropogenic management practices and N deposition need to be considered when estimating the effects of N deposition on the biogeochemical cycle of a forest ecosystem.     

Atmospheric methane uptake by tropical montane forest soils and the contribution of organic layers

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH₄), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH₄ consumption (6.2 Tg CH₄ year^?1), limited data are available on CH₄ exchange from tropical montane forests. We present the results of an extensive study on CH₄ exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH₄ sinks, with decreasing annual uptake rates from 5.9 kg CH₄–C ha^?1 year^?1 at 1,000 m to 0.6 kg CH₄–C ha^?1 year^?1 at 3,000 m. Topography had no effect on soil atmospheric CH₄ uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH₄ uptake rates, mineral nitrogen content of the mineral soil and with CO₂ emissions indicated that the largest CH₄ uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH₄ uptake was N limited instead of inhibited by NH₄ ⁺. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH₄ budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.     

Soil system budgets of N,Si and P in an agricultural irrigated watershed: surplus,differential export and underlying mechanisms

Ongoing socio-economic and climatic changes can differentially affect the biogeochemistry of the key nutrients nitrogen (N), silica (Si), and phosphorus (P) by altering their soil budgets, their transfer to aquatic environments and their ecological stoichiometry. This may lead to cascade consequences for aquatic communities and biogeochemical processes. Soil budgets, river export, and N, Si, and P ecological stoichiometry were assessed in a heavy impacted basin (Mincio River, Italy) in two decades (1991–2000; 2001–2010). The main aim was to analyse element-specific mechanisms of terrestrial-aquatic transport and retention within aquatic habitats. Budget results suggest a net accumulation (inputs exceeding outputs) of all nutrients in agricultural lands, mainly due to livestock manure, with a reduction for N (196 kg N ha^?1 year^?1 in 2000, and 132 kg N ha^?1 year^?1 in 2010), and constant values for Si (up to 3 kg Si ha^?1 year^?1) and P (43 kg P ha^?1 year^?1) along the study period. River export of N and P accounted for 3–27% and?~?2% of N and P soil net accumulation, respectively, while Si export was significantly greater (25 kg Si ha^?1 year^?1) than Si net accumulation on farmlands. The stoichiometry of net nutrient accumulation in soils was not reflected by the stoichiometry of nutrient riverine export, due to element-specific mechanisms. We speculate that N and Si vertical and horizontal mobilization is increased by the irrigation loop, while P retention is favored by limited erosion due to limited slopes in the Mincio River basin. The simultaneous analysis of N, Si and P allows us to better understand the different paths, transformation and retention mechanisms at the watershed scale.     

Carbon cycling and sequestration in a Japanese red pine (Pinus densiflora) forest on lava flow of Mt. Fuji

Biometric-based carbon flux measurements were conducted in a pine forest on lava flow of Mt. Fuji, Japan, in order to estimate carbon cycling and sequestration. The forest consists mainly of Japanese red pine (Pinus densiflora) in a canopy layer and Japanese holly (Ilex pedunculosa) in a subtree layer. The lava remains exposed on the ground surface, and the soil on the lava flow is still immature with no mineral soil layer. The results showed that the net primary production (NPP) of the forest was 7.3 ± 0.7 t C ha^?1 year^?1, of which 1.4 ± 0.4 t C ha^?1 year^?1 was partitioned to biomass increment, 3.2 ± 0.5 t C ha^?1 year^?1 to above-ground fine litter production, 1.9 t C ha^?1 year^?1 to fine root production, and 0.8 ± 0.2 t C ha^?1 year^?1 to coarse woody debris. The total amount of annual soil surface CO₂ efflux was estimated as 6.1 ± 2.9 t C ha^?1 year^?1, using a closed chamber method. The estimated decomposition rate of soil organic matter, which subtracted annual root respiration from soil respiration, was 4.2 ± 3.1 t C ha^?1 year^?1. Biometric-based net ecosystem production (NEP) in the pine forest was estimated at 2.9 ± 3.2 t C ha^?1 year^?1, with high uncertainty due mainly to the model estimation error of annual soil respiration and root respiration. The sequestered carbon being allocated in roughly equal amounts to living biomass (1.4 t C ha^?1 year^?1) and the non-living C pool (1.5 t C ha^?1 year^?1). Our estimate of biometric-based NEP was 25 % lower than the eddy covariance-based NEP in this pine forest, due partly to the underestimation of NPP and difficulty of estimation of soil and root respiration in the pine forest on lava flows that have large heterogeneity of soil depth. However, our results indicate that the mature pine forest acted as a significant carbon sink even when established on lava flow with low nutrient content in immature soils, and that sequestration strength, both in biomass and in soil organic matter, is large.     

Nitrogen deposition promotes ecosystem carbon accumulation by reducing soil carbon emission in a subtropical forest

Background and aims

Tropical and subtropical forests are experiencing high levels of atmospheric nitrogen (N) deposition, but the responses of such forests ecosystems to N deposition remain poorly understood.

Methods

We conducted an 8-year field experiment examining the effect of experimental N deposition on plant growth, soil carbon dioxide efflux, and net ecosystem production (NEP) in a subtropical Chinese fir forest. The quantities of N added were 0 (control), 60, 120, and 240 kg ha^?1 year^?1.

Results

NEP was lowest under ambient conditions and highest with 240 kg of N ha^?1 year^?1 treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2 to 16.4 kg C per kg N added in comparison with control. In addition, N deposition treatments significantly decreased heterotrophic respiration (by 0.69–1.85 t C ha^?1 year^?1) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots.

Conclusions

Our findings suggest that the young Chinese fir forest is carbon source and N deposition would sequester additional atmospheric CO₂ at high levels N input, mainly due to reduced soil CO₂ emission rather than increased plant growth, and the amount of sequestered C depended on the rate of N deposition.     

Lignin content and chemical characteristics in maize and wheat vary between plant organs and growth stages: consequences for assessing lignin dynamics in soil

In forest ecosystems, the silicon (Si) mass-balance at the watershed scale can be strongly influenced by readily soluble Si components, such as dissolved Si, adsorbed Si, amorphous silica (biogenic and pedogenic opal) and short-range ordered aluminosilicates. The aim of the present study is to (a) identify the components of the readily soluble Si pool in the Cambisol found below three tree species, under homogeneous soil and climate conditions, and (b) study the impact of Si recycling by tree species on the Si pools. We therefore measured the concentrations of Si extracted by Na₂CO₃ (Si_alk), oxalate (Si_ox) and CaCl₂. The Si_alk concentration decreased from the humus layer to 15 cm depth and then slightly increased until a depth of 75 cm. In the humus layer, the Si_alk concentration consisted mainly of phytoliths and differed significantly between tree species (expressed as mg SiO₂ g^-1): Douglas fir (14.5?±?0.65) > European beech (11.8?±?0.30) > Black pine (5.4?±?0.31). Below 7.5 cm, the Si_alk content did not differ significantly between tree species, and the Si_ox content, increasing significantly, was mainly comprised of Si adsorbed onto amorphous Fe oxides. These results suggest that (a) tree species can impact the readily soluble Si content in the topsoil, through different rates of Si uptake and phytolith restitution by the vegetation, and (b) the soil’s readily soluble Si pool is mainly comprised of phytoliths and adsorbed Si. Here, the readily soluble Si pool is thus controlled by both the iron dynamics and Si biocycling.     

Short-term simulated nitrogen deposition increases carbon sequestration in a Pleioblastus amarus plantation

In order to understand the influence of nitrogen (N) deposition on the key processes relevant to the carbon (C) balance in a bamboo plantation, a two-year field experiment involving the simulated deposition of N in a Pleioblastus amarus plantation was conducted in the rainy region of SW China. Four levels of N treatments: control (no N added), low-N (50 kg N ha^?1 year^?1), medium-N (150 kg N ha^?1 year^?1), and high-N (300 kg N ha^?1 year^?1) were set in the present study. The results showed that soil respiration followed a clear seasonal pattern, with the maximum rates in mid-summer and the minimum in late winter. The annual cumulative soil respiration was 585?±?43 g CO₂-C m^?2 year^?1 in the control plots. Simulated N deposition significantly increased the mean annual soil respiration rate, fine root biomass, soil microbial biomass C (MBC), and N concentration in fine roots and fresh leaf litter. Soil respirations exhibited a positive exponential relationship with soil temperature, and a linear relationship with MBC. The net primary production (NPP) ranged from 10.95 to 15.01 Mg C ha^?1 year^?1 and was higher than the annual soil respiration (5.85 to 7.62 Mg C ha^?1 year^?1) in all treatments. Simulated N deposition increased the net ecosystem production (NEP), and there was a significant difference between the control and high N treatment NEP, whereas, the difference of NEP among control, low-N, and medium-N was not significant. Results suggest that N controlled the primary production in this bamboo plantation ecosystem. Simulated N deposition increased the C sequestration of the P. amarus plantation ecosystem through increasing the plant C pool, though CO₂ emission through soil respiration was also enhanced.     

Fate of ammonium 15N in a Norway spruce forest under long-term reduction in atmospheric N deposition

In the last decades, in particular forest ecosystems became increasingly N saturated due to elevated atmospheric N deposition, resulting from anthropogenic N emission. This led to serious consequences for the environment such as N leaching to the groundwater. Recent efforts to reduce N emissions raise the question if, and over what timescale, ecosystems recover to previous conditions. In order to study the effects on N distribution and N transformation processes under the lowered N deposition treatment, we investigated the fate of deposited NH₄ ⁺-¹⁵N in soil of a N-saturated Norway spruce forest (current N deposition: 34 kg ha^?1 year^?1; critical N load: 14 kg ha^?1 year^?1), where N deposition has been reduced to 11.5 kg ha^?1 year^?1 since 14.5 years. We traced the deposited ¹⁵N in needle litter, bulk soil, and amino acids, microbial biomass and inorganic N in soil. Under reduced N deposition, 123 ± 23% of the deposited N was retained in bulk soil, while this was only 72 ± 15% under ambient deposition. We presume that with reduced deposition the amount of deposited N was small enough to become completely immobilized in plant and soil and no leaching losses occurred. Trees receiving reduced N deposition showed a decline in N content as well as in ¹⁵N incorporation into needle litter, indicating reduced N plant uptake. In contrast, the distribution of ¹⁵N within the soil over active microbial biomass, microbial residues and inorganic N was not affected by the reduced N deposition. We conclude that the reduction in N deposition impacted only plant uptake and drainage losses, while microbial N transformation processes were not influenced. We assume changes in the biological N turnover to start with the onset of the decomposition of the new, N-depleted litter.     

Inter-Annual Precipitation Variability Decreases Switchgrass Productivity from Arid to Mesic Environments

Cellulosic biofuels are an important source of renewable biomass within the alternative energy portfolio. Switchgrass (Panicum virgatum L.), a perennial C₄ grass native to North America, is widely studied as a biofuel feedstock for its consistently high yields and minimal input requirements. The influences of precipitation amount and temporal variability on the fertilizer response of switchgrass productivity are not fully understood. Moreover, global climate models predict changes in rainfall patterns towards lower and increasingly variable soil water availability in several productive areas worldwide, which may impact net primary production of biofuel crops. We conducted a meta-analysis of aboveground net primary production of switchgrass from 48 publications encompassing 82 different locations, 11 soil types, 52 switchgrass cultivars, fertilizer inputs between 0 to 896 kg N ha^?1 year^?1, and 1 to 6 years of annual productivity measures repeated on the same stand. Productivity of the lowland ecotype doubled with N rates >?131 kg N ha^?1 year^?1, but upland ecotype productivity increased only by 50%. Results showed an optimum N rate of 30 to 60 kg N ha^?1 year^?1 for both ecotypes, after which biomass gain per unit of N added decreased. Growing season precipitation (GSPPT) and inter-annual precipitation variability (inter-PPTvar) affected both ecotypes similarly. Long-term mean annual precipitation (MAP) differentially affected lowland and upland productivity, depending on the N level. Productivity responses to MAP and GSPPT were similar for both upland and lowland ecotypes at none or low N rates. When N increased beyond 60 kg N ha^?1 year^?1, lowland cultivars had a greater growth response to MAP than uplands. Productivity increased with increasing GSPPT and MAP and had a positive linear response to MAP ranging from 600 to 1200 mm year^?1. One third of the variability in switchgrass production was accounted for by inter-PPTvar. After accounting for MAP, sites with higher inter-PPTvar had lower switchgrass productivity than sites with lower inter-PPTvar. Increased inter-annual variation in precipitation reduced production of both ecotypes. Predicted changes in the amount and timing of precipitation thus likely will exert greater influence on production of upland than lowland ecotypes of switchgrass.     

Modeling denitrification in a tile-drained, corn and soybean agroecosystem of Illinois, USA

Denitrification is known as an important pathway for nitrate loss in agroecosystems. It is important to estimate denitrification fluxes to close field and watershed N mass balances, determine greenhouse gas emissions (N₂O), and help constrain estimates of other major N fluxes (e.g., nitrate leaching, mineralization, nitrification). We compared predicted denitrification estimates for a typical corn and soybean agroecosystem on a tile drained Mollisol from five models (DAYCENT, SWAT, EPIC, DRAINMOD-N II and two versions of DNDC, 82a and 82h), after first calibrating each model to crop yields, water flux, and nitrate leaching. Known annual crop yields and daily flux values (water, nitrate-N) for 1993–2006 were provided, along with daily environmental variables (air temperature, precipitation) and soil characteristics. Measured denitrification fluxes were not available. Model output for 1997–2006 was then compared for a range of annual, monthly and daily fluxes. Each model was able to estimate corn and soybean yields accurately, and most did well in estimating riverine water and nitrate-N fluxes (1997–2006 mean measured nitrate-N loss 28 kg N ha^?1 year^?1, model range 21–28 kg N ha^?1 year^?1). Monthly patterns in observed riverine nitrate-N flux were generally reflected in model output (r ² values ranged from 0.51 to 0.76). Nitrogen fluxes that did not have corresponding measurements were quite variable across the models, including 10-year average denitrification estimates, ranging from 3.8 to 21 kg N ha^?1 year^?1 and substantial variability in simulated soybean N₂ fixation, N harvest, and the change in soil organic N pools. DNDC82a and DAYCENT gave comparatively low estimates of total denitrification flux (3.8 and 5.6 kg N ha^?1 year^?1, respectively) with similar patterns controlled primarily by moisture. DNDC82h predicted similar fluxes until 2003, when estimates were abruptly much greater. SWAT and DRAINMOD predicted larger denitrification fluxes (about 17–18 kg N ha^?1 year^?1) with monthly values that were similar. EPIC denitrification was intermediate between all models (11 kg N ha^?1 year^?1). Predicted daily fluxes during a high precipitation year (2002) varied considerably among models regardless of whether the models had comparable annual fluxes for the years. Some models predicted large denitrification fluxes for a few days, whereas others predicted large fluxes persisting for several weeks to months. Modeled denitrification fluxes were controlled mainly by soil moisture status and nitrate available to be denitrified, and the way denitrification in each model responded to moisture status greatly determined the flux. Because denitrification is dependent on the amount of nitrate available at any given time, modeled differences in other components of the N cycle (e.g., N₂ fixation, N harvest, change in soil N storage) no doubt led to differences in predicted denitrification. Model comparisons suggest our ability to accurately predict denitrification fluxes (without known values) from the dominant agroecosystem in the midwestern Illinois is quite uncertain at this time.     

Decreased Silica Land–sea Fluxes through Damming in the Baltic Sea Catchment – Significance of Particle Trapping and Hydrological Alterations

We tested the hypothesis that reservoirs with low water residence time and autochthonous production influence river biogeochemistry in eutrophied river systems draining cultivated watersheds. The effect of a single artificial water reservoir and consecutive reservoirs on silica (Si) river fluxes is exemplified by the moderately dammed Vistula River and the heavily regulated Daugava River that are compared with the practically undammed Oder River. The sum of the discharge weighted annual mean biogenic silica (BSi) and dissolved silicate (DSi) concentrations in the rivers Oder, Vistula and Daugava were about 160 μ M (40 + 120 μ M), 150 μ M (20 + 130 μ M) and 88 μ M (6 + 82 μ M), respectively. Assuming BSi and DSi concentrations as observed in the Oder River as typical for eutrophied but undammed rivers, complete trapping of this BSi could have lowered Si fluxes to the Baltic Sea from rivers with cultivated watersheds by 25%. The superimposed effect of hydrological alterations on reduced Si land–sea fluxes is demonstrated by studies in the boreal/subarctic and oligotrophic rivers Kalixälven and Lueälven. The DSi yield of the heavily dammed Luleälven (793 kg km^?2 yr^?1) constituted only 63% of that was found in the unregulated Kalixälven (1261 kg km^?2 yr^?1), despite the specific runoff of the Luleälven (672 mm m^?2 yr^?1) being 19% higher than that of theKalixälven (563 mm m^?2 yr^?1); runoff normalized DSi yield of the former, regulated watershed, was only half the DSi yield of the latter, unperturbed watershed. Based on these findings, it is hypothesized here that perturbed surface water–groundwater interactions are the major reasons for the reduced annual fluctuations in DSi concentrations as also seen in the heavily dammed and eutrophic river systems such as the Daugava and Danube.     

Phosphorus cycling in a small watershed in the Brazilian Cerrado: impacts of frequent burning

Plant productivity in many tropical savannas is phosphorus limited. The biogeochemical cycling of P in these ecosystems, however, has not been well quantified. In the present study, we characterized P stocks and fluxes in a well-preserved small watershed in the Brazilian Cerrado. As the Cerrado is also a fire-dominated ecosystem, we measured the P stocks and fluxes in a cerrado stricto sensu plot with complete exclusion of fire for 26 years (unburned plot) and then tested some predictions about the impacts of fire impacts on P cycling in an experimental plot that was burned three times since 1992 (burned plot). The unburned area is an ecosystem with large soil stocks of total P (1,151 kg ha^?1 up to 50 cm depth), but the largest fraction is in an occluded form. Readily extractable P was found up to 3 m soil depth suggesting that deep soil is more important to the P cycle than has been recognized. The P stock in belowground biomass (0?C800 cm) was 9.9 kg ha^?1. Decomposition of fine litter released 0.97 kg P ha^?1 year^?1. Fluxes of P through bulk atmospheric deposition, throughfall and litter leachate were very low (0.008, 0.006 and 0.028 kg ha^?1 year^?1, respectively) as was stream export (0.001 kg ha^?1 year^?1). Immobilization of P by microbes during the rainy season seems to be an important mechanism of P conservation in this ecosystem. Fire significantly increased P flux in litter leachate to 0.11 kg ha^?1 year^?1, and added 1.2 kg ha^?1 of P in ash deposition after fire. We found an increase of P concentration in soil solution at 100 cm depth (from 0.03 ??g l^?1 in unburned plot to 0.3 ??g l^?1 in the burned plot). In surface soils (0?C10 cm) of the burned plot, fire decreased the concentrations of extractable organic-P fractions, but did not significantly increase inorganic-P fractions. The reduction of extractable soil organic P in the burned plot in topsoil and the increase of P in the soil solution at greater depths indicated a reduction of P availability and may increase P fixation in deep soils. Repeated fire events over the long term may result in significant net loss of available forms of phosphorus from this ecosystem.     

Quantifying the contribution of land use to N<Subscript>2</Subscript>O,NO and CO<Subscript>2</Subscript> fluxes in a montane forest ecosystem of Kenya

Increasing demand for food and fibre by the growing human population is driving significant land use (LU) change from forest into intensively managed land systems in tropical areas. But empirical evidence on the extent to which such changes affect the soil-atmosphere exchange of trace gases is still scarce, especially in Africa. We investigated the effect of LU on soil trace gas production in the Mau Forest Complex region, Kenya. Intact soil cores were taken from natural forest, commercial and smallholder tea plantations, eucalyptus plantations and grazing lands, and were incubated in the lab under different soil moisture conditions. Soil fluxes of nitrous oxide (N₂O), nitric oxide (NO) and carbon dioxide (CO₂) were quantified, and we approximated annual estimates of soil N₂O and NO fluxes using soil moisture values measured in situ. Forest and eucalyptus plantations yielded annual fluxes of 0.3–1.3 kg N₂O–N ha^?1 a^?1 and 1.5–5.2 kg NO–N ha^?1 a^?1. Soils of commercial tea plantations, which are highly fertilized, showed higher fluxes (0.9 kg N₂O–N ha^?1 a^?1 and 4.3 kg NO–N ha^?1 a^?1) than smallholder tea plantations (0.1 kg N₂O–N ha^?1 a^?1 and 2.1 kg NO–N ha^?1 a^?1) or grazing land (0.1 kg N₂O–N ha^?1 a^?1 and 1.1 kg NO–N ha^?1 a^?1). High soil NO fluxes were probably the consequence of long-term N fertilization and associated soil acidification, likely promoting chemodenitrification. Our experimental approach can be implemented in understudied regions, with the potential to increase the amount of information on production and consumption of trace gases from soils.     

Simulated nitrogen deposition significantly suppresses the decomposition of forest litter in a natural evergreen broad-leaved forest in the Rainy Area of Western China

Background and aims

Litter, an essential component of forest ecosystems, plays an important role in maintaining soil fertility, sequestering carbon (C) and improving soil biodiversity. However, litter decomposition is affected by increased nitrogen (N) deposition. Numerous reports have presented N deposition experiments in different forest ecosystems to investigate the effects of N deposition on litter decomposition, but the effects remain unclear, especially in ecosystems receiving increasingly higher levels of ambient N deposition. To address this gap, we performed a litterbag experiment to understand the effects of increasing N deposition on the litter decomposition process in natural evergreen broad-leaved forest in the Rainy Area of Western China.

Methods

A 2-year field litter decomposition experiment was conducted using the litterbag method. Four levels of N deposition were established: control (CK; 0 kg·N·ha^?1·year^?1), low N deposition (LN; 50 kg·N·ha^?1·year^?1), medium N deposition (MN; 150 kg·N·ha^?1·year^?1), and high N deposition (HN; 300 kg·N·ha^?1·year^?1). The simulated N depositions ranged from 50% to 320% of the ambient rate of wet N deposition.

Results

Simulated N deposition significantly increased the remaining mass, C, N, lignin and cellulose of the litter. The LN treatment decreased the remaining phosphorus (P); conversely, the HN treatment increased it. In the late stage of the study period, the mass remaining was positively closely correlated to the lignin and cellulose remaining during the decomposition process.

Conclusions

Simulated N deposition significantly suppressed the litter decomposition in the natural evergreen broad-leaved forest, despite the high rate of ambient N deposition, and the inhibitory effects increased with the N deposition levels. The suppressive effect of N deposition on litter decomposition may be primarily explained by the inhibition of lignin and cellulose degradation by the exogenous inorganic N. With ongoing N deposition in future, N deposition may have a potentially significant impact on C and N cycles in such forest ecosystems.

     

Fine-root seasonal pattern,production and turnover rate of European beech (Fagus sylvatica L.) stands in Italy Prealps: Possible implications of coppice conversion to high forest

Abstract

The aim of this study was to investigate the possible effects of coppice conversion to high forest on the beech fine-root systems. We compared the seasonal pattern of live and dead fine-root mass (d < 2 mm), production and turnover in three beech stands that differed in management practices. Tree density was higher in the 40-year-old coppice stand than in the stands that were converted from coppice to high forest in 1994 and 2004, respectively. We found that a reduction in tree density reduced the total fine-root biomass (Coppice stand, 353.8 g m^?2; Conversion 1994 stand, 203.6 g m^?2; Conversion 2004 stand, 176.2 g m^?2) which continued to be characterised by a bimodal pattern with two major peaks, one in spring and one in early fall. Conversion to high forest may also affect the fine-root soil depth distribution. Both fine-root production and turnover rate were sensitive to management practices. They were lower in the Coppice stand (production 131.5 g m^?2 year^?1; turnover rate 0.41 year^?1) than in the converted stands (1994 Conversion stand: production 232 g m^?2 year^?1, turnover rate 1.06 year^?1; 2004 Conversion stand: production 164.2 g m^?2 year^?1, turnover rate 0.79 year^?1).     

Effect of nitrogen and phosphorus fertilization on the composition of rhizobacterial communities of two Chilean Andisol pastures

The effect of nitrogen (N) and phosphorus (P) fertilization on composition of rhizobacterial communities of volcanic soils (Andisols) from southern Chile at molecular level is poorly understood. This paper investigates the composition of rhizobacterial communities of two Andisols under pasture after 1- and 6-year applications of N (urea) and P (triple superphosphate). Soil samples were collected from two previously established sites and the composition of rhizobacterial communities was determined by denaturing gradient gel electrophoresis (PCR–DGGE). The difference in the composition and diversity between rhizobacterial communities was assessed by nonmetric multidimensional scaling (MDS) analysis and the Shannon–Wiener index. In Site 1 (fertilized for 1 year), PCR–DGGE targeting 16S rRNA genes and MDS analysis showed that moderate N application (270 kg N ha^?1 year^?1) without P significantly changed the composition of rhizobacterial communities. However, no significant community changes were observed with P (240 kg P ha^?1 year^?1) and N–P application (270 kg N ha^?1 year^?1 plus 240 kg P ha^?1 year^?1). In Site 2 (fertilized for 6 years with P; 400 kg P ha^?1 year^?1), PCR–DGGE targeting rpoB, nifH, amoA and alkaline phosphatase genes and MDS analysis showed changes in rhizobacterial communities only at the highest rate of N application (600 kg N ha^?1 year^?1). Quantitative PCR targeting 16S rRNA genes also showed higher abundance of bacteria at higher N application. In samples from both sites, the Shannon–Wiener index did not show significant difference in the diversity of rhizobacterial communities. The changes observed in rhizobacterial communities coincide in N fertilized pastures with lower soil pH and higher pasture yields. This study indicates that N–P application affects the soil bacterial populations at molecular level and needs to be considered when developing fertilizer practices for Chilean pastoral Andisols.     

Role of six European tree species and land‐use legacy for nitrogen and water budgets in forests

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30‐year‐old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small‐leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallø during 2 years. Mean annual percolation below the root zone (mm yr^?1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)? spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha^?1 yr^?1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha^?1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha^?1 yr^?1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)? maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallø. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short‐term perspective (30 years) of these stands.