Soil Nitrogen and Sulfur Leaching in a Subtropical Forest at a Transition State under Decreasing Atmospheric Deposition

Anthropogenic emissions of nitrogen- (N) and sulfur (S)-containing pollutants have declined across China in recent years. However, the responses of N and S depositions and dynamics in soil remain unclear in subtropical forests. In this study, the wet and throughfall depositions of dissolved inorganic N (DIN) and SO42− were continuously monitored in a mildly polluted subtropical forest in Southeast China in 2017 and 2018. Moreover, these solutes in soil water along the soil profile were monitored in 2018. Throughfall deposition of DIN and S decreased by 59% and 53% in recent 3 years, respectively, which can be majorly attributed to the decreases in wet depositions of NO3− and SO42−. Meanwhile, NH4+ deposition remained relatively stable at this site. Even though N deposition in 2018 was below the N saturation threshold for subtropical forests, significant N leaching still occurred. Excess export of N occurred in the upper soil layer (0–15 cm), reaching 6.86 ± 1.54 kg N/ha/yr, while the deeper soil (15–30 cm) was net sink of N as 8.29 ± 1.71 kg N/ha/yr. Similarly, S was excessively exported from the upper soil with net flux of 14.7 ± 3.15 kg S/ha/yr, while up to 6.37 ± 3.18 kg S/ha/yr of S was retained in the deeper soil. The significant N and S leaching under declined depositions suggested that this site possibly underwent a transition state, recovering from historically high acid deposition. Furthermore, the rainfall intensity remarkably regulated leaching and retention of SO42− and DIN at this site. The impacts of climate changes on N and S dynamics require further long-term monitoring in subtropical forests.

Bulk deposition of base cations in a subalpine natural forest landscape estimated by canopy budget model: A focus on the forest edge canopy

Calculations of base cation inputs of loads in forest edge canopies are rare, although forest edge canopies play a paradoxical role in the effective capture of atmospheric deposition. Throughfall deposition and canopy exchange of base cations were studied with a continuous throughfall investigation under a natural forest edge and a closed canopy in a subalpine forest over a period of 2 years. Compared with precipitation, the concentration of base cations in the throughfall of both canopies was enriched as expected, but the enrichment level in the forest edge was less than that in the closed canopy. Moreover, the throughfall deposition of base cation fluxes in the closed canopy (35.19 kg ha-1 y-1) was slightly higher than that in the forest edge canopy (33.50 kg ha-1 y-1). Seasonally, the base cation input in the rainy season was 2.32–2.70 times higher than that in the snowy season in throughfall in forest edge canopy and the closed canopy. Furthermore, the canopy budget model suggested that the direction and magnitude of canopy exchange and dry deposition controlled the net throughfall fluxes (NTF) of base cations, and obvious differences could be observed between the canopy and seasonal scales. Compared with other elements, K and Mg leached from the main canopy exchange process, while Ca was absorbed by both canopy types in the annual NTF. These results highlight the spatial variability of base cation chemical characteristics, enhance cognitive the deposition of nutrients and regulatory factors in different forest landscapes, preferably guide the formulation forest ecological management strategies.

Good Agreement Between Modeled and Measured Sulfur and Nitrogen Deposition in Europe, in Spite of Marked Differences in Some Sites

Atmospheric nitrogen and sulfur deposition is an important effect of atmospheric pollution and may affect forest ecosystems positively, for example enhancing tree growth, or negatively, for example causing acidification, eutrophication, cation depletion in soil or nutritional imbalances in trees. To assess and design measures to reduce the negative impacts of deposition, a good estimate of the deposition amount is needed, either by direct measurement or by modeling. In order to evaluate the precision of both approaches and to identify possible improvements, we compared the deposition estimates obtained using an Eulerian model with the measurements performed by two large independent networks covering most of Europe. The results are in good agreement (bias <25%) for sulfate and nitrate open field deposition, while larger differences are more evident for ammonium deposition, likely due to the greater influence of local ammonia sources. Modeled sulfur total deposition compares well with throughfall deposition measured in forest plots, while the estimate of nitrogen deposition is affected by the tree canopy. The geographical distribution of pollutant deposition and of outlier sites where model and measurements show larger differences are discussed.

Nickel in forests – a short review on its distribution and fluxes

The distribution and cycling of nickel (Ni) in forests is greatly affected by their proximity to emission sources of the metal. The throughfall deposition is always richer in Ni than the bulk deposition. It can be inferred that some dry deposition enriches the throughfall. In remote forested areas, the hydrological fluxes of Ni do not differ a lot from those in litterfall. In addition, the current year needles in conifers have higher concentrations than the older needles, a sign of absorption and mobility of the metal. In contrast, near an industrial Ni source the older needles accumulate much more of the metal. The Ni content in bark tissue can be used to map the deposition distribution of the metal around an area (rural or urban). The concentrations of Ni in forest soils is also dependent on their distances from the Ni emission sources and the nature of the soil parent material. The Ni concentrations increase with soil depth due to the geogenic origin of the metal. Low pH greatly enhances the mobility of the metal in soils, much more than the leachability of organic matter.

Molybdenum in forests-A short review

<p>This review deals with the importance of molybdenum (Mo) in nitrogen fixation in forests as well as the distribution of Mo in the various components of a forest ecosystems and its cycling. With regard to nitrogen (N) fixation in tropical forests, the limiting factor is the availability of phosphorus in soils. For the temperate and boreal forests, the Mo availability is more crucial. The canopies of the tropical forests retain Mo from precipitation, which is used for N fixation by the microorganisms living there. There is also retention of Mo in throughfall deposition by the canopy of temperate forests but so far, there has been no information as to the biological processes taking place there. The contribution of Mo in bulk and throughfall deposition in the Mo cycling is more important than once thought. There is great variability in Mo concentrations in tree leaves either conifers or broadleaves and even within the same species. In soils, the organic matter, clay and the oxides of Al, Fe and Mn play a predominant role in preventing Mo from leaching.</p>

The fluxes and distribution of molybdenum in a mountainous fir forest in two hydrological years

<p>The distribution, stocks and fluxes of Mo were examined in a mountainous fir forest in Greece in two hydrological years. In both years, the fluxes of Mo in throughfall were lower than those in bulk deposition implying Mo retention and probably uptake by tree canopies. The Mo concentration was appreciable in soil solution but its fluxes were lower than those in throughfall. The Mo fluxes in both bulk and throughfall deposition were higher than those in litterfall, a finding that stresses the importance of the hydrological cycle for this metal. The Mo concentration in the older twigs in the standing trees were higher than the current year ones most probably due to the existence of dry deposition. The concentrations of total Mo in soils reflected the nature of the sedimentary rocks underlying the soil and increased with depth following the clay concentration gradient. The available (oxalate extractable) Mo in soils ranged from 3% to 14% of the total Mo. In terms of the stocks of Mo, the soil had by far the highest values in both total and available Mo. The needs of the fir plants for Mo uptake are covered by the available Mo in the mineral soil layers, whereas the Mo content in the organic horizon may not be adequate for this purpose.</p>

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

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.