The Impact of Nitrogen on Forest Soils and Feedbacks on tree Growth

The effects of pollutant nitrogen on forest soils and the potential feedbacks on tree growth are discussed using data from recent plot and catchment manipulation studies. Results indicate that N applied to the soil affects both soil N transformations and base cation status in some forest soils. Whilst reductions in tree growth are infrequently reported, a greater understanding of the effect of increased N deposition on soil N transformations, and associated changes in soil acidification and nutritional balance, is essential if sensitive stands are to identified. In particular, the factors controlling soil N accumulation rates and thus the onset of nitrate leaching are not clearly understood with increased nitrogen availability potentially increasing nitrogen accumulation due to abiotic fixation and lignolytic enzyme suppression, whilst the onset of nitrification as the C/N ratio of forest floor material declines, may reduce N retention efficiency. The switch from increased to decreased tree growth rate in response to N additions in some experiments highlights the need for long-term studies as a necessary component of future research.     

Regional assessment of the temporal trends in soil acidification in Southern Sweden,using the safe model

The dynamic soil acidification model SAFE was applied to 44 forested sites in Skåne, southern Sweden, using available Swedish databases on present soil status, vegetation and deposition. Time series of deposition were derived for each site from present deposition in a generalized fashion by dividing deposition into different classes and scaling with deposition trends from the literature. This study connects the current status of the soil and the soil development with critical load maps calculated with the steady-state model PROFILE.The model was calibrated against measurements of present base saturation from the Swedish Forest Inventory. Model output was compared with available measurements of soil water chemistry.     

Effects of Acid Deposition on Forest Soils in Northernmost Russia: Modelled and Field Data

In addition to strong natural stresses forest ecosystems in the Kola Subarctic, Russia, receive high loads of sulphur and heavy metals from the nickel smelter. To estimate soil response to acid deposition we compared the soil field data along a pollution gradient and simulated time effects. Multivariate technique was applied to investigate spatial distribution of soil field data. Time response of soils to acid deposition was evaluated with the SMART model. According to field observations there is no evidence for strong soil acidification effects close to the smelter. Concentrations of exchangeable Ca and base saturation increase, while acidity decrease in lower soil mineral horizons towards the pollution source. However, some features seem to reflect the early stages of the started acidification. Most soil profiles have low pH values. Despite increasing of exchangeable Ca and Mg towards the smelter in lower mineral horizons due to geological inheritance, they do not reveal the same trends in the upper ones. Concentration of exchangeable K in organic horizons decreases towards the smelter, thus confirming the starting acidification. As result, exchangeable base cations are depleted in the considerable part of shallow soil profiles. According to model simulation the present acid load does not effect considerably on forest soils in background areas, however, dramatic shift in soil chemistry near the smelter is expected within several decades. Due to low pool of exchangeable base cations and low weathering rate continued acid deposition can lead to increased soil acidification and nutrient imbalance.     

Changes in soil properties and site productivity caused by red alder

Red alder (Alnus rubra Bong.) is well recognized as an effective host plant for the symbiotic fixation of N. While this fixation process leads to the rapid accumulation of N within the ecosystem, it also enhances nutrient accumulation in biomass and soil organic matter and increases nitrification and cation leaching. We hypothesized that changes in soil properties resulting from these processes would decrease site productivity for second rotation red alder. Adjacent stands of 55 yr old alder and Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) were studied at the Thompson Research Center on the Cedar River Watershed in western Washington, USA.The presence of red alder caused the following soil changes: decreased soil solution pH, increased CEC, increased exchangeable acidity accompanied by a decreased soil pH and base saturation. This decreased soil and soil solution pH resulted in increased A1 concentration in the soil solution and on exchange sites as well as decreased P availability. To determine the effect of these changes on the productivity of the 2nd rotation alder forest, a species conversion experiment was initiated 5 yr ago. Results from this conversion study clearly indicated that the first rotation red alder forest has caused a relative decrease in the productivity of the second rotation red alder plantation. Compared to the growth of red alder on the former Douglas fir site, the second rotation red alder on the former red alder site exhibited 339 less height growth and 759, less aboveground biomass accumulation after 5 yr. Future research will focus on identifying those factors causing this lower productivity including P availability, soil acidity and Al toxicity, cation availability, and competition with other vegetation.     

Changes in soil properties and site productivity caused by red alder

Red alder (Alnus rubra Bong.) is well recognized as an effective host plant for the symbiotic fixation of N. While this fixation process leads to the rapid accumulation of N within the ecosystem, it also enhances nutrient accumulation in biomass and soil organic matter and increases nitrification and cation leaching. We hypothesized that changes in soil properties resulting from these processes would decrease site productivity for second rotation red alder. Adjacent stands of 55 yr old alder and Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) were studied at the Thompson Research Center on the Cedar River Watershed in western Washington, USA. The presence of red alder oaused the following soil changes: decreased soil solution pH, increased CEC, increased exchangeable acidity accompanied by a decreased soil pH and base saturation. This decreased soil and soil solution pH resulted in increased A1 concentration in the soil solution and on exchange sites as well as decreased P availability. To determine the effect of these changes on the productivity of the 2nd rotation alder forest, a species conversion experiment was initiated 5 yr ago. Results from this conversion study clearly indicated that the first rotation red alder forest has caused a relative decrease in the productivity of the second rotation red alder plantation. Compared to the growth of red alder on the former Douglas fir site, the second rotation red alder on the former red alder site exhibited 33% less height growth and 75% less aboveground biomass accumulation after 5 yr. Future research will focus on identifying those factors causing this lower productivity including P availability, soil acidity and Al toxicity, cation availability, and competition with other vegetation.     

Acid deposition and forest nutrient imbalances: A modeling approach

Leaching of nutrients in areas with high acid deposition could be one of the causes to the nutritional imbalance observed in the tree leaf biomass in these areas. A simulation model is developed in order to study the interaction between soil and vegetation in acidified forests. A tree growth model, based on the nutrient productivity concept, is supplemented with soil processes to form an integrated plant-soil model containing the most important soil processes for acidification. The model is able to link decreased Mg availability in the soil and reduced growth caused by Mg deficiency. Simulation results show that the time and pH at which the stand will start to experience Mg deficiency is dependent on quantity and type of acid deposition, growth rate of the stand, initial base saturation and initial fraction of Mg on the cation exchange complex. The total amount of Mg initially present is not decisive for the time at which Mg deficiency will appear. The conceptual idea of nutrient imbalances in forest stands caused by acid deposition induced leaching is supported by this model.     

Nitrogen loading levels affect abundance and composition of soil ammonia oxidizing prokaryotes in semiarid temperate grassland

Purpose  

Global nitrogen deposition has profound impact on the terrestrial ecosystem including the semiarid temperate grassland, causing vegetation community shifts and soil acidification. Little is known regarding the effect of nitrogen (N) deposition on the belowground microbial communities. This study aimed to examine the response of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to added N in semiarid temperate grassland.     

Alberta油砂地区在两种水文流域森林土壤酸化敏感性研究

Input of large amounts of N and S compounds into forest ecosystems through atmospheric deposition is a significant risk for soil acidification in the oil sands region of Alberta. We evaluated the sensitivity of forest soils to acidification in two watersheds （Lake 287 and Lake 185） with contrasting hydrological regimes as a part of a larger project assessing the role of N and S cycling in soil acidification in forest ecosystems. Fifty six forest soil samples were collected from the two watersheds by horizon from 10 monitoring plots dominated by either jack pine （Pinus banksiana） or aspen （Populus tremuloides）. Soils in the two watersheds were extremely to moderately acidic with pH （CaCl2） ranging from 2.83 to 4.91. Soil acid-base chemistry variables such as pH, base saturation, Al saturation, and acid-buffering capacity measured using the acetic acid equilibrium procedure indicated that soils in Lake 287 were more acidified than those in Lake 185. Acid-buffering capacity decreased in the order of forest floor 〉 subsurface mineral soil 〉 surface mineral soil. The most dramatic differences in percent Ca and Al saturations between the two watersheds were found in the surface mineral soil horizon. Percent Ca and Al saturation in the surface mineral soil in Lake 287 were 15% and 70%, respectively; the percent Ca saturation value fell within a critical range proposed in the literature that indicates soil acidification. Our results suggest that the soils in the two watersheds have low acid buffering capacity and would be sensitive to increased acidic deposition in the region.     

The Role of Phosphorus Availability in the Response of Soil Nitrogen Cycling,Understory Vegetation and Arbuscular Mycorrhizal Inoculum Potential to Elevated Nitrogen Inputs

The impacts of increased nitrogen (N) inputs into temperate ecosystems via atmospheric nitrogen deposition on nitrogen cycling and nitrogen retention have been described in a variety of ecosystem types. The role of secondary nutrients such as phosphorus (P) in ecosystem responses to increased N inputs is less well-understood. N and P availability are likely to interact to influence ecosystem productivity and N cycling rates, and this interaction would be expected to vary as N inputs increase. Furthermore, N and P inputs may affect plant-mycorrhizal associations and the ability of arbuscular mycorrhizae (AM) to colonize roots. We added nitrogen (97 kg ha^-1 yr^-1) and phosphorus (30 kg ha^-1 yr^-1) to an oak-maple forest in southwestern Virginia (U.S.A.) from 1994 through 1996. Inorganic nitrogen concentrations, net nitrogen mineralization, net nitrification rates and arbuscular mycorrhizal inoculum potential (MIP) were assessed during the growing season in 1996. Responses of the understory vegetation and soil N cycling to N addition suggested that the ecosystem was strongly N-limited. Nitrogen cycling rates were not affected by P inputs, though P addition increased P availability and decreased MIP. It was hypothesized that P availability may have more significant influences on N cycling and the plant-mycorrhizal association in ecosystems showing stronger symptoms of nitrogen saturation. Results suggest that the use of P fertilization would be effective in alleviating P-deficiency in vegetation receiving elevated atmospheric N deposition, but perhaps at the cost of benefits that associations with arbuscular mycorrhizae provide.     

Experimental acidification of alpine catchments at Sogndal,Norway: Results after 8 years

Manipulations with whole catchments were initiated in Norway in 1983 (RAIN project Reversing Acidification In Norway) to obtain direct experimental evidence relating to the reversibility of soil and water acidification, rate of change, and the relative roles of sulfur and nitrogen. We present here results for soil and runoff chemistry during 8 years of acid addition at Sogndal, a pristine acid-sensitive site in central Norway characterized by gneissic bedrock, thin and patchy soils, and alpine vegetation. Catchment SOG2 receives 100 meq m^?2 yr^?1 H₂SO₄, catchment SOG4 receives a 1∶1 mixture of H₂SO₄ and HNO₃, while catchments SOG1 and SOG3 serve as untreated controls. Acid is applied to the snowpack in April and in 5 portions of 11 mm of pH 3.2 acidified lakewater during the snowfree period. The 8-years of acid addition have caused major changes in runoff chemistry. Concentrations of sulfate and base cations have increased while acid neutralizing capacity (ANC) has decreased. Henriksen's F-factor (change in concentration of non-marine Ca+Mg divided by change in concentration of non-marine SO₄) is about 0.35, but is expected to decrease as soil acidification proceeds. Runoff is acidic, aluminum-rich, and toxic to fish and other aquatic organisms. Repeated soil sampling indicates no dramatic trends related to treatment. Year-to-year variations are large, and mask changes expected. The input-output budgets indicate that over the 8-yr period Ca has been depleted by about 5% of the total soil pool of exchangeable Ca. The observed trends are consistent with response predicted by MAGIC, a process-oriented model of soil and water acidification. The gradual increase in nitrate flux from catchment SOG4 may be the first indication of ‘nitrogen saturation’ induced simply by increasing nitrogen deposition.