Hans‐Örjan Nohrstedt

Response of Coniferous Forest Ecosystems on Mineral Soils to Nutrient Additions: A Review of Swedish Experiences

Nitrogen (N) is the only nutrient that promotes forest growth when given individually. An extra stem growth of 15 m 3 ha -1 is obtained during a 10 yr period following an application of 150 kg N ha -1 . Larger growth increases have often been the result of more intensive N fertilization. Lime or wood ash give a minor growth stimulation on sites with a carbon (C) to N ratio below 30 in the humus layer, while the opposite effect prevails on N-poor sites. Nutrients given as soluble fertilizers are readily taken up by trees. Boron deficiency may be induced in northern Sweden after N fertilization or liming. The ground vegetation may be altered by single-shot N fertilization, but long-term effects occur only for intensive regimes. Lime or wood ash may modify the flora if soil pH is significantly altered: the change will be in response to N availability. Fruit-body production of mycorrhizal fungi is disfavoured by chronic N input, but also by lime or ash. However, the mycorrhizal structures on root tips are less affected. Faunistic studies are not common and those present are mostly devoted to soil fauna. A practical N dose of 150 kg N ha -1 has no clear effect, but higher doses may reduce the abundance in some groups. Hardened wood ash does not significantly affect the soil fauna. Lime favours snails and earthworms, while other groups are often disfavoured. The response of aquatic fauna to terrestrial treatments has hardly been studied. N fertilization generally results in insignificant effects on fish and benthic fauna. Lime and wood ash reduce the acidity of the topsoil, but practical doses (2-3 t ha -1 ) are too low to raise the alkalinity of runoff unless outflow areas are treated. N fertilizer use in forestry and N-free fertilizers lack effects on acidification. N fertilization may, however, be strongly acidifying if nitrification is induced and followed by nitrate leaching. N fertilization often results in increased long-term C retention in trees and soil, but does not promote significant N 2 O losses. N may temporarily reduce CH 4 oxidation in soil, but there are indications of a long-term opposite effect. Lime and poorly hardened wood ash may cause losses of C from N-rich soils. Only a few per cent of added N are leached to surface water following practical N fertilization, while N-free fertilizers do not induce N leaching. Soil incubations and soil-water studies suggest an increased risk for nitrate formation and leaching where lime or wood have been added to N-rich soils, but increased leaching to surface water has not been observed. Wood ash causes a temporal increase in bioavailability of cadmium (Cd). Other fertilizers may indirectly increase the availability of heavy metals. Wood ash may contain radioactive caesium 137Cs, but addition of such ash does not increase radioactivity in plants and soil.

Main Findings and Future Challenges in Forest Nutritional Research and Management in the Nordic Countries

Previous research has documented that nitrogen fertilization can lead to a significant tree growth increase in large parts of Scandinavia. Nutrient management of Nordic forests is currently under debate, mainly because environmental values are believed to be unnecessarily endangered by these actions. Nutrient management including fertilization, liming, recycling of wood ash or application of alternative nutrient sources can counteract nutrient deficiencies and imbalances, and be used for compensating the nutrients removed through harvesting. For successful implementation of nutritional management measures, several questions should still be studied. These questions mainly concern diagnosis of the need for applications, nutritional sustainability and treatment effects on the forest ecosystem and the surrounding environment.

Soil microbial activity in eleven Swedish coniferous forests in relation to site fertility and nitrogen fertilization

The effect of forest fertilization on soil microbial activity and biomass was studied in 11 coniferous forests with different site fertility. Soil microbial activity was measured as respiration rate and the microbial biomass was estimated using ATP content and substrate‐induced respiration rate (SIR). At all sites, except those with the highest site fertility, the microbial activity and biomass were reduced in the fertilized treatments compared with the control. The general trend indicated that the lower the site fertility, the more pronounced was the reduction of microbial biomass and activity. Some possible explanations for this effect are discussed.

Effects of repeated nitrogen fertilization with different doses on soil properties in a Pinus sylvestris stand

Studies of soil and soil water (50 cm) properties were performed six years after the final fertilization in an old field experiment. During a 20‐year period plots had been fertilized three times with different doses of ammonium nitrate. The highest dose was 600 kg N ha‐1, meaning that 1800 kg N ha‐1 was given during the period. A substantial accumulation of organic matter had occurred in the forest floor as a result of the fertilization. At the dose giving the maximal response the humus layer had almost doubled its content of carbon and nitrogen, when comparing with the control. The pH was unaffected by the dose in the humus layer and in the eluvial layer. In the upper part of the spodic horizon the pH had decreased 0.2–0.3 units at intermediate doses. The amounts of exchangeable base cations were not changed as a result of the fertilization. The pH of the soil water was similar over all doses. The concentration of nitrate‐N was below the detection limit for all doses except for the highest.

Soil water chemistry as affected by liming and im fertilization at two Swedish coniferous forest sites

Lime (1 000 kg/ha), NH4NO3 (150 kg N/ha) or a combination of both, were added to plots in two middle‐aged coniferous forest stands. Lysimeter studies were performed during four to six years after treatment. The concentration of inorganic N did not increase after addition of lime only. NH4NO3 fertilization, alone or in combination with lime, temporarily increased inorganic N, especially NO3. All treatments increased the acidity during the whole observation period when compared to the control. Addition of lime did not increase concentrations of Ca and Mg in the soil water. In contrast, N fertilization significantly increased the concentrations of both elements. The increases followed exactly the pattern of increases in NO3 concentrations. More research is needed to explain soil water acidification by a low lime dose. Application practices as regards placement within different parts of watersheds are an important subject to study. With our present knowledge, a low dose of lime cannot be recommended to counte...

Soil chemistry in a Pinus sylvestris stand after repeated treatment with two types of ammonium nitrate fertilizer

Soil chemistry was studied in a field experiment in a 50‐year old Pinus sylvestris stand. Plots had been fertilized five times during a 20‐year period with a total of 810 kg N ha‐1 in the form of ammonium nitrate, either with or without dolomite enrichment (totally 120 kg Ca and 60 kg Mg ha‐1). The experiment had a randomized block design, including a total of six replicates. Soil sampling was performed in 1989, 14 months after the latest fertilizer addition. Samples were taken from the mor layer, the eluvial layer, and the three. top 3‐cm layers of the spodic horizon. Both fertilizers increased concentrations and amounts of C and N in the mor layer. They also resulted in a small increase of the pH in this layer (0.2–0.3 units). In the uppermost 3 cm of the spodic horizon, a very minor decrease was detected ( ≤ 0.1 unit). Despite the pH increase in the mor layer, the amount of titrable acidity tended to increase in this layer (30% over control for both fertilizers; not significant). For the whole analysed...

Nitrogen fixation (C2H2‐reduction) in birch litter

Nitrogen fixation (C2H2) by free‐living heterotrophic microorganisms in the upper part of the forest soil (0–3 cm) was, on average, twenty times higher below birch than below pine or spruce. The highest fixation that occurred was 1.4 kg Nha−1 during the growth period. In most cases the fixation of nitrogen was concentrated to older leaf litter. No activity could be detected in green leaves taken from the canopy or in recently fallen yellow leaves. Relatively low activities in the soil were obtained at sites in the southern part of Sweden, which corresponded to elevated concentrations of lead and copper in the green birch leaves.

Effects of Previous High N Addition on Nutrient Conditions in Above-ground Biomass of a Picea abies Stand in Sweden

Deposition of air pollutants, together with intensified harvesting practices, has led to concern about the declining base cation status and the acidifying effects in the soil, with possible implications for silviculture. The aim of this study was to analyse the effects of a previous high N addition on the nutritional status in the above - ground part of a tree stand. We sampled various biomass fractions in a highly productive Norway spruce (Picea abies (L.) Karst.) stand in the province of Värmland, western Sweden. The experimental plots had been fertilized with urea four times at intervals of 5 yrs, the last time being 13 yrs before our study. The total amount of N added was in the range 480 - 2400 kg ha?1. Despite the high N addition and an insignificant growth increase following N input, there were no obvious residual changes of nutrient concentrations in the above - ground parts of trees.

Effects on water chemistry, benthic invertebrates and brown trout following forest fertilization in Central Sweden

Two coniferous forest drainage areas in central Sweden were partially fertilized with ammonium nitrate and calcium ammonium nitrate respectively, both at a dose of 150 kg N ha‐1. During the following years observations were made on stream water chemistry, invertebrates and brown trout (Salmo trutta L.). Upstream stations were used as controls. Very high concentrations of inorganic N (up to 45 mg l‐1) were recorded immediately after the fertilization. Thereafter, concentration decreased rapidly but remained elevated during the whole study period. Acidity conditions (pH, alkalinity, aluminium) were unaffected by both treatments. The only registered effect on the benthic fauna was a three‐ to five‐fold increase of drifting invertebrates during the first four‐five days after the treatment. However, this did not reduce the population density at the treated stations. No effects on population of trout were recorded.

A field study on forest floor respiration response to artificial heavy metal contaminated acid rain

Over a period of five months, the forest floor of a coniferous stand was irrigated 20 times with a total of 100 mm of artificial rain in addition to the normal precipitation (302 mm). The experiment was made with a fully randomized two‐factorial design. The two treatment factors were acid (a mixture of 2/3 sulphuric and 1/3 nitric acid) and heavy metals (cadmium and copper). Both factors were tested at three levels, whereby altogether nine combinations of acid and metal load could be studied. The highest load corresponded to 100 times the actual wet deposition of acid and heavy metals. The forest floor respiration was measured on intact cores (radius and depth = 3 cm) respectively one day after and 7 months after the final irrigation which took place in October. There was no statistically significant effect on respiration of any of the treatments examined. The pH of the forest floor was slightly lowered by the most acid treatment. This effect was statistically significant four days after the final treatme...