Hans-Örjan Nohrstedt

Nitrogen losses and soil water acidity after clear-felling of fertilized experimental plots in a Pinus sylvestris stand

Abstract Studies of losses of nitrogen to water and air (denitrification) were performed during 3 years following a clearcut of a previously heavily fertilized Pinus sylvestris stand. During a 20-year period prior to the harvest, plots had been fertilized three times with different doses of ammonium nitrate (0–600 kg N ha−1 each time). Soil studies conducted during the last summer before clearcutting showed that the soil nitrogen pool had almost doubled in some plots. Organic nitrogen was the dominant nitrogen fraction in the soil water in most plots. This fraction was independent of the fertilizer dose. Before harvesting and during two subsequent years, nitrate only occurred at the plots which had received the maximum dose (3 × 600 kg N ha−1). The concentrations of the different nitrogen fractions in soil water tended to decrease on all plots from pre-clearcut conditions during the first 2 years. During the third year, which had normal precipitation, increases in nitrate were recorded at the plots with the three highest doses. This was the case especially for the highest dose, for which the concentration rose from 0.24 mg nitrate-N l−1 in the previous year to 2.2 mg l−1. The results as regards the effect of the clearcut on soil water pH were somewhat contradictory, but a minor pH increase was indicated. Before clear-felling there was no correlation between pH of soil water and fertilizer dose. During the years following the clearcut, pH was negatively correlated with dose. The reason for these changes could not be explained solely by nitrification. Losses of N2O-N were estimated to be less than 0.1 kg ha−1 year−1 during the first 2 years after clearcut. Denitrification measured with the acetylene block technique revealed no higher values, pointing to an insignificant N2 loss. The losses were independent of the previous fertilizer dose. Despite a huge build-up of the nitrogen storage from previous fertilizations, nitrogen losses to water and air were insignificant during the first 2 years after the harvest. During the third year, which was wetter than the two previous years, increased nitrate concentrations were recorded for the highest dose for the first time. Owing to the cold site and the low precipitation for 2 years after the clearcut, further measurements will be necessary before more definite conclusions can be made.

Effects of stump-treatment substances for root-rot control on ground vegetation and soil properties in a Picea abies forest in Sweden.

There is increasing commercial interest in treating stumps to restrict the spread of root rot [Heterobasidion annosum (Fr.) Bref.] when thinning Swedish coniferous forests during the summer. Both chemical and biological substances are used for this purpose. During the treatment, however, a large proportion of the substance applied is spilled beside the stumps. A field study was conducted on the effects of stump-treatment substances on various ground-vegetation species in a Picea abies (L.) Karst. forest in Sweden. Three different substances commonly used in forestry were studied: urea solution (0.23 kg N m-2), borate solution (10 g B m-2) and a fungal preparation of Phlebiopsis gigantea (Fr.) Jül. spores (1 g spores m-2). The principal objectives were to assess whether any of the substances were harmful to plants and whether plant species differed in their sensitivity. Both borate and urea solution caused severe damage to most ground-vegetation species tested. Bryophytes were affected more strongly than vascular plants and urea was slightly more toxic than borate. Treatments with P. gigantea caused no obvious damage. The size and persistence of chemical changes in the soil induced by the treatments were also analysed. Transient changes were apparent in topsoil where borate or urea had been added. Very high concentrations of B were initially observed where borate had been applied, and even after 1 yr they were slightly higher than the threshold concentration at which plant injuries are expected. Urea treatment initially resulted in a pH increase of 2 units and a substantial increase in soil ammonium content. After 1 yr these effects had largely disappeared, although some increase in ammonium was still detectable.

EFFECTS OF LIMING AND FERTILIZATION (N, PK) ON CHEMISTRY AND NITROGEN TURNOVER IN ACIDIC FOREST SOILS IN SW SWEDEN

SW Sweden has very acidic forest soils because of deposition ofair-borne pollutants. Large-scale liming and fertilization have been proposed as countermeasures against a possible future development of forest decline. To test the effects of suggested treatments, liming (3 or 6 t ha1) and fertilization with easily soluble PK (25 or 50 kg P, 80 or 160 kg K ha1) or N(20 kg N ha1 annually in the form of NH4 NO3) were applied in different combinations in four experiments in 30–60 yr-old Picea abies forests in SW Sweden. Four yearsafter the initial application of the fertilizers, samples were taken from the O-horizon and the two uppermost 5 cm thick layersof the mineral soil. Their pH(H2O) and easily extractable Ca, Mg, K, P and inorganic N contents were analyzed. Samples werealso incubated to estimate net N mineralization and potential nitrification rates. Liming increased the pH by 0.6–1 unit in the O-horizon, and by 0.1 unit in the mineral soil. The Ca + Mg content increased by 15–25 kmolc ha1 (4–8 foldincrease) in the O-horizon of the limed plots, while an increaseof 5 kmolc ha1 (two-fold increase) was observed in theuppermost 5 cm of the mineral soil. Liming did not affect extractable P, K or inorganic N contents. Net N mineralization and potential nitrification rates in the O-horizon were enhanced 1.5- and 6-fold, respectively, by liming, but it had no apparenteffect in the mineral soil. N fertilization caused a slight increase (1.5 kg ha1) in the content of inorganic N, buthad no effects on the other variables measured. The amount ofextractable P was raised by 16 kg ha1 in plots given the high P dose (50 kg ha1), but no other effects of PK fertilization were detected.

Effect of liming and N-fertilization on denitrification and N2-fixation in an acid coniferous forest floor

Abstract The effect of dolomite on denitrification and nitrogen fixation was studied in a field experiment in central Sweden. The experiment had a split-plot design, including fertilization with ammonium nitrate. Samples were taken from the forest floor during the growing season following the last treatment. Addition of dolomite (2800 kg ha−1) raised the pH of the forest floor (3 cm thick) by 0.7-1.1 unit from an original level of 4.4. The lower figure was obtained when lime was added in the same year as analysis and the higher figure when liming took place the previous year. Fertilization with ammonium nitrate decreased the pH by 0.3-0.4 units at the first two samplings. At the third sampling pH had risen 0.3 units, and at the final sampling 4 months after the fertilization there was no remaining effect. Nitrous oxide could not be detected in any of the cores that were incubated. This also applied to the nitrogen-fertilized plots. Although cores were taken soon after rainfall, the air-filled pore space never decreased below 75%, which probably constitutes the major limiting factor for denitrification in this field experiment. Strong effects on nitrogen fixation by lime and fertilizer were indicated. Lime increased nitrogen fixation in a close relationship with what could be predicted from the pH-increase. Fertilization was strongly inhibitory in nearly all cases, even when the concentration of ammonium and nitrate had decreased to about the control level.

The fate of 137Cs in coniferous forests following the application of wood-ash.

In the future, it may become common practice in Swedish forestry to recycle wood-ash, a waste product of the combustion of bio-fuel. As a consequence of the Chernobyl radioactive fallout in 1986, large areas of central Sweden were contaminated. Application of recycled wood-ash, originating from contaminated areas, to a previously uncontaminated forest, risks an increase in the concentration of radioactive 137Cs. We measured 137Cs radioactivity in different parts of coniferous forests in seven field experiments. Measurements of radioactivity were made 5-8 years after an application of wood-ash equivalent to 3000 kg ha(-1). The sites, in a north-south transect across Sweden, have a background radioactivity ranging from 0 to 40 kBq m(-2), the higher levels are mainly a result of the Chernobyl fall-out. Depending on its origin, the radioactivity of the applied wood-ash ranged from 0.0 to 4.8 kBq kg(-1), corresponding to 0.0-1.44 kBq m(-2). In autumn 1999, samples were taken from the soil, field vegetation, needles and twigs and the levels of 137Cs determined. In addition, soil samples were analysed for extractable K. The highest 137Cs concentration was found in the soil. At six of the seven sites there were no statistically significant effects of wood-ash application on 137Cs activity. This was despite the fact that the wood-ash had, in one case, added the same amount of radioactivity as the background. However, at one site with intermediate 137Cs deposition (10-20 kBq m(-2)), there was a statistically significant decrease in 137Cs radioactivity in the soil, needles and twigs from the plots treated with wood-ash. The decrease in radioactivity was partly due to the fact that one of the main constituents of wood-ash is K, which is antagonistic to 137Cs. Based on our results, it appears that application of wood-ash containing 137Cs does not necessarily increase the 137Cs radioactivity in plants and soil. However, some of the observed effects could be a result of the low number of replicates used in this study.