Pertti J. Martikainen

Nitrous oxide production and nitrification in acidic soil from a dutch coniferous forest

Abstract The production of nitrous oxide and nitrification were studied using aerobic incubations of soil sampled from the F-layer of a nitrogen-saturated Douglas fir stand (The Netherlands). The nitrous oxide production (3 ng N 2 O-N g −1 dry soil h −1 ) in the acid soil (pH 3.7) could be inhibited by 90% using low (3 Pa) or high (2.5 kPa) acetylene concentrations. The experiments with aerobic soil slurries showed that N;0 production at pH 4 was 4–8 times higher than at pH 6. Also the nitrification rate was higher at pH 4 than at pH 6. The ratio of N 2 O-N to NO 3 − -N was higher at pH 4 (0.33–0.47%) than at pH 6 (0.04–0.18%). Acetylene (2.5 kPa) or nitrapyrin (10μgml −1 ) inhibited N 2 O (40–96%) and NO 3 − production (79–100%) in the soil slurries indicating the importance of chemolithotrophic bacteria. The results showed that N 2 O is produced in a nitrogen-saturated acid forest soil in North Western Europe even if well-aerated. The aggregated ammonium oxidizers ( Nitrosospira ) that are responsible for ammonium oxidation in these acid soils may have importance in N 2 O production. The possible significance of the aggregation of Nitrosospira cells for N 2 O production is discussed.

Evaluation of the fumigation-extraction method for the determination of microbial C and N in a range of forest soils

The release of extractable C and N by chloroform fumigation of 10 forest and two arable soils was compared with microbial C measured by direct microscopic counting (membrane filter technique). The soils varied with respect of pH (3.9–6.8) and content of organic C (2.6–36%). The amount of microbial C based on microscopic counts ranged from 332 to 1342 μ g cm−3. The release of C caused by fumigation correlated well with the results obtained from direct microscopic counting (R2 = 0.87). The regression model calculated for microbial C is Cm = (1.30 Cf + 309) μg cm−3 soil, where subscript m refers to the values for microscopic counting and subscript f to the release caused by fumigation. Assuming that fumigation causes a release of microbial N in the same proportion as for microbial C, the following regression model for microbial N was obtained: Nm (1.38 Nf + 45.3) fig cm−3 soil. The average amount of microbial C determined with the fumigation-extraction method for coniferous forest soils, deciduous forest soils and arable soils was 1.19, 1.13 and 1.40% of total C, respectively. The avarage amount of microbial N in these soils was 5.9, 3.4 and 2.5% of total N. The results suggest that the fumigation-extraction method can be used for the determination of microbial C and N in different types of forest soil.

Nitrification in two coniferous forest soils after different fertilization treatments

The effects of seven different fertilization treatments on nitrification in the organic horizons of a Myrtillus-type (MT) and a Calluna-type pine forest in southern Finland were studied. No (NO−3 + NO−2)-N accumulated in unfertilized soils during 6 weeks at 14 or 20°C in the laboratory. Net nitrification was stimulated by urea in both soils (but more in the MT pine forest soil) and to a lesser degree by wood ash but not by ammonium nitrate or nitroform (ureaformaldehyde). Nitrification was not detected in nitroform fertilized soils although ammonium accumulation was high during incubation. In the MT pine forest soil, net nitrification appeared to be stimulated by apatite, biotite and micronutrients. Nitrapyrin inhibited nitrification indicating that it was carried out by autotrophic nitrifiers. In the urea-fertilized MT pine forest soil, nitrification took place at an incubation temperature of 0°C. Accumulation of (N0−3 + NO−2)-N was highest in soil sampled at < 10°C.

Mineralization of carbon and nitrogen in soil samples taken from three fertilized pine stands: Long-term effects

Seven years after fertilization the rate of CO2 production in the soil samples taken from the organic horizons of a poor pine forest site (Calluna vulgaris site type), treated with urea or ammonium nitrate with lime, was lower than that in the unfertilized soil. The same trend was also observed in samples of theEmpetrum-Calluna site type 14 years after fertilization. In the more fertileVaccinium myrtillus site type these rapidly-soluble N fertilizers had a long-term enhancing effect on the production of CO2. Apatite and biotite eliminated the decreasing effect of urea on the production of CO2. One reason for this might be the long-term increase in soil pH caused by apatite and biotite, or their constituents (Ca, Mg, K, P). Nitroform (a slow-releasing N fertilizer) had no statistically significant effect on the production of CO2 in soil samples from any of the forest types. Despite the high N mineralization in the samples from nitroform fertilized soils there was no nitrification, and the high content of total N indicated that after nitroform fertilization the losses of N were low.The correlation between the net mineralization values for C (CO2 production) and N was poor. However, multiple linear regression analysis, which also took into account the effect of nutrients and pH, indicated that there was a link between the mineralization of C and N.

Numbers of autotrophic nitrifiers and nitrification in fertilized forest soil

Abstract The most probable number (MPN) method was used to estimate how numbers of autotrophic nitrifiers in Myrtillus-type and Calluna-type pine forest soils in southern Finland were affected by seven different fertilization treatments. No NH + 4 oxidizers and only a few hundred NO 2 −1 oxidizers g −1 of soil were found in unfertilized organic (O) horizons. Ammonium nitrate and nitroform (ureaformaldehyde) had hardly any effect on the nitrifiers. Urea, alone or applied together with apatite + biotite or with apatite + biotite + micronutrients, increased numbers of NH 4 + and NO 2 − oxidizers. Wood ash, alone or with apatite, also had a stimulative effect. The effects of the stimulative fertilizers were less in the A 2 horizon than in the O horizon. The MPN counts were considerably affected by the duration of incubation: counts of NH 4 + oxidizers kept increasing for at least 8 weeks and counts of NO 2 − oxidizers for at least 15 weeks. These MPN counts were compared with earlier results from incubation experiments on the same soils to find out how they reflect changes in soil nitrification after fertilization.

Methane oxidation in soil profiles of Dutch and Finnish coniferous forests with different soil texture and atmospheric nitrogen deposition

Abstract We studied methane oxidation capacity in soil profiles of Dutch and Finnish coniferous forests. The Finnish sites ( n = 9) had nitrogen depositions from 3 to 36 kg N ha −1 a −1 . The deposition of N on the Dutch sites ( n = 13) was higher ranging from 50 to 92 kg N ha −1 a −1 . The Dutch sites had also limed counterparts. Methane oxidation rates were determined by incubating soil samples in the laboratory at + 15°C with 10 μl CH 4 l −1 (10 ppmv CH 4 ). In general, CH 4 oxidation rates were highest in the uppermost mineral layers. The average CH 4 oxidation rate in the Finnish mineral soils was three times higher than that in the Dutch soils. The litter layers did not oxidize CH 4 . In the Netherlands all organic horizons had a negligible capacity to oxidize CH 4 . However, some Finnish organic horizons showed high CH 4 oxidation capacity. In the Netherlands, in contrast to Finland, there were some soil profiles lacking CH 4 oxidation. Higher contents of nitrate and ammonium, as well as greater production of nitrous oxide (N 2 O) and lower production of carbon dioxide in the Dutch than in the Finnish forest soils reflected the high N deposition rate in the Netherlands. Not only the N deposition, but also the highly sorted soil texture (fine sand) with low amounts of both coarse and fine particles is an important reason for the low CH 4 oxidation in the Dutch soils. The proportions of fine and coarse particles, both well represented in moraine soils typical in northern Europe, correlated positively with the CH 4 oxidation. Fine particles provide a good surface for microbial growth. Coarse particles, on the other hand, enhance diffusion of CH 4 and oxygen into the soil. Methane oxidation in the Dutch mineral soils was slightly enhanced by liming.

Nitrification in forest soil of different pH as affected by urea, ammonium sulphate and potassium sulphate

Abstract Nitrification was inhibited by ammonium sulphate and potassium sulphate added to soil from the organic horizon (pH 4.7) of a Myrtillus-type pine forest. Urea did not inhibit nitrification. Soil pH was slightly decreased by the salts but increased by urea. The salts increased soil electrical conductivity more than urea did. The inhibition of nitrification following salt treatments was probably due to a decrease in soil pH and not to osmotic effects. In acid conditions, the salts had a less inhibitory effect on CO 2 production than on nitrification, indicating that nitrifying bacteria were more sensitive than other organisms to the salts.

Mineralization of C and N and nitrification in Scots pine forest soil treated with nitrogen fertilizers containing different proportions of urea and its slow-releasing derivative, ureaformaldehyde

The effects of fast-release urea (U), slow-release ureaformaldehyde (Nitroform, NF) and a mixture of the two in different proportions (NF100%, NF80%, NF50%, NF25%) on soil microbial activities were studied. Urea fertilization enhances nitrification which can cause nitrogen leaching. Our aim was to find whether it is possible to avoid unwanted effects of urea by using mixtures of organic N compounds of different solubility. Net nitrification and mineralization of N and C were determined during aerobic laboratory incubation of soil samples taken 3 months after fertilization. The numbers of autotrophic nitrifiers were estimated by a Most Probable Number (MPN) method and the number of heterotrophic bacteria by plate count. Ion-exchange resin bags placed in the soil below the organic horizon were used to estimate movements of nutrients in soil. The soil pH and the number of heterotrophic bacteria increased in soils receiving high amounts of fast-release N, but CO2 production did not increase. The concentration of exchangeable NH4+ and the number of ammonium oxidizers in the soil were related to the proportion of urea in the applied mixtures; highest numbers were found in the urea and lowest numbers in the NF100-treated soils. The low amount of NO3− found in the soil and accumulated in the resins indicates low nitrification activity in this site. However, the number of ammonium oxidizers indicates that the increase in nitrifier population correlates with the release rate of NH4+. The slow-release NF did not increase nitrification activity and it had a positive effect on microbial activity even at high doses, hence it seems a possible alternative as a slow-release N fertilizer in forestry.

Nitrification in forest soil after refertilization with urea or urea and dicyandiamide

Abstract The effects of urea refertilization on nitrification activity and the influence of a nitrification inhibitor, dicyandiamide, added together with urea were studied in an acid forest soil. Net nitrification and mineralization of N were determined by aerobic incubation of soil samples and numbers of autotrophic nitrifiers by the most probable number (MPN) method. Refertilization with urea enhanced nitrification activity more than single urea application. The higher number of autotrophic nitrifiers in the previously fertilized soil compared to the soil receiving urea for the first time indicated that a nitrifier population was built up after the first urea addition. Dicyandiamide application seemed to help sustain raised soil nitrogen concentrations after urea fertilization since in the previously fertilized soils the ammonium concentrations were higher after urea and dicyandiamide treatment than after refertilization with urea alone.

Mineralization of carbon and nitrogen in acid forest soil treated with fast and slow-release nutrients

The effects of slow (apatite, biotite) and fast-release nutrients (P, K, Mg) on C and N mineralization in acid forest soil were studied. These nutrients were applied alone or together with urea or urea and limestone. The production of CO2 in the soil samples taken one and three growing seasons after the application was lower in the soils treated with the fast-release nutrients than in the untreated soils. Similar reduction of microbial activity was not seen after the apatite and apatite+biotite treatments. In the first growing season, urea and urea+limestone enhanced CO2 production, but after three growing seasons, the opposite was true. Apatite and apatite+biotite added together with urea did not compensate for the decreasing effect of urea on the CO2 production. The addition of fast-release salts increased somewhat the concentration of NHinf4sup+in the soil and more NH4+ accumulated during laboratory incubation in the soil samples taken one growing season after the application. The urea addition immediately increased the concentrations of NH4+ and of NO3− in the soil, but, three growing seasons after application, urea had only a slight increasing effect on mineral N content of the soil. Slow-release nutrients seem to have a more favourable effect than fast-release salts on nutrient turnover in acid forest soil.