Folia Miorobiol. 24, 403--407 (1979)

Acetylene Reduction (Dinitrogen Fixation) and Nitrification in Soil as Affected by the Structural Aggregate Size V. ~KRDLETA, A. HYNDR.~.KOV.~ and M. N~McovX D61aartment of Plant ~Vuttition Physiology, Institute of ~xt~rimental Botany, GzawhoslotJak Academy of Sciences, 166 30 Prague 6 l~eceived Jttcte 26, 197.8

ABSTIIACT. The effect of size of struetural aggregates on the intensity of nitrification and nitrogenase (nitrogen: acetylene oxidoreduetase) activity was investigated in three soile. I n two of them the nitrogenase activity was limited b y addition of glueeee. Aggregates of a larger diameter ( 2 - - 4 mm) exhibited a considerably higher nitrogenece activity t h a n those with a diameter smaller t h a n 2 m m . This effect was even more pronounced when the sell ~amples were repeatedly intensively aerated. On the contrary, smaller aggregates (0.5--2 ram) exhibited more intensive nitrifie~tion.

Nitrification and biological dinitrogen fixation belong to fundamental processes of the nitrogen cycle in nature. With respect to certain biological factors (e.g. soil aeration and level of utilizable carbon and nitrogen source) these processes represent opposite poles. The intensity of nitrification depends on a high degree of soil aeration (Po2) and the level of utilizable ammonium ion as a substrate (Alexander 1977; Sharma and Ahler 1977). Dinitrogen fixation, its reduction at the enzyme and to a certain extent at the cellular level is, on the contrary, associated with anaerobic or microaerophilic conditions. Soil aggregates contain microzones of a different degree of aeration, so t h a t for instance zones of nitrification and denitrification are separated (Greenwood 1962a, b). The effect of size of structural aggregates on nitrification and non-symbiotic dinitrogen fixa$ion under aerobic conditions with nitric-oxide reductase (nitrogen:aceCylene oxidoreductase, nitrogenase, EC 1.7.99.2)was investigated in the present communication. MATERIALS AND METHODS

Soils. Soils were taken in three localities: 1) Reservation K o d a -- rendzina on limestone: samples were taken from horizon A (0--100 mm), humus content 3 ~/o, Nt 0.79 ~/o, Cox 9.96 ~/o, CaCO3 12 0/o; pHKcl 7.2; accessible P~Os 0.6 and K 2 0 13.3 mg/100 g; dominant growth Quercus pubescens without the herbal level; the soil was designated QP.

4114 V. ~ K R D L E T A et al.

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2) K o d a -- rendzina on limestone, cultural soil: samples were taken under lucernegrass t u r f from horizon A; other characteristics were similar to those in soil QP; the soil was designated Koda. 3) Libezniee near Prague -- ehernozem on loess: samples were taken from horizon A (0--300 mm) below the permanent turf, humus content 2.5 ~o, Nt 0.20 %, Cox 1.94 ~ , CaCOa traces; pHKcl 6.8; accessible P205 4.7 and KeO 8.9 mg/100 g soil; the soil was designated Libezniee. Air-dried soil was sieved and divided into four fractions according to the size of structural aggregates (0.5--1; 1--2; 2--3; 3 - - 4 mm). Immediately prior to experiments the dry weight of the individual samples was determined and the soil was wetted with distilled water or enriched with a glucose solution to 25 ~o moisture or supplemented with 1 ~ glucose (W/W). The moisture of the samples was monitored during the experiment and its decrease was compensated b y adding distilled water to constant mass. Wetted or glucose-enriched 10-g samples were incubated in 100-ml serum flasks in a thermostat at 25 • 1 ~ The thermostat was modified as a moist chamber. Nitrogenase assay. After preincubation the flasks were gas-tight closed with rubber stoppers and the nitrogenase activity was assayed (Dart et al. 1975). Reduction of acetylene was followed in two ways: a) Acetylene was added to the samples after a 1, 2, 3, 6, 9 and 14-d preineubation and after determining the production of ethylene the same samples were used for determination of the production of nitrates. b) The samples were incubated every 8 h with acetylene and after each assay of the nitrogenase activity t h e y were intensively evacuated and flushed with air (5 • one-rain intervals). Incubation with acetylene was always 1 h in a water bath at 25 ~ The nitrogene activity was expressed as the amount of ethylene in nmol (g dry w t . ) - l h -1. Sensitivity of detection was 0.6 pmol ml-1. Nitrate determination. Immediately after sampling of the incubation atmosphere a soil water extract (1 : 3, V/W) with a correction for the sample humidity was prepared b y 1-h shaking. Concentration of the nitrate ion was measured potentiometrically in the filtered extract using NOa- ion selective electrode Corning 476109. The method of the external standard and of standard addition were applied (Hubs and Bernagzik 1976). As the results obtained b y the two methods were similar the external standard was used in further experiments. The results were expressed in ppm~o3 soil dry weight and evaluated b y variation analysis on the basis of which values of LSDp

Acetylene reduction (dinitrogen fixation) and nitrification in soil as affected by the structural aggregate size.

Folia Miorobiol. 24, 403--407 (1979) Acetylene Reduction (Dinitrogen Fixation) and Nitrification in Soil as Affected by the Structural Aggregate Size...
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