Summary. Long-term effects of 12 inorganic pollu- tants on N transformations were studied in a sandy cambisol. As, Be, Br, Cd, Cr, F, Pb, Hg, Se, Sn, and.
BiologyandFertility of S o i l s
Biol Fertil Soils (1989) 7:254-258
© Springer-Verlag 1989
Long-term effects of different inorganic pollutants on nitrogen transformations in a sandy cambisol B.-M. Wilke Lehrstuhl fiir Bodenkunde und Bodengeographie, Universit~it Bayreuth und Institut ftir Landschaftsbau TU Berlin, Albrecht-Thaer-Weg 4, D-1000 Berlin 33
Summary. Long-term effects of 12 inorganic pollutants on N transformations were studied in a sandy cambisol. As, Be, Br, Cd, Cr, F, Pb, Hg, Se, Sn, and V were added to the soil as inorganic salts in 1975 and 1976. Soil samples were taken in 1984 to determine total N mineralisation and nitrification. All pollutants except Se and Sn inhibited N mineralisation. The most toxic elements under investigation were Be and Hg. Nitrification was reduced to a lower degree than total N mineralisation. As, Be, Cd, Cr, F, Pb, Se, and Sn failed to inhibit this process at all. It is assumed that nitrifying bacteria became adapted to these pollutants in the course of time. The arginine-ammonification method was less sensitive in detecting the effects of pollutants on N transformation than the N mineralisation test.
Key words: N-mineralisation - Nitrification Arginine ammonification - Inorganic pollutants
In recent decades the soils of industrial countries have been increasingly contaminated with inorganic pollutants, mostly from exhaust gases, mine spills, and application of sewage sludges. Numerous investigations have shown the adverse effects of pollutants on microbial processes in soils (Babich and Stotzky 1980; Tyler 1981; Wilke 1986). Inhibition of microbial activity was mainly influenced by the amount of pollutant added, the type of soil, and the chemical form of the pollutant. Soil N transformation is interesting because almost all the N in epipedons is found in organic compounds. Several workers have investigated the influence of inorganic pollutants, mainly heavy metals, on N mineralisation and nitrification and have reported somewhat mixed results. Tyler et al. (1974) studied the ef-
fects of Cd and Pb salts on nitrification in a mull soil in a 7-week aerobe incubation experiment. It showed a depression in NO~- accumulation with increasing Pb levels up to 10g Pb kg -1 soil. However, with 25 g Pb, slightly more NO~- accumulated than in the controls. Liang and Tabatabai investigated the influence of 19 trace metals on N mineralisation (1977) and nitrification (1978), with the metals added at the level of 5 ~mol g-I, and found inhibitory effects in every case. The most toxic ions were AGO), Hg(II), Cu(II) and Cd(II). Chang and Broadbent (1982) studied the influence of six metals on N mineralisation and nitrification in a silt loam in a 12-week incubation experiment. All metals were inhibitory at 400 mg kg-1. Among the metals, the sequence of decreasing inhibition was Cr > Cd > Cu > Zn > Mn > Pb. In contrast, Quraishi and Cornfield (1971) reported that the addition of Cu up to 10000 mg kg -1 stimulated N mineralisation. Wilke (1988a) also showed, in a 14-day perfusion experiment, that Se stimulated N mineralisation in mull, moder and mor soils. Incubation studies using heavy metals indicated that nitrification was more sensitive to pollutants than total N mineralisation (Liang and Tabatabai 1977, 1978; Ghiashuddin and Cornfield 1979; Chang and Broadbent 1982; Rother et al. 1982). In contrast, a 9-month experiment carried out under field conditions by Tonner et al. (1985) and a long-term study by Wilke (1988 b) revealed that ammonification was more reduced than nitrification due to Cu, Cd, Ni, Pb, and Zn additions. Therefore, the aim of the present investigation was to study the long-term effects of various inorganic pollutants on both total N mineralisation and nitrification. Since the effects of pollutants depend mainly on their soluble fraction rather than on their total content in soil, special attention was paid to the water-soluble fraction of each pollutant. Moreover, the influ-
255 ence of inorganic pollutants on arginine ammonificat i o n w a s s t u d i e d . T h i s n e w m e t h o d was p r o p o s e d b y A l e f a n d K l e i n e r (1986) as a s i m p l e w a y o f e s t i m a t i n g m i c r o b i a l p o t e n t i a l s in soils a n d m a y substitute for the time-consuming determination of N mineralisation.
Materials and methods The A horizon (0-25 cm) of a sandy cambisol from a field trial of the Biologische Bundesanstalt (Berlin-West) was used. It contained 9°70 clay, 12°/0 silt, 79070 sand, 1.2°/0 organic C, 0.080/0 total N and had a pH (CaC12) of 6.0. The cation exchange capacity was 10.3 mEq/100 g. In 1975 the soil was placed in concrete frames of 1 × 1 × 1 m (1 m3). The underlying material was a layer of peat (about 15 cm thick). Cr, Cd, Hg, and Pb were added as chlorides, Be, Ni, and Sn as sulphates, and As, Br, F, Se, and V as Na salts. All compounds were mixed with the A horizon in one to three additions, depending on the amount added, in 1975 and 1976 (Kloke 1986). All treatments were carried out fourfold in two concentrations, 16 controls remaining untreated. In the following years tomatoes, potatos, beans, rye, wheat, barley, and rape seed were grown on the plot. Weeds were removed occasionally. The soil was dug up twice a year in spring and autumn. Samples for mineralisation experiments and chemical analyses were obtained by taking four subsamples from the epipedons of the As-, Cd-, F-, Ni-, Pb-, and Se-treated plots and half of the controls in March, 1984. The plots amended with Be, Br, Cr, Hg, Sn, and V and the other half of the controls were sampled in November, 1984. For arginine ammoniflcation tests, samples were collected in September, 1987. For microbial measurements, the field-moist soils were passed through a 2-mm sieve and stored at a temperature of - 1 8 °C in the dark. For chemical analysis the samples were air-dried before being sieved.
Microbiological methods. Before incubation the soil samples were allowed to thaw at 4°C in a refrigerator for 4 days. The effects of the pollutants on N mineralisation were determined by the amounts of nitrogen (NH~- and NO3) mineralised by the amended and un-
amended soils. In this work, 3 ml deionized water was added to a 10-g sample of field-moist soil in a 100-ml Erlenmeyer flask and incubated at 25 °C. After 28 days mineralised NH~ and NO~- were extracted with 50 ml of 1 M K2SO4 and measured photometrically as described by Beck (1983). All experiments were carried out with six replicates. Two replicates were frozen at the beginning of the incubation to determine initial NH~ and NO~-. Nitrification was determined according to Beck (1976). Ten grams of soil were saturated with 3 ml destilled water in 100-ml flasks and 1 ml of a 1% (NH4)2SO4 solution was added as the NH~ source. The flasks were stoppered and incubated at 25 °C. After 21 days mineralised NH~ and nitrified NO~- were determined as described above. Nitrification was expressed as the percentage of mineralised NH~. Arginine ammonification was determined according to the method proposed by Alef and Kleiner (1986). The method is based on ammonification of arginine by heterotrophic soil bacteria. Two grams of sieved, moist soil were placed in centrifuge tubes (12 ml), locked with rubber stoppers, and incubated at a temperature of 30°C for 30rain. Then 0.5 ml of arginine solution (100mg Larginine in 50 ml H20) was added dropwise to the soil. After 3 h of incubation (30 °C) the samples were mixed with 8 ml of 2 M KC1 for 15 min on a horizontal shaker (180 movements/min) and centrifuged for 10 rain at 5000 rpm. After centrifugation 0.5 ml of the clear supernatant was used for photometrical NH~ determinations. The percentage inhibition of N transformations by each pollutant was calculated from (A-B/A) 100, where A is the amount mineralised in the controls and B is the amount mineralised in the treated soils.
Chemical analyses. Total contents of pollutants except F and Se were determined in an aqua regia extract by atomic absorption spectrometry. Total amounts of F were analysed in a NaOH extract by an ion-selective electrode (H/ini 1975). Total Se was determined by neutron-activation analysis. Mobile fractions of pollutants (perfusion extracts) were extracted using a perfusion apparatus as described by Wilke (1982). A 50-g soil sample was placed in the perfusion apparatus and percolated with 100 ml distilled water for 4 days. Element concentrations in the percolates were determined according to the above-mentioned methods. pH was measured with a glass electrode in 0.01 M CaC12 solution (soil:solution ratio 1:2.5). The cation exchange capacity was
Table 1. Long-term effects of inorganic pollutants on N mineralisation (N-min) in a sandy cambisol 8 or 9 years after the last treatment Element
Amount added (mg/kg)
Soil conc. a 1984 (mg/kg)
PL b (mg/1)
N-min (p.gN day- 1)
Control As I As II Cd I Cd II FI F II NiI NilI Pb I Pb II Se II
0 50 300 50 200 500 2000 100 400 1000 4000 40
-
0.68 1.92 0.08 0.23 19 37 0.23 1.60 0.07 0.23 0.16
6.23 3.74 3.05 2.82 0.75 2.74 2.30 6.73 0.13 4.24 3.49 6.24
22.7 52.8 45 209 219 529 86 316 810 3800 7.4
Inhibition c (%)
0a 40b 51 b 55b 88c 56b 63c -8a 79b 32a 44b 0a
Element
Amount added (mg/kg)
Soil conc. a 1984 (mg/kg)
PL b (mg/1)
N-min (~tgN day- 1)
Inhibition c (%)
Control BeI Be II BrI Br II Cr I Cr II Hg I HglI Sn I Sn II VII
0 30 80 60 240 300 800 50 200 117 467 400
-
-
13.5 38.0 5.8 7.8 288 884 54 180 129 452 122
0.009 0.032 0.024 0.050 0.051 0.096 0.009 0.046 < 0.01 < 0.01 6.4
7.34 4.24 3.86 8.84 4.01 7.80 6.76 5.50 1.11 7.44 6.84 6.91
0a 43b 48 b - 19a 46b -5a 9a 26b 85c 0a 8a 7a
a conc., concentration b PL, concentration measured in perfusion extract c Differences are significantly different (t-test, P
