Abstract. The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism ...
EFFECTS
OF ACIDIFICATION
AND NITROGEN
AND LIMING ON CARBON
MINERALIZATION
AND SOIL ORGANISMS
IN
MOR HUMUS
T R Y G G V E P E R S S O N , H E L l e N E L U N D K V I S T , A N D E R S WIRI~N, R I I T T A H Y V O N E N , and B E N G T WESSI~N
Dept of Ecology and Environmental Research, Swedish University of Agricultural Sciences, Box 7072, S-750 07 Uppsala, Sweden
Received October 7, 1988; revised March 31, 1989) Abstract. The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism biomass. Intact soil cores from F / H layers in a Norway spruce (C:N=31) and a Scots pine (C:N=44) stand in central Sweden were treated in the laboratory for 55 days with deionized water (control), weak H2SO 4 (successively applied as 72 mm of acid rain of pH 3.1), strong H2SO 4 (applied as a single high dose of pH 1), and lime (CaCO3). Strong acidification reduced C mineralization and increased net N mineralization in both soils. Weak acidification resulted in similar but less pronounced effects. Liming initially stimulated C mineralization rate, but the rates declined, indicating that an easily available C source was successively used up by the microorganisms. Liming also increased net N mineralization in the C:N=31 humus, but not significantly in the C:N-44 humus. Strong acidification generally affected the amounts of FDA-active fungal hyphae, nematodes and enchytraeids more than the other treatments did. The increases in net N mineralization after acidification and liming could only partly be explained by the decreases in biomass N in soil organisms. Mineralization of biomass N from killed soil organisms could at the most explain up to about 30% of the increase in net N mineralization after strong acidification. Most of the effects on N mineralization seemed to depend on the fact that acidification reduced and liming increased the availability of C and N to the microorganisms. Furthermore, acidification seemed to reduce the incorporation of N from dead organisms into the soil organic matter and, thereby, make the N compounds more readily available to microbial decomposition and mineralization.
1. Introduction
Acid deposition is now affecting large areas of Europe and North America. Several studies have indicated that experimental acidification can cause changes in the soil biota (e.g. BSgtth et al., 1980), reduce C mineralization (Bryant et al., 1979; Klein et al., 1984) and, at least temporarily, stimulate N mineralization (Tamm, 1976; Tamm et al., 1977; Strayer et al., 1981) and tree growth (Tamm and Wiklander, 1980; Tveite, 1980). Other studies have shown that N mineralization can be depressed by experimental acidification (Francis, 1982; Klein et al., 1984). Liming counteracts soil acidification by increasing base saturation and soil pH. Application of lime to acid forest soils affects soil biota (see reviews by S6derstr6m (1984) and Persson (1988)) and is suggested to stimulate C mineralization (e.g. Adams and Cornforth, 1973; Mai and Fiedler, 1978; N6mmik, 1978; Lohm et al., 1984). Several authors have found that liming increases net N mineralization (e.g. Nyborg and Hoyt, 1978; Sahrawat et al., 1985) or decreases net N mineralization Water, Air, and Soil Pollution 45: 77-96, 1989. © 1989 Kluwer Academic Publishers. Printed in the Netherlands.
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T R Y G G V E PERSSON ET AL.
(e.g., N6mmik, 1968, 1978; Tamm and Pettersson, 1969; Adams and Cornforth, 1973; Popovi6, 1975). N6mmik (1979) observed that liming had a tendency to stimulate N mineralization in mor humus with a C:N ratio lower than 30, while higher ratios were correlated with reduced mineralization rates. The soil organism biomass has a low C:N ratio, often around 5 (McGill et al., 1981), and can contain as much as 4 to 5% of total N in forest soils (Persson, 1983). Disturbances affecting soil organism biomass might, therefore, have a great impact on net N mineralization (Sparling and Ross, 1988). Tamm et al. (1977) hypothesized that the increase in net N mineralization after application of high doses of acid in field experiments could depend on a partial mortality of the microfiora followed by mineralization of biomass N. Aber et al. (1982) believed that this was a short-term phenomenon depending on the unnaturally high dose of acid, and suggested that long-term increases in N mineralization were very unlikely to occur. According to their hypothesis, the decrease in C mineralization following acidification should be accompanied by a corresponding decrease in net N mineralization. Such a decrease in both C and N mineralization was found by Klein et al. (1984). The increase in net N mineralization after liming of mor humus with C:N ratios 30 reported by N6mmik (1979), could also depend on responses by the microbial biomass. Because the C:N ratio of the soil organism biomass is low, a treatment that stimulates microbial growth is likely to cause N immobilization if the microbial substrate has a high C:N ratio. In order to check earlier findings and to test the importance of the soil organism biomass as a N source, a short-term laboratory experiment was conducted, in which two contrasting humus materials, one with a moderately low C:N ratio (for Swedish conditions) and one with a high C:N ratio, were used. The following questions were addressed: (1) Will acidification reduce and liming increase C mineralization? (2) Will application of a single high dose of H2SO4 kill soil organisms and thereby increase net N mineralization? (3) Will application of small doses of H2SO4 reduce N mineralization without affecting the biomass N of the soil organisms? (4) Will application of lime increase net N mineralization in mor humus with low C:N ratios and decrease net N mineralization in mor humus with high C:N ratios, and can these changes be explained by increases in the biomass of soil organisms? The soil organisms studied were fungi, nematodes and enchytraeids. Fungi generally have a much higher biomass than bacteria and the nematodes and enchytraeids together make up more than 50% of the soil animal biomass in the humus layer of this type of coniferous forest (Persson et al., 1980).
EFFECTS OF ACIDIFICATION AND LIMING
79
2. Materials and Methods 2.1. MATERIALS F / H - l a y e r blocks were taken from two coniferous forests in central Sweden, a 130yr-old Scots pine (Pinus sylvestris L.) stand at Jfidra~s (60o49 ' N, 16o30 ' E, 185 m above M.S.L.) (described by Persson et al., 1980) and a 70-yr-old Norway spruce (Picea abies Karst.) stand at Fexboda (60o08 ' N, 17°30'1 E, 40 m above M.S.L.). Both soils are spodosols, but the J~idrahs humus is a mor (C:N = 44) and the Fexboda humus is a moder-like mor (C:N = 31). The Fexboda humus was taken in March and the J~idraS.s humus in April 1983. The F / H layer blocks were cut to fit into 50 cm 2 cylindrical containers, and roots larger than 1 m m in diameter were removed. The containers were of SAN (standard styrene/acryl nitrile) plastic. Lids with rubber septa and rubber sealings were used to close the containers periodically, enabling us to take gas samples for respiration measurements. The total volume of the closed container was 466 m L (Figure 1). Each humus sample put into the container had a fresh weight (fwt) of 70 g and a thickness of about 3 cm. Mean dry weight (dwt) measured at the end of the experiment was a b o u t 14 g, and loss on ignition was about 80% of dwt for both humus types. The water content fluctuated between 60% and 100% of water
A
B i I
"
J
t
Rubber septum
Pi peffe
i
Grass beads
Net
Fig. 1. The container used in the study. (A) Open container during pipetting. (B) Closed container before gas sampling.
80
TRYGGVE PERSSONET AL. TABLE I Treatments per container (50 cm 2) in the 55-day-experiment. Water (pure or as a solute) was applied every day during the first 35 days and every second day during the next 20 days. Code
Applied at the start
Added with each watering
Total amounts applied per container
W H
-
8 mL H20 0.0043 mmol H2SO 4
SH
0.783 mmol H2SO4 in 10 mL H20 (pH 1) + 10 mL H20 1.5 g (15 mmol) CaCO3 applied on the humus surface
360 mL H20 0.194 mmol HzSO4 + 360 mL H20 0.783 mmol H2SO 4 + 360 mL H20
Ca
in 8 mL H20 (pH 3.1) 8 mL H20 8 mL H20
15 mmol CaCO3 + 360 mL H20
holding capacity because of periodical waterings (see below). Each humus sample was placed on a terylene net resting on a layer of 5 mm glass beads in the bottom part of the container (Figure 1). 2.2. TREATMENTS The humus materials were treated as indicated in Table I and kept at 15 °C for 55 days. Humus that only received deionized water was used as a control (W). In the H treatment, acidified water (pH 3.1) was applied throughout the experiment. In the SH treatment, a single dose of 10 mL of 0.078 M H2SO4 (pH approximately 1) was applied at the start of the experiment followed by another 10 mL of deionized water immediately afterwards. In the Ca treatment, CaCO 3 powder was applied to the surface of the humus material. After the initial application, the SH and Ca treatments received deionized water equal to the W treatment. After each watering the leachates were removed with a pipette and kept at -20 °C. Each treatment contained 15 replicates. The humus materials of about 14 g dwt per container corresponded to a humus layer of 2.8 kg dwt m -2 with an organic content of about 80%. The waterings simulated a rainfall of 72 mm during the experimental period of 55 days, which is a fairly normal precipitation in central Sweden. The H treatment was equivalent to a total application of 38 kg H 2 S O 4 ha -1 or about 0.8 kmol H ÷ ha -1, which is about the same as the annual atmospheric deposition of H + ions in central Sweden. The SH treatment corresponded to a total application of 150 kg H2SO4 ha I or 3 kmol H + ha -~, which is equal to the highest dose annually applied in a field experiment at Norrliden reported by Tamm et al. (1977). The Ca treatment corresponded to 3000 kg C a C O 3 ha -1. 2.3. CO2 MEASUREMENTS Most of the time the humus containers were open, which meant that some water was lost through evaporation between the waterings. At certain intervals the
EFFECTS OF ACIDIFICATIONAND LIMING
81
containers were closed for 2 hr, after which gas samples were taken by means of evacuated blood sampling tubes (Venoject T-273, Terumo Corp., Tokyo, Japan). The tubes were never stored for more than one week to minimize losses of CO2". A 1.00 mL subsample was injected into a gas chromatograph equipped with a hot wire detector. The mass of C evolved per container and hr was calculated as[
12×I06(C-Co) P ( ~ f R c -
+ A×Vaq)
t
where R c = ~g C container -I hr -1, C = sample concentration of CO2 (mL mL-1), C o - CO2 concentration (mL mL -1) in the container directly after closure, t = time between closure and gas sampling (hr), Vg = gas volume (mL) in the container, Vaq = water volume (L) in the container, A = pH-dependent CO2-absorption factor, where:
+ KIK2 + K1K2K3 A =
30, thus, had to be rejected, as regards intact h u m u s samples. We conclude that m o s t o f the effects f o u n d on C and N mineralization after the acid and lime treatments seem to depend on the fact that acidification reduces and liming increases the availability of C and N to the microorganisms. In addition, the data indicate that acidification reduces the i n c o r p o r a t i o n of N c o m p o u n d s f r o m dead organisms into the soil organic matter and, thereby, results in increased availability of substances rich in N to the microorganisms and increased net N mineralization.
Acknowledgments We are grateful to B. Berg and S.I. Nilsson for constructive criticism of the manuscript, R A r o n s s o n , T. Gr6nkvist and C. Lundkvist for technical assistance, G. S u n n e r s t r a n d for m a k i n g the illustrations and N. Rollison for the linguistic revision. We t h a n k C.O. T a m m and B. Popovi6 for stimulating discussions and for allowing us to use their field experiments, S. A n d e r s s o n for c o m m e n t s on bacteria, and referees for constructive suggestions. G r a n t s for the study were received f r o m the Swedish Council for Forestry and Agricultural Research.
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