- Email: [email protected]
Nitrification and ammonification in acid forest litter and humus as affected by peptone and ammonium-n amendment
Soil Biol. Biochem. Vol. 18, No. I, pp. 45-51, 1986 Printed in Great Britain. All rights reserved
0038-0717/86 $3.00 + 0.00 Copyright 4 1986 Pergamon Press Ltd
N I T R I F I C A T I O N A N D A M M O N I F I C A T I O N IN ACID FOREST LITTER A N D H U M U S AS A F F E C T E D BY P E P T O N E A N D A M M O N I U M - N A M E N D M E N T J. A. ADAMS* Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, U K .
(Accepted 27 August 1985) Summary--Samples of strongly acid forest litter and humus from beneath Sitka spruce, heather, Scots pine and larch from two sites in north-east Scotland were incubated aerobically at 20'C in the laboratory. At the Glen Tanar site, spruce litter and larch humus showed significant nitrification and ammonification whereas spruce humus and Scots pine humus produced only NH~--N. Heather humus showed no net mineralization. At the Fetteresso site, application of fertilizer N, P and K to Sitka spruce up to 3 yr previously, significantly stimulated the production of NO;-N in both litter and humus. Amendment of the samples with organic N as peptone caused significant increases in NO~-N production in those samples that already showed nitrification. The increases in NO;-N generally represented a low proportion of the added peptone-N. Amendment with NH~--N as (NH4)2SO4 either had no effect or significantly reduced NOi--N production (in larch humus). The results suggest the occurrence of heterotrophic nitrification in some of these forest samples. Net immobilization of NH g-N was typically greater in NH4+-N amended than in peptone amended samples, except for heather humus which showed complete immobilization of both N sources. Total mineral N produced ~t the end of the aerobic incubation was correlated (P < 0.01) with NH4+-N produced during a 30-day anaerobic incubation at 30-C. Net NO~--N production was greater in litter than in the corresponding humus samples and was correlated (P < 0.001) with initial organic N soluble in 1 m KCI. INTRODUCTION Most forest soils contain considerable amounts of nitrogen in the forest floor. However, an inadequate supply of available mineral N is recognized as one of the nutritional factors constraining forest growth in temperate and high latitudes. Differences between agricultural and forest soils suggest that factors known to affect N availability and the ionic forms of N present in agricultural soils, such as C-to-N ratio, pH and NH4+-N substrate limitation, may not all similarly affect N availability in forest soils. In agricultural soils of near neutral or slightly acid pH, N O 3-N is usually the main ionic form of mineral N present with only low amounts of NH4+-N. This is usually considered to be due to the relative kinetics of the ammonification and nitrification steps of the autotrophic nitrification process (Alexander, 1965). In acidic forest soils, NH4+-N is typically the main ionic form present. Nitrate may be absent, or if present often occurs in lower concentrations than NH~--N (Keeney, 1980; Robertson, 1982). Many forests provide acid soil conditions which are not conducive to or prevent autotrophic nitrification (Keeney, 1980). However, the presence of NO~--N and its formation in strongly acid soils has been described by several workers including Weber and Gainey (1962), Ishaque and Cornfield (1972, 1974), Focht and Verstraete (1977) and Williams (1983). The possibility of a heterotrophic pathway of N O ; - N formation in strongly acid nitrifying soils has been suggested (Alexander, 1965; Focht and Ver*On leave from: Department of Soil Science, Lincoln College, Canterbury, New Zealand. 45
straete, 1977; Belser, 1979). The use of organic N and N H + - N amendment as a means of investigating the possibility of heterotrophic nitrification in acid soil environments has been suggested by Focht and Verstraete (1977). They noted that N O 3 - N formation should be related to the amount of organic N present and that the addition of NH4~-N should inhibit or have no effect on the nitrification process. Using this approach, Van de Dijk and Troelstra (1980) demonstrated heterotrophic nitrification in an acid heath soil (pH 4.3) in the Netherlands. I present the results of two laboratory incubation studies. The first investigated possible factors affecting the amounts and forms of N mineralized during aerobic and anaerobic incubation of various litter and humus samples from forests in north-east Scotland. The second investigated the effects of amending these samples with organic N and NH4+-N substrates. METHODS
Experimental sites and sampling Two experimental areas were selected in the Grampian region of Scotland; Fetteresso, Mearns Forest (U.K. National Grid Reference N O 8186) and Craigendinnie Hill, Glen Tanar (U.K. National Grid Reference N O 5196). At Fetteresso, two sites were selected in a stand of Sitka spruce (Picea sitchensis (Bong) Carr.) planted in 1947 on a brown forest soil. Samples were collected in April 1984 from the L and F + H horizons of an untreated control plot and an N P K treated plot forming part of a fertilizer experiment studying nutrient cycling (Williams, 1983). The N P K treated plot had received a total of
J. A. A D A M S
46
Table I. Site characteristics and fertilizer history Fertilizer element (kg ha i)
Vegetation
I
Sitka spruce
1947
Strichen
230
None
2
Sitka spruce
1947
Strichen
230
N 800 P 400 K 600
L and F+H L and F+H
Natural vegetation 1870 1957
Countesswells
370
None
H
Countesswells Countesswells
375 200
None None
1951
Countesswells
200
None
F+ H L and F+H H
Site
Soil association t
Altitude (m)
Planting date
Horizons sampled
Fetteresso
Glen Tanar 3
Heather
4 5
Scots pine Sitka spruce
6
Larch
~Glentworth and Muir (1963).
8 0 0 k g N h a i as NH4NO 3, 4 0 0 k g P h a - I as unground phosphate rock (Gafsa, 12.2% P) and 600 kg K ha ~as KC1 split as four equal applications in the spring or early summers of 1973, 1979, 1980 and 1981. At Glen Tanar, sites were selected under heather (Calhma t,ulgaris), in an old Scots pine (Pinus syh'estris) stand, in a 1957 Sitka spruce stand and in a 1951 larch (Larix decidua) stand. All four sites were on the Countesswells association (Glentworth and Muir, 1963). Details of the sites and the organic horizons sampled are given in Table 1. At all sites, samples were collected from the appropriate soil horizons within an area of approximately 0.5 m-'. The L horizon material consisted of relatively undecomposed needle litter and overlaid the more humified F + H horizon material which was not separated. Twigs, stones and large roots were removed by sieving samples ( < 5 ram). Samples were stored at 4 C at field moisture content.
hwubation I Aerobic incubations were conducted in quadruplicate using 5g of litter and 10 g of humus samples. Incubations were carried out in the dark at 20°C in glass bottles covered with transparent polythene film. Samples were watered weekly to maintain their original water content, which represented from 50 to 60% of field capacity for the litter samples and from 70 to 90% of field capacity for the humus samples. Samples were removed for extraction and analysis of mineral N (NH4+-N, NO£-N and NO3--N) at 14, 27, 41 and 55 days. Two of the 4 replicates removed at each day were extracted by shaking for 1 h with 50 ml of 1 M KC1. The other 2 replicates were shaken similarly with distilled water, after which the pH of the suspension was measured. Analytical grade KCI was then added to each bottle to give a KCI concentration of 1 u and the sample reshaken for a further hour. Extracts were filtered and stored at 4°C before analysis.
Incubation 2 Each of the forest floor samples was amended with organic N (as peptone, 13.4% N) or with NH+-N (as (NH4)2SO4), both added at a rate of 100mg N kg -j dry weight. Amending treatments were applied as a
2 ml aqueous solution to the forest floor samples, with unamended (control) samples receiving 2 ml of distilled water. The samples were then allowed to revert to their original water content during the incubation. The forest floor samples were incubated aerobically in triplicate for 28 days as for incubation 1. All samples were extracted with 1 M KCI for I h. The unamended forest floor samples were also incubated anaerobically under waterlogged conditions at 30°C for 30 days. Studies had shown this period to be necessary for near complete mineralization in these kinds of samples (B. L. Williams, personal communication). Mineralized N was determined as NH~-N.
Chemical analyses Concentrations of NH~-N, NO~-N and NO3--N were determined colorimetrically (Crooke and Simpson, 1971; Keeney and Nelson, 1982; Henriksen and Selmer-Olsen, 1970). Organic N soluble in KCI was determined at Day 0 on unamended samples from incubation 2, by digesting an aliquot of the KC1 extract in a concentrated H2SO4-H.,O2(30% w/v)-Se mixture at 330°C (Wall et al., 1975); tests have shown that this method does not recover NO~--N (B. L. Williams, unpublished data). Organic N was taken as the difference between the NH~--N content of the KC1 extract after and before digestion. Standards were digested with the samples. Subsamples of the litter and humus were dried at 70°C and then ground lightly ( < 2 ram), followed by fine grinding (<250/1m). Available P levels were determined on the < 2 mm material as Bray 2 P (Bray and Kurtz, 1945). Total P, Ca, Mg and K were determined on the finely-ground material after digestion in a cone. H2SO4-H202-Se mixture, as used for organic N. Phosphorus was determined in all extracts by the method of John (1970). Calcium, Mg and K were determined by flame emission and absorption spectrometry. Total C and N contents were determined on a Hewlett-Packard 185 automatic C, H, N analyser.
Statistical analysis Differences in NO~--N and NH4+-N production by the litter and humus samples after 55 days (Incu-
47
N i t r i f i c a t i o n a n d a m m o n i f i c a t i o n in the forest f l o o r Table 2. Chemical properties of the sampled sites
C/N
Bray 2P (mg kg i)
Total P (mg kg ~)
Ca (%)
Total Mg (%)
K (%)
Soluble organic N (mg kg i)
86 19 560 544
1200 880 1580 1550
0.41 0.30 0.59 0.66
0.052 0.058 0.046 0.044
0.29 0.47 0.21 0.18
18 0 64 15
II 17 60 26 9
700 580 960 740 630
0.04 0.20 0.55 0.41 0.23
0.029 0.060 0.058 0.058 0.037
0.34 0.06 0.07 0.08 0.44
14 38 106 21 22
Site and sample
pH
Total N (%)
Fetteresso I Unfertilized spruce L Unfertilized spruce F + H 2 Fertilized spruce L Fertilized spruce F + H
4.56 4.17 4.51 4.23
1.70 1.44 1.76 1.60
28 27 28 29
Glen Tanar 3 Heather H 4 Scots pine F + H 5 Spruce L Spruce F + H 6 Larch H
3.88 3.89 4.57 3.97 4.05
0.66 1.36 1.44 1.44 0.56
33 35 34 31 14.5
bation 1) were tested for significance by analysis of variance followed by comparison of means using Scheffe's test. Differences between the effects of the amending treatments on net NO3--N production and NH4+-N production or immobilization during Incubation 2 were similarly tested. In both cases, NO3-N and NH4+-N production were treated separately for each sample. RESULTS
Chemical properties of samples from the different sites are shown in Table 2. All samples are extremely acid with litter pH values around 4.50 and humus pH values ranging from 3.88 to 4.23. The pH values of the spruce litter and humus at Fetteresso were similar at both the fertilized and unfertilized sites. Bray P and total P levels show a marked effect of the rock phosphate addition, as do total Ca levels to a lesser extent. Total N and K levels at both sites show no resultant effect of the considerable applications of these elements although soluble organic N levels were higher at the fertilized site. The chemistry of the humus and litter at the four Glen Tanar sites shows some differences attributable to the different plant species. The heather humus is characterized by a low total N content and a very low total Ca content, while the larch humus also has a low total N content but in conjunction with a low C-to-N ratio of 14.5; the other samples have ratios ranging from 31 to 35. Both heather and larch humus also have low available P levels. The Glen Tanar Scots pine and spruce samples are similar in chemistry to the unfertilized spruce at Fetteresso, except that the former samples have low total K contents. Incubation 1
Incubation of litter and humus showed considerable differences in the absolute and relative extents of ammonification and nitrification between the different sites (Fig. 1). Nitrite-N was not detected in any sample at any day. At Fetteresso, both litter and humus from the unfertilized plot produced mainly NH~-N, with NO~--N production reaching 4 0 of the total mineral N in the litter but being negligible in the humus. Samples from the fertilized plot showed significant NO~--N production, increasing from 20 to 2 5 0 of total mineral N at Day 0 to approximately 4 0 0 by Day 55 in both litter and humus. Total mineral N produced during 55 days in litter or humus did not
differ significantly between the fertilized and unfertilized plots. Ammonium-N and NO~--N concentrations were significantly higher in litter than in humus of the Fetteresso samples at Day 0, but the differences were not significant at Day 55. Rates of production of NH+-N in both litter and humus, and NOj--N in the litter of the unfertilized plot, fitted linear regression equations (P < 0.05). In the humus of the fertilized plot, NO3-N production was linear (P < 0.05) during the incubation, while NH~--N production was linear (P < 0.01) during the first 41 days but levelled off at 55 days. In the samples from Glen Tanar, only those from beneath spruce and larch produced significant amounts of NOj--N during incubation. The spruce litter produced considerably more NOj--N than spruce humus, although the latter produced the greater amount of total mineral N during 55 days. Nitrate-N production in the spruce litter was similar to that in the fertilized spruce plot at Fetteresso, reaching approximately 4 0 0 of the total mineral N at Day 55, whereas NO3-N production in the spruce humus never exceeded 5% of total mineral N. Larch humus produced NO~-N and NH1-N linearly (P < 0.01) during the incubation, with NO~--N reaching 3 2 0 of total mineral N at Day 55. The Scots pine humus produced a small but non-significant amount of NH~-N during the first 27 days of the incubation, but levels then remained relatively constant through to 55 days. Heather humus produced no NH4+-N or NO_f-N during the incubation. The pH of the heather and spruce humus at Glen Tanar (Table 2) remained constant throughout the incubation. The remaining humus samples showed pH drops of between 0.2 and 0.3 units. Litter samples showed a larger pH drop of about 0.5 units. Incubation 2
The net production of NO3-N and net production or immobilization of NH4+-N in samples amended with organic N and NH~'-N are shown in Table 3. During the 28 days of this incubation, unamended samples produced comparable amounts of net NO~-N to those produced during the first 27 days of the first incubation (Fig. 1; Table 3). Exceptions were the spruce litter from Glen Tanar and spruce litter and humus samples from the fertilized Fetteresso site. In these samples, NO~--N production was greater in the second incubation, and differences in N H ~ - N production also occurred.
48
J.A. ADAMS Fetteresso
Glen
NH:-N
Tanar
( m g kg - I )
Spruce o--o
300
-!~i
Unfertilized
Fertl.ljzed
L
o--o o--a +--+ =--= o--o
o
L+:+N / e
Spruce L .. F+H Larch H Scots pine F+H Heather H
e /
/ / / / /
/
200
2
. o/, J
I / / / a/ / /
// /
100
-"
.e
e" 0 ~
/ /
/
/
o~
,~
,,
..0
/ ~÷
/
~o
-" "~4~~ Ip"
NO~- N (rag kg-')
200
o
100
/
/
o/
~o , 0 ~
i
0
14
27
41
55
0
~o
/
~
e~--.~--.--o~a~
!
i
i
14
27
41
~ 0
i 55
Days
Fig. I. NH;-N and NO3-N produced during aerobic incubation of unamended samples at 20C (Incubation 1). No NO;-N was present from day 0 to day 55 in humus from beneath heather (site 5) or Scots pine (site 6). In samples that showed nitrification, amendment with peptone produced small but significant increases (P <0.05) in NO~--N production in all but one sample, the fertilized spruce humus, where the increase was not significant. In the Fetteresso spruce samples, amendment with NH~-N gave NO~--N levels not significantly different from those in the untreated samples. In the Glen Tanar spruce litter, NO~--N produced after NH4+-N amendment was lower than, but not significantly different from, that produced after treatment with peptone. In the larch humus, NH4+-N amendment gave significantly lower NO~--N production than occurred in the untreated sample. The effects of amendment on NH4+-N production or immobilization after incubation were varied. Heather humus and spruce litter from Glen Tanar completely immobilized both added peptone-N and NH~--N. Litter and humus from the unfertilized
spruce at Fetteresso showed similar NH4+-N production with little or no immobilization from peptone or NH4+-N amendment. The remaining humus and litter samples all showed some immobilization of added N, with immobilization being greatest in the NH4+-N amended samples.
DISCUSSION Samples from the fertilized and unfertilized Fetteresso spruce plots show contrasting patterns of N H 2 - N and NO~--N production during aerobic incubation (Fig. I). Although they produced similar amounts of total mineral N, samples from the unfertilized plot produced dominantly NH4+-N whereas those from the fertilized plot produced both NH2--N and NO~--N. Williams (1983) found a similar result when incubating samples collected from this site in 1974 after the first fertilizer application. However, the
Nitrification and ammonification in the forest floor
49
Table 3. Net production of NO~ -N and net production or immobilization of NH~*-N (rag kg ~ after 28 days at 20 C of samples amended with peptone or NHa+-N at a rate of 100 mg N kg ~ oven-dry litter or humus Site and sample
None
Amendment Peptone
NH2-N
FCIICFCSSO
I Unfertilized spruce L Unfertilized spruce F + H 2 Fertilized spruce L Fertilized spruce F + H Glen Tartar 3 Heather H 4 Scots pine F + H 5 Spruce L Spruce F + H 6 Larch H
NO 3 -N NHa+-N NO~-N NHa+-N NO~-N NH~-N NO 3 -N NH~-N
13ab I 96a 0.4a 80a 175a 112a 56a 63a
20b 233a 0.3a 147b 227b 149a 69a 97b
9.7a 237a 0.3a 151 b 185a 69a 58a 35c
NO~-N NH~-N NO~ -N N Ha+-N NO~-N NH~-N NO~ -N NH~+-N NO~-N NH~-N
0a 0a 0.1a 4a 298a la 2.8a 51a 34a 21a
0a 1.3a 0.1a 80b 396b 3a 3.2a 100b 38b 93b
0a - 100b 0.1a 58c 359ab -99b 0.1b 68ab 15c 39c
~For each sample, numbers followed by the same letter are not significantly different at P < 0.05; NO~ -N and NHa+-N values were treated separately for statistical analysis.
amount of NO~-N produced during the incubation, and the proportion of the total mineral N it represents, are greater in this study. This may reflect a further stimulation of nitrification by the additional fertilizer applications in 1979, 1980 and 1981. Total N and K differed little between the fertilized and unfertilized plots (Table 2), suggesting that substantial losses of both elements may have occurred from the fertilized plot. In contrast, total and available P and total Ca levels are much higher in the litter and humus of the fertilized plot, showing that much of the added fertilizer P remains within the nutrient cycle. In samples from the fertilized plot, the high proportion (35%) of the total P that is extracted by the Bray 2 reagent, compared with that from the unfertilized plot (2-7%), suggests that much of the added fertilizer may be still present as largely unchanged rock phosphate. This is supported by the significant correlation between Bray P and total Ca in the litter and humus (Table 4). Williams (1983) suggested that a reduction in acidity caused by the increased calcium content in the fertilized plots had stimulated NO3--N production. The Fetteresso samples used in this study had similar pH values, total N contents, C-to-N ratios and total Mg contents (Table 2). However, there are large differences in P content and availability between the two plots. Low P availability has been suggested as a possible explanation for the absence of nitrification in some forest soils (Robertson, 1982). It is clear from Williams (1983) and the results here that the application of fertilizer has stimulated the production of NO~--N in spruce litter and humus at Fetteresso. Comparison of results from the Fetteresso sites with those from the Glen Tartar spruce litter and larch humus suggests that the factors controlling NO~-N production may be more complex than simply P availability or acidity. The spruce litter from Glen Tanar produced similar amounts of NO£-N to that from the fertilized Fetteresso plot, but had total S B B I81--D
and available P levels similar to those of the unfertilized Fetteresso litter which produced mainly NH4*-N (Fig. 1, Table 2). The larch humus, which also produced significant amounts of NO3 -N had the lowest available P level of all the incubated samples and was strongly acidic (Table 2). Linear correlation analysis was used to identify soil properties affecting the extent of ammonification and nitrification and the amounts of N mineralized in aerobic and anaerobic incubations (Table 4). Nitrogen mineralized anaerobically ranged from 200mgkg -~ in heather humus to 1720mgkg -~ in spruce litter from the unfertilized Fetteresso plot. Levels were higher in litter than humus samples. A highly significant correlation (P < 0.01) was obtained between N mineralized anaerobically in 30 days at 3 0 C and that mineralized aerobically, taken as the sum of NH+-N and N O ; - N present at the start of the incubation plus that formed during 28 days at 2 0 C . This further confirms that N mineralized anaerobically may be a useful index of N availability in forest soils, as suggested by Shumway and Atkinson (1978), Powers (1980) and Smith et al. (1981). Sample pH was the only chemical property measured that correlated significantly with N mineralized anaerobically. Sample pH was also highly significantly correlated with N mineralized aerobically, as were total N and Ca at the 5% probability level. Initial N content and pH have previously been shown to be important in the aerobic mineralization of N in forest soils, together with C-to-N ratio (Keeney, 1980). Except for the larch humus, C-to-N ratios here covered a limited range (27-35) and showed no correlation with any of the measures of inorganic N production. Net NH~--N production during aerobic incubation for 28 days was significantly correlated with total N and P contents (Table 4), confirming the importance of initial N content in the ammonification of forest floor samples (Keeney, 1980). Nitrate-N production
J. A. ADAMS
50
~z 0
during 28 days was significantly correlated with pH, and very highly significantly correlated with soluble organic N levels at the start of the incubation. The importance of this last correlation was amplified by the experiment investigating the effects of amendment with peptone and NH4+-N. The observed increase in N O , - N production in the nitrifying samples following peptone amendment contrasts with the lack of significant response to NH4+-N amendment in the spruce samples, and the apparent inhibition in NO 3 -N production in the larch humus. This pattern of response is that suggested by Focht and Verstraete (1977) as being typical of a process of heterotrophic nitrification. The very highly significant correlation between NO~ -N production during aerobic incubation and initial soluble organic N levels further supports the possibility of a heterotrophic pathway of N O ; - N formation in those litter and humus samples where nitrification occurs. Nitrate-N production in the forest floor samples following peptone amendment represents 4. 7, 13, 52 and 98% of the added peptone in the larch humus, unfertilized spruce litter, fertilized spruce humus, fertilized spruce litter, and Glen Tanar spruce litter respectively. The low amounts of NO 3-N produced in most of the forest floor samples suggest that the factor determining whether or not nitrification occurs in a particular forest floor sample may be the amount of readily mineralizable carbon available for heterotrophic utilization rather than the supply of an organic N source. The stimulation of NO3-N production in the fertilized spruce litter and humus at Fetteresso might then be a consequence of litter-fall containing less lignin and more readily metabolizable carbon sources following the fertilizer applications. The other interesting feature of the amendment experiment is the apparently greater immobilization in the majority of samples of added NH4+-N than of N H ; - N resulting from ammonification of the added peptone. The effect is typically statistically significant (Table 3) and has been reproduced in other incubation experiments using the Glen Tanar spruce litter and larch humus (J. A. Adams, unpublished data).
§
'
.~z
o m
~z
Y.
0
~5-~ z.o
o
~
--
c~
c=;
c~
c~
g o
--o
"~-o
z.o
c~
I
o
,~. ~.,
--c~o
z
--
c:; I
I
I
[
E i
0
[-
c~
ii E o
~
-
~ 0
c~
..,m
iI
g Z 6 c~ II
,,:/
v
i
ca
z 0
0',
Z ~ 0 0 0 0 0 0 ~ 0 I
0
~
: ~
•
~
•
0
V
Acknowledgements--I thank Dr B. L. Williams for advice and assistance, Miss Jean Cooper for the statistical analyses and Mr B. F. L. Smith for the C and N analyses. The award of a sabbatical leave grant by the Council of Lincoln College and of a Commonwealth Bursary by the Royal Society and Nuffield Foundation are gratefully acknowledged. Thanks also to the Conservator (East Scotland Conservancy), Forestry Commission and to Glen Tartar Estates for permission to take samples at the two sites. REFERENCES
~
_
~-~-~
~
v c~
I
B
a,,. e
q::
z
Zz~0~zz.~~
z
Alexander M. (1965) Nitrification. In Soil Nitrogen (W. V. Bartholomew and F. E. Clark, Eds), pp. 307-343. American Society of Agronomy, Madison, Wisconsin. Belser L. W. (1979) Population ecology of nitrifying bacteria. Annual Review of Microbiology 33, 309-333. Bray R. H. and Kurtz L T. (1945) Determination of total, organic and available forms of phosphorus in soils. Soil Science 59, 39-45. Crookc W. M. and Simpson W. E. (1971) Determination of ammonium in Kjeldahl digests of crops by an automated procedure. Journal of Science Food and Agriculture 22, 9-10.
Nitrification and ammonification in the forest floor Focht D. D. and Verstraete W. (1977) Biochemical ecology of nitrification and denitrification. Adt,ances in Microbial Ecology 1, 135-214. Glentworth R. and Muir J. W. (1963) The soils of the country around Aberdeen, Inverurie and Fraserburgh. Memoir of the Soil Survey of Scotland. H.M.S.O. Edinburgh. Henriksen A. and Selmer-Olsen A. R. (1970) Automated methods for determining nitrite and nitrate in water and soil extracts. Analyst 95, 514-518. Ishaque M. and Cornfield A. H. (1972) Nitrogen mineralization and nitrification during incubation of East Pakistan "Tea" soils in relation to pH. Plant and Soil 37, 91-95. lshaque M. and Cornfield A. H. (1974) Nitrogen mineralization and nitrification in relation to incubation temperature in an acid Bangladesh soil lacking autotrophic nitrifying organisms. Tropical Agriculture 51, 37-41. John M. K. (1970) Colorimetric determination of phosphorus in soil and plant materials with ascorbic acid. Soil Science 109, 214-220. Keeney D. R. (1980) Prediction of soil nitrogen availability in forest ecosystems: a literature review. Forest Science 26, 159-17!. Keeney D. R, and Nelson D. W. (1982) Nitrogen-inorganic forms. In Methods of Soil Analysis, Part 2 (A. L. Page, Ed.), pp. 643-698. American Society of Agronomy and Soil Science Society of America, Madison, Wisconsin.
51
Powers R. F. (1980) Mineralizable soil nitrogen as an index of nitrogen availability to forest trees. Soil Science Society of America Journal 44, 1314-1320. Robertson G. P. (1982) Nitrification in forested ecosystems. Philosophical Transactions of the Royal Society London Series B 296, 445-457. Shumway J. and Atkinson W. A. (1978) Predicting nitrogen fertiliser response in unthinned stands of Douglas-fir. Communications in Soil Science and Plant Analysis 9, 529-539. Smith J. L., McNeal B. L., Owens E. J. and Klock G. O. (I 981 ) Comparison of nitrogen mineralized under anaerobic and aerobic conditions for some agricultural and forest soils of Washington. Communications in Soil Science and Plant Analysis 12, 997-1009. Van de Dijk S, J. and Troelstra S. R. (1980) Heterotrophic nitrification in a heath soil demonstrated by an h~ situ method. Plant and Soil 57, 11-21. Wall L. L., Gehrke C. W., Neuner T. E., Cathey R. D. and Rexroad P. R. (1975) Total protein nitrogen; evaluation and comparison of four different methods. Journal ~[ the Association of Official Agricultural Chemists 58, 807-81 I. Weber D. F. and Gainey P. L. (1962) Relative sensitivity of nitrifying organisms to hydrogen ions in soils and solutions. Soil Science 94, 138-145. Williams B. L. (1983) Nitrogen transformations and decomposition in litter and humus from beneath closed-canopy Sitka spruce. Forestry 56, 17-32.
0038-0717/86 $3.00 + 0.00 Copyright 4 1986 Pergamon Press Ltd
N I T R I F I C A T I O N A N D A M M O N I F I C A T I O N IN ACID FOREST LITTER A N D H U M U S AS A F F E C T E D BY P E P T O N E A N D A M M O N I U M - N A M E N D M E N T J. A. ADAMS* Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, U K .
(Accepted 27 August 1985) Summary--Samples of strongly acid forest litter and humus from beneath Sitka spruce, heather, Scots pine and larch from two sites in north-east Scotland were incubated aerobically at 20'C in the laboratory. At the Glen Tanar site, spruce litter and larch humus showed significant nitrification and ammonification whereas spruce humus and Scots pine humus produced only NH~--N. Heather humus showed no net mineralization. At the Fetteresso site, application of fertilizer N, P and K to Sitka spruce up to 3 yr previously, significantly stimulated the production of NO;-N in both litter and humus. Amendment of the samples with organic N as peptone caused significant increases in NO~-N production in those samples that already showed nitrification. The increases in NO;-N generally represented a low proportion of the added peptone-N. Amendment with NH~--N as (NH4)2SO4 either had no effect or significantly reduced NOi--N production (in larch humus). The results suggest the occurrence of heterotrophic nitrification in some of these forest samples. Net immobilization of NH g-N was typically greater in NH4+-N amended than in peptone amended samples, except for heather humus which showed complete immobilization of both N sources. Total mineral N produced ~t the end of the aerobic incubation was correlated (P < 0.01) with NH4+-N produced during a 30-day anaerobic incubation at 30-C. Net NO~--N production was greater in litter than in the corresponding humus samples and was correlated (P < 0.001) with initial organic N soluble in 1 m KCI. INTRODUCTION Most forest soils contain considerable amounts of nitrogen in the forest floor. However, an inadequate supply of available mineral N is recognized as one of the nutritional factors constraining forest growth in temperate and high latitudes. Differences between agricultural and forest soils suggest that factors known to affect N availability and the ionic forms of N present in agricultural soils, such as C-to-N ratio, pH and NH4+-N substrate limitation, may not all similarly affect N availability in forest soils. In agricultural soils of near neutral or slightly acid pH, N O 3-N is usually the main ionic form of mineral N present with only low amounts of NH4+-N. This is usually considered to be due to the relative kinetics of the ammonification and nitrification steps of the autotrophic nitrification process (Alexander, 1965). In acidic forest soils, NH4+-N is typically the main ionic form present. Nitrate may be absent, or if present often occurs in lower concentrations than NH~--N (Keeney, 1980; Robertson, 1982). Many forests provide acid soil conditions which are not conducive to or prevent autotrophic nitrification (Keeney, 1980). However, the presence of NO~--N and its formation in strongly acid soils has been described by several workers including Weber and Gainey (1962), Ishaque and Cornfield (1972, 1974), Focht and Verstraete (1977) and Williams (1983). The possibility of a heterotrophic pathway of N O ; - N formation in strongly acid nitrifying soils has been suggested (Alexander, 1965; Focht and Ver*On leave from: Department of Soil Science, Lincoln College, Canterbury, New Zealand. 45
straete, 1977; Belser, 1979). The use of organic N and N H + - N amendment as a means of investigating the possibility of heterotrophic nitrification in acid soil environments has been suggested by Focht and Verstraete (1977). They noted that N O 3 - N formation should be related to the amount of organic N present and that the addition of NH4~-N should inhibit or have no effect on the nitrification process. Using this approach, Van de Dijk and Troelstra (1980) demonstrated heterotrophic nitrification in an acid heath soil (pH 4.3) in the Netherlands. I present the results of two laboratory incubation studies. The first investigated possible factors affecting the amounts and forms of N mineralized during aerobic and anaerobic incubation of various litter and humus samples from forests in north-east Scotland. The second investigated the effects of amending these samples with organic N and NH4+-N substrates. METHODS
Experimental sites and sampling Two experimental areas were selected in the Grampian region of Scotland; Fetteresso, Mearns Forest (U.K. National Grid Reference N O 8186) and Craigendinnie Hill, Glen Tanar (U.K. National Grid Reference N O 5196). At Fetteresso, two sites were selected in a stand of Sitka spruce (Picea sitchensis (Bong) Carr.) planted in 1947 on a brown forest soil. Samples were collected in April 1984 from the L and F + H horizons of an untreated control plot and an N P K treated plot forming part of a fertilizer experiment studying nutrient cycling (Williams, 1983). The N P K treated plot had received a total of
J. A. A D A M S
46
Table I. Site characteristics and fertilizer history Fertilizer element (kg ha i)
Vegetation
I
Sitka spruce
1947
Strichen
230
None
2
Sitka spruce
1947
Strichen
230
N 800 P 400 K 600
L and F+H L and F+H
Natural vegetation 1870 1957
Countesswells
370
None
H
Countesswells Countesswells
375 200
None None
1951
Countesswells
200
None
F+ H L and F+H H
Site
Soil association t
Altitude (m)
Planting date
Horizons sampled
Fetteresso
Glen Tanar 3
Heather
4 5
Scots pine Sitka spruce
6
Larch
~Glentworth and Muir (1963).
8 0 0 k g N h a i as NH4NO 3, 4 0 0 k g P h a - I as unground phosphate rock (Gafsa, 12.2% P) and 600 kg K ha ~as KC1 split as four equal applications in the spring or early summers of 1973, 1979, 1980 and 1981. At Glen Tanar, sites were selected under heather (Calhma t,ulgaris), in an old Scots pine (Pinus syh'estris) stand, in a 1957 Sitka spruce stand and in a 1951 larch (Larix decidua) stand. All four sites were on the Countesswells association (Glentworth and Muir, 1963). Details of the sites and the organic horizons sampled are given in Table 1. At all sites, samples were collected from the appropriate soil horizons within an area of approximately 0.5 m-'. The L horizon material consisted of relatively undecomposed needle litter and overlaid the more humified F + H horizon material which was not separated. Twigs, stones and large roots were removed by sieving samples ( < 5 ram). Samples were stored at 4 C at field moisture content.
hwubation I Aerobic incubations were conducted in quadruplicate using 5g of litter and 10 g of humus samples. Incubations were carried out in the dark at 20°C in glass bottles covered with transparent polythene film. Samples were watered weekly to maintain their original water content, which represented from 50 to 60% of field capacity for the litter samples and from 70 to 90% of field capacity for the humus samples. Samples were removed for extraction and analysis of mineral N (NH4+-N, NO£-N and NO3--N) at 14, 27, 41 and 55 days. Two of the 4 replicates removed at each day were extracted by shaking for 1 h with 50 ml of 1 M KC1. The other 2 replicates were shaken similarly with distilled water, after which the pH of the suspension was measured. Analytical grade KCI was then added to each bottle to give a KCI concentration of 1 u and the sample reshaken for a further hour. Extracts were filtered and stored at 4°C before analysis.
Incubation 2 Each of the forest floor samples was amended with organic N (as peptone, 13.4% N) or with NH+-N (as (NH4)2SO4), both added at a rate of 100mg N kg -j dry weight. Amending treatments were applied as a
2 ml aqueous solution to the forest floor samples, with unamended (control) samples receiving 2 ml of distilled water. The samples were then allowed to revert to their original water content during the incubation. The forest floor samples were incubated aerobically in triplicate for 28 days as for incubation 1. All samples were extracted with 1 M KCI for I h. The unamended forest floor samples were also incubated anaerobically under waterlogged conditions at 30°C for 30 days. Studies had shown this period to be necessary for near complete mineralization in these kinds of samples (B. L. Williams, personal communication). Mineralized N was determined as NH~-N.
Chemical analyses Concentrations of NH~-N, NO~-N and NO3--N were determined colorimetrically (Crooke and Simpson, 1971; Keeney and Nelson, 1982; Henriksen and Selmer-Olsen, 1970). Organic N soluble in KCI was determined at Day 0 on unamended samples from incubation 2, by digesting an aliquot of the KC1 extract in a concentrated H2SO4-H.,O2(30% w/v)-Se mixture at 330°C (Wall et al., 1975); tests have shown that this method does not recover NO~--N (B. L. Williams, unpublished data). Organic N was taken as the difference between the NH~--N content of the KC1 extract after and before digestion. Standards were digested with the samples. Subsamples of the litter and humus were dried at 70°C and then ground lightly ( < 2 ram), followed by fine grinding (<250/1m). Available P levels were determined on the < 2 mm material as Bray 2 P (Bray and Kurtz, 1945). Total P, Ca, Mg and K were determined on the finely-ground material after digestion in a cone. H2SO4-H202-Se mixture, as used for organic N. Phosphorus was determined in all extracts by the method of John (1970). Calcium, Mg and K were determined by flame emission and absorption spectrometry. Total C and N contents were determined on a Hewlett-Packard 185 automatic C, H, N analyser.
Statistical analysis Differences in NO~--N and NH4+-N production by the litter and humus samples after 55 days (Incu-
47
N i t r i f i c a t i o n a n d a m m o n i f i c a t i o n in the forest f l o o r Table 2. Chemical properties of the sampled sites
C/N
Bray 2P (mg kg i)
Total P (mg kg ~)
Ca (%)
Total Mg (%)
K (%)
Soluble organic N (mg kg i)
86 19 560 544
1200 880 1580 1550
0.41 0.30 0.59 0.66
0.052 0.058 0.046 0.044
0.29 0.47 0.21 0.18
18 0 64 15
II 17 60 26 9
700 580 960 740 630
0.04 0.20 0.55 0.41 0.23
0.029 0.060 0.058 0.058 0.037
0.34 0.06 0.07 0.08 0.44
14 38 106 21 22
Site and sample
pH
Total N (%)
Fetteresso I Unfertilized spruce L Unfertilized spruce F + H 2 Fertilized spruce L Fertilized spruce F + H
4.56 4.17 4.51 4.23
1.70 1.44 1.76 1.60
28 27 28 29
Glen Tanar 3 Heather H 4 Scots pine F + H 5 Spruce L Spruce F + H 6 Larch H
3.88 3.89 4.57 3.97 4.05
0.66 1.36 1.44 1.44 0.56
33 35 34 31 14.5
bation 1) were tested for significance by analysis of variance followed by comparison of means using Scheffe's test. Differences between the effects of the amending treatments on net NO3--N production and NH4+-N production or immobilization during Incubation 2 were similarly tested. In both cases, NO3-N and NH4+-N production were treated separately for each sample. RESULTS
Chemical properties of samples from the different sites are shown in Table 2. All samples are extremely acid with litter pH values around 4.50 and humus pH values ranging from 3.88 to 4.23. The pH values of the spruce litter and humus at Fetteresso were similar at both the fertilized and unfertilized sites. Bray P and total P levels show a marked effect of the rock phosphate addition, as do total Ca levels to a lesser extent. Total N and K levels at both sites show no resultant effect of the considerable applications of these elements although soluble organic N levels were higher at the fertilized site. The chemistry of the humus and litter at the four Glen Tanar sites shows some differences attributable to the different plant species. The heather humus is characterized by a low total N content and a very low total Ca content, while the larch humus also has a low total N content but in conjunction with a low C-to-N ratio of 14.5; the other samples have ratios ranging from 31 to 35. Both heather and larch humus also have low available P levels. The Glen Tanar Scots pine and spruce samples are similar in chemistry to the unfertilized spruce at Fetteresso, except that the former samples have low total K contents. Incubation 1
Incubation of litter and humus showed considerable differences in the absolute and relative extents of ammonification and nitrification between the different sites (Fig. 1). Nitrite-N was not detected in any sample at any day. At Fetteresso, both litter and humus from the unfertilized plot produced mainly NH~-N, with NO~--N production reaching 4 0 of the total mineral N in the litter but being negligible in the humus. Samples from the fertilized plot showed significant NO~--N production, increasing from 20 to 2 5 0 of total mineral N at Day 0 to approximately 4 0 0 by Day 55 in both litter and humus. Total mineral N produced during 55 days in litter or humus did not
differ significantly between the fertilized and unfertilized plots. Ammonium-N and NO~--N concentrations were significantly higher in litter than in humus of the Fetteresso samples at Day 0, but the differences were not significant at Day 55. Rates of production of NH+-N in both litter and humus, and NOj--N in the litter of the unfertilized plot, fitted linear regression equations (P < 0.05). In the humus of the fertilized plot, NO3-N production was linear (P < 0.05) during the incubation, while NH~--N production was linear (P < 0.01) during the first 41 days but levelled off at 55 days. In the samples from Glen Tanar, only those from beneath spruce and larch produced significant amounts of NOj--N during incubation. The spruce litter produced considerably more NOj--N than spruce humus, although the latter produced the greater amount of total mineral N during 55 days. Nitrate-N production in the spruce litter was similar to that in the fertilized spruce plot at Fetteresso, reaching approximately 4 0 0 of the total mineral N at Day 55, whereas NO3-N production in the spruce humus never exceeded 5% of total mineral N. Larch humus produced NO~-N and NH1-N linearly (P < 0.01) during the incubation, with NO~--N reaching 3 2 0 of total mineral N at Day 55. The Scots pine humus produced a small but non-significant amount of NH~-N during the first 27 days of the incubation, but levels then remained relatively constant through to 55 days. Heather humus produced no NH4+-N or NO_f-N during the incubation. The pH of the heather and spruce humus at Glen Tanar (Table 2) remained constant throughout the incubation. The remaining humus samples showed pH drops of between 0.2 and 0.3 units. Litter samples showed a larger pH drop of about 0.5 units. Incubation 2
The net production of NO3-N and net production or immobilization of NH4+-N in samples amended with organic N and NH~'-N are shown in Table 3. During the 28 days of this incubation, unamended samples produced comparable amounts of net NO~-N to those produced during the first 27 days of the first incubation (Fig. 1; Table 3). Exceptions were the spruce litter from Glen Tanar and spruce litter and humus samples from the fertilized Fetteresso site. In these samples, NO~--N production was greater in the second incubation, and differences in N H ~ - N production also occurred.
48
J.A. ADAMS Fetteresso
Glen
NH:-N
Tanar
( m g kg - I )
Spruce o--o
300
-!~i
Unfertilized
Fertl.ljzed
L
o--o o--a +--+ =--= o--o
o
L+:+N / e
Spruce L .. F+H Larch H Scots pine F+H Heather H
e /
/ / / / /
/
200
2
. o/, J
I / / / a/ / /
// /
100
-"
.e
e" 0 ~
/ /
/
/
o~
,~
,,
..0
/ ~÷
/
~o
-" "~4~~ Ip"
NO~- N (rag kg-')
200
o
100
/
/
o/
~o , 0 ~
i
0
14
27
41
55
0
~o
/
~
e~--.~--.--o~a~
!
i
i
14
27
41
~ 0
i 55
Days
Fig. I. NH;-N and NO3-N produced during aerobic incubation of unamended samples at 20C (Incubation 1). No NO;-N was present from day 0 to day 55 in humus from beneath heather (site 5) or Scots pine (site 6). In samples that showed nitrification, amendment with peptone produced small but significant increases (P <0.05) in NO~--N production in all but one sample, the fertilized spruce humus, where the increase was not significant. In the Fetteresso spruce samples, amendment with NH~-N gave NO~--N levels not significantly different from those in the untreated samples. In the Glen Tanar spruce litter, NO~--N produced after NH4+-N amendment was lower than, but not significantly different from, that produced after treatment with peptone. In the larch humus, NH4+-N amendment gave significantly lower NO~--N production than occurred in the untreated sample. The effects of amendment on NH4+-N production or immobilization after incubation were varied. Heather humus and spruce litter from Glen Tanar completely immobilized both added peptone-N and NH~--N. Litter and humus from the unfertilized
spruce at Fetteresso showed similar NH4+-N production with little or no immobilization from peptone or NH4+-N amendment. The remaining humus and litter samples all showed some immobilization of added N, with immobilization being greatest in the NH4+-N amended samples.
DISCUSSION Samples from the fertilized and unfertilized Fetteresso spruce plots show contrasting patterns of N H 2 - N and NO~--N production during aerobic incubation (Fig. I). Although they produced similar amounts of total mineral N, samples from the unfertilized plot produced dominantly NH4+-N whereas those from the fertilized plot produced both NH2--N and NO~--N. Williams (1983) found a similar result when incubating samples collected from this site in 1974 after the first fertilizer application. However, the
Nitrification and ammonification in the forest floor
49
Table 3. Net production of NO~ -N and net production or immobilization of NH~*-N (rag kg ~ after 28 days at 20 C of samples amended with peptone or NHa+-N at a rate of 100 mg N kg ~ oven-dry litter or humus Site and sample
None
Amendment Peptone
NH2-N
FCIICFCSSO
I Unfertilized spruce L Unfertilized spruce F + H 2 Fertilized spruce L Fertilized spruce F + H Glen Tartar 3 Heather H 4 Scots pine F + H 5 Spruce L Spruce F + H 6 Larch H
NO 3 -N NHa+-N NO~-N NHa+-N NO~-N NH~-N NO 3 -N NH~-N
13ab I 96a 0.4a 80a 175a 112a 56a 63a
20b 233a 0.3a 147b 227b 149a 69a 97b
9.7a 237a 0.3a 151 b 185a 69a 58a 35c
NO~-N NH~-N NO~ -N N Ha+-N NO~-N NH~-N NO~ -N NH~+-N NO~-N NH~-N
0a 0a 0.1a 4a 298a la 2.8a 51a 34a 21a
0a 1.3a 0.1a 80b 396b 3a 3.2a 100b 38b 93b
0a - 100b 0.1a 58c 359ab -99b 0.1b 68ab 15c 39c
~For each sample, numbers followed by the same letter are not significantly different at P < 0.05; NO~ -N and NHa+-N values were treated separately for statistical analysis.
amount of NO~-N produced during the incubation, and the proportion of the total mineral N it represents, are greater in this study. This may reflect a further stimulation of nitrification by the additional fertilizer applications in 1979, 1980 and 1981. Total N and K differed little between the fertilized and unfertilized plots (Table 2), suggesting that substantial losses of both elements may have occurred from the fertilized plot. In contrast, total and available P and total Ca levels are much higher in the litter and humus of the fertilized plot, showing that much of the added fertilizer P remains within the nutrient cycle. In samples from the fertilized plot, the high proportion (35%) of the total P that is extracted by the Bray 2 reagent, compared with that from the unfertilized plot (2-7%), suggests that much of the added fertilizer may be still present as largely unchanged rock phosphate. This is supported by the significant correlation between Bray P and total Ca in the litter and humus (Table 4). Williams (1983) suggested that a reduction in acidity caused by the increased calcium content in the fertilized plots had stimulated NO3--N production. The Fetteresso samples used in this study had similar pH values, total N contents, C-to-N ratios and total Mg contents (Table 2). However, there are large differences in P content and availability between the two plots. Low P availability has been suggested as a possible explanation for the absence of nitrification in some forest soils (Robertson, 1982). It is clear from Williams (1983) and the results here that the application of fertilizer has stimulated the production of NO~--N in spruce litter and humus at Fetteresso. Comparison of results from the Fetteresso sites with those from the Glen Tartar spruce litter and larch humus suggests that the factors controlling NO~-N production may be more complex than simply P availability or acidity. The spruce litter from Glen Tanar produced similar amounts of NO£-N to that from the fertilized Fetteresso plot, but had total S B B I81--D
and available P levels similar to those of the unfertilized Fetteresso litter which produced mainly NH4*-N (Fig. 1, Table 2). The larch humus, which also produced significant amounts of NO3 -N had the lowest available P level of all the incubated samples and was strongly acidic (Table 2). Linear correlation analysis was used to identify soil properties affecting the extent of ammonification and nitrification and the amounts of N mineralized in aerobic and anaerobic incubations (Table 4). Nitrogen mineralized anaerobically ranged from 200mgkg -~ in heather humus to 1720mgkg -~ in spruce litter from the unfertilized Fetteresso plot. Levels were higher in litter than humus samples. A highly significant correlation (P < 0.01) was obtained between N mineralized anaerobically in 30 days at 3 0 C and that mineralized aerobically, taken as the sum of NH+-N and N O ; - N present at the start of the incubation plus that formed during 28 days at 2 0 C . This further confirms that N mineralized anaerobically may be a useful index of N availability in forest soils, as suggested by Shumway and Atkinson (1978), Powers (1980) and Smith et al. (1981). Sample pH was the only chemical property measured that correlated significantly with N mineralized anaerobically. Sample pH was also highly significantly correlated with N mineralized aerobically, as were total N and Ca at the 5% probability level. Initial N content and pH have previously been shown to be important in the aerobic mineralization of N in forest soils, together with C-to-N ratio (Keeney, 1980). Except for the larch humus, C-to-N ratios here covered a limited range (27-35) and showed no correlation with any of the measures of inorganic N production. Net NH~--N production during aerobic incubation for 28 days was significantly correlated with total N and P contents (Table 4), confirming the importance of initial N content in the ammonification of forest floor samples (Keeney, 1980). Nitrate-N production
J. A. ADAMS
50
~z 0
during 28 days was significantly correlated with pH, and very highly significantly correlated with soluble organic N levels at the start of the incubation. The importance of this last correlation was amplified by the experiment investigating the effects of amendment with peptone and NH4+-N. The observed increase in N O , - N production in the nitrifying samples following peptone amendment contrasts with the lack of significant response to NH4+-N amendment in the spruce samples, and the apparent inhibition in NO 3 -N production in the larch humus. This pattern of response is that suggested by Focht and Verstraete (1977) as being typical of a process of heterotrophic nitrification. The very highly significant correlation between NO~ -N production during aerobic incubation and initial soluble organic N levels further supports the possibility of a heterotrophic pathway of N O ; - N formation in those litter and humus samples where nitrification occurs. Nitrate-N production in the forest floor samples following peptone amendment represents 4. 7, 13, 52 and 98% of the added peptone in the larch humus, unfertilized spruce litter, fertilized spruce humus, fertilized spruce litter, and Glen Tanar spruce litter respectively. The low amounts of NO 3-N produced in most of the forest floor samples suggest that the factor determining whether or not nitrification occurs in a particular forest floor sample may be the amount of readily mineralizable carbon available for heterotrophic utilization rather than the supply of an organic N source. The stimulation of NO3-N production in the fertilized spruce litter and humus at Fetteresso might then be a consequence of litter-fall containing less lignin and more readily metabolizable carbon sources following the fertilizer applications. The other interesting feature of the amendment experiment is the apparently greater immobilization in the majority of samples of added NH4+-N than of N H ; - N resulting from ammonification of the added peptone. The effect is typically statistically significant (Table 3) and has been reproduced in other incubation experiments using the Glen Tanar spruce litter and larch humus (J. A. Adams, unpublished data).
§
'
.~z
o m
~z
Y.
0
~5-~ z.o
o
~
--
c~
c=;
c~
c~
g o
--o
"~-o
z.o
c~
I
o
,~. ~.,
--c~o
z
--
c:; I
I
I
[
E i
0
[-
c~
ii E o
~
-
~ 0
c~
..,m
iI
g Z 6 c~ II
,,:/
v
i
ca
z 0
0',
Z ~ 0 0 0 0 0 0 ~ 0 I
0
~
: ~
•
~
•
0
V
Acknowledgements--I thank Dr B. L. Williams for advice and assistance, Miss Jean Cooper for the statistical analyses and Mr B. F. L. Smith for the C and N analyses. The award of a sabbatical leave grant by the Council of Lincoln College and of a Commonwealth Bursary by the Royal Society and Nuffield Foundation are gratefully acknowledged. Thanks also to the Conservator (East Scotland Conservancy), Forestry Commission and to Glen Tartar Estates for permission to take samples at the two sites. REFERENCES
~
_
~-~-~
~
v c~
I
B
a,,. e
q::
z
Zz~0~zz.~~
z
Alexander M. (1965) Nitrification. In Soil Nitrogen (W. V. Bartholomew and F. E. Clark, Eds), pp. 307-343. American Society of Agronomy, Madison, Wisconsin. Belser L. W. (1979) Population ecology of nitrifying bacteria. Annual Review of Microbiology 33, 309-333. Bray R. H. and Kurtz L T. (1945) Determination of total, organic and available forms of phosphorus in soils. Soil Science 59, 39-45. Crookc W. M. and Simpson W. E. (1971) Determination of ammonium in Kjeldahl digests of crops by an automated procedure. Journal of Science Food and Agriculture 22, 9-10.
Nitrification and ammonification in the forest floor Focht D. D. and Verstraete W. (1977) Biochemical ecology of nitrification and denitrification. Adt,ances in Microbial Ecology 1, 135-214. Glentworth R. and Muir J. W. (1963) The soils of the country around Aberdeen, Inverurie and Fraserburgh. Memoir of the Soil Survey of Scotland. H.M.S.O. Edinburgh. Henriksen A. and Selmer-Olsen A. R. (1970) Automated methods for determining nitrite and nitrate in water and soil extracts. Analyst 95, 514-518. Ishaque M. and Cornfield A. H. (1972) Nitrogen mineralization and nitrification during incubation of East Pakistan "Tea" soils in relation to pH. Plant and Soil 37, 91-95. lshaque M. and Cornfield A. H. (1974) Nitrogen mineralization and nitrification in relation to incubation temperature in an acid Bangladesh soil lacking autotrophic nitrifying organisms. Tropical Agriculture 51, 37-41. John M. K. (1970) Colorimetric determination of phosphorus in soil and plant materials with ascorbic acid. Soil Science 109, 214-220. Keeney D. R. (1980) Prediction of soil nitrogen availability in forest ecosystems: a literature review. Forest Science 26, 159-17!. Keeney D. R, and Nelson D. W. (1982) Nitrogen-inorganic forms. In Methods of Soil Analysis, Part 2 (A. L. Page, Ed.), pp. 643-698. American Society of Agronomy and Soil Science Society of America, Madison, Wisconsin.
51
Powers R. F. (1980) Mineralizable soil nitrogen as an index of nitrogen availability to forest trees. Soil Science Society of America Journal 44, 1314-1320. Robertson G. P. (1982) Nitrification in forested ecosystems. Philosophical Transactions of the Royal Society London Series B 296, 445-457. Shumway J. and Atkinson W. A. (1978) Predicting nitrogen fertiliser response in unthinned stands of Douglas-fir. Communications in Soil Science and Plant Analysis 9, 529-539. Smith J. L., McNeal B. L., Owens E. J. and Klock G. O. (I 981 ) Comparison of nitrogen mineralized under anaerobic and aerobic conditions for some agricultural and forest soils of Washington. Communications in Soil Science and Plant Analysis 12, 997-1009. Van de Dijk S, J. and Troelstra S. R. (1980) Heterotrophic nitrification in a heath soil demonstrated by an h~ situ method. Plant and Soil 57, 11-21. Wall L. L., Gehrke C. W., Neuner T. E., Cathey R. D. and Rexroad P. R. (1975) Total protein nitrogen; evaluation and comparison of four different methods. Journal ~[ the Association of Official Agricultural Chemists 58, 807-81 I. Weber D. F. and Gainey P. L. (1962) Relative sensitivity of nitrifying organisms to hydrogen ions in soils and solutions. Soil Science 94, 138-145. Williams B. L. (1983) Nitrogen transformations and decomposition in litter and humus from beneath closed-canopy Sitka spruce. Forestry 56, 17-32.