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Soil denitrification rates in a subalpine watershed of the eastern Sierra Nevada
Forest Ecoioa and Management, 50 ( 1992) 2 17-23 1 Elsevier :ienrP: Publishers B.V., Amsterdam
217
Soil denitrification rates ina sdmlpine watershed ofthe eastern RF. Walker, SE Hixson and C.M. Skau University of Nevudi:, Rena, Deparrmeni &Range, U’ildlife and Forestry, 1000 Valley Road, Rena. NV89512, USA
(Accepted 18 June 199 1)
ABSTRACT Walker, R.F., Hixson, SE. anti Skau, C.M., 1992. Soil denitrification rates in a subalpine watershed of the eastern Sierra Nevada. For. Ecol. Manage.. 50: 2 17-23 1. Denitrification rates in soils of six subalpine plant communities in an eastern Sierra Nevada watershed were determined by the acetylene inhibition method. Soil atmosphere samnles were collected monthly from June 1986 throu& May 1987 in a rioarian, wet meadow, dry meadow, north-facing forest, south-facing forest and barren site and analy;:ed for NzO content using gas chromatography. Soil temperature, moisture, organic matter, C, N, C:N ratio, NO3-N and pH were examined to assess their effects on denitrification rates, Mean denitrification rates for the year varied from 103.3 pg m-* h-’ in the north-facing forest to 12i!.2 @gm-* h-’ at the barren site, but did not differ significantly among any of the six plant communities. However. comparisons among months within individual communities revealed that the denitrZcation rates in each community varied significantly over the year, and in three of the six sites sign&ant correlations between denitrilication rates and other soil parameters were detected. Soil acidity >uaspositively correlated with denitrilication rate in the riparian and wet meadow communities, and in the dry meadow, soil moisture was positively correlated while soil temperature and organic matter were negatively correlated with denitrification rate. Comparisons among sites within individual rronths revealed significant differences in denitrification rates in June, September, October and January, but no single site consistently exhibited the highest or lowest rate in all 4 months, and only in Octi:ber, when denitritication rates were positively correlated with soil temperature and moisture, was variation in denitrification rates among sites explained by other soil parameters. For the six plant cc inmuniiies overall, soil denitrilication rates were highly variable from June to October, increased sh‘itply from October to December, and then declined from December to May.
INTRODUCTION
Soil denitrification has been intensively studied since Balderston et al. (1976), Yoshinari et al. (1977), Yeomans and Beauchamp (1978), Ryden Correspondence to: R.F. Walker, University of Nevada, Reno, Department of Range, Wildlife and Forestry, 1000 Valley Road, Reno, NV 895 i 2, USA. *Research supported by the Nevada Agricultural Experiment Station and Mclntire-Stennis Cooperative Forestry Research Program Projects 6 i:! and 6 19, University of Nevada, Reno.
0 1992 Elsevier Science Publishers B.V. All rights r:rserved 0378-I 127/92/$05.00
218
R.F. WALKER
ET AL.
et al. ( 1979a) and Walter et al. ( 1979) experimented with acetylene and its inhibitory effects on the reduction of NzO to NZ. However, conclus:ons regarding the soil parameters affecting denitrification have often conflicted. Robertson and Tiedje ( 1984) suggested that N03-N production may be the principal determinate of N20 loss, and Robertson et al. ( 1987) concluded that N03-N supply limited denitrification in a southeastern forest on a clearcut site. However, Myrold ( 1988 ) reported only a weak correlation in agricultural soils and Muller et al. ( 1980) found no correlation in acid soils between denitrification and soil NO,-N content. Lack of a strong correlation between denitritication and this seemingly pivotal factor may be the result of available C being the more limiting parameter (Ryden et al., 1979b; Rolston, 1981; Sextone et al., 1985; Myrold, 1988), although Gamble et al. (1977) found that not all denitrifiers use C as a substrate. Organic matter has also been found to be directly related to denitritication (Dubey and Fox, 1974), and the irregular distribution of particulate organic matter may be a major factor influencing the spatial variability of this process (Parkin, 1987). Stefanson ( 1972 ) observed a strong interrelationship between soil moisture and organic matter as determinates of denitrification, but under anaerobic conditions, N20 evolution has been found to be completely controlled by the amount of easily decomposed organic matter (Burford and Bremner, 1975 ). The anaerobic conditions necessary for denitrification may develop as a result of high O2 consumption around organic matter (Parkin, 1987 ) or as a result of increasing soil moisture content. Several studies have indicated a direct relationship between increasing soil moisture and enhanced denitrification (Stefanson, 1972; Burford and Stefanson, 1973; Denmead et al., 1979; Ryden et al., 1979b; Ryden and Lund, 1980). However, other studies indicate that denitrification is not always correlated with soil moisture or rainfall (Dowdell and Smith, 1974) and concluded that this anomaly may be due to the limitation of available C (Sextone et al., 1985) or low soil temperatures (Mosier et al., 1986). Grundmann et al. ( 1988) found a narrow window of soil moisture and soil N&-N contents where the combined effects of these two parameters produced peaks in denitrificaiion. While soil temperature has generally been demonstrated to have a positive correlation with denitrification rate (Dowdell and Smith, 1974; Ryden et al., 1978, 1979a; Denmead et al., 1979 ), negative correlations have also been reported (Greenlee, 1985; Myrold, 1988) but it was concluded that the cause was a strong negative correlation between soil temperature and soil moisture. Denitrification has also been reported to increase with decreasing soil acidity (Broadbent and Clark, 1965). Muller et al. ( 1980) found a high correlation between denitrification rate and pH in strongly acid forest, mine and agricultural soils. Dubey and Fox ( 1974), however, found that raising the pH of subsoil only enhanced denitrifrcation in that stratum slightly, and Robert-
DENITRIFICATION
RATES
IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
219
son and Tiedje ( 1984) concluded that pH was not well correlated with NzO production. Little research pertaining to denitrification under the stressful conditions of high altitude watersheds has been undertaken. Subsequently, the intent of this study was to investigate denitrification rates and the influence of soil temperature, moisture, organic matter, total C and N, C: N ratio, N03-N and pH on this process in six subalpine plant communities of the eastern Sierra Nevada. MATERIALS AND METHODS
Site description The study site was located near the base of an 80 h: watershed at an elevation of 2134 m on the east slope of the Carson Rr.nge in Nevada, USA (39”7’30”N, 119”53’ 1O”W). The six plant communities studiec! comprised a riparian, wet meadow, dry meadow, north-facing forest, south-
Principal plant species
Riparian
.4lnus tenuifolia Nun., Corms stolonifera Michx., Poa sandbergii Vasey, Juncus spp., Agrostis spp. P. sandbergii Vasey, Juncus ens(folius Wikstr., Agrosh spp. P. sandbergii Vasey, Juncus spp. Abies concoiur (Gord. and Glend.) Lindl., Pinus jeffieeyi Grev. and Balf., Libocedrus decurrens Torr. A. concolor (Gord. and Glend. ) Lindl., 7. j@‘@~i Grev. and Bait: No vegetation
Wet meadow Dry meadow North forest South forest Barren
220
R.F. WALKER
ET AL.
probes constructed of drip irrigation tubing perforated at 5-cm intervals along one side (Ryden et al., 1979a; Greenlee, 1985). The probes were inserted 1 m into the ground with the perforated side facing the chamber to permit CzHz to be supplied to the soil atmosphere. Acetylene was simultaneously supplied to the 12 probes at each site at a rate of 600 ml min- ’beginning 30 min prior to sampling to allow the &Hz to diffuse through the soil profile. During l-h sampling periods, N20 gas samples were drawn from each chamber at a rate of 300 ml min- ‘, passed through tubes of drierite and ascarite to remove Hz0 and CQz gases and adsorbed in vials containing 5 8, molecular sieve material (Ryden et al., 1978). Three Reotemp Model H bimetal coil thermometers (Reotemp Instrument Corp., San Diego, CA) per chamber;were used to measure soil temperature at 5, 10 and 25 cm depths. The Campbell Pacific Neutron Model 503 DR hydroprobe moisture depth gauge (CPN Corp., Pacheco, CA) was employed to measure soil moisture at 15 cm intervals on a percent by volume basis to a depth of 90 cm. Each plant community had three neutron access tubes constructed of 0.13 cm thin-walled aluminum tubing with a 4.83 cm i.d. installed near the chambers. All soil temperature and moisture measurements were taken at the mid-point of each I-h sampling period. Owing to equipment malfunction, soil moisture data are incomplete for certain sites during the months of November and January. Five soil subsamples, collected near each chamber to a depth of 20 cm, were combined into one composite sample per chamber. Samples were transported to the laboratory on ice, oven-dried at 38 “C for 48 h, ground to pass a No. 10 (2.0 mm opening) sieve and shipped to A&L Agricultural Laboratories (Memphis, TN) for analysis of selected chemical properties. Methods of analyses were as follows: percent organic matter and total C by the WalkleyBlack method, calorimetric and titration modifications, respectively; total N by macroXjeldah1 digestion; N@-N by use of a specific ion electrode after extraction with CaSO,; and pH by use of a glass electrode on a 1: 1 mixture (by weight ) of soil and distilled water (American Society of Agronomy, 1965 ). Total C and N values were used to calculate C : N ratios. Laboratory equipment and procedures Laboratory procedures used in this study were described by Focht ( 1978 ), Ryden et al. ( 1978) and Greenlee ( 1985). Nitrous oxide gas was released from the molecular sieve material and allowed to equilibrate overnight. The following day, 0.5 ml NzO gas samples were injected into a gas chromatograph column of SO/ 100 mesh Porapak Q material (500 cm long by 0.2 1 cm i.d.) heated to 50°C. Nitrous oxide gas was detected by a 63Ni electron capture detector heated to 350°C. Argon was the carrier gas with a flow rate of 25 ml min-I. At first, 0.2,0.5 and 1.0 ml of an 11.8 ppm NzO standard were
DENITRIFICATION
RATES IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
221
injected after every three NzO samples. It was later determined that only the 0.5 ml standard injection was necessary, but the standard was then injected after each sample. During analysis of the August samples, the 11.8 ppm N20 standard was exhausted and replaced with a 13.2 ppm NzO standard. Each sample and standard was injected until at least two N20 peak measurements came within 10% of each other. An average sample peak height was then calculated and compared with standard peak heights. Using the standard tank as the source, the field set-up was duplicated in the laboratory in order to calculate recovery rates, which were approximately 90%. Statistical anal@ All data underwent analysis of variance, and differences among means were assessed using the LSD test. Lognormal transformations were performed on denitritication rates (Parkin et al., 1988) and arcsine transformations on soil organic matter and total C and N. Stepwise regressions were performed to ascertain the relationships between denitrification rate and the other soil parameters studied when LSD tests revealed significant differences in denitrification rates among or within sites. A significance level of 605 was selected for all analyses, which were accomplished using the Statistical Analysis System (SAS Institute Inc., Gary, NC). RESULTS
Mean soil denitrification rates for the year did not differ significantly (PI 0.05 ) among the six plant communities, although all other soil parameters examined varied among sites (Table 2 ) . However, denitrilication rates did vary significantly among months at each individual site, and at three of the six sites this variation was related to certain chemical and/or physical parameters. Soil acidity was positively correlated with denitrification rate in both the riparian (r*=0.38, P=O.O34) and the wet meadow communities (r*=0.38, EO.042). A more complex relationship between denitrification and other soil parameters was found in the dry meadow, where soil moisture at 15 cm (r* = 0.15, P= 0.048) was positively correlated while soil temperature at 10 cm (r* = 0.39, P= 0.030) and soil organic matter content ( r*= 0.24, P~O.037) were negatively correlated with denitrification rate. No significant correlations between denitrilication rate and the other soil parameters examined were detected in the north-facing forest, south-facing forest or barren site. The absence of significant differences in denitrification rates among the six plant communities for the year did not preclude such differences in individual months (Fig. 1). In June, the denitrification rate in the north-facing forest exceeded those in the riparian, wet meadow and south-facing forest commu-
-
Denitrification rate (pg m-* h-r) 34.8a 28.8b 12.6~ 11.3c 12.8~ 12.8~
42.7a 31.4b 15.6~ I I .9d 14.6~ 12.6cd
41.9a 39Sb l9.lc ll.Oe 12.96 I l.9de
60cm 36.0b 40.2a 31.2b 9.4d 12.3~ 12.6~
90cm
I I.la 5.6b 4.3c 3.ld 3.5cd 3.0d
Organic matter (%)
6.5a 3.4b 2.5~ 1.8d 2.0cd I .6d
Total C (%)
not differ significantly according to the LSD test (a=0.05).
6.0ab 7.5ab 8.7a 5.0b 7.lab 8.2a
30cm
I5 cm
25 cm
5 cm
IOcm
Soil moisture (% by volume)
Soil temperature (“C)
6.4c Riparian 105.3a s.9c 8.0bc Wet meadow I04.4a 7.lbc l3.9a Dry meadow 106Sa l0.6a 4.8~ North forest 4.6~ 103.3a South forest 103.7a 7.7bc 7.5abc I I .4ab Barren i’20.2a 9.6ab ___ ‘Means within a coKmn sharing a common letter do
Site
0.34a 0.14b O.lOc 0.06d 0.07cd 0.07cd
Total N (0~)
20.0d 24.6~ 25.6bc 29.3a 28.7bc 30.6a
C:N ratio
5.3a 4.6a 3.9b 3.6b 3.8b 3.8b
N09-N (ppm)
6.4b 6.7a 6.2bc 5.7d 6.0~ 6.2bc
pH
Mean denitrification rates and selected physical and chemical properties of soils of six subalpine Sierra Nevada plant communities for the year June 1986 through May 1987”
TABLE 2
DENlTRIFICATlON
RATES IN SUBALPINE
PLANTS
IN SIERRA
223
NEVADA
I
0 0 @ a
Riparian Wet Meadow DrvMeadow N.‘Forest S. Forest ? ?Barren
200 -
A
180 160 +h
140 -
c? &
120 -
2 aJ 2 g ._ ;;i 2 ‘2
ab ab
100 SO-
b b b
60-
‘2 a” 40
ab b
June
Jllly
Aug
Sept
0ct
Nov
Dee
Jan
Feb March
April
May
Fig. 1. Soil denitrification rates in six subalpine plant communities of the eastern Sierra Nevada from June 1986 through May 1987. Within each month, means sharing a common letter do not differ significantly according to the LSD test (~1~0.05).
nities by 75% or more. The denitrification rate at the barren site in September was 85% or more higher than those at either of the meadow or forest sites, and that at the riparian site was 119% or more higher than those of either forest site. In October, the denitrification rate in the dry meadow exceeded that in the wet meadow by 165%, and in January, denitrification rates at the barren and south-facing forest sites were 57% or more higher than that in the north-facing forest. Only in October, however, was the variation in denitrification rates among sites explained by variation in any of the other soil parameters investigated. Denitrification was positively correlated with both soil temperature at 10 cm (r*=0.86, PzO.008) and soil moisture at 90 cm (Y*= 0.12, P= 0.027 ) during this month. In months other than the four indicated above, denitrification rates did not differ significantly among the six plant communities, although the other soil parameters examined varied among sites (Table 3 ) in almost every month of the study. For the six plant communitic overall, a seasonal pattern in soil denitrilication was somewhat evident. From June to October, denitrification rates
I3.Oc l2.k 31.3a 9.ld 18.7b 33.3a
13.0d 10.8d 28.3b l2.2d 18.0~ 33.0a
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
June
July
August
September
7.8d l6.0b 2l.Oa lO.5c li.ac I l.Oc
l6.5b 12.8~ l9.7a l4.7bc l9.3a 2l.Oa
5cm
Site
Sampling date
l3.3b l6.7a 9.7de 11.8bc I I .Ocd
8.5e
31a 20b 5e 6d IOC IOC
31a I 1.8bc Ilb l4.5a 5e lO.Sc 5e l2.7b IOC I3.3ab 9d
7.Sd
l3.Od l5.0cd l8.2b l2.7d 16.3bc 20.7a
32a 28b 9e Ilc Ilc IOd
ll.5d l3.3c l5.7b l0.3d 14.2bc I9.0a
l1.7d ll.Sd 22.7b ll.Od l5.7c 27.0a l4.8d l3.3e 16.3~ 13.7de l7.7b l9.5a
32a 31b 6e ad l2c l2c
15cm
11.8bc 13.5b 17.0a 1o.oc 13.7b lass
25 cm
39a 16b 7e ad l2c ad
39a 28b 7f 8e l3c 9d
39a Sib af IOd l3c 9e
40a 33b 9f ’Qe ISC Ild
30 cm
40a 34b 9d ae l2c 9d
41a 35b 1Od 9e 12c 9e
35b 38a l9c af Ild 9e
35b 38a 19c 9e 12d 9e
l3a 7b 5bc 3c 4bc 3c
15a 6b 5b 3b 4b 4b
l4a 6b 5b 3b 4b 3b
34b 38a 22c 12f lad 13e
40a 37b 16d llf 17c 12e 34b 38a 2lc 9f 12d 10e
I5a 7b 5b 3b 4b 4b
90 cm
60 cm
40a 35b lld 9e 13c 9e
Organic matter (%)
Soil moisture (% by volume)
l2.3d l2.0d 22.3b 9.8d la.7c 25.3a
IOcm
Soil temperature ( “C)
Selected physical and chemical prrperties of soils in six subalpine Sierra Nevada plant communi?ies’
TABLE 3
0.38a 0.13b 0.09b 0.05b 0.07b 0.05b
0.38a 0.14b 0.09b 0.06b 0.07b 0.05b
8.57a 3.77b 2.63b l.70b 2.50b 2.20b 7.77a 4.07b 2.73bc I .83c 2.20bc I .93c
22c 25c 39a 33b 35ab 41a
0.38a 0. I5b 0.07b 0.06b 0.07b 0.04b 8.00a 3.90b 2.87b l.87b 2.40b I .80b
22c 32ab 29b 36a 31ab 36a
23c 26c 30bc 3lbc 38ab 43a
20b 23b 27ab 25ab 34a 32a
C:N ratio
0.45a 0.17b 0.12b 0.06b 0.08b 0.07b
Total N (%)
8.50a 3.76b 3.07b l.60b 2.60b 2.lOb
Total C (%)
6.3s 5.3ab 5.0b 5.3ab 5.0b 5.7ab
6.0a 6.0a 4.7b 4.7b 5.7ab 5.0ab
7.7a 6.7ab 6.Oab 5.0b 5.7b 6.0ab
6.Oa 5.>a 5.7~ 5.3a 4.7a 5.3a
NOj-N (ppm)
6.5a 6.7a 6.4a 5.7b 6.2a 6.3a
6.5ab 6.9a 6.3ab 5.ac 6.2b 6.4ab
6.5a 6.7a 6.3a 5.7b 6.3a 6.2a
6.2a 6.7a 6.2a 5.7b b.i, 6.2a
pH
F 4 F E s 2 F
F
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
October
November
December
January
-0.lab -0.2a - 0.7ab -l.Sb O.Oa -l.Oab
19b
I/lb
35c 39b 54a 9d
39a 39a 29b 8c
42a 31b 19c 9e 10d
34a 30b 18c 8e lid
2.2a 1.5ab 1.7ab -0.2ab 2.la -1.3b
35c 39b 55a 9e lld 9e
41a 38b 23c 8e 12d 8e
43a 31b 19c 8f 12d 9e
O.Oa O.Oa -0.2a -1.3a 0.2a -1.3a
llb 9c
12c 8d
12c 10d
34a 30b 16c 6f 10d 9e
1.8bc
2.2a 2.2a 2.3a 2.3a 2.5a 1.5a
0.5ab 0.7ab 1.Oa 1.Oa 0.6ab O.Ob
21b
41a
33a
0.5a 0.6a 0.8a 0.8a 0.8a -0.2a
2.8b 2.5bc
35a
41a 35a
35c 39b 58a 8f 12d 9e
41a 32b 19c 10f 12d Ile
4Ja 39b 24~ 8e 12d 8e
33a 30b 13c 8f IOe i2d
9c 4d
1.5c 2.0bc 5.0a
I .2b 2.7b 9.3a 2.3b 1.7b 2.5b
5.3cd G.5ab 7.0a 4.3d 5.7bc 4.5d 3.3ab 3.5ab 4.3a 3.0b 3.7ab 4.2a
4.3c 4.2~ 13.0a 3.2c 7.7b 7.Sb
3.5d 3.7d I8.2a 2.5d 7.5c 12.Sb
1 la 6b Sbc 3c 4bc 3c
9a 5b 4bc 2c 3c 2c
IOa 6b 4bc 3c 2c 2c
12a 7b 5bc 3c 4bc 3c
6.20a 3.33b 2.60bc 1.9oc 2.30bc 1.5oc
0.41a 0.17b 0.12bc 0.08bc 0. IObc 0.06~
6.7a 3.7b 3.3b 2.7b 3.0b 3.0b
4.la 4.3a 3.0b 3.0b 3.0b 2.3b
7.7a 6.7a 5.0b 5.0b 5.0b 4.7b
16d 5.3a 20cd 4.7a 2lbc 3.3b 25ab 3.Ob 24abc 3.Ob 27a 3.3b
18a 32a 19a 22a 19a 20a
0.29a 0.12b 0.12b 0.05b 0.08b 0.07b
5.17a 3.03b 2.30bc 1.23~ I .47c 1.37c
21c 29b 28bc 36ab 34ab 37a
0.32a 18bc 0.16b 21ab 0.13br 16~ 0.06d 24a 0.07cd 19bc 0.07cd 2 lab
0.34a 0.14b O.lOb 0.05b 0.07b 0.05b
5.83a 3.30b 2.13bc 1.43c 1.4oc 1.37c
6.90a 3.87b 2.lObc 1.80~ 2.20bc 1.9oc
6.3a 6.6a 6.la 5.5b 6.0a 6.la
6.3ab 6.6a 6.2abc 5.9c 6.0bc 6.labc
6.3a 6.6a 5.6~ 5.6~ 6.4a 6.0b
6.6ab 6.9a 6.5ab 5.8d 6.1~ 6.4b
;-” tl *
c z 5 $ g ?J 5 ? $ 2 5. $ $
Z
g Z 2 ii P+ 5
lO.Ob 12.8b 19.2a 0.2c 2.2c
1.2c
ll.3d 17.3bc 20.3b 8.5d l4.k 24.Oa
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
April
May
lO.Od 14.k l7.7b 7.7e 10.8d 20.8a
6.3c 8.8b 12.3a -0.5e 3.7d 2.5d
0.5b 4.Oa 0.8b -0.2b 0.5b O.Ob
10.3b I?.Oab 13.3a 6.8~ lO.7b l2.7a
4.2bc 4.5b 7.7a 0.2d 3.Oc 5.0b
1.2bc 2.8a 2.5ab 0.3c 1.3abc 0.7c
36a 34b 9e 8f 12d 14.c
37a 34b 17e 22c 17d 13f
45a 36b 19f 21d 20e 22c
0.29a 0.13b O.lObc 0.05bc 0.06bc 0.04~
4.77a 2.53b 2.73b 1.83b 2.07b I .47b 6.lOa 3.23b 2.27bc 6.27a 1.57c 1.3oc
8a 4b 5b 3b 4b 3b 1 la 6b 4bc 2c 3c 2c
39b 44a 31c 1Of 12e l8d
45a 43b 2lc l6e 13f ISd 45a 43b 19c 12f 13e 14d
45a 37b 20d 19e l8f 2lc 45a 35b ISC lie 14d ISC
39b 44a 26c IOf 12e 16d
0.33a O.ISb 0.07b 0.08b 0.07b 0.04b
5.87a 3.1Ob 2.03b 2.4Ob 1.87b 1.30b
IOa 5b 4b 4b 3b 5b
38c 42b 43a IOf 12e 2Od
O.23a 0.13b O.lOb 0.05c 0.06~ 0.05c
0.23a 0.13b O.lObc O.lObc 0.07bc o.oc
43b 44a 22c l7e 13f 19d
3.87a 2.63b 1.97bcd 2.47bc 1.87cd I .43d
53a 37b 22c l9e 20d 19e
7a 5b 3c 4b 3c 3c
38c 43b 54a 9f 12e 17d
43a 42b 24c ISe 13f 17d
26a 2Sa 23a 23a 24a 24a
19c 21bc 27abc 36a 32ab 34a
19c 2lbc 27ab 30ab 27ab 33a
18b 2lb 20b 25a 27a 26a
C:N ratio
2.3a 2.3a 2.Oa %.Oa 2.7a 2.0a
3.Oa 3.0a 2.Ob 2.Ob 2.7a 2.0b
3.Oab 3.Oab 3.3a 2.7ab 2.3b 3.3a
4.3a 4.3a 3.3b 3.0b 2.7b 3.0b
NO,-N (ppm)
‘Means within a column and measurement period sharing a common letter do not differ significantly according to the LSD test (~~~0.05).
O.Ob 6.7a O.2b -0.2b O.Ob -0.5b
Riparian Wet meadow Dry meadow North forest South forest Bdrrcn
March
0.2ab 0.7a 0.2ab -0.8~ -0.2bc -0.2bc
Total N (%)
45a 35b 22c 19e 19e 21d
38a 34b 21d 20e 19f 26c
-O.Ab 3.0a 0.3b O.Ob -0.7b 0.2b
O.Oa 0.3a -0.7ab -2.2b -2.Ob -0.5ab
Riparia!: Wet meadow Dry meadow North forest South forest Barren
February
9Ocm
Total C (0~)
60 cm
Organic matter (o/o)
30 cm
l5cm
25 cm
‘cm
1Ocm
Soil moisture (% by volume)
Soil temperature ( C)
Site
Sampling date
TABLE 3 (continued)
6.5a 6.6a 6.2a 5.5b 6.0ab 6.3a
6.5ab 6.88 6.2ab 5.6~ 6.lb 6.3ab
6.4ab 6.6a 6.2bc 5.6d 6.1~ 6.3bc
6.3a 6.6a 6.2a 6.la 5.6b &la
pH
; e $
k
DENITRIFICATION
RATES IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
221
were highly v-ariable, but then increased sharply from October to December. Pronounced deciine in denitrification rates from December to February was followed by a fiu-ther but more gradual decrease from February through May. DISCUSSION AND CONCLUSIONS
Comparisons of soil denitrifkation rates over a 12 month period in six subalpine Sierra Nevada plant communities revealed that the mean rates for the year did not vary significantly among sites. However, environmental conditions in the six communities differed, and variation in the denitrification rates of individual communities over the 12 months was not related to the same soil parameters at all sites. At the riparian and wet meadow sites, denitrification was positively correlated with soil acidity, although the magnitude or the variation in pH was relatively small. Subsequently, it is possible that denitrificacion was actually related to soil microbe activity on soluble C substrate rather than soil acidity, and increased microbe respiration resulted in increased acidity as follows (Donahue et al., 1983): COz+H20=H++HC0~ Thus, these small changes in soil pH may have been a function of COZ production from C mineralization by soil microbes rather than a factor influencing denitrification rates in the relatively wet riparian and wet meadow communities. Nevertheless, no significant correlation between denitrification and total C was found at any site in this study, and furthermore, some denitrifying bacteria do not use C as a substrate (Gamble et al.. 1977 ). In the dry meadow, which was generally intermediate in soil moisture content among the six communities, denitrification rate increased with increasing soil moisture at the 15 cm depth as expected, but was also negalively correlated with soil temperature at 10 cm and organic matter. However, denitrification may be influenced more by soii moisture than by temperature as suggested by Myrold ( 1988 ) and Greenlee ( 1985 ), and thus the increased denitrification rate during the colder months of the year may have been the result of increased soil moisture content during these months rather than declining temperatures. The negative correlation between denitrification rate and soil organic matter content in the dry meadow may have resulted from a decrease in organic mat” ;r mineralization and a subsequent reduction in soil NOX-N available for denitritication. Robertson et al. ( 1987) identified low N03-N availability as a factor limiting denitritication, but in this study no significant correlation was detected between denitrification rate and soil N03-N at any site. Denitriikation rates at the three driest sites, i.e. both forest sites and the barren site, were not found to be influenced over the 12 months by any other soil parameter studied. Eithe: parameters other than those examined affected the rates at these three rites, or denitrification at these sites responded to different pa-
228
R.F. WALKER
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rameters as environmental conditions changed during the year, confounding any easily discernible relationship. The difference in denitrification rate between the dry and wet meadow communities in October was largely accounted for by differences in soil temperature, and to a lesser degree soil moisture. Soil temperature, and its effects on denitrifying bacteria, solubility of NzO in water and diffusion of N20 through the soil profile, has been linked to denitrification in several studies (Ryden et al., 1978, 1979a; Blackmer et al., 1982 ). Thus, the positive correlation between denitrificzttion rate and temperature at I(! cm found here is not surprising. The positive correlation between denitrification rate and soil moisture at 90 cm in October may have reflected the higher moisture content at this depth in the dry meadow. Dubey and Fox ( 1974) found that denitrification can occur at lower depths if sufficient substrate for bacteria is available. Denitrification has been reported to be highly variable in aerobic soils (Burford and Stefanson, 1973 ), where it is largely confined to anaerobic microsites within the fine pores and aggregates of soils that are aerobic overall. In this study, this variability was particularly evident from June through October, the period of maximum aeration. However, denitrification rates in the relatively moist riparian and wet meadow soils were not consistently higher than those of the other sites during this period, as would be expected. In a process reported earlier by Bowden and Bormann ( 1986 ), NzO may have been flushed from the soil of the riparian community, located adjacent to a perennial stream, and from that of the wet meadow, located in a seep zone near the base of a bill, by lateral movement of subsurface water. This would result in underestimation of the denitrification rates at these two sites. Overall, denitrification rates rose sharply from October to December despite the declining soil temperatures during this period. Denitrifying bacterial populations have been found to differ in their response to different temperatures (Powlson et al., 1988 ), and there is evidence that multiple denitrifying bacterial populations are present in many soils (Gamble et al., 1977). Thus, it is possible that the increasing denitrification rates found here in late fall resulted from the activity of a bacterial population adapted to colder soil temperatures. Except for the dry meadow, these rising denitrification rates did not reflect increasing soil moisture, as this study was conducted during a drought year and soil moisture content remained relatively constant during these months. There was a sharp overall decrease in the rates of denitrification from December to February, followed by further gradual decline through May. From December to February, extremely cold soil temperatures may have precipitated the sharp drop in denitrification rates, as the soils of all six communities were near or below freezing during this period. However, soil NOA-N concentrations were also generally declining, a trend that continued through May.
DENITRIFICATION
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During the winter, cold soil temperatures likely caused a reduction of organic matter mineralization and thus N03-N production, and in the spring, plant uptake may have further decreased N03-N concentration. Subsequently, declining soil N03-N concc &rations likely contributed to the overall decline in denitrification rates during winter and spring. Previous studies have reported potential limitations imposed by N03-N supply on denitrification (Robertson and Tiedje, 1984; Robertson et al., 1987 ), including those resulting from plant NO,-N absorption (Ryden, I 98 1: Haider et al., 1987 ). Denitrification in wildland soils is a complex and variable process, responding to different environmental stimuli and the interactions of these stimuli in diverse habitats. These results illustrate the temporal and spatial variability of soil denitrification in a subalpine watershed of the Sierra Nevada. Despite the diversity of plant community types represented in this watershed, however, soil denitritication rates over the long-term were not found to vary appreciably among these plant communities. ACKNOWLEDGMENTS
The authors wish to thank G.C. Miller, Y.O. Koh and C.E. Crume of the University of Nevada, Reno for their assistance during this research.
REFERENCES American Society of Agronomy, 1965. In: C.A. Black (Editor), Methods of Soil Analysis. Part 2. Am. Sot. Agron., Madison, WI. Balderston, W.L., Sherr, B. and Payne, W.J., 1976. Blockage by acetylene of nitrous oxide reduction in Pseudo~nonasperfecfornarinus. Appl. Environ. Microbial., 31: 504-508. Blackmer, A.M., Robbins, S.G. and Bremner, J.M., 1982. Diurnal variability in rate of emission of nitrous oxide from soiis. Soil Sci. Sot. Am. J., 46: 937-942. Bowden, W.B. and Bormann, F.H., 1986. Transport and loss of nitrous oxide in soil water after forest clear-cutting. Science, 233: 867-869. Broadbent, F.E. and Clark, F.E., 1965. Denitrification. in: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen. Am. Sot. Agron., Madison, WI, pp. 344-359. Burford, J.R. and Bremner, J.M., 1975. Relationships between the denitrilication capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Biol. Biochem., 7: 389-394. Burford, J.R. and Stefanson, R.C., 1973. Measurement of gaseous losses of nitrogen from soils. Soil Biol. Biochem., 5: 133-141. Denmead, O.T., 1979. Chamber systems for measuring nitrous oxide emission from soils in the field. Soil Sci. Sot. Am. J., 43: 89-95. Denmead, O.T., Freney, J.R. and Simpson, J.R., 1979. Studies of nitrous oxide emission from a grass sward. Soil Sci. Sot. Am. J., 43: 726-728. Donahue, R.L., Miller, R.W. and Shickluna, J.C., 1983. Soils: An Introduction to Soils and Plant Growth. Prentice-Hall, Englewood Cliffs, NJ. Dowdell, R.J. and Smith, K.A., 1974. Field studies of the soil atmosphere: II. Occurrence of nitrous oxide. J. Soil Sci., 25: 231-238.
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Dubey, H.D. and Fox, R.H., I9 Y-l.Denitrification losses from humid tropical soils of Puerto Rico. Soil Sci. Sot. Am. Proc.. iU: 9 17-920. Focht, D.D., 1978. Methods for aG,iiysis of denitrification in soils. In: D.R. Nielson and J.G. MacDonald (Editors), Nitrog;: :‘t the Environment. Vol. 2. Academic Press, New York, pp. 433-490. Gamble, T.N., Betlach, M.R. and Txdjije, J.M., 1977. Numerically dominant denitrifying bacteria from world soils. Appl. Envjr.in. Microbial., 33: 926-939. Greenlee, D.L., 1985. Denitrilication ! .IICSof a mountain meadow near Lake Tahoe. M.S. Thesis. University of Nevada, Reno, NV. Grundmann, G.L., Rolston, D.E. and Kachanoski, R.G., 1988. Field soil properties influencing the variability of denitrification gas fluxes. Soil Sci. Sot. Am. J., 52: 1351-l 355. Haider, K., Moiser, A. and Heinemeyer. 0.: 1987. The effect of growing plants on denitrification at high soil nitrate concentrations. Soil Sci. Sot. Am. J., 5 I: 97-102. Mosier, A.R., Guenzi, W.D. and Schweizer, E.E., 1986. Soil losses of dinitrogen and nitrous oxide from irrigated crops in northeastern Colorado. Soil Sci. Sot. Am. J., 50: 344-348. Muller, M.M., Sundman, V. and Skujins, .I., 1980. Denitrilication in low pH spodosols and peats determined with the acetylene in ,bition method. Appl. Environ. Microbial., 40: 235239. Myrold, D.D., 1988. Denitrification in ryegrass and winter wheat cropping systems of western Oregon. Soil Sci. Sot. Am. J., 52: 412-416. Parkin, T.B., 1987. Soil microsites as a source of denitrification variability. Soil Sci. Sot. A,?. J., 51: 1194-l 199. Parkin, T.B., Meisinger, J.J., Chester, ST., S:“arr. J.L. and Robinson, J.A.. 1988. Evaluation of statistical estimation methods for lognormally distributed variables. Soil Sci. Sot. Am. J., 52: 323-329. Powlson, D-S., Saffigna, P.G. and Kragt-Cotiaar, M.. 1988. Denitrilication at sub-optimal temperatures in soils from different climatic zones. Soil Biol. Biochem., 20: 7 19-723. Robertson, G.P. and Tiedje, J.M., 1984. Denitsification and nitrous oxide production in successional and old-growth Michigan forests. Soil Sci. Sot. Am. J., 48: 383-389. Robertson, G.P., Vitousek, P.M., Matson, P.A. and Tiedje, J.M., 1987. Denitrification in a clearcut loblolly pine (Pinus &e&z L.) plantation in the southeastern U.S. Plant Soil, 97: 119-129. Ralston, D.E., 198 1. Nitrous oxide and nitrogen gas production in fertilizer loss. In: C.C. Delwiche (Editor), Denitrilication, Nitrilicatton and Atmospheric Nitrous Oxide. Wiley, New York, pp. 127-149. Ryden, J C., I98 I. N20 exchange between a grassland soil and the atmosphere. Nature. 292: 235-237. Ryden, J.C. and Lund, L.J., 1980. Nature anti ;_a:cnt of directly measured denitrilication losses m units. Soil Sci. Sot. Am. J., 44: 505-5 11. from some irrigated vegetable crop pro& Ryden. J.C., Lund. L.J. and Focht, D.D.. -Direct in-field measurement of nitrous oxide flux from soils. Soil Sci. Sot. Am. J.. -i_ -737. Ryden, J.C., Lund, L.J. and Foch?, Dm1 9.~.Direct measurement of denitrilication loss from soils: I. Laboratory evaluarioii km inhibition of nitrous oxide reduction. Soil Sci. Sot. Am. J., 43: 104-l 10. Ryden, J.C.. Lund, L.J., Letey, J. and Fob. !P.. 1979a. Direct measurement of denitrifcation loss from soils: II. Developmem > .&cation of field methods. Soil Sci. Sot. Am. J., 43: 1 IO-1 18. Sextone, A.J.. Parkin, T.B. and Tiedje, J.M.. 1985. Temporal response of soil denitrification rates to rainfall and irrigation. Soil Set. Sot. Am. J., 49: 99-193. Stefanson. R.C.I 1972. Soil denitrification i-i sealed soil-plant systems: 1. Effect of plants, soil water content and soil organic matter comcn?. Plant Soil. 33: 113-l 27.
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Walter, H.M., Keeney, D.R. and Fillet-y, I.R., 1979. Inhibition of nitrification by acetylene. Soil Sci. Sot. Am. J., 43: 195-196. Yeomans, J.C. and Beauchamp, E.G., 1978. Limited inhibition of nitrous oxide reduction in soil in the presence of acetylene. Soil Biol. Biochem., 10: 5 17-S 19. Yoshinari, T., Hynes, R. and Knowles, R., 1977. Acetylene inhibition of nitrous oxide reduction and measurement of denitrification and nitrogen fixation in soil. Soil Biol. Biochem., 9: 177-183.
217
Soil denitrification rates ina sdmlpine watershed ofthe eastern RF. Walker, SE Hixson and C.M. Skau University of Nevudi:, Rena, Deparrmeni &Range, U’ildlife and Forestry, 1000 Valley Road, Rena. NV89512, USA
(Accepted 18 June 199 1)
ABSTRACT Walker, R.F., Hixson, SE. anti Skau, C.M., 1992. Soil denitrification rates in a subalpine watershed of the eastern Sierra Nevada. For. Ecol. Manage.. 50: 2 17-23 1. Denitrification rates in soils of six subalpine plant communities in an eastern Sierra Nevada watershed were determined by the acetylene inhibition method. Soil atmosphere samnles were collected monthly from June 1986 throu& May 1987 in a rioarian, wet meadow, dry meadow, north-facing forest, south-facing forest and barren site and analy;:ed for NzO content using gas chromatography. Soil temperature, moisture, organic matter, C, N, C:N ratio, NO3-N and pH were examined to assess their effects on denitrification rates, Mean denitrification rates for the year varied from 103.3 pg m-* h-’ in the north-facing forest to 12i!.2 @gm-* h-’ at the barren site, but did not differ significantly among any of the six plant communities. However. comparisons among months within individual communities revealed that the denitrZcation rates in each community varied significantly over the year, and in three of the six sites sign&ant correlations between denitrilication rates and other soil parameters were detected. Soil acidity >uaspositively correlated with denitrilication rate in the riparian and wet meadow communities, and in the dry meadow, soil moisture was positively correlated while soil temperature and organic matter were negatively correlated with denitrification rate. Comparisons among sites within individual rronths revealed significant differences in denitrification rates in June, September, October and January, but no single site consistently exhibited the highest or lowest rate in all 4 months, and only in Octi:ber, when denitritication rates were positively correlated with soil temperature and moisture, was variation in denitrification rates among sites explained by other soil parameters. For the six plant cc inmuniiies overall, soil denitrilication rates were highly variable from June to October, increased sh‘itply from October to December, and then declined from December to May.
INTRODUCTION
Soil denitrification has been intensively studied since Balderston et al. (1976), Yoshinari et al. (1977), Yeomans and Beauchamp (1978), Ryden Correspondence to: R.F. Walker, University of Nevada, Reno, Department of Range, Wildlife and Forestry, 1000 Valley Road, Reno, NV 895 i 2, USA. *Research supported by the Nevada Agricultural Experiment Station and Mclntire-Stennis Cooperative Forestry Research Program Projects 6 i:! and 6 19, University of Nevada, Reno.
0 1992 Elsevier Science Publishers B.V. All rights r:rserved 0378-I 127/92/$05.00
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et al. ( 1979a) and Walter et al. ( 1979) experimented with acetylene and its inhibitory effects on the reduction of NzO to NZ. However, conclus:ons regarding the soil parameters affecting denitrification have often conflicted. Robertson and Tiedje ( 1984) suggested that N03-N production may be the principal determinate of N20 loss, and Robertson et al. ( 1987) concluded that N03-N supply limited denitrification in a southeastern forest on a clearcut site. However, Myrold ( 1988 ) reported only a weak correlation in agricultural soils and Muller et al. ( 1980) found no correlation in acid soils between denitrification and soil NO,-N content. Lack of a strong correlation between denitritication and this seemingly pivotal factor may be the result of available C being the more limiting parameter (Ryden et al., 1979b; Rolston, 1981; Sextone et al., 1985; Myrold, 1988), although Gamble et al. (1977) found that not all denitrifiers use C as a substrate. Organic matter has also been found to be directly related to denitritication (Dubey and Fox, 1974), and the irregular distribution of particulate organic matter may be a major factor influencing the spatial variability of this process (Parkin, 1987). Stefanson ( 1972 ) observed a strong interrelationship between soil moisture and organic matter as determinates of denitrification, but under anaerobic conditions, N20 evolution has been found to be completely controlled by the amount of easily decomposed organic matter (Burford and Bremner, 1975 ). The anaerobic conditions necessary for denitrification may develop as a result of high O2 consumption around organic matter (Parkin, 1987 ) or as a result of increasing soil moisture content. Several studies have indicated a direct relationship between increasing soil moisture and enhanced denitrification (Stefanson, 1972; Burford and Stefanson, 1973; Denmead et al., 1979; Ryden et al., 1979b; Ryden and Lund, 1980). However, other studies indicate that denitrification is not always correlated with soil moisture or rainfall (Dowdell and Smith, 1974) and concluded that this anomaly may be due to the limitation of available C (Sextone et al., 1985) or low soil temperatures (Mosier et al., 1986). Grundmann et al. ( 1988) found a narrow window of soil moisture and soil N&-N contents where the combined effects of these two parameters produced peaks in denitrificaiion. While soil temperature has generally been demonstrated to have a positive correlation with denitrification rate (Dowdell and Smith, 1974; Ryden et al., 1978, 1979a; Denmead et al., 1979 ), negative correlations have also been reported (Greenlee, 1985; Myrold, 1988) but it was concluded that the cause was a strong negative correlation between soil temperature and soil moisture. Denitrification has also been reported to increase with decreasing soil acidity (Broadbent and Clark, 1965). Muller et al. ( 1980) found a high correlation between denitrification rate and pH in strongly acid forest, mine and agricultural soils. Dubey and Fox ( 1974), however, found that raising the pH of subsoil only enhanced denitrifrcation in that stratum slightly, and Robert-
DENITRIFICATION
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IN SUBALPINE
PLANTS
IN SIERRA
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219
son and Tiedje ( 1984) concluded that pH was not well correlated with NzO production. Little research pertaining to denitrification under the stressful conditions of high altitude watersheds has been undertaken. Subsequently, the intent of this study was to investigate denitrification rates and the influence of soil temperature, moisture, organic matter, total C and N, C: N ratio, N03-N and pH on this process in six subalpine plant communities of the eastern Sierra Nevada. MATERIALS AND METHODS
Site description The study site was located near the base of an 80 h: watershed at an elevation of 2134 m on the east slope of the Carson Rr.nge in Nevada, USA (39”7’30”N, 119”53’ 1O”W). The six plant communities studiec! comprised a riparian, wet meadow, dry meadow, north-facing forest, south-
Principal plant species
Riparian
.4lnus tenuifolia Nun., Corms stolonifera Michx., Poa sandbergii Vasey, Juncus spp., Agrostis spp. P. sandbergii Vasey, Juncus ens(folius Wikstr., Agrosh spp. P. sandbergii Vasey, Juncus spp. Abies concoiur (Gord. and Glend.) Lindl., Pinus jeffieeyi Grev. and Balf., Libocedrus decurrens Torr. A. concolor (Gord. and Glend. ) Lindl., 7. j@‘@~i Grev. and Bait: No vegetation
Wet meadow Dry meadow North forest South forest Barren
220
R.F. WALKER
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probes constructed of drip irrigation tubing perforated at 5-cm intervals along one side (Ryden et al., 1979a; Greenlee, 1985). The probes were inserted 1 m into the ground with the perforated side facing the chamber to permit CzHz to be supplied to the soil atmosphere. Acetylene was simultaneously supplied to the 12 probes at each site at a rate of 600 ml min- ’beginning 30 min prior to sampling to allow the &Hz to diffuse through the soil profile. During l-h sampling periods, N20 gas samples were drawn from each chamber at a rate of 300 ml min- ‘, passed through tubes of drierite and ascarite to remove Hz0 and CQz gases and adsorbed in vials containing 5 8, molecular sieve material (Ryden et al., 1978). Three Reotemp Model H bimetal coil thermometers (Reotemp Instrument Corp., San Diego, CA) per chamber;were used to measure soil temperature at 5, 10 and 25 cm depths. The Campbell Pacific Neutron Model 503 DR hydroprobe moisture depth gauge (CPN Corp., Pacheco, CA) was employed to measure soil moisture at 15 cm intervals on a percent by volume basis to a depth of 90 cm. Each plant community had three neutron access tubes constructed of 0.13 cm thin-walled aluminum tubing with a 4.83 cm i.d. installed near the chambers. All soil temperature and moisture measurements were taken at the mid-point of each I-h sampling period. Owing to equipment malfunction, soil moisture data are incomplete for certain sites during the months of November and January. Five soil subsamples, collected near each chamber to a depth of 20 cm, were combined into one composite sample per chamber. Samples were transported to the laboratory on ice, oven-dried at 38 “C for 48 h, ground to pass a No. 10 (2.0 mm opening) sieve and shipped to A&L Agricultural Laboratories (Memphis, TN) for analysis of selected chemical properties. Methods of analyses were as follows: percent organic matter and total C by the WalkleyBlack method, calorimetric and titration modifications, respectively; total N by macroXjeldah1 digestion; N@-N by use of a specific ion electrode after extraction with CaSO,; and pH by use of a glass electrode on a 1: 1 mixture (by weight ) of soil and distilled water (American Society of Agronomy, 1965 ). Total C and N values were used to calculate C : N ratios. Laboratory equipment and procedures Laboratory procedures used in this study were described by Focht ( 1978 ), Ryden et al. ( 1978) and Greenlee ( 1985). Nitrous oxide gas was released from the molecular sieve material and allowed to equilibrate overnight. The following day, 0.5 ml NzO gas samples were injected into a gas chromatograph column of SO/ 100 mesh Porapak Q material (500 cm long by 0.2 1 cm i.d.) heated to 50°C. Nitrous oxide gas was detected by a 63Ni electron capture detector heated to 350°C. Argon was the carrier gas with a flow rate of 25 ml min-I. At first, 0.2,0.5 and 1.0 ml of an 11.8 ppm NzO standard were
DENITRIFICATION
RATES IN SUBALPINE
PLANTS
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221
injected after every three NzO samples. It was later determined that only the 0.5 ml standard injection was necessary, but the standard was then injected after each sample. During analysis of the August samples, the 11.8 ppm N20 standard was exhausted and replaced with a 13.2 ppm NzO standard. Each sample and standard was injected until at least two N20 peak measurements came within 10% of each other. An average sample peak height was then calculated and compared with standard peak heights. Using the standard tank as the source, the field set-up was duplicated in the laboratory in order to calculate recovery rates, which were approximately 90%. Statistical anal@ All data underwent analysis of variance, and differences among means were assessed using the LSD test. Lognormal transformations were performed on denitritication rates (Parkin et al., 1988) and arcsine transformations on soil organic matter and total C and N. Stepwise regressions were performed to ascertain the relationships between denitrification rate and the other soil parameters studied when LSD tests revealed significant differences in denitrification rates among or within sites. A significance level of 605 was selected for all analyses, which were accomplished using the Statistical Analysis System (SAS Institute Inc., Gary, NC). RESULTS
Mean soil denitrification rates for the year did not differ significantly (PI 0.05 ) among the six plant communities, although all other soil parameters examined varied among sites (Table 2 ) . However, denitrilication rates did vary significantly among months at each individual site, and at three of the six sites this variation was related to certain chemical and/or physical parameters. Soil acidity was positively correlated with denitrification rate in both the riparian (r*=0.38, P=O.O34) and the wet meadow communities (r*=0.38, EO.042). A more complex relationship between denitrification and other soil parameters was found in the dry meadow, where soil moisture at 15 cm (r* = 0.15, P= 0.048) was positively correlated while soil temperature at 10 cm (r* = 0.39, P= 0.030) and soil organic matter content ( r*= 0.24, P~O.037) were negatively correlated with denitrification rate. No significant correlations between denitrilication rate and the other soil parameters examined were detected in the north-facing forest, south-facing forest or barren site. The absence of significant differences in denitrification rates among the six plant communities for the year did not preclude such differences in individual months (Fig. 1). In June, the denitrification rate in the north-facing forest exceeded those in the riparian, wet meadow and south-facing forest commu-
-
Denitrification rate (pg m-* h-r) 34.8a 28.8b 12.6~ 11.3c 12.8~ 12.8~
42.7a 31.4b 15.6~ I I .9d 14.6~ 12.6cd
41.9a 39Sb l9.lc ll.Oe 12.96 I l.9de
60cm 36.0b 40.2a 31.2b 9.4d 12.3~ 12.6~
90cm
I I.la 5.6b 4.3c 3.ld 3.5cd 3.0d
Organic matter (%)
6.5a 3.4b 2.5~ 1.8d 2.0cd I .6d
Total C (%)
not differ significantly according to the LSD test (a=0.05).
6.0ab 7.5ab 8.7a 5.0b 7.lab 8.2a
30cm
I5 cm
25 cm
5 cm
IOcm
Soil moisture (% by volume)
Soil temperature (“C)
6.4c Riparian 105.3a s.9c 8.0bc Wet meadow I04.4a 7.lbc l3.9a Dry meadow 106Sa l0.6a 4.8~ North forest 4.6~ 103.3a South forest 103.7a 7.7bc 7.5abc I I .4ab Barren i’20.2a 9.6ab ___ ‘Means within a coKmn sharing a common letter do
Site
0.34a 0.14b O.lOc 0.06d 0.07cd 0.07cd
Total N (0~)
20.0d 24.6~ 25.6bc 29.3a 28.7bc 30.6a
C:N ratio
5.3a 4.6a 3.9b 3.6b 3.8b 3.8b
N09-N (ppm)
6.4b 6.7a 6.2bc 5.7d 6.0~ 6.2bc
pH
Mean denitrification rates and selected physical and chemical properties of soils of six subalpine Sierra Nevada plant communities for the year June 1986 through May 1987”
TABLE 2
DENlTRIFICATlON
RATES IN SUBALPINE
PLANTS
IN SIERRA
223
NEVADA
I
0 0 @ a
Riparian Wet Meadow DrvMeadow N.‘Forest S. Forest ? ?Barren
200 -
A
180 160 +h
140 -
c? &
120 -
2 aJ 2 g ._ ;;i 2 ‘2
ab ab
100 SO-
b b b
60-
‘2 a” 40
ab b
June
Jllly
Aug
Sept
0ct
Nov
Dee
Jan
Feb March
April
May
Fig. 1. Soil denitrification rates in six subalpine plant communities of the eastern Sierra Nevada from June 1986 through May 1987. Within each month, means sharing a common letter do not differ significantly according to the LSD test (~1~0.05).
nities by 75% or more. The denitrification rate at the barren site in September was 85% or more higher than those at either of the meadow or forest sites, and that at the riparian site was 119% or more higher than those of either forest site. In October, the denitrification rate in the dry meadow exceeded that in the wet meadow by 165%, and in January, denitrification rates at the barren and south-facing forest sites were 57% or more higher than that in the north-facing forest. Only in October, however, was the variation in denitrification rates among sites explained by variation in any of the other soil parameters investigated. Denitrification was positively correlated with both soil temperature at 10 cm (r*=0.86, PzO.008) and soil moisture at 90 cm (Y*= 0.12, P= 0.027 ) during this month. In months other than the four indicated above, denitrification rates did not differ significantly among the six plant communities, although the other soil parameters examined varied among sites (Table 3 ) in almost every month of the study. For the six plant communitic overall, a seasonal pattern in soil denitrilication was somewhat evident. From June to October, denitrification rates
I3.Oc l2.k 31.3a 9.ld 18.7b 33.3a
13.0d 10.8d 28.3b l2.2d 18.0~ 33.0a
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
June
July
August
September
7.8d l6.0b 2l.Oa lO.5c li.ac I l.Oc
l6.5b 12.8~ l9.7a l4.7bc l9.3a 2l.Oa
5cm
Site
Sampling date
l3.3b l6.7a 9.7de 11.8bc I I .Ocd
8.5e
31a 20b 5e 6d IOC IOC
31a I 1.8bc Ilb l4.5a 5e lO.Sc 5e l2.7b IOC I3.3ab 9d
7.Sd
l3.Od l5.0cd l8.2b l2.7d 16.3bc 20.7a
32a 28b 9e Ilc Ilc IOd
ll.5d l3.3c l5.7b l0.3d 14.2bc I9.0a
l1.7d ll.Sd 22.7b ll.Od l5.7c 27.0a l4.8d l3.3e 16.3~ 13.7de l7.7b l9.5a
32a 31b 6e ad l2c l2c
15cm
11.8bc 13.5b 17.0a 1o.oc 13.7b lass
25 cm
39a 16b 7e ad l2c ad
39a 28b 7f 8e l3c 9d
39a Sib af IOd l3c 9e
40a 33b 9f ’Qe ISC Ild
30 cm
40a 34b 9d ae l2c 9d
41a 35b 1Od 9e 12c 9e
35b 38a l9c af Ild 9e
35b 38a 19c 9e 12d 9e
l3a 7b 5bc 3c 4bc 3c
15a 6b 5b 3b 4b 4b
l4a 6b 5b 3b 4b 3b
34b 38a 22c 12f lad 13e
40a 37b 16d llf 17c 12e 34b 38a 2lc 9f 12d 10e
I5a 7b 5b 3b 4b 4b
90 cm
60 cm
40a 35b lld 9e 13c 9e
Organic matter (%)
Soil moisture (% by volume)
l2.3d l2.0d 22.3b 9.8d la.7c 25.3a
IOcm
Soil temperature ( “C)
Selected physical and chemical prrperties of soils in six subalpine Sierra Nevada plant communi?ies’
TABLE 3
0.38a 0.13b 0.09b 0.05b 0.07b 0.05b
0.38a 0.14b 0.09b 0.06b 0.07b 0.05b
8.57a 3.77b 2.63b l.70b 2.50b 2.20b 7.77a 4.07b 2.73bc I .83c 2.20bc I .93c
22c 25c 39a 33b 35ab 41a
0.38a 0. I5b 0.07b 0.06b 0.07b 0.04b 8.00a 3.90b 2.87b l.87b 2.40b I .80b
22c 32ab 29b 36a 31ab 36a
23c 26c 30bc 3lbc 38ab 43a
20b 23b 27ab 25ab 34a 32a
C:N ratio
0.45a 0.17b 0.12b 0.06b 0.08b 0.07b
Total N (%)
8.50a 3.76b 3.07b l.60b 2.60b 2.lOb
Total C (%)
6.3s 5.3ab 5.0b 5.3ab 5.0b 5.7ab
6.0a 6.0a 4.7b 4.7b 5.7ab 5.0ab
7.7a 6.7ab 6.Oab 5.0b 5.7b 6.0ab
6.Oa 5.>a 5.7~ 5.3a 4.7a 5.3a
NOj-N (ppm)
6.5a 6.7a 6.4a 5.7b 6.2a 6.3a
6.5ab 6.9a 6.3ab 5.ac 6.2b 6.4ab
6.5a 6.7a 6.3a 5.7b 6.3a 6.2a
6.2a 6.7a 6.2a 5.7b b.i, 6.2a
pH
F 4 F E s 2 F
F
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
October
November
December
January
-0.lab -0.2a - 0.7ab -l.Sb O.Oa -l.Oab
19b
I/lb
35c 39b 54a 9d
39a 39a 29b 8c
42a 31b 19c 9e 10d
34a 30b 18c 8e lid
2.2a 1.5ab 1.7ab -0.2ab 2.la -1.3b
35c 39b 55a 9e lld 9e
41a 38b 23c 8e 12d 8e
43a 31b 19c 8f 12d 9e
O.Oa O.Oa -0.2a -1.3a 0.2a -1.3a
llb 9c
12c 8d
12c 10d
34a 30b 16c 6f 10d 9e
1.8bc
2.2a 2.2a 2.3a 2.3a 2.5a 1.5a
0.5ab 0.7ab 1.Oa 1.Oa 0.6ab O.Ob
21b
41a
33a
0.5a 0.6a 0.8a 0.8a 0.8a -0.2a
2.8b 2.5bc
35a
41a 35a
35c 39b 58a 8f 12d 9e
41a 32b 19c 10f 12d Ile
4Ja 39b 24~ 8e 12d 8e
33a 30b 13c 8f IOe i2d
9c 4d
1.5c 2.0bc 5.0a
I .2b 2.7b 9.3a 2.3b 1.7b 2.5b
5.3cd G.5ab 7.0a 4.3d 5.7bc 4.5d 3.3ab 3.5ab 4.3a 3.0b 3.7ab 4.2a
4.3c 4.2~ 13.0a 3.2c 7.7b 7.Sb
3.5d 3.7d I8.2a 2.5d 7.5c 12.Sb
1 la 6b Sbc 3c 4bc 3c
9a 5b 4bc 2c 3c 2c
IOa 6b 4bc 3c 2c 2c
12a 7b 5bc 3c 4bc 3c
6.20a 3.33b 2.60bc 1.9oc 2.30bc 1.5oc
0.41a 0.17b 0.12bc 0.08bc 0. IObc 0.06~
6.7a 3.7b 3.3b 2.7b 3.0b 3.0b
4.la 4.3a 3.0b 3.0b 3.0b 2.3b
7.7a 6.7a 5.0b 5.0b 5.0b 4.7b
16d 5.3a 20cd 4.7a 2lbc 3.3b 25ab 3.Ob 24abc 3.Ob 27a 3.3b
18a 32a 19a 22a 19a 20a
0.29a 0.12b 0.12b 0.05b 0.08b 0.07b
5.17a 3.03b 2.30bc 1.23~ I .47c 1.37c
21c 29b 28bc 36ab 34ab 37a
0.32a 18bc 0.16b 21ab 0.13br 16~ 0.06d 24a 0.07cd 19bc 0.07cd 2 lab
0.34a 0.14b O.lOb 0.05b 0.07b 0.05b
5.83a 3.30b 2.13bc 1.43c 1.4oc 1.37c
6.90a 3.87b 2.lObc 1.80~ 2.20bc 1.9oc
6.3a 6.6a 6.la 5.5b 6.0a 6.la
6.3ab 6.6a 6.2abc 5.9c 6.0bc 6.labc
6.3a 6.6a 5.6~ 5.6~ 6.4a 6.0b
6.6ab 6.9a 6.5ab 5.8d 6.1~ 6.4b
;-” tl *
c z 5 $ g ?J 5 ? $ 2 5. $ $
Z
g Z 2 ii P+ 5
lO.Ob 12.8b 19.2a 0.2c 2.2c
1.2c
ll.3d 17.3bc 20.3b 8.5d l4.k 24.Oa
Riparian Wet meadow Dry meadow North forest South forest Barren
Riparian Wet meadow Dry meadow North forest South forest Barren
April
May
lO.Od 14.k l7.7b 7.7e 10.8d 20.8a
6.3c 8.8b 12.3a -0.5e 3.7d 2.5d
0.5b 4.Oa 0.8b -0.2b 0.5b O.Ob
10.3b I?.Oab 13.3a 6.8~ lO.7b l2.7a
4.2bc 4.5b 7.7a 0.2d 3.Oc 5.0b
1.2bc 2.8a 2.5ab 0.3c 1.3abc 0.7c
36a 34b 9e 8f 12d 14.c
37a 34b 17e 22c 17d 13f
45a 36b 19f 21d 20e 22c
0.29a 0.13b O.lObc 0.05bc 0.06bc 0.04~
4.77a 2.53b 2.73b 1.83b 2.07b I .47b 6.lOa 3.23b 2.27bc 6.27a 1.57c 1.3oc
8a 4b 5b 3b 4b 3b 1 la 6b 4bc 2c 3c 2c
39b 44a 31c 1Of 12e l8d
45a 43b 2lc l6e 13f ISd 45a 43b 19c 12f 13e 14d
45a 37b 20d 19e l8f 2lc 45a 35b ISC lie 14d ISC
39b 44a 26c IOf 12e 16d
0.33a O.ISb 0.07b 0.08b 0.07b 0.04b
5.87a 3.1Ob 2.03b 2.4Ob 1.87b 1.30b
IOa 5b 4b 4b 3b 5b
38c 42b 43a IOf 12e 2Od
O.23a 0.13b O.lOb 0.05c 0.06~ 0.05c
0.23a 0.13b O.lObc O.lObc 0.07bc o.oc
43b 44a 22c l7e 13f 19d
3.87a 2.63b 1.97bcd 2.47bc 1.87cd I .43d
53a 37b 22c l9e 20d 19e
7a 5b 3c 4b 3c 3c
38c 43b 54a 9f 12e 17d
43a 42b 24c ISe 13f 17d
26a 2Sa 23a 23a 24a 24a
19c 21bc 27abc 36a 32ab 34a
19c 2lbc 27ab 30ab 27ab 33a
18b 2lb 20b 25a 27a 26a
C:N ratio
2.3a 2.3a 2.Oa %.Oa 2.7a 2.0a
3.Oa 3.0a 2.Ob 2.Ob 2.7a 2.0b
3.Oab 3.Oab 3.3a 2.7ab 2.3b 3.3a
4.3a 4.3a 3.3b 3.0b 2.7b 3.0b
NO,-N (ppm)
‘Means within a column and measurement period sharing a common letter do not differ significantly according to the LSD test (~~~0.05).
O.Ob 6.7a O.2b -0.2b O.Ob -0.5b
Riparian Wet meadow Dry meadow North forest South forest Bdrrcn
March
0.2ab 0.7a 0.2ab -0.8~ -0.2bc -0.2bc
Total N (%)
45a 35b 22c 19e 19e 21d
38a 34b 21d 20e 19f 26c
-O.Ab 3.0a 0.3b O.Ob -0.7b 0.2b
O.Oa 0.3a -0.7ab -2.2b -2.Ob -0.5ab
Riparia!: Wet meadow Dry meadow North forest South forest Barren
February
9Ocm
Total C (0~)
60 cm
Organic matter (o/o)
30 cm
l5cm
25 cm
‘cm
1Ocm
Soil moisture (% by volume)
Soil temperature ( C)
Site
Sampling date
TABLE 3 (continued)
6.5a 6.6a 6.2a 5.5b 6.0ab 6.3a
6.5ab 6.88 6.2ab 5.6~ 6.lb 6.3ab
6.4ab 6.6a 6.2bc 5.6d 6.1~ 6.3bc
6.3a 6.6a 6.2a 6.la 5.6b &la
pH
; e $
k
DENITRIFICATION
RATES IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
221
were highly v-ariable, but then increased sharply from October to December. Pronounced deciine in denitrification rates from December to February was followed by a fiu-ther but more gradual decrease from February through May. DISCUSSION AND CONCLUSIONS
Comparisons of soil denitrifkation rates over a 12 month period in six subalpine Sierra Nevada plant communities revealed that the mean rates for the year did not vary significantly among sites. However, environmental conditions in the six communities differed, and variation in the denitrification rates of individual communities over the 12 months was not related to the same soil parameters at all sites. At the riparian and wet meadow sites, denitrification was positively correlated with soil acidity, although the magnitude or the variation in pH was relatively small. Subsequently, it is possible that denitrificacion was actually related to soil microbe activity on soluble C substrate rather than soil acidity, and increased microbe respiration resulted in increased acidity as follows (Donahue et al., 1983): COz+H20=H++HC0~ Thus, these small changes in soil pH may have been a function of COZ production from C mineralization by soil microbes rather than a factor influencing denitrification rates in the relatively wet riparian and wet meadow communities. Nevertheless, no significant correlation between denitrification and total C was found at any site in this study, and furthermore, some denitrifying bacteria do not use C as a substrate (Gamble et al.. 1977 ). In the dry meadow, which was generally intermediate in soil moisture content among the six communities, denitrification rate increased with increasing soil moisture at the 15 cm depth as expected, but was also negalively correlated with soil temperature at 10 cm and organic matter. However, denitrification may be influenced more by soii moisture than by temperature as suggested by Myrold ( 1988 ) and Greenlee ( 1985 ), and thus the increased denitrification rate during the colder months of the year may have been the result of increased soil moisture content during these months rather than declining temperatures. The negative correlation between denitrification rate and soil organic matter content in the dry meadow may have resulted from a decrease in organic mat” ;r mineralization and a subsequent reduction in soil NOX-N available for denitritication. Robertson et al. ( 1987) identified low N03-N availability as a factor limiting denitritication, but in this study no significant correlation was detected between denitrification rate and soil N03-N at any site. Denitriikation rates at the three driest sites, i.e. both forest sites and the barren site, were not found to be influenced over the 12 months by any other soil parameter studied. Eithe: parameters other than those examined affected the rates at these three rites, or denitrification at these sites responded to different pa-
228
R.F. WALKER
ET AL.
rameters as environmental conditions changed during the year, confounding any easily discernible relationship. The difference in denitrification rate between the dry and wet meadow communities in October was largely accounted for by differences in soil temperature, and to a lesser degree soil moisture. Soil temperature, and its effects on denitrifying bacteria, solubility of NzO in water and diffusion of N20 through the soil profile, has been linked to denitrification in several studies (Ryden et al., 1978, 1979a; Blackmer et al., 1982 ). Thus, the positive correlation between denitrificzttion rate and temperature at I(! cm found here is not surprising. The positive correlation between denitrification rate and soil moisture at 90 cm in October may have reflected the higher moisture content at this depth in the dry meadow. Dubey and Fox ( 1974) found that denitrification can occur at lower depths if sufficient substrate for bacteria is available. Denitrification has been reported to be highly variable in aerobic soils (Burford and Stefanson, 1973 ), where it is largely confined to anaerobic microsites within the fine pores and aggregates of soils that are aerobic overall. In this study, this variability was particularly evident from June through October, the period of maximum aeration. However, denitrification rates in the relatively moist riparian and wet meadow soils were not consistently higher than those of the other sites during this period, as would be expected. In a process reported earlier by Bowden and Bormann ( 1986 ), NzO may have been flushed from the soil of the riparian community, located adjacent to a perennial stream, and from that of the wet meadow, located in a seep zone near the base of a bill, by lateral movement of subsurface water. This would result in underestimation of the denitrification rates at these two sites. Overall, denitrification rates rose sharply from October to December despite the declining soil temperatures during this period. Denitrifying bacterial populations have been found to differ in their response to different temperatures (Powlson et al., 1988 ), and there is evidence that multiple denitrifying bacterial populations are present in many soils (Gamble et al., 1977). Thus, it is possible that the increasing denitrification rates found here in late fall resulted from the activity of a bacterial population adapted to colder soil temperatures. Except for the dry meadow, these rising denitrification rates did not reflect increasing soil moisture, as this study was conducted during a drought year and soil moisture content remained relatively constant during these months. There was a sharp overall decrease in the rates of denitrification from December to February, followed by further gradual decline through May. From December to February, extremely cold soil temperatures may have precipitated the sharp drop in denitrification rates, as the soils of all six communities were near or below freezing during this period. However, soil NOA-N concentrations were also generally declining, a trend that continued through May.
DENITRIFICATION
RATES
IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
229
During the winter, cold soil temperatures likely caused a reduction of organic matter mineralization and thus N03-N production, and in the spring, plant uptake may have further decreased N03-N concentration. Subsequently, declining soil N03-N concc &rations likely contributed to the overall decline in denitrification rates during winter and spring. Previous studies have reported potential limitations imposed by N03-N supply on denitrification (Robertson and Tiedje, 1984; Robertson et al., 1987 ), including those resulting from plant NO,-N absorption (Ryden, I 98 1: Haider et al., 1987 ). Denitrification in wildland soils is a complex and variable process, responding to different environmental stimuli and the interactions of these stimuli in diverse habitats. These results illustrate the temporal and spatial variability of soil denitrification in a subalpine watershed of the Sierra Nevada. Despite the diversity of plant community types represented in this watershed, however, soil denitritication rates over the long-term were not found to vary appreciably among these plant communities. ACKNOWLEDGMENTS
The authors wish to thank G.C. Miller, Y.O. Koh and C.E. Crume of the University of Nevada, Reno for their assistance during this research.
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Dubey, H.D. and Fox, R.H., I9 Y-l.Denitrification losses from humid tropical soils of Puerto Rico. Soil Sci. Sot. Am. Proc.. iU: 9 17-920. Focht, D.D., 1978. Methods for aG,iiysis of denitrification in soils. In: D.R. Nielson and J.G. MacDonald (Editors), Nitrog;: :‘t the Environment. Vol. 2. Academic Press, New York, pp. 433-490. Gamble, T.N., Betlach, M.R. and Txdjije, J.M., 1977. Numerically dominant denitrifying bacteria from world soils. Appl. Envjr.in. Microbial., 33: 926-939. Greenlee, D.L., 1985. Denitrilication ! .IICSof a mountain meadow near Lake Tahoe. M.S. Thesis. University of Nevada, Reno, NV. Grundmann, G.L., Rolston, D.E. and Kachanoski, R.G., 1988. Field soil properties influencing the variability of denitrification gas fluxes. Soil Sci. Sot. Am. J., 52: 1351-l 355. Haider, K., Moiser, A. and Heinemeyer. 0.: 1987. The effect of growing plants on denitrification at high soil nitrate concentrations. Soil Sci. Sot. Am. J., 5 I: 97-102. Mosier, A.R., Guenzi, W.D. and Schweizer, E.E., 1986. Soil losses of dinitrogen and nitrous oxide from irrigated crops in northeastern Colorado. Soil Sci. Sot. Am. J., 50: 344-348. Muller, M.M., Sundman, V. and Skujins, .I., 1980. Denitrilication in low pH spodosols and peats determined with the acetylene in ,bition method. Appl. Environ. Microbial., 40: 235239. Myrold, D.D., 1988. Denitrification in ryegrass and winter wheat cropping systems of western Oregon. Soil Sci. Sot. Am. J., 52: 412-416. Parkin, T.B., 1987. Soil microsites as a source of denitrification variability. Soil Sci. Sot. A,?. J., 51: 1194-l 199. Parkin, T.B., Meisinger, J.J., Chester, ST., S:“arr. J.L. and Robinson, J.A.. 1988. Evaluation of statistical estimation methods for lognormally distributed variables. Soil Sci. Sot. Am. J., 52: 323-329. Powlson, D-S., Saffigna, P.G. and Kragt-Cotiaar, M.. 1988. Denitrilication at sub-optimal temperatures in soils from different climatic zones. Soil Biol. Biochem., 20: 7 19-723. Robertson, G.P. and Tiedje, J.M., 1984. Denitsification and nitrous oxide production in successional and old-growth Michigan forests. Soil Sci. Sot. Am. J., 48: 383-389. Robertson, G.P., Vitousek, P.M., Matson, P.A. and Tiedje, J.M., 1987. Denitrification in a clearcut loblolly pine (Pinus &e&z L.) plantation in the southeastern U.S. Plant Soil, 97: 119-129. Ralston, D.E., 198 1. Nitrous oxide and nitrogen gas production in fertilizer loss. In: C.C. Delwiche (Editor), Denitrilication, Nitrilicatton and Atmospheric Nitrous Oxide. Wiley, New York, pp. 127-149. Ryden, J C., I98 I. N20 exchange between a grassland soil and the atmosphere. Nature. 292: 235-237. Ryden, J.C. and Lund, L.J., 1980. Nature anti ;_a:cnt of directly measured denitrilication losses m units. Soil Sci. Sot. Am. J., 44: 505-5 11. from some irrigated vegetable crop pro& Ryden. J.C., Lund. L.J. and Focht, D.D.. -Direct in-field measurement of nitrous oxide flux from soils. Soil Sci. Sot. Am. J.. -i_ -737. Ryden, J.C., Lund, L.J. and Foch?, Dm1 9.~.Direct measurement of denitrilication loss from soils: I. Laboratory evaluarioii km inhibition of nitrous oxide reduction. Soil Sci. Sot. Am. J., 43: 104-l 10. Ryden, J.C.. Lund, L.J., Letey, J. and Fob. !P.. 1979a. Direct measurement of denitrifcation loss from soils: II. Developmem > .&cation of field methods. Soil Sci. Sot. Am. J., 43: 1 IO-1 18. Sextone, A.J.. Parkin, T.B. and Tiedje, J.M.. 1985. Temporal response of soil denitrification rates to rainfall and irrigation. Soil Set. Sot. Am. J., 49: 99-193. Stefanson. R.C.I 1972. Soil denitrification i-i sealed soil-plant systems: 1. Effect of plants, soil water content and soil organic matter comcn?. Plant Soil. 33: 113-l 27.
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RATES IN SUBALPINE
PLANTS
IN SIERRA
NEVADA
231
Walter, H.M., Keeney, D.R. and Fillet-y, I.R., 1979. Inhibition of nitrification by acetylene. Soil Sci. Sot. Am. J., 43: 195-196. Yeomans, J.C. and Beauchamp, E.G., 1978. Limited inhibition of nitrous oxide reduction in soil in the presence of acetylene. Soil Biol. Biochem., 10: 5 17-S 19. Yoshinari, T., Hynes, R. and Knowles, R., 1977. Acetylene inhibition of nitrous oxide reduction and measurement of denitrification and nitrogen fixation in soil. Soil Biol. Biochem., 9: 177-183.