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Phytotoxic effect of Barley, oat, and wheat-straw mulches in eastern quebec forest plantations 2. Effects on nitrification and black spruce (Picea mariana) seedling growth
Forest Ecology and Management, 29 (1989) 295-310 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
295
P h y t o t o x i c Effect of Barley, Oat, and W h e a t - S t r a w Mulches in Eastern Quebec Forest Plantations 2. Effects on Nitrification and B l a c k Spruce (Picea mariana) Seedling G r o w t h R. JOBIDON 1, J.R. THIBAULT2 and J.A. FORTIN 2
IMinist~re de l'Energie et des Ressources, Direction Recherche et Ddveloppement, Service de l'Am~lioration des Arbres, 2700 Einstein, Sainte-Foy, Qudbec, GIP 3 W8 (Canada) 2Centre de Recherche en Biologie Foresti~re, Facult~ de Foresterie et G~od~sie, Universitd Laval, Qudbec, GIK 7P4 (Canada) (Accepted 11 January 1989)
ABSTRACT Jobidon, R., Thibault, J.R. and Fortin, J.A., 1989. Phytotoxic effect of barley, oat, and wheatstraw mulches in eastern Quebec forest plantations. 2. Effects on nitrification and black spruce ( Picea mariana ) seedling growth. For. Ecol. Manage., 29: 295-310. Red raspberry (Rubus idaeus L.), a common forest weed especially in clearcut areas of northeastern America, is known to possess definite nitrophilous habit. In a companion paper we showed that barley, oat, and wheat straws used as cover mulches inhibited red-raspberry establishment and N nutrition after clearcutting and site preparation. As part of the mechanisms involved, we hypothesized that an inhibition of nitrification would prevent the establishment of red raspberry. Effects of barley, oat, and wheat-straw cover-mulches were evaluated under field conditions. We monitored soil NO3-N production, the abundance of NH+-oxidizers, and nitrate reductase activity (NRA) in R. idaeus foliage as an indication of the availability of nitrate in soil. Over two consecutive years, the use of straw mulches significantly reduced NO3-N soil production and NH +-oxidizers. The five phenolic acids, p-coumaric, syringic, vanillic, ferulic and p-hydroxybenzoic, responsible for the toxicity of the straws, significantly inhibited NH +-oxidizers at concentrations ranging from 10-3M to 10-6M in a laboratory experiment. The NRA measured during the 1987 growing-season in red-raspberry foliage confirmed the results obtained for NOz-N content of soil. The results suggest that the inhibition of NO3-N production under straw mulching restricted, to some extent, the establishment of red-raspberry seedlings. Other mechanisms could also be partially responsible for the inhibition observed, and are discussed. The effect of straw mulches was also evaluated on shoot growth and basal stem-diameter growth and on mineral nutrition (total N, P, K, Ca, Mg and Mn) of black-spruce (Picea mariana (Mill) BSP) seedlings over the two successive growing-seasons (1986 and 1987 ) after straw application. The straw treatments enhanced shoot and stem-diameter growth of P. mariana seedlings. Treated black-spruce seedlings showed a significantly higher foliar N-content than control seedlings. There were no consistant significant differences between treated and control seedlings for the other nutrients analysed. The promotion of black-spruce seedling growth and foliar N-content is associated with a reduction in competitive vegetation and also an inhibition of nitrification from the straws.
0378-1127/89/$03.50
© 1989 Elsevier Science Publishers B.V.
296
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
INTRODUCTION The effect of forest clearcutting on soil nutrient dynamics has received much attention during the last decades. Production of nitrate often increases rapidly after disturbance of the forest floor (Romell, 1935; Likens et al., 1970; Vitousek et al., 1982). The rapid increase in nitrification generally observed after a major disturbance such as clearcutting could be attributed to: 1) variations of abiotic factors; 2 ) reduction in plant uptake of nitrogen; 3) increase of organicmatter decomposition and nitrogen mineralization; and 4) the removal of sources of potentially inhibitory biochemicals (Vitousek and Mellilo, 1979; Vitousek, 1981). The extent of nutrient loss, especially of NO3-N from the forest ecosystem after clearcutting should decrease rapidly according to the rate of revegetation (Marks and Bormann, 1972). Thus, the role of successional species in the maintenance of soil nutrient status becomes increasingly important if devegetation becomes more frequent (Marks, 1974). The successional pattern of species after forest disturbances in eastern North America was studied by Marks ( 1974 ) who, among others (e.g., Whitney, 1978) noted the importance of red raspberry (Rubus idaeus L.) as a pioneer and opportunistic species in disturbed forests. This species is also known by European and North American foresters to possess definite nitrophilous habits (Hesselman, 1917; Tamm, 1974; Whitney, 1978; Tilman, 1987). In a companion study (Jobidon et al., 1989, this volume), we demonstrated the allelopathic inhibition of red-raspberry seedling growth by using cover mulches of barley, oat, and wheat straws, in laboratory and field experiments. Under laboratory conditions, aqueous extracts of barley, oat, and wheat straws significantly inhibited shoot elongation and completely inhibited root development ofR. idaeus propagules. Under field conditions, red raspberry seedlings failed to establish in the first growing-season after the straw placement and very few plants were established in the second growing season. The use of a control mulch allowed the separation of physical and chemical effects of the straws (Putnam and De Frank, 1983). The treatments significantly reduced N nutrition in R./daeus plants, as compared to both control mulch and control. Moreover, the treatments reduced the establishment of other opportunistic species (e.g. Epilobium angustifolium L. ) . The allelopathic effect of barley, oat, and wheat straws has been documented in agriculture (Rice 1984; Putnam and Weston, 1986) but has never been used in forest plantations to inhibit the establishment of unwanted vegetation after clearcutting and site preparation. Numerous studies have demonstrated that use of straw mulches on the soil surface generally results in the reduction of nitrate formation (e.g. Albrecht and Uhland, 1925; Rice, 1984). Our hypothesis was that inhibition of R. /daeus establishment in recent clearcut and prepared sites of eastern Quebec was, at least partially, due to an inhibition of nitrification and nitrifying bacteria by allelopathic substances
PHYTOTOXICITY OF STRAW MULCHES TO PICEA
297
involved in decomposing barley, oat, and wheat straws. As part of this study we also evaluate the effect of the straws on growth of planted black-spruce seedlings (Picea mariana (Mill.) BSP). Among the most noticeable effects of allelopathy previously reported are seed germination inhibition, growth inhibition, and impaired mineral nutrition (Rice, 1984). So the potential effects of the straws on black spruce seedlings were determined by rate of height increase and mineral nutrition status of the seedlings during the two successive years following the treatments.
MATERIALSAND METHODS
The study sites The three experimental sites (referred to as P1, P2, and P3) used in this study are described by Jobidon et al. (1989, this volume ). Analyses for ammonium-nitrogen and nitrate-N in the soil Soil NH4- and NO3-N were determined during the summers 1986 and 1987 in each plot of the three sites, as follows. Since NO3-N is a highly mobile ion in the soil, the soil samples were collected after a period of 5 consecutive days without rainfall. Sampling was done for one site at a time. From each plot, about 10 g of a composite soil sample was directly extracted in the field, in 100 ml of 2M KC1 solution (Keeney and Nelson, 1982) and the operation repeated ten times. Each composite soil sample was the result of ten randomly distributed and thoroughly mixed soil cores from the treated and control furrows (015 cm). The 400-ml polypropylene jars containing the soil KC1 suspension were vigourously shaken for 1 min and placed in a portable ice-box at 10 ° C, until refrigeration at 3 °C shortly afterwards. The procedure followed was according to the recommendations of Keeney and Nelson (1982). After being shaken, the soil KC1 suspension was filtered through W h a t m a n No. 42 filter paper to a volume of 500 ml with 2M KC1 solution. Aliquots of the filtrates were stored in a refrigerator (3 °C ) until the analyses were performed, usually within 2-5 days. The soil was oven-dried at 85 °C for 72 h and weighed. Ammonium- and nitrate-N were determined spectrophotometrically using an automated Flow Injection Analyser (Tecator FIA star model No 5020 equipped with a Tecator 5007 Sampler and a Tecator 5022 Detector Controller). Ammonium-N was determined by the indophenol-blue reaction at 636 nm. The FIA used was equipped with a Cd column to reduce NO~ to NO2, which was then measured at a wavelength of 540 nm. The blank consisted of 2M KC1 solution filtered through W h a t m a n No. 42 filter paper. Soil samples were col-
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R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
lected each time conditions of no rainfall during 5 consecutive days occurred; i.e. in 1986, June and July for the P1 site, and July for the P2 and P3 sites, and in 1987, June, July and August for the P1 and P2 sites and June for the P3 site.
Determination of NH +-oxidizers Determination of the abundance of NH +-oxidizers was performed in June and August 1986 and in June 1987 for every plot of the three study sites, and repeated three times. For each plot, 15 evenly distributed soil samples (0-15 cm) were thoroughly mixed and the composite soils were put in plastic bags. These were incubated on the day of sampling for determination of the NH +oxidizing population by the most probable number (MPN) method of Alexander (1982), according to the recommendations of Schmidt and Belser (1982). The blank consisted of uninoculated control tubes. The incubation was performed at 28 ° C in the dark for a period of 6 weeks.
Effect of phenolic acids on NH +-oxidizers In August 1986, 15 evenly distributed soil samples (0-15 cm) collected from each of the three control plots of the three study sites were thoroughly mixed and dept in plastic bags. Sterile solutions ofp-coumaric, syringic, vanillic, ferulic, and p-hydroxybenzoic acids were prepared. The effects of four concentrations ( 10 -~, 10 -4, 10 -5, and 10 -6 M) of each phenolic acid were tested on the development of the NH4 +-oxidizing population, as above. All tubes were inoculated with the same soil buffer suspension. The control consisted of an incubation of the same soil buffer suspension without the addition of phenolic acids. A first blank consisted of uninoculated control tubes with the phenolic acids, and a second blank consisted of uninoculated control tubes without the addition of phenolic acids. The tubes were placed in the dark at 28°C for a period of 6 weeks. The experiment was repeated three times.
Nitrate reductase activity (NRA) in red-raspberry foliage Nine R. idaeus seedlings were randomly sampled with root systems from each plot of the three sites between 0700 and 0900 (Rhoden et al., 1987) and put in a portable ice-box at 5 ° C. The seedlings were sampled in June, July, and August 1987. The NRA was immediately determined on three composite foliage samples from three seedlings by the in-vivo method developed for Deschampsiaflexuosa by Havill et al. (1974) and described by HSgberg et al. (1986). Composite samples of 0.1-g (fresh weight) leaves were cut into 1-2-mm-long segments and transferred to test tubes. Five ml of incubation medium were added and the test tubes were shaken for 1 h at 28 ° C in darkness. A reagent of 1 ml 1% (w/w) sulfanilic acid in 3.2MHC1 and 1 ml 0.01% (w/w) N- (1-naphthyl ) ethylenediamine (NED) was added to a 1-ml aliquot of assay medium for spectrophotometric (540 mm) determination of the nitrite produced. The blank consisted of the same solution with 1 ml of deionized water instead of the NED
PHYTOTOXICITYOF STRAWMULCHESTOPICEA
299
c o m p o n e n t . T h e a c t i v i t y is e x p r e s s e d t h r o u g h o u t as t h e a m o u n t o f n i t r i t e p r o d u c e d g - 1 f r e s h w e i g h t h - 1.
Black-spruce seedling growth and [oliar mineral content During mid-May 1986, all the sites were planted with P. mar/ana seedlings grown in containers under same environmental conditions, and which were about 10-12 cm in height at planting time. The seedlings were planted manually in the center of the furrows at 2-m intervals. Black-spruce seedling growth and foliar mineral content were measured at the end of 1986 and 1987 growing-seasons. In September 1986, 40 black-spruce seedlings were randomly harvested from each plot for a total of 240 seedlings per site, and shoot growth-increment was measured. The seedlings were randomly paired and oven-dried at 60 oC for 72 h. The needless were then carefully separated from other plant parts, ground to 40 mesh, and kept in plastic bottles until analysed, generally within a few weeks. Foliage was analysed for total nitrogen (N) by the micro-Kjeldahl method, phosphorus (P) by the vanadomolybdophosphoric-yellow method, and for potassium (K), calcium (Ca), magTABLE 1
Mean concentration 1 (p.p.m. g-1 dry weight) of NHa-N and NO3-N in the upper portion of the soil profiles (0-15 cm) for the 1986 growing-season Site
Treatment 2
P1
BA OA WH MX CM C
P2
BA OA WH MX CM C
P3
BA OA WH MX CM C
June
July
NH,+
N0~-
NH +
N0~-
19.27ab 16.57a 21.08bc 24.96cd 28.60de 31.46e
1.04ab 0.00a 0.00a 1.10ab 5.06c 4.09bc
16.06abc 28.56efg 28.02efg 26.76ef 13.79ab 10.56a
5.20cd 0.89ab 6.01d 6.69d 3.07abcd 2.21abcd
21.51d 16.53bc 23.49fg 30.79def 17.55bcd 18.72cd
0.79ab 6.60d 0.39ab 2.84abcd 6.67d 6.57d
34.73g 32.69fg 24.80def 26.71ef 27.59efg 29.91fg
4.35bcd 1.60abc 0.00a 5.13cd 5.86d 4.31bcd
n.a.
n.a.
n.a.
n.a.
1Each value represents mean of 10 analyses. Values followed by the same letter do not differ (P < 0.05 ) using Duncan's new multiple range test. n.a.: not available (see text for details ). eBA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control
300
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
nesium (Mg), and manganese (Mn) by standard atomic absorption techniques. In September 1987, 20 P. mariana seedlings were randomly harvested from each plot, for a total of 120 seedlings per site. Shoot growth-increment and basal stem-diameter of the seedlings were measured. The seedlings were then randomly paired, and the current-year needles were separated from the 1-year-old needles, and oven-dried at 60 ° C for 72 h for nutrient analyses (total N, P, K, Ca, Mg, Mn, as above). RESULTS
Effects on nitrification, NH4-oxidizers, and NRA During the 1986 and 1987 growing-seasons, the mean NO3-N content of soil from treated plots was slightly lower than that of both the control mulch and the control. However, the different treatments did not produce the same level of inhibition of nitrification within and between the sites (Tables 1 and 2). Oat and wheat straws produced the greatest reduction in NO3-N in soil during June 1986 at the P1 site, and only oat straw remained inhibitory in July, as TABLE 2 Mean concentration 1 (p.p.m. g-1 dry weight) of NHa-N and NO3-N in the upper portion of the soil profiles (0-15 cm) for the 1987 growing-season Site
Treatment 2
June
July
August
NH +
N0~-
NH +
N0~-
NH4+
N0~-
P1
BA OA WH MX CM C
14.20ab 16.55b 12.60ab ll.06ab 14.22ab 16.93b
8.95abc 5.70a 2.76a 5.41a 24.90e 23.56de
12.69abc 10.19a 10.9lab 10.92ab 17.07d 17.81d
5.23ab 4.21a 5.82ab 4.82a 22.11c 20.41c
ll.02bcd 12.05de 15.49f 11.26cd 8.97b 9.30bc
16.25ab 22.20bc 23.81bc 21.97bc 42.25d 28.58c
P2
BA OA WH MX CM C
12.10ab 12.49ab ll.96ab 10.86ab 9.91a 9.45a
9.13abc 7.23abc 7.93abc 6.37ab 18.68d 19.26de
15.35cd 12.93abc 14.67cd 13.29abc 13.87bc ll.49ab
18.55c 10.72b 19.83c 10.15b 29.84d 34.74e
13.50ef 12.82de 14.53ef 12.47de 5.51a 6.18a
12.62a 10.63a 10.31a 10.84a 14.15a 15.66a
P3
BA OA WH MX CM C
10.19a 14.80b 12.84ab ll.21ab 15.00b 12.80ab
6.17a 4.19a 3.42a 5.10a 11.42bc 12.51c
n.a.
n.a.
n.a.
n.a.
1Each value represents mean of 10 analyses. Values followed by the same letter do not differ (P < 0.05 ) using Duncan's new multiple range test. n.a.: not available (see text for details). 2BA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control.
PHYTOTOXICITYOF STRAWMULCHESTO PICEA
301
TABLE 3 Mean number 1 of NH4 +-oxidizers (MPN method) in the upper portion of the soil profiles (0-15 cm) for the 1986 and 1987 growing-seasons Site
Treatment 2
Spoil sampling data June 1986
August 1986
June 1987
P1
BA OA WH MX CM C
893* 927* 1027" 1053" 9420 10967
1123" 1267" 986* 1033" 11213 12453
2313" 3790 3027* 2974* 14703 16150
P2
BA OA WH MX CM C
3300* 4800* 1816" 3937* 14813 16667
2893* 5423 3216" 4857 13706 15367
3720* 4903 4150" 3833* 15670 17421
P3
BA OA WH MX CM C
2660* 1933" 2767* 1056" 12500 14333
3327* 2893* 1933" 2327* 14230 13821
3870* 4541 4013" 3977* 18627 17430
1Each value represents mean of 3 repetitions. Within each site and sampling date, statistical t value* indicates significant difference between numbers of NH4 +-oxidizers in treatment and control(c) group at P<0.05. 2BA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control. TABLE 4 Mean number 1 of NH4 +-oxidizers (MPN method) in control and treatment cultures with phenolic acids Treatment
P-Coumaric acid P-Hydroxybenzoic acid Ferulic acid Vanillic acid Syringic acid Phenolic acids mixture
Concentration (M)
Control
×10 -3
×10 -4
×10 -5
×10 -6
160" 590* 130" 233* 173" 99*
253* 687 284* 400* 217" 363*
293* 417" 363* 473* 310" 960
383* 1123 209* 960 690 703
5387 5387 5387 5387 5387 5387
1Each value represents mean of 3 repetitions. Statistical t value *indicates significant difference between numbers of NH4 +-oxidizers in treatment and control groups at P < 0.05.
302
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
TABLE 5 Average nitrate reductase activity (NRA 1, #mol NO2- g-~ h -~) in R. idaeus leaves for the P1, P2, and P3 sites in June, July and August of 1987 Site
Treatment2
NRA June
July
August
P1
BA OA WH CM C
4.54d 4.09d 2.97bc 6.21d 4.47d
1.84bcde 2.15de 1.62abc 4.99h 2.71f
2.19cd 2.27cde 1.98cd 4.82h 3.03f
P2
BA OA WH CM C
2.19b 2.44b 2.12b 5.45d 3.52cd
1.95cde 1.69bcd 1.66bcd 4.19g 4.32g
2.20cde 1.97bcd 1.77abc 4.32g 4.26g
P3
BA OA WH CM C
1.21a 1.16a 1.00a 3.79cd 3.51cd
1.22a 1.25a 1.54ab 1.37ab 2.15e
1,33a 1.41a 1.49ab 2.32de 2.61e
1Each value represents mean of 3 repetitions, each repetition analysed 3 times. For one month, values followed by the same letter do not differ (P<0.05), using Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw :CM: control mulch; C: control. TABLE 6 Mean 1 shoot increment (cm) and basal diameter (mm) of black-spruce seedlings, at the first and second growing-season after straw placement Site
Treatment2
Shoot increment
Basal diameter
1986
1987
1987
P1
BA OA WH CM C
9.7d 8.1c 8.6cd 4.3a 7.5bc
17.1d 14.7c 15.4c 10.8ab 12.7b
6.40f 4.37abc 5.21cde 3.57a 4.44bc
P2
BA OA WH CM C
7.5bc 8.4cd 8.2cd 5.2a 7.6bc
16.7d 15.4c 16.4c 10.7a 12.6b
6.35f 5.56de 6.66f 4.39bc 4.50bc
P3
BA OA WH CM C
7.9bc 8.3cd 8.1c 6.5b 8.3cd
16.2c 15.2cd 15.2cd 11.9ab 12.6b
6.03ef 4.70bcd 5.81ef 4.15ab 4.35abc
1Means followed by the same letter do not differ (P<0.05), according to Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw; CM: control mulch; C: control.
PHYTOTOXICITY OF STRAWMULCHESTO PICEA
303
compared to control plots. Barley and wheat straws produced significant reductions in NO3-N content of soil at the P2 site, and only wheat straw was inhibitory at the P3 site (Table 1 ). There were no significant differences in NO3-N or NH4-N content of soil between the control mulch and the control during the 1986 and 1987 growing-seasons (Tables 1 and 2). For both study years, NH4-N content of soil was slightly higher in the treated plots than in either control. Nitrate production increased during the 1987 growing-season, as compared to 1986. The increases were more pronounced for the control plots than for the treated ones, indicating that the treatments slightly delayed the production of NO3-N. Most treatments significantly reduced NH +-oxidizers in both 1986 and 1987 (Table 3). The five phenolic acids tested significantly reduced the mean number of NH+-oxidizers (Table 4). Even at a concentration of 10-6M, p-coumaric and ferulic acids remained inhibitory to the NH +oxidizing populations. The nitrate reductase activity in red-raspberry foliage from treated plots was significantly lower during 1987 than that of both controls (Table 5). TABLE 7 Mean 1 nutrient content (p.p.m. ex. N, % ) in black-spruce needles, the first growing-season after straw placement Site
Treatment 2
Nutrient
N
P
K
Ca
Mg
Mn
P1
BA OA WH MX CM C
2.06fg 1.76bc 1.88cde 1.76bc 1.05a 1.61b
1 813de 1 950e 1 731bcd 1 749cd 1 752cd 1 941de
5 716ab 6 345def 5 942abcd 6 166bcde 6 136bcd 6 431def
5 896fgh 5 800efgh 5 779efgh 5 711defgh 5 374bcdef 5 016ab
1 23lab 1 254abcd 1 302abcde 1 288abcd 1 186 a 1 239abc
1 542cd 1 482bcd 1 408bcd 1 250b 1 358bc 1 629d
P2
BA OA WH MX CM C
2.08fg 1.99ef 2.18g 2.01ef 1.27 1.63b
2 l13f 2 l13f 1 972ef 1 940de 1 797d 1 857de
5 904abc 9 019g 6 363def 6 150bcde 6 314cdef 6 691f
6 788j 6 254hij 6 593ij 6 298hij 5 149abcd 4 601a
1 430ef 1 355bcdef 1 377def 1 478f 1 204a 1 179a
1 365bcd 1 363bcd 1 494bcd 1 340bc 1 336bc 1 502bcd
P3
BA OA WA MX CM C
1.83cd 1.94def 1.83cd 1.76bc 1.03a 1.46
1 598ab 1 537a 1 716bcd 1 810d 1 627abc 1 769cd
6 662ef 6 759f 6 431def 6 772f 5 615a 5 798abc
5 788efgh 5 087abc 6 126ghi 5 645cdefg 5 241bcde 5 452bcdef
1 467f 1 355bcdef 1 389ef 1 375cdef 1 270abcd 1 290abcde
824a 645a 1 437bcd 1 650d 1 221b 1 611cd
IEach value represents mean of 20 analyses. Values followed by the same letterdo not differ (P< 0.05), according to Duncan's new multple range test. 2BA: barley straw; OA: oat straw; W H : wheat straw; M X : misture of the 3 kinds of straws; C M : control mutch; C: control.
304
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
TABLE 8 Mean nutrient content 1 (p.p.m., ex. N, % ) in black-spruce current needles and 1-year-old needles, the second growing-season after straw placement
Needle age
Treatment 2
Nutrient N
P
Current needles
BA OA WH CM C
1.98a 1.86a 1.98a 1.67b 1.88a
2 2 3 2 3
1-year-old needles
BA OA WH CM C
1.64y 1.46x 1.35x 1.44x 1.48x
2 334x 2 216x 2 l13x 2 238x 2 252x
883a 863a 140a 830a 144a
K
Ca
Mg
Mn
5 690a 6 152ab 6 238ab 5 808ab 6 449b
6 899b 6 242ab 6 372ab 6 460ab 5 775a
1 200a 1 268a 1 230a 1 207a 1 171a
2 042a 1950a 2 424a 2 442a 2 528a
5 338x 5 155x 5 009x 4 468y 5 138x
8 471x 8 561x 8 144x 10 239y 8 950xy
891x 907x 718y 991x 882x
2 815x 2 667x 3 382xy 3 934y 4 002y
1Each value represents mean of 20 analyses (P1 and P2 sites). For one needle age group, means followed by the same letter do not differ ( P < 0 . 0 5 ) , according to Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw; CM: control mulch; C: control.
Effects on black-spruce seedling growth and mineral nutrition Application of straw to the soil surface caused a significant increase in mean shoot and basal stem-diameter growth of black-spruce seedlings (Table 6). For the 1986 and 1987 growing-seasons, mean shoot growth-increments of treated seedlings were 8.3 and 17.8 cm respectively, and reached 6.6 and 12.6 cm respectively for control seedlings (values compiled from Table 6). Basal stem diameter of treated and control seedlings were 5.68 and 4.43 ram, respectively (values compiled from Table 6). For both study years, responses ofP. mar/ana seedlings to the treatments were similar between sites and between treatments. There were no cases of impaired mineral nutrition in black-spruce seedlings growing on the treated plots, for either study year 1986 (Table 7) or 1987 (Table 8). On the contrary, treated black-spruce seedlings showed a significantly higher foliar N content than control seedlings, for the 1986 growingseason. For the three study sites and the four straw treatments, the 1986 blackspruce needle-N content was 1.92 %, as compared to 1.57% for the control seedlings (data compiled from Table 7). For the 1987 growing season, N content in current needles was not significantly higher than in the control. The other nutrients analysed (P, K, Ca, Mg, and Mn) showed no significant differences between treated and control seedlings, for both study years, except for Mn on site P3 with barley and oat straw (Table 7).
PHYTOTOXICITY OF STRAW MULCHES TO PICEA
305
DISCUSSION The results showed that NO3-N production in treated plots was inhibited, or at least delayed, as compared to control plots, during the course of this study. The inhibition of nitrification could not be attributed to low soil pH, or differences in pH values between treated and control plots (Jobidon et al., 1989). Work at Hubbard Brook (U.S.A.) indicated that nitrification can proceed in the acid soils (pH below 5.0) of cutover hardwood forests (Smith et al., 1968; Likens et al., 1969; Vitousek, 1977). Such nitrification at low pH may be the result of acid-adapted varieties of autotrophic nitrifiers (Alexander, 1977), or the result of heterotrophic nitrification (Lang and Jagnow, 1986), or both. Moreover, differences observed in NOa-N production, the abundance of NH +oxidizers and NRA in R./daeus foliage could not be attributed to differences in any soil characteristics within the sites (Jobidon et al., 1989). The cover straw mulches used in this study inhibited, to some extent, the NH +-oxidizers. Many studies have shown that low numbers of nitrifiers could be a direct result of toxic effects from secondary plant chemicals both in agroecosystems (Rice, 1984), and forests (Lodhi, 1978; White 1986). A drastic reduction in numbers ofNH + -oxidizers upon addition of various phenolic acids was observed, even at a concentration as low as 10-6M. The five phenolic acids used were identified as the toxic substances involved in the allelopathic activity of the straws used (Guenzi and McCalla, 1966). However, we did not establish if these acids occurred in toxic concentrations in the field experiment. Nitrate reductase is an inducible enzyme (Beevers and Hageman, 1980). Recently, HSgberg et al. (1986) pointed out the positive correlation between NRA and the availability of NOa-N in forest soils. The low rates of NRA observed in red-raspberry foliage from treated plots during the 1987 growingseason confirmed the results obtained from the determinations of soil NO3-N content and the abundance of NH+-oxidizers. The low NRA obtained on treated plots is in accordance with the low N concentration measured in the R. idaeus leaves from treated plots (Jobidon et al., 1989). The nitrophilous character of red raspberry was established by a number of researchers (e.g., Hesselman, 1917; Tamm, 1974; Whitney, 1978; Tilman, 1987) with respect to its establishment after forest disturbance. In a related study (Jobidon et al., 1989), we demonstrated the inhibition of red-raspberry establishment on the treated plots. Moreover, foliar-nutrient analyses revealed a significant reduction (33%) in N content ofR. idaeus seedlings growing on the treated plots, as compared to control seedlings. The nitrophilous character of red raspberry, combined with the low NO3-N produced could, at least partially, explain the differences previously observed in red-raspberry establishment. From the results obtained in this study and the previous one (Jobidon et al., 1989) we could assume that the inhibition observed in R./daeus seedling establishment was, at least partially, the result of: 1 ) a direct phytotoxic effect
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R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
due to the straws; and 2) an inhibitory effect of the straws on nitrification processes. However, other mechanisms could also have played a role in the inhibition observed. Although this present study was not designed to evaluate the effect of the straws on other factors than the fate of N, the treatments have produced other direct or indirect effects, such as conserving soil moisture, regulation of soil surface temperature, and reduction of soil erosion. These factors could in turn influence nitrogen mineralization and nitrification. Conservation (or addition) of organic matter on the soil surface after clearcutting and site preparation could enhance N immobilization. Vitousek and Matson (1984) pointed out the importance of conserving organic matter after clearcutting because microbial uptake of N during decomposition of organic matter was the most important process retaining N. However, the low rates of NO3-N content of soil in treated plots could not be attributed to increased Nimmobilization, since soil from both control mulch and control had similar N content. Kimber (1973) pointed out that soil surface application of wheat straw depressed germination and growth of wheat, while immobilization of N occurred mainly when the straw was mixed with the soil. Production of inhibitors could be due to microorganisms involved in straw decomposition, as in the production of patulin by PeniciUium urticae growing on wheat-straw residues (Norstadt and McCalla, 1963). The toxic effect of patulin has been demonstrated on seed germination and early growth of bioassay species (Ellis and McCalla, 1973). Patulin, and other microbially produced phytotoxins - e.g. the phytotoxin acetic acid produced during straw decomposition (Lynch, 1977 ) - could also contribute to the inhibition of nitrification observed, and inhibit red-raspberry establishment. For both 1986 and 1987 growing seasons, shoot growth increment of P. mariana seedlings was similar among the straw treatments, regardless of study sites. The straw treatments did not inhibit black-spruce seedling growth. There are some cases of significant growth enhancement on the treated plots which could be observed during both study years. Basal diameter of black-spruce seedlings was higher on treated plots than on control plots. Seedling stem diameter was somewhat more responsive than height to the treatments applied. This could be the result of reducing competitive vegetation in plots covered with straw mulches. Haywood (1986) and Zutter et al. (1986) also noted similar trend in diameter response to reduction in interfering vegetation. The differences observed in black-spruce needle N content could not be attributed to differences in soil N content prior to straw placement, or to differences in other soil characteristics between treated and control plots (Jobidon et al., 1989). The promotion in N nutrition and growth of treated black-spruce seedlings could be the result of conservation of N in the form of NH4-N rather than in the easily leachable NO3-N form, or the result of reduced competition for light, nutrients, and water on treated plots, or both. Unfortunately, the experiment was not designed to separate those two effects. From an ecological
PHYTOTOXICITY OF STRAW MULCHES TO PICEA
307
point of view, it is of interest to note that N nutrition of a pioneer species, red raspberry, was significantly depressed by the treatments (Jobidon et al., 1989) whereas N nutrition of black spruce, a climax species, was significantly enhanced. The only reasonable explanation for the opposite response seems to be that R. idaeus has adapted well to rapidly released N in the form of NO~-. In contrast, it appears that black spruce has evolved as an efficient user of NH +. This is of importance for conservation of nitrogen within the site, especially in areas where N nutrition is critical. Such an opposite response in N nutrition indirectly support the hypothesis of Rice and Pancholy (1972) that inhibition of nitrification increased as succession progressed towards climax. It would be of interest to verify if the opposite response obtained in N nutrition between red-raspberry (species forming vesicular-arbuscular mycorrhizae) and black spruce (species forming ectomycorrhizae) could be partially explained by their respective symbiotic association. The nutrient status of treated black-spruce seedlings, for both study years, is in accordance with the adequate levels suggested by Swan (1970) for this species. However, the 1986 needle N content in both control seedlings could be noted as critical or low, according to Swan (1970), and could not be attributed to differences in environmental conditions during seedling production. The question of nutrient ratios has been extensively investigated by Ingestad (1967, 1979) in relation to optimum growth of seedlings. For birch, pine and spruce seedlings, optimum nutrition occurred at the following relative levels: N = 100, P = 13, K = 6 5 , C a = 6 , Mg--8.5. For the 1986 growing season, we obtained the following levels for treated seedlings: N = 1 0 0 , P = 9 . 6 , K = 4 0 , Ca-- 31.2, Mg = 7.1; and for control seedlings (control mulch not included): N = 100, P = 12, K = 40.2, Ca = 32, Mg = 7.8. For the 1987 growing season (current needles ), the levels were as follows for treated seedlings: N = 100, P = 15.3, K = 31.1 C a = 33.5, M g = 6.4; and for control seedlings (control mulch not included): N = 100, P = 16.7, K=34.3, Ca=30.7, Mg=6.2. For both study years, there are no noticeable differences in relative nutrient levels between treated and control seedlings, indicating that treatments did not affect balance of plant macronutrients. The levels obtained in K and Ca differed, to some extent, from the levels proposed by Ingestad (1967, 1979) but for species other than black spruce. However, the relative levels obtained in K and Ca are in accordance with Weetman (1968) for black-spruce seedlings. The results obtained demonstrated an allelopathic effect of the straws on nitrification processes and on R. idaeus establishment during the two consecutive years of this study. However, other factors, abiotic and biotic, could have exerted direct or indirect effect, on nitrification and on red-raspberry seed germination and growth. The treatments did not impair P. mariana seedling growth.
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R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
ACKNOWLEDGEMENTS
This study was funded by the Gouvernement du Qudbec, Ministate de l'Energie et des Ressources, Direction Recherche et Ddveloppement, to whom we are grateful. We are indebted to Mr. Georges Fortier, Head of the Office des Producteurs de Bois de La Pocati~re, for providing us the experimental sites and to the Canadian Forestry Service for their invaluable assistance. The authors thank Drs. Michel Boudoux, Gilles D. Leroux, Azim U. Mallik, Alan R. Putnam, and Gilles Vall~e for helpful suggestions and critical review of earlier version of the manuscript, Dr. Claude Camir~ for discussions, and Mr. Alain Brousseau and Mr. Rdal Mercier for technical advice. We also thank anonymous reviewers for valuable suggestions on the manuscript.
REFERENCES
Albrecht, W.A. and Uhland, R.E., 1925. Nitrate accumulation under the straw mulch. Soil Sci., 20: 253-267. Alexander, M., 1977. Introduction to Soil Microbiology (2nd edition). Wiley, New York, 467 pp. Alexander, M., 1982. Most probable number method for microbial populations. In: A.L. Page, R.H. Miller, and D.R. Keeney (Editors), Methods of Soil Analysis. Part 2, Chemical and Microbiological Properties. Am. Soc. Agron., Agron. Monogr. No. 9 (2nd edition), pp. 815-820. Beevers, L. and Hageman, R.H., 1980. Nitrate and nitrite reduction. In: B.J. Miflin (Editor), The Biochemistry of Plants, Vol. 5. Academic Press, New York, pp. 115-168. Ellis, J.R. and McCalla, T.M., 1973. Effects of patulin and method of application on growth stages of wheat. Appl. Microbiol., 25: 562-566. Guenzi, W.D. and McCalla, T.M., 1966. Phenolic acids in oats, wheat, sorghum and corn residues and their phytotoxicity. Agron. J., 58: 303-304. Havill, D.C., Lee, J.A. and Stewart, G.R., 1974. Nitrate utilization by species from acidic and calcareous soils. New Phytol., 73: 1221-1231. Haywood, J.D., 1986. Response of planted Pinus taeda L. to brush control in northern Louisiana. For. Ecol. Manage., 15: 129-134. Hesselman, H., 1917. Studien fiber die Nitrabildung in nattirlichen Boden und ihre Beduntung in pflanzenSkologischer Hinsicht. Medd. Statens Skogst'6rsiiksanst. Stockh., 13-14: 297-527. H~igberg, P., Granstr6m, A., Johansson, T., Lundmark-Thelin, A. and Niisholm, T., 1986. Plant nitrate reductase activity as an indicator of availability of nitrate in forest soils. Can. J. For. Res., 16: 1165-1169. Ingestad, T., 1967. Methods for uniform optimum fertilization of forest tree plants. Proc. 14th IUFRO Congr., 3: 265-269. Ingestad, T., 1979. Mineral nutrient requirements of Pinus sylvestris and Picea abies seedlings. Physiol. Plant., 45: 373-380. Jobidon, R., Thibault, J.R. and Fortin, J.A., 1989. Phytotoxic effect of barley, oat, and wheat straw mulches in eastern Quebec forest plantations. I. Effects on red raspberry (Rubus idaeus) For. Ecol. Manage., 29: 277-294. Keeney, D.R. and Nelson, D.W., 1982. Nitrogen - Inorganic forms. In: A.L. Page, R.H. Miller and D.R. Keeney (Editors), Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties, Am. Soc. Agron., Agron. Monogr. No. 9 (2nd edition), pp. 643-698.
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Kimber, R.W.L., 1973. Phytotoxicity from plant residues. III. The relative effect of toxins and nitrogen immobilization on the germination and growth of wheat. Plant Soil, 38: 543-555. Lang, E. and Jagnow, G., 1986. Fungi of a forest soil nitrifying at low pH values. Microbiol. Ecol., 38: 257-265. Likens, G.E., Bormann, F.H. and Johnson, N.M., 1969. Nitrification: importance to nutrient losses for a cutover forest ecosystem. Science, 163: 1205-1206. Likens, G.E., Bormann, F.H., Johnson, N.M., Fisher, D.W. and Pierce, R.S., 1970. Effects of forest ecosystem. Ecol. Monogr., 40: 23-47. Lodhi, M.A.K., 1978. Inhibition of nitrifying bacteria, nitrification and mineralization in spoil soils as related to their successional stages. Bull. Torrey Bot. Club, 106: 284-289. Lynch, J.M., 1977. Phytotoxicity of acetic acid produced in the anaerobic decomposition of wheat straw. J. Appl. Bacteriol., 42: 81-87. Marks, P.L., 1974. The role of pin cherry (Prunus pennsylvanica L. ) in the maintenance of stability in northern hardwood ecosystems. Ecol. Monogr., 44: 73-88. Marks, P.L. and Bormann, F.H., 1972. Revegetation following forest cutting: mechanisms for return to steady-state nutrient cycling. Science, 176: 914-915. Norstadt, F.A. and McCalla, T.M., 1963. Phytotoxic substances from a species of Penicillium Science, 140: 410-411. Putnam, A.R. and De Frank, J., 1983. Use of phytotoxic plant residues for selective weed control. Crop. Protect., 2: 173-181. Putnam, A.R. and Weston, L.A. 1986. Adverse impacts of allelopathy in agricultural systems. In: A.R. Putnam and C.S. Tang (Editors), The Science of Allelopathy. Wiley, New York, pp. 4356. Rice, E.L., 1984. Allelopathy, 2nd ed., Academic Press, Orlando, F1, 422 pp. Rice, E.L. and Pancholy, S.K. 1972. Inhibition of nitrification by climax ecosystems. Am. J. Bot., 59: 1033-1040. Rhoden, E.G., Croy, L.I. and Doolittle, G., 1987. Seasonal and diurnal variation in nitrate reductase activity of cowpeas. Plant Soil, 102: 17-19. Romell, L.G., 1935. Ecological problems of the humus layer in the forest. Cornell Univ. Agric. Exp. Stn. Mem., 170, 28 pp. Schmidt, E.L. and Belser, L.W., 1982. Nitrifying bacteria. In: A.L. Page, R.H. Miller and D.R. Keeny (Editors), Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. Am. Soc. Agron., Agron. Monogr. No. 9 (2nd Edition), pp. 1027-1042. Smith, W.H., Bormann, F.H. and Likens, G.E., 1968. Response of chemoautotrophic nitrifiers to forest cutting. Soil Sci., 106: 471-473. Swan, H.S.D., 1970. Relationships between nutrient supply, growth and nutrient concentrations in the foliage of black spruce and jack pine. Pulp Pap. Res. Inst. Can., Woodlands Pap., 19, 46 pp. Tamm, C.O., 1974. Experiments to analyse the behaviour of young spruce forest at different nutrient levels. In: Proc. First International Congress of Ecology - Structure, Functioning and Management of Ecosystems. The Hague, 8-14 September 1974. Pudoc, Wageningen, pp. 266272. Tilman, D., 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecol. Monogr., 57: 189-214. Vitousek, P.M., 1977. The regulation of element concentrations in mountain streams in the northeastern United States. Ecol. Monogr., 47: 65-87. Vitousek, P.M., 1981. Clear-cutting and the nitrogen cycle. In: F.E. Clark and T. Rosswall (Editors), Terrestrial Nitrogen Cycles. Ecol. Bull. (Stockh.), 33: 631-642. Vitousek, P.M. and Matson, P.A., 1984. Mechanisms of nitrogen retention in forest ecosystems: a field experiment. Science, 225: 51-52. Vitousek, P.M. and Mellilo, J.M,, 1979. Nitrate losses from disturbed forests: patterns and mechanisms. For Sci., 25: 605-619.
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Vitousek, P.M. Gosz, J.R., Grier, C.C., Mellilo, J.R. and Reiners, W.A., 1982. A comparative analysis of potential nitrification and nitrate mobility in forest ecosystems. Ecol. Monogr., 52: 155-177. Weetman, G.F., 1968. The nitrogen fertilization of three black spruce stands. Pulp Pap. Res. Inst. Can., Woodlands Pap., 6, 45 pp. White, C.S., 1986. Volatile and water-soluble inhibitors of nitrogen mineralization and nitrification in a ponderosa pine ecosystem. Biol. Fert. Soils, 2: 97-104. Whitney, G.G., 1978. A demographic analysis of Rubus idaeus L. and Rubus pubescens RAF. The reproductive traits and population dynamics of two temporally isolated members of the genus Rubus. Ph.D. Thesis, Yale University, 139 pp. Zutter, B.R., Glover, G.R. and Gjerstad, D.H., 1986. Effects of herbaceous weed control using herbicides on a young loblolly pine plantation. For. Sci., 32" 882-899.
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P h y t o t o x i c Effect of Barley, Oat, and W h e a t - S t r a w Mulches in Eastern Quebec Forest Plantations 2. Effects on Nitrification and B l a c k Spruce (Picea mariana) Seedling G r o w t h R. JOBIDON 1, J.R. THIBAULT2 and J.A. FORTIN 2
IMinist~re de l'Energie et des Ressources, Direction Recherche et Ddveloppement, Service de l'Am~lioration des Arbres, 2700 Einstein, Sainte-Foy, Qudbec, GIP 3 W8 (Canada) 2Centre de Recherche en Biologie Foresti~re, Facult~ de Foresterie et G~od~sie, Universitd Laval, Qudbec, GIK 7P4 (Canada) (Accepted 11 January 1989)
ABSTRACT Jobidon, R., Thibault, J.R. and Fortin, J.A., 1989. Phytotoxic effect of barley, oat, and wheatstraw mulches in eastern Quebec forest plantations. 2. Effects on nitrification and black spruce ( Picea mariana ) seedling growth. For. Ecol. Manage., 29: 295-310. Red raspberry (Rubus idaeus L.), a common forest weed especially in clearcut areas of northeastern America, is known to possess definite nitrophilous habit. In a companion paper we showed that barley, oat, and wheat straws used as cover mulches inhibited red-raspberry establishment and N nutrition after clearcutting and site preparation. As part of the mechanisms involved, we hypothesized that an inhibition of nitrification would prevent the establishment of red raspberry. Effects of barley, oat, and wheat-straw cover-mulches were evaluated under field conditions. We monitored soil NO3-N production, the abundance of NH+-oxidizers, and nitrate reductase activity (NRA) in R. idaeus foliage as an indication of the availability of nitrate in soil. Over two consecutive years, the use of straw mulches significantly reduced NO3-N soil production and NH +-oxidizers. The five phenolic acids, p-coumaric, syringic, vanillic, ferulic and p-hydroxybenzoic, responsible for the toxicity of the straws, significantly inhibited NH +-oxidizers at concentrations ranging from 10-3M to 10-6M in a laboratory experiment. The NRA measured during the 1987 growing-season in red-raspberry foliage confirmed the results obtained for NOz-N content of soil. The results suggest that the inhibition of NO3-N production under straw mulching restricted, to some extent, the establishment of red-raspberry seedlings. Other mechanisms could also be partially responsible for the inhibition observed, and are discussed. The effect of straw mulches was also evaluated on shoot growth and basal stem-diameter growth and on mineral nutrition (total N, P, K, Ca, Mg and Mn) of black-spruce (Picea mariana (Mill) BSP) seedlings over the two successive growing-seasons (1986 and 1987 ) after straw application. The straw treatments enhanced shoot and stem-diameter growth of P. mariana seedlings. Treated black-spruce seedlings showed a significantly higher foliar N-content than control seedlings. There were no consistant significant differences between treated and control seedlings for the other nutrients analysed. The promotion of black-spruce seedling growth and foliar N-content is associated with a reduction in competitive vegetation and also an inhibition of nitrification from the straws.
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R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
INTRODUCTION The effect of forest clearcutting on soil nutrient dynamics has received much attention during the last decades. Production of nitrate often increases rapidly after disturbance of the forest floor (Romell, 1935; Likens et al., 1970; Vitousek et al., 1982). The rapid increase in nitrification generally observed after a major disturbance such as clearcutting could be attributed to: 1) variations of abiotic factors; 2 ) reduction in plant uptake of nitrogen; 3) increase of organicmatter decomposition and nitrogen mineralization; and 4) the removal of sources of potentially inhibitory biochemicals (Vitousek and Mellilo, 1979; Vitousek, 1981). The extent of nutrient loss, especially of NO3-N from the forest ecosystem after clearcutting should decrease rapidly according to the rate of revegetation (Marks and Bormann, 1972). Thus, the role of successional species in the maintenance of soil nutrient status becomes increasingly important if devegetation becomes more frequent (Marks, 1974). The successional pattern of species after forest disturbances in eastern North America was studied by Marks ( 1974 ) who, among others (e.g., Whitney, 1978) noted the importance of red raspberry (Rubus idaeus L.) as a pioneer and opportunistic species in disturbed forests. This species is also known by European and North American foresters to possess definite nitrophilous habits (Hesselman, 1917; Tamm, 1974; Whitney, 1978; Tilman, 1987). In a companion study (Jobidon et al., 1989, this volume), we demonstrated the allelopathic inhibition of red-raspberry seedling growth by using cover mulches of barley, oat, and wheat straws, in laboratory and field experiments. Under laboratory conditions, aqueous extracts of barley, oat, and wheat straws significantly inhibited shoot elongation and completely inhibited root development ofR. idaeus propagules. Under field conditions, red raspberry seedlings failed to establish in the first growing-season after the straw placement and very few plants were established in the second growing season. The use of a control mulch allowed the separation of physical and chemical effects of the straws (Putnam and De Frank, 1983). The treatments significantly reduced N nutrition in R./daeus plants, as compared to both control mulch and control. Moreover, the treatments reduced the establishment of other opportunistic species (e.g. Epilobium angustifolium L. ) . The allelopathic effect of barley, oat, and wheat straws has been documented in agriculture (Rice 1984; Putnam and Weston, 1986) but has never been used in forest plantations to inhibit the establishment of unwanted vegetation after clearcutting and site preparation. Numerous studies have demonstrated that use of straw mulches on the soil surface generally results in the reduction of nitrate formation (e.g. Albrecht and Uhland, 1925; Rice, 1984). Our hypothesis was that inhibition of R. /daeus establishment in recent clearcut and prepared sites of eastern Quebec was, at least partially, due to an inhibition of nitrification and nitrifying bacteria by allelopathic substances
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297
involved in decomposing barley, oat, and wheat straws. As part of this study we also evaluate the effect of the straws on growth of planted black-spruce seedlings (Picea mariana (Mill.) BSP). Among the most noticeable effects of allelopathy previously reported are seed germination inhibition, growth inhibition, and impaired mineral nutrition (Rice, 1984). So the potential effects of the straws on black spruce seedlings were determined by rate of height increase and mineral nutrition status of the seedlings during the two successive years following the treatments.
MATERIALSAND METHODS
The study sites The three experimental sites (referred to as P1, P2, and P3) used in this study are described by Jobidon et al. (1989, this volume ). Analyses for ammonium-nitrogen and nitrate-N in the soil Soil NH4- and NO3-N were determined during the summers 1986 and 1987 in each plot of the three sites, as follows. Since NO3-N is a highly mobile ion in the soil, the soil samples were collected after a period of 5 consecutive days without rainfall. Sampling was done for one site at a time. From each plot, about 10 g of a composite soil sample was directly extracted in the field, in 100 ml of 2M KC1 solution (Keeney and Nelson, 1982) and the operation repeated ten times. Each composite soil sample was the result of ten randomly distributed and thoroughly mixed soil cores from the treated and control furrows (015 cm). The 400-ml polypropylene jars containing the soil KC1 suspension were vigourously shaken for 1 min and placed in a portable ice-box at 10 ° C, until refrigeration at 3 °C shortly afterwards. The procedure followed was according to the recommendations of Keeney and Nelson (1982). After being shaken, the soil KC1 suspension was filtered through W h a t m a n No. 42 filter paper to a volume of 500 ml with 2M KC1 solution. Aliquots of the filtrates were stored in a refrigerator (3 °C ) until the analyses were performed, usually within 2-5 days. The soil was oven-dried at 85 °C for 72 h and weighed. Ammonium- and nitrate-N were determined spectrophotometrically using an automated Flow Injection Analyser (Tecator FIA star model No 5020 equipped with a Tecator 5007 Sampler and a Tecator 5022 Detector Controller). Ammonium-N was determined by the indophenol-blue reaction at 636 nm. The FIA used was equipped with a Cd column to reduce NO~ to NO2, which was then measured at a wavelength of 540 nm. The blank consisted of 2M KC1 solution filtered through W h a t m a n No. 42 filter paper. Soil samples were col-
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lected each time conditions of no rainfall during 5 consecutive days occurred; i.e. in 1986, June and July for the P1 site, and July for the P2 and P3 sites, and in 1987, June, July and August for the P1 and P2 sites and June for the P3 site.
Determination of NH +-oxidizers Determination of the abundance of NH +-oxidizers was performed in June and August 1986 and in June 1987 for every plot of the three study sites, and repeated three times. For each plot, 15 evenly distributed soil samples (0-15 cm) were thoroughly mixed and the composite soils were put in plastic bags. These were incubated on the day of sampling for determination of the NH +oxidizing population by the most probable number (MPN) method of Alexander (1982), according to the recommendations of Schmidt and Belser (1982). The blank consisted of uninoculated control tubes. The incubation was performed at 28 ° C in the dark for a period of 6 weeks.
Effect of phenolic acids on NH +-oxidizers In August 1986, 15 evenly distributed soil samples (0-15 cm) collected from each of the three control plots of the three study sites were thoroughly mixed and dept in plastic bags. Sterile solutions ofp-coumaric, syringic, vanillic, ferulic, and p-hydroxybenzoic acids were prepared. The effects of four concentrations ( 10 -~, 10 -4, 10 -5, and 10 -6 M) of each phenolic acid were tested on the development of the NH4 +-oxidizing population, as above. All tubes were inoculated with the same soil buffer suspension. The control consisted of an incubation of the same soil buffer suspension without the addition of phenolic acids. A first blank consisted of uninoculated control tubes with the phenolic acids, and a second blank consisted of uninoculated control tubes without the addition of phenolic acids. The tubes were placed in the dark at 28°C for a period of 6 weeks. The experiment was repeated three times.
Nitrate reductase activity (NRA) in red-raspberry foliage Nine R. idaeus seedlings were randomly sampled with root systems from each plot of the three sites between 0700 and 0900 (Rhoden et al., 1987) and put in a portable ice-box at 5 ° C. The seedlings were sampled in June, July, and August 1987. The NRA was immediately determined on three composite foliage samples from three seedlings by the in-vivo method developed for Deschampsiaflexuosa by Havill et al. (1974) and described by HSgberg et al. (1986). Composite samples of 0.1-g (fresh weight) leaves were cut into 1-2-mm-long segments and transferred to test tubes. Five ml of incubation medium were added and the test tubes were shaken for 1 h at 28 ° C in darkness. A reagent of 1 ml 1% (w/w) sulfanilic acid in 3.2MHC1 and 1 ml 0.01% (w/w) N- (1-naphthyl ) ethylenediamine (NED) was added to a 1-ml aliquot of assay medium for spectrophotometric (540 mm) determination of the nitrite produced. The blank consisted of the same solution with 1 ml of deionized water instead of the NED
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299
c o m p o n e n t . T h e a c t i v i t y is e x p r e s s e d t h r o u g h o u t as t h e a m o u n t o f n i t r i t e p r o d u c e d g - 1 f r e s h w e i g h t h - 1.
Black-spruce seedling growth and [oliar mineral content During mid-May 1986, all the sites were planted with P. mar/ana seedlings grown in containers under same environmental conditions, and which were about 10-12 cm in height at planting time. The seedlings were planted manually in the center of the furrows at 2-m intervals. Black-spruce seedling growth and foliar mineral content were measured at the end of 1986 and 1987 growing-seasons. In September 1986, 40 black-spruce seedlings were randomly harvested from each plot for a total of 240 seedlings per site, and shoot growth-increment was measured. The seedlings were randomly paired and oven-dried at 60 oC for 72 h. The needless were then carefully separated from other plant parts, ground to 40 mesh, and kept in plastic bottles until analysed, generally within a few weeks. Foliage was analysed for total nitrogen (N) by the micro-Kjeldahl method, phosphorus (P) by the vanadomolybdophosphoric-yellow method, and for potassium (K), calcium (Ca), magTABLE 1
Mean concentration 1 (p.p.m. g-1 dry weight) of NHa-N and NO3-N in the upper portion of the soil profiles (0-15 cm) for the 1986 growing-season Site
Treatment 2
P1
BA OA WH MX CM C
P2
BA OA WH MX CM C
P3
BA OA WH MX CM C
June
July
NH,+
N0~-
NH +
N0~-
19.27ab 16.57a 21.08bc 24.96cd 28.60de 31.46e
1.04ab 0.00a 0.00a 1.10ab 5.06c 4.09bc
16.06abc 28.56efg 28.02efg 26.76ef 13.79ab 10.56a
5.20cd 0.89ab 6.01d 6.69d 3.07abcd 2.21abcd
21.51d 16.53bc 23.49fg 30.79def 17.55bcd 18.72cd
0.79ab 6.60d 0.39ab 2.84abcd 6.67d 6.57d
34.73g 32.69fg 24.80def 26.71ef 27.59efg 29.91fg
4.35bcd 1.60abc 0.00a 5.13cd 5.86d 4.31bcd
n.a.
n.a.
n.a.
n.a.
1Each value represents mean of 10 analyses. Values followed by the same letter do not differ (P < 0.05 ) using Duncan's new multiple range test. n.a.: not available (see text for details ). eBA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control
300
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
nesium (Mg), and manganese (Mn) by standard atomic absorption techniques. In September 1987, 20 P. mariana seedlings were randomly harvested from each plot, for a total of 120 seedlings per site. Shoot growth-increment and basal stem-diameter of the seedlings were measured. The seedlings were then randomly paired, and the current-year needles were separated from the 1-year-old needles, and oven-dried at 60 ° C for 72 h for nutrient analyses (total N, P, K, Ca, Mg, Mn, as above). RESULTS
Effects on nitrification, NH4-oxidizers, and NRA During the 1986 and 1987 growing-seasons, the mean NO3-N content of soil from treated plots was slightly lower than that of both the control mulch and the control. However, the different treatments did not produce the same level of inhibition of nitrification within and between the sites (Tables 1 and 2). Oat and wheat straws produced the greatest reduction in NO3-N in soil during June 1986 at the P1 site, and only oat straw remained inhibitory in July, as TABLE 2 Mean concentration 1 (p.p.m. g-1 dry weight) of NHa-N and NO3-N in the upper portion of the soil profiles (0-15 cm) for the 1987 growing-season Site
Treatment 2
June
July
August
NH +
N0~-
NH +
N0~-
NH4+
N0~-
P1
BA OA WH MX CM C
14.20ab 16.55b 12.60ab ll.06ab 14.22ab 16.93b
8.95abc 5.70a 2.76a 5.41a 24.90e 23.56de
12.69abc 10.19a 10.9lab 10.92ab 17.07d 17.81d
5.23ab 4.21a 5.82ab 4.82a 22.11c 20.41c
ll.02bcd 12.05de 15.49f 11.26cd 8.97b 9.30bc
16.25ab 22.20bc 23.81bc 21.97bc 42.25d 28.58c
P2
BA OA WH MX CM C
12.10ab 12.49ab ll.96ab 10.86ab 9.91a 9.45a
9.13abc 7.23abc 7.93abc 6.37ab 18.68d 19.26de
15.35cd 12.93abc 14.67cd 13.29abc 13.87bc ll.49ab
18.55c 10.72b 19.83c 10.15b 29.84d 34.74e
13.50ef 12.82de 14.53ef 12.47de 5.51a 6.18a
12.62a 10.63a 10.31a 10.84a 14.15a 15.66a
P3
BA OA WH MX CM C
10.19a 14.80b 12.84ab ll.21ab 15.00b 12.80ab
6.17a 4.19a 3.42a 5.10a 11.42bc 12.51c
n.a.
n.a.
n.a.
n.a.
1Each value represents mean of 10 analyses. Values followed by the same letter do not differ (P < 0.05 ) using Duncan's new multiple range test. n.a.: not available (see text for details). 2BA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control.
PHYTOTOXICITYOF STRAWMULCHESTO PICEA
301
TABLE 3 Mean number 1 of NH4 +-oxidizers (MPN method) in the upper portion of the soil profiles (0-15 cm) for the 1986 and 1987 growing-seasons Site
Treatment 2
Spoil sampling data June 1986
August 1986
June 1987
P1
BA OA WH MX CM C
893* 927* 1027" 1053" 9420 10967
1123" 1267" 986* 1033" 11213 12453
2313" 3790 3027* 2974* 14703 16150
P2
BA OA WH MX CM C
3300* 4800* 1816" 3937* 14813 16667
2893* 5423 3216" 4857 13706 15367
3720* 4903 4150" 3833* 15670 17421
P3
BA OA WH MX CM C
2660* 1933" 2767* 1056" 12500 14333
3327* 2893* 1933" 2327* 14230 13821
3870* 4541 4013" 3977* 18627 17430
1Each value represents mean of 3 repetitions. Within each site and sampling date, statistical t value* indicates significant difference between numbers of NH4 +-oxidizers in treatment and control(c) group at P<0.05. 2BA: barley straw; OA: oat straw; WH: wheat straw; MX: straw mixture; CM: control mulch; C: control. TABLE 4 Mean number 1 of NH4 +-oxidizers (MPN method) in control and treatment cultures with phenolic acids Treatment
P-Coumaric acid P-Hydroxybenzoic acid Ferulic acid Vanillic acid Syringic acid Phenolic acids mixture
Concentration (M)
Control
×10 -3
×10 -4
×10 -5
×10 -6
160" 590* 130" 233* 173" 99*
253* 687 284* 400* 217" 363*
293* 417" 363* 473* 310" 960
383* 1123 209* 960 690 703
5387 5387 5387 5387 5387 5387
1Each value represents mean of 3 repetitions. Statistical t value *indicates significant difference between numbers of NH4 +-oxidizers in treatment and control groups at P < 0.05.
302
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
TABLE 5 Average nitrate reductase activity (NRA 1, #mol NO2- g-~ h -~) in R. idaeus leaves for the P1, P2, and P3 sites in June, July and August of 1987 Site
Treatment2
NRA June
July
August
P1
BA OA WH CM C
4.54d 4.09d 2.97bc 6.21d 4.47d
1.84bcde 2.15de 1.62abc 4.99h 2.71f
2.19cd 2.27cde 1.98cd 4.82h 3.03f
P2
BA OA WH CM C
2.19b 2.44b 2.12b 5.45d 3.52cd
1.95cde 1.69bcd 1.66bcd 4.19g 4.32g
2.20cde 1.97bcd 1.77abc 4.32g 4.26g
P3
BA OA WH CM C
1.21a 1.16a 1.00a 3.79cd 3.51cd
1.22a 1.25a 1.54ab 1.37ab 2.15e
1,33a 1.41a 1.49ab 2.32de 2.61e
1Each value represents mean of 3 repetitions, each repetition analysed 3 times. For one month, values followed by the same letter do not differ (P<0.05), using Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw :CM: control mulch; C: control. TABLE 6 Mean 1 shoot increment (cm) and basal diameter (mm) of black-spruce seedlings, at the first and second growing-season after straw placement Site
Treatment2
Shoot increment
Basal diameter
1986
1987
1987
P1
BA OA WH CM C
9.7d 8.1c 8.6cd 4.3a 7.5bc
17.1d 14.7c 15.4c 10.8ab 12.7b
6.40f 4.37abc 5.21cde 3.57a 4.44bc
P2
BA OA WH CM C
7.5bc 8.4cd 8.2cd 5.2a 7.6bc
16.7d 15.4c 16.4c 10.7a 12.6b
6.35f 5.56de 6.66f 4.39bc 4.50bc
P3
BA OA WH CM C
7.9bc 8.3cd 8.1c 6.5b 8.3cd
16.2c 15.2cd 15.2cd 11.9ab 12.6b
6.03ef 4.70bcd 5.81ef 4.15ab 4.35abc
1Means followed by the same letter do not differ (P<0.05), according to Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw; CM: control mulch; C: control.
PHYTOTOXICITY OF STRAWMULCHESTO PICEA
303
compared to control plots. Barley and wheat straws produced significant reductions in NO3-N content of soil at the P2 site, and only wheat straw was inhibitory at the P3 site (Table 1 ). There were no significant differences in NO3-N or NH4-N content of soil between the control mulch and the control during the 1986 and 1987 growing-seasons (Tables 1 and 2). For both study years, NH4-N content of soil was slightly higher in the treated plots than in either control. Nitrate production increased during the 1987 growing-season, as compared to 1986. The increases were more pronounced for the control plots than for the treated ones, indicating that the treatments slightly delayed the production of NO3-N. Most treatments significantly reduced NH +-oxidizers in both 1986 and 1987 (Table 3). The five phenolic acids tested significantly reduced the mean number of NH+-oxidizers (Table 4). Even at a concentration of 10-6M, p-coumaric and ferulic acids remained inhibitory to the NH +oxidizing populations. The nitrate reductase activity in red-raspberry foliage from treated plots was significantly lower during 1987 than that of both controls (Table 5). TABLE 7 Mean 1 nutrient content (p.p.m. ex. N, % ) in black-spruce needles, the first growing-season after straw placement Site
Treatment 2
Nutrient
N
P
K
Ca
Mg
Mn
P1
BA OA WH MX CM C
2.06fg 1.76bc 1.88cde 1.76bc 1.05a 1.61b
1 813de 1 950e 1 731bcd 1 749cd 1 752cd 1 941de
5 716ab 6 345def 5 942abcd 6 166bcde 6 136bcd 6 431def
5 896fgh 5 800efgh 5 779efgh 5 711defgh 5 374bcdef 5 016ab
1 23lab 1 254abcd 1 302abcde 1 288abcd 1 186 a 1 239abc
1 542cd 1 482bcd 1 408bcd 1 250b 1 358bc 1 629d
P2
BA OA WH MX CM C
2.08fg 1.99ef 2.18g 2.01ef 1.27 1.63b
2 l13f 2 l13f 1 972ef 1 940de 1 797d 1 857de
5 904abc 9 019g 6 363def 6 150bcde 6 314cdef 6 691f
6 788j 6 254hij 6 593ij 6 298hij 5 149abcd 4 601a
1 430ef 1 355bcdef 1 377def 1 478f 1 204a 1 179a
1 365bcd 1 363bcd 1 494bcd 1 340bc 1 336bc 1 502bcd
P3
BA OA WA MX CM C
1.83cd 1.94def 1.83cd 1.76bc 1.03a 1.46
1 598ab 1 537a 1 716bcd 1 810d 1 627abc 1 769cd
6 662ef 6 759f 6 431def 6 772f 5 615a 5 798abc
5 788efgh 5 087abc 6 126ghi 5 645cdefg 5 241bcde 5 452bcdef
1 467f 1 355bcdef 1 389ef 1 375cdef 1 270abcd 1 290abcde
824a 645a 1 437bcd 1 650d 1 221b 1 611cd
IEach value represents mean of 20 analyses. Values followed by the same letterdo not differ (P< 0.05), according to Duncan's new multple range test. 2BA: barley straw; OA: oat straw; W H : wheat straw; M X : misture of the 3 kinds of straws; C M : control mutch; C: control.
304
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
TABLE 8 Mean nutrient content 1 (p.p.m., ex. N, % ) in black-spruce current needles and 1-year-old needles, the second growing-season after straw placement
Needle age
Treatment 2
Nutrient N
P
Current needles
BA OA WH CM C
1.98a 1.86a 1.98a 1.67b 1.88a
2 2 3 2 3
1-year-old needles
BA OA WH CM C
1.64y 1.46x 1.35x 1.44x 1.48x
2 334x 2 216x 2 l13x 2 238x 2 252x
883a 863a 140a 830a 144a
K
Ca
Mg
Mn
5 690a 6 152ab 6 238ab 5 808ab 6 449b
6 899b 6 242ab 6 372ab 6 460ab 5 775a
1 200a 1 268a 1 230a 1 207a 1 171a
2 042a 1950a 2 424a 2 442a 2 528a
5 338x 5 155x 5 009x 4 468y 5 138x
8 471x 8 561x 8 144x 10 239y 8 950xy
891x 907x 718y 991x 882x
2 815x 2 667x 3 382xy 3 934y 4 002y
1Each value represents mean of 20 analyses (P1 and P2 sites). For one needle age group, means followed by the same letter do not differ ( P < 0 . 0 5 ) , according to Duncan's new multiple range test. 2BA: barley straw; OA: oat straw; WH: wheat straw; CM: control mulch; C: control.
Effects on black-spruce seedling growth and mineral nutrition Application of straw to the soil surface caused a significant increase in mean shoot and basal stem-diameter growth of black-spruce seedlings (Table 6). For the 1986 and 1987 growing-seasons, mean shoot growth-increments of treated seedlings were 8.3 and 17.8 cm respectively, and reached 6.6 and 12.6 cm respectively for control seedlings (values compiled from Table 6). Basal stem diameter of treated and control seedlings were 5.68 and 4.43 ram, respectively (values compiled from Table 6). For both study years, responses ofP. mar/ana seedlings to the treatments were similar between sites and between treatments. There were no cases of impaired mineral nutrition in black-spruce seedlings growing on the treated plots, for either study year 1986 (Table 7) or 1987 (Table 8). On the contrary, treated black-spruce seedlings showed a significantly higher foliar N content than control seedlings, for the 1986 growingseason. For the three study sites and the four straw treatments, the 1986 blackspruce needle-N content was 1.92 %, as compared to 1.57% for the control seedlings (data compiled from Table 7). For the 1987 growing season, N content in current needles was not significantly higher than in the control. The other nutrients analysed (P, K, Ca, Mg, and Mn) showed no significant differences between treated and control seedlings, for both study years, except for Mn on site P3 with barley and oat straw (Table 7).
PHYTOTOXICITY OF STRAW MULCHES TO PICEA
305
DISCUSSION The results showed that NO3-N production in treated plots was inhibited, or at least delayed, as compared to control plots, during the course of this study. The inhibition of nitrification could not be attributed to low soil pH, or differences in pH values between treated and control plots (Jobidon et al., 1989). Work at Hubbard Brook (U.S.A.) indicated that nitrification can proceed in the acid soils (pH below 5.0) of cutover hardwood forests (Smith et al., 1968; Likens et al., 1969; Vitousek, 1977). Such nitrification at low pH may be the result of acid-adapted varieties of autotrophic nitrifiers (Alexander, 1977), or the result of heterotrophic nitrification (Lang and Jagnow, 1986), or both. Moreover, differences observed in NOa-N production, the abundance of NH +oxidizers and NRA in R./daeus foliage could not be attributed to differences in any soil characteristics within the sites (Jobidon et al., 1989). The cover straw mulches used in this study inhibited, to some extent, the NH +-oxidizers. Many studies have shown that low numbers of nitrifiers could be a direct result of toxic effects from secondary plant chemicals both in agroecosystems (Rice, 1984), and forests (Lodhi, 1978; White 1986). A drastic reduction in numbers ofNH + -oxidizers upon addition of various phenolic acids was observed, even at a concentration as low as 10-6M. The five phenolic acids used were identified as the toxic substances involved in the allelopathic activity of the straws used (Guenzi and McCalla, 1966). However, we did not establish if these acids occurred in toxic concentrations in the field experiment. Nitrate reductase is an inducible enzyme (Beevers and Hageman, 1980). Recently, HSgberg et al. (1986) pointed out the positive correlation between NRA and the availability of NOa-N in forest soils. The low rates of NRA observed in red-raspberry foliage from treated plots during the 1987 growingseason confirmed the results obtained from the determinations of soil NO3-N content and the abundance of NH+-oxidizers. The low NRA obtained on treated plots is in accordance with the low N concentration measured in the R. idaeus leaves from treated plots (Jobidon et al., 1989). The nitrophilous character of red raspberry was established by a number of researchers (e.g., Hesselman, 1917; Tamm, 1974; Whitney, 1978; Tilman, 1987) with respect to its establishment after forest disturbance. In a related study (Jobidon et al., 1989), we demonstrated the inhibition of red-raspberry establishment on the treated plots. Moreover, foliar-nutrient analyses revealed a significant reduction (33%) in N content ofR. idaeus seedlings growing on the treated plots, as compared to control seedlings. The nitrophilous character of red raspberry, combined with the low NO3-N produced could, at least partially, explain the differences previously observed in red-raspberry establishment. From the results obtained in this study and the previous one (Jobidon et al., 1989) we could assume that the inhibition observed in R./daeus seedling establishment was, at least partially, the result of: 1 ) a direct phytotoxic effect
306
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
due to the straws; and 2) an inhibitory effect of the straws on nitrification processes. However, other mechanisms could also have played a role in the inhibition observed. Although this present study was not designed to evaluate the effect of the straws on other factors than the fate of N, the treatments have produced other direct or indirect effects, such as conserving soil moisture, regulation of soil surface temperature, and reduction of soil erosion. These factors could in turn influence nitrogen mineralization and nitrification. Conservation (or addition) of organic matter on the soil surface after clearcutting and site preparation could enhance N immobilization. Vitousek and Matson (1984) pointed out the importance of conserving organic matter after clearcutting because microbial uptake of N during decomposition of organic matter was the most important process retaining N. However, the low rates of NO3-N content of soil in treated plots could not be attributed to increased Nimmobilization, since soil from both control mulch and control had similar N content. Kimber (1973) pointed out that soil surface application of wheat straw depressed germination and growth of wheat, while immobilization of N occurred mainly when the straw was mixed with the soil. Production of inhibitors could be due to microorganisms involved in straw decomposition, as in the production of patulin by PeniciUium urticae growing on wheat-straw residues (Norstadt and McCalla, 1963). The toxic effect of patulin has been demonstrated on seed germination and early growth of bioassay species (Ellis and McCalla, 1973). Patulin, and other microbially produced phytotoxins - e.g. the phytotoxin acetic acid produced during straw decomposition (Lynch, 1977 ) - could also contribute to the inhibition of nitrification observed, and inhibit red-raspberry establishment. For both 1986 and 1987 growing seasons, shoot growth increment of P. mariana seedlings was similar among the straw treatments, regardless of study sites. The straw treatments did not inhibit black-spruce seedling growth. There are some cases of significant growth enhancement on the treated plots which could be observed during both study years. Basal diameter of black-spruce seedlings was higher on treated plots than on control plots. Seedling stem diameter was somewhat more responsive than height to the treatments applied. This could be the result of reducing competitive vegetation in plots covered with straw mulches. Haywood (1986) and Zutter et al. (1986) also noted similar trend in diameter response to reduction in interfering vegetation. The differences observed in black-spruce needle N content could not be attributed to differences in soil N content prior to straw placement, or to differences in other soil characteristics between treated and control plots (Jobidon et al., 1989). The promotion in N nutrition and growth of treated black-spruce seedlings could be the result of conservation of N in the form of NH4-N rather than in the easily leachable NO3-N form, or the result of reduced competition for light, nutrients, and water on treated plots, or both. Unfortunately, the experiment was not designed to separate those two effects. From an ecological
PHYTOTOXICITY OF STRAW MULCHES TO PICEA
307
point of view, it is of interest to note that N nutrition of a pioneer species, red raspberry, was significantly depressed by the treatments (Jobidon et al., 1989) whereas N nutrition of black spruce, a climax species, was significantly enhanced. The only reasonable explanation for the opposite response seems to be that R. idaeus has adapted well to rapidly released N in the form of NO~-. In contrast, it appears that black spruce has evolved as an efficient user of NH +. This is of importance for conservation of nitrogen within the site, especially in areas where N nutrition is critical. Such an opposite response in N nutrition indirectly support the hypothesis of Rice and Pancholy (1972) that inhibition of nitrification increased as succession progressed towards climax. It would be of interest to verify if the opposite response obtained in N nutrition between red-raspberry (species forming vesicular-arbuscular mycorrhizae) and black spruce (species forming ectomycorrhizae) could be partially explained by their respective symbiotic association. The nutrient status of treated black-spruce seedlings, for both study years, is in accordance with the adequate levels suggested by Swan (1970) for this species. However, the 1986 needle N content in both control seedlings could be noted as critical or low, according to Swan (1970), and could not be attributed to differences in environmental conditions during seedling production. The question of nutrient ratios has been extensively investigated by Ingestad (1967, 1979) in relation to optimum growth of seedlings. For birch, pine and spruce seedlings, optimum nutrition occurred at the following relative levels: N = 100, P = 13, K = 6 5 , C a = 6 , Mg--8.5. For the 1986 growing season, we obtained the following levels for treated seedlings: N = 1 0 0 , P = 9 . 6 , K = 4 0 , Ca-- 31.2, Mg = 7.1; and for control seedlings (control mulch not included): N = 100, P = 12, K = 40.2, Ca = 32, Mg = 7.8. For the 1987 growing season (current needles ), the levels were as follows for treated seedlings: N = 100, P = 15.3, K = 31.1 C a = 33.5, M g = 6.4; and for control seedlings (control mulch not included): N = 100, P = 16.7, K=34.3, Ca=30.7, Mg=6.2. For both study years, there are no noticeable differences in relative nutrient levels between treated and control seedlings, indicating that treatments did not affect balance of plant macronutrients. The levels obtained in K and Ca differed, to some extent, from the levels proposed by Ingestad (1967, 1979) but for species other than black spruce. However, the relative levels obtained in K and Ca are in accordance with Weetman (1968) for black-spruce seedlings. The results obtained demonstrated an allelopathic effect of the straws on nitrification processes and on R. idaeus establishment during the two consecutive years of this study. However, other factors, abiotic and biotic, could have exerted direct or indirect effect, on nitrification and on red-raspberry seed germination and growth. The treatments did not impair P. mariana seedling growth.
308
R. JOBIDON, J.R. THIBAULT AND J.A. FORTIN
ACKNOWLEDGEMENTS
This study was funded by the Gouvernement du Qudbec, Ministate de l'Energie et des Ressources, Direction Recherche et Ddveloppement, to whom we are grateful. We are indebted to Mr. Georges Fortier, Head of the Office des Producteurs de Bois de La Pocati~re, for providing us the experimental sites and to the Canadian Forestry Service for their invaluable assistance. The authors thank Drs. Michel Boudoux, Gilles D. Leroux, Azim U. Mallik, Alan R. Putnam, and Gilles Vall~e for helpful suggestions and critical review of earlier version of the manuscript, Dr. Claude Camir~ for discussions, and Mr. Alain Brousseau and Mr. Rdal Mercier for technical advice. We also thank anonymous reviewers for valuable suggestions on the manuscript.
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