- Email: [email protected]
Soil microbiological and chemical effects of a nitrogen-fixing shrub in poplar plantations in semi-arid region of Northeast China
European Journal of Soil Biology 46 (2010) 325e329
Contents lists available at ScienceDirect
European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Soil microbiological and chemical effects of a nitrogen-fixing shrub in poplar plantations in semi-arid region of Northeast China Rong Mao a, b, De-Hui Zeng a, *, Gui-Yan Ai a, Dan Yang a, b, Lu-Jun Li a, b, Yun-Xia Liu a, b a b
Institute of Applied Ecology, Chinese Academy of Sciences, 72 Wenhua Rd, Shenyang 110016, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2009 Received in revised form 25 May 2010 Accepted 28 May 2010 Available online 9 June 2010 Handling editor: Christoph Tebbe
Nitrogen (N)-fixing species have a function to enrich N in soil. Mixing N-fixing shrub species into poplar stands can be assumed as a measure to increase productivity while improving soil fertility. To verify this assumption and to understand the temporal influences of N-fixing shrub species mixed into poplar plantations on soil fertility, we investigated selected soil chemical and microbial properties in pure poplar (Populus xiaozhuanica W. Y. Hsu et Liang) and mixed poplareseabuckthorn (Hippophae rhamnoides L.) stands at ages of five and 15 years in a semi-arid region of Northeast China. Both stands at age of five have similar values of aboveground biomass, total soil organic C concentration, total N concentration, microbial biomass C, and metabolic quotient; however, at age of 15, these values except for soil metabolic quotient are significantly greater in mixed poplareseabuckthorn stand than in pure poplar stand. The soil metabolic quotient is lower in the former stand than in the latter stand. Our results suggest that, in semi-arid regions, mixing N-fixing shrub species into poplar plantations can improve soil fertility in a long run rather than in a short term; therefore, mixing N-fixing shrub species into poplar stands is an option to improve soil fertility and increase productivity in a long run. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Mixed forest N mineralization Pure forest Soil metabolic quotient Soil microbial biomass
1. Introduction Fast-growing forest plantations have been recognized to be an effective measure to solve the shortage of wood in many parts of the world [8,10]. However, frequent harvest-related nutrient losses can lead to declines in soil fertility and biomass productivity over successive rotations [16,25]. Therefore, nitrogen (N)-fixing plants have been recommended for mixed plantations with valuable timber trees to offset potential soil degradation and increase stand productivity [9,15,25,26]. Compared with pure plantation, the presence of nitrogen-fixing plants can promote nutrient cycling, increase soil N pool, improve soil N availability and enhance overall plantation yields [4,6,33]. Poplar (Populus spp.), a fast-growing species, has been planted on a large scale in most parts of China, especially in arid and semiarid areas [17]. The presence of N-fixing plant species in poplar plantations has been shown to maintain soil fertility and stand productivity [13,19,20]. Previous studies have suggested that introduction of N-fixing species into poplar plantations could
* Corresponding author. Tel.: þ86 24 83970220; fax: þ86 24 83970394. E-mail address: [email protected] (D.-H. Zeng). 1164-5563/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2010.05.005
enhance both litter quality and quantity [13,19] and increase aboveand below-ground productivity [20]. However, the effect of Nfixing species on soil microbial properties is not well understood in mixed poplar plantations. Considering the critical roles of soil microbial biomass and activity in regulating nutrient cycling and maintaining soil fertility [1,11], a better knowledge of soil microbial properties may help to evaluate the effect of N-fixing species on soil fertility and thus to improve stand productivity in poplar plantations. Seabuckthorn (Hippophae rhamnoides L.) is an actinorhizal shrub species characterized by its ability to form a symbiosis with the nitrogen-fixing actinobacteria Frankia. This association leads to the formation of nitrogen-fixing root nodules and thus improves soil nutrient status [19]. Therefore, seabuckthorn has been widely introduced into poplar stands in arid and semi-arid regions of China [19,20]. In the present study, we investigated pH, total organic carbon (TOC) and total nitrogen (TN) concentrations, microbial biomass carbon (MBC), metabolic quotient (qCO2), and net N mineralization rate (Nmin) in soils from 5- and 15-year-old pure poplar (Populus xiaozhuanica W. Y. Hsu et Liang) plantations and mixed poplar-seabuckthorn plantations in a semi-arid region of Northeast China. Our objective was to examine the impact of N-fixing shrub species on soil chemical and microbial properties in poplar stands in relation to stand age.
326
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
2. Materials and methods 2.1. Study site and experimental design The study was conducted in Jianping County (40170 e42 200 N, 119100 e120 020 E), west of Liaoning Province, Northeast China. The study site belongs to the semi-arid monsoon climate of temperate zone. The mean annual temperature is 6.5 C, precipitation is 467 mm (more than 60% falling between June and August), and frost-free period is 148 days. The soil is a sandy soil and classified as skeletol primitive soil in the Chinese Soil Classification, which corresponds to Fluvisols in the FAO Soil Classification. To quantify changes in soil chemical and microbial properties following introduction of seabuckthorn into poplar plantations, 5and 15-year-old mixed poplar plantations and adjacent pure poplar plantations were chosen for investigation. For each stand age, three pairs of pure and mixed poplar sites were identified in September 2008. Each pair of pure and mixed poplar stands was adjacent and 30 m apart, and all replicated paired plots were within 500 m. All sites were located on relatively flat terrain and similar in soil type, slope and land management history. The soils of all the treatments were developed on the same parent material that was fluvial deposits. Based on the oral history, the lands had been cultivated for more than 100 years before afforestation, and poplar stands were in a first-rotation cycle. The stocking density of pure poplar stands was approximately 830 trees ha1. Poplar and seabuckthorn were mixed in the proportion of 50:50 (830 trees ha1 of each species) in manner between lines, and the stocking density of 5-year-old mixed poplar stands was approximately 1660 trees ha1. With stand development, due to the excessive shading by poplar or water and nutrient competition, about 55 percent of seabuckthorn did not survive in 15-year-old mixed poplar stands. Therefore, the stocking density of poplar and seabuckthorn in the 15-year-old mixed stands were about 830 and 370 trees ha1, respectively. Each of the plantations was established during April through planting nurseryraised 2-year-old seedlings in previous dug pits. The size of pits was 40 cm 40 cm 40 cm for poplar and 30 cm 30 cm 30 cm for seabuckthorn, respectively. Pruning of about 25% of the poplar canopy was conducted 5 years after planting in late October, and the pruning materials were removed from the stands. No fertilizers were applied during the forest establishment and management. Dominant understory species in the poplar plantations included Artemisia scoparia, Beckmannia syzigachne, Cleistogenes squarrosa and Setaria viridis. 2.2. Sample collection and analyses Three plots of 15 m 15 m were established for each treatment in September 2008. In each plot, diameter at breast height (1.3 m)
and tree height were measured for poplar, and basal diameter and height for seabuckthorn. Biomass estimates of poplar and seabuckthorn were calculated using published species-specific allometric equations [17,21]. The herbaceous biomass and forest floor mass were sampled in five randomly selected quadrats (50 cm 50 cm) in each plot, then oven-dried to a constant mass at approximately 65 C for 48 h, and weighed. Litter samples were ground for determination of organic C and total N concentrations. Detailed description of the plantations is shown in Table 1. Soil samples from 0e15 cm depth were collected in September 2008 (before the onset of foliar litterfall) and June 2009 (during growing season). For obtaining a representative sample, six soil cores (4 cm in diameter) were collected using “Z” pattern in each plot and thoroughly mixed to form a composite sample. After removing the plant roots, macrofauna and debris, the soil was sieved (<2 mm) and divided into two subsamples. One subsample was stored at 4 C until analysis for soil MBC, basal respiration rate and Nmin. The second subsample was air-dried at room temperature (20 C) for chemical analysis. Soil pH, TOC and TN were analyzed only for air-dried soil samples taken on September 2008, as we did not expect the changes in these chemical properties to be detectable in such a short period. Measurements of microbial properties commenced within four days of soil collection. Soil pH was measured in a 1:2.5 soil:water suspension. Air-dried samples were ground and passed through a 0.25 mm sieve for measurement of soil TOC and TN concentrations. Soil TOC was determined using the dichromate wet oxidation method of Walkley and Black [23]. Soil TN was analyzed by the Kjeldahl method [5] using a continuous-flow autoanalyzer (AutoAnalyzer III, Bran þ Luebbe GmbH, Germany). Soil MBC was determined according to the chloroform fumigation-extraction method of Vance et al. [30] modified by Lin et al. [18]. Briefly, three portions of fresh soil (equivalent to 20 g ovendried soil) were weighed into 100 mL beakers and fumigated under dark at 25 C for 24 hours with ethanol-free chloroform. After fumigant removal, the soil was extracted with 50 mL 0.5 M K2SO4 for 30 min. Three portions of non-fumigated soil were extracted simultaneously when fumigation commenced. The organic C in the extracts was measured by the dichromate oxidation method. Soil MBC was calculated by: MBC ¼ EC/0.38, where EC is the difference in organic C concentration between the fumigated and unfumigated soils, and 0.38 is the extractable part of microbial biomass C [30]. Soil qCO2 was calculated by dividing the mean values of basal respiration rate by the corresponding MBC, and expressed as mg CO2eC g1 MBC h1. Basal respiration rate was measured by the method of Salamanca et al. [28]. Field-moist soil equivalent to 20 g oven-dried soil was preincubated in 500 mL flask at 25 C and 60% water-holding capacity (WHC) for three days. After the
Table 1 Stand characteristics of pure and mixed poplar plantations with different ages. Stand age (year)
Stand type
5
Pure plantation Mixed plantation Pure plantation Mixed plantation
15 Two-way ANOVA results Stand age Stand type Stand age Stand type
Aboveground biomass (Mg ha1) 8.96(0.20) 9.17(1.09) 120.51(2.30) 170.33(11.96)*
<0.001 0.003 0.004
Forest floor C pool (Mg ha1) 0.30(0.02) 0.26(0.02) 2.11(0.07) 3.72(0.16)**
<0.001 <0.001 <0.001
Forest floor N pool (kg ha1) 8.07(0.55) 6.65(0.36) 54.62(2.48) 115.03(2.90)***
<0.001 <0.001 <0.001
Forest floor C/N ratio 37.2(0.8) 39.2(0.5) 38.6(0.7)** 32.4(0.6)
0.003 0.012 <0.001
Data represent means and values in parentheses are standard errors (n ¼ 3). Significant differences between pure and mixed poplar plantations at same stand age are indicated by *P < 0.05, **P < 0.01, ***P < 0.001.
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
preincubation, the flask was opened for two hours to supply samples with adequate oxygen. A plastic vial containing 10 mL 0.2 M NaOH solution was placed in each flask to trap the evolved CO2. Subsequently, the flask was closed again and incubated for another three days under the same condition. The evolved CO2 trapped in NaOH was determined by back titration with 0.1 M HCl after precipitating the carbonate with excessive 1 M BaCl2 solution. Soil Nmin was measured by the method described in Shi et al. [29]. Fresh soil equivalent to 20 g oven-dried soil was extracted with 50 mL 2 M KCl for one hour to determine the initial inorganic þ N concentration (NHþ 4 eN þ NO3 eN). Soil NH4 eN and NO3 eN concentrations were analyzed by the colorimetric method using a continuous-flow autoanalyzer. Another sample equivalent to 20 g oven-dried soil was placed in 100 mL beaker and adjusted to 60% WHC with distilled water and aerobically incubated at 25 C for 28 days. The total weight of the beaker containing the incubation soil was recorded. Soil moisture was checked by weighing and adjusting to 60% WHC with distilled water at an interval of threeefive days during the incubation period. At the end of incubation, the soil was analyzed for inorganic N concentration as described above. Soil Nmin was calculated by the difference between the initial and final inorganic N concentrations and expressed as mg N g1 soil d1.
327
floor C (P < 0.01, Table 1) and N (P < 0.001, Table 1) pools in 15-yearold mixed plantations were significantly greater than the corresponding pure plantations, but the opposite was true for forest floor C/N ratio (P < 0.01, Table 1). 3.2. Soil chemical properties For each stand type, soil TOC and TN concentrations showed a significant increase with stand age (P < 0.001, Table 2). Moreover, stand age and stand type had a significant interaction on soil TOC (P ¼ 0.045, Table 2) and TN (P < 0.001, Table 2) concentrations, but had no significant interaction on soil pH (P ¼ 0.468, Table 2) and C/N ratio (P ¼ 0.065, Table 2). There were no significant differences in soil pH, TOC and TN concentrations, and C/N ratio between 5-year-old pure and mixed poplar stands (P > 0.05, Table 2). Soil TOC (P < 0.05) and TN (P < 0.01) concentrations in 15-year-old mixed poplar stands were significantly higher than those in 15-year-old pure poplar stands (Table 2). In contrast, soil pH (P < 0.05) and C/N ratio (P < 0.05) of 15-year-old mixed poplar stands were significantly lower than those of the corresponding pure poplar stands (Table 2). 3.3. Soil microbial biomass C, metabolic quotient and net N mineralization rate
2.3. Statistical analyses Data were statistically analyzed using SPSS (v. 13.0), and the accepted significance level was a ¼ 0.05. Two-way analysis of variance (ANOVA) was used to analyze the effects of stand age and stand type on stand characteristics and soil chemical properties. Repeated measures ANOVA was used to examine the effects of stand age and stand type on soil MBC, qCO2 and Nmin over the sampling dates. One-way ANOVA was used to determine significant difference in soil properties between stand types for each stand age within the same sampling date. Data were tested for normality using the KolmogoroveSmirnov test, and all data were conformed to a normal distribution (data not shown). 3. Results 3.1. Aboveground biomass, forest floor C and N pools In both pure and mixed poplar stands, aboveground biomass, forest floor C and N pools significantly increased with stand age (P < 0.001, Table 1). In addition, stand age and stand type had a significant interaction on aboveground biomass (P ¼ 0.004, Table 1), forest floor C (P < 0.001, Table 1) and N (P < 0.001, Table 1) pools, and forest floor C/N ratio (P < 0.001, Table 1). No significant differences in aboveground biomass, forest floor C and N pools, and forest floor C/N ratio existed between 5-year-old pure and mixed poplar stands (P > 0.05, Table 1). Aboveground biomass (P < 0.05, Table 1), forest
Soil MBC and Nmin significantly increased with stand development in both pure and mixed poplar plantations (P < 0.001, Table 3), and the opposite was true for soil qCO2 (P < 0.001, Table 3). However, stand age, stand type and sampling date had no significant interactive effects on soil MBC (P ¼ 0.093, Table 3), qCO2 (P ¼ 0.588, Table 3) and Nmin (P ¼ 0.483, Table 3). No significant differences in soil MBC and qCO2 existed between 5-year-old pure and mixed poplar stands in two sampling dates (P > 0.05, Fig. 1a, b). Soil MBC of 15-year-old mixed poplar stands was significantly greater than that of the corresponding pure poplar stands in two sampling dates (P < 0.05, Fig. 1a), whereas soil qCO2 of 15-year-old mixed poplar stands was significantly lower than that of the corresponding pure poplar stands (P < 0.05, Fig. 1b). In September 2008, there was no significant difference in Nmin between pure and mixed poplar stands at each stand age (P > 0.05, Fig. 1c). However, Nmin of 5- and 15-year-old mixed poplar stands was significantly higher than that of the corresponding pure poplar stands in June 2009, respectively (P < 0.05, Fig. 1c). 4. Discussion Many studies suggested that the presence of N-fixing species in forest plantations could increase soil TOC and TN storage [19,27,33] and enhance stand productivity [13,20]. In our study, aboveground biomass, soil TOC and TN concentrations of 15-year-old mixed
Table 2 Soil chemical properties under pure and mixed poplar plantations with different ages. Stand age (year)
Stand type
pH
TOC (g kg1)
TN (g kg1)
C/N ratio
5
Pure plantation Mixed plantation Pure plantation Mixed plantation
8.49(0.03) 8.45(0.06) 8.35(0.01)* 8.26(0.03)
2.37(0.55) 1.97(0.03) 4.83(0.19) 5.94(0.24)*
0.42(0.04) 0.29(0.06) 0.53(0.04) 0.96(0.06)**
5.6(1.3) 7.4(1.7) 9.2(0.4)* 6.2(0.5)
0.002 0.111 0.468
<0.001 0.289 0.045
<0.001 0.018 <0.001
0.313 0.616 0.065
15 Two-way ANOVA results Stand age Stand type Stand age Stand type
Data represent means and values in parentheses are standard errors (n ¼ 3). Significant differences between pure and mixed poplar plantations at same stand age are indicated by *P < 0.05, **P < 0.01. TOC total organic C concentration, TN total N concentration.
328
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
Table 3 Results (P-values) of repeated measures ANOVAs on the effects of stand age (A), stand type (T), sampling date (D) and their interactions on MBC, qCO2 and Nmin. Treatments
MBC
qCO2
Nmin
A T D AT AD TD ATD
<0.001 0.009 <0.001 <0.001 <0.001 0.106 0.093
<0.001 0.003 0.734 0.055 0.092 0.291 0.588
<0.001 0.005 0.008 0.024 0.096 0.008 0.483
MBC microbial biomass carbon, qCO2 metabolic quotient, Nmin net N mineralization rate.
poplar stands were higher than those of the corresponding pure poplar stands. However, there were no significant differences in aboveground biomass, soil TOC and TN concentrations between 5year-old pure and mixed poplar plantations. This could be explained by the stand age and hence the quantity and quality of organic matter input to soil. Rothe et al. [27] found that, compared with pure Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands, a significant C accretion in mixed red alder (Alnus rubra Bong.) and Douglas-fir stands would require about 30 years. He et al. [13] showed that N fixed by seabuckthorn increased with stand age, and that rate of N-fixation ranged from 3.7 kg ha1 year1 in 1year-old mixed poplar stands to 84.8 kg ha1 year1 in 7-year-old mixed poplar stands in the same region as our study. Additionally, the seabuckthorn occupied 18% of total aboveground biomass in 5year-old mixed poplar plantations (data not shown) and may compete with poplar for soil water and nutrients. Therefore, in the 5-year-old mixed poplar stands, low rate of N-fixation and interspecific competition limited plant net primary productivity and did not increase organic matter input to soil via plant residues (Table 1). Moreover, compared with pure poplar stands, high intensity of soil disturbance during site preparation in mixed poplar stands may lead to a higher rate of organic C and N losses [24]. In our study, about 55 percent of seabuckthorn shrubs did not survive in
MBC -1 (µ g C g )
240
a
15-year-old mixed poplar stands, which means that high mixing proportion of seabuckthorn shrubs would be unnecessary at the initial stage of stand establishment. If true, soil disturbance during site preparation and interspecific competition between poplar and seabuckthorn would decrease. In 15-year-old mixed poplar stands, the increased rate of N-fixation, enhanced soil N availability and reduced interspecific competition promoted net primary productivity and increased both quantity and quality of plant litter input (Table 1). In addition, leaf litter with high N concentration contributed to increased buildup of humus and produced Nenriched soil organic matter [3,27]. Therefore, 15-year-old mixed stands had significantly greater soil TOC and TN concentrations than did the corresponding pure stands. Soil microbial biomass acts both as sources and sinks of plant available nutrients, and also can regulate soil nutrient transformation [1,11]. The pool size of soil microbial biomass depends on the quantity of organic matter input [1,14]. Compared with 5-yearold pure poplar plantations, the corresponding mixed poplar plantations did not increase stand productivity and enhance organic matter input to soil (Table 1). Therefore, there was no significant difference in soil MBC between 5-year-old pure and mixed poplar plantations in two sampling dates. However, a larger pool size of MBC in 15-year-old mixed stands was consistent with the study of Hart et al. [12]. This was due to the increased plant productivity and organic matter input (Table 1). Soil qCO2 reflects the quantity and quality of soil organic matter, soil nutrient availability, microbial substrate utilization efficiency and ecosystem stability [1,2]. Compared with 15-year-old pure plantations, a decline in soil qCO2 in the corresponding mixed plantations suggested that microbes were more efficient in utilizing available C. This could be attributed to the enhanced quality of soil organic matter input (low C/N ratio) [29]. However, the presence of seabuckthorn in 5-year-old mixed poplar stands could not improve soil organic matter quality and thus enhance microbial utilization efficiency. Previous studies found that mixing N-fixing species into forest plantations could increase soil N mineralization and improve soil N status [4,12,27]. In our study, soil Nmin of 5- and 15-year-old mixed
Pure Mixed
5-year-old
15-year-old **
180
*
120 60
-1
qCO2
-1
(mg CO2-C g MBC h )
0 4 3
(µ g N g d )
***
1 0
c *
0.27
-1
-1
**
2
0.36 Nmin
b
0.18
*
0.09 0.00
September
June
September
June
Sampling season Fig. 1. Soil microbial biomass carbon (MBC) (a), metabolic quotient (qCO2) (b) and net N mineralization rate (Nmin) (c) under pure and mixed poplar plantations. Values are means and bars represent standard errors (n ¼ 3). Bars followed by asterisks indicate significant differences between pure and mixed poplar plantations at same stand age for a given sampling date, *P < 0.05, **P < 0.01, ***P < 0.001.
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
poplar stands were greater than those of corresponding pure poplar stands only in June 2009. The difference in soil Nmin on specific sampling date may be due to difference in availability of labile N substrates [22]. In June 2009, a relatively high Nmin in mixed stands could be attributed to enhanced quantity and quality of forest litter and soil organic matter [7,27]. High-quality litter (low C/N ratio) may accelerate decomposition and decrease microbial immobilization of N, and result in enhanced Nmin and plant available N [22,32]. However, forest floor mass was at a minimum for deciduous species before the onset of foliar litterfall [31]. Therefore, there was no significant difference in Nmin between pure and mixed poplar stands at each stand age in September 2008. 5. Conclusions Our observations revealed that there were increases in aboveground biomass, soil TOC and TN concentrations and MBC, and a decrease in qCO2 in 15-year-old mixed poplar stands, compared to corresponding pure poplar stands. However, no significant differences in stand productivity, soil TOC and TN concentrations, MBC and qCO2 existed between 5-year-old pure and mixed poplar plantations. Our results suggest that, in semi-arid regions, mixing N-fixing shrub species into poplar plantations can improve soil fertility in a long run rather than in a short term; therefore, mixing N-fixing shrub species into poplar stand is an option to increase productivity while improving soil fertility in a long run. Meanwhile, our findings highlight that appropriate mixing proportions of Nfixing species and stock density in mixed poplar plantations should be considered to minimize interspecific competition, and to increase long-term soil fertility and stand productivity. Acknowledgments This work was funded by the National Key Technologies R&D Program of China (No. 2006BAD03A0502) and the National Natural Science Foundation of China (No. 30872011). We thank two anonymous reviewers, Christoph C. Tebbe and Xinhua Zhou for their helpful remarks on an earlier version of this manuscript, He-Ming Lin and Jing-Shi Li for laboratory analyses, and other colleagues who participated in the field work. References [1] A.S.F. Araujo, V.B. Santos, R.T.F. Monteiro, Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piaui state, Brazil. Eur. J. Soil. Biol. 98 (2008) 225e230. [2] F. Bastida, A. Zsolnay, T. Hernández, C. García, Past, present and future of soil quality indices: a biological perspective. Geoderma 147 (2008) 159e171. [3] B. Berg, C.A. McClaugherty, A.V. De Santo, D.W. Johnson, Humus buildup in boreal forests: effects of litterfall and its N concentration. Can. J. Forest Res. 31 (2001) 988e998. [4] D. Binkley, P. Sollins, R. Bell, D. Sachs, D.D. Myrold, Biogeochemistry of adjacent conifer and alder-conifer stands. Ecology 73 (1992) 2022e2033. [5] J.M. Bremner, Nitrogen-total. in: D.L. Sparks, A.L. Page, P.A. Helmke, R.H. Loeppert, P.N. Soltanpour, M.A. Tabatabai, C.T. Johnston, M.E. Sumner (Eds.), Methods of Soil Analysis. Part 3, Chemical Methods, Soil Science Society of America Book Series, Number 5, 1996, pp. 1085e1122 Wisconsin, USA. [6] M.D. Busse, M.F. Jurgensen, D.S. Page-Dumroese, R.F. Powers, Contribution of actinorhizal shrubs to site fertility in a Northern California mixed pine forest. Forest Ecol. Manag. 244 (2007) 68e75.
329
[7] R. Dinesh, M.A. Suryanarayana, S.G. Chaudhuri, T.E. Sheeja, K.N. Shiva, Longterm effects of leguminous crops on biochemical and biological properties in the organic and mineral layers of soils of a coconut plantation. Eur. J. Soil Biol. 42 (2006) 147e157. [8] T.M. Fenning, J. Gershenzon, Where will the wood come from? Plantation forests and the role of biotechnology. Trends Biotechnol. 20 (2002) 291e296. [9] D.I. Forrester, J. Bauhus, A.L. Cowie, J.K. Vanclay, Mixed-species plantations of Eucalyptus with nitrogen-fixing trees: a review. Forest Ecol. Manag. 233 (2006) 211e230. [10] T.R. Fox, Sustained productivity in intensively managed forest plantations. Forest Ecol. Manag. 138 (2000) 187e202. [11] G. Jia, B. Liu, G. Wang, B. Zhang, The microbial biomass and activity in soil with shrub (Caragana korshinskii K.) plantation in the semi-arid loess plateau in China. Eur. J. Soil Biol. 46 (2010) 6e10. [12] S.C. Hart, D. Binkley, D.A. Perry, Influence of red alder on soil nitrogen transformations in two conifer forests of contrasting productivity. Soil Biol. Biochem. 29 (1997) 1111e1123. [13] X. He, C. Zhang, S. Yang, Y. Zhang, D. Su, H. Liu, Role of nitrogen fixing trees in mixed forest. I. Nitrogen fixation and tree growth of mixed stand with Hippophae rhamnoides. Chin. J. Appl. Ecol. 7 (4) (1997) 354e358. [14] H. Insam, Are the soil microbial biomass and basal respiration governed by the climate regime? Soil Biol. Biochem. 22 (1990) 525e532. [15] M.J. Kelty, The role of species mixtures in plantation forestry. Forest Ecol. Manag. 233 (2006) 195e204. [16] J.P. Kimmins, Evaluation of consequences for future tree productivity of loss of nutrients in whole tree harvesting. Forest Ecol. Manag. 1 (1977) 169e183. [17] W. Liang, H. Hu, F. Liu, D. Zhang, Research advance of biomass and carbon storage of poplar in China. J. For. Res. 17 (1) (2006) 75e79. [18] Q.M. Lin, Y.G. Wu, H.L. Liu, Modification of fumigation extraction method for measuring soil microbial biomass carbon. Chin. J. Ecol. 18 (2) (1999) 63e66. [19] S. Liu, Effects of seabuckthorn (Hippophae rhamnoides L.) on nutrient distribution and biological cycling of poplar plantations in dry subhumid area of China. Acta Ecol. Sin. 19 (4) (1999) 534e542. [20] S. Liu, Effects of seabuckthorn on tree growth and biomass production of poplar plantations in a subhumid-arid area of China. Acta Phytoecol. Sin. 24 (2) (2000) 169e174. [21] Z. Liu, G. Gao, Y. Lu, X. Li, Comparison of biomass and its estimation of Hippophae rhamoides L. under different climatic and soil conditions. J. Nanjing For. Univ.: Nat. Sci. 31 (1) (2007) 37e41. [22] D.C. McKinley, C.W. Rice, J.M. Blair, Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes. Soil Biol. Biochem. 40 (2008) 2627e2633. [23] D.W. Nelson, L.E. Sommers, Total carbon, organic carbon, and organic matter. in: D.L. Sparks, A.L. Page, P.A. Helmke, R.H. Loeppert, P.N. Soltanpour, M.A. Tabatabai, C.T. Johnston, M.E. Sumner (Eds.), Methods of Soil Analysis. Part 3, Chemical Methods, Soil Science Society of America Book Series, Number 5, 1996, pp. 961e1010 Wisconsin, USA. [24] K.I. Paul, P.J. Polglase, J.G. Nyakuengama, P.K. Khanna, Change in soil carbon following afforestation. Forest Ecol. Manag. 168 (2002) 241e257. [25] D.A. Perry, The scientific basis of forestry. Annu. Rev. Ecol. Syst. 29 (1998) 435e466. [26] A. Rothe, D. Binkley, Nutritional interactions in mixed species forests: a synthesis. Can. J. Forest Res. 31 (2001) 1855e1870. [27] A. Rothe, J.K. Cromack, S.C. Resh, E. Makeneci, Y. Son, Soil carbon and nitrogen changes under Douglas-fir with and without red alder. Soil Sci. Soc. Am. J. 66 (2002) 1988e1995. [28] E.F. Salamanca, M. Raubuch, R.G. Joergensen, Relationships between soil microbial indices in secondary tropical forest soils. Appl. Soil Ecol. 21 (2002) 211e219. [29] W. Shi, H. Yao, D. Bowman, Soil microbial biomass, activity and nitrogen transformations in a turfgrass chorosequence. Soil Biol. Biochem. 38 (2006) 311e319. [30] E.D. Vance, P.C. Brookes, D.S. Jenkinson, An extraction method for measuring soil microbial biomass C. Soil Biol. Biochem. 19 (1987) 703e707. [31] L. Vesterdal, I.K. Schmidt, I. Callesen, L.O. Nilsson, P. Gundersen, Carbon and nitrogen in forest floor and mineral soil under six common European tree species. Forest Ecol. Manag. 255 (2008) 35e48. [32] P.M. Vitousek, Nutrient cycling and nutrient use efficiency. Am. Nat. 119 (1982) 553e572. [33] J.G. Vogel, S.T. Gower, Carbon and nitrogen dynamics of boreal jack pine stands with and without a green alder understory. Ecosystems 1 (1998) 386e400.
Contents lists available at ScienceDirect
European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Soil microbiological and chemical effects of a nitrogen-fixing shrub in poplar plantations in semi-arid region of Northeast China Rong Mao a, b, De-Hui Zeng a, *, Gui-Yan Ai a, Dan Yang a, b, Lu-Jun Li a, b, Yun-Xia Liu a, b a b
Institute of Applied Ecology, Chinese Academy of Sciences, 72 Wenhua Rd, Shenyang 110016, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2009 Received in revised form 25 May 2010 Accepted 28 May 2010 Available online 9 June 2010 Handling editor: Christoph Tebbe
Nitrogen (N)-fixing species have a function to enrich N in soil. Mixing N-fixing shrub species into poplar stands can be assumed as a measure to increase productivity while improving soil fertility. To verify this assumption and to understand the temporal influences of N-fixing shrub species mixed into poplar plantations on soil fertility, we investigated selected soil chemical and microbial properties in pure poplar (Populus xiaozhuanica W. Y. Hsu et Liang) and mixed poplareseabuckthorn (Hippophae rhamnoides L.) stands at ages of five and 15 years in a semi-arid region of Northeast China. Both stands at age of five have similar values of aboveground biomass, total soil organic C concentration, total N concentration, microbial biomass C, and metabolic quotient; however, at age of 15, these values except for soil metabolic quotient are significantly greater in mixed poplareseabuckthorn stand than in pure poplar stand. The soil metabolic quotient is lower in the former stand than in the latter stand. Our results suggest that, in semi-arid regions, mixing N-fixing shrub species into poplar plantations can improve soil fertility in a long run rather than in a short term; therefore, mixing N-fixing shrub species into poplar stands is an option to improve soil fertility and increase productivity in a long run. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Mixed forest N mineralization Pure forest Soil metabolic quotient Soil microbial biomass
1. Introduction Fast-growing forest plantations have been recognized to be an effective measure to solve the shortage of wood in many parts of the world [8,10]. However, frequent harvest-related nutrient losses can lead to declines in soil fertility and biomass productivity over successive rotations [16,25]. Therefore, nitrogen (N)-fixing plants have been recommended for mixed plantations with valuable timber trees to offset potential soil degradation and increase stand productivity [9,15,25,26]. Compared with pure plantation, the presence of nitrogen-fixing plants can promote nutrient cycling, increase soil N pool, improve soil N availability and enhance overall plantation yields [4,6,33]. Poplar (Populus spp.), a fast-growing species, has been planted on a large scale in most parts of China, especially in arid and semiarid areas [17]. The presence of N-fixing plant species in poplar plantations has been shown to maintain soil fertility and stand productivity [13,19,20]. Previous studies have suggested that introduction of N-fixing species into poplar plantations could
* Corresponding author. Tel.: þ86 24 83970220; fax: þ86 24 83970394. E-mail address: [email protected] (D.-H. Zeng). 1164-5563/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2010.05.005
enhance both litter quality and quantity [13,19] and increase aboveand below-ground productivity [20]. However, the effect of Nfixing species on soil microbial properties is not well understood in mixed poplar plantations. Considering the critical roles of soil microbial biomass and activity in regulating nutrient cycling and maintaining soil fertility [1,11], a better knowledge of soil microbial properties may help to evaluate the effect of N-fixing species on soil fertility and thus to improve stand productivity in poplar plantations. Seabuckthorn (Hippophae rhamnoides L.) is an actinorhizal shrub species characterized by its ability to form a symbiosis with the nitrogen-fixing actinobacteria Frankia. This association leads to the formation of nitrogen-fixing root nodules and thus improves soil nutrient status [19]. Therefore, seabuckthorn has been widely introduced into poplar stands in arid and semi-arid regions of China [19,20]. In the present study, we investigated pH, total organic carbon (TOC) and total nitrogen (TN) concentrations, microbial biomass carbon (MBC), metabolic quotient (qCO2), and net N mineralization rate (Nmin) in soils from 5- and 15-year-old pure poplar (Populus xiaozhuanica W. Y. Hsu et Liang) plantations and mixed poplar-seabuckthorn plantations in a semi-arid region of Northeast China. Our objective was to examine the impact of N-fixing shrub species on soil chemical and microbial properties in poplar stands in relation to stand age.
326
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
2. Materials and methods 2.1. Study site and experimental design The study was conducted in Jianping County (40170 e42 200 N, 119100 e120 020 E), west of Liaoning Province, Northeast China. The study site belongs to the semi-arid monsoon climate of temperate zone. The mean annual temperature is 6.5 C, precipitation is 467 mm (more than 60% falling between June and August), and frost-free period is 148 days. The soil is a sandy soil and classified as skeletol primitive soil in the Chinese Soil Classification, which corresponds to Fluvisols in the FAO Soil Classification. To quantify changes in soil chemical and microbial properties following introduction of seabuckthorn into poplar plantations, 5and 15-year-old mixed poplar plantations and adjacent pure poplar plantations were chosen for investigation. For each stand age, three pairs of pure and mixed poplar sites were identified in September 2008. Each pair of pure and mixed poplar stands was adjacent and 30 m apart, and all replicated paired plots were within 500 m. All sites were located on relatively flat terrain and similar in soil type, slope and land management history. The soils of all the treatments were developed on the same parent material that was fluvial deposits. Based on the oral history, the lands had been cultivated for more than 100 years before afforestation, and poplar stands were in a first-rotation cycle. The stocking density of pure poplar stands was approximately 830 trees ha1. Poplar and seabuckthorn were mixed in the proportion of 50:50 (830 trees ha1 of each species) in manner between lines, and the stocking density of 5-year-old mixed poplar stands was approximately 1660 trees ha1. With stand development, due to the excessive shading by poplar or water and nutrient competition, about 55 percent of seabuckthorn did not survive in 15-year-old mixed poplar stands. Therefore, the stocking density of poplar and seabuckthorn in the 15-year-old mixed stands were about 830 and 370 trees ha1, respectively. Each of the plantations was established during April through planting nurseryraised 2-year-old seedlings in previous dug pits. The size of pits was 40 cm 40 cm 40 cm for poplar and 30 cm 30 cm 30 cm for seabuckthorn, respectively. Pruning of about 25% of the poplar canopy was conducted 5 years after planting in late October, and the pruning materials were removed from the stands. No fertilizers were applied during the forest establishment and management. Dominant understory species in the poplar plantations included Artemisia scoparia, Beckmannia syzigachne, Cleistogenes squarrosa and Setaria viridis. 2.2. Sample collection and analyses Three plots of 15 m 15 m were established for each treatment in September 2008. In each plot, diameter at breast height (1.3 m)
and tree height were measured for poplar, and basal diameter and height for seabuckthorn. Biomass estimates of poplar and seabuckthorn were calculated using published species-specific allometric equations [17,21]. The herbaceous biomass and forest floor mass were sampled in five randomly selected quadrats (50 cm 50 cm) in each plot, then oven-dried to a constant mass at approximately 65 C for 48 h, and weighed. Litter samples were ground for determination of organic C and total N concentrations. Detailed description of the plantations is shown in Table 1. Soil samples from 0e15 cm depth were collected in September 2008 (before the onset of foliar litterfall) and June 2009 (during growing season). For obtaining a representative sample, six soil cores (4 cm in diameter) were collected using “Z” pattern in each plot and thoroughly mixed to form a composite sample. After removing the plant roots, macrofauna and debris, the soil was sieved (<2 mm) and divided into two subsamples. One subsample was stored at 4 C until analysis for soil MBC, basal respiration rate and Nmin. The second subsample was air-dried at room temperature (20 C) for chemical analysis. Soil pH, TOC and TN were analyzed only for air-dried soil samples taken on September 2008, as we did not expect the changes in these chemical properties to be detectable in such a short period. Measurements of microbial properties commenced within four days of soil collection. Soil pH was measured in a 1:2.5 soil:water suspension. Air-dried samples were ground and passed through a 0.25 mm sieve for measurement of soil TOC and TN concentrations. Soil TOC was determined using the dichromate wet oxidation method of Walkley and Black [23]. Soil TN was analyzed by the Kjeldahl method [5] using a continuous-flow autoanalyzer (AutoAnalyzer III, Bran þ Luebbe GmbH, Germany). Soil MBC was determined according to the chloroform fumigation-extraction method of Vance et al. [30] modified by Lin et al. [18]. Briefly, three portions of fresh soil (equivalent to 20 g ovendried soil) were weighed into 100 mL beakers and fumigated under dark at 25 C for 24 hours with ethanol-free chloroform. After fumigant removal, the soil was extracted with 50 mL 0.5 M K2SO4 for 30 min. Three portions of non-fumigated soil were extracted simultaneously when fumigation commenced. The organic C in the extracts was measured by the dichromate oxidation method. Soil MBC was calculated by: MBC ¼ EC/0.38, where EC is the difference in organic C concentration between the fumigated and unfumigated soils, and 0.38 is the extractable part of microbial biomass C [30]. Soil qCO2 was calculated by dividing the mean values of basal respiration rate by the corresponding MBC, and expressed as mg CO2eC g1 MBC h1. Basal respiration rate was measured by the method of Salamanca et al. [28]. Field-moist soil equivalent to 20 g oven-dried soil was preincubated in 500 mL flask at 25 C and 60% water-holding capacity (WHC) for three days. After the
Table 1 Stand characteristics of pure and mixed poplar plantations with different ages. Stand age (year)
Stand type
5
Pure plantation Mixed plantation Pure plantation Mixed plantation
15 Two-way ANOVA results Stand age Stand type Stand age Stand type
Aboveground biomass (Mg ha1) 8.96(0.20) 9.17(1.09) 120.51(2.30) 170.33(11.96)*
<0.001 0.003 0.004
Forest floor C pool (Mg ha1) 0.30(0.02) 0.26(0.02) 2.11(0.07) 3.72(0.16)**
<0.001 <0.001 <0.001
Forest floor N pool (kg ha1) 8.07(0.55) 6.65(0.36) 54.62(2.48) 115.03(2.90)***
<0.001 <0.001 <0.001
Forest floor C/N ratio 37.2(0.8) 39.2(0.5) 38.6(0.7)** 32.4(0.6)
0.003 0.012 <0.001
Data represent means and values in parentheses are standard errors (n ¼ 3). Significant differences between pure and mixed poplar plantations at same stand age are indicated by *P < 0.05, **P < 0.01, ***P < 0.001.
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
preincubation, the flask was opened for two hours to supply samples with adequate oxygen. A plastic vial containing 10 mL 0.2 M NaOH solution was placed in each flask to trap the evolved CO2. Subsequently, the flask was closed again and incubated for another three days under the same condition. The evolved CO2 trapped in NaOH was determined by back titration with 0.1 M HCl after precipitating the carbonate with excessive 1 M BaCl2 solution. Soil Nmin was measured by the method described in Shi et al. [29]. Fresh soil equivalent to 20 g oven-dried soil was extracted with 50 mL 2 M KCl for one hour to determine the initial inorganic þ N concentration (NHþ 4 eN þ NO3 eN). Soil NH4 eN and NO3 eN concentrations were analyzed by the colorimetric method using a continuous-flow autoanalyzer. Another sample equivalent to 20 g oven-dried soil was placed in 100 mL beaker and adjusted to 60% WHC with distilled water and aerobically incubated at 25 C for 28 days. The total weight of the beaker containing the incubation soil was recorded. Soil moisture was checked by weighing and adjusting to 60% WHC with distilled water at an interval of threeefive days during the incubation period. At the end of incubation, the soil was analyzed for inorganic N concentration as described above. Soil Nmin was calculated by the difference between the initial and final inorganic N concentrations and expressed as mg N g1 soil d1.
327
floor C (P < 0.01, Table 1) and N (P < 0.001, Table 1) pools in 15-yearold mixed plantations were significantly greater than the corresponding pure plantations, but the opposite was true for forest floor C/N ratio (P < 0.01, Table 1). 3.2. Soil chemical properties For each stand type, soil TOC and TN concentrations showed a significant increase with stand age (P < 0.001, Table 2). Moreover, stand age and stand type had a significant interaction on soil TOC (P ¼ 0.045, Table 2) and TN (P < 0.001, Table 2) concentrations, but had no significant interaction on soil pH (P ¼ 0.468, Table 2) and C/N ratio (P ¼ 0.065, Table 2). There were no significant differences in soil pH, TOC and TN concentrations, and C/N ratio between 5-year-old pure and mixed poplar stands (P > 0.05, Table 2). Soil TOC (P < 0.05) and TN (P < 0.01) concentrations in 15-year-old mixed poplar stands were significantly higher than those in 15-year-old pure poplar stands (Table 2). In contrast, soil pH (P < 0.05) and C/N ratio (P < 0.05) of 15-year-old mixed poplar stands were significantly lower than those of the corresponding pure poplar stands (Table 2). 3.3. Soil microbial biomass C, metabolic quotient and net N mineralization rate
2.3. Statistical analyses Data were statistically analyzed using SPSS (v. 13.0), and the accepted significance level was a ¼ 0.05. Two-way analysis of variance (ANOVA) was used to analyze the effects of stand age and stand type on stand characteristics and soil chemical properties. Repeated measures ANOVA was used to examine the effects of stand age and stand type on soil MBC, qCO2 and Nmin over the sampling dates. One-way ANOVA was used to determine significant difference in soil properties between stand types for each stand age within the same sampling date. Data were tested for normality using the KolmogoroveSmirnov test, and all data were conformed to a normal distribution (data not shown). 3. Results 3.1. Aboveground biomass, forest floor C and N pools In both pure and mixed poplar stands, aboveground biomass, forest floor C and N pools significantly increased with stand age (P < 0.001, Table 1). In addition, stand age and stand type had a significant interaction on aboveground biomass (P ¼ 0.004, Table 1), forest floor C (P < 0.001, Table 1) and N (P < 0.001, Table 1) pools, and forest floor C/N ratio (P < 0.001, Table 1). No significant differences in aboveground biomass, forest floor C and N pools, and forest floor C/N ratio existed between 5-year-old pure and mixed poplar stands (P > 0.05, Table 1). Aboveground biomass (P < 0.05, Table 1), forest
Soil MBC and Nmin significantly increased with stand development in both pure and mixed poplar plantations (P < 0.001, Table 3), and the opposite was true for soil qCO2 (P < 0.001, Table 3). However, stand age, stand type and sampling date had no significant interactive effects on soil MBC (P ¼ 0.093, Table 3), qCO2 (P ¼ 0.588, Table 3) and Nmin (P ¼ 0.483, Table 3). No significant differences in soil MBC and qCO2 existed between 5-year-old pure and mixed poplar stands in two sampling dates (P > 0.05, Fig. 1a, b). Soil MBC of 15-year-old mixed poplar stands was significantly greater than that of the corresponding pure poplar stands in two sampling dates (P < 0.05, Fig. 1a), whereas soil qCO2 of 15-year-old mixed poplar stands was significantly lower than that of the corresponding pure poplar stands (P < 0.05, Fig. 1b). In September 2008, there was no significant difference in Nmin between pure and mixed poplar stands at each stand age (P > 0.05, Fig. 1c). However, Nmin of 5- and 15-year-old mixed poplar stands was significantly higher than that of the corresponding pure poplar stands in June 2009, respectively (P < 0.05, Fig. 1c). 4. Discussion Many studies suggested that the presence of N-fixing species in forest plantations could increase soil TOC and TN storage [19,27,33] and enhance stand productivity [13,20]. In our study, aboveground biomass, soil TOC and TN concentrations of 15-year-old mixed
Table 2 Soil chemical properties under pure and mixed poplar plantations with different ages. Stand age (year)
Stand type
pH
TOC (g kg1)
TN (g kg1)
C/N ratio
5
Pure plantation Mixed plantation Pure plantation Mixed plantation
8.49(0.03) 8.45(0.06) 8.35(0.01)* 8.26(0.03)
2.37(0.55) 1.97(0.03) 4.83(0.19) 5.94(0.24)*
0.42(0.04) 0.29(0.06) 0.53(0.04) 0.96(0.06)**
5.6(1.3) 7.4(1.7) 9.2(0.4)* 6.2(0.5)
0.002 0.111 0.468
<0.001 0.289 0.045
<0.001 0.018 <0.001
0.313 0.616 0.065
15 Two-way ANOVA results Stand age Stand type Stand age Stand type
Data represent means and values in parentheses are standard errors (n ¼ 3). Significant differences between pure and mixed poplar plantations at same stand age are indicated by *P < 0.05, **P < 0.01. TOC total organic C concentration, TN total N concentration.
328
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
Table 3 Results (P-values) of repeated measures ANOVAs on the effects of stand age (A), stand type (T), sampling date (D) and their interactions on MBC, qCO2 and Nmin. Treatments
MBC
qCO2
Nmin
A T D AT AD TD ATD
<0.001 0.009 <0.001 <0.001 <0.001 0.106 0.093
<0.001 0.003 0.734 0.055 0.092 0.291 0.588
<0.001 0.005 0.008 0.024 0.096 0.008 0.483
MBC microbial biomass carbon, qCO2 metabolic quotient, Nmin net N mineralization rate.
poplar stands were higher than those of the corresponding pure poplar stands. However, there were no significant differences in aboveground biomass, soil TOC and TN concentrations between 5year-old pure and mixed poplar plantations. This could be explained by the stand age and hence the quantity and quality of organic matter input to soil. Rothe et al. [27] found that, compared with pure Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands, a significant C accretion in mixed red alder (Alnus rubra Bong.) and Douglas-fir stands would require about 30 years. He et al. [13] showed that N fixed by seabuckthorn increased with stand age, and that rate of N-fixation ranged from 3.7 kg ha1 year1 in 1year-old mixed poplar stands to 84.8 kg ha1 year1 in 7-year-old mixed poplar stands in the same region as our study. Additionally, the seabuckthorn occupied 18% of total aboveground biomass in 5year-old mixed poplar plantations (data not shown) and may compete with poplar for soil water and nutrients. Therefore, in the 5-year-old mixed poplar stands, low rate of N-fixation and interspecific competition limited plant net primary productivity and did not increase organic matter input to soil via plant residues (Table 1). Moreover, compared with pure poplar stands, high intensity of soil disturbance during site preparation in mixed poplar stands may lead to a higher rate of organic C and N losses [24]. In our study, about 55 percent of seabuckthorn shrubs did not survive in
MBC -1 (µ g C g )
240
a
15-year-old mixed poplar stands, which means that high mixing proportion of seabuckthorn shrubs would be unnecessary at the initial stage of stand establishment. If true, soil disturbance during site preparation and interspecific competition between poplar and seabuckthorn would decrease. In 15-year-old mixed poplar stands, the increased rate of N-fixation, enhanced soil N availability and reduced interspecific competition promoted net primary productivity and increased both quantity and quality of plant litter input (Table 1). In addition, leaf litter with high N concentration contributed to increased buildup of humus and produced Nenriched soil organic matter [3,27]. Therefore, 15-year-old mixed stands had significantly greater soil TOC and TN concentrations than did the corresponding pure stands. Soil microbial biomass acts both as sources and sinks of plant available nutrients, and also can regulate soil nutrient transformation [1,11]. The pool size of soil microbial biomass depends on the quantity of organic matter input [1,14]. Compared with 5-yearold pure poplar plantations, the corresponding mixed poplar plantations did not increase stand productivity and enhance organic matter input to soil (Table 1). Therefore, there was no significant difference in soil MBC between 5-year-old pure and mixed poplar plantations in two sampling dates. However, a larger pool size of MBC in 15-year-old mixed stands was consistent with the study of Hart et al. [12]. This was due to the increased plant productivity and organic matter input (Table 1). Soil qCO2 reflects the quantity and quality of soil organic matter, soil nutrient availability, microbial substrate utilization efficiency and ecosystem stability [1,2]. Compared with 15-year-old pure plantations, a decline in soil qCO2 in the corresponding mixed plantations suggested that microbes were more efficient in utilizing available C. This could be attributed to the enhanced quality of soil organic matter input (low C/N ratio) [29]. However, the presence of seabuckthorn in 5-year-old mixed poplar stands could not improve soil organic matter quality and thus enhance microbial utilization efficiency. Previous studies found that mixing N-fixing species into forest plantations could increase soil N mineralization and improve soil N status [4,12,27]. In our study, soil Nmin of 5- and 15-year-old mixed
Pure Mixed
5-year-old
15-year-old **
180
*
120 60
-1
qCO2
-1
(mg CO2-C g MBC h )
0 4 3
(µ g N g d )
***
1 0
c *
0.27
-1
-1
**
2
0.36 Nmin
b
0.18
*
0.09 0.00
September
June
September
June
Sampling season Fig. 1. Soil microbial biomass carbon (MBC) (a), metabolic quotient (qCO2) (b) and net N mineralization rate (Nmin) (c) under pure and mixed poplar plantations. Values are means and bars represent standard errors (n ¼ 3). Bars followed by asterisks indicate significant differences between pure and mixed poplar plantations at same stand age for a given sampling date, *P < 0.05, **P < 0.01, ***P < 0.001.
R. Mao et al. / European Journal of Soil Biology 46 (2010) 325e329
poplar stands were greater than those of corresponding pure poplar stands only in June 2009. The difference in soil Nmin on specific sampling date may be due to difference in availability of labile N substrates [22]. In June 2009, a relatively high Nmin in mixed stands could be attributed to enhanced quantity and quality of forest litter and soil organic matter [7,27]. High-quality litter (low C/N ratio) may accelerate decomposition and decrease microbial immobilization of N, and result in enhanced Nmin and plant available N [22,32]. However, forest floor mass was at a minimum for deciduous species before the onset of foliar litterfall [31]. Therefore, there was no significant difference in Nmin between pure and mixed poplar stands at each stand age in September 2008. 5. Conclusions Our observations revealed that there were increases in aboveground biomass, soil TOC and TN concentrations and MBC, and a decrease in qCO2 in 15-year-old mixed poplar stands, compared to corresponding pure poplar stands. However, no significant differences in stand productivity, soil TOC and TN concentrations, MBC and qCO2 existed between 5-year-old pure and mixed poplar plantations. Our results suggest that, in semi-arid regions, mixing N-fixing shrub species into poplar plantations can improve soil fertility in a long run rather than in a short term; therefore, mixing N-fixing shrub species into poplar stand is an option to increase productivity while improving soil fertility in a long run. Meanwhile, our findings highlight that appropriate mixing proportions of Nfixing species and stock density in mixed poplar plantations should be considered to minimize interspecific competition, and to increase long-term soil fertility and stand productivity. Acknowledgments This work was funded by the National Key Technologies R&D Program of China (No. 2006BAD03A0502) and the National Natural Science Foundation of China (No. 30872011). We thank two anonymous reviewers, Christoph C. Tebbe and Xinhua Zhou for their helpful remarks on an earlier version of this manuscript, He-Ming Lin and Jing-Shi Li for laboratory analyses, and other colleagues who participated in the field work. References [1] A.S.F. Araujo, V.B. Santos, R.T.F. Monteiro, Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piaui state, Brazil. Eur. J. Soil. Biol. 98 (2008) 225e230. [2] F. Bastida, A. Zsolnay, T. Hernández, C. García, Past, present and future of soil quality indices: a biological perspective. Geoderma 147 (2008) 159e171. [3] B. Berg, C.A. McClaugherty, A.V. De Santo, D.W. Johnson, Humus buildup in boreal forests: effects of litterfall and its N concentration. Can. J. Forest Res. 31 (2001) 988e998. [4] D. Binkley, P. Sollins, R. Bell, D. Sachs, D.D. Myrold, Biogeochemistry of adjacent conifer and alder-conifer stands. Ecology 73 (1992) 2022e2033. [5] J.M. Bremner, Nitrogen-total. in: D.L. Sparks, A.L. Page, P.A. Helmke, R.H. Loeppert, P.N. Soltanpour, M.A. Tabatabai, C.T. Johnston, M.E. Sumner (Eds.), Methods of Soil Analysis. Part 3, Chemical Methods, Soil Science Society of America Book Series, Number 5, 1996, pp. 1085e1122 Wisconsin, USA. [6] M.D. Busse, M.F. Jurgensen, D.S. Page-Dumroese, R.F. Powers, Contribution of actinorhizal shrubs to site fertility in a Northern California mixed pine forest. Forest Ecol. Manag. 244 (2007) 68e75.
329
[7] R. Dinesh, M.A. Suryanarayana, S.G. Chaudhuri, T.E. Sheeja, K.N. Shiva, Longterm effects of leguminous crops on biochemical and biological properties in the organic and mineral layers of soils of a coconut plantation. Eur. J. Soil Biol. 42 (2006) 147e157. [8] T.M. Fenning, J. Gershenzon, Where will the wood come from? Plantation forests and the role of biotechnology. Trends Biotechnol. 20 (2002) 291e296. [9] D.I. Forrester, J. Bauhus, A.L. Cowie, J.K. Vanclay, Mixed-species plantations of Eucalyptus with nitrogen-fixing trees: a review. Forest Ecol. Manag. 233 (2006) 211e230. [10] T.R. Fox, Sustained productivity in intensively managed forest plantations. Forest Ecol. Manag. 138 (2000) 187e202. [11] G. Jia, B. Liu, G. Wang, B. Zhang, The microbial biomass and activity in soil with shrub (Caragana korshinskii K.) plantation in the semi-arid loess plateau in China. Eur. J. Soil Biol. 46 (2010) 6e10. [12] S.C. Hart, D. Binkley, D.A. Perry, Influence of red alder on soil nitrogen transformations in two conifer forests of contrasting productivity. Soil Biol. Biochem. 29 (1997) 1111e1123. [13] X. He, C. Zhang, S. Yang, Y. Zhang, D. Su, H. Liu, Role of nitrogen fixing trees in mixed forest. I. Nitrogen fixation and tree growth of mixed stand with Hippophae rhamnoides. Chin. J. Appl. Ecol. 7 (4) (1997) 354e358. [14] H. Insam, Are the soil microbial biomass and basal respiration governed by the climate regime? Soil Biol. Biochem. 22 (1990) 525e532. [15] M.J. Kelty, The role of species mixtures in plantation forestry. Forest Ecol. Manag. 233 (2006) 195e204. [16] J.P. Kimmins, Evaluation of consequences for future tree productivity of loss of nutrients in whole tree harvesting. Forest Ecol. Manag. 1 (1977) 169e183. [17] W. Liang, H. Hu, F. Liu, D. Zhang, Research advance of biomass and carbon storage of poplar in China. J. For. Res. 17 (1) (2006) 75e79. [18] Q.M. Lin, Y.G. Wu, H.L. Liu, Modification of fumigation extraction method for measuring soil microbial biomass carbon. Chin. J. Ecol. 18 (2) (1999) 63e66. [19] S. Liu, Effects of seabuckthorn (Hippophae rhamnoides L.) on nutrient distribution and biological cycling of poplar plantations in dry subhumid area of China. Acta Ecol. Sin. 19 (4) (1999) 534e542. [20] S. Liu, Effects of seabuckthorn on tree growth and biomass production of poplar plantations in a subhumid-arid area of China. Acta Phytoecol. Sin. 24 (2) (2000) 169e174. [21] Z. Liu, G. Gao, Y. Lu, X. Li, Comparison of biomass and its estimation of Hippophae rhamoides L. under different climatic and soil conditions. J. Nanjing For. Univ.: Nat. Sci. 31 (1) (2007) 37e41. [22] D.C. McKinley, C.W. Rice, J.M. Blair, Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes. Soil Biol. Biochem. 40 (2008) 2627e2633. [23] D.W. Nelson, L.E. Sommers, Total carbon, organic carbon, and organic matter. in: D.L. Sparks, A.L. Page, P.A. Helmke, R.H. Loeppert, P.N. Soltanpour, M.A. Tabatabai, C.T. Johnston, M.E. Sumner (Eds.), Methods of Soil Analysis. Part 3, Chemical Methods, Soil Science Society of America Book Series, Number 5, 1996, pp. 961e1010 Wisconsin, USA. [24] K.I. Paul, P.J. Polglase, J.G. Nyakuengama, P.K. Khanna, Change in soil carbon following afforestation. Forest Ecol. Manag. 168 (2002) 241e257. [25] D.A. Perry, The scientific basis of forestry. Annu. Rev. Ecol. Syst. 29 (1998) 435e466. [26] A. Rothe, D. Binkley, Nutritional interactions in mixed species forests: a synthesis. Can. J. Forest Res. 31 (2001) 1855e1870. [27] A. Rothe, J.K. Cromack, S.C. Resh, E. Makeneci, Y. Son, Soil carbon and nitrogen changes under Douglas-fir with and without red alder. Soil Sci. Soc. Am. J. 66 (2002) 1988e1995. [28] E.F. Salamanca, M. Raubuch, R.G. Joergensen, Relationships between soil microbial indices in secondary tropical forest soils. Appl. Soil Ecol. 21 (2002) 211e219. [29] W. Shi, H. Yao, D. Bowman, Soil microbial biomass, activity and nitrogen transformations in a turfgrass chorosequence. Soil Biol. Biochem. 38 (2006) 311e319. [30] E.D. Vance, P.C. Brookes, D.S. Jenkinson, An extraction method for measuring soil microbial biomass C. Soil Biol. Biochem. 19 (1987) 703e707. [31] L. Vesterdal, I.K. Schmidt, I. Callesen, L.O. Nilsson, P. Gundersen, Carbon and nitrogen in forest floor and mineral soil under six common European tree species. Forest Ecol. Manag. 255 (2008) 35e48. [32] P.M. Vitousek, Nutrient cycling and nutrient use efficiency. Am. Nat. 119 (1982) 553e572. [33] J.G. Vogel, S.T. Gower, Carbon and nitrogen dynamics of boreal jack pine stands with and without a green alder understory. Ecosystems 1 (1998) 386e400.