Nitrogen addition enhances home-field advantage during litter decomposition in subtropical forest plantations

Soil Biology & Biochemistry 90 (2015) 188e196

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Nitrogen addition enhances home-field advantage during litter decomposition in subtropical forest plantations Zaipeng Yu a, Zhiqun Huang a, *, Minhuang Wang a, Ruiqiang Liu a, Lujia Zheng a, Xiaohua Wan a, Zhenhong Hu a, Murray R. Davis b, Teng-Chiu Lin c a

Key Laboratory for Subtropical Mountain Ecology (Ministry of Science and Technology and Fujian Province Funded), College of Geographical Sciences, Fujian Normal University, Fuzhou, China Scion, P.O. Box 29237, Fendalton, Christchurch 8540, New Zealand c Department of Life Science, National Taiwan Normal University, No. 88, Section 4, DingChow Road, Taipei 11677, Taiwan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 May 2015 Received in revised form 14 July 2015 Accepted 30 July 2015 Available online 18 August 2015

Nitrogen (N) exerts strong effects on litter decomposition through altering microbial abundance and community composition. However, the effect of N addition on plantesoil interactions such as home-field advantage (HFA: enhanced decomposition at a home environment compared to a guest environment) in relation to litter decomposition remains unclear. To fill this knowledge gap, we conducted a reciprocal litter transplant plus N addition experiment in Mytilaria laosensis and Cunninghamia lanceolata plantations for two years in subtropical China where anthropogenic N input is amongst the highest in the world. We found positive HFA effects (in which the calculation incorporates litter of both species) with litter mass loss 11.2% faster at home than in the guest environment in the N addition (50 kg N ha
1 yr
1) treatment, but no significant HFA effects were found in the control treatment. The magnitude of the HFA effect on carbon (C) release increased with N addition, while that on N release decreased. The HFA effects on phosphorus, potassium, calcium, sodium, and magnesium release were positive overall, but varied through time and the magnitude of the effects were different among elements. The greater HFA effects in the N addition treatment were associated with greater differences in microbial biomass and community composition between home and guest environments than in the control treatment. Our results indicate that anthropogenic N enrichment could lead to enhanced HFA effects, through modification of microbial communities, and thereby affect C sequestration and N cycling in subtropical forests. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Cunninghamia lanceolata Microbial community composition Nutrient release Reciprocal litter transplant

1. Introduction The return of carbon (C) and nutrients to the soil through litter decomposition is a fundamental ecological process closely linked to C sequestration through its effects on soil organic matter formation and turnover (Kramer et al., 2003; Hobara et al., 2013). Litter decomposition is largely affected by environmental conditions (e.g., temperature and precipitation) on a global scale, by litter quality on a local scale (Melillo et al., 1982; Coûteaux et al., 1995; Zhang et al., 2008), and is mediated by soil organisms, nutrient availability and
lez and Seastedt, their interactions (Berg and Matzner, 1997; Gonza 2011; Rinkes et al., 2013). Several recent studies have illustrated that interactions between soil biota and litter quality may have

* Corresponding author. Tel.: þ86 591 83434802; fax: þ86 591 83441543. E-mail address: [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.soilbio.2015.07.026 0038-0717/© 2015 Elsevier Ltd. All rights reserved.

additive effects on litter decomposition (Ayres et al., 2009a). It has also been suggested that litter decomposition responds positively to increased soil nitrogen (N) availability (Norris et al., 2013). Home-field advantage (HFA) (i.e., litter decomposes faster when placed in its habitat of origin than when placed out of its habitat of origin) has been a salient topic in studies of litter decomposition in recent years (Ayres et al., 2009a; Perez et al., 2013; Chomel et al., 2015; Veen et al., 2015a). Some researchers have proposed that HFA is a previously unrecognized factor in explaining variability in litter decomposition at the global scale (Wang et al., 2012; Veen et al., 2015a). The HFA hypothesis is based on the assumption that decomposer communities are functionally equivalent rather than dissimilar, but specialize in decomposing the plant species above them (Ayres et al., 2009a, 2009b; Keiser et al., 2014). However, although a number of studies have reported positive HFA effects (Kagata and Ohgushi, 2012; Wang et al., 2012; Wallenstein et al., 2013; Austin et al., 2014), studies that have found no or

Z. Yu et al. / Soil Biology & Biochemistry 90 (2015) 188e196

negative effects are common. For example, St. John et al. (2011) found no HFA effects in a grassland-forest reciprocal litter transplant study. Perez et al. (2013) found little support for HFA effects in a plant successional gradient from open grasslands to forests. Freschet et al. (2012) reported a continuum from positive to negative interactions between specific litters and decomposer communities, as specific litter and the ecosystem litter layer become increasingly dissimilar in quality. The lack of universal positive HFA effects suggest that the occurrence and magnitude of HFA effects likely vary among ecosystem types, succession stages, plant species and climate zones (Gießelmann et al., 2011; Milcu and Manning, 2011; Makkonen et al., 2012; Austin et al., 2014). At a given site, HFA effects may also vary at different stages of decomposition (Ayres et al., 2009a), but the mechanisms remain poorly understood as most studies are based on single point in time sampling (Veen et al., 2015a). Moreover, the magnitude or even the occurrence of HFA may be different if different aspects of litter decomposition, such as biomass and nutrient release, are considered because different elements may be immobilized or released at different rates and the rates may change through time (Ayres et al., 2009a; Fujii and Takeda, 2010; Aponte et al., 2012; Wang et al., 2012; Liu et al., 2015). For example, N and phosphorus (P) are often immobilized in early stages and released in later stages, while C is decomposed more rapidly in early stages (Wang et al., 2012). The differences in HFA effects among different elements and decomposition stages have rarely been examined. Thus, although the role of HFA on litter decomposition is widely recognized and supported by many studies, the conditions of its occurrence, its magnitude and interactions with other factors are not well understood (Gießelmann et al., 2011; St. John et al., 2011; Veen et al., 2015a, 2015b). Nitrogen enrichment from both atmospheric deposition and fertilization is becoming a major environmental issue in regions undergoing rapid industrialization such as China (Liu et al., 2013). Anthropogenic N input could either accelerate or inhibit decomposition through modification of decomposer communities in different decay stages (Berg and Matzner, 1997; Carreiro et al., 2000; Hobbie, 2008; Hobbie et al., 2012). Liu et al. (2015) found that N addition significantly enhanced nutrient release through increased soil microbial biomass and enhanced leaf litter quality. Knorr et al. (2005) reported that the responses to N addition depend on levels of N deposition, fertilization and litter quality. Studies have examined the effects of N additions and HFA on decomposition separately, but the effects of N addition on litter decomposition through its influence on HFA have only been examined in a few studies (Vivanco and Austin, 2011; Allison et al., 2013). In addition, although HFA effects have been examined in a variety of ecosystems, studies from the tropical and subtropical regions are rare (Veen et al., 2015a). More studies on how N affects the occurrence, direction and magnitude of HFA from ecosystems in diverse regions are needed for a thorough understanding of the effects of anthropogenic N enrichment on C and nutrient cycling. Forest plantations in subtropical China have been recognized as an important C pool at the global scale (Xu, 2011; Huang et al., 2013a). Plantations of Cunninghamia lanceolata in subtropical China alone (9.11 million ha) account for approximately 18% and 5% of all forest plantations in China and the world, respectively (Huang et al., 2013b). Because timber production of C. lanceolata decreases significantly in consecutive rotations (Wei et al., 2012), plantations of native broadleaved tree species alone or in mixtures with coniferous species are encouraged in China (Xu, 2011). Several studies have compared litter decomposition between different forest types using reciprocal transplant experiments (Yang et al., 2004; Liu et al., 2005; Wang et al., 2008), and some have examined litter decomposition in response to N addition in subtropical

189

China (Mo et al., 2006). One study examined HFA effects in two types of natural forests in central China (Wang et al., 2015) but to our knowledge, no studies have examined HFA effects in forest plantations of China. We conducted a litter decomposition experiment with reciprocal litter transplant plus N addition treatments in C. lanceolata and broadleaved Mytilaria laosensis plantations in subtropical China where the atmospheric N deposition rate reaches 30e50 kg N ha
1 yr
1. The objectives of this study were to 1) explore the effects of litter quality, habitat (home vs. guest environments) and microbial community composition on decomposition rate and 2) evaluate the effects of N addition on microbial community composition and litteresoil interactions during litter decomposition. Specifically we tested the following hypotheses. First, because the two forests had very different soil properties and litter quality (Huang et al., 2013b) we expected that HFA effects would occur in both C. lanceolata and M. laosensis forest plantations. Second, because numerous studies have found that low quality recalcitrant litter (Ayres et al., 2009a; Milcu and Manning, 2011; Keiser et al., 2014) is likely to have greater HFA effects than high quality litter, we hypothesized that N addition would improve litter quality (Allison et al., 2013; Liu et al., 2015), and thus decrease HFA effects. Third, because different nutrients/elements are released or immobilized at different rates and the rates vary with time, we hypothesized that HFA effects would differ among elements and change through time. 2. Materials and methods 2.1. Site description The experiment was conducted at Xiayang, northwestern Fujian Province of southeastern China (26 480 N, 117 580 E) between July 2012 and June 2014. The experimental site is located on a deep red soil classified as a sandy clay loam Ferric Acrisol according to the FAO/UNESCO classification (Huang et al., 2013b). The site is characterized by a humid subtropical climate with short and mild winters (with occasional frost) in January and February, and long, hot and humid summers between June and October. Spring and autumn are warm transitional periods. During the two-year study period, mean annual precipitation was 1747 mm and mean annual temperature was 20  C. In October 1996, seedlings of C. lanceolata and M. laosensis were planted on a site where C. lanceolata had previously been grown and harvested in April 1995. The 5-ha area was divided equally for the planting of monocultures of the two species. The seedlings were planted at 2 m  2 m intervals giving a seedling density of 2500 individuals ha
1. 2.2. Experimental design, litterbag preparation and sampling In June 2012, before N addition, top soil samples (10 cm) were collected from three soil cores in each plot for analyses of the initial soil pH and concentrations of total C, total N, mineral N (NHþ 4 eN, NO
3 eN) and dissolved organic C and N. Soil samples were transported to the laboratory and stored at 4  C for less than 2 days prior to processing. The moist soils were sieved through 2 mm mesh to remove large pieces of organic debris prior to analysis. Six 2 m  2 m plots were established in each of the M. laosensis and C. lanceolata plantations (Fig. S1). Three of the six plots of each species were randomly assigned as N addition plots which received 50 g N m
2 yr
1 in the form of granular NH4NO3 from the beginning of the study while no N addition was applied to the other three plots. The inorganic N addition was divided into six applications and applied evenly every two months. Each of the six plots was split into two equal parts. In July 2012, fifteen C. lanceolata litterbags

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were placed on one part and fifteen M. laosensis litterbags were place on the other part yielding 360 litterbags (2 host species  2 litter species  2 N treatments  3 replication  15 bags) in total. Freshly fallen leaf litter was collected using litter traps located in C. lanceolata and M. laosensis plantations near the experimental site between March 2012 and June 2012. The litter was air dried and stored prior to field placement. Litterbags, 20  20 cm, were constructed from 1-mm nylon window screen material. Twelve evenly distributed 0.25 cm  0.25 cm holes were cut on the upper side of each litterbag to minimize isolation of the litter from large soil fauna. Twenty grams of air-dried litter of C. lanceolata or M. laosensis were placed in each litterbag. Before placement, the forest floor litter layer and humus layer were carefully removed so that the litterbags could easily contact the mineral soil. Plastic strings stretched over the litterbags were used to fix the bags in position. Two litterbags, one containing C. lanceolata and the other M. laosensis litter, were randomly harvested from each plot at approximately 2, 4, 6, 8, 10, 12, 14, 16, 18 and 24 months after the initial placement. At the 8, 12 and 16 month collections, subsamples of litter were prepared to determine microbial community composition by phospholipid fatty acid analysis (PLFA). 2.3. Laboratory analysis Litter was removed from the litterbags, cleaned of ingrown roots, animals and soil, oven-dried at 65  C for at least 72 h to a constant mass, and weighed. Dried litter samples (including initial litter samples) were ground into a fine powder using a ball mill and analyzed for total carbon (C) and nitrogen (N) using an Elemental EL MAX CNS analyzer (Elementary, Hanau, Germany). Lignin content was assayed using the acid detergent method (Rowland and Roberts, 1994). For determination of phosphorus (P) concentration, litter samples (0.25 g each) were digested in a 10 mL mixture of sulphuric and perchloric acid (5:1) at 300  C and the P concentration in the digested solution was measured by a continuous flow analyzer (Sanþþ, Skalar, Netherlands) (Zarcinas et al., 1987). For potassium (K), calcium (Ca), sodium (Na), and magnesium (Mg) determination, 0.2 g litter samples were digested with 4 mL of concentrated nitric acid heated at 120  C, and 30% hydrogen peroxide (H2O2) was repeatedly added until the digested solution was colourless (Jones Jr., 2001). After H2O2 digestion, the cation concentrations were measured by atomic absorption spectroscopy (TAS-990, Beijing, China). Subsamples from litterbags were freeze-dried and quickly ground using agate mortar and pestle (Hobbie et al., 2012). Lipids were extracted from litter (0.25 g) using a single-phase chloroformemethanolephosphate buffer extraction after White et al. (1979) and slightly modified (after a pilot run) to maximize extraction of fatty acids from the litter. Fatty acids were extracted with a one-phase solvent consisting of a 1:2:0.8 mixture of chloroform, methanol and phosphate buffer (pH 7.4). Litter samples were extracted with 20 mL of the solvent in a shaker for 24 h. The samples were centrifuged at 1000 g for 10 min, and the supernatant was removed. The remaining litter was re-extracted with 10 mL of the same extraction solvent for another 12 h. The supernatant was removed after centrifuging and the two extracts were combined and then evaporated under N2 to a volume of 1 mL. The phospholipids in the concentrated extract were separated on silicic acid columns by sequentially eluting with organic solvents of increasing polarity and amended with a nonadecanoic acid standard (100 ml). They were then saponified and methylated, forming fatty acid methyl esters (FAMEs). Individual FAMEs were identified by gas chromatography (Hewlett Packard 5890 GC, equipped with 6890 series injector, flame ionization detector and

an Ultra 2 capillary column; 25 m  0.2 mm, 0.33 mm film thickness) based on their retention times and in combination with the MIDI Sherlock Microbial Identification System (MIDI Inc., Newark, DE). Total lipid abundance was calculated as the sum of lipids of which chain length was from C10 to C20 and could be measured as microbial biomass. Gram-positive bacteria were represented by all iso and anteiso branch chain fatty acids (Denef et al., 2009; Landesman and Dighton, 2010), whereas Gram-negative bacteria were represented by monounsaturated and cyclopropane fatty acids (Frostegård et al., 2011; Ushio et al., 2013). C18:1u9 and C18:2u6,9 were used as indicators of fungi, while C16:1u5 was used to indicate arbuscular mycorrhizal fungi (Swallow et al., 2009). The PLFAs 10Me16:0, 10 Me17:0 and 10 Me18:0 were used to indicate soil actinomycetes. The abundance of individual PLFAs was calculated as the absolute amount of C (nmol PLFA-C g
1 litter or soil) and then converted to mole percentage PLFA-C. Fungi:bacteria and Gram-positive:Gram-negative bacteria ratios were calculated as the sum of C18:1u9 and C18:2u6,9/(sum of all bacterial lipid), and (sum of the branched lipids)/(sum of the monounsaturated and cyclopropyl lipids), respectively. 2.4. Data analysis We fitted litter mass remaining at the end of the experiment to an exponential decay model, X ¼ e
kt, where X is the proportion of initial mass remaining at time t, and k is the decomposition constant in the exponential model. The best fit model was determined using Akaike's Information Criteria, where a difference between two candidate models of 3 was used to indicate a significant difference in model fit (Hobbie et al., 2012). To determine the overall HFA effects (where both litter species are considered), we calculated the HFA index (HFAI) following Ayres et al. (2009b). First the relative mass loss was calculated as:

ARMLa ¼

Aa  100 Aa þ Ba

where, ARMLa represents the relative mass loss of litter (or nutrient) from species A at site a, and Aa and Ba represent the percent (of initial) litter mass (or nutrient) loss of plant species A and B, respectively, decomposing at site a. The HFA index (HFAI) was then calculated as:

 HFAI ¼

ARMLa þ BRMLb 2

  ARMLb þ BRMLa

 100
100

where HFAI represents the percent enhanced mass loss of litter (or nutrient) when it decomposes at home versus in a guest environment and is a net value for both species (A and B) in the reciprocal transplant. We calculated HFAI for each sampling time to determine temporal changes in HFA effects. We also determined the HFA of the two litter species by calculating the mean HFA (% increase in k value at home compared to in guest environment) for each litter type separately following Vivanco and Austin (2008) and Austin et al. (2014): Mean HFA ¼ (khome
kguest)/kguest  100 where, khome and kguest is the decay constant of a given species at home and in guest environment, respectively. ANOVAs with repeated measurements were used to determine the effects of sampling time (repeated factor), habitat (home vs. guest environment) and species on litter mass, C and N loss. One-way ANOVAs were used to compare the magnitude of

Z. Yu et al. / Soil Biology & Biochemistry 90 (2015) 188e196

0.6

HFAI of litter mass, C and nutrient decay between control and N addition plots. One-way ANOVAs were also used to examine habitat effects on decay constants and microbial biomass. Redundancy analysis (Mayor et al., 2012) was performed to examine the effects of habitat on microbial community composition and relationships between PLFAs and litter nutrient concentrations. Statistical analyses were performed using SPSS 17.5 for Windows and Canoco for Windows 4.5 (version 4.5, Microcomputer Power, NY, USA).

Initial litter chemistry and soil properties were different between M. laosensis and C. lanceolata plantations (Table 1). Compared to the C. lanceolata litter, the M. laosensis litter had lower lignin, cellulose, P, and Ca concentrations and lower lignin: N ratio but higher concentrations of K, Mg and Na (Table 1). Soils in M. laosensis plantations had higher total C and N concentrations and C:N ratio but lower NO
3 , DOC, and DON concentrations than soils in C. lanceolata plantations (Table 1). 3.2. Decomposition rates and mass loss patterns M. laosensis litter had a greater k than C. lanceolata both in the control and N addition plots and both at the home and guest environments (Fig. 1). The decay constant of M. laosensis litter was greater at home than in the guest environment in N addition plots but not in control plots (i.e., without N addition) (Fig. 1). In contrast, no difference in k of C. lanceolata litter was detected between home and guest environments in either control or N addition plots (Fig. 1). The decay of litter mass of both species varied significantly through time. It decreased quickly in the first year and then more gradually in the second year both at home and in the guest environment (Fig. 2). The mass loss of M. laosensis litter was greater at home than in the guest environment in the N addition plots but not in the control plots (Fig. 2). The mass loss of C. lanceolata litter was not different between home and guest environments in either the control plots or N addition plots (Fig. 2).

Table 1 Initial chemical properties (mean ± standard error) of leaf litter and soils (0e10 cm depth) in M. laosensis and C. lanceolata plantations. F, df and P values from ANOVAs are shown and significant differences are indicated by bold-face P values.

Litter Lignin (%) Cellulose (%) C (g/kg) N (g/kg) P (g/kg) K (g/kg) Ca (g/kg) Mg (g/kg) Na (g/kg) C/N lignin/N Soil NHþ 4 (mg/kg) NO
3 (mg/kg) Mineral N (mg/kg) DOC (mg/kg) DON (mg/kg) Total N% Total C% C/N

C. lanceolata

F

df

± ± ± ± ± ± ± ± ± ± ±

0.5 0.6 8 0.2 0.01 0.03 0.23 0.02 0.07 1.7 0.6

36 23 497 7.3 0.11 0.7 3.9 1.0 0.4 78 50

± ± ± ± ± ± ± ± ± ± ±

0.4 0.3 2 0.2 0.01 0.01 0.15 0.01 0.03 3.3 1.2

1.038 2.718 0.071 0.281 1.844 0.818 1.000 1.279 5.189 1.706 3.696

8 8 8 8 7 7 7 7 7 8 8

<0.001 <0.001 0.895 0.320 <0.001 <0.001 0.001 <0.001 0.010 0.712 <0.001

14.6 2.6 17.2 43.8 8.4 0.19 3.0 16.0

± ± ± ± ± ± ± ±

2.5 1.3 2.3 4.9 1.5 0.01 0.2 0.5

12.3 6.5 18.8 58.5 13.7 0.15 2.1 13.8

± ± ± ± ± ± ± ±

1.6 1.0 1.8 7.2 1.1 0.01 0.1 0.2

0.596 5.876 0.283 5.113 8.496 13.258 15.530 16.749

10 10 10 10 10 10 10 10

0.458 0.036 0.606 0.047 0.015 <0.001 <0.001 0.001

b

c

c

-1

k (yr )

c

0.0

MG CG c

Control

N addition

Fig. 1. Litter decay constants (k) of M. laosensis and C. lanceolata at home and in the guest environment without (control) and with N addition. MH: M. laosensis litter in home environment; MG: M. laosensis litter in guest environment (C. lanceolata); CH: C. lanceolata litter in home environment; CG: C. lanceolata litter in guest environment (M. laosensis). Error bars are ±1 stand error. Bars without a letter in common are significantly different at P < 0.05.

3.3. Carbon and nutrient decay patterns The C decay of both M. laosensis and C. lanceolata litter varied significantly through time and was similar to the pattern of litter mass loss (Fig. 2, Table 2). M. laosensis had greater C mass loss (i.e. less C remaining) than C. lanceolata in both the control plots and N addition plots (Fig. 2). The C decay of C. lanceolata litter was greater at home than in the guest environment but only in the N addition plots. However, C decay of M. laosensis litter showed no significant

Control

100 90

90

80

80

70

70

60

60

50

50

100

MH MG CH CG

40 0

200

400

600

800 0 100

80

80

60

60

40

40

20

20

0 140

N addition

100

40

P

28 14 489 7.5 0.08 4.3 2.4 1.3 0.7 78 38

b

MH CH

0.2

N mass remaining

M. laosensis

b

Litter mass remaining

3.1. Initial litter and soil properties

a

0.4

C mass remaining

3. Results

191

0

200

400

600

0 800 0 140

120

120

100

100

80

80

60

60

40 0

200

400 600 Time (days)

40 800 0

200

400

600

800

200

400

600

800

400 600 Time (days)

800

200

Fig. 2. The change over time of litter, N and C mass remaining (% of initial mass) for M. laosensis and C. lanceolata litter at home and guest environments in control and with N addition treatments. MH: M. laosensis litter in home environment; MG: M. laosensis litter in guest environment (C. lanceolata); CH: C. lanceolata litter in home environment; CG: C. lanceolata litter in guest environment (M. laosensis). Bars are ±1 standard error.

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Table 2 Effects (indicated by P values from ANOVA with repeated measurements) of sampling time, habitat, litter type, and their interactions on litter mass and C and N release in control and N addition treatments in M. laosensis and C. lanceolata plantations. Significant differences are indicated by bold-face P values.

0.005 0.003 0.006

0.907 0.036 <0.001

<0.001 0.128 0.007

0.745 0.005 0.800

0.995 0.040 0.026

<0.001 0.116 <0.001

0.854 0.400 0.553

0.040 0.903 0.135

0.247 0.927 0.225

0.608 0.333 0.243

3.4. HFA of mass loss and C release When litter of both species was included in the calculation, the HFAI for litter mass loss in the control plots was negative during the first 400-days of decomposition, which turned slightly positive at later stages, whereas it was always positive in the N addition plots throughout the 2-yr period (Fig. 4). Average HFAI of mass loss across the multiple sampling times was higher in the N addition plots (11.2) than the control plots (
3.5) (Table 3). The HFAI of C loss was also greater in the N addition plots than in the control plots (Table 3). When each litter type was considered separately, the mean HFA of M. laosensis litter mass loss was 5.3% and 23.9% in the control plots and the N addition plots, respectively (Fig. 5). For C. lanceolata litter it was
6.1% and 3.9% in the control plots and the N addition plots, respectively (Fig. 5).

Control

500

P (% of initial)

300 200 100

300 200 100

0 500 0

200

400

0 800 0 500

600

200

400

600

800

200

400

600

800

200

400

600

800

200

400

600

800

400 600 Time (days)

800

400 P (% of initial)

400

300

300 200

200

100

100

0 120 0

200

400

0 800 0 120

600

90 60 30 0 120 0

200

400

60 30 0 250 0

200

400

90 60 30

0 800 0 120

600

90

90 60 30 0 800 0 250

600

200

200

150 100 50

150 100 50

0

When litter of both species was taken into consideration, the HFAI of N loss was in general positive in the control plots but it was much greater in the second year (up to 21%) than the first year (less than 5%) while in the N addition plots it was generally small (within ±10%) (Fig. 4). The average HFAIs of P, Mg, and Na were positive in

MH MG CH CG

400

400

0

3.5. HFA of nutrient mass loss

N addition

500

P (% of initial)

differences between the two habitats in either the control plots or the N addition plots (Fig. 2). The decay of N varied greatly through time (Fig. 2, Table 2). Both M. laosensis and C. lanceolata litter immobilized N in the early decay stages at both home and guest environments and in both the control and the N addition plots (Fig. 2). In the control plots, N was immobilized in M. laosensis litter for 400 days at the home environment, compared to 245 days when transplanted to the C. lanceolata plantation. For C. lanceolata litter, N immobilization lasted for 300 days at both home and guest environments. Nitrogen addition increased the magnitude of N immobilization for both M. laosensis and C. lanceolata litter (Fig. 2). The magnitude of nutrient release or immobilization differed among P and the cation elements and was different between the two species for some elements (Fig. 3). Phosphorus was immobilized in the litter of both species in the first 475-days of decay, then released slowly (Fig. 3). In contrast, Ca and Mg in the litter of both species exhibited net release both at home and in guest environments throughout the 2-yr period (Fig. 3). In C. lanceolata litter, K content, due to immobilization, was as much as three times that of the initial mass after 245e475 days of decay both at home and in guest environments. In contrast 50% of the K in M. laosensis litter was quickly released in the first 70 days of decomposition, then released gradually throughout the study period (Fig. 3). Litter Na content was only 20% of the initial content at the end of the first year, then litter immobilized Na to the initial content or greater at the end of the second year in both species (Fig. 3).

P (% of initial)

0.044 0.627 0.027

Time  habitat  litter type

P (% of initial)

<0.001 <0.001 <0.001

Habitat  litter type

P mass remaining

0.213 0.115 0.303

Time  litter type

K mass remaining

<0.001 <0.001 <0.001

Time  habitat

Litter type

Ca mass remaining

Habitat

Mg mass remaining

Time

Na mass remaining

Control Litter mass N mass C mass N addition Litter mass N mass C mass

200

400 600 Time (days)

0 800 0

200

Fig. 3. The change over time of litter phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) mass remaining (% of initial) for M. laosensis and C. lanceolata litter at home and guest environments in control and with N addition treatments. MH: M. laosensis litter in home environment; MG: M. laosensis litter in guest environment (C. lanceolata); CH: C. lanceolata litter in home environment; CG: C. lanceolata in guest environment (M. laosensis). Bars are ±1 standard error.

Z. Yu et al. / Soil Biology & Biochemistry 90 (2015) 188e196 30 10

C

20 10

0 -10

0 Control N addition

-20 -30 30 0

200

400

600

800 N

20

-10 -20 90 0

200

400

600

800

30

0

0

-10

-30

80 0

200

400

600

800 K

40

30 0

200

400

600

800 Ca

20 10

0

0 -10

-40

-20

-80 60 0

200

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600

40

-30 800 120 0 Mg 80

200

400

600

800 Na

40

20 0

0

-20

-40 0

200

400 Time (days)

600

0

800

200

400 Time (days)

600

800

Fig. 4. The temporal patterns of home-field advantage indices (HFAI) for mass and nutrient loss of M. laosensis and C. lanceolata litter in the control (open circles) and N addition (filled circles) treatments.

Table 3 Home-field advantage index of litter mass and nutrient release averaged across multiple sampling times (mean ± standard error) in control and N addition treatments. F, df and P values are presented and significant differences are indicated by bold-face P values. Mass/nutrients

Control

Mass C N P K Ca Mg Na


3.5
0.9 7.4 6.9
6.0
7.1 16.7 12.2

± ± ± ± ± ± ± ±

N addition 2.6 3.4 2.7 4.3 13.9 6.3 10.6 5.8

11.2 12.2
0.2 18.7 5.3
4.9 8.4 20.9

± ± ± ± ± ± ± ±

2.0 3.2 1.8 12.9 20.8 7.3 12.1 17.3

F

df

P

0.620 0.103 1.529 2.388 0.452 0.035 0.008 3.534

18 14 14 8 8 6 8 8

<0.001 0.013 0.037 0.407 0.665 0.840 0.622 0.645

both the control and N addition plots (Fig. 4). HFAI was mostly near zero for K in the control plots, except at 475 days of decomposition during which it was highly negative. In comparison, it was highly positive at 245 days but highly negative at 475 days of 40 a

Mean HFA

30

Control N addition

20 10

3.6. Microbial community composition and abundance

P

60

10

HFAI

decomposition in the N addition plots. In contrast to the other nutrients, Ca had a negative HFAI during the first year of decomposition. The HFAI for N was greater in the control plots than in the N addition plots, but for the other nutrient elements there was no difference between the control and N addition plots (Table 3).

30 Litter mass

20

193

Redundancy analysis indicated that there were significant differences in microbial community composition between home and guest environments both with and without N addition (Fig. 6). In the control plots, C. lanceolata litter had different microbial community composition between home and guest environments but with some overlap between the two (Fig. 6a). In contrast, also in the control treatment, M. laosensis litter had very different microbial communities between home and guest environments that did not overlap at all (Fig. 6a). When subjected to N addition both species showed no overlap in microbial community between home and guest environments (Fig. 6b). Moreover, when C. lanceolata litter was transplanted to the M. laosensis site the microbial community composition largely overlapped with that of M. laosensis litter. Similarly when M. laosensis litter was transplanted to the C. lanceolata site its microbial community composition largely overlapped with that of C. lanceolata litter (Fig. 6b). In other words, after N addition, the guest environment exerted a stronger control on microbial community composition than the home environment did. Litter microbial community composition was significantly related to the concentrations of P, K, Ca, and Mg and the ratios of C: N and N: P in the control plots (Fig. 6a; Table S1). However, with N addition litter microbial community composition was only significantly related to concentrations of P and the ratios of C: N and N: P (Fig. 6b; Table S1). Together, all of the substrate properties explained 59.7% (with axis one explaining 49.0% and axis two explaining an additional 10.7%) of the variance of microbial composition in the control plots and 63.1% (with axis one explaining 54.1% and axis two explaining additional 9.0%) in the N addition plots (Fig. 6). The microbial biomass in the litter of the two species responded differently to the transplanted environment in both control plots and N addition plots (Fig. S2). In the control plots, the total biomass and the biomass of fungi and bacteria of C. lanceolata litter were significantly lower in the guest environment than at home after 8 months of transplant while the opposite was true for the M. laosensis. In the N addition plots, the total biomass and the biomass of fungi and bacteria of M. laosensis litter were significantly lower in the guest environment than at home after 8 months of decay, while the total biomass and the biomass of bacteria in C. lanceolata litter were significantly lower in the guest environment than at home after 16 months of decay. The ratio of bacteria to fungi significantly decreased over the decay period in M. laosensis litter, but in C. lanceolata litter, the ratio did not change over time (Fig. S2). 4. Discussion

b

b

0 -10

c

-20 M.laosensis

C. lanceolata

Fig. 5. The mean home-field advantage (Mean HFA) for M. laosensis and C. lanceolata litter mass loss in control and N addition treatments. Error bars are ±1 stand error. Bars without a letter in common are significantly different at P < 0.05.

4.1. HFA of litter mass and C loss in the control Contrary to our prediction, we found no overall HFA effects (i.e., litter of both species were included in the calculation of HFAI) on litter mass and C mass loss in the control plots. The result is consistent with several studies that reported no or negative HFA effects despite large differences in litter quality and habitat environment (St John et al., 2011; Freschet et al., 2012; Perez et al., 2013). The lack of HFA effects may result from a stronger litter

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Z. Yu et al. / Soil Biology & Biochemistry 90 (2015) 188e196

A meta-analysis of litter decomposition indicated that litter quality has only weak or no influence on HFAI of litter mass decay at the global scale (Veen et al., 2015a). When calculated separately for individual species we found positive HFA effects of litter mass decay for M. laosensis but negative effects for C. lanceolata in the control plots (i.e. without N addition) (Fig. 5). Our results agree with several studies that reported species-specific HFA effects (Perez et al., 2013; Austin et al., 2014). Chomel et al. (2015) recently reported that spruce litter exhibited positive HFA effects while poplar litters exhibited negative HFA effects. Our results also indicate that litter quality could be an important factor in determining HFA effects at local scales. Although the microbial community composition of M. laosensis litter (Fig. 6a) and the microbial biomass of both species in the control plots differed significantly between home and guest environments at early decay stages (Fig. S2), these differences did not substantially affect decomposition rates of the two species (Fig. 1), resulting in the lack of HFA effects. There are several possible explanations for this. First, although transplanting resulted in changes in microbial biomass, it may have little effect on abundance of soil organisms specialized in decomposing litter of the two species. As a result, the transplant effect on litter decay is minimized. Second, the decomposer communities, especially fungi, probably have broad functional capacities and are capable of decomposing the two litter types (St John et al., 2011; Keiser et al., 2014; Chomel et al., 2015). Therefore, litter of both species required no specialized decomposers localized at any particular site to decompose the litter (Ayres et al., 2009a, 2009b) and as such changes in microbial community composition and biomass did not lead to changes in decomposition rates. Third, although the original litter at the site receiving guest litter was removed before litterbag placement, subsequent litter fall could still add a substantial amount of the original litter to the site and as such affect the magnitude or even the occurrence of HFA (St John et al., 2011; Perez et al., 2013). Fourth, the warm and moist climate at our study site is likely suitable for maintaining diverse and large amounts of decomposers that could breakdown litter of diverse quality very quickly, obscuring the HFA effects (Gießelmann et al., 2011; Veen et al., 2015a). 4.2. HFA effects in response to N addition

Fig. 6. Redundancy analysis ordination biplot of PLFA profiles indicating the relationships between the variation of microbial community composition and substrate quality [concentrations of litter carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), N:P ratio and C:N ratio] in decomposing M. laosensis and C. lanceolata litter in (a) control plots and (b) N addition plots. Solid red arrows denote substrate quality indicators, with the longest independent variable vectors being most strongly related to variation in microbial community composition. MH: M. laosensis at home; MG: M. laosensis litter in guest environment (C. lanceolata); CH: C. lanceolata at home; CG: C. lanceolata in guest environment (M. laosensis). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

quality control than habitat control on litter decomposition. In our study, litter type was a significant factor affecting litter mass and C loss both in the control plots and in the N addition plots. It was also a significant factor affecting N mass loss in the control (Table 2). In contrast, habitat type (home versus guest environments) was a significant factor for litter mass and C decay only in the N addition plots and it was not a significant factor affecting N loss (Table 2).

The finding of enhanced HFA effects for both litter and C mass loss of both species with N addition requires that our second hypothesis that N addition would lead to smaller HFA effects must be rejected. Our result is in contrast to the weak positive HFA effects when litter was fertilized by N reported for a temperate forest in southern California (Allison et al., 2013) and the disappearance of HFA effects with N addition for Nothofagus species in Argentina (Vivanco and Austin, 2011). N deposition is relatively low at the two sites, 5e17 kg N ha
1 yr
1 in southern California (Fenn et al., 2010) and even lower in the unpolluted Nothofagus forest in Argentina. Thus, N addition greatly enhanced litter decomposition (by 46% in the Nothofagus forest) and as such may obscure the differences in decomposition rate associated with differences in litter quality. In contrast, our study site receives greater N deposition (30e50 kg N ha
1 yr
1) so that decomposition is probably less N limited. The lack of N limitation is supported by the lack of a significant relationship between microbial community composition and litter N concentration (Fig. 6). Although N addition can accelerate decomposition, the increased decomposition is probably not sufficient to obscure the control of litter quality on decomposition unless N is limited. The drastically different microbial communities and decreased microbial biomass in guest compared to home environments for

Z. Yu et al. / Soil Biology & Biochemistry 90 (2015) 188e196

both litter types in the N addition plots suggests that N enrichment led to stronger site-specific microbial communities (Allison et al., 2013; Veen et al., 2015a). Studies suggest that increased N availability could stimulate the decomposition of soluble fractions in early decay stages (Berg and Matzner, 1997; Hobbie, 2008; Hobbie et al., 2012), but the effect could vary with original soil N availability (Norris et al., 2013). The greater microbial biomass in the early decay stages in N enriched environments has also been reported for northern forests in the United States (Hobbie et al., 2012). In contrast, a meta-analysis using 82 published field studies found that N enrichment could reduce microbial biomass (Treseder, 2008). The discrepancy suggests that local scale patterns could be very different from those found at broader scales. Differences in litter quality likely contributed to the larger fungi biomass in the early than late decay stages in M. laosensis litter and the opposite pattern in C. lanceolata litter (i.e., lower fungi biomass in the early decay stages). C. lanceolata litter is in general more recalcitrant to decomposition so that it was colonized by fungi decomposers, which are specialized in decomposing recalcitrant materials, at later decay stages compared to M. laosensis. Our study illustrates that N addition can change the relationships between microbial community composition and concentrations of K, Ca, and Mg (Fig. 6). The added N may compensate for nutrient demand of microorganisms in decomposing organic C (Güsewell and Freeman, 2005; Güsewell and Gessner, 2009; Kaiser et al., 2014) and as such weaken the community composition relationship with some nutrients. The shifted microbial community composition as a consequence of N addition probably contributed to the accelerated decomposition and thereby enhanced HFA effects. The temporal patterns of the HFA index clearly show that the magnitude and direction of HFA changed considerably over time both with and without N addition (Fig. 3) possibly because of differences in decomposition rates between early and late decomposition stages. The HFA effects are more likely to be detected at the more rapidly decomposing early stages than the more slowly decomposing late stages (Ayres et al., 2009a). Therefore, the magnitudes (regardless of directions) of HFA for both litter mass and C loss were more variable in early than late decay stages (Fig. 4). 4.3. HFA of nutrient release The greater and earlier immobilization of N at home than in the guest environment likely led to the positive HFA effects for N mass loss in the control plots (Hunt et al., 1988; Ayres et al., 2009a). However, when litters were subjected to N addition, the overall HFA was negative possibly because decomposers can easily immobilize N in an N enriched environment resulting in the lack of advantage of being decomposed at home. For individual species, our results revealed that M. laosensis litter immobilized N earlier and in greater amounts at its home environment than in the guest environment both in the control plots and in the N addition plots. In contrast, C. lanceolata litter did not show significant changes of N immobilization with N addition (Fig. 2). Further, the duration of immobilization was longer in M. laosensis litter than in C. lanceolata litter. The results suggest that the pattern of N immobilization is speciesspecific possibly because of differences in litter nutrient composition. The more positive HFAIs of P, K, Mg, and Na release in early than late decay stages were possibly associated with litter mass loss which was more rapid in the early than late decay stages especially with N addition (Fig. 4). Our result of highly variable patterns of HFA over time and among elements (Fig. 3) supports our third hypothesis and highlights that patterns of litter mass loss at home and in guest environments do not present a complete

195

picture of HFA. Our study also highlights that results from a single sampling time could miss the dynamic nature of HFA in litter decomposition. 5. Conclusions Our study illustrates the change from limited HFA effects to highly positive HFA effects in the decomposition of M. laosensis and C. lanceolata litter in subtropical plantation forests with N addition. Compared to the control plots, the greater difference in microbial community composition between home and guest environments in the N addition plots likely contributed to the enhanced HFA effects. The result is in contrast to studies which found weak, or no HFA effects under N fertilization. The results suggest that the response of HFA to N deposition is species-specific and/or varies with levels of N deposition. Furthermore, the positive response to N addition of HFA effects in C loss and negative response of HFA effects in N release suggest that N addition could affect C sequestration and N cycling through its effects on litter decomposition. Acknowledgements The research was supported by the National Natural Science Foundation of China (41371269), the National “973” Program of China (2014CB954002), and the Program for New Century Excellent Talents in University and the Science Foundation for Excellent Talents of Fujian Province, China. We thank Dr Pei-Jen Lee Shaner from National Taiwan Normal University for her input which has helped to improve this paper. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.soilbio.2015.07.026. References Allison, S.D., Lu, Y., Weihe, C., Goulden, M.L., Martiny, A.C., Treseder, K.K., Martiny, J.B.H., 2013. Microbial abundance and composition influence litter decomposition response to environmental change. Ecology 94, 714e725. ~o
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