Nitrogen Mineralization and Leaching in the Early Stages of a Subtropical Reforestation in Southern China

R E S E A R C H A R T I C L E

Nitrogen Mineralization and Leaching in the EarlyStages of a Subtropical Reforestation in SouthernChinaFaming Wang,1,2 Weixing Zhu,3 Hanping Xia,1 Shenglei Fu,1 and Zhian Li1,4

Abstract

Nitrogen cycling is a critical component in plantations,yet the spatial and temporal variations of N transforma-tions in different stages of reforestation are often poorlyknown. Here we report the seasonal variation of soil insitu net N mineralization, net nitrification and N leach-ing in five young plantations (two monocultures of exoticspecies: Eucalyptus urophylla and Acacia crassicarpa; anative species monoculture; a 10-species mixture and a30-species mixture) and a shrubland (without experimen-tal planting) in subtropical southern China. Our resultsshow that net N mineralization and nitrification rates inthe E. urophylla monoculture (13.5 and 9.98 kg N ha−1

yr−1, respectively) were the lowest among six plantingtreatments, less than 1/3 of those in the 10-species and

30-species mixtures. Two exotic fast-growing monocultureshad 10–60% lower soil extractable nitrate concentrationsthan the native plantations and shrubland and had thelowest nitrogen leaching losses. The leguminous A. crassi-carpa monoculture did not have higher soil N availabilityin comparison with non-leguminous species. Both N miner-alization and nitrification varied seasonally; soil moistureseemed to be important in controlling these temporal vari-ations. This study highlights that in the early stages ofreforestaion, a better understanding of plant species effectson soil N cycling would be beneficial to forest manage-ment decisions and could provide a critical foundation foradvancing restoration practices.

Key words: exotic species, N mineralization, N leaching,native species, nitrification, reforestation, southern China.

Introduction

Human activities have greatly changed land-use patternsworldwide, with millions of hectares of land being deforestedor degraded (Vitousek 1994). Thus, restoration at a global scaleis critical for the sustainability of the earth ecosystem andthe long-term well-being of humanity (Gardiner et al. 2003).Afforestation of former agricultural land, rehabilitation ofdegraded natural ecosystems and conversion of single-speciesto mixed-species plantations are among the major types ofrestoration practices currently being implemented throughoutthe world (Lamb 1998).

An important aspect of forest restoration is to understand thedynamics of key nutrients and the feedbacks between differenttree species planted and soil nutrient cycling. Because nitro-gen (N) is a limiting nutrient in most terrestrial ecosystems(Verhoeven & Schmitz, 1991; LeBauer & Treseder, 2008),

1 Heshan National Field Research Station of Forest Ecosystem, South ChinaBotanical Garden, Chinese Academy of Sciences, Guangzhou, 510650, P.R. China2 Graduate University of Chinese Academy of Sciences, Beijing, 100049,P.R. China3 Department of Biological Sciences, State University of New York-Binghamton,Binghamton, NY 13902, U.S.A.4 Address correspondence to Z. Li, email [email protected]

© 2010 Society for Ecological Restoration Internationaldoi: 10.1111/j.1526-100X.2009.00642.x

soil N availability exerts strong controls on primary produc-tion not only in temperate and boreal forests (Reich et al.1997), but also in tropical and subtropical forests (Davidsonet al. 2004; LeBauer & Treseder 2008; Siddique et al. 2008).For example, due to fast growth of plants, soil N availabil-ity usually becomes a limiting factor of lumber production ineucalypt plantations (Nzila et al. 2002; Smethurst et al. 2004).Soil N availability could also regulate the rate and trajectoryof ecosystem development in other types of tropical and sub-tropical plantations (Yu & Peng 1996; Smith et al. 1998). Insubtropical southern China, the amelioration of N fertility byleguminous Acacia spp. has been found to stimulate the colo-nization of native species and increase native biodiversity (Yu& Peng 1996). In tropical Costa Rica, N availability has beenfound to limit plant growth and affect species composition anddiversity (Robertson 1984).

Nitrogen mineralization, a major process supplying mineralN to plants in terrestrial ecosystems, is a microbial processthat is regulated by many abiotic and biotic factors (plantspecies composition, soil microorganisms, soil temperature,moisture, pH and the quantity and quality of SOM, amongothers). Previous studies have shown that plant species havemajor impacts on soil N mineralization (Vuuren et al. 1992;Knoepp & Swank 1998). Vuuren et al. (1992) found that earlysuccession species tended to maintain lower N mineralization

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Nitrogen Mineralization and Leaching in Reforestation

rates than the late succession species. In Brazil, the dominanceof legume trees in a mixed plantation resulted in higher soilnitrification over nitrate immobilization (Siddique et al. 2008).

Southern China had 25 million hectares forest plantation,with most of them established as monocultures of introducedexotic plants (SFA 2005; Ren et al. 2007a). In recent years,due to increasing concerns on biodiversity, native speciesplantation was encouraged in establishing “Ecological forests”in southern China. The cycling of N in these forest plantationsis of particular interest because most of plantations in southernChina are generally located on nutrient-poor soils (Ren et al.2007a). However, the relationships between plant speciescomposition and diversity and soil N cycling are poorlyknown. A better understanding on such plant–soil interactionsis important for promoting restoration on degraded lands,designing better sustainable agroforestry systems, selectingspecies for biodiversity considerations, and improving currentbiogeochemical models (Russell et al. 2007).

In 2005, five plantations and one unplanted, later natu-rally colonized shrubland were established in the southernChina restoration field station for studying the feedbacksbetween aboveground plant species composition and diversityand belowground soil nutrient cycling, microbial diversity, andC and N accumulations. The study employed rigorous experi-mental design with plot size of 1 ha each to accommodate “realfield” conditions and to allow multiple experimental projects.In this paper, we report a full-year study of N mineraliza-tion and N leaching measurements in the early stages of theseplantations. We hypothesized that: (1) exotic Eucalyptus andAcacia were different from native species in their influence onsoil N transformation; (2) N mineralization, nitrification andleaching loss would vary seasonally, in line with the abioticchanges of temperature and precipitation; and (3) the plantingtreatment effects (i.e. exotics vs. natives) on soil N processeswere seasonally dependent.

Methods

Study Area

The experimental site is located at the Heshan National FieldResearch Station of Forest Ecosystem (HNFRS, 60.7 m a.s.l.,112◦50′E, 22◦34′N), in the subtropical region of southernChina. The soil type is acrisols developed from sandstone,with a pH of about 4.0. The climate of the region istypically subtropical monsoon, with mean annual temperatureof 22.6◦C. The warm and humid growing season (from Aprilto September) covers the summer (June to August), part ofspring (March to May) and fall (September to November),whereas the cool and dry season (from October to March)covers the winter (December to February) and part of fall andspring (Fig. 1). In 2007, the highest monthly precipitation wasfound in August and the lowest in November (Fig. 1; Chenet al. 2009).

In 2005, an ecological restoration project was initiated at thestation. In March, an area of 50 ha monoculture Masson pine(Pinus massoniana) plantation was cut and residue burned.

Figure 1. Monthly soil temperature (measured at 5 cm soil depth) andrainfall at HNFRS in 2007.

Five tree plantations plus an unplanted control (later naturallydeveloped into a shrubland) were established after the burningwithout soil ripping. Each treatment was replicated three times,with each replicate plot an area of 1 ha (total 18 plots).All plots were randomly distributed in the 50 ha study area.The five plantations were: Eucalyptus urophylla (Myrtaceae)monoculture, Acacia crassicarpa (Leguminosae) monoculture,Castanopsis hystrix (Fagaceae) native species monoculture,10-species mixture and 30-species mixture (Table 1). Treesaplings in tube stocks with similar height (50–100 cm) wereplanted at 3 × 2 m spacing in each plot. In mixture treatments,different species were planted randomly. Table 2 containsdetailed plant information in the experimental plots sincethe planting. In 2007, all A. crassicarpa roots were foundnodulated and the nodules varied from 1 to 2 mm in lengthand 0.5 to 1 mm in width (Wang, personal observation).

Soil Collection and Analyses

In 2007, an in situ soil-core technique (Raison et al. 1987)was used to estimate soil net nitrogen mineralization andnitrification rates. Briefly, in each replicate plot, nine subplotswere randomly located. At each subplot, three polyvinylchloride tubes 4.6 cm (diameter) × 15 cm (height) werehammered into a depth of 10 cm. One of the three tubes fromeach subplot was retrieved and sent to the laboratory (S0).The other two tubes were incubated in situ for 1 month beforebeing retrieved. Of these two incubation tubes, one had anopen top to allow rainfall to enter and pass (S1), whereas theother had a lid on the top and some holes on the sidewall foraeration (S2). Initial soil samplings were made in March, June,September, and December, at the beginnings of the four typicalseasons of spring, summer, fall, and winter in this region.

All soil cores were stored at 4◦C and extracted for mineral Nwithin 48 hours after the sampling. Before extraction, each ofthe nine cores from the same plot was divided into two sections(0–5 and 5–10 cm) and soils from the same section (from thesame plot) were pooled and mixed thoroughly. Bulk densitywas calculated based on soil weight. Visible roots and stoneswere removed manually. Twenty grams of manually sortedfresh soil from each section were extracted with 100 mL of

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Table 1. Species planted in the five reforestation treatments in 2005 at HNFRS in southern China.

Species Name Family Name Plantations Species Characteristics

Eucalyptus urophylla Myrtaceae Monoculture Exotic, fast growthAcacia crassicarpa Leguminosa Monoculture Exotic, fast growthCastanopsis hystrix Fagaceae Monoculture∗ Native speciesLiquidambar formosana Hamamelidaceae 10-species mixture∗ Native speciesMagnoliaceae glanca Magnoliaceae 10-species mixture Exotic speciesMachilus chinensis Lauraceae 10-species mixture Native speciesCinnamomum burmanii Lauraceae 10-species mixture Native speciesTsoongiodendron odorum Magnoliaceae 10-species mixture Native speciesJacaranda acutifolia Bignoniaceae 10-species mixture Exotic speciesBischofia javanica Euphorbiaceae 10-species mixture Native speciesSchima superba Theaceae 10-species mixture Native speciesDillenia indica Dilleniaceae 10-species mixture Exotic speciesMichelia macclurei Magnoliaceae 30-species mixture Native speciesOrmosia pinnata Leguminosae 30-species mixture Native speciesDelonix regia Leguminosae 30-species mixture Exotic speciesSterculia lanceolata Sterculiaceae 30-species mixture Native speciesGarcinia oblongifolia Guttiferae 30-species mixture Native speciesGarcinia cowa Guttiferae 30-species mixture Native speciesPterocareus indicus Leguminosae 30-species mixture Exotic speciesDracontomelon dao Anacardiaceae 30-species mixture Native speciesElaeocarpus japonicus Elaeocarpaceae 30-species mixture Native speciesCinnamomum parthenoxylon Lauraceae 30-species mixture Native speciesRadermachera sinica Bignoniaceae 30-species mixture Native speciesMaesa japonica Myrsinaceae 30-species mixture Native speciesDolichandrone caudafelina Bignoniaceae 30-species mixture Native speciesMichelia chapensis Magnoliaceae 30-species mixture Native speciesSyzygium cumini Myrtaceae 30-species mixture Native speciesElaeocarpus apiculatus Elaeocarpaceae 30-species mixture Native speciesGrevillea robusta Proteaceae 30-species mixture Exotic speciesCastanopsis fissa Fagaceae 30-species mixture native speciesAcronychia pedunculata Rutaceae 30-species mixture Native speciesSchefflera octophylla Araliaceae 30-species mixture Native species

∗ Castanopsis hystrix was also used in the 10-species mixture, and all plants in the 10-species mixture were also used in the 30-species mixture.

Table 2. Vegetation description of the five reforestation treatments (plus a shrubland control) at HNFRS (data were collected in October 2006, 1 yearafter initial planting, unless otherwise noted).

Overstory Tree Species Major Understory Species

Plantations Name Height (m) dbh(cm) Coverage (%) Name Coverage (%)

Eucalyptus urophylla E. urophylla 3.31 3.01 51 Rhodomyrtus tomentosa 15(11.9)∗ (9.1)∗ Miscanthus sinensis 4

Acacia crassicarpa A. crassicarpa 2.77 2.28 62 Rhodomyrtus tomentosa 10(6.3)∗ (6.4)∗ Miscanthus sinensis 5

Clerodendrum fortunatum 2Castanopsis hystrix C. hystrix 0.79 0.80 1–2 Dicranopteris dichotoma 15

Rhodomyrtus tomentosa 4Baeckea frutescens 4

10 mixture 10 species <1.0 — 1–2 Dicranopteris dichotoma 18Rhodomyrtus tomentosa 9Baeckea frutescens 3

30 mixture 30 species <1.0 — 1–2 Dicranopteris dichotoma 20Miscanthus sinensis 5Rhodomyrtus tomentosa 3

Shrubland Trema tomentosa and Litsea cubeba <2.0 — 2 Dicranopteris dichotoma 50Miscanthus sinensis 20Rhodomyrtus tomentosa 3

∗ Data in parentheses were recorded in January 2008, 34 months after the initial planting.

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2 M KCl solution (1:5 ratio) and filtered (Shuangquan quan-titative filter paper 202#). Concentrations of ammonium andnitrate in the filtered solution were determined by a flow injec-tion autoanalyzer (FIA, Lachat Instruments, Loveland, CO,U.S.A.): ammonium by the salicylate–nitroprusside methodand nitrate by the sulfanilamide method after the Cd-corereduction to nitrite. Soil moisture was determined by weightloss after oven drying at 105◦C for 24 hours.

Net N mineralization was calculated as the increase ofammonium plus nitrate N between the initial soil sample (S0)and the incubated sample (S2), whereas net nitrification wascalculated as the increase of nitrate N. Nitrogen leaching wascalculated as the difference of the ammonium plus nitrate Nbetween the covered soil core (S2) and the open core (S1).Net N mineralization, net nitrification and leaching rates in theunit of mg N m−2 month−1 (0–10 cm soil) were computed asthe summation in the 0–5 and 5–10 cm layers. Although themethod used here may contain some “artificial” effects (e.g.root severing and the exclusion of root on soil moisture) on soilN processes (Jussy et al. 2004), many researchers believed thatsuch effects were minor and that in situ incubation providedthe best estimates of N availability in forest soils (Adams et al.1989; Zhu & Carreiro 2004). For leaching, the method usedhere represented the upper most potential estimation becausethe exclusion of plant roots could dramatically increase theactual leaching loss (Raison et al. 1987).

Annual net N mineralization and nitrification rates in the0–10 cm soil were calculated as:

A =∑

Si × 3/100

Where A represents the annual net N mineralization andnitrification rates (kg N ha−1 yr−1), Si represents the measuredmonthly N mineralization and nitrification rates (mg N m−2

month−1) in each season. Three and 1/100 represents theconversion constant from month to season, and from mg N/m2

to kg N/ha, respectively. N mineralization and nitrificationrates over the growing season were estimated from the ratesmeasured in spring, summer, and fall.

General soil chemical properties were determined using thesoil samples collected in March 2007. Soil pH was measured

at 1:5 mixture of soil:deionized water. Soil available P wasextracted with Bray-2 solution (Bray & Kurtz 1945) anddetermined by the molybdate blue colorimetric method. Soilswere ground to pass through a 0.25-mm sieve and thenanalyzed for total N (TN), total P (TP), and organic matter.Total N and TP concentration was determined by the micro-Kjeldahl digestion, followed by colorimetric determination onLachat FIA. Soil organic carbon (SOC) was determined bywet combustion method, with SOM calculated as: SOM =1.73 × SOC (Liu et al. 1996). Soil C/N ratio was calculatedas the SOC to soil TN mass ratio.

Statistical Analyses

The rates of soil N mineralization, nitrification, N leach-ing loss, and the concentrations of extractable ammoniumand nitrate were analyzed by a two-way analysis of variance(ANOVA) with planting treatment (five plantations plus oneshrubland) and season as the main factors. Least significantdifference (LSD) post hoc test was used to compare the effectsof planting treatment on the above variables at each season.General soil properties (pH, available P, TN, TP, organic mat-ter content (SOM), C/N mass ratio and bulk density) wereanalyzed by a two-way ANOVA testing plant and soil layer(0–5 and 5–10 cm layers) effect, followed by the LSD posthoc test. Variance equality of data was tested by Levence’stest. Because the variances of N mineralization, nitrification,N leaching, extractable ammonium, and nitrate were not homo-geneous, a non-parameter rank analysis (Scheirer–Ray–Hare’stest) was used for these variables (Scheirer et al. 1976). Allanalyses and computations were performed on SPSS 15.0(SPSS Inc., Chicago, IL, U.S.A.) and Excel 2003 (MicrosoftCorp., Redmond, WA, U.S.A.).

ResultsGeneral Soil Properties

Soil pH, available P, TN, TP, organic matter content (SOM),and C/N mass ratio were not significantly different amongdifferent planting treatments in both 0–5 and 5–10 cm soils(Table 3, data in 5–10 cm soils are not shown). Above

Table 3. General soil chemical and physical properties (0–5 cm soils) 2 years after the initial planting at HNFRS.

Variables EU AC CH 10-Mixed 30-Mixed SL

Soil pH 4.01 ± 0.05 3.87 ± 0.08 3.90 ± 0.06 3.85 ± 0.08 3.93 ± 0.08 3.91 ± 0.05Available P (mg/kg) 1.36 ± 0.03 1.33 ± 0.52 0.79 ± 0.40 1.56 ± 0.32 0.79 ± 0.47 1.10 ± 0.42Total N (g/kg) 0.79 ± 0.09 0.75 ± 0.10 0.98 ± 0.25 0.93 ± 0.22 0.88 ± 0.18 0.53 ± 0.07SOM (g/kg) 31.3 ± 7.5 38.1 ± 3.2 31.1 ± 5.4 39.9 ± 4.4 30.5 ± 3.9 24.6 ± 4.3Total P (g/kg) 0.23 ± 0.07 0.36 ± 0.02 0.18 ± 0.04 0.23 ± 0.03 0.18 ± 0.05 0.32 ± 0.08C/N ratio 23.4 ± 6.3 30.1 ± 3.2 21.5 ± 6.0 31.6 ± 3.8 21.6 ± 5.0 28.4 ± 6.4Bulk density (g/cm3) 0.99bc ± 0.01 0.96c ± 0.02 1.01bc ± 0.02 1.04ab ± 0.04 1.09a ± 0.02 0.99bc ± 0.02

EU, Eucalyptus urophylla monoculture; AC, Acacia crassicarpa monoculture; CH, Castanopsis hystrix monoculture; 10-mixed, 10-species mixture; 30-mixed, 30-speciesmixture; SL, unplanted shrubland.Data are means ± SE, n = 3. All data came from the soil sampled in March 2007 except the bulk density values, which were the means from soil samples collected in June,September, and December 2007. Superscript letters indicated a significant difference (p = 0.05) was detected by one-way ANOVA and means sharing the same superscript letterwere not significantly different at p = 0.05 (LSD).



variables in the 5–10 cm soils were significantly lower thanthose in the 0–5 cm soils (p < 0.05 for all variables).

Nitrogen Mineralization and Nitrification

As we hypothesized, both of planting treatment and season hadsignificant effects on the in situ rates of net N mineralization(p < 0.05 and p < 0.001, respectively, Fig. 2a). In March,June, and September, net N mineralization rates were the low-est in the fast-growing E. urophylla monoculture and highestin the 10-species mixture (Fig. 2a). The difference betweenthe E. urophylla monoculture and the 10-species mixture wasstatistically significant in spring (p < 0.05, Fig. 2a). Anotherexotic species, A. crassicarpa monoculture, had the secondlowest rates of N mineralization most of the time. These resultswere partly consistent with our first hypothesis. Seasonal vari-ation of N mineralization was very large (p < 0.001, Fig. 2a).Nitrogen mineralization rates were the highest in March, thensteadily decreased in June and September, until all measuredrates became negative in December (Fig. 2a). Although bothplanting treatment and season significantly affected the soil Nmineralization rate, there was no interaction of these two mainfactors (Fig. 2a).

Patterns of soil net nitrification were similar to net N min-eralization, with both planting treatment and seasonal effectsbeing highly significant but without interaction (p < 0.01 andp < 0.001, respectively, Fig. 2b), indicating a close linkagebetween the two processes. There was a significant differencebetween the 10-species mixture and the E. urophylla mono-culture in the spring and summer months (p < 0.05, Fig. 2b).The leguminous A. crassicarpa always had intermediate ratesof nitrification and was not significantly different from otherplanting treatments (Fig. 2b).

Annual net N mineralization and nitrification rates were boththe lowest in the E. urophylla monoculture (13.5 and 9.98 kgN ha−1 yr−1, respectively), and were significantly lower thanthose in the 10-species mixture (p < 0.05, Table 4). Theleguminous A. crassicarpa plantation had intermediate ratesof N mineralization (25.4 kg N ha−1 yr−1) and nitrification(19.2 kg N ha−1 yr−1). The annual rates were affected by thenegative values in December, when plants were in dormancyand high levels of microbial N immobilization seemed to beoccurring, likely fueled by extremely high concentrations ofammonium at the beginning of the winter incubation (Fig. 3a).We thus also calculated the N mineralization and nitrificationrates based on the first three sampling dates (growing season Nproduction most relevant to plant uptake) and included those inTable 4. The growing season N mineralization and nitrificationhad similar patterns as the annual rates. Annual growingseason net N mineralization rates between the E. urophyllamonoculture (24.6 kg N ha−1 yr−1) and the 10 species mixture(58.1 kg N ha−1 yr−1) showed a statistically significant (p <

0.05) difference.

Nitrogen Leaching

The calculated potential leaching rates were highly variableboth within treatment and among treatments and showed

no overall statistical difference among plantation types buta strong seasonal difference (p < 0.001, Fig. 2c). In June,as much as 746 mg N m−2 was leached out from the 10-species mixed plantation, which was about 500 mg N m−2

higher than those from the E. urophylla monoculture and theA. crassicarpa monoculture (p < 0.05, Fig. 2c). The highestleaching occurred in June, during the rainy season, and thelowest in December, the dry season (Figs. 1 & 2c). Therewas also no interaction between planting treatment and season(Fig. 2c).

Extractable Soil Inorganic N

Over the entire year, the differences of soil extractableammonium among planting treatments were not significant ineither 0–5 or 5–10 cm soils (Fig. 3a, data of 5–10 cm arenot shown), but the seasonal effects were highly significantin both soil layers (p < 0.001, Fig. 3a). Although noduleswere observed in the roots of A. crassicarpa, soil ammoniumunder A. crassicarpa was not significantly different fromother planting treatments. In the 0–5 cm soils, the highestammonium concentration was found in December, which wasat least 1.5 mg/kg higher than those in other seasons (Fig. 3a).The 5–10 cm soils had the similar inter-species and seasonalpatterns as the 0–5 cm soils.

Unlike ammonium, there was a significant difference insoil extractable nitrate among different plantations (p < 0.001for both layers, Fig. 3b), whereas the seasonal effects wereonly significant in the 0–5 cm soils (p < 0.01, Fig. 3b, datain 5–10 cm were not shown). Nitrate concentrations werelower in the exotic E. urophylla and A. crassicarpa plantationsthan those in the native species plantations and the shrubland(Fig. 3b). In June, surface layer (0–5 cm) nitrate concentrationunder the 10-species mixture was markedly higher than thoseunder the E. urophylla monoculture and the A. crassicarpamonoculture (p < 0.05; Fig. 3b). As for seasonal variations,the highest nitrate concentration in 0–5 cm soils was found inJune and the lowest in September (Fig. 3b), but the differencebetween the seasons was generally small. In the 5–10 cmsoils, the pattern of nitrate among plantations and seasons wassimilar to those in the 0–5 cm layer although both plantationand seasonal differences were much smaller.

Discussion

N Mineralization and Nitrification

Soil net N mineralization rates under E. urophylla monoculturewere the lowest among the six planting treatments (Table 4)and were 30–60% lower than those under the native mix-ture plantations and unplanted shrubland. These results areconsistent with our hypothesis that exotic E. urophylla couldgreatly change soil N transformations in comparison withnative species. O’Connell (2003) reported that in southwesternAustralia, 6–11 years eucalypt plantations had 30% lower Nmineralization rate than unplanted pasture. After O’Connell’s



Figure 2. Soil net N mineralization (a), net nitrification (b), and potential inorganic N leaching loss (c) in the 0–10 cm soils of six reforestationtreatments at HNFRS in 2007. Data are means + SE (n = 3). Bars in a particular sample time sharing the same superscript letter were not significantlydifferent at p = 0.05 (LSD). Overall plantation and seasonal effects from the two-way ANOVA were provided for each sub-figure.



Table 4. The annual and growing season (without winter data) net N mineralization, nitrification, and N leaching rates (kg N ha−1 yr−1) in the 0–10 cmsoils in 2007 at HNFRS.

Net N Mineralization (kg N ha−1 yr−1 ) Nitrification (kg N ha−1 yr−1 ) N Leaching (kg N ha−1 yr−1 )

Plantation Annual Growing Seasons Annual Growing Seasons Annual

EU 13.5b ± 0.42 24.6b ± 0.72 9.98b ± 1.19 9.18b ± 0.81 11.7 ± 0.94AC 25.4ab ± 5.57 34.9ab ± 5.58 19.2b ± 5.52 18.0b ± 5.40 11.1 ± 4.37CH 20.0b ± 12.2 32.9b ± 13.0 18.0b ± 8.23 18.2b ± 9.00 21.9 ± 14.210-mixed 45.3a ± 9.96 57.1a ± 6.66 38.0a ± 6.45 37.8a ± 4.77 30.3 ± 3.4130-mixed 30.1ab ± 7.12 42.5ab ± 6.12 24.4ab ± 4.88 22.0ab ± 3.45 12.7 ± 3.13SL 25.0ab ± 6.88 36.0ab ± 7.38 26.2ab ± 5.58 24.6ab ± 3.87 25.7 ± 10.6

EU, Eucalyptus urophylla monoculture; AC, Acacia crassicarpa monoculture; CH, Castanopsis hystrix monoculture; 10-mixed, 10-species mixture; 30-mixed, 30-speciesmixture; SL, unplanted shrubland.Data are means ± SE, n = 3. Superscript letters indicate a significant difference (p = 0.05) detected by the one-way ANOVA. Means sharing the same superscript letter werenot significantly different at p = 0.05 (LSD).

Figure 3. Soil extractable ammonium (a) and nitrate (b) in the 0–5 cmsoils under the six reforestation treatments at HNFRS in 2007. Data aremeans + SE (n = 3). Bars in a particular sample time sharing the samesuperscript letter were not significantly different at p = 0.05 (LSD).

study, Mendham et al. (2004) investigated the difference ofsoil N mineralization between 11–12 years eucalypt planta-tion and pasture in 10 more sites, and found an even greatermagnitude of decline (50%). Consistent with the above stud-ies, our results indicated that eucalypt plantation could reducesoil N mineralization even 2–3 years after their planting.One explanation for this is that eucalypt litters cause soil

N immobilization. Aggangan et al. (1999) reported markedreductions in N mineralization when eucalypt litter was addedto pasture soil incubated in the laboratory. They hypothesizedthat leaching of soluble carbon compounds from the eucalyptlitter could contribute to the immobilization of soil mineralN. The work by Corbeels et al. (2003) also indicated thateucalypt leaf residues, whether applied to the soil surface orincorporated into soil, caused immobilization of mineral Nand acted as an effective sink for mineral N. Therefore, thisimmobilization process is an important cause of the low soilN mineralization in eucalypt plantations and could result inshort-term limitations in mineral N supply for plant growth(O’Connell et al. 2003).

Despite being an N-fixing species (nodules were observedin 2007), A. crassicarpa monoculture did not have higher Nmineralization or nitrification rates in comparison with non-fixing native species. This result was in contrast to the initialhypothesis 1 and was also different from those reported byLi et al. (2001), who found that soil N mineralization wasthe highest in the 13-year-old Acacia plantations in a nearbyreforestation site, compared with other non-legume plantationsin the region. The higher litter mass and litter N content ofAcacia spp. might be responsible for the high N mineralizationrates in the medium-age Acacia stands. Many studies showedthat forest floor litter mass and litter quality were key factorsdetermining soil N mineralization rates (Steltzer & Bowman1998; Booth et al. 2005). In the present study, the litterfall inAcacia plantations was limited (Wang, personal observation).We also did not find any significant relationships betweenN mineralization rates and soil quality indices, such as C/Nratios, SOM, and TN. As strong correlations between Nmineralization and soil indices are likely the result of long-term feedbacks between litterfall, microbial mineralization,and plant nutrient uptake, in the longer time frame, Acaciaplantation could still contribute positively to soil N availability.

Although N mineralization rate was the highest in the 10-species mixture (45.3 kg N ha−1yr−1), overall the rates werelow. In a climax monsoon evergreen broadleaf forest, 45miles from our study site, Fang (2006) reported that monthlyN mineralization rate in summer was about 6.7 kg N ha−1

month−1, which was 10% higher than that in our 10-species



mixture (5.73 kg N ha−1 month−1), and over two timeshigher than those in the E. urophylla monoculture (2.29 kgN ha−1 month−1). The values reported in our study were alsolower than those found in a nearby 23 years restoration site(7.41–11.3 kg N ha−1 month−1 in June, unpublished data).Li et al. (2006) studied an evergreen broad-leave forest inYunnan, China, and found that annual N mineralization in0–15 cm soils was 159.12 kg N ha−1. Compared with thesenatural and old growth forests, soils in our young plantationswere less fertile, with lower SOM, lower TN and loweravailable N. Such low-fertility soils are typical in the region,widely distributed in the subtropical southern China (Renet al. 2007a).

As reported in other studies (Knoepp & Swank 1998; Zhu& Carreiro 1999), soil nitrification and N mineralization inthis study are significantly correlated (r2 = 0.73, p < 0.01,n = 72). Previous studies have shown that soil ammoniumgoverned the process of nitrification (Robertson 1984; Zhu &Carreiro 1999). Pandey et al. (2007) observed that high soilammonium led to a rapid increase in nitrification. However, atthe onset of winter, we observed that higher soil ammoniumconcentration was associated with a lower rate of nitrification,suggesting that other environmental factors, such as soilmoisture, also exert controls on nitrification. On the otherhand, the large accumulation of ammonium at the beginningof winter could also stimulate net N immobilization andlead to calculated negative N mineralization rates in winter(Figs. 2 & 3)

Seasonal changes of N mineralization and nitrificationwere often ascribed to seasonal variations of temperatureand moisture (Maithani et al. 1998; Smith et al. 1998). Inthis study, there was a significant quadratic relationshipbetween soil moisture and N mineralization (r2 = 0.33 andp < 0.001, data not presented), indicating the regulation ofN mineralization by soil moisture. The rapid reduction ofN transformations in winter can be explained by the alteredtemperature and water availability, which directly regulate soilmicrobial activity (Yan et al. 2008). However, in the earlystages of plantation, other factors, including the substrateavailability (i.e. SOM content) may also be important. Forexample, the greater N mineralization in spring may be due tothe accumulation of organic matter from the previous growingseason, with minimal microbial activities in winter due to lowtemperature and in our sites, low precipitation (Fig. 1). On theother hand, we did not find any significant plantation × seasoninteractions for N mineralization and nitrification, rejecting thethird hypothesis (Fig. 2).

Nitrogen Leaching

Soil N leaching quantified by the method used here (thedifference between capped and open incubation cores, seeMethods) is an estimate of N leaching potential, without theN uptake by plants (Raison et al. 1987). Thus, it may over-estimate the actual leaching rates, but it is a reliable (andsimple) way to compare N leaching potential among differentplantations and seasons. Nitrogen leaching differed strongly

among seasons but no statistical difference was found amongdifferent plantation treatments (Fig. 2c). Seasonal variation ofrainfall mainly accounted for the dynamics of N leaching. Inthis region, most rainfall events happen between April andSeptember (Fig. 1), a time of strong plant demand on N.Fang et al. (2008) also found peak N leaching in the rainygrowing season using a zero-tension lysimeter technique inthe regional climax forests, suggesting hydrologic-driven Nloss in the rainy growing season could override plant-drivenN uptake and lead to a large amount of N loss in southernChina, even under N-limited conditions.

As reported in other studies (Wang et al. 2006), there weresignificant relationships between annual N leaching rates andannual N mineralization rates (r2 = 0.42 and p < 0.001),and nitrification rates (r2 = 0.42 and p < 0.001), suggestingclose connections among various N fluxes. In both spring andsummer, leaching was significantly lower in the fast-growingeucalypt plantations than the 10-species mixture and the nativeC. hystrix monoculture. This is in line with the lower Nmineralization, nitrification, and extractable nitrate found inthe eucalypt soils (Figs. 2 & 3).

Implications for Reforestation

The results of this study suggested that N availability couldlimit productivity in the early stage of reforestation, especiallyfor the two fast-growing exotic species. The annual N demandin a 3-year-old E. urophylla monoculture was estimated at48 kg ha−1yr−1 in southern China (Lin et al. 2002). The lowN mineralization rate (13.5 kg ha−1yr−1) and nitrate content(Fig. 3) found in the soils of the E. urophylla monoculturein this study suggests an N limitation for plant growth.The similar low N mineralization rate was also observed inother studies. In Brazil, Goncalves et al. (1999) reported a16 kg N ha−1yr−1 N mineralization rate in a 21-month-oldE. grandis monoculture. Soil fertilization practices shouldbe considered to address the apparent N limitation in theseyoung eucalypt plantations. In addition, residue managementduring soil preparation may be another way to retain N.In the Brazil study, Goncalves et al. (1999) suggested thatthe residue was an important source for soil N cycling andplant uptake, and burning of residues lead to substantialN volatilization to the atmosphere. In western Australia,O’Connell et al. (2004) found that residue retention couldgreatly increase N mineralization in the second rotation ofE. globulus plantations.

Nitrogen is not the only limiting soil nutrient in these youngplantations. In south China, phosphorus is often deficientin soils (Xu et al. 2002). Phosphorus is a key nutrientthat constrains productivity of plantations, especially eucalyptplantation (Xu et al. 2002, 2005). In this study, the soilavailable P in the 0–5 cm layer was less than 1.5 mg/kg, andits concentration declined greatly with soil depth (Table 3).However, in a nearby 22-year-old forest, soil available P inthe 0–10 cm soil layer was over 3.1 mg/kg (Xiong et al.2008). We also found that the average P concentrations in theliving leaf of these young E. urophylla and A. crassicarpa



plantations were 0.95 and 0.71 mg/g, respectively (Zhanget al. unpublished data), which were much lower than theconcentrations in fertilized sites of this region (Liang et al.2004; Wang et al. 2005).

Selection of plant species plays an important role in for-est restoration and the probability of restoration success willincrease by selecting species most suitable to the environ-mental conditions at the site (Eviner & Hawkes 2008). Theuse of exotic species, especially fast-growing species, is ingreat dispute, largely due to biodiversity concern (Lamb 1998;D’antonio & Meyerson 2002). In this study, although lowersoil N availability was found under the exotic plantations, thelow N leaching loss in these plantations could be the result ofN retention in the biomass and could be helpful in restoringdegraded land in the region, especially coupled with the secondrotation of native plantations (Ren et al. 2007b). Therefore,the use of exotic species versus native species in reforesta-tion should be site dependent and should be considered over alonger time frame. In poor soils with high leaching potential,although native species may be better suited for biodiversityand future ecosystem development, exotic species grown herewere better at reducing nutrient loss. Thus, evaluation of thepros and cons of exotic species versus native species selectionin reforestation is critical.

Implications for Practice

• In forest restoration, the plant–soil interaction shouldbe well considered. A thorough understanding of plantspecies effects on soil N availability could guide man-agement decisions and provide a critical foundation foradvancing restoration strategies.

• The fast-growing Eucalyptus species are used worldwidefor timber plantation and reforestation. Based on theresults of this study and others, in the early stages ofEucalyptus plantation, soil fertilization (N and P) appearswarranted.

• Due to their lower N leaching and higher growth rates,Eucalytus and Acacia species may be used as pioneerspecies in reforestation, especially for restoring degradedland severely impacted by rainfall leaching.

• Native species mixture and naturally colonized shrublandmay have higher levels of soil N availability; however,the high potential N leaching loss may lead to soilnutrition depletion in the early stages of reforestationand retard forest development later.

Acknowledgments

The project was supported by the National Natural ScienceFoundation of China (30630015), the National Basic ResearchProgram of China (973 Program 2009CB421101) and Chi-nese Academy of Sciences Knowledge Innovation Program(KSCX2-SW-133, KZCX2-YW-413). We would also like tothank Miranda Kearney at Binghamton University, and twoanonymous reviewers who helped to improve the quality of

this paper. [Grant number for the National Natural ScienceFoundation of China corrected since online publication dateof March 12, 2010]

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Nitrogen Mineralization and Leaching in the Early Stages of a Subtropical Reforestation in Southern China