Acta Ecologica Sinica 30 (2010) 123–128

Contents lists available at ScienceDirect

Acta Ecologica Sinica

journal homepage: www.elsevier .com/locate /chnaes

Nitrogen released from different plant residues of the Loess Plateau and theiradditions on contents of microbial biomass carbon, nitrogen in soil

Zhou Jian-Bin *, Chen Xing-Li, Zhang Ying-Li, Liu Jian-LiangCollege of Resource and Environment Science, Northwest A&F University, Yangling, Shaanxi 712700, China

a r t i c l e i n f o

Keywords:Loess Plateau

Plant residuesN mineralizationSoil microbial biomass carbon and nitrogen

1872-2032/$ - see front matter � 2010 Ecological Socdoi:10.1016/j.chnaes.2010.04.001

* Corresponding author.E-mail address: jbzhou@nwsuaf.edu.cn (J.-B. Zhou

a b s t r a c t

An incubation method was used to investigate the nitrogen release characteristics from the residue of tenplant species which commonly grow in the northern part of the Loess Plateau. The effect of the residue onsoil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) was also determined.There were significant differences in the total N content and the C/N ratios among the different types ofplant residue. The total N content of the residues ranged from 6.61 to 32.78 g kg�1. The C/N ratio of theresidue ranged from 14 to 65. There was an immediate increase in soil N after alfalfa, erect milkvetch, andkorshinsk peashrub residue was added to the soil. In contrast, soil N decreased after elm, sea buckthorn,and wild peach residue was added to the soil. The soil N content remained relatively low for 14–34 daysand then increased. This indicated that N immobilization occurred during the early portion of the incu-bation period when elm, sea buckthorn and wild peach residue was added to the soil. Soil N levels werelow during the entire incubation period when simon poplar, locust, Stipa bungeana, and old world blue-stem residue were added to the soil. The addition of plant residue significantly increased SMBC and SMBNin all treatments. The SMBC and SMBN values were greatest in treatments containing plant residue withhigh total N content and low C/N ratios. The C/N ratios of korshinsk peashrub, sea buckthorn, and wildpeach residues were similar, but the amount of N released from these residues and the effects of the res-idue on SMBC and SMBN in soil were significantly different. This indicates that not only the C/N ratio butalso the chemical composition of the plant residue affected decomposition. It is important to consider Cand N release characteristics from plant residue in order to adjust the C and N balance of soil when rev-egetating degraded ecosystems.

� 2010 Ecological Society of China. Published by Elsevier B.V. All rights reserved.

1. Introduction

The amounts and types of plant residues returned into in terres-trial ecosystem affect the fertility of soil [1]. Microbes in ecosystemplay an important role in element and energy flows in soil ecosys-tem as the driving forces for transformation of plant residues insoil; and nutrients in microbial biomass also involve in the nutrientcycles in soil ecosystem [2]. Therefore, soil microbial biomass isused as an important index to evaluate the effects of different cul-tivation and management on quality and function of soil ecosys-tem [3–6].

The Loess Plateau is a region suffering from very serious soilerosion in the world due to the deteriorating vegetation by defor-estation and inadequate cultivation management. Severe soil ero-sion results in the divided land, poor soil, and weak stabileecosystem. In recent years, ecological restoration is a key issue indeveloping the west region of China [6]. Different measures ofreestablishing the vegetation in the region, such as change the

iety of China. Published by Elsevie

).

marginal crop land into forest and grassland with trees, shrubsand grass, are used to control soil and water erosion. There has aseries of researches to study the effects of ecological restorationon soil physical and chemical properties in the region [7,8]. Nitro-gen is a factor limited the plant growth in most ecosystems. How-ever, the nitrogen release from the residues of these plants is lessunderstood, and there is also less research to study the effects ofaddition of different residues on soil microbial biomass carbonand nitrogen.

Therefore, the objectives of this study were to evaluate the ef-fects of different residues of plant species from the Loess Plateauon immobilization and mineralization of N in soil and on soilmicrobial biomass carbon and nitrogen.

2. Materials and methods

2.1. Materials

The soil used was sampled from Shenmu Experimental Stationof Institute of Soil and Water Conservation, CAS, in fall of 2006.

r B.V. All rights reserved.

124 J.-B. Zhou et al. / Acta Ecologica Sinica 30 (2010) 123–128

The previous crop was soybean (Glycine max (L.) Merr), and soilwas Typ-Los-Orthic Entisols. Soil was air-dried, and sieved in2 mm. The basic physical–chemical properties of the soils were:organic carbon, 4.87 g kg�1; total N, 0.57 g kg�1; NO3-N, 32.99mg kg�1; NH4-N, 1.88 mg kg�1; pH 7.90; Olsen-P, 2.58 mg kg�1;clay, 14.7% (<0. 001 mm); slit, 51.1% [9].

Different residues of plant species (trees, shrubs, and grasses)were also sampled around the station, three tree species includedlocust (Robinia pseudoacacia), simon poplar (Populus.simonii Carr)and elm (Ulmus pumila L.); three shrubs were peashrub (Caraganakorshinskii Kom.), Sea buckthorn (Hippophae rhammoides L.), wildpeach (Prunus davidiana (Carr.) Franch); four grasses were Stipabungeana (Stipa bungeana), old world bluestem (Bothriochloaischaemum (L.) Keng), Milkvetch (Astragalus adsurgens Pall), alfalfa(Medicago sativa L.).

These plants are the native species in the region. When sam-pling, the foliar were taken from trees and shrubs, and; the above-ground were taken for grasses. And the samples were washed, andair-dried, and then dried in 60 �C oven, and sieved into 0.5–1.0 mmfor use. And the contents of total C, N and lignin in the residues arepresented in Table 1.

2.2. Incubation experiment

Before the incubation experiment, air-dried soil was pre-incu-bated at 70% of field capacity for a week to activate the soil micro-bial activity. Moisture soil equal to 600 g of air-dried soil wasmixed with 1% of different residues of plant, and added into anincubation box, and incubated in 28 �C incubator. Soil withoutaddition of plant residues was as control. Three replicates for eachtreatment. Sub-sample was taken from each incubation box at 0, 1,3, 6, 10, 16, 24, 34, 46, and 60 days of incubation to determine con-tents of soil microbial biomass carbon and nitrogen. Water wasadded by weight method every 3 days to keep the soil moisturestable during the incubation.

2.3. Determination methods

Soil moisture was determined with oven drying method, andnutrients in soil and plant were determined by conventional meth-od [10]. The lignin was determined with iodometry method [11].Soil microbial biomass carbon and nitrogen were determined withfumigation and extraction method [12].

Briefly, soil sample was fumigated with CH3OH-free CHCl3 for24 h at 25 �C (fumigated samples) to destroy cell membranes ofsoil microorganisms (unfumigated soil as the control). After re-moval of the CHCl3 vapor, the fumigated and unfumigated sampleswere extracted with 0.5 M K2SO4 solution. Total C in the filtrate

Table 1Chemical component properties of the plant residues.

Plant residues

Trees A Ulmus pumila L.B Populus.simonii CarrC Robinia pseudoacacia

Shrubs D Caragana korshinskii KomE Hippophae rhammoides L.F Prunus davidiana (Carr.) Franch

Grasses G Stipa bungeanaH Bothriochloa ischaemum (L.) KengI Astragalus adsurgens PallJ Medicago sativa L.

Average

Coefficient of variation (%)

was determined with TOC analyzer (Phoenix 8000); and totalsoluble N in the filtrate was oxidized with alkaline persulfate oxi-dation, then measured by dual-wavelength ultraviolet spectropho-tometer [13]. Both SMBC and SMBN were calculated as thedifferences in filtrates between fumigated and unfumigated soil.A kC and kN factor for SMBC and SMBN were both 0.45 [12].

2.4. Statistics

Differences between different treatments were evaluated withSAS (8.0) for each examined parameter using the Duncan honestlysignificant difference test at 95% significance level.

3. Results

3.1. Chemical compositions of residues of different plant species

The coefficients of variation (CV) of N contents in the plant res-idues was as high as 47.4%, and alfalfa with the highest N content32.78 g kg�1, old world bluestem with the lowest N 6.61 g kg�1

(Table 1). Compared to the N contents in plant residues, the varia-tion of C contents in plant residues was relatively low (CV = 5.85%);C content in S. bungeana was the highest (499.46 g kg�1), and si-mon poplar was the lowest (414.44 g kg�1). Therefore, it resultedin high variation of C/N ratio of plant residues (CV = 65.8%); thehighest was old world bluestem (C/N = 65.49), the lowest was al-falfa C/N 14.19. The C/N ratio of simon poplar was the highestamong the three tree’s species, next were locust and elm. The var-iation of C/N ratio of three shrubs was small, with average of 15.64.For grasses, the C/N ratios of S. bungeana and old world bluestemwere higher than alfalfa and Milkvetch.

The average lignin contents in the residues was 27.11%, and thehighest was simon poplar (30.89%), the lowest was milkvetch(24.10%), with CV 8.2%. The lignin contents of simon poplar in threetree species was the highest, and next were locust and elm; thelignin contents in three shrub species followed the order: pea-shrub > Seabuckthorn > wild peach. For grasses the lignin contentsof S. bungeana and old world bluestem were higher than alfalfa andMilkvetch.

3.2. Mineral N in different treatments

The differences of mineral N in soil with and without addition ofplant residues during the incubation indicate the net mineraliza-tion of the residues. Positive value indicated the release of N fromthe residues, and negative value indicated the immobilization of N.Additions of simon poplar and locust resulted in net immobiliza-tion of N during the incubation, and N immobilization by simon

TOC (g kg�1) Total N (g kg�1) C/N ratios Lignin (%)

422.58 25.36 16.66 25.20414.44 9.02 45.97 30.89437.73 14.92 29.33 27.78

466.85 30.95 15.08 29.59464.70 29.71 15.64 27.14458.53 28.29 16.21 25.76

499.46 9.86 50.67 27.54432.69 6.61 65.49 28.61427.93 27.01 15.84 24.10464.98 32.78 14.19 24.47

448.99 21.45 28.51 27.11

5.84 47.43 65.81 8.23

-60

-40

-20

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

Incubation time (d)

Min

eral

ized

N (

mg•

kg-1

)

A B C

-60

-40

-20

0

20

40

60

80

100

120

140

Incubation time (d)

Min

eral

ized

N (

mg•

kg-1

)

D E F

-60

-40

-20

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

Incubation time (d)

Min

eral

ized

N (

mg•

kg-1

)

G H I J

0 10 20 30 40 50 60

a b

c

Fig. 1. Nitrogen mineralization from the different plant residues during the incubation. (A) Ulmus pumila L., (B) Robinia pseudoacacia, (C) Populus.simonii Carr, (D) Caraganakorshinskii Kom, (E) Hippophae rhammoides L., (F) Prunus davidiana (Carr.) Franch, (G) Stipa bungeana, (H) Bothriochloa ischaemum (L.) Keng, (I) Astragalus adsurgens Pall, (J)Medicago sativa L. (the same below).

J.-B. Zhou et al. / Acta Ecologica Sinica 30 (2010) 123–128 125

poplar was higher than locust. For elm, net N immobilization oc-curred during the early 20 days of the incubation, then N releaseoccurred (Fig. 1a).

There were significant differences in N release from residues ofthree shrub species during the incubation (Fig. 1b). For peashrub,net N mineralization was positive, indicating the release of N fromthe residues during the entire incubation. For Seabuckthorn andwild peach, N immobilization occurred before 15 days and 34 daysof incubation, respectively. After that, the immobilized N wasreleased.

During the incubation, addition of S. bungeana and old worldbluestem resulted in the net immobilization of N (Fig. 1c), and Nimmobilized by S. bungeana was higher than old world bluestem.Addition of alfalfa and milkvetch resulted in net releases of N dur-ing incubation, and N released from alfalfa was significantly higherthan milkvetch (P < 0.01).

3.3. Soil microbial biomass C (SMBC) in different treatments

Compared to the treatment without addition of residues, addi-tion of different plant residues increased the contents of SMBC dur-ing the incubation (Fig. 2). During the initial stage of incubation,SMBC increased sharply, and peaked at 6 days of incubation, andthen declined gradually.

The increases of SMBC in elm treatment during the initial stagewas higher than simon poplar and locust (Fig. 2a); and as the incu-bation continuing, the SMBC in elm treatment decreased; and itwas significantly higher than simon poplar and locust treatmentduring the first 30 days of the incubation. The SMBC contents in si-mon poplar and locust followed a same trend during the incuba-

tion; at the end of incubation, the average of SMBC of locusttreatment was higher than simon poplar treatment (P < 0.05).

For three shrub species (Fig. 2b), SMBC contents of peashruband wild peach treatments were significantly higher than Seabuck-thorn (P < 0.05) during the incubation. For three grass species(Fig. 2c), SMBC contents of milkvetch and alfalfa followed a sametrend, in the early 30 days of the incubation, SMBC in these treat-ments were significantly higher than S. bungeana and old worldbluestem (P < 0.05).

At the end of incubation, SMBN contents in soils added with dif-ferent plant species was shrubs (average, 269.68 mg kg�1) > grass(244.67 mg kg�1) > trees (231.26 mg kg�1), and the difference wasstatistically significant.

3.4. Soil microbial biomass N (SMBN) in different treatments

Compared to the treatment without addition of the residues,addition of different plant residues increased the contents of SMBNduring the incubation (Fig. 3). As the continuing of the incubation,SMBN declined gradually, but was still higher than treatment with-out addition of residues.

SMBN content in elm treatment was higher than simon poplarand locust treatments (Fig. 3a). During the first 35 days of the incu-bation, SMBN in locust treatment was higher than simon poplartreatment (P < 0.05). During the late stage of the incubation, thedifferences between different residues of three tree species becamesmaller. The SMBN contents in peashrub and wild peach treat-ments were significantly higher than Seabuckthorn treatment(P < 0.05). At the end of the incubation, the SMBN contents in pea-shrub and wild peach treatments were 68.8% and 49.2% higher

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 Incubation time (d)

Soi

l mic

robi

al b

iom

ass

C (

mg•

kg-1

)

CK A

B C

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60

Incubation time (d)

Soi

l mic

robi

al b

iom

ass

C (

mg•

kg-1

)

CK G H

I J

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60Incubation time (d)

Soi

l mic

robi

al b

iom

ass

C(m

g•kg

-1)

CK D

E F

a b

c

Fig. 2. Changes of soil microbial biomass C under different treatments during the incubation. CK = soil (the same below).

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 Incubatin time (d)

Soil

mic

robi

al b

iom

ass

N (

mg•

kg-1

)

CK A

B C

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

Incubatin time (d)

Soil

mic

robi

al b

iom

ass

N (

mg•

kg-1

)

CK GH IJ

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60Incubatin time (d)

Soil

mic

robi

al b

iom

ass

N (

mg•

kg-1

)

CK D

E F

a b

c

Fig. 3. Changes of soil microbial biomass N under different treatments during the incubation.

126 J.-B. Zhou et al. / Acta Ecologica Sinica 30 (2010) 123–128

Table 2Correlation analysis among soil microbial biomass carbon and nitrogen after 60-day incubation, mineralized N from plant residues and its chemical components.

SMBC SMBN N-mineralizedfrom residues

C/N ratio Lignin ofresidues

Organic-C ofresidues

Total-N ofresidues

SMBC 1

SMBN 0.773** (n = 100) 1

N-mineralized �0.065 (n = 100) 0.325** (n = 100) 1

C/N ratio �0.687* (n = 10) �0.683* (n = 10) �0.712* (n = 10) 1

Lignin of residues �0.712* (n = 10) �0.583 (n = 10) �0.488 (n = 10) 0.565 (n = 10) 1

Organic-C of residues 0.310 (n = 10) 0.404 (n = 10) 0.254 (n = 10) �0.069 (n = 10) �0.071 (n = 10) 1

Total-N of residues 0.807** (n = 10) 0.800** (n = 10) 0.841** (n = 10) �0.946** (n = 10) �0.578 (n = 10) 0.202 (n = 10) 1

* Indicate the significant difference at P < 0.05.** Indicate the significant difference at P < 0.01.

J.-B. Zhou et al. / Acta Ecologica Sinica 30 (2010) 123–128 127

than Seabuckthorn treatment, respectively. During most of incuba-tion, the SMBN contents in alfalfa and milkvetch treatments werehigher than S. bungeana and old world bluestem treatments(Fig. 3c); and SMBN of alfalfa was higher than other grass species.

At the end of 60 days incubation, SMBN in soil added with thedifferent residues of plant species followed the order of shrubs (aver-age, 53.44 mg kg�1) > grass(43.28 mg kg�1) > tree(30.37 mg kg�1),and the differences between each species was statisticallysignificant.

3.5. Correlationships between SMBC, SMBN and mineralized N andchemical composition of residues

There were significantly positive correlationships between totalN in the residues and SMBC, SMBN and mineralized N and chemicalcomposition of residues (Table 2); and negative correlationshipswas found between total N in the residues and C/N ratio (P <0.05), and the correlationship between total N and lignin was notstatistically significant. SMBN also had a positive correlationshipswith mineralized N from the residues and SMBC (P < 0.01), andSMBC’s correlationships with mineralized N was not statisticallysignificant.

4. Discussion

There are many researches to study the N release from differentplant residues [9,14,15]. Addition of residues with higher C/N ratioresulted in N immobilization due to high energy for microbes; onthe contrast, addition of residues with low C/N ratio (e.g., legume)increased N released from the residues [16]. However, the study onthe N release from different plant residues of Loess Plateau is lim-ited. This study showed that after addition of residues of alfalfa,milkvetch, and peashrub, N released from these residues sincethe initial of the incubation. For elm, Sea buckthorn and wild peachtreatments, N immobilization occurred during the first 14 to34 days of incubation, then released. And for simon poplar, locust,S. bungeana and old world bluestem treatments, N from these res-idues did not release during the entire incubation, mineral N in soilwas immobilized. There was positive correlationship between theN released from residues and their total N content; and negativelycorrelationship with the C/N ratio (Table 2). It indicates that the Nreleased from the residues is closely related to total N and C/N ratioof the residues.

Addition of the plant residues significantly increased the con-tents of SMBC, SMBN in soil. It is consistent with the other studies[1,17–19]. This is related to the increasing activity of microbes byaddition of organic matter. However, different residues of plantspecies had various effects on the contents of SMBC, SMBN in soil.After the incubation, generally, addition of the residues with higher

N content and low C/N ratio resulted in higher contents of SMBC,SMBN in soil than the residues with lower N content and higherC/N ratio. For example, at the end of incubation, the SMBC, SMBNin soil added the alfalfa (with the lowest C/N ratio, 14.19) werehigher than the old world bluestem residues (with the highest C/N ratio, 65.47). There was negatively correlationship between theSMBC, SMBN in soil and total N and C/N ratio of plant residues (Ta-ble 2). It indicates that the C/N ratio of plant residues was the mainreason for the differences in SMBC, SMBN in soil [20]. This is re-lated to the higher decomposition rate of low C/N ratio residueafter addition into soil [21].

The C/N ratios of three shrubs (Sea buckthorn, peashrub andwild peach) was similar (C/N, 15–16). However, after addition ofthese residues to soil, there were significant differences in SMBC,SMBN; and N released from these residues were also significantdifferences (Fig. 3b). It indicates that the C/N ratio of residues isnot only one factor affecting their decomposition; and other chem-ical compositions of residues, such as cellulose, hemicelluloses, andlignin, etc., also affect their decomposition [22–25]. Therefore, it isneeded to study the relationship between the different chemicalcompositions of residues from the Loess Plateau and their decom-position rates.

Water stress is known as a limited factor for reestablishing thevegetation on the Loess Plateau. Therefore, different types of plantsincluding trees, shrubs, or grasses are chosen at the basis of waterresource in different regions [26–28]. This study indicated that dif-ferent residues of plant species had different chemical properties(Table 1), and N releasing characteristic, and their addition onSMBC, SMBN were also different. Undoubtedly, these differences(especially in N) have great effects on transformations of organicmatter and energy flow in soil–plant ecosystem. Therefore, it issuggested that during reestablishing the vegetation on the LoessPlateau, the characteristics of C and N decomposition from differ-ent residues of plant is needed to consider to adjusting C and N cy-cles in soil ecosystem.

In this study, we used the incubation method to evaluate the ef-fects of addition of plant residues from the Loess Plateau on min-eral N, SMBC, SMBN in soil. It is clear that the transformation ofplant residues in soil in the field is affected by environmental fac-tor, such as soil moisture, temperature, etc. Therefore, long termfield study is needed to study the decomposition of plant residuesin soil and their effects on soil fertility.

Acknowledgements

This research was supported by the National Natural ScienceFoundation of China (No. 40571087) and National TechnologyR&D Pillar Program in the 11th Five Year Plan of China(2007BAD89B02).

128 J.-B. Zhou et al. / Acta Ecologica Sinica 30 (2010) 123–128

References

[1] C.J. Hu, B.J. Fu, T.T. Jin, et al., Effects of vegetation restoration on soil microbialbiomass carbon and nitrogen in hilly areas of Loess Plateau, Chinese Journal ofApplied Ecology 20 (1) (2009) 45–50.

[2] J.A. Harris, Measurements of the soil microbial community for estimating thesuccess of restoration, European Journal of Soil Science 54 (4) (2003) 801–808.

[3] Y.M. Li, J.C. Hu, S.L. Wang, et al., Function and application of soilmicroorganisms in forest ecosystem, Chinese Journal of Applied Ecology 15(10) (2004) 1943–1946.

[4] J.B. Zhou, Z.J. Chen, S.X. Li, Contents of soil microbial biomass nitrogen and itsmineralized characteristics and relationships with nitrogen supplying abilityof soils, Acta Ecologica Sinica 21 (10) (2001) 1718–1723.

[5] B.F. Rogers, R.L. Tate III., Temporal analysis of the soil microbial communityalong a top sequence in Pineland soils, Soil Biology and Biochemistry 33 (10)(2001) 1389–1401.

[6] J. Xue, G.B. Liu, Q.H. Dai, et al., Effect of different vegetation restoration modelson soil microbial biomass in eroded hilly Loess Plateau, Chinese Journal ofApplied Ecology 19 (3) (2008) 517–523.

[7] Z.M. Wen, F. Jiao, B.Y. Liu, et al., Natural vegetation restoration and soil nutrientdynamics of abandoned farmlands in forest-steppe zone on Loess Plateau,Chinese Journal of Applied Ecology 16 (11) (2005) 2025–2029.

[8] J. Gong, L.D. Chen, B.J. Fu, et al., Effects of land use and vegetation restorationon soil quality in a small catchment of the Loess Plateau, Chinese Journal ofApplied Ecology 15 (12) (2004) 2292–2296.

[9] Z.Y. Li, S.Q. Li, S.X. Li, Organic N mineralization in typical soils of the LoessPlateau, Acta Ecologica Sinica 28 (10) (2008) 4940–4950.

[10] S.D. Bao, Agrochemical Analysis of Soil, China Agricultural Press, Beijing, 2000.[11] X.H. Boqnhok, The Analyzing Method of Plant Biochemistry, Science Press,

Beijing, 1981.[12] J.S. Wu, The Determining Method and Application of Soil Microbial Biomass,

Meteorology Press, Beijing, 2006.[13] J.B. Zhou, S.X. Li, Choosing of a proper oxidizer for alkaline persulfate oxidation

to determining total nitrogen in solution, Journal of Plant Nutrition andFertilizer Science 4 (3) (1998) 299–304.

[14] S.Q. Li, S.X. Li, Priming effect of ammonium nitrogen fertilizer on soil nitrogenunder waterlogged condition, Plant Nutrition and Fertilizer Science 7 (4)(2001) 361–367.

[15] C.Y. Lu, X. Chen, Mineralization process of soil organic nitrogen in differentfertilizer systems and organic materials with different C/N ratios, ChineseJournal of Soil Science 34 (4) (2003) 265–270.

[16] G. Yang, X.Y. He, K.L. Wang, et al., Effects of vegetation types on soil microbialcarbon, nitrogen and soil respiration, Chinese Journal of Soil Science 39 (1)(2008) 189–191.

[17] G.T. Li, Z.J. Zhao, Y.F. Huang, et al., Effect of straw returning on soil nitrogentransformation, Journal of Plant Nutrition and Fertilizer Science 8 (2) (2002)162–167.

[18] M. Helfrich, B. Ludwig, M. Potthoff, et al., Effect of litter quality and soil fungion macroaggregate dynamics and associated partitioning of litter carbon andnitrogen, Soil Biology and Biochemistry 40 (2008) 1823–1835.

[19] C.H. Wang, X.R. Xing, X.G. Han, Advances in study of factors affecting soil Nmineralization in grassland ecosystems, Chinese Journal of Applied Ecology 15(11) (2004) 2184–2188.

[20] X.F. Jiang, J. Luo, Q.W. Huang, et al., Effect of different organic–inorganic mixedfertilizer application on pepper yield and soil microbial properties, Journal ofPlant Nutrition and Fertilizer Science 14 (4) (2008) 766–773.

[21] V. Zeller, R.D. Bardgett, U. Tappeiner, Site and management effects on soilmicrobial properties of subalpine meadows: a study of land abandonmentalong a northsouth gradient in the European Alps, Soil Biology andBiochemistry 33 (2001) 639–649.

[22] C.T. Jagadish, C.M. Subhash, K. Shyam, Influence of straw size on activity andbiomass of soil microorganisms during decomposition, European Journal ofSoil Biology 37 (2001) 157–160.

[23] F. Du, Z.S. Liang, X.X. Xu, et al., The community biomass of abandoned farmland and its effects on soil nutrition in the Loess Hilly Region of NorthernShaanxi, China, Acta Ecologica Sinica 27 (5) (2007) 1673–1683.

[24] Y.L. Hu, S.L. Wang, Y. Huang, et al., Effects of litter chemistry on soil biologicalproperty and enzymatic activity, Acta Ecologica Sinica 25 (10) (2005) 2662–2667.

[25] E.D. Vance, F.S. Chapin III., Substrate limitations to microbial activity in taigaforest floors, Soil Biology and Biochemistry 33 (2) (2001) 173–188.

[26] H.B. Zhao, G.B. Liu, M.X. Xu, Review on vegetation restoration on nutrientchanges in watershed of Loess Hilly Region, Bulletin of Soil and WaterConservation 24 (2) (2004) 72–75.

[27] Y.H. Ma, S.L. Guo, Y.L. Yang, et al., Influence of vegetation types on soil organicC at Yangou catchment in the Loess hilly-gully region, Journal of NaturalResources 22 (1) (2007) 97–105.

[28] X.P. Zhang, G.H. Yang, D.X. Wang, et al., Effect of different vegetationrestoration models on soil microbial characters in the gully region of LoessPlateau, Journal of Northwest A&F University (Nat. Sci. Ed.) 36 (5) (2008) 149–159.