Effects of weed communities with various species numbers on soil features in a subtropical orchard ecosystem

Agriculture, Ecosystems and Environment 102 (2004) 377–388

Effects of weed communities with various species numbers on soil features in a subtropical orchard ecosystem Xin Chen a,∗ , Jianjun Tang b , Zhiguo Fang a , Katsuyoshi Shimizu c a

c

Agroecology Institute, College of Life Science, Zhejiang University, Huajiachi Campus, 268 Kaixuan Road, 310029 Hangzhou City, Zhejiang Province, PR China b Institute of Plant Science, College of Life Science, Yuquan Campus, Zhejiang University, 38 Zheda Road, 310027 Hangzhou City, Zhejiang Province, PR China Laboratory of Comparative Environmental Agronomy, Institute of Agriculture and Forestry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan Received 5 August 2002; received in revised form 5 August 2003; accepted 11 August 2003

Abstract Recent emphasis on species diversification in sustainable agriculture highlights the importance of elucidating how species number and diversity affect soil nutrient processes. Effects of weed species numbers on soil carbon, nitrogen and arbuscular mycorrhizal fungi (AMF) were studied in field experiments during 1998–2001 in a subtropical citrus orchard situated at Changshan County (28◦ 54 N, 118◦ 30 E) in the southwestern Zhejiang Province, PR China. Twelve native weed species were selected for the experiment based on their characteristics of nitrogen fixation, root system type and phenological period. Six kinds of weed communities (groups) of either 0, 1, 2, 4, 8 or 12 weed species were formed by selectively removing the unwanted species. After establishing each artificial weed communities, plant biomass, plant nitrogen, soil organic matter, soil total N, soil microbial C and N, number of AMF spore were measured in May and October 1999–2001. The results demonstrated a strong influence of weed communities with different species numbers on soil N, C and soil microbes because of weed biomass that was returned to soil. Species numbers increasing from 0 to 4 and 8 to 12 enhanced plant biomass significantly that directly affected soil C and N. Microbial biomass C and N increased significantly with species numbers increasing from 0 to 12 in the early growing season but not in the late growing season possibly due to the competition for N and other nutrients between plants and microbes. Results implied that in a community with a few plant species, increasing species numbers plays a determining role in soil C and N by increasing plant biomass, in a species rich community, however species characteristics are important for determining soil C and N. The numbers of AMF spores increased significantly with increasing species number, which may contribute to the mycorrhizal colonization of cultivated plants. © 2003 Elsevier B.V. All rights reserved. Keywords: Weed community; Plant biomass and plant nitrogen; Soil nitrogen and soil organic matter; Microbial biomass carbon and nitrogen; Subtropical citrus orchard; Arbuscular mycorrhizal fungi

Abbreviations: N, nitrogen; C, carbon; AMF, arbuscular mycorrhizal fungi Corresponding author. Tel.: +86-571-86971154; fax: +86-571-87997840. E-mail addresses: [email protected] (X. Chen), [email protected] (J. Tang), [email protected] (K. Shimizu). ∗

0167-8809/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2003.08.006

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1. Introduction The hilly and mountainous area in southern China is potentially the most productive region of agricultural development and accounts for 64% of total land in this zone. Utilization of the mountain and hilly resources to develop agriculture is an important way to feed the growing population in this area. As the utilization of the slope land resources for agriculture, soil erosion, nutrient losses, degradation of soil organic matter (SOM) and soil microbial activity decreasing became the major problems due to the complete removal of primary vegetation and underdeveloped cultivated vegetation (Zhao, 1995). In order to tackle these problems and maintain the land productivity, it is imperative to conserve native biodiversity, increase vegetation coverage and improve soil quality and stress tolerance of soil on slope. Weeds are an integral component of agroecosystems and play an important role in diversifying of the land. Evidence from field experiments shows that weeds can be used to increase species diversity of an ecosystem, reduce pest density, and maintain soil fertility (Risch, 1983; Lagerlof and Wallin, 1993; Wyss, 1996; McLaughlin and Mineau, 1995). In the preliminary experiments, we found that weeds improved the orchard ecosystem resistance to soil erosion in the rainstorm season and tolerance to soil drought in hot–dry period (Chen et al., 1999; Chen and Fang, 2002). Most of native weed species are hosts to arbuscular mycorrhizal fungi (AMF) (Chen et al., 2001). Some non-symbiotic microorganisms in the rhizosphere of weeds enhance phosphorus solubility (Chen et al., 2002). The present study aims to examine the potential effects of weed species and their combinations in enhancing soil quality and assess how many weed species and what species will benefit orchard ecosystem stability. 2. Materials and methods

in southeastern China (28◦ 54 N, 118◦ 30 E). After complete removal of primary vegetation, young citrus plants (Citrus changshan-huyou Y.B. Chang) were transplanted in the spring of 1997 at the density of 5 m ×4 m with only about 10% vegetation coverage of the land. The red soils in the region are equivalent to Ultisols and Oxisols in US soil taxonomy, which are composed of 70.50% clay, 10.63% silt, 18.79% sand and with a pH 5.4. The soil had 44.32 ± 5.16 mg kg−1 extractable N, 9.27 ± 0.78 mg kg−1 extractable P, and 54.6 ± 5.34 mg kg−1 extractable K. The native weedy species that dominated in the site were in the families of Gramineae, Leguminosae, Asteraceae, Polygonaceae, Lamiaceae, Euphorbriaceae, Violaceae, Molluginaceae, Ranunculaceae, Primulaceae, Brasssicaceae, Cyperaceae, and Amaranthaceae. Twelve commonly existing weed species in the citrus orchard were selected for the experiment, based on the plant characteristics for nitrogen fixation, phenological period, root system type, and symbiosis. The characteristics of these species are described in Table 1. Six treatments representing common weed communities in the orchard (0, 1, 2, 4, 8, and 12 weed species combination) were used (Table 2). Except T1 and T2 , each group constituted of annual weeds and biennial weeds that co-exist temporally in May and June. All the groups except T1 included leguminous weeds. The experimental design was a randomized complete block with six treatments and four replications. There were a total of 24 plots in the experimental field site. Plots were 15 m × 8 m and separated by 2 m wide buffer strips. Weed seed banks were developed by sowing mixtures of the weed seeds into soil in each plot in October 1998. Weed seedling density was thinned to 1500 plants of the total community per plot. Other weed species were removed manually. Weed residues were plowed back into soil and weed seed banks were redeveloped at the end of each October of the experimental years. 2.2. Measurement of plant biomass and plant nitrogen

2.1. Experimental site and experimental design The experiments were conducted in a newly developed orchard from October 1998 to December 2001. The experimental site covers about 150 ha on a hilly region, situated in southwestern Zhejiang Province,

Plant samples were collected in May, coinciding with vigorous growth of summer weeds (annual weeds) and winter weeds (biennial weeds) were maturing, and at the end of October when summer weeds matured in each year of 1999–2001. Thirteen

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Table 1 Description of some characteristics of the weeds used in the experiment during 1999–2001a Species

Artemisia argyi Levl. Et Vant. Conyza canadensis (L.) Cronq. Digitaria ciliaris (Retx.) Koel. Eragrostis pilosa (Linn.) Beauv. Euphorbia supina Raf. Gnaphalium affine D. Don Kummerowia striata (Thunb.) Schindl. Oxalis corniculata L. Phyllanthus urinaria L. Poa annua L. Trifolium repens L. Vicia hirsuta (Linn.) S.E. Gray a

Height (cm)

Root system

Growth duration

30–50 50–100 30–80 20–50 5–20 10–30 5–20 5–15 10–30 8–30 5–15 10–30

T T F F T T T F T F F F

March–October April–October April–October March–October March–October November–June March–October Perennial March–October November–May Perennial November–May

Nitrogen fixation

Mycorrhizal colonization

N-L N-L N-L N-L N-L N-L L N-L N-L N-L L L

High Moderate Poor High Poor Moderate High High Poor High Moderate High

Treatment 1

2

3

4

× ×

×

×

×

× ×

5

× × × × × × × ×

6 × × × × × × × × × × × ×

T: tap root system; F: fibrous system; L: leguminous weed; N-L: non-leguminous weed.

samples with an area of 1 m2 were taken from each plot. The aboveground plant biomass of each weed species was separated, dried and weighed. The root systems of each weed species was removed from the soil and washed, dried, and weighed separately. The biomass measured in May and October were summed as a year’s total plant biomass. Each sample of plant species was oven-dried, ground. Plant nitrogen content was analyzed by Kjeldahl procedures. 2.3. Soil sampling Soil samples were collected in May and October of each year during 1999–2001. For each plot, 20 soil cores (2.5 cm diameter) were taken to a depth of 20 cm and were then composited. All samples were immediately transported to the laboratory and stored at 4 ◦ C. Half of each sample was used to determining microbial biomass carbon (MBC) and soluble C, microbial

biomass nitrogen (MBN), soil NH4 -N, NO3 -N and numbers of AMF spores within 3 days after sampling. The remaining half of each composited sample was air-dried and used for measurement of total nitrogen (TN) and total SOM. 2.4. Measurement of soil total N and SOM in the soil Soil total nitrogen was analyzed by Kjeldahl procedures (K2 SO4 –CuSO4 –Se digestion and 2200 Kjeltech Auto Distillation). Total SOM was determined by using the K2 CrO4 –H2 SO4 Oil-Bath-Heating method described by Lu (2000). 2.5. Measurements of MBC, soluble C, MBN and soluble N MBC and MBN were determined by a chloroform fumigation–extraction method adopted from Vance

Table 2 Design and their description of species combination of weed community in the experiments Treatment

Treatment description

T1 T2 T3 T4 T5

Weeds were manually removed. Soil remained bare during the experiment One species (K. striata) mixture between citrus tree rows Two species (K. striata and P. annua) mixture between tree rows Four species (K. striata, P. annua, E. pilosa and T. repens) mixture between tree rows Eight species (K. striata, P. annua, T. repens, E. pilosa, E. supina, V. hirsuta, G. affine and O. corniculata) mixture between tree rows Twelve weed spices (K. striata, P. annua, T. repens, E. pilosa, E. supina, V. hirsuta, G. affine, O. corniculata, C. canadensis, A. argyi, D. ciliaris and P. urinaria) mixture between tree rows

T6

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et al. (1987) and Wu et al. (1990). Briefly, a moist soil sample equivalent to 10 g dry soil was extracted with 40 ml 0.5 M K2 SO4 , shaken for 30 min, and filtered (no. 42 Whatman paper) on a vacuum extraction set. A second moist soil sample equivalent to 10 g dry soil was fumigated with chloroform for 24 h and extracted with 40 ml 0.5 M K2 SO4 as above. Extracted samples for determining MBC and MBN were kept frozen until analyzed. MBC and soluble C of non-fumigated soil samples were measured using Shimadzu total organic carbon analyzer (Shimadzu TOC-505, Shimadzu Scientific Instruments Inc.). MBC was calculated from the total dissolved organic C by using a Kec -factor of 0.45 (Yao et al., 2000). MBN in extracted samples was determined by Kjeldahl procedures. Soluble NH4 -N and NO3 -N in the extracted samples of non-fumigated soil were analyzed by a diffusion-conductivity analyzer (Carlson, 1978). 2.6. Numbers of AMF spore measurement Spore of mycorrhizal fungi were collected by the wet-sieving and decanting method (Gerdemann and Nicolson, 1963) using a moist soil sample equivalent to 100 g dry soil and sieves of 53, 106, and 1680 ␮m mesh. Spores were counted and identified by using dissection microscopes. 2.7. Data analysis All the experimental data were analyzed using a general linear model (GLM) design command in SPSS Version 10.0 (SPSS Inc., Standard Version) for one-way analysis of variance (ANOVA). Least significant difference (LSD) at 5% confidence level was used for comparisons of treatments. Correlation of plant biomass and SOM was analyzed by mixed model procedure.

3. Results 3.1. Plant biomass and nitrogen retention in plant biomass Species number had significant effects on top and root plant biomass. Data from 1999 to 2001 showed

that the total aboveground biomass and root biomass increased significantly when the species numbers of weed community increased (Fig. 1, P < 0.001). In the first year of the experiment, there was no significant difference of biomass aboveground between treatments 3 and 4 (P = 0.310), treatments 3 and 5 (P = 0.196), treatments 4 and 5 (P = 0.77), but in 2000 and 2001 only treatments 4 and 5 was found no significant difference (P = 0.392 in 2000 and P = 0.513 in 2001). N retention in plant biomass aboveground increased significantly as the species numbers increased (Fig. 1, P < 0.001), but there was no significant difference between treatments 4 and 5 throughout the experiment (P = 0.053 in 1999, P = 0.191 in 2000, P = 0.531 in 2001). The component of plant biomass and plant nitrogen aboveground in each treatment of 2001 showed that species number increasing contributed to plant biomass and plant nitrogen in treatments 2–4 (Figs. 2 and 3), but not in treatments 5 and 6. In treatment 2 with only one summer weed species (Kummerowia striata), the formation of plant biomass and plant nitrogen were only in the period of April–October. There was no any coverage during November–March. In treatment 3, there were two species (summer weed K. striata and winter weed Poa annua) with different growth duration (Tables 1 and 2). The plant biomass and plant nitrogen were formed in a whole year, resulted in higher plant biomass and plant nitrogen in treatment 3 than that in treatment 2. In treatment 4, there were four species differed in growth duration and morphological and physiological characteristics (Tables 1 and 2). The plant biomass and plant nitrogen formed greatly in these four species combination by making full use of light, nutrient, water and other resources. In treatment 5, the characteristics of species (four summer weed species and four winter weed species) were similar to treatment 4 (Tables 1 and 2), so there was no significant difference of plant biomass and plant nitrogen between treatments 4 and 5 at the same plant density. In treatment 6, the enhancement of plant biomass and plant nitrogen was due to the larger and competitive weeds, Artemisia argyi, Digitaria ciliaris and Conyza canadensis, were added to the weed community (Figs. 1–3).

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Fig. 1. Plant biomass for both aboveground and belowground and nitrogen retention in plant under different community with various species richness during 1999–2001. Values are means ± S.E.

Fig. 2. Species contribution to total aboveground biomass under different community with various species richness during 1999–2001.

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Fig. 3. Species contribution to total nitrogen in aboveground part under different community with various species richness during 1999–2001.

3.2. Carbon and nitrogen in soil

by treatments in 2000 and 2001 and October 1999, but no significant difference was found in May 1999 (Fig. 4b). In both May and October 2001, soil soluble C without species covered was lower significant than that of the soil with species but weed species number had little influence on soil soluble C (Table 3). Sampling in early growing season (May) showed that NH4 -N and NO3 -N increased significantly with the species

SOM increased significantly as weed species numbers increased both in May and October 2000 and 2001, but there was no significant difference in 1999 (Fig. 4a). Correlation analysis of data from 2000 to 2001 showed that SOM in every plot was significantly correlated to total biomass of weed community (P = 0.001, Fig. 5). Soil total N was influenced significantly

Table 3 Influence of species number in the weed community on soil soluble carbon, NH4 -N and NO3 -N sampled in May and October 2001a Species number

May 2001 (mg kg−1 ) NH4 -N

0 1 2 4 8 12

2.85 6.47 7.07 7.91 8.42 10.17 a

Values are mean ± S.E.

± ± ± ± ± ±

October 2001 (mg kg−1 )

NO3 -N 0.22 0.30 0.95 1.10 0.21 0.43

5.26 10.73 11.84 15.60 17.31 18.90

± ± ± ± ± ±

Soluble C 0.78 1.74 1.99 1.95 1.96 1.79

59.59 76.78 93.67 82.90 94.98 88.80

± ± ± ± ± ±

7.01 5.67 13.00 2.71 15.52 11.30

NH4 -N 2.68 4.45 5.67 7.11 7.72 6.45

± ± ± ± ± ±

NO3 -N 0.16 0.22 0.16 0.96 0.18 0.19

5.06 10.50 11.09 10.11 10.10 7.65

± ± ± ± ± ±

Soluble C 0.78 1.74 1.21 1.31 1.29 1.69

48.69 67.94 78.61 64.70 67.07 63.85

± ± ± ± ± ±

4.47 10.41 6.50 2.37 6.28 4.41

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Fig. 4. Comparison of soil total nitrogen and SOM measured in May and October under different community with various species richness during 1999–2001. Values are means ± 1S.E. (a) Soil organic carbon; (b) soil total nitrogen.

numbers from 0 to 12. But in late growing season (October) species numbers reduced NH4 -N and NO3 -N as the number of species increased from 4 to 12 (Table 3). 3.3. Microbial biomass C and microbial biomass N The number of plant species significantly impacted on MBC, although some differences existed. MBC significantly increased as the number of plant species increased in May 2000 and 2001. Similar trend ex-

isted in samples of May 1999, although less significant (Fig. 6a). However, a different pattern emerged in the October samples. MBC increased with plant species number when the number of plant species was less than 8. An increase of plant species number from 8 to 12 did not increase MBC in 1999–2001 (Fig. 6a). Sampling in early growing season (May) showed that MBN increased significantly with the species numbers. But in late growing season (October) species numbers decreased MBN as the number of species increased from 8 to 12 (Fig. 6b).

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Soil organic matter (g/kg)

20

15

10 y = 0.0125x + 9.1549 2 R = 0.7621 5

0 0

100

200

300

400

500

600

700

Plant biomass aboveground (g/m2) Fig. 5. Linear regression model describing the relationship between aboveground part biomass and SOM under different community with various species richness based on the data from 2000 to 2001.

3.4. AMF spore The number of AMF spores in the treatments with more plant species was significantly higher than those with fewer species and the control (without weeds) throughout the experiment (Fig. 7, P < 0.001). The numbers of spores observed in May were significantly lower than in October (Fig. 7, P < 0.001). 4. Discussion 4.1. Species number and characteristics and biomass Experiments have demonstrated that losses in biodiversity can lead to reductions in biomass production (Tilman et al., 1996, 1997; Hooper and Vitousek, 1997; Hector et al., 1999; Loreau et al., 2001). Results from the present studies indicated that species number and characteristics affected interactively on plant biomass. In the communities with less species number (treatments 2–4 in this study), species number increasing contributed to the plant biomass enhancement. But

in the communities with rich species (treatments 5 and 6 in this study), species characteristics played more important role in the weed community biomass formation. 4.2. Species number and characteristics and soil quality Changes in soil nitrogen, SOM and soil MBC and MBN are important reflections of soil quality (Kennedy and Smith, 1995). Plant species and plant diversity may affect soil mineralization (van der Krift and Berendse, 2001), microbial activity (Jackson et al., 1989; Spehn et al., 2000) and nutrient cycling (Hooper and Vitousek, 1998). Our present experiment showed that weeds kept in orchard increased the soil quality by returning back the carbon and nitrogen that were fixed by the weeds. Weeds could promote the soil MBC and MBN by influencing the nitrogen dynamic in soil. Sampling in May demonstrated that increasing weed species including legumes (Tables 1 and 2) from 1 to 12 enhanced SOM, NH4 -N and NO3 -N and soil total N (Fig. 4, Table 3) because of carbon and

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385

Fig. 6. Comparison of soil microbial carbon and soil microbial nitrogen measured in May and October under different community with various species richness during 1999–2001. Values are means ± 1S.E. (a) MBC; (b) MBN.

nitrogen fixation by weeds. Thus higher N and C in soil increased MBC and MBN in soil (Fig. 6). However, the samples of October indicated that weed species with 12 species decreased MBC and MBN (Fig. 6). The possible reasons were that larger and competitive species A. argyi, D. ciliaris and C. canadensis were added to this treatment. With well-developed root systems these competitive weeds had a strong ability to acquire available N from soil and grew up very fast and resulted in high plant biomass and plant nitrogen (Figs. 2 and 3). Then less soluble nitrogen was left in

soil for microorganism (Table 3). After plowing back into the soil, the huge weed residue in the soil decomposed gradually, releasing the nutrients. The decomposition process became more active in the next spring when the temperature in the soil became warmer. The enriched organic carbon and nitrogen source led to a rapid increase of MBC and MBN in May. In late autumn, because of the full absorption of soil nitrogen and carbon by those competitive weeds, the soil MBC and MBN decreased in October. It was suggested that changed species number and component

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Fig. 7. Difference of AMF spore number measured in May and October in the soil with various plant species richness treatment during 1999–2001. Value are means ± 1S.E.

of weed community influenced available N in soil. Competition between plants and microbes for nutrients may limit the formation of MBC and MBN. 4.3. Plant species and AMF spores Burrows and Pfleger (2002) have reported that increasing plant species richness was correlated with increases in AMF sporulation and species numbers as well as changes in AMF community composition. Our present study showed that AMF spore numbers increased significantly as the plant species number increased (Fig. 7). These spore number increasing may be due to the direct effects of increased numbers of plant species on density of plant roots available for colonization since all species used in this experiment are mycorrhizal plants (Table 1). A large number of spores may enhance mycorrhizal colonization of cultivated plants. Weed species and cultivated plants may be infected by the same species of mycorrhizal fungi and the plants could be interconnected by a common

mycelial thread (Simard et al., 1997). The results imply that weed species maintaining in orchard may be favorable for AMF propagation and mycorrhizal symbiosis formation of cultivated plants. Spores of AMF show seasonal patterns of abundance in natural environment (Douds and Millner, 1999). This experiment showed a similar pattern that spore numbers measured in October were much higher than that measured in May (Fig. 7), indicating that spores may produce in autumn. 4.4. Species diversity needed in upland orchard system How many species should be kept in an ecosystem to maintain the highest level of primary production or ecosystem stability has been a concern recently (McCan, 2000). Hooper and Vitousek (1997) demonstrated that net primary production and nutrient retention in an ecosystem increase as the number of plant species increases. Agronomists found that crop

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production increased as crop species number increased from 1 to 5 species in the agricultural system (Cai, 2000). Results from our experiments indicated that there were two patterns for the response of soil N and C to changes of weed species richness and composition. For C and N dynamics in soil, weed composition explained more of the variation than did the species number present. We observed an increase of soil N and C, MBC and MBN in soil pool size as the species increased from 0 to 8, but a decrease from 8 to 12. This can be explained by different composition of the weed communities. In the treatments 4 and 5, weed communities are formed mainly of leguminous weed and small, short weeds that absorb less nutrients from soil (Tables 1–3), but in plot with 12 species. Three large and competitive weed species, A. argyi, C. Canadensis and D. ciliaris, were adopted, resulting in huge nutrient consumption (Tables 1 and 2, Figs. 1 and 3). The effects of differences in community composition are widely recognized in intercropping and agroforestry (Vandermeer, 1990). To improve total yield, much time and expense were invested on finding species or genetic varieties that combine in more diverse agroecosystems (Pimm, 1997; Zhu et al., 2000). Weeds were also considered to be a way to diversify agricultural systems recently (Altieri, 1999; Wyss, 1996; Moore, 2000). Our experiment suggested that conserved weeds could be considered as an intermediate pool to mediate soil N and C and soil microbes for cultured plants in the newly developed orchard where coverage of cultured plants was lower and most of the soil exposed to air. Weeds fixed C and N and minimized nutrient losing with runoff and leaching. Weed plants were buried into soil near the root systems of cultured trees, and decomposed by microbes and then release of N, C and other nutrients for growing plants. However, some weed species or weed communities occurring in agricultural systems may produce a negative effect on the growth of cultivated plants (Chen and Fang, 2002). Our preliminary experiment showed that the growth of young trees were influenced significantly by some weed species, for example, the fruit plant height and canopy width in the plots that maintaining A. argyi, C. canadensis and D. ciliaris were significant lower than that maintaining other species (Chen and Fang, 2002). Therefore, aggressive species should be avoided when weeds were maintained in agroecosystem.

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5. Conclusions Both numbers and characteristics of weed species affected on weed community biomass. Increasing species numbers contributed to the biomass enhancement of weed communities with less species numbers, but in species richness communities, species composition determined the biomass formation. In the new developed orchard where coverage of cultured plants was lower and most of the soil exposed to air, weeds could increase SOM, soil total nitrogen and microbial activities. The conserved weeds in a new developed orchard could be considered as an intermediate pool to mediate soil N and C for cultured plants. Weeds absorbed soil nutrients and prevented nutrients from losing with runoff and leaching. When weed plants were buried into soil and decomposed by soil microbes, the released nutrients were favorable for the growth of cultured plants.

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