sustainability
Article Below-Ground Interspecific Competition of Apple (Malus pumila M.)–Soybean (Glycine max L. Merr.) Intercropping Systems Based on Niche Overlap on the Loess Plateau of China
Yubo Sun 1, Huaxing Bi 1,2,3,4,5,6,*, Huasen Xu 7 ID , Hangqi Duan 1, Ruidong Peng 1 and Jingjing Wang 1
1 College of Water and Soil Conservation, Beijing Forestry University, Beijing 100083, China; [email protected] (Y.S.); [email protected] (H.D.); [email protected] (R.P.); [email protected] (J.W.) 2 Ji County Station, Chinese National Ecosystem Research Network (CNERN), Beijing 100083, China 3 Beijing Collaborative Innovation Center for Eco-environmental Improvement with Forestry and Fruit Trees, Beijing 102206, China 4 Key Laboratory of State Forestry Administration on Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China 5 Beijing Engineering Research Center of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China 6 Engineering Research Center of Forestry Ecological Engineering, Ministry of Education, Beijing Forestry University, Beijing 100083, China 7 College of Resources and Environmental Sciences, China Agricultural University, Beijing 100094, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-6233-6756
Received: 31 July 2018; Accepted: 20 August 2018; Published: 24 August 2018
Abstract: To provide a scientific basis and technical support for agroforestry management practices, such as interrow configuration and soil water and fertilizer management, a stratified excavation method was performed both to explore the fine-root spatial distribution and niche differentiation and to quantify the below-ground interspecific competition status of 3-, 5-, and 7-year-old apple (Malus pumila M.)–soybean (Glycine max L. Merr.) intercropping systems and monocropping systems. The fine roots of older trees occupied a larger soil space and had both a greater fine-root biomass density (FRMD) and a greater ability to reduce the FRMD of soybean, but this ability decreased with the distance from the apple tree row. Similarly, the FRMD of apple trees was also adversely affected by soybean plants, but this effect gradually increased with a decrease in tree age or with the distance from the tree row. Compared with that of the 3- and 5-year-old monocropped apple trees, the FRMD of the 3- and 5-year-old intercropped apple trees increased in the 40–100 cm and 60–100 cm soil layers, respectively. However, compared with that of the 7-year-old apple and soybean monocropping systems, the FRMD of the 7-year-old intercropped apple trees and soybean plants decreased in each soil layer. Compared with that of the corresponding monocropped systems, the fine-root vertical barycenter (FRVB) of the intercropped apple trees displaced deeper soil and that of the intercropped soybean plants displaced shallower soil. Furthermore, the FRVB of both intercropped apple trees and intercropped soybean plants displaced shallower soil with increasing tree age. Intense below-ground interspecific competition in the 3-, 5-, and 7-year-old apple–soybean intercropping systems occurred in the 0–40 cm soil layer at distances of 0.5–0.9, 0.5–1.3, and 0.5–1.7 m from the apple tree row, respectively.
Keywords: fine-root biomass density; fine-root vertical barycenter; niche differentiation; niche overlap; below-ground interspecific competition
Sustainability 2018, 10, 3022; doi:10.3390/su10083022 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 3022 2 of 14
1. Introduction Agroforestry management plays an important role in both alleviating conflicts between forestry and agriculture and improving land productivity. However, these advantages may be offset by competition between trees and crops [1]. Therefore, taking full advantage of trees and crops, minimizing competition among species, and maximizing the use of available resources are key to improving the yield and overall productivity of agroforestry systems [2,3]. Competition between trees and crops for soil water and nutrients in agroforestry systems is often more intense than is competition for light [4–7]. The spatial distribution and morphology of the roots of trees and crops in an agroforestry system not only determine individual competitive ability to reach below-ground resources [8–10], but also constitute the main factor that determines the degree of below-ground competition among species [1]. Because of their morphological and physiological plasticity, the fine roots of trees and crops have the inherent capability to adapt to the spatial heterogeneity of soil water and nutrients in agroforestry systems [3]. Competition between trees and crops species reduces the quantity of fine roots [10–12]. Some studies reported that the fine roots of wheat (Triticum aestivum L.) decreased with increasing jujube tree (Zizyphus jujuba Mill.) age [13,14]. In response to below-ground competition for soil water and nutrients, the fine roots of trees move to deeper soil [6,11,15] or shallower soil [13], but the fine roots of crop species move to shallower soil [12,13]. The responses of root niche differentiation to resource availability and/or plant competition constitute the main mechanism used by plant roots to avoid competition [4,16,17]. To reduce competition between trees and crops for below-ground resources, the roots of ideal agroforestry trees are mostly distributed away from the top of the soil profile and instead gather resources from deep soil layers so that the crop species can use the resources that are near the soil surface [18]. However, since the roots of trees always dominate at all soil depths in agroforestry systems, there is no spatial separation between the rooting zones of trees and crops [10]. The niche differentiation of the fine roots of trees and crops ultimately determines the degree of fine-root niche overlap in agroforestry systems. Therefore, it is necessary to understand the niche differentiation of the fine roots of both trees and crop species and to analyze the adaptive tactics of the fine-root niche responses to below-ground interspecific competition in agroforestry intercropping systems. Both the quantity and niche differentiation of the fine roots of each component of an agroforestry system change with increasing tree age. However, no reports exist on the status of niche differentiation in response to increasing tree age. The degree of overlap of neighboring root systems is particularly interesting when competitive relationships are considered [19]. According to theoretical ecology, competition occurs under conditions of an interspecific insufficient supply of resources and niche overlap [20,21]. Under agroforestry management conditions in which no irrigation is applied in the arid and semi-arid regions of the Loess Plateau of China, water is the main limiting factor for plant growth, and farmland nutrients are often insufficient [22]. Therefore, the competition between trees and crops for soil water and nutrients in intercropping systems in this region is determined mainly by the niche characteristics of their roots [22]. Xu et al. [23] reported that the relative distance of the fine-root vertical barycenter (FRVB) between walnut trees (Juglans regia L.) and soybean plants increased gradually as the distance from the walnut tree row increased in the region. This means that the niche overlap between the fine roots of walnut and soybean decreased with the distance from the walnut tree row and that the below-ground interspecific competition decreased gradually. To the authors’ knowledge, no study has investigated the below-ground competition status in agroforestry systems, especially in response to increasing tree age. The Loess Plateau is one of the main apple-producing areas in China, and intercropping apple trees with soybean plants, which is an effective way to increase land use efficiency and increase farmers’ economic returns, represents one of the major agroforestry systems in the region. However, the extensive management practices for agroforestry systems lead to intense competition among species for soil water and nutrients, which results in low overall economic efficiency and affects the efficient and Sustainability 2018, 10, 3022 3 of 14 sustainable development of agroforestry systems. Therefore, after analyzing the spatial distribution of fine roots both in apple–soybean intercropping systems and their corresponding monocropping systems, it was possible to characterize the niche differentiation of intercropped apple trees and intercropped soybean plants and further quantitatively determine the below-ground competition status among species. The results from this study provide a scientific basis and technical support for management practices, such as interrow configuration and water and fertilizer management practices.
2. Materials and Methods
2.1. Experimental Site This study was conducted at the Experimental Demonstration Base of Water Resource Saving Agroforestry System (36◦010 N, 110◦430 E) in Ji County, Shanxi Province, China. Ji County lies in a typical hill and gully region of the Loess Plateau; the climate is temperate continental monsoon. The annual precipitation is 521 mm, and the potential evaporation is 1729 mm. The annual mean temperature is 9.9 ◦C, and the annual cumulative temperature greater than 10 ◦C is 3358 ◦C. The mean number of sunlit hours is 2563.8 h, and there are 172 frost-free days. The soil parent material is loess, and the soil is uniform. The soil properties within the 0–100 cm layer are as follows: 1.32 g·cm−3 bulk density, pH = 7.97, 0.81 g·kg−1 total N, 19.7 g·kg−1 available P, 235.7 mg·kg−1 available K, and 13.5 g·kg−1 organic C. The major economic species of trees planted for use in agroforestry systems include apple (Malus pumila M.), walnut (Juglans regia L.), and apricot (Prunus armeniaca L.), and the major intercropped crop species include soybean (Glycine max L. Merr.), peanut (Arachis hypogaea L.), and maize (Zea mays L.). The trees and crops depend mainly on rain as a source of water, and no irrigation was available at any of the experimental areas throughout the year under study.
2.2. Materials and Experimental Design The experiment was conducted in August 2017. There were seven treatments in this study: apple–soybean intercropping systems (the apple trees were 3, 5, or 7 years old), apple monocropping systems (the apples trees were 3, 5, or 7 years old), and soybean monocropping systems. The apple trees were planted at a spacing of 4.0 × 5.0 m in an east-west direction in 2010, 2012, and 2014. In August 2017, the average tree crown width of the 3-, 5-, and 7-year-old apple trees was 1.3, 1.7, and 2.0 m, respectively, and the tree heights were 3.2, 3.6, and 3.7 m, respectively. The intercropped apple trees were continually intercropped with soybean crops, and the monocropped apple trees were not intercropped with any species. The soybean plants were cultivated at a spacing of 0.3 × 0.4 m and were situated 0.5 m from the apple tree row. The monocropped soybean plants were planted on farmland near the apple orchards. The same agricultural management practices were used for all treatments. The experimental design included three replications per treatment. The apple–soybean intercropping treatments included a 12.0 × 15.0 m plot that consisted of fourteen apple trees intercropped with soybean plants. The monocropped apple tree treatments were the same size (12.0 × 15.0 m) but consisted of only fourteen apple trees; the monocrop soybean treatment was also the same size (12.0 × 15.0 m). The area within 0.5 m of the apple tree row was used for fine-root sampling (4.0 × 4.0 m) in both the apple–soybean intercropping system and the apple tree monocropping system treatments (see Figure1). The fine-root sampling area was divided into ten equally sized sections denoted as S1, S2, S3, S4, S5, N5, N4, N3, N2, and N1. Among them, S1, S2, S3, S4, and S5 were south of the apple tree row at a distance of 0.5–0.9, 0.9–1.3, 1.3–1.7, 1.7–2.1, and 2.1–2.5 m, respectively, and N1, N2, N3, N4, and N5 were north of the apple tree row at a distance of 0.5–0.9, 0.9–1.3, 1.3–1.7, 1.7–2.1, and 2.1–2.5 m, respectively. Three sections (4.0 × 0.4 m) were randomly selected in the soybean monocropping treatment. The vertical soil profile in each section was divided into five soil layers (0–20, 20–40, 40–60, 60–80, and 80–100 cm) to excavate and collect the apple roots and soybean roots. Sustainability 2018, 10, x FOR PEER REVIEW 4 of 14
divided into five soil layers (0–20, 20–40, 40–60, 60–80, and 80–100 cm) to excavate and collect the Sustainability 2018, 10, 3022 4 of 14 apple roots and soybean roots.
FigureFigure 1.1. CoreCore fine-root fine-root sampling sampling plot plot of ofthe the apple– apple–soybeansoybean intercropping intercropping system system treatments. treatments. The Thefine-root fine-root sampling sampling areas areas in inthe the intercropping intercropping area area were were divided divided into into ten equally sizedsized sectionssections denoteddenoted asas S1,S1, S2,S2, S3,S3, S4,S4, S5,S5, N5,N5, N4, N4, N3, N3, N2, N2, and and N1. N1.
RootRoot samplessamples werewere collectedcollected andand placedplaced intointo meshmesh bagsbags (0.15(0.15 mmmm pores),pores), afterafter whichwhich thethe samplessamples werewere carefullycarefully cleanedcleaned withwith taptap waterwater toto removeremove anyany adheredadhered soilsoil particlesparticles andand thenthen soakedsoaked inin waterwater forfor 2424 h.h. The finefine roots ( ≤22 mm) mm) of of the the apple apple trees trees and and soybean soybean plants plants were were then then measured measured with with an anelectronic electronic Vernier Vernier caliper. caliper. Dark Dark roots, roots, partially partially decayed decayed roots, roots, brittle brittle roots andand otherother extraneousextraneous materialsmaterials werewere removed. removed. All All the the root root samples samples were were weighed weighed immediately immediately after after drying drying at at 70 70◦C °C for for 48 48 h. h. 2.3. Fine-Root Biomass Density 2.3. Fine-Root Biomass Density The fine-root biomass density (FRMD) is an index used to measure the growth of fine roots of plantsThe and fine-root is calculated biomass via thedensity following (FRM equation:D) is an index used to measure the growth of fine roots of plants and is calculated via the following equation: FRMD = W ,/V (1) = d s FRMD WVds, (1) · −3 where FRMD is the fine-root biomass density (g m−3 ), Wd represents the fine-root dry weight (g), where FRMD is the fine-root biomass3 density (g·m ), Wd represents the fine-root dry weight (g), and and Vs represents the soil volume (m ). Vs represents the soil volume (m3). 2.4. Fine-Root Niche Differentiation 2.4. Fine-Root Niche Differentiation The depth of the fine-root vertical barycenter (FRVB) can be used to quantify the vertical barycenter The depth of the fine-root vertical barycenter (FRVB) can be used to quantify the vertical of both tree and crop roots to explore their niche differentiation [12,23]. The FRVB of the apple trees barycenter of both tree and crop roots to explore their niche differentiation [12,23]. The FRVB of the and soybean plants was then calculated via the following equation: apple trees and soybean plants was then calculated via the following equation: r r FRVB = D P (2) FRVB =∑ DiPi , (2) = ii i i=1,1 wherewhere FRVB FRVB is is the the depth depth of of the the fine-root fine-root vertical vertical barycenter barycenter (cm), (cm),r r( r(r≤ ≤ 5)5) representsrepresents thethe soilsoil layer,layer,D Di i isis thethe depthdepth ofof thethe middlemiddle ofof thethei thith soil soil layer layer (cm),(cm), andandP Pi i isis thethe proportionproportion ofof FRMDFRMD inin thethei thith soilsoil layerlayer toto thethe total total FRMD FRMD (0–100 (0–100 cm). cm).
2.5.2.5. Below-GroundBelow-Ground InterspecificInterspecific CompetitionCompetition IntensityIntensity Index Index TheThe nicheniche overlapoverlap formulaformula [[24]24] cancan bebe usedused toto effectivelyeffectively calculatecalculate thethe degreedegree ofof competitioncompetition amongamong species species for for common common resources resources under under conditions conditions of of resource resource shortage shortage [25 ,[25,26].26]. The The formula formula can can be SustainabilitySustainability 20182018,, 1010,, x 3022 FOR PEER REVIEW 55 of of 14 14 be used to calculate the below-ground interspecific competition intensity of apple–soybean used to calculate the below-ground interspecific competition intensity of apple–soybean intercropping intercropping systems. The equation is as follows: systems. The equation is as follows: rrr BICII= PP / v P22 P , rAj Sju r Ajr Sj (3) jjj===111u 2 2 BICII = ∑ PAjPSj/t∑ PAj ∑ PSj (3) where BICII (0 ≤ BICII ≤ 1) represents thej=1, below-groundj=1 intej=rspecific1 competition intensity index (BICII) between apple trees and soybean plants, r (r ≤ 5) represents the soil layer, PAj represents the where BICII (0 ≤ BICII ≤ 1) represents the below-ground interspecific competition intensity index proportion of apple tree FRMD in the jth soil layer to the total apple tree FRMD (0–100 cm), and PSj (BICII) between apple trees and soybean plants, r (r ≤ 5) represents the soil layer, P represents the represents the proportion of soybean FRMD in the jth soil layer to the total soybean FRMDAj (0–100 cm). proportion of apple tree FRMD in the jth soil layer to the total apple tree FRMD (0–100 cm), and PSj 2.6.represents Statistical the Analyses proportion of soybean FRMD in the jth soil layer to the total soybean FRMD (0–100 cm). 2.6. StatisticalAn analysis Analyses of variance (ANOVA) was performed using SPSS 22.0 (IBM Inc., Armonk, NY, USA). The FRMD, FRVB, and BICII of each treatment were described by mean values (n = 3) An analysis of variance (ANOVA) was performed using SPSS 22.0 (IBM Inc., Armonk, NY, USA). followed by standard deviations. Differences between the apple trees or soybean plants at different The FRMD, FRVB, and BICII of each treatment were described by mean values (n = 3) followed by distances or depths were analyzed by one-way ANOVA, and significant differences between the standard deviations. Differences between the apple trees or soybean plants at different distances or depths mean values were compared by the least significant difference (LSD) test. Paired-sample t-tests were were analyzed by one-way ANOVA, and significant differences between the mean values were compared used to examine differences in FRMD, FRVB, and BICII between the treatments. The statistical by the least significant difference (LSD) test. Paired-sample t-tests were used to examine differences in results accompanied by error bars and significance level (p) are shown, and differences were FRMD, FRVB, and BICII between the treatments. The statistical results accompanied by error bars and considered statistically significant when p ≤ 0.05. significance level (p) are shown, and differences were considered statistically significant when p ≤ 0.05.
3.3. Results Results
3.1.3.1. Horizontal Horizontal Fine-Root Fine-Root Distribution Distribution TheThe FRMD FRMD of of the the intercropped intercropped apple apple trees trees was was less less than than that that of ofthe the monocropped monocropped apple apple trees trees of theof thesame same age age(see (seeFigure Figure 2A–C).2A–C). In general, In general, the FRMD the FRMD (the mean (the mean value value of the of 0–100 the 0–100cm depth) cm depth) of the 3-,of the5-, and 3-, 5-, 7-year-old and 7-year-old intercropped intercropped apple appletrees treeswas 3.49, was 3.49,6.14, 6.14,and and8.46 8.46g·m− g3 ·lessm−3 thanless thanthat thatof the of correspondingthe corresponding monocropped monocropped apple apple trees, trees, respectively respectively.. In addition, In addition, the FRMD the FRMD north (the north mean (the value mean ofvalue Sections of Sections N1–N5) N1–N5) of the of apple the apple tree treerow rowwas was grea greaterter than than that that of ofthe the south south (the (the mean mean value value of of SectionsSections S1–S5) S1–S5) of of the the row row in in the the 3-, 3-, 5-, 5-, and and 7-year-old 7-year-old intercropped intercropped apple apple trees; trees; the the values values were were 7.10, 7.10, 11.46,11.46, and 11.66%11.66% greatergreater on on the the north north side side than than on theon souththe south side, side, respectively. respectively. The closer The thecloser section the sectionwas to was the appleto the treeapple row, tree the row, greater the greater the FRMD the FR ofMD the of apple the apple trees. trees. In each In section,each section, compared compared with withthat ofthat the of corresponding the corresponding monocropped monocropped apple trees, apple the trees, FRMD the of 3-,FRMD 5-, and of 7-year-old3-, 5-, and intercropped 7-year-old intercroppedapple trees decreased apple trees (see Figure decreased2A–C). (see Compared Figure with 2A–C). the corresponding Compared with monocropped the corresponding apple trees, monocroppedthe proportion apple of the trees, FRMD the reduction proportion in 3-, of 5-, andthe 7-year-oldFRMD reduction intercropped in 3-, apple5-, and trees 7-year-old increased intercroppedwith the distance apple from trees the increased apple tree with row, the and distance the proportion from the ofapple the FRMDtree row, reduction and the inproportion each section of thedecreased FRMD withreduction increasing in each tree section age. decreased with increasing tree age.
Figure 2. Cont. SustainabilitySustainability 20182018,, 1010,, x 3022 FOR PEER REVIEW 66 of of 14 14
FigureFigure 2. 2. HorizontalHorizontal distributions distributions of of fine-root fine-root biomass biomass density density in in ( (AA)) apple apple trees trees monocropped monocropped for for 3 3 yearsyears and and 3-year-old 3-year-old apple trees intercropped with soybeansoybean plants;plants; ((BB)) appleapple treestrees monocropped monocropped for for 5 5years years and and 5-year-old 5-year-old apple apple trees intercroppedtrees intercropped with soybean with plants;soybean and plants; (C) apple and trees (C) monocroppedapple trees monocroppedfor 7 years and for 7-year-old 7 years and apple 7-year-old trees intercropped apple trees withintercropped soybean with plants. soybean The error plants. bars The indicate error barsthe standardindicate the deviations. standard Thedeviations. means withThe means different wi lettersth different within letters a row within andcolumn a row and are column statistically are statisticallysignificant (significantp < 0.05). (p < 0.05).
The FRMD of the intercropped soybean plants was less than that of the monocropped soybean The FRMD of the intercropped soybean plants was less than that of the monocropped soybean plants (see Figure 3). In addition, the FRMD of the northern apple tree row was greater than that of plants (see Figure3). In addition, the FRMD of the northern apple tree row was greater than that of the the southern soybean plants intercropped among 3-, 5-, and 7-year-old apple trees; the values were southern soybean plants intercropped among 3-, 5-, and 7-year-old apple trees; the values were 0.89, 0.89, 1.68, and 1.92% greater on the north side than on the south side, respectively. Overall, the 1.68, and 1.92% greater on the north side than on the south side, respectively. Overall, the FRMD of the FRMD of the soybean plants intercropped with 3-, 5-, and 7-year-old apple trees was 2.74, 5.48, and soybean plants intercropped with 3-, 5-, and 7-year-old apple trees was 2.74, 5.48, and 9.14 g·m−3 less 9.14 g·m−3 less than that of the monocropped soybean plants, respectively. The closer the section was than that of the monocropped soybean plants, respectively. The closer the section was to the apple to the apple tree row, the lower the FRMD of the intercropped soybean plants. Compared with the tree row, the lower the FRMD of the intercropped soybean plants. Compared with the monocropped monocropped soybean plants, the FRMD of the soybean plants intercropped with 3-, 5-, and soybean plants, the FRMD of the soybean plants intercropped with 3-, 5-, and 7-year-old apple trees was 7-year-old apple trees was significantly lower (p < 0.05) in the 0.5–1.3, 0.5–1.7, and 0.5–2.5 m soil significantly lower (p < 0.05) in the 0.5–1.3, 0.5–1.7, and 0.5–2.5 m soil layers, respectively. Compared layers, respectively. Compared with the monocropped soybean plants, the proportion of the FRMD with the monocropped soybean plants, the proportion of the FRMD reduction in each section increased reduction in each section increased with increasing tree age. Overall, in the apple–soybean with increasing tree age. Overall, in the apple–soybean intercropping systems, the FRMD values of intercropping systems, the FRMD values of the 3-and 5-year-old apple trees were less than those of the 3-and 5-year-old apple trees were less than those of the corresponding soybean cropping systems, the corresponding soybean cropping systems, while the FRMD values of the 7-year-old apple trees while the FRMD values of the 7-year-old apple trees were much larger than those of the corresponding were much larger than those of the corresponding soybean plants. soybean plants. Sustainability 2018, 10, x FOR PEER REVIEW 7 of 14
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Figure 3. Horizontal distributions of fine-root biomass density in monocropped soybean plants and soybean plants intercropped with 3-, 5-, and 7-year-old apple trees. The error bars indicate the standard deviations. The means with different letters within a row and column are statistically
significant (p < 0.05). FigureFigure 3. 3. HorizontalHorizontal distributions distributions of of fine-root fine-root biomas biomasss density density in in monocropped monocropped soybean soybean plants plants and and 3.2. Vertical Finesoybeansoybean-Root plantsplants Distribution intercroppedintercropped with with 3-, 3-, 5-, and5-, and 7-year-old 7-year-old apple apple trees. trees. The error The barserror indicate bars indicate the standard the standarddeviations. deviations. The means The with means different wi lettersth different within aletters row and within column a row are statistically and column significant are statistically (p < 0.05). The FRMDsignificant of the (p
Figure 4. Cont. Sustainability 2018, 10, x FOR PEER REVIEW 8 of 14 Sustainability 2018, 10, 3022 8 of 14
Figure 4.Figure Vertical 4. Vertical distributions distributions of offine-root fine-root biomassbiomass density density in (A )in apple (A) treesapple monocropped trees monocropped for 3 years for 3 years andand 3-year-old 3-year-old appleapple trees trees intercropped intercropped with soybean with soybean plants; (B) appleplants; trees (B monocropped) apple trees for monocropped 5 years and for 5-year-old apple trees intercropped with soybean plants; and (C) apple trees monocropped for 7 years and 5 years and 5-year-old apple trees intercropped with soybean plants; and (C) apple trees 7-year-old apple trees intercropped with soybean plants. The error bars indicate the standard deviations. monocroppedThe means for with7 years different and letters 7-year-old within a rowapple and tr columnees intercropped are statistically with significant soybean (p < 0.05). plants. The error bars indicate the standard deviations. The means with different letters within a row and column are statisticallyThe significant FRMD of the (p soybean< 0.05). plants decreased with increasing soil depth (see Figure5). The FRMD of the soybean plants was concentrated within the 0–20 cm soil layer, where the FRMD of both the The monocroppedFRMD of the soybean soybean plants plants and those decreased intercropped with withincreasing 3-, 5-, and soil 7-year-old depth apple(see Figure trees accounted 5). The FRMD of the soybeanfor 66.46, plants 69.46, was 70.09, concentrated and 73.80% of within the total the FRMD, 0–20 cm respectively. soil layer, Compared where the with FRMD that of of the both the monocropped soybean plants, the degree of the FRMD reduction of the soybean plants intercropped monocropped soybean plants and those intercropped with 3-, 5-, and 7-year-old apple trees with 3-, 5-, and 7-year-old apple trees increased with increasing soil depth, and the degree of reduction accountedin for each 66.46, soil layer 69.46, increased 70.09, with and increasing 73.80% treeof the age. total FRMD, respectively. Compared with that of the monocropped soybean plants, the degree of the FRMD reduction of the soybean plants intercropped with 3-, 5-, and 7-year-old apple trees increased with increasing soil depth, and the degree of reduction in each soil layer increased with increasing tree age. SustainabilitySustainability2018 2018, ,10 10,, 3022 x FOR PEER REVIEW 9 of9 of14 14 Sustainability 2018, 10, x FOR PEER REVIEW 9 of 14
Figure 5. Vertical distributions of fine-root biomass density in monocropped soybean plants and FigureFigure 5. 5.Vertical Vertical distributions distributions of fine-rootof fine-root biomass bioma densityss density in monocropped in monocroppe soybeand soybean plants plants and soybean and soybean plants intercropped with 3-, 5-, and 7-year-old apple trees. The error bars indicate the standard plantssoybean intercropped plants intercropped with 3-, 5-, with and 3-, 7-year-old 5-, and 7-year-old apple trees. apple The trees. error The bars error indicate bars theindicate standard the standard deviations. deviations. The means with different letters within a row and column are statistically significant (p < Thedeviations. means with The differentmeans with letters different within letters a row within and column a row areand statistically column are significant statistically (p significant< 0.05). (p < 0.05). 0.05). 3.3. Fine-Root Niche Differentiation 3.3. Fine-Root Niche Differentiation 3.3. Fine-Root Niche Differentiation The FRVB of both apple trees and intercropped soybean plants were tended to increase in The FRVB of both apple trees and intercropped soybean plants were tended to increase in depth depthThe with FRVB the of distance both apple from trees the and apple intercropped tree row soybean (see Figure plants6). were The tended FRVB to of increase the intercropped in depth with the distance from the apple tree row (see Figure 6). The FRVB of the intercropped apple trees was applewith the trees distance was deeper from the than apple that tree of row the (see corresponding Figure 6). The monocropped FRVB of the intercropped apple trees apple in each trees section. was deeper than that of the corresponding monocropped apple trees in each section. The FRVB of the Thedeeper FRVB than of that the monocroppedof the corresponding apple monocroppe trees increasedd apple with trees increasing in each treesection. age, The while FRVB that of ofthe the monocropped apple trees increased with increasing tree age, while that of the intercropped apple trees monocropped apple trees increased with increasing tree age, while that of the intercropped apple trees intercroppeddecreased. Generally, apple trees the decreased. relative distances Generally, between the relative the 3-, 5-, distances and 7-year-old between intercropped the 3-, 5-, and apple 7-year-old trees decreased. Generally, the relative distances between the 3-, 5-, and 7-year-old intercropped apple trees intercroppedand the corresponding apple trees monocropped and the corresponding apple trees monocropped were 7.03, 4.02, apple and 0.97 trees cm, were respectively. 7.03, 4.02, In and general, 0.97 cm, and the corresponding monocropped apple trees were 7.03, 4.02, and 0.97 cm, respectively. In general, respectively.compared with In that general, of the compared monocropped with soybean that of theplants, monocropped the FRVB of soybeanthe soybean plants, plants the intercropped FRVB of the compared with that of the monocropped soybean plants, the FRVB of the soybean plants intercropped soybeanwith 3-, plants5-, and intercropped 7-year-old withapple 3-,trees 5-, andwas 7-year-old shallower, apple and treesthe relative was shallower, distances and between the relative the with 3-, 5-, and 7-year-old apple trees was shallower, and the relative distances between the distancesintercropped between and monocropped the intercropped soybean and plants monocropped were 1.54, soybean 2.47, and plants 4.53 cm, were respectively 1.54, 2.47, (see and Figure 4.53 cm, intercropped and monocropped soybean plants were 1.54, 2.47, and 4.53 cm, respectively (see Figure respectively6). The relative (see distance Figure 6of). the The FRVB relative between distance the intercropped of the FRVB apple between trees the and intercropped corresponding apple soybean trees 6). The relative distance of the FRVB between the intercropped apple trees and corresponding soybean andplants corresponding increased with soybean the distance plants from increased the apple with tr theee distancerow and decreased from the apple with treeincreasing row and tree decreased age. In plants increased with the distance from the apple tree row and decreased with increasing tree age. In withaddition, increasing the relative tree age. distance In addition, north of thethe relativeapple tree distance row was north slightly of the greater apple than tree that row south was slightlyof the addition, the relative distance north of the apple tree row was slightly greater than that south of the greaterrow in thanthe 3-, that 5-, southand 7-year-old of the row apple–soybean in the 3-, 5-, andintercropping 7-year-old systems; apple–soybean the values intercropping were 0.34, 0.09, systems; and row in the 3-, 5-, and 7-year-old apple–soybean intercropping systems; the values were 0.34, 0.09, and the0.61 values cm greater were on 0.34, the 0.09, north and side 0.61 than cm on greater the south on theside, north respectively. side than on the south side, respectively. 0.61 cm greater on the north side than on the south side, respectively.
Figure 6. Depths of the fine-root vertical barycenter for apple trees and soybean plants with the distance FigureFigure 6. DepthsDepths ofof the fine-root vertical barycenterbarycenter forfor appleapple treestrees andand soybeansoybean plantsplants withwith thethe from the apple tree row. The error bars indicate the standard deviations. distancedistance from the apple tree row. The error bars indicateindicate thethe standardstandard deviations.deviations. Sustainability 2018, 10, 3022 10 of 14 Sustainability 2018, 10, x FOR PEER REVIEW 10 of 14
3.4.3.4. Below-Ground Below-Ground Interspe Interspecificcific Competition Competition Intensity Intensity TheThe BICII ofof thethe apple–soybean apple–soybean intercropping intercropping systems systems decreased decreased with with the distancethe distance from from the apple the appletree row tree (see row Figure (see Figure7). The 7). BICII The BICII was slightlywas slightly greater greater south south of the of apple the apple tree tree row row than than north north of the of therow, row, but but the differencethe difference was was not significantnot significant (p > 0.05).(p > 0.05). The The BICII BICII increased increased significantly significantly (p < 0.05)(p < 0.05) with withage ofage the of apple the apple trees. trees. The horizontal The horizontal distribution distribution of the BICIIof the significantly BICII significantly differed differed (p < 0.05) (p between < 0.05) betweensections insections the areas in the whose areas distance whose distance ranged from rang 0.5–2.5ed from m 0.5–2.5 from them from tree row.the tree Moreover, row. Moreover, the BICII thevalues BICII of thevalues 3-, 5-,of andthe 7-year-old3-, 5-, and apple–soybean 7-year-old apple–soybean intercropping intercropping systems were systems much greater were much in the greater0.5–0.9, in 0.5–1.3, the 0.5–0.9, and 0.5–1.7 0.5–1.3, m and sections 0.5–1.7 than m sections in the other than sections. in the other sections.
FigureFigure 7. Below-groundBelow-groundinterspecific interspecific competition competition intensity intensity index index of 3-, of 5-, 3-, and 5-, 7-year-old and 7-year-old apple–soybean apple– soybeanintercropping intercropping systems. systems. The error The bars error indicate bars indicate the standard the standard deviations. deviations.
4. Discussion 4. Discussion
4.1.4.1. Horizontal Horizontal Fine-Root Fine-Root Distribution Distribution TheThe FRMD of thethe appleapple treestrees and and soybean soybean plants plants in in the the apple–soybean apple–soybean intercropping intercropping systems systems was wasrelated related to the to age the ofage the of apple the apple trees andtrees the and distance the distance from the from apple the treeapple row. tree Compared row. Compared with those with of thosethe younger of the younger apple trees, apple the trees, fine rootsthe fine of roots the older of the apple older trees apple occupied trees occupi a largered soila larger space soil and space had andboth had a greater both a quantity greater quantity of fine roots of fine (see roots Figure (see2A–C) Figure and 2A–C) a greater and abilitya greater to shortenability to the shorten fine roots the fineof soybean roots of plants soybean (see plants Figure 3(see). Similar Figure results 3). Similar have beenresults reported have been in jujube–wheat reported in intercroppingjujube–wheat intercroppingsystems [14]. Insystems addition, [14]. the Infine addition, roots ofthe the fine apple root treess of the were apple also trees adversely were also affected adversely by the affected soybean byplants, the soybean and this plants, effect gradually and this increasedeffect gradually with a decreaseincreased inwith tree a age decrease or with in increasing tree age distanceor with increasingfrom the apple distance tree from row the (see apple Figure tree2A–C). row Overall,(see Figure the 2A–C). FRMD Overall, values of the the FRMD 3-, 5-, values and 7-year-old of the 3-, −3 5-,intercropped and 7-year-old apple treesintercropped were 3.49, apple 6.14, andtrees 8.46 were g·m −3.49,3 less 6.14, than thoseand 8.46 of corresponding g·m less than monocropped those of correspondingapple trees, respectively monocropped (see Figure apple2 A–C).trees, respective The FRMDly values (see Figure of the soybean2A–C). The plants FRMD intercropped values of with the −3 soybean3-, 5-, and plants 7-year-old intercropped apple trees with were 3-, 5-, 2.74, and 5.48, 7-year-old and 9.14 apple g·m− 3treesless thanwere those2.74, of5.48, the and monocropped 9.14 g·m lesssoybean than plants,those respectivelyof the monocropped (see Figure soybean3). Therefore, plants, competition respectively in (see fine-root Figure growth 3). Therefore, occurred competitionbetween the applein fine-root trees and growth soybean occurred plants. Thebetw competitivenesseen the apple of trees soybean and was soybean greater thatplants. of appleThe competitivenessin the 3-and 5-year-old of soybean appl—soybean was greater intercropping that of apple systems, in the while 3-and the competitiveness5-year-old appl–-soybean of soybean intercroppingwas less than thatsystems, of apple while in the the 7-year-old competitiveness apple–soybean of soybean intercropping was less systems. than that The of FRMD apple values in the of 7-year-oldthe 3-and 5-year-oldapple–soybean intercropped intercropping apple trees systems. were less The than FRMD those of values the corresponding of the 3-and soybean 5-year-old plants, intercroppedwhile the FRMD apple of thetrees 7-year-old were less apple than treesthose was of the much corresponding greater than soybean that of the plants, corresponding while the soybean FRMD ofplants the 7-year-old (see Figure apple2A–C trees and Figurewas much3). Consequently, greater than plantsthat of with the corresponding a greater quantity soybean of fine plants roots (see are Figureslikely to 2A–C be more and competitive3). Consequently, than plantsplants withwith fewera grea fineter quantity roots, which of fine is roots similar are to likely the conclusions to be more competitiveof Schroth [ 27than] and plants Zhang with et fewer al. [13 fine]. The roots, farther whic theh is section similar was to the to theconclu applesions tree of row, Schroth the fewer [27] and the Zhangfine roots et al. of [13]. the apple The farther trees and the thesection greater was the to finethe rootsapple oftree the row, soybean the fewer plants. the This fine phenomenon roots of the apple trees and the greater the fine roots of the soybean plants. This phenomenon indicated that the fine roots of the soybean plants were inhibited by the apple trees. This inhibition is a response to Sustainability 2018, 10, 3022 11 of 14 indicated that the fine roots of the soybean plants were inhibited by the apple trees. This inhibition is a response to strong below-ground interspecific competition [13], and it potentially increases the competitive absorption of soybean for soil water and nutrients [28] and reflects a positive response to the weakened competitiveness of apple trees. The direct reason for spatial differences in soybean root distributions is soil spatial heterogeneity [29]. In this study, the FRMD for both apple trees and soybean plants north of the apple tree row was greater than that south of the apple tree row. This discrepancy might be affected by canopy shading and it resulted in a better soil water status north of the tree row than south of the row [30], which caused the fine roots on the north of the tree row to grow vigorously.
4.2. Vertical Fine-Root Distribution The fine roots of the intercropped apple trees were concentrated within the 0–60 cm soil layer, and those of the intercropped apple trees and soybean plants were distributed mostly within the 20–40 cm and 0–20 cm soil layers (see Figure4A–C and Figure5), respectively. These results indicated that the vertical distribution of the fine roots between the apple trees and soybean plants were both skewed and overlapped. Soybean plants use mainly shallow soil resources, while apple trees can use relatively deep soil resources. Compared with those of the apple trees and soybean plants in this study, the fine roots of both walnut and soybean were distributed relatively more in the 0–20 cm soil layer in a walnut–soybean intercropping system in the same region [23]. Therefore, compared with that of the walnut–soybean intercropping system, the fine-root distribution of the apple–soybean intercropping systems displayed a better spatial structure and reduced competition for shallow soil resources among the apple trees and soybean plants. Compared with the FRMD of the monocropped apple trees, those of the 3-and 5-year-old intercropped apple trees were larger in the 40–100 cm and 60–100 cm soil layers, respectively (see Figure4A,B). This difference is direct evidence of the niche differentiation of fine roots in apple–soybean intercropping systems. When competition is unavoidable, plants exhibit greater root development in the deep layers of the soil to obtain more resources [31,32]. However, the FRMD of the 7-year-old intercropped apple trees was lower than that of the monocropped apple trees in each soil layer (see Figure4C), which might occur because the 7-year-old apple trees had a greater quantity of fine roots and because the competitiveness of soybean was much weaker than that of apple. Similarly, Bolte and Villanueva [33] reported that the fine roots of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were greater in pure forests than in mixed forests. The FRMD of the intercropped soybean plants was lower than that of the monocropped soybean plants in each soil layer (see Figure5). Similarly, Zamora et al. [ 3] reported that the fine roots of cotton (Gossypium hirsutum L.) intercropped with pecan (Carya illinoensis K. Koch) were less abundant than those of monocropped systems in each soil layer. Interspecific interactions between trees and crops may result in increased capture of growth-limiting resources [34,35].
4.3. Fine-Root Niche Differentiation Responses in terms of root morphological plasticity to resource availability and/or plant competition constitute the main mechanism used by plant roots to avoid competition [16,17]. Changes in root morphology cause niche differentiation in plants, and ecologists believe that niche differentiation is the key to species coexistence [36]. It was observed that interspecific interactions in apple–soybean intercropping systems caused the fine roots of the intercropped apple trees to move toward deep soil and caused the fine roots of the intercropped soybean plants to move toward the soil surface (see Figure6). Many researchers have reached similar conclusions [ 6,11,12,15]. This niche differentiation of fine roots is conducive to alleviating below-ground interspecific competition and improving the efficiency of deep soil resource utilization in apple–soybean intercropping systems. Nevertheless, Zhang et al. [13] reported that the fine roots of intercropped jujube presented a shallower distribution in the soil profile than did those of monocropped jujube. This discrepancy may be due to differences in jujube’s own characteristics or resources. Furthermore, in this study, the competitiveness of the soybean fine roots gradually weakened with increasing tree age, while the competitiveness Sustainability 2018, 10, 3022 12 of 14 of the apple fine roots gradually increased. Therefore, the fine roots of both intercropped apple trees and intercropped soybean plants were gradually displaced to the shallow soil with an increase in tree age (see Figure6), which was an adaptive tactic of the fine roots of each component in the apple–soybean intercropping systems in response to intense below-ground interspecific competition. The differentiation of the intercropped apple and intercropped soybean fine-root niches ultimately determined the degree of niche overlap in the apple–soybean intercropping systems. The niche overlap between the intercropped apple trees and the corresponding soybean plants decreased with increasing distance from the apple tree row and increased with increasing tree age.
4.4. Below-Ground Interspecific Competition Intensity Below-ground competition is most likely to occur when two species absorb soil resources from the same soil layer or at the same time [37]. The competition between populations must meet the conditions for population niche overlap and an insufficient supply of shared resources [20,21]. Under the conditions of non-irrigated agroforestry management in the arid and semi-arid regions of the Loess Plateau, soil water and farmland nutrients are often insufficient [21]. Therefore, the niche overlap of the fine-root components of apple–soybean intercropping systems on the Loess Plateau will inevitably lead to competition for soil water and soil nutrients. The results from this study showed that the components of the 3-, 5-, and 7-year-old apple–soybean intercropping systems competed for soil water and nutrients at distances of 0.5–1.7, 0.5–2.1, and 0.5–2.5 m from the apple tree row (see Figure7), respectively. The below-ground interspecific competition intensity increased with increasing tree age, which was caused by the niche differentiation between the apple trees and soybean plants. The below-ground interspecific competition intensity was slightly greater south of the apple tree row than north of the row. Furthermore, a relatively greater intensity of below-ground interspecific competition in the 3-, 5-, and 7-year-old apple–soybean intercropping systems for soil water and nutrients occurred within distances of 0.5–0.9, 0.5–1.3, and 0.5–1.7 m from the apple tree row (see Figure7), respectively. These ranges were related not only to the age of the apple trees but also to the variety of intercropping crops and planting density of the apple trees and crop species. Niche overlap is the degree of shared use of the same resource by two species or the frequency of encounter between two species for the same resource [38,39]. Therefore, the degree of below-ground interspecific competition in the apple–soybean intercropping system was determined by the degree of common use for resources within the same soil layer by the apple trees and soybean plants.
5. Conclusions In conclusion, the fine roots of the apple trees and soybean plants were distributed mostly within the 20–40 cm and 0–20 cm soil layers, respectively. In addition, the intercropped apple trees and intercropped soybean plants had lower FRMD values than did the monocropped apple trees and monocropped soybean plants, respectively. Compared with that of the corresponding monocropped systems, the FRVB of the intercropped apple trees displaced deeper soil and that of the intercropped soybean plants displaced shallower soil. Furthermore, the FRVB of both intercropped apple trees and intercropped soybean plants displaced shallow soil with increasing tree age. The niche differentiation of the fine roots caused the niche overlap between the intercropped apple trees and the intercropped soybean plants to decrease as the distance from the apple tree row increased, while the overlap increased with increasing tree age. To effectively alleviate the below-ground interspecific competition in apple–soybean intercropping systems and obtain increased production, farmers should appropriately increase the distance between soybean plants and apple tree rows. Soybean plants intercropped with 3-, 5-, and 7-year-old apple trees should be planted in areas at distances of 0.9, 1.3, and 1.7 m away from apple tree rows, respectively. Furthermore, farmers should also increase water and fertilizer inputs within the 0–40 cm soil layer in intercropping areas. Irrigation and fertilization can be appropriately increased as the tree age increases, and the input should increase with the distance from the apple tree row; in addition, inputs south of apple tree rows should be slightly greater than those north of Sustainability 2018, 10, 3022 13 of 14 the rows. Additional research is needed on the amount of fertilizer and irrigation that should be reasonably invested.
Author Contributions: Y.S. and H.B. conceived and designed the experiments; Y.S., H.D., R.P., and J.W. performed the experiments; Y.S. analyzed the data; Y.S. wrote the paper; Y.S., H.B., and H.X. revised the paper. Funding: This work was financially supported by the National Natural Science Foundation of China (No. 31470638), the National Key Technology R & D Program of the Ministry of Science and Technology of China (No. 2015BAD07B0502), the National Key R & D Program of China (No. 2016YFC0501704) and the Beijing Municipal Education Commission (CEFF-PXM2018_014207_000024). Acknowledgments: The authors are grateful for the support received from the Ji County Forest Ecosystem Research Station. Many thanks are given to the anonymous reviewers and the editors for their helpful comments. Conflicts of Interest: The authors declare no conflicts of interest.
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