The Nutrient Status of Plant Roots Reveals Competition Intensities in Rubber Agroforestry Systems
Total Page：16
File Type：pdf, Size：1020Kb
Article The Nutrient Status of Plant Roots Reveals Competition Intensities in Rubber Agroforestry Systems Junen Wu 1,* , Huanhuan Zeng 2,3,4, Fan Zhao 2,3, Chunfeng Chen 2,3, Xiaojin Jiang 2,3, Xiai Zhu 2,3, Pingyuan Wang 2,3, Zhixiang Wu 5 and Wenjie Liu 2,3,* 1 Faculty of Geography, Yunnan Normal University, Kunming 650050, China 2 CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun 666303, China; [email protected] (H.Z.); [email protected] (F.Z.); [email protected] (C.C.); [email protected] (X.J.); [email protected] (X.Z.); [email protected] (P.W.) 3 Center of Plant Ecology, Core Botanical Gardens, Chinese Academy of Sciences, Menglun 666303, China 4 University of Chinese Academy of Sciences, Beijing 100049, China 5 Rubber Research Institute, Chinese Academy of Tropical Agricultural Sciences, Danzhou 571737, China; [email protected] * Correspondence: [email protected] (J.W.); [email protected] (W.L.); Tel.: +86-871-65941198 (J.W.); +86-691-8715071 (W.L.) Received: 10 October 2020; Accepted: 30 October 2020; Published: 31 October 2020 Abstract: Controversial competition theories may confuse the current understanding of belowground plant competition and thus result in incorrect diagnoses and mitigation strategies for nutrient competition. As such, the management of nutrient competition is a major challenge in the application and development of rubber agroforestry systems (AFSs). To explore the effects of plant competition on the nutrient status of rubber AFSs, this study measured the carbon, nitrogen, and phosphorus concentrations of the litter and soil and in plant leaves, stems, and roots from five rubber plantations (i.e., rubber monocultures and rubber mixed with cocoa, coffee, tea, and Flemingia macrophylla (Willd.) Merr., 1910)). The relative competition intensity indexes were calculated to evaluate the competition intensity of each mixed-species system, and Bayesian networks were established to investigate the linkage effects of interspecific competition for nutrients. This study demonstrated that rubber trees had weak competition with cocoa trees, moderate competition with F. macrophylla and tea trees, and intense competition with coffee trees. With the increase in competition intensity, the negative effects of interspecific competition on soil gradually offset the improvement in soil nutrients achieved with intercropping. Nitrogen and phosphorous translocation from the stems to the roots was enhanced by competition. However, enhanced nutrient allocation to roots may have led to insufficient nitrogen and phosphorous supplies in plant leaves. The quality of the litter therefore decreased because the nutrient status of fallen leaves determines the initial litter conditions. Such consequences may reduce the release of nutrients from the litter to the soil and thus increase soil nutrient depletion. This study revealed that competition effects were most obvious for the root nutrient status, followed by the stem and leaf nutrient statuses. Moreover, this study further demonstrated that the nutrient concentration of plant roots can better indicate the intensity of nutrient competition than the nutrient concentration of other plant organs. Keywords: agroforestry systems; Bayesian network; belowground competition; nutrient cycling; plant nutrient allocation; plant–soil interactions Forests 2020, 11, 1163; doi:10.3390/f11111163 www.mdpi.com/journal/forests Forests 2020, 11, 1163 2 of 17 Forests 2020, 11, x FOR PEER REVIEW 2 of 18 1. Introduction Introduction Understanding plantplant competition competition can can reveal reveal how varioushow various plant species plant maintainspecies theirmaintain coexistence their coexistencein natural ecosystems in natural [1 ]ecosystems and help guide [1] theand design help guide and management the design ofand artificial management ecosystems of [2artificial,3]. The ecosystemslatter advantage [2,3]. has The more latter practical advantage significance—for has more example,practical insignificance—for the construction example, of rubber-based in the constructionagroforestry systemsof rubber-based (AFSs; Figure agroforestry1a). Such systems artificial (A forestsFSs; Figure could solve 1a). Such the disadvantages artificial forests of rubber could solvemonocultures the disadvantages and provide of arubber series ofmonocultures benefits to rubber and provide cultivation a series areas of [4 benefits]; therefore, to rubber rubber cultivation AFSs have areasbeen labeled[4]; therefore, as having rubber a high AFSs economic have been potential labele andd as a having high biodiversity a high economic and being potential environmentally and a high biodiversityfriendly and usefuland being in sustainable environmentally development friendly in manyand useful studies in [ 2sustainable,4]. However, development the practical in benefits many studiesof many [2,4]. rubber However, AFSs rarely the practical meet the benefits expectations of ma ofny farmers. rubber AFSs One ofrarely the most meet important the expectations reasons of is farmers.belowground One of competition, the most important which isreasons caused is bybelo improperwground combinations competition, which of species is caused andresults by improper in the poorcombinations growth of of both species the treesand andresults the intercropsin the poor [2 ].gr Therefore,owth of both this issuethe trees affects and the the harvests intercrops of rubber [2]. AFSsTherefore, and impedes this issue their affects promotion. the harvests of rubber AFSs and impedes their promotion. Figure 1. ((aa)) Schematic Schematic diagram diagram of of an an agroforestry agroforestry system; system; ( (b)) the the common common nutrient nutrient cycling cycling processes processes in an agroforestry system. Although manymany researchersresearchers have have recognized recognized that that competition competition is anis an important important consideration consideration in the in thedesign design of AFSs, of AFSs, “how “how plants plants compete compete with otherwith othe plantsr plants below below the ground” the ground” is still ais controversial still a controversial topic in topicagroforestry in agroforestry and ecological and ecological studies [ 5studies–10]. This [5–10]. is mainly This is because mainly belowground because belowground competition competition is covert [ 1is], covertthe heterogeneities [1], the heterogeneities of environmental of environmental resources are resources complex are [11 ],complex and conventional [11], and experimentalconventional experimentalapproaches, suchapproaches, as time-point such as biomass-basedtime-point bioma methods,ss-based methods, reveal only reveal competitive only competitive outcomes outcomes rather ratherthan competitive than competitive processes processes [10]. [10] Therefore,. Therefore, controlling controlling competition competition among among plants plants is is di difficultfficult inin most AFSs [[2],2], even though relevant studies have been conducted over over multiple multiple decades decades [3,10]. [3,10]. For For example, example, increasing the application of fertilizer is regarded as a useful means of weakening plant competition for soil nutrients [3,12]. [3,12]. However, However, an increasing number of competition experiments suggest that the general intensity ofof interspecificinterspecific competitioncompetition in in plant plant communities communities is is not not weakened weakened by by the the addition addition of ofnutrients nutrients [13 –[13–15].15]. Other Other methods, methods, such such as root as pruning root pruning or root or segregation, root segregation, may be emayffective. be However,effective. However,such methods such are methods impractical are imprac becausetical they because require they long-term require long-t humanerm interference human interference and may also and a mayffect plantalso affect growth. plant Thus, growth. to understand Thus, to understand plant competition plant competition for nutrients for andnutrients test the and related test the means related of meanscontrolling of controlling competition competition in AFSs, the in AFSs, intensity the ofintensity this competition of this competition must first must be recognized. first be recognized. Nutrient diagnosis would would seem seem to to be an idealideal approachapproach forfor recognizingrecognizing the intensitiesintensities of plantplant nutrient competition because it it is generally applied in the management of fertilization on on plantations [16]. [16]. However, only leaf nutrient diagnosis is widely applied to determine if nutrient resources on rubber plantations are maintained at an optimum level fo forr plant growth; thus, information on the nutrient status of other parts of mixed-species AFSs, which could be used to estimate the intensities of plant competition in in the the AFSs AFSs [4,12], [4,12 is], scarce is scarce [3,16]. [3, 16Therefor]. Therefore,e, if only if leaf only nutrients leaf nutrients are considered are considered in nutrient in competitionnutrient competition experiments, experiments, the functions the functions of nutrient of nutrient storage, storage, nutrient nutrient translocation, translocation, and and nutrient homeostasis in plants and the linkage effects effects of plant interactions on nutrient cycling (Figure 11b)b) wouldwould be totally neglected [[17,18].17,18]. To explore suitable indicators to estimate the intensity of competition for nutrients in rubber AFSs and and the the effects