|Productivity formation in relation to soil organic carbon stabilization and its mechanisms in dryland intercropping systems
|Place of Conferral
|旱地农业 dryland agriculture 间作系统 intercropping system 种间相互作用 interspecific interaction 生产力 productivity 碳周转 SOC turnover
间作系统（intercropping system）是通过种间相互作用和生物多样性效应（补 偿效应和选择效应）提高土地生产力和资源利用效率的种植模式，对农田生态系 统碳过程和土壤碳封存有显著影响。过去几十年，尽管有大量试验研究了间作系 统的产量优势和根际过程，但旱地间作系统生产力形成的资源依赖性、物种配置 依赖性及地上地下部分调控机理等系统探索尚未见报道，该问题与土壤有机碳稳 定性机制密切相关。不同作物配置的竞争力差异和可利用资源差异（比如水分和 养分）是否重塑种间互作关系？这种变化如何影响系统生产力形成？进而如何影 响土壤有机碳（soil organic carbon, SOC）周转和稳定性？上述三个相对独立、但 紧密关联的问题既是农业生态学中的基础科学问题，也是全球变化生态学关注的 热点问题。 本研究以旱地间作系统为研究对象，围绕旱地间作系统生产力形成和土壤有 机碳稳定性机制这个关键科学问题，在黄土高原半干旱雨养农业区开展了连续 4 年（2019-2022）的长期定位观测试验，完成了 4 个相对独立、但紧密关联的大田 试验。本研究以玉米、小麦、大豆和豌豆等作物物种为试验材料，设置了 3 个传 统的大田试验（两年和四年重复）和 1 个多年长期观测试验，融合了生态学、植 物生理学和土壤学经典方法，开展交叉学科探索。主要得出如下结果： （1）旱地间作系统生产力形成具有明显的水分依赖性特征，土壤水分条件 的改善降低了物种间的竞争强度。 在玉米-蚕豆间作系统中，覆膜和不覆膜条件下土壤水分有效性对种间互作 关系和系统生产力带来显著影响。低土壤含水量（不覆膜）使间作系统净效应（NE） 为负、土地当量比（LER）小于 1，造成间作系统产量损失。提高土壤水分有效 性（覆膜）扭转了这一趋势，使间作系统增产（NE>0 和 LER>1）。然而，无论土 壤水分含量如何，蚕豆均为优势种，而玉米为劣势种。低土壤水分提高了选择效 应对生产力的贡献（p0.05）。与大豆间作显著促进了玉米和小麦根系向根际土壤可溶性有机碳 （DOC）和易氧化有机碳（EOOC）的输入（p<0.05），从而提高了玉米和小麦条 带土壤微生物活性（MBC 和 MBN）、氮素矿化速率和无机氮水平。然而这使土 壤碳排放较单作系统显著提高 10.6%。相反，与小麦间作显著降低了玉米条带土 壤无机氮含量和土壤储水量，造成玉米根系向土壤输入的生物量和不稳定碳 （DOC 和 EOOC）含量显著下降。这一趋势进一步降低了微生物活性，从而使 土壤碳排放下降了 7.4%（p<0.05）。 （4）种间互作通过影响土壤养分有效性和微生物活性显著改变了作物根系 III 碳输入、激发效应和碳排放过程，从而影响根际碳周转过程。 通过 13C 标记手段对不同种植模式下各作物根际碳周转过程，尤其是对激发 效应和碳排放的驱动过程及其机制进行了探究。研究发现，禾-豆间作体系中玉 米和小麦根际沉积碳（13C）输入和土壤碳排放平均增加了27.5%（p<0.05）和12.7% （p<0.05），但是较单作显著降低了根际激发效应约 27.3%（p<0.05）。相反，间 作大豆的根际沉积碳平均降低了 15.0%，而根际激发效应和土壤碳排放分别平均 提高了 32.0%（p<0.05）和 29.1%（p<0.05）。来自小麦的种间竞争显著降低了玉 米根际沉积碳输入、根系呼吸和碳排放（p<0.05），但是对激发效应影响不显著。 与大豆间作显著提高了玉米和小麦根际矿物氮和有效磷含量，这一方面降低了禾 本科作物根际微生物对养分的需求，从而降低了微生物对有机质分解造成的激发 效应。然而，与大豆间作改善了禾本科作物根际土壤养分状况，产生了作物根系 到土壤微生物的正向级联效应，使根系呼吸、微生物活性和易分解碳获取酶活性 都显著提升，从而提高了土壤碳排放。而属于大豆生态位的养分被禾本科作物汲 取后，大豆不得不通过介导微生物胞外酶分泌，加速 SOM 分解来维持自身需求， 从而提高了根际激发效应强度和碳排放。 （5）旱地禾豆间作系统有效第促进了土壤养分富集和 POM 分解，从而提 高不稳定碳向 MAOC 的转化。 本研究在 2019-2022 年定位观测禾-豆间作系统和相应单作系统的基础上， 对 13C 在不同土壤碳库中的同化和分配，以及 13C 标记的秸秆在田间分解中碳氮 流失进行了系统性观测。结果表明，在 4 年尺度上，与单作相比，所有禾-豆间 作系统均显著提高了禾本科作物生物量、氮和磷吸收量，表现出种间促进。常规 禾-豆间作系统根区土壤氮素含量显著增加，而 SOC 含量并没有出现显著性变化。 13C 标记试验结果表明，在所有禾-豆间作处理中，更多的 13C 被转移到矿物结合 态有机质中（MAOM）中，而较少保留在颗粒态有机质中（POM）。同时土壤微 生物固定的 13C 更多，而在 DOC 中保留的 13C 较少。另外，在禾-豆间作系统中， 秸秆分解造成的 13C 和 N 的流失更多。造成以上现象的原因在于种间促进介导 的土壤氮素富集和土壤酸化（pH 降低），降低了土壤和微生物碳氮比，从而加速 了土壤中易分解有机质的分解和作物秸秆的分解。 （6） 旱地禾-豆间作加速了微生物碳泵效应，促进了土壤 POC 向 MAOC 的转变，然而这一过程由于较低的微生物碳利用效率造成了深层土壤碳损失。 为了从更长时间尺度上理解禾-豆间作系统生产力优势以及对土壤有机碳库 周转和稳定性的影响。本研究在长达 10 年（2013-2022）的禾-豆间作系统及相应 IV 的单作系统中发现（试验 4），玉米-大豆和小麦-大豆间作系统的 10 年平均土地 当量比（LER）分别达到了 1.34 和 1.35，净效应（NE）达到了 3.72 t ha-1 和 1.47 t ha-1，表明禾-豆间作系统具有极大的增产潜力。另一方面，与禾本科单作相比， 禾-豆间作对表层土壤（0-40cm）有机碳含量无显著性影响，然而深层土壤（40- 100cm）有机碳出现显著性下降（4.6%）。在浅层土壤中，禾-豆间作系统土壤 fPOC 和 oPOC 较单作系统分别显著下降 16.8%和 5.3%，而 MAOC 较单作显著上升了 5.1%。深层土壤 fPOC 和 oPOC 较浅层下降幅度更大。与单作系统相比，禾-豆间 作系统浅层土壤 TN 和无机氮分别上升了 13.9%和 13.1%，而土壤 C/N 比下降了 16.7%（P<0.05），深层土壤也表现出相似的趋势。禾-豆间作较单作显著提高了土 壤微生物活性，降低了微生物生物量 C/N 比，这一趋势刺激了微生物碳分解酶活 性（BG+CBH）提高，使土壤微生物呼吸（MR）上升了 34.7%。然而，在深层土 壤中，禾-豆间作具有较高的微生物熵（qMBC）和代谢熵（qCO2），表明具有较 低的碳利用效率。 综上所述，间作系统在旱地农业生态系统中对作物水分利用与生产力形成的 关系以及对旱地土壤碳周转的生物地球化学过程的影响，并揭示了机理。研究结 果对于理解间作系统在旱地农业生态系统所提供的生态系统服务功能具有重要 意义，也为旱地农业生态系统可持续性集约化生产提供了理论依据。
Intercropping system is a farming mode that improves land productivity and resource utilization efficiency through interspecific interactions (competition and facilitation) and biodiversity effects (compensation and selection effects). It can gnerate significant impacts on carbon process and soil carbon sequestration in farmland ecosystems. Over the past few decades, although a large number of experiments have been conducted to explore the yield advantages and rhizosphere processes of intercropping systems, there have been little systematic explorations on the resource dependence, species configuration dependence, and regulation mechanisms connecting aboveground and underground parts about productivity formation in dryland intercropping systems. This issue is closely related to the stability mechanism of soil organic carbon (SOC). Do the differences in competitiveness and available resources (such as water and nutrients) among different crop configurations reshape interspecific interactions? How does this change affect the formation of system productivity? How does it affect the turnover and stability of SOC? These three relatively independent but closely related issues are not only basic scientific issues in agricultural ecology, but also hot issues of global change ecology. Taking the dryland intercropping systems as the research object, this study is aimed at the key scientific issues of productivity formation and SOC stability mechanism in the dryland intercropping systems. Long-term filed observations were conducted in the semi-arid rainfed agricultural area of the Loess Plateau for six consecutive years (2017-2022). Six relatively independent but closely related field experiments were completed. In this study, maize, wheat, soybean, pea and other crop species were used as experimental materials. Five traditional field experiments (twoyear repeated observations) and one multi-year long-term observation experiment were set up. The classic methods in ecology, plant physiology and soil science were VI integrated to carry out interdisciplinary exploration. The main results are as follows: (1) The formation of productivity in dryland intercropping systems was highly dependent on water availability, and the improvement of soil water status reduced the intensity of competition among species. In the maize-soybean intercropping system, soil water availability under film mulching and non-film mulching conditions generated significant impacts on interspecific interactions and system productivity. Low soil moisture content (without mulching) resulted in a negative net effect (NE) and a land equivalent ratio (LER) of less than 1 in the intercropping system, resulting in a loss of yield in the intercropping system. Improving soil water availability (mulching) reversed this trend and increased yield in intercropping systems (NE>0 and LER>1). However, regardless of soil moisture, soybean was the dominant species, while maize was the inferior one. Low soil moisture increased the contribution of selection effect to productivity (p0.05). Intercropping with soybean significantly increased the input of dissolved organic carbon (DOC) and easily oxidized organic carbon (EOOC) from maize and wheat roots into rhizosphere soil (p<0.05), and thus increased the microbial activity (MBC and MBN), N mineralization rate and inorganic N level in maize and wheat strip soil. However, it significantly increased soil carbon emission by 10.6% compared with monoculture system. In contrast, wheat turned to significantly lower soil inorganic N content and soil moisture in maize strips owing to belowground competition, which decreased labile C input (DOC and EOOC) and microbial activity. As such, carbon emission was significantly lowered by 7.4%. (4) Interspecific interaction significantly changed the process of crop root carbon input, priming effect and carbon emission by affecting soil nutrient availability and microbial activity, thereby affecting the rhizosphere carbon turnover process. The rhizosphere carbon turnover process of each crop under different planting patterns was explored by 13C labeling, especially the driving process and mechanism of priming effect and carbon emissions. The results showed that the rhizosphere C input and soil C emission of maize and wheat were increased by 27.5% (p<0.05) and 12.7% (p<0.05) VIII on average, but the rhizosphere priming effect (RPE) was significantly decreased by 27.3% (p<0.05) compared with that of monoculture system. In contrast, rhizodeposited C was decreased by 15.0% on average, while RPE and soil C emission were increased by 32.0% (p<0.05) and 29.1% (p<0.05), respectively. Interspecific competition from wheat significantly reduced C input, root respiration and C emissions of maize (p<0.05), but had no significant effect on RPE. Intercropping with soybean significantly increased the rhizosphere mineral N and available P contents of maize and wheat, which reduced the rhizosphere microorganisms' demand for SOM decomposition, thus reducing the RPE. However, the improved soil nutrient status produced a positive cascade effect between crop roots and soil microorganisms, which improved root respiration, microbial activities, and carbon degrading enzyme activities (β-glucosidase and βcellulolytic enzyme), thus enhancing soil C emissions. However, after the nutrients of soybean niche are absorbed by cereal crops, they need to maintain their own demand by mediating microbial extracellular enzyme secretion and accelerating SOM decomposition, thus increasing the intensity of RPE and C emission. (5) The dryland cereal-legume intercropping system effectively promoted soil nutrient enrichment and POM decomposition, thereby improving the conversion of unstable C to MAOC. In this study, the assimilation and allocation of 13C in different soil C pools and the C and N loss from 13C labeled straw decomposition was observed in the field based on the location observation of cereal-soybean intercropping system and corresponding monoculture system during 2019-2022. At the four-year scale, plant facilitation was constantly observed since plant biomass, N and P uptake benefits were significantly promoted in the legume-led intercropping relative to the monoculture. While total SOC was not evidently improved in cereal-soybean intercropping systems, the soil N was significantly enriched in the root-zone soils. In the 13C tracking trial, more labeled 13C was translocated to soil MAOM and less retained in soil POM. As such, more 13C was immobilized and saved by soil microorganisms and less was retained in the labile C pool. In the straw decomposition trial, more 13C was lost from the straw following 150-day landfill in the cereal-legume intercropping systems. Plant facilitation-mediated soil N enrichment and acidification were key mechanism lowering soil and microbial C/N, and thus accelerated the label SOM and straw decomposition. (6) The dryland cereal-legume intercropping accelerated the microbial carbon pump effect and promoted the transition from soil POC to MAOC, but this process IX caused deep soil carbon loss due to low microbial carbon use efficiency. In order to understand the productivity advantages of cereal-legume intercropping systems and their effects on SOC pool turnover and stability on longer scales, a long-term (10 years) experiment was established from 2013 to 2022. Results found that the 10-year average LER of maize-soybean and wheat-soybean intercropping systems reached 1.34 and 1.35, and NE reached 3.72 t ha-1 and 1.47 t ha-1 , respectively, indicating that the cereallegume intercropping system has great potential for yield increase. On the other hand, compared with cereal monoculture, cereal-legume intercropping had no significant effect on the SOC content of surface soil (0-40 cm), but SOC decreased significantly (4.6%) in deep soil (40-100 cm). In surface soil, fPOC and oPOC decreased by 16.8% and 5.3% respectively, while MAOC increased by 5.1% compared with monocropping system. Deep soil fPOC and oPOC decline more than surface soil. Compared with the monocropping system, TN and inorganic nitrogen increased by 13.9% and 13.1%, respectively, while the soil C/N ratio decreased by 16.7% (p<0.05), and deep soils showed a similar trend. Compared with monoculture, cereal-legume intercropping significantly improved soil microbial activity and reduced the C/N ratio of microbial biomass, which stimulated the activity of microbial carbonase (BG+CBH) and increased soil microbial respiration (MR) by 34.7%. However, in deep soils, cereallegume intercropping has higher microbial entropy (qMBC) and metabolic entropy (qCO2), indicating lower carbon use efficiency. To sum up, in the past six years, this study comprehensively analyzed the impact of intercropping system on the relationships between crop water use and productivity formation in dryland farming ecosystem, as well as on the biogeochemistry process of soil carbon turnover in dryland. The relevant mechanisms were revealed and discussed. The findings prove to be of great significance for understanding the ecosystem services of intercropping systems in the dryland agricultural ecosystems. They also provide a theoretical basis for the sustainable and intensive production of the dryland agricultural ecosystem.
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|王伟. 旱地间作系统生产力形成和土壤有机碳稳定机制[D]. 兰州. 兰州大学,2023.
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