稻田伴生浮萍碳、氮汇及对水稻产量影响的研究进展

2023-12-28景立权李凡赵一函王训康赵福成赖上坤孙小淋王云霞杨连新

中国农业科学 2023年23期

景立权，李凡，赵一函，王训康，赵福成，赖上坤，孙小淋，王云霞，杨连新

1扬州大学农学院/江苏省作物遗传生理重点实验室/江苏省作物栽培生理重点实验室/江苏省粮食作物现代产业技术协同创新中心，江苏扬州 225009；2浙江省农业科学院玉米与特色旱粮研究所，浙江东阳 322100；3江苏省农业科学院宿迁农科所，江苏宿迁 223800；4上海市农业科学院生态环境保护研究所，上海奉贤 201403；5扬州大学环境科学与工程学院，江苏扬州 225009

浮萍是一种常见于静水环境中的水体漂浮微观植物。以大气CO2浓度增高为主导致的温度上升为特征的气候变化威胁着粮食安全。或因气候变暖及灌溉水体富营养化等，近年来我国稻田浮萍伴生有逐年加重趋势。本文综述了浮萍对稻田的影响，发现了一些重要信息：浮萍伴生降低稻田水体温度0.8—2.76 ℃及pH 0.10—0.45，改变了微生物群落结构，减少稻田NH3挥发18.2%—59.0%，提高氮利用率17.2%—78.0%，结果增加了稻田氮汇及稻谷产量（9.0%—34.6%）；伴生浮萍生长繁殖快，其年产生物量可达8×103—13×103kg·hm-2，碳汇几乎与当季水稻相当；水稻浮萍的互利共生总体大于竞争，二者伴生呈现了稻田生态系统对环境变化适应的现象。但未来本领域仍需深入研究，包括浮萍伴生，特别是与环境因子互作（高温及高CO2浓度等）条件下，对稻田生态环境变化、水稻生长、产量、品质的影响及机制和可能带给稻田的风险等，为未来基于水稻-浮萍等生物协作开发适应气候及环境变化、维持农业可持续发展的稻作技术提供理论支撑。

浮萍；水稻；碳汇；氮汇；产量

0 引言

浮萍（L.）是繁殖快、富有营养、遍布全球的水体漂浮微观植物，常见于静水环境中[1-2]。或因气候变化及灌溉水体富营养化等，近年来我国稻田浮萍伴生有逐年加重趋势：据本团队对海南（图1-a）、浙江、上海、江苏（图1-b、d、e、f）、吉林（图1-c）、黑龙江等共126个独立田块的调查，稻田浮萍发生概率达69.8%，而极端高温的2022年[3]相对更为严重（镇江扬州由2021年的70.8%增至2022年80.0%）；且本团队基地也发现，环境CO2浓度及温度增高条件下浮萍生长繁殖速率有倍增趋势（图1-d—1-f）——种种迹象表明稻田伴生浮萍的发生总体随施肥水平[4]、大气CO2浓度及温度[5]的升高而加重。

图a，b分别为海南三亚及江苏镇江生产稻田——浮萍发生严重；图c为吉林乾安生产田——浮萍发生率低；图d为水稻大田试验对照区（常规处理，江苏扬州）——浮萍生长繁殖相对缓慢；图e为高温（+1℃）下，浮萍生长繁殖相对加快；图f为高CO2浓度（+200 μmol·mol-1）及高温（+1℃）同时处理下浮萍生长繁殖最快

全球CO2排放量持续增加，近年来屡创历史新高[6]。以大气CO2为代表的温室气体及其导致的气温上升是全球气候变化的基本特征[7-9]，是极端气候事件发生的关键诱因，抑或是刺激浮萍大面积发生的重要因素，剧烈地影响着粮食生产[3,10]。2020年，我国政府庄严承诺“双碳”计划，实现“双碳”目标，一要“减排”，二要“碳汇”[11]。陆地生态系统固碳减排被认为是一种经济可行、环境友好的中和CO2的有效途径，是实现自然“增汇”的主体。尽管增加森林、草地等植被面积是实现“双碳”目标的重要举措，但这也加剧了其“与粮争地”的矛盾，农田生态碳汇依然是我国陆地生态系统固碳减排的关键[12-13]。

水稻为全球67.0%的人口供应主食[14]，为超过全球一半的人口提供蛋白及矿质元素等基本营养[15]。稻田占据我国33.0%的总耕地面积，生产了占全球29.0%的稻谷[16]。与欧、美发达国家不同，我国人多地少，包括固碳减排在内的所有涉农问题的解决都必须基于我国粮食的绝对安全。在野外条件下，浮萍伴生可显著降低水体温度[17]及pH[18-19]，其生长快、固氮能力强[20-23]，年产生物量达8×103—13×103kg·hm-2[24]——伴生浮萍降温幅度同21世纪中后期温度增幅[25]相似，而其碳、氮汇能力则与当季水稻相当。如能与稻互利共生，稻田伴生浮萍不仅是一种理想的生态碳汇载体，也将是一种优异的稻田应对气候及环境变化的媒介，这与我国“双碳”战略吻合。浮萍在基础学科领域的研究已取得重大进展，发达国家产业化利用发展迅速[26-28]。本文从浮萍的植物学特征、生育特点和稻田碳、氮汇方面剖析其与水稻生长的关系，展望本领域未来的研究方向，以期为稻田碳、氮固定及水稻增产增效提供参考，为基于生物协作的适应未来气候及环境变化稻作技术开发提供理论支撑。

1 浮萍植物学特征及生育特点

浮萍，学名L.，单子叶纲天南星目浮萍科，结构简单，是目前世界上最小的单子叶开花植物[29-30]，已广泛应用于植物生理学、分子生物学及生态学等研究领域[28]。浮萍通常仅由3—6 mm×2—15 mm 的叶状体和长1—5 cm、直径小于0.5 mm的1条或多条细根（甚至无根）组成（图2）[5]，叶状体呈绿色，背面有时呈紫色。浮萍全世界共有5属，其中青萍和紫萍在我国较为常见。与紫萍相比，青萍适应温度及pH范围大，营养物质耐受性强，化学需氧量浓度范围广[27]，因此在碳、氮固定及适应力方面更有优势[28]。浮萍富含蛋白质、氨基酸、胡萝卜素及碳水化合物等[31-33]，其含水率因品种及生长环境差异而不同，通常为86.0%—97.0%。多数浮萍蛋白含量为干重的15.0%—45.0%[1,29,34]，理想条件下可达26.3%— 45.5%，介于苜蓿（20.0%）和大豆（41.7%）[35]之间，而粗纤维含量仅为5.0%—15.0%，是一种天然绿肥[36]。浮萍亦广泛富集水体重金属、矿质元素和有机污染物等[26,36-37]，鉴于此，美国已将其推荐为深度净化水体氮、磷及其资源化利用的革新技术载体[38]。

图a、b引自AN等[1]，为不同时期浮萍的形态学特征；c、d为江苏省作物遗传生理重点实验室及实验田模拟图

浮萍有无性和有性两种生殖方式，生命力强，在5—34 ℃、pH 4.5—9.0环境下均可生长繁殖[5]，20—30 ℃、pH 6.5—7.5时生命活动最为旺盛[39]。浮萍可通过根或叶状体直接从空气中吸收CO2，从水中吸取氮、磷、钾等营养物质，由其叶绿体通过光合作用合成并积累有机物[29,40]。通常情况下，浮萍以成熟叶状体分生芽孢进行无性繁殖为主，在低于5 ℃时进入休眠状态，在其夹囊或开口内形成休眠芽。浮萍休眠芽小、通气组织少、细胞壁厚，逆境（或冬季）时可脱离母体沉入水底，逆境解除后利用储藏在休眠体内的淀粉发芽而形成完整植株。浮萍的植物学特征和生育特点为稻田生态及资源化利用奠定了基础。

2 国内外浮萍研究与应用情况

美国最早（1925年，SAEGER[41]）发表了相关浮萍的研究。目前，美国罗格斯大学和中国科学院成都生物研究所保存众多浮萍资源。中国科学院其他单位（植物研究所、水生生物研究所及青岛生物能源与过程研究所）及海南大学也有部分保存[28]。就全球研究热度和水平来说，美国均居首位，是浮萍研究的主导国家，其次集中在欧、日等发达地区，尽管我国关于浮萍的发文量位列全球前五，但研究水平相对还较低[5]。

目前浮萍研究热点主要集中在污水净化及能源利用方面[5,42]，而在应对环境污染、全球气候变暖及食物短缺方面浮萍也具有巨大的潜力[43-45]。因此，2014年美国能源部重点资助了有关浮萍的基础研究，计划从浮萍基因组学和分子生物学方面率先取得突破，这也意味着浮萍产业化应用的大门打开了[1]。作为食品开发的潜力植物，浮萍研究也得到了荷兰政府资助[28]。但在研究浮萍的所有领域中，农学相关研究很少，仅占5.0%[5]，且大多集中在伴生浮萍截留水体氮、磷等矿质元素流失及抑制杂草方面，而基于农学、农艺学等的稻田伴生浮萍碳、氮汇及其直接对水稻生长影响方面的研究则相对缺乏。

3 稻田伴生浮萍碳、氮汇及对水稻产量的影响

3.1 伴生浮萍碳汇及机理

适宜条件下，浮萍可呈指数增长[46-47]，《本草纲目》记载“一叶经宿，即生数叶”，是目前已知生长最快的植物。浮萍无性繁殖时，一个叶状体母芽体内一般包括6—7个处于不同发育阶段的芽孢，通常繁殖一代仅需2—7 d[28,48]，甚至不足1 d[49-50]。研究表明，在实验室条件下，浮萍物质积累可达每周1—2 kg·m-2（鲜重）[50-51]，生产淀粉的能力平均是玉米的5—6倍[47]，经过诱导后淀粉含量甚至可达干重的65.0%[52]，固碳能力极强，是一种理想的生物碳汇载体。与其他植物相比，浮萍科退化严重，除快速生长繁殖外，浮萍已摆脱大部分基因的束缚[1,53]，这或是浮萍生长繁殖快的本质原因；同时浮萍的生长与繁殖受品种和环境影响也很大[54-55]，这是建立稻-萍生物协作稻田生态关系的基础。浮萍在稻田中的生长与繁殖，可循环生产并积累有机物[56]，其腐解后作为绿肥回归稻田[36,57]，碳汇的同时增加稻田有机质。

3.2 伴生浮萍氮汇及机理

粮食供应、能源危机、氮肥利用及环境污染一直是我国经济社会发展的潜在矛盾。化肥贡献了全球一半以上的粮食增产[58]，而化肥工业本身高耗能、高碳排。农田是全球氮污染及温室气体排放的主要来源[13,20,59-61]。据早期研究估算，氮污染危害抵消了全球0.3%—3.0%的GDP[62]，与其对作物增产效益相抵[63]。我国农田施肥量大[58]，化肥利用率低（仅为40.2%[64]，远低于发达国家的52.0%—67.0%[63]），地区间不平衡（22.0%—70.0%）[65]，氮损失严重[39,63]，对环境的负面影响大[58,66]。我国化肥约30.0%用于水稻生产[39]，提高稻田化肥利用率对我国农业可持续绿色发展、环境保护及碳、氮汇等意义重大[58]。

区别于其他作物，水稻耗水多[67]，水稻整个生育期稻田多有水层覆盖。NH3挥发是稻田氮肥损失最主要途径之一[68]，占氮总损失的40.0%— 50.0%[39]，由于高温高湿气候，我国南方稻田更为严重[69]。浮萍的氮含量接近3.0%，高于一般的湿地植物，包括水稻[70-71]，氮汇能力很强。前人研究证明（表1），浮萍伴生可降低稻田水体温度[18-19,21]及pH（0.10—0.45），降低NH3挥发达18.2%—59.0%，进而减少稻田总氮损失11.2%—13.6%[20]，结果提高氮肥利用率17.2%—78.0%，最终增加了稻谷产量，增幅达9.0%—34.6%（表1）。

浮萍伴生主要通过以下三个方面提高稻田氮利用率：①浮萍植株体上下通透，其根系及叶面等均具有发达的气体交流系统，可将空气中的氧气通过叶片及根系转移至根际水中，在浮萍植株体附近形成微生物膜，为氮代谢微生物的活动提供适宜的环境，从而促进氮形式的转化，降低水体铵态氮浓度，抑制氮的挥发[20,74-75]；②同时浮萍对水体铵态氮也具有较好的吸收和吸附作用[75]，浮萍与水体进行物质交换，主要截留并吸收原本因NH3挥发而损失的氮（几乎不与稻争肥），其凋亡后又将吸收的氮素重新释放至稻田，浮萍的生长与腐解对水体氮含量的变化起到了“缓冲”作用，进而促进氮向土壤转移[20]，形成稻田“氮库”效应，满足水稻生育后期对氮的需求，从而提高稻田氮利用率[72]；③此外，包括浮萍在内的水体漂浮微观植物与水稻伴生时，其物理覆盖降低了水体受光强度，抑制了迟发杂草光合作用，增加了CO2的溶解，进而降低水体pH及温度[18,21]，降温幅度可达0.86—2.76 ℃[19]。科学家的深入研究表明，浮萍伴生导致的水体pH、温度变化对减少稻田氮损失起关键作用[20]，通常两个指标的大小与稻田NH3挥发速率保持显著正相关关系[40,68]。与浮萍相似，同样作为漂浮植物的满江红，尽管植物学及形态学等特征与浮萍迥异，但也具有相似的稻田氮汇能力[57,76-78]。这些说明浮萍与其他微观漂浮植物作用机理相似，除物理覆盖外，对稻田还包括明显的“透膜”效应：吸光、阻光、透气及碳、氮汇等，并保持与水体进行物质交换。

表1 浮萍伴生对稻田生态环境因子及水稻产量的影响

——表示未被作者报道；↓ 抑制或降低；↑ 促进或提高 —— This data wasn’t reported by its authors; ↓ Suppress or reduce; ↑ Promote or enhance

3.3 微生物及化感物质

浮萍伴生可通过稻田碳、氮汇及抑制病、草害等间接影响水稻生长[79-81]，但更为复杂的是，其也可通过自身及自身寄宿的微生物群落释放次级代谢产物直接作用于水稻的生长。研究发现，稻田伴生浮萍寄生的微生物菌落与水稻极为相似[30]，这就意味着各自微生物的丰缺彼此影响各自的生命活动：HUANG等[81]研究证实，从水稻器官分离出的254种优势菌落（）可完全移植于无菌浮萍上，这些细菌分泌的生长素可同时促进稻、萍的生长；LU等[23]也发现，浮萍根际寄宿的微生物分泌的豆甾醇具有明显促进反硝化细菌脱氮及其生物膜形成，进而促进氮在稻田水体的形式转化。当稻田伴生浮萍密度过高时也会产生一些化感物质，如酚酸、长链脂肪酸、乙烯及其他生长素类物质等[52,82]，其中的脂肪酸甲酯及脂肪酸酰胺可刺激反硝化细菌脱氮[83]，而浮萍腐解也可增加水体疏水性酸（hydrophobic acids）和类海水腐殖质（marine humic-like substances）浓度，进而促进氮向土壤转移，改变水体微环境及微生物生存状态[52,68]，抑制与其竞争光照及营养的杂草的生长[84-85]。这些化感物质自身具有易降解的特点[86]，目前尚未发现它们威胁水稻生长的报道，但对浮萍自身的生命活动具有明显的调控作用，如，有学者证实，浮萍在含有细胞分裂素的水体中生存时间被大大延长[43]。大多数化感物质的生物化学结构及功能是一致的，同一种物质往往对不同植物有相同的作用，浮萍与水稻通过化感物质相互作用程度如何？目前尚不清晰。

3.4 浮萍伴生对水稻产量的影响及机理

近20年，稻田浮萍伴生直接对水稻生长及产量影响的报道很少，国内外仅发现9例（表1），其中7例[12,17,19-20,39,72-73]证实促进水稻产量的形成，平均增产幅度15.8%，1例认为不利于水稻生长[44]，而1例效应不明显[36]。2008年，广西大学实验结果证实，浮萍处理提高了土壤氮、磷、钾及有机质含量，促进了水稻后期的抗倒伏能力形成，同时提高了水稻产量构成的4个因子，与稻糠配合施用效果更好，综合增产稻谷高达11.1%[39]；2009年，浙江大学报道，不同氮水平下浮萍伴生降低了稻田水体pH、温度及NH3挥发平均分别为0.45、0.9—2.1 ℃和33.2%—53.7%，结果使水稻增产9.4%—9.8%[17]；2017年，中国科学院南京土壤研究所探索稻田土壤氮流失规律时发现，伴生浮萍在水稻生育的前期截留并吸收原本因NH3挥发而损失的氮，后期通过干湿交替或其他栽培措施的调控使浮萍逐渐凋亡，并将浮萍吸收的氮重新释放至稻田，满足水稻生育后期对氮素的需求，从而提高氮利用率35.0%—78.0%，增产水稻9.0%—10.0%[72]；2019，南京林业大学联合中山大学等报道，在配施生物炭的情况下浮萍可提高稻田氮利用率17.2%，增产水稻10.9%[12]；而与其相似的满江红也具有相同的效果[57,76-77]；2021年，处于亚热带海洋性季风气候的上海交通大学研究浮萍对稻田杂草多样性时发现，浮萍覆盖大幅降低了稻田杂草密度（60.3%—90.4%），极大地缓解了水稻与杂草的资源（肥、光、水及空间等）竞争，从而提高了水稻的每穗粒数（33.7%）和穗重（28.2%），最终使水稻增产28.0%[73]；次年，其团队深入分析表明，水稻产量的增加主要与稻田生态环境因子值降低有关：浮萍伴生降低了水/土温度（0.86—2.76 ℃）、透光率（98.0%）、溶氧量（8.9%）及pH（0.32—0.39）等[19]；WANG等[20]在太湖地区的实验结果也证明了稻田伴生浮萍增产稻谷的结论，且其还观察到在不施肥的条件下浮萍尽管仍呈现了增产趋势，但并未达显著水平（见[20]，Fig.3-e）。2003年，中国水稻所也曾观察到了类似结果：在不施氮的条件下，浮萍伴生尽管抑制了稻田杂草的生长，但对水稻产量并无显著影响[36]，这说明浮萍伴生对水稻的增产效果与施肥量有关。当然也有个别报道视萍为杂草，因其与稻竞争养分[87]，降低稻田水/土温度[44]，特别是降低我国北方稻田的水/土温度[88]等，从而抑制了水稻分蘖与生长[44]，造成稻谷减产，且这种不利影响随浮萍覆盖率增加而增大。综上，多数学者证实了稻田稻-萍的生物协作关系[89]，但或因当地土壤肥力、温光条件、肥水管理及浮萍伴生程度等的差异，少数情况下伴生浮萍与水稻也存在竞争关系。然而，浮萍特别是生长失控下的浮萍对稻田综合风险的系统评估目前尚未见报道。

4 未来展望

气候及稻田生态环境在持续变化，以往学者多从温、水、肥、管等非生物及抗性育种等方面研究稻田生态系统应对变化的措施[10,90]，而基于稻-萍生物协作的研究相对较少。有限的报道认为伴生浮萍对稻田影响很大：降低水/土温度（或可对冲高温对稻田的负面影响）及pH，固定碳、氮，提高氮利用率，进而大幅增加稻谷产量——稻田生态系统对气候及环境变化呈现了明显的适应现象。浮萍在基础学科领域的研究已取得重大进展，且在发达国家的产业化利用方面发展迅速，而在农业领域的理论研究及推广则很少，导致生产上人们多错误地视浮萍为杂草而用药除之，这不仅提高了稻田农药用量及人工成本，还加重了潜在的环境压力。鉴于浮萍特性及其在稻田生态系统中应用的巨大潜力，笔者认为未来可从以下几个方面深入开展稻-萍等生物协作研究。

4.1 系统调查浮萍伴生对稻田生态环境因子的影响规律

浮萍伴生可通过改变生态环境因子间接影响稻田生态，也可通过自身及自身寄宿的微生物释放化感物质直接作用于水稻，深入开展浮萍伴生，特别是与环境因子互作（高温及高CO2浓度等）下，对稻田生态环境影响的研究显得尤为重要，具体包括：①不同程度不同时期的浮萍伴生对稻田水/土温度、pH、透光率、溶氧量及水资源利用率的影响规律；②浮萍伴生对稻田微生物优势群落的影响规律；③浮萍伴生对稻田水体化感物质的影响规律，包括稻、萍及相关寄宿微生物释放的激素等；④稻田浮萍伴生条件下，关键营养元素（氮、磷、钾及微量矿质元素等）在稻、萍、水体和土壤之间的转移及转化关系；⑤深入对比及分析以上指标对水稻生命活动的影响规律，确定其中的关键因子。

4.2 基于农学及农艺学，深入研究浮萍伴生对水稻生长、产量及稻米品质的影响及机制

目前研究浮萍伴生对水稻的影响仅集中在产量表观性状方面，鲜有涉及水稻生长、品质及其形成机制方面的深入探索。因此，在浮萍伴生对水稻直接及间接的影响下，系统调查水稻响应及机理显得尤为迫切，包括：水稻生长规律（分蘖机制、倒伏抗性及干物质积累规律等）、稻米产量形成机制（叶片光合及籽粒灌浆生理等）、稻米不同品质指标响应规律及机制；同时研究不同稻田生态环境下浮萍的生长繁殖规律及控制技术，从农学及农艺学栽培调控角度为稻-浮等互利共生的稻田生物协作生态系统的建立提供理论支撑。

4.3 科学调研，准确评估浮萍伴生对稻田可能造成的风险

类似并区别于大豆-根瘤菌的互利共生，稻田伴生浮萍与水稻或也存在竞争关系，伴生浮萍的快速生长及大量繁殖抑或给稻田带来潜在的风险，准确评估这一风险显得尤为重要，具体包括：①系统调查全国浮萍发生情况及对稻田及水稻的表观性状影响规律；②深入分析浮萍伴生条件下，稻田虫、草、特别是病害发生规律及动力学特征；③开展长期定位试验，综合分析稻田伴生浮萍碳、氮汇变化及对稻田水分利用、土壤营养及生态的影响规律，特别是稻田重金属积累与转运规律等。

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Research Progress on the Carbon and Nitrogen Sink of Duckweed Growing in Paddy and Its Effects on Rice Yield

JING LiQuan1, LI Fan1, ZHAO YiHan1, WANG XunKang1, ZHAO FuCheng2, LAI ShangKun3, SUN XiaoLin4,WANG YunXia5, YANG LianXin1

1Agricultural College of Yangzhou University/Jiangsu Key Laboratory of Crop Genetics and Physiology/Jiangsu Key Laboratory of Crop Cultivation and Physiology/Jiangsu Co-Innovation Center for Modern Production Technology of Grain Crops, Yangzhou 225009, Jiangsu;2Institute of Maize and Featured Upland Crops, Zhejiang Academy of Agricultural Sciences, Dongyang 322100, Zhejiang;3Suqian Institute, Jiangsu Academy of Agricultural Sciences, Suqian 223800, Jiangsu;4Eco-Environmental Protection Research Institute, Shanghai Academy of Agricultural Sciences, Shanghai 201403;5College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225009, Jiangsu

Duckweed (L.) is a floating microscopic plant that is usually found in standing water. Climate change is characterized by rising temperature, which is mainly due to increasing atmospheric CO2concentration, and it poses potential risks to food production. Owing to factors such as climate warming and/or the eutrophication of water, duckweed growth in paddy fields has shown an increasing trend year by year in China. This paper focused on the impacts of duckweed on paddy fields and highlighted some vital trends. Duckweed reduced the water temperature of paddy by 0.86-2.76 ℃ and the pH value by 0.10-0.45, changed the structure of microbial community, reduced the NH3volatilization by 18.2%-59.0%, and increased the nitrogen utilization rate by 17.2%-78.0%. As a result, the nitrogen sink of paddy increased and the rice yield rose by 9.0%-34.6% upon duckweed growing in paddy. Duckweed grew and reproduced rapidly, and its annual biomass could reach 8×103-13×103kg·hm-2, making its carbon sink almost equal to that of rice in the same season. The mutualism between duckweed and rice was greater than its competition, and the coexistence of duckweed and rice in paddy showed an adaptation of the rice field ecosystem to environmental changes. Future research in this field should focus on the effect and its mechanism of duckweed on the paddy environment changes, rice growth, yield, and quality, and the risks which might bring to the paddy fields, especially the interaction with environmental factors (elevated temperature and CO2concentration, etc.). Such research would provide theoretical support for the sustainable agricultural development of rice farming technology based on biological collaboration, such as rice-duckweed, which can adapt to future changes in climate and environment.

duckweed (L.); rice; carbon sink; nitrogen sink; yield