山中玩物杂言

[宋] 戴表元

蚯蚓食土壤，蚩蚩何所为。

蜩蝉饮用露，蜕上高高枝。

卑高慕其仙，飘飖安可追，

倘令智相及，无羡亦无悲

Dirt Is Not Dead: How Land Use Affects the Living Soil

作者：Jes HinesFranciska T. De Vries

ABSTRACT摘要：

人类利用土地种植农作物以获取食物，我们所采用的种植方法会影响到生活在土壤里的生物。土壤生物（在生态环境中）起着重要的作用，例如分解有机物，释放养分供植物生长。通过在农田中施加更多的农药和化肥，我们便可以在较小的空间内生产出更多的农作物。但这些行为方法也会危害到土壤里的生物体本身以及它们的机能。相比之下，我们可以采用更温和的方法种植农作物，这样对土壤生物有利，但这种方法需要更多的土地。世界各国的人们都需要农作物所带来的食物以过上健康的生活。正是因为我们共享同一片土地，当我们寻思着如何利用这片土地时，我们需要知道农业是如何影响土壤生物的。

Humans use land to grow crops for food, and the farming methods we use can influence the organisms that live in the soil. Soil organisms do important work, like decomposing organic matter and releasing nutrients for plant growth. By adding more pesticides and fertilizers to farmland, we can produce more crops in a smaller space. But those methods can also harm soil organisms and the work that they do. In contrast, we can use gentler methods to grow crops, which are better for soil animals, but those methods require more land. People in all countries need food from crops to live healthy lives. Because we all share one land surface, when we decide how to use land, we need to remember how agriculture influences soil animals

01

7.8 BILLION PEOPLE, AND COUNTING, EAT FOOD THAT GROWS IN SOIL

78亿人以土壤中生长出来的食物为食，且这个数字仍在不断上升

有78亿人，他们都需要吃健康的食物。蔬菜和其他农作物是健康饮食的一部分。可是，那么多人生产食物是要付出环境代价的。首先我们需要有足够多的农田，还需要能保持环境健康的耕作方法。生存在土壤里的生物体可以告知我们，我们的耕作方式对环境是好是坏。

土壤里生活着数十亿的生物体，例如细菌、真菌、线虫及足类生物（图1）。它们和我们一样，要摄取食物以促进生长，在吸入氧气的同时排除二氧化碳。当土壤生物进行它们的生命进程时，它们从食物中释放出对植物生长有用的营养物质（图1，小图）。它们还将有机物（即死亡的植物和生物体）在整片土地中混合在一起。它们改善了土壤的结构，从而改善了植物的水分供应。土壤生物为帮助植物生长出人类喜欢吃的植物提供资源，而土壤可提供重要的生态系统服务，如水的过滤和养分的循环，它们有助于维系人类的生存与健康。

There are 7.8 billion people and all of them need to eat healthy food. Vegetables and other crops are part of a healthy diet. But producing food for so many people comes with a cost to the environment. First, we need enough farmland and then we need farming practices that keep the environment healthy. The organisms that live in soil can tell us if our farming practices are good or bad for the environment.

In the soil, there are billions of organisms, such as bacteria, fungi, nematodes, and isopods that live in soil (Figure 1). They all eat, and grow, and use oxygen and release carbon dioxide, just like we do. When soil organismsAll of the types of life forms that live below ground. This includes single celled organisms like bacteria, and multi-cellular organisms like earthworms, protozoa, nematodes, and mites. carry out these life processes, they release nutrients from their food that are useful for plant growth (Figure 1, inset). They also mix organic matter (i.e., dead plants and organisms) throughout the soil. They improve the structure of the soil, which improves water availability for plants. Soil organisms supply the resources that help plants grow the food that humans like to eat. Soils provide important ecosystem servicesThe important things environments do for people. In soils ecosystems, examples include recycling nutrients, retaining and draining water, and mixing organic matter., such as water filtration and nutrient recycling, that help keep humans alive and healthy.

Figure 1 - The soil food web is composed of many kinds of soil organisms: (1) bacteria, (2) fungi, (3) arbuscular mycorrhizal fungi, (4) protozoans, (5) bacteria-feeding nematodes, (6) fungal-feeding nematodes, (7) root-feeding nematodes, (8) collembolans, (9) predatory nematode, and (10) predatory mites.

图1:土壤食物网是由多种多样的土壤生物体组成的 （1）细菌，（2）真菌，（3）丛枝菌根真菌，（4）原生动物，（5）食菌线虫，（6）食真菌线虫（7）食根线虫，（8）弹尾虫，（9）捕食性线虫（10）捕食性螨虫

Arrows point from the organisms that are eaten to the organisms that eats them. Interaction pathways form three main energy channels: bacteria-based channel (orange arrows), fungal-based channel (yellow arrows), and root-based channel (green arrows). The magnifying glass shows that nutrients and energy flow through

h each soil organism when it eats and respires.

箭头从被捕食的生物指向捕食它们的生物。相互作用的分支形成了三个主要的能量通道：基于细菌的通道（橙色箭头），基于真菌的通道（黄色箭头），基于根的通道（绿色箭头）。放大镜展现当土壤生物进食和呼吸时，养分和能量流经的每个土壤生物体。

02

SOIL ORGANISMS FORM A FOOD WEB UNDER YOUR FEET

土壤里的生物在你的脚下形成一个食物网

土壤里的生物并不是单独工作的，它们在一个被称为土壤食物网的相互作用脉络中相互联系。一个物种的丰度变化会影响到捕食者和被捕食者的丰度，这种相互作用链被成为能量通道（图1）。土壤生物网里有三个主要的能量通道：一个以细菌为营养物，另一个以真菌为营养物，第三个则以植物根部为营养物提供能量。细菌和真菌以有机物，如死去的真菌和枯萎的叶子为食，而后它们会被其他生物体吃掉。还有一些生物体直接以活根为食，如食根线虫（一种微小的虫子），而后它们也会被其他生物体吃掉。当人类利用土地的方式中断了这些能量通道，如翻耕土壤或给土壤施加肥料，可能会改变土壤生物网提供其通常的生态系统服务的能力。可是，有多少物种受到人类的影响？我们对土壤生物正常运行的影响又有多大？

The organisms in the soil do not work alone; they are connected with each other in a network of interactions called the soil food webThe network of feeding interactions among soil organisms. The interactions start from three main resources (roots, fungi, bacteria) that form energy channels.. Changes in the abundance of one species can affect the abundance of their predators and prey, in a chain of interactions called an energy channel (Figure 1). Soil food webs have three main energy channels: one fueled by bacteria, another fueled by fungi, and a third fueled by plant roots. Bacteria and fungi eat organic matter, such as dead roots and leaves, and later they are eaten by other organisms. There are also organisms that feed directly on living roots, such as root-eating nematodes (tiny worms), which are also eaten by other animals. When humans use the land in a way that disrupts these energy channels (such as by tillingPreparing soil for agriculture by plowing or digging. Tilling buries weeds and makes it easier for crop roots to enter the soil. the soil or adding fertilizers), they may alter the ability of soil food webs to provide their usual ecosystem services. But how many species are affected by humans and how strong is our effect on the processes soil organisms normally perform?

03

AN EXPERIMENT TESTING THE EFFECTS OF LAND USE ON SOIL FOOD WEBS

测试土地利用对土壤食物网的影响的实验

科学家们假定不同类型的土地利用会影响土壤食物网的结构。他们认为土地的集中利用，如翻耕土地会减少土壤生物的数量，同时也会限制土壤所提供的生态系统服务。科学家们还认为当农民采用大面积土地利用的方式时，就如大面积种植草坪和牧草，土壤里的生物体会变得更加丰富。在大面积的利用土地中，翻耕次数减少，施加化肥的量也降低了，但是要生产出同样数量的粮食则需要更多的土地。他们认为，土壤生物在自然草原上将会是生活得最好的，因为在那里农民不会利用土地种植庄稼。

Scientists hypothesized that different types of land useHow the natural environment is managed and modified. Examples in text include Intensive land use, extensive land use, and natural areas. would affect the structure of the soil food webs (Figure 2) [1]. They thought that intensive land use, such as tilling the land, would reduce the number of soil organisms and limit the ecosystem services provided by soils [1]. Scientists also thought that soil organisms would be more abundant when farmers applied extensive land use practices (such as hay meadows and pasture). In extensive land use, there is less plowing and less fertilizer added, but more land area is needed to produce the same amount of food. They thought that soil organisms would do best in natural grasslands, where farmers do not use the land to grow crops [1].

·Figure 2 - Three common ways that natural environments are used.

·Extensive land use does not involve much plowing and harvesting or the addition of lots of fertilizers and pesticides, and it produces diverse ecosystem services. Medium-intensity land use needs some plowing and harvesting and yields intermediate harvests. High-intensity land use requires lots of work plowing and harvesting, as well as lots of chemical use, to produce a lot of crops from a smaller area. In high-intensity land use, some ecosystem services are reduced in favor of high-output of crop production.

·图2 -- -- 利用自然环境的三种常见形式。

粗放型的土地利用不需要大量的耕种和收割，也不需要添加大量的化肥和杀虫剂，它能产生多种生态系统服务。中等强度密集型的土地利用需要一些耕种和收割，中等产量。高强度密集型土地利用需要大量的耕种和收割工作以及使用大量的化学品，以在较小的面积上生产大量的作物。在高强度的土地利用中，一些生态系统服务减少，而有利于农作物的高产。

 为了使用多样化的环境条件（如温度、降水、土壤质地等）的场地为目标，科学家团队小组前往了欧洲四个国家：瑞典、捷克、英国和希腊。在每个国家里，他们去了三种土地利用类型的场所：高强度密集农业地带、中等强度密集农业地带和天然草地（图2）。在每个地点里，他们对包括真菌、细菌、原生动物、线虫、蚯蚓、竹节虫、螨虫和弹尾虫等土壤生物进行测量。为了了解不同土地利用方式所提供的生态系统服务有何不同，他们还对一些土壤生物主导的一些重要过程进行了测量。

With the goal of using sites exposed to a broad range of environmental conditions (temperature, precipitation, soil texture, etc.), a team of scientists went to four countries across Europe: Sweden, Czechia, United Kingdom, and Greece. In each country, they went to sites with three types of land use: high-intensity agriculture, medium-intensity agriculture, and natural grasslands [1] (Figure 2). At each site, they measured a variety of soil organisms, including fungi, bacteria, protozoa, nematodes, earthworms, enchytraeids, mites, and collembolans. To find out how the ecosystem services differed between the land uses, they also measured some of the important processes performed by soil organisms.

04

INTENSIVE FARMING REDUCES BIODIVERSITY IN THE SOIL

精耕细作（高强度密集型）减少了土壤中的生物多样性

科学家们发现，土地使用类型对他们在土壤中发现的生物体种类和数量起着重要的作用。更密集的土地利用降低了土壤食物网的生物多样性：存在的生物群落减少了，而在这些生物群落中，物种也减少了。他们还发现，在有高强度中等强度密集型土地利用的地方，大多数的土壤生物群的总重量都较低。以植物根部作为营养物的生物减少得最多，以细菌和真菌为食的生物受影响较小。这可能是因为在中、高强度土地利用中，翻耕土壤对植物根部有极大的影响，这对以植物根部作为营养物的生物有长期影响。但这种变化有多少是由于翻耕给土壤带来的直接影响，又有多少是由于土壤食物网的变化呢？

The scientists found that the type of land use was important for the numbers and types of organisms that they found in the soil. They found that more intensive land use reduced the biodiversity of the soil food web: there were fewer groups of organisms present, and within these groups, there were also fewer species [2]. They also found that the total weight of most groups of soil organisms was lower where there was high- and medium-intensity land use. The organisms that are fueled by plant roots were the most reduced, and organisms that eat bacteria and fungi were affected less. This is probably because tilling the soil has strong effects on plant roots in medium- and high-intensity land use, which has extended effects on the organisms that are fueled by plant roots. But how much of the change was due to the direct effects of the tilling on the soil, and how much was due to changes in the soil food webs?

05

PROCESSES CARRIED OUT BY SOIL ORGANISMS

由土壤生物体主导的一些过程

为了回答上述问题，科学家们研究了土壤生物与这些生物所主导的过程之间的关系。科学家们重点研究了两个过程：呼吸作用和氮的循环。当我们谈论微小的生物体时，呼吸作用是利用氧气并释放二氧化碳，以产生生物生长和执行其功能所需的能量的过程。氮是包括植物在内的所有生命的基本元素。它在环境中通过几个步骤参与循环。矿化是氮元素从生物体释放到土壤中的过程（图3），通常是在生物体腐烂的时候矿化。土壤中的氮可以通过浸出（即被水冲出土壤）或反硝化作用而流失，在反硝化作用中，氮变成一种由细菌产生的称为一氧化二氮（N2O）的气体，从土壤中泄漏到大气中。

To evaluate the question above, the scientists looked at relationships between groups of soil organisms and the processes that these organisms carry out. The scientists focused on two processes: respiration and the cycling of nitrogen. When we are talking about tiny organisms, respiration is the name of the process that uses oxygen and releases carbon dioxide, to produce the energy that the organisms need to grow and perform their functions. Nitrogen is an essential element for all life, including plants. It cycles through the environment in several steps. Mineralization is the process by which nitrogen is released from organisms into the soil (Figure 3), usually when they decay. Nitrogen in the soil can be lost through leaching, which means being flushed out of the soil by water, or by denitrification, in which the nitrogen is turned into a gas called nitrous oxide (NO2) produced by bacteria and escapes from the soil into the atmosphere.

·Figure 3 - Nitrogen is transformed by soil food webs so that it can move through plants, water, or air.

Soil organic matter (dead plants and animals) is mineralized to inorganic nitrogen by the feeding activity of soil organisms. Inorganic nitrogen in soil can be taken up by plants and fungi, or it can be lost from the soil by either leaching into ground water or by conversion to N2O gas by bacteria, in a process called denitrification.

·图3-氮元素被土壤食物网转化以便于可通过植物，水和空气流动。

·土壤有机物（枯萎的植物和死去的动物）在土壤生物的摄食活动中被矿化为无机氮。土壤中的无机氮可以被植物和真菌吸收，也可以通过溶入地下水或通过细菌转化为N2O气体从土壤中流失，这一过程被称为反硝化。

05

NATURAL GRASSLANDS ENHANCE SOIL RESPIRATION

天然草场可增强土地呼吸作用

首先，科学家们发现，在天然草场的土壤生物的呼吸作用更强，土壤中的蚯蚓也更多。蚯蚓把土壤混合在一起，这么做刺激了其他土壤生物的活动。就像我们一样，当土壤生物变得更活跃时，它们需要吃更多的东西，呼吸更多的空气。当有更多的有机物作为食物时，就会有更多更活跃的土壤生物。由于天然草场的有机物比中等强度密集农业地带的田地要高，所以天然草场的呼吸作用要高一些。

First, scientists found that the respiration of soil organisms was higher in natural grasslands and where there were more earthworms in the soil. Earthworms mix the soil, and by doing this, they stimulate the activity of other soil organisms. Just like us, when soil organisms become more active, they need to eat more and respire more. When there is more organic matter available as food, there will be a greater number of active soil organisms. Because natural grasslands have higher organic matter than the fields used for medium- and high-intensity agriculture, respiration was higher in these sites.

06

MORE NITROGEN FLOWS INTO AND OUT OF INTENSIVELY MANAGED FARMS

更多氮元素流入和流出密集型管理的农场

当科学家们研究氮循环时，他们发现，在细菌能量通道较强的食物网中，氮的矿化程度较高「见土壤食物网中的术语表」。在土地利用密集的地区，细菌通常会非常多，它们在那里快速生长，快速死亡，并将体内软组织中的氮元素释放到土壤中。

在他们测量了释放到土壤中的矿化氮后，科学家们想知道土地利用如何影响氮元素的下一步去向。他们发现：在土地利用密集度较低的地区，生存着大量的丛枝菌根真菌，当浸出到水中的氮元素较少时，意味着有更多的氮元素保留在土壤中供植物利用。农民们想通过限制氮元素的浸出量以便于大部分氮元素可以被农作物所利用，而不是在田地里流失。丛枝菌根真菌是一种特殊的真菌，它可以与植物的根相连接从而帮助植物吸收包括氮元素在内的养分。这些真菌可能会吸收土壤里的氮元素，防止氮元素浸出而流失。因此，丛枝菌根真菌的存在可能是一个减少（氮）渗滤到不同类型的土地利用中的良好指标。

为了评估脱氮量（化学上称之为反硝化作用），科学家们测量了一氧化二氮气体（N2O）的浓度。他们发现，在N2O浓度较低的地方，鞭毛虫的数量增加了——鞭毛虫这种微小的生物在土壤里的水中借助自己的尾巴推动前进，同时它们以细菌为食。重要的是，实际上反硝化作用并不是由鞭毛虫完成的，而是由一组特殊的细菌完成的。由于在反硝化细菌不丰富的地区鞭毛虫数量的增加，我们或许可以将鞭毛虫的丰度作为其他过程（如反硝化）和土壤生物（如反硝化细菌）的重要的指标。了解低浓度的反硝化细菌在土壤食物网中会发生什么是很重要的，因为当氮元素以N2O的形式流失在土壤之外时，会使气候变化变得一团糟。N2O是一种温室效应气体，其导致温室效应的强度是二氧化碳的314倍！

When scientists looked at the cycling of nitrogen, they found that nitrogen mineralization was higher in food webs with a stronger bacterial energy channel (see soil food web in glossary). Bacteria are often very abundant in areas with intensive land use, where they grow quickly and die quickly, releasing the nitrogen from their tissues into the soil.

After they measured the mineralized nitrogen that was released into the soil, the scientists wanted to know how land use affected where the nitrogen went next. They found that, in areas with less intensive land use where there were a lot of arbuscular mycorrhizal fungi, there was less leaching of nitrogen into the water, meaning that more nitrogen stayed in the soil where plants could use it. Farmers want to limit the amount of nitrogen leaching, so that most of the nitrogen can be used by crop plants instead of flowing out of the fields. Arbuscular mycorrhizal fungi are a special group of fungi that attach to plant roots and help the plant take up nutrients, including nitrogen. These fungi might take up nitrogen from the soil and prevent it from being lost through leaching. So, the presence of arbuscular mycorrhizal fungi could be a good indicator of reduced leaching across different types of land use.

To evaluate the amount of denitrification, scientists measured the concentration of nitrous oxide gas (N2O). They found lower N2O concentrations where there was an increased number of flagellates—tiny creatures that use a tail to propel themselves forward in the water that is within the soil and that eat bacteria. Importantly, denitrification is not actually done by flagellates, but by a specific group of bacteria. Because flagellate increased in areas where denitrifying bacteria were not abundant, we may be able to use flagellate abundance as an important indicator of other processes (e.g., denitrification) and soil organisms (e.g., denitrifying bacteria). Understanding what is happening in soil food webs with low concentrations of denitrifying bacteria is important because, when nitrogen leaves the soil as N2O, it can make climate change worse. N2O is a greenhouse gas that is 314 times stronger than CO2!

06

INCREASE SOIL BIODIVERSITY TO MINIMIZE THE IMPACTS OF INTENSIVE FARMING

增加土壤生物多样性，最大限度地减少密集型耕作的影响

有关利用土地种植蔬菜和其他农作物的“正确”方式的问题是一场持续存在的科学和社会辩论。人类如何给数十亿的人们生产出足够的健康食物同时把对环境的影响度最小化呢？科学家们了解到，土壤生物对人类所造成的环境变化反应很敏感。耕作方式不仅影响单个生物体，还影响复杂的物种的相互作用脉络。这种相互作用脉络的变化会影响到能量如何在生态系统中流动，哪些养分会得以保留，哪些养分会消失，以及农民将不得不依赖于对土壤施加化肥和农药。尽管集约利用土地所占用的面积较少，这是积极的一面，但是它也带来了许多负面影响，因为它减少了土壤生物的多样性且增加了养分和碳元素的流失。反过来讲，较低强度的土地利用所生产的农作物较少，但对环境更友好。

所以，一个解决方案就是：恢复密集型土地利用地区的土壤生物多样性，使其减少对化肥和农药的依赖。这样对环境有好处，也对人类有利。因此，如果你有种植自己的蔬菜的机会，你将保护环境，因为你不会通过高强度密集型种植方式来生产农作物。如果你没有机会种植蔬菜，你仍然可以通过购买非集约化农田生产的蔬菜和其他农作物来保护环境。渐渐地，我们便可以让我们的地球越来越健康。

The question of the “right” way to use land to grow vegetables and other crops is an ongoing scientific and social debate. How can humans produce healthy and enough food for billions of people with the minimal impact to the planet? Scientists are learning that soil organisms are sensitive to changes humans make to the environment. Farming practices do not just influence individual organisms, they influence a complex network of species interactions. Changes in this interaction network affect how energy flows through an ecosystem, which nutrients are retained, which nutrients are lost, and how farmers will have to rely on fertilizers and pesticides. While intensive land use takes up less land area, which is positive, it also causes many negative impacts since it decreases soil biodiversity and increases losses of nutrients and carbon. In opposition, lower intensity land use produces fewer crops but is more environmentally-friendly. So, one solution is to restore soil biodiversity in intensive land use areas to make them less dependent on fertilizers and pesticides, which is good for the environment and therefore for humans. So, if you have the possibility to grow your own vegetables you will be protecting the environment because you will not be producing them through intensive agriculture practices. If you cannot plant your vegetables, you can still protect the environment if you can buy vegetables and other crops that are not produced in intensive farmland. Little by little we can make our planet healthier and healthier.

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附上作者简介：

Jes是一名生态学家，在德国生物多样性综合研究中心（iDiv）工作。她的研究重点是生物多样性是如何应对环境的变化的。她对复杂系统如何生长以及物种如何影响生态系统中营养物质、能量和信息的流动感兴趣。

Franciska de Vries是阿姆斯特丹大学的教授。她的研究重点是了解植物和土壤微生物之间的相互作用如何应对气候变化，以及这如何影响生态系统的功能。

原文：https://kids.frontiersin.org/article/10.3389/frym.2020.549486