Composting and Phytoremediation to Regenerate Soils

Research Paper

Composting and Phytoremediation to Regenerate Soils

 

Author: Jim Mac Donald                                                                       Date: 1 January 2022

 

Abstract:

A wholistic view for the restoration and rejuvenation of degraded and polluted soils. The purpose of this paper is to discuss the current methods used to regenerate soils and to create a strategy that can be used by incorporating them together to create a method that can be replicated by farmers everywhere.

In part A, we will explore the concept of creating compost in general and the method of creating compost directly in your soils. In part B, we will also explore the technology of phytoremediation, which is using plants to clean up our soils and environment. Finally in part C, we will bring both concepts together to create a practical plan that farmers can use to add nutrients back into their soils as well as undertaking long term restoration and rejuvenation of their land, all at the same time.

Part A: Composting

Compost is the preferred material used by farmers, gardeners, and land restorers as they strive to improve the quality of their soil. Composting is a method to add minerals and organic matter to the soil. Composting is both an art and a science which must come together to create a healthy and effective compost material. Sadly, with the advent of synthetic fertilizers, the use of compost by farmers has diminished over time, resulting in very degraded and contaminated farmlands across the globe. The high cost and possible shortage of nitrogen-based fertilizers over the coming years will force more and more farmers to look for alternative ways to provide the nutrients needed to grow crops.

Background

Compost is produced by either the hot or cold composting process. Producing compost on an industrial scale uses the hot composting method. Composting is the biological decomposition of organic matter under aerobic conditions. This contrasts with fermentation or anaerobic decomposition that takes place under anaerobic conditions. Controlling the process requires that the temperature and oxygen levels be maintained for optimum conditions. The temperature regime involves both mesophilic (30 – 40 degrees celcius) and thermophilic (50 – 60 degrees celcius) conditions. Under mesophilic temperatures, biological decomposition of the organic matter is more effective and rapid. Under mesophilic conditions, the rate of decomposition is rapid initially, and the easily decomposable materials, carbohydrates, proteins, and fats, are metabolized by a myriad of organisms. (Wu, et al., 2017)

The composting process is driven by microbes and their secreted enzymes that play a key role in the decomposition and mineralization of organic wastes for the success of composting. Microbial community structure is influenced by environmental conditions, such as temperature, pH, moisture content, and nutrient content. (Wu, et al., 2017)

The quality of the compost produced is dependant on the initial materials used, what you put in is what you get out. Not all compost is therefore made equal. The compost is generally applied onto the soils prior to planting. The nutritional value and the quantity of compost used will determine the yield of any crops grown. The application of the compost requires specialized equipment and as the demand for compost increases so too will the cost of purchasing the compost.

 

The cold composting process is what nature does naturally. All the plant organic matter as well as the manure from animals is slowly decomposed by the microbes and insects that naturally occur in the soil. This is how nature recycles. In forests the organic materials from both plants and animals are slowly decomposed by the microbes and insect life in and around the soil. It is a slow and natural process.

As farmers we have two choices, either buy compost to add to the soils or build your own compost in your soils. One method of adding organic matter and in particular nitrogen back into the soils is using the method of green manure cropping. This is commonly used by organic growers as a way of adding nitrogen to their soils.

Green manure cropping is a form of cold composting and is done in situ on the farmland. This involves planting a single legume crop in the off season and ploughing the plants back into the soils. The organic matter degrades slowly over time providing nutrition for the subsequent crops. Green manure crops are commonly associated with organic agricultural practices. Typically, a green manure crop is grown for a specific period of time, and then it is ploughed under and incorporated into the soil while it is green or shortly after flowering. The practice of green manuring in agriculture can be traced back to the fallow cycle of crop rotation where mainly the legume family was used to allow soils to recover. It is obtained in two ways: by growing green manure crops or by collecting green leaves (along with twigs) from plants grown in wastelands, field bunds, and forests for incorporating into topsoil (Sullivan, 2003).

The single downfall of green manure cropping is the use of only one variety of plant. This method is only done to add nitrogen back into the soils through the root system. It is a method used to either reduce or eliminate the use of fertilizers. The plants and the roots are also creating a limited environment in the rhizosphere for bacteria to thrive. Only those bacteria associated with that specific plant will be able to thrive.

(Song, et al. 2007a) found that rhizosphere communities of wheat, maize and faba bean did not differ, and were more affected by the soil type rather than plant species. But when intercropped, significant differences in community composition were found between the wheat-faba bean and wheat-maize intercrops compared with their sole crops. Bacterial diversity was found to be higher in crop mixtures although evenness was equal to sole crops with some new species found in mixtures, which were not present in monocultures (Song, et al., 2007a) Figure 1.

Figure 1: Source: (Ehrmann & Ritz, 2014)

Soil organic matter has long been considered the key factor influencing the quality of soil. The organic matter is provided by the plant matter. Soil organic matter is a source of and a sink for plant nutrients in soils and is important in maintaining soil tilth, aiding infiltration of air and water, promoting water retention, and reducing erosion (Gregorich, et al., 1994).

Again, we come back to the same principle of making compost, what you put in is what you get out. If only one legume crop is planted as your green manure crop, the organic matter introduced into the soils from those plants will be extremely limited, only the nutrients available from that crop will be decomposed and added to you soil. The soil will have limited or poor organic matter and limited nutrients. As the roots of plants provide an environment that encourages certain bacteria to thrive, only planting one type of plant, will also only encourage the growth of a few types of bacteria. This limits the variety of bacteria in the soil and thus inhibits the conversion of the minerals into a form that is available for the plants to use. The result is a soil with a limited variety in both organic material as well as limited bacteria and soil biology. Compost has been created in your soils, but of an inferior quality.

Using the same principle as the green manure cropping, but this time, planting a variety of different plants species is called multi-cropping or mixed cropping. It will take the farmer the same effort and time to plant a mixed variety of seeds as it would to plant just one seed variety. The cost of the mixed seeds will be marginally more expensive, but the benefits will far outweigh the additional cost.

By planting a mixed crop of twenty different varieties of plants, the organic matter that is produced will provide a wide range of nutrients. The roots of each plant variety will create the conditions for different bacteria to thrive. The interaction between each plant and their root systems will also create new conditions where other bacteria and soil biology can thrive. In this instance of nature, one plus one, does not equal just two, but rather an exponential effect on the complexity of the soil biology. The quality and quantity of compost that is created in your soils through multi-cropping will be far superior to any compost that can purchased. The addition cost of the seed varieties pales into insignificance to the diverse organic matter and soil biology that is created. A literature survey shows potential advantages such as (1) higher overall productivity, (2) better control of pests and diseases, (3) enhanced ecological services and (4) greater economic profitability (Malézieux, et al., 2009)

This method of multi or mixed cropping is a fantastic way for farmers to create their own compost directly in their own soils. This method allows farmers to work with nature, allowing nature to do what she knows best. Please read the book “Dirt to Soil” by Gabe Brown. Gabe details his practical experience of using multi-cropping as well as incorporating animals to rejuvenate his soils. This method can be done anywhere in the world and in all soil types. Research the type of plants that will grow well in your particular area and soil type.

Part B: Phytoremediation

Phytoremediation is the use of plants and their associated microbes for environmental clean-up. This technology makes use of the naturally occurring processes by which plants and their microbial rhizosphere flora degrade and sequester organic and inorganic pollutants. Phytoremediation is an efficient clean-up technology for a variety of organic and inorganic pollutants. (Pilon- Smits, 2005)

Contaminated soils through industrial heavy metal pollution or degraded soils due to the overuse of synthetic fertilizers often requires extensive remediation work. The outcomes are often not achieved due to the high costs involved.

In the past 10 years phytoremediation has gained acceptance as a technology and has been acknowledged as an area of research. There has already been a substantial increase in our knowledge of the mechanisms that underlie the uptake, transport, and detoxification of pollutants by plants and their associated microbes. Still, large gaps in our knowledge await further research. Phytoremediation efficiency is still limited by a lack of knowledge of many basic plant processes and plant-microbe interactions. (Pilon- Smits, 2005)

Plants are generally considered as a food source for humans and animals and are not considered as clean-up resources. In dealing with problems of pollution and contamination, we like to develop overly complex solutions to solve the problem. These solutions are often touted by corporations because money can be made in the process. Nature provides us with an amazingly simple solution. The use of plants as bioremediation partners.

Phytoremediation can be used for solid, liquid, and gaseous substrates. Polluted soils and sediments have been phytoremediated at military sites (TNT, metals, organics), agricultural fields (herbicides, pesticides, metals, selenium), industrial sites (organics, metals, arsenic), mine tailings (metals), and wood treatment sites. Polluted waters that can be phytoremediated include sewage and municipal wastewater (nutrients, metals), agricultural runoff/drainage water (fertilizer nutrients, metals, arsenic, selenium, boron, organic pesticides, and herbicides), industrial wastewater (metals, selenium), coal pile runoff (metals), landfill leachate, mine drainage (metals), and groundwater plumes (organics, metals). (Pilon- Smits, 2005)

One of the main advantages of phytoremediation is that it can be done in situ, on the farm or directly in contaminated industrial land. This makes the process of clean-up very cost effective and minimizes the exposure of the contaminants to other people, animals, and the surrounding environment. Phytoremediation is a “green” solution to the clean and restore the soils and surrounding ecosystems.

The integral mechanism of phytoremediation occurs in the rhizosphere of plant roots. Rhizosphere remediation occurs completely without plant uptake of the pollutant in the area around the root. The rhizosphere extends approximately 1 mm around the root and is under the influence of the plant. Plants release a variety of photosynthesis-derived organic compounds in the rhizosphere that can serve as carbon sources for heterotrophic fungi and bacteria. (Bowen & Rovira, 1991). As much as 20% of carbon fixed by a plant may be released from its roots. (Olson, et al., 2003)

The rhizosphere is where all the action occurs. Every root of the plant will create different conditions in the rhizosphere. These different conditions are also created along the entire length of the root. The roots of each plant grow according to the needs and requirements of that specific plant. The conditions within the rhizosphere at every point along the root as well as the conditions created by the different architecture of the roots, creates a root-bacteria relationship. Figure 2.

Figure 2.

The soil structure, Ph, soil type, minerals and organic matter around the roots also creates unique conditions for the root-bacteria interrelationship. Figure 3.

Figure 3.

The main principle behind using Phytoremediation, is the use of a large variety of plants. Remediating a field with over one hundred distinct species of plants will create trillions and trillions of unique conditions where the interaction of the roots and bacteria will be able to degrade the unwanted organic compounds. The interactions that take place in the soil between the roots, bacteria and surrounding soil types are too complex to discuss in detail in this report. The knowledge of these interactions is also limited at present to design phytoremediation strategies specific for each farm. Therefore, this general approach of using multiple plants is recommended as well as having the trust in nature that she knows what to do. Use nature as your partner to help remediate the soils.

Part C:  Ecological Farming

In part A, we introduced the mechanisms of composting and the method of creating compost directly within your soils. Growing a mixed green manure crop with up to twenty different varieties of plants will produce highly nutritious organic matter and a wide array of minerals directly in the soil. This mixed cropping will also increase the complexity of the biology and bacteria within the soil.

In part B, we introduced the relatively new technology of phytoremediation, which is using different plants and their root systems to clean contaminated soils. Mixed green cropping and phytoremediation is one and the same principle of using the diversity of plants to rejuvenate your soils. This method of multi-cropping provides the following benefits for the farmer and the environment in general

• Produce nutrient rich organic compost directly in the soils.
• Increasing bacterial and biological activity within the soils which in turn will create more bio-available minerals for the plants.
• The increased root structures and rhizosphere activity increases the biological activity in the soils and degrades the organic contaminants with the soil.
• Cost effective compared to synthetic fertilizer applications
• The higher organic matter allows for increased infiltration and storage of rainwater in the soils.
• Due to the variety of plants, the above ground biomass provides food and shelter for a wider range of insects and birds.
• The overall health of the soils and surrounding ecosystems are improved.
• Create soils and surrounding environments that are resilient to the ever-changing climate.

The future of agriculture will be dependent on taking a wholistic view of the multitude of ecosystems involved in farming and producing food. By bringing the method of creating compost directly in your soils with the understanding of phytoremediation, farmers can produce food while at the same time build healthy ecosystems in the soils and surrounding areas.

References:

Bowen GC, Rovira AD. 1991. The rhizosphere—the hidden half of the hidden half. In Plant Roots—The Hidden Half, eds. Y Waisel, A Eshel, U Kaffkafi, pp. 641–69. New York: Marcel Dekker

Ehrmann, & Ritz, K. (2014). Plant: soil interactions in temperate multi-cropping production systems. Plant and Soil, 376(1/2), 1–29. https://doi.org/10.1007/s11104-013-1921-8

Gabe Brown, Dirt to Soil, Chelsea Green Publishing, 2018. ISBN 976-1-61358-763-1

Gregorich, E.G., Carter, M.R., Angers, D.A., Monreal, C.M., Ellert, B.H., 1994. Towards a minimum data set to assess soil organic matter quality in agricultural soils. Can. J. Soil Sci. 74, 367–385.

Malézieux, Crozat, Y., Dupraz, C., Laurans, M., Makowski, D., Ozier-Lafontaine, H., Rapidel, B., de Tourdonnet, S., & Valantin-Morison, M. (2009). Mixing plant species in cropping systems: concepts, tools, and models. A review. Agronomy for Sustainable Development, 29(1), 43–62. https://doi.org/10.1051/agro:2007057

Olson PE, Reardon KF, Pilon-Smits EAH. 2003. Ecology of rhizosphere bioremediation. In Phytoremediation: Transformation and Control of Contaminants, ed. SC McCutcheon, JL Schnoor, pp. 317–54. New York: Wiley

Pilon-Smits. (2005). Phytoremediation. Annual Review of Plant Biology, 56(1), 15–39. https://doi.org/10.1146/annurev.arplant.56.032604.144214

Song YN, Zhang FS, Marschner P, Fan FL, Gao HM, Bao XG, Sun JH, Li L (2007a) Effect of intercropping on crop yield and chemical and microbiological properties in rhizosphere of wheat (Triticum aestivum L.), maize (Zea mays L.), and faba bean (Vicia faba L.). Biol Fertil Soils 43:565–574

Sullivan, P. G. 2003. Overview of cover crops and green manures (Fundamentals of sustainable agriculture). NCAT Agriculture Specialist. p. 16. (on-line access: http://www.attra.ncat.org/attra-pub/PDF/covercrop.pdf.

Wu, He, H., Inthapanya, X., Yang, C., Lu, L., Zeng, G., & Han, Z. (2017). Role of biochar on composting of organic wastes and remediation of contaminated soils—a review. Environmental Science and Pollution Research International, 24(20), 16560–16577. https://doi.org/10.1007/s11356-017-9168-1

The Plasma Understanding of this Paper

 

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