7.2: Carbon and Nutrient Flows in History

Did you ever wonder why some civilizations were able to sustain agriculture for large populations while others exhausted their soils? One key component is the natural flow of carbon and nutrients. In the early days of agriculture, the productive areas were generally in low-lying locations where rivers and streams converged and flooded low-lying soils with water that contained sediments eroded from upriver soils. This annual flooding provided repeated deposits of nutrients and organic matter contained in the river’s sediments. For example, the Nile River Basin is over 1.2 million square miles in size and reaches from east-central Africa all the way to the Mediterranean. Through erosion and leaching (even under natural conditions), each area in the basin contributes small amounts of minerals (nutrients) and organic matter (carbon) that converge into the narrow downstream valley as sediment (Figure 7.1). Through the monsoonal rains in the upper basin, an annual supply of naturally fertile sediments (alluvium) was deposited on fields in the lower Nile valley and delta. This sustained a large population for several millennia. Other similar major confluence areas that were centers of ancient civilizations:

• The Indo-Gangetic Plain in parts of current day Pakistan, India, Bangladesh and Nepal, which is supplied by rivers—the Indus, Ganges and Brahmapatura—and sediment derived from the Himalayas
• The North China Plain, which is supplied by the sediment-laden Yellow River from the loess plateau in inner China
• The land between the rivers Euphrates and Tigris (Mesopotamia) in present day Syria and Iraq, which contains sediment derived from the Armenian Highlands in Turkey
• The Valley of Mexico, where ancient Lake Texcoco was fed by rivers from the surrounding fertile volcanic mountains and supported sustained wetland crop production by the Aztec civilization using raised beds (chinampas)
• Many other larger or smaller zones of alluvial deposits that were settled by tribes, including Native Americans, where they provided fertile soils and nearby sources of fish and land animals

Note

Alluvial soils are formed from sediments deposited along the banks of streams and rivers, and in the deltas where flowing water meets the still water of a lake or ocean and sediments settle to the bottom. They tend to be very fertile because of the small-size mineral particles, organic matter (carbon) and nutrients deposited over long periods of time.

Figure 7.1. Basin-wide nutrient, carbon and water flows converge towards productive confluence zones.

The continuous water, carbon and nutrient supplies allowed for highly productive crop production but also came with frequent flooding. In the past century, dams and levees have been constructed to reduce the impacts of flooding (and oftentimes to generate energy as well), but this means that the benefits of soil rejuvenation have ceased. Moreover, these ancient confluence zones have also become the most urbanized areas in the world, further reducing agricultural land areas. Notably, the lake in central Mexico was drained and is now occupied by the megalopolis Mexico City.

Contrasting these convergence zones in valleys and deltas, there are other regions from which the water and sediments originate, and where carbon, nutrients and water are lost. These are extensive areas away from the valleys and deltas, typically hilly or mountainous, from which resources tend to move away due to runoff, erosion and leaching. Their losses are gains for the regions downriver. In ancient times, these less productive areas were mostly used for pasturing where low-producing perennial vegetation was still valuable for extensive animal grazing, supporting small populations. Whenever such lands were taken into crop production, soils soon became exhausted from tillage, carbon and nutrient exports off the farm, and high soil erosion (Figure 7.2). Many regions thereby became unsuitable for annual crops and were converted back to pasture or to tree or vine crops (olives, grapes) that grow with low soil fertility (Figure 7.2). These farming conditions could not support large civilizations and often resulted either in declines or conquests. Notably, the low agricultural potential of the degraded hills of central Italy drove Roman conquests of the Egyptian breadbasket in the lower Nile.

Carbon and Nutrient Concentration in Soils by Human Activity

Figure 7.2. Degraded land on Crete, Greece, with olive trees.

With growing global populations, more of the marginal areas were brought into production. Some were naturally productive (e.g., grassland areas in the central United States and Asia), while others were more fragile (e.g., the eastern United States). Before the availability of nutrient replenishment with artificial fertilizers, farmers sometimes built soil fertility through periodic flooding and deliberately bringing organic materials and nutrients to their crop fields, sometimes even creating soils so strongly influenced by this human activity that they’re called Anthrosols. One example is the so-called plaggen soil of northwestern Europe (Figure 7.3). These are found on low-fertility sandy soils that were not good for crop production but were suitable for pasturing. Farmers would keep areas from forest succession by cutting away the heath sod, containing plants of low nutritional value and palatability, and promoting new vegetation that could feed grazing animals, mostly sheep. But there was still a need to grow food crops. Therefore, the slices of rich sod were brought to barns where they were used as bedding for the overnighting sheep. The sod was further enriched with the sheep excrements (containing carbon and nutrients harvested from the pastures during the day), creating fertile compost that was in turn applied onto the small fields that were used to grow crops for human consumption. In this crop-pasture system, the carbon and nutrients were partly cycled on the pastures and partly flowed in a way that concentrated onto crop fields. This human ingenuity allowed for sustained animal and crop production on naturally marginal soils.

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Figure 7.3. Two cases of soils enhanced through human activity, producing anthropogenic soil (from left): Plaggen soil in Belgium (photo by Karen Vancampenhout) and Terra Preta del Indio soil in Brazil (photo by Biqing Liang).

Figure 7.3. Two cases of soils enhanced through human activity, producing anthropogenic soil (from left): Plaggen soil in Belgium (photo by Karen Vancampenhout) and Terra Preta del Indio soil in Brazil (photo by Biqing Liang)

There are other examples of “dark earths” found in many settled areas around the world, notably the Amazonian Terra Preta soils that were enriched with char (as we discuss in Chapter 2). In this case, food and fuel were collected from the surrounding rainforest and were concentrated onto the soils in and around the ancient settlements. Unlike the plaggen soils, the charring of some of the organic materials created very stable organic matter that, centuries later, still keeps the soil fertile. There are many other examples of the concentration of carbon and nutrients around population settlements, including early New Englanders who used byproducts from the bountiful cod fishing industry to enhance the fertility of their croplands.

Why is this historical perspective relevant? Because today there are good opportunities to enhance soil fertility by better using organic materials. In fact, most organic farmers do just that. They bring organic materials that are often considered “wastes” onto their fields to replace the nutrients that are exported with the crops (they especially need phosphorus and potassium). Typically, this is done through compost made with tree leaves and food wastes in urban areas or through excess manure from livestock farms. Like in the past, these farmers are taking advantage of the availability of organic materials and nutrients external to their farms and are bringing them onto their fields to build soil fertility, as we discuss in detail in part 3 of this book. Different rotations and integrating cropping and livestock also offer many opportunities to “grow your own” soil organic matter and improve nutrient cycling.

Figure 7.4. The patterns of nutrient and carbon flows change over time. Modified from Magdoff, Lanyon, and Liebhardt (1997).