Follow the Energy: How Cropping Systems Fundamentally Differ From Natural Systems

Energy flow drives everything in an ecosystem. As with nitrogen, following the energy through an ecosystem reveals to us how that ecosystem works. Here, I want to compare how the energy flows through cropping systems to how it flows through unmanaged natural systems. This comparison will show us why cropping systems rarely match the abundance of organisms or the nutrient cycling observed in their natural counterparts, and why this tradeoff is inherent to the production of food. Let’s take a look.

Energy Flow and Net Primary Production (NPP)

Our energy begins at the sun. Sunlight shoots into space arrives at the earth’s surface eight minutes later, falling on the leaves of plants. Photosynthesis in the plants converts this solar energy to chemical energy — energy stored in the chemical bonds of sugars and other carbohydrates.

This capture of solar energy as chemical energy is called primary production, with plants being the primary producers because they make that initial conversion of solar energy into chemical energy (read: food) for the rest of us secondary organisms that can’t photosynthesize.

Ecologists call this accumulation of energy Net Primary Production (NPP):

“Net primary production…represents the amount of energy stored in plant material annually that is available as a source of food and fiber for the planet including its human inhabitants.”

(DeLucia et al., 2014)

Sometimes scientists record NPP as units of dry plant biomass generated per unit area and time. Other times they record it as just the carbon portion of that dry plant biomass. Either way, NPP is a way of measuring the sunlight-powered plant production of an area or system per unit area and time, and is very much affected by local land and climate factors.

For example, a tropical rainforest might have an NPP around 9.8 tons of dry matter per acre per year, whereas a temperate grassland might have an NPP around 2.7, and a desert ecosystem an NPP around 0.4 (Whittaker and Likens, 1975).

It’s this variation in NPP that then drives variation in how much life is supported in these different environments. There’s a lot more creatures big and small in a tropical rainforest than a desert and that’s due to the underlying difference in the NPP available to feed them. Consequently, this energy flow from the sun to plants is the fundamental process powering life on earth, both in natural systems and cropping systems. Without this process, plants, animals, people, and nature as we know it would not exist. Further, the more energy that can be captured through this process, the more life and biological processes are possible. 

“Net primary production is the basis of all ecosystem services”

(Cherlet et al., 2018)

“NPP is a fundamental ecological parameter that provides information about the health and status of vegetative communities “

(Reichle, 2023)

How does NPP differ between cropping systems and natural systems?

Depending on management, the NPP of annual cropping systems can be less than, equal to, or more than the NPP of the native vegetation which it replaced (DeFries et al., 1999; Prince et al., 2001; Monfreda et al., 2008; von Haden and Dornbush, 2017). Typically, however, the more modernized and intensive the agricultural system, the higher the NPP will be compared to the native vegetation it replaced. The use of crops specifically bred for and managed for production and the use of fertilizers and irrigation to reduce nutrient and water limitations push the NPP up from what would be observed in an unmanaged, natural system (Bradford et al., 2005; Prince et al., 2001).

Modern agricultural systems are, by design, often more productive in terms of biomass than the native vegetation they replaced. And yet, farms typically support less wildlife and have lower soil organic matter than the native vegetation they replaced. How can farms produce more biomass and therefore more stored energy than nature, but then seem to support less life on the farm? The answer is simply because most of that biomass, that stored energy is harvested and exported off the farm to support life elsewhere. Harvest changes everything.

How does harvest change everything?

In natural systems, most of the produced biomass stays within the system—with the small amount of biomass that is exported being moved by migratory animals, water or wind. This means that most of the NPP of natural systems, ends up feeding the secondary organisms in the local area. The bugs, the birds, the mammals, the reptiles, etc.—they all get supported by the local NPP. Further, either in the form of decaying plant matter, animal matter, or feces, most of the local NPP eventually goes on to help maintain local soil organic matter levels.

In contrast, in a cropping system, we harvest and export a large proportion of the biomass produced by crops—exporting about 50% of the aboveground biomass of grain crops like corn, rice, and wheat, and over 95% of aboveground biomass for crops like lettuce or hay. In agriculture, we grow the crops for the express purpose of harvesting them to export them off the farm, often to distant populations to feed the secondary organisms there, to feed us. And this happens every year, year-after-year, by design.

Even though cropping systems can produce as much NPP as unmanaged ecosystems (and sometimes more), and so ecologically function just as well on the productivity side, the harvest and export of most of this energy means that cropping systems will function differently than unmanaged ecosystems on the consumption side.

Agriculture is fundamentally a redistribution of energy and nutrient resources both spatially and temporally. The more NPP humans take for human use, the less there is available in that moment for local secondary organisms. It is unsurprising to find animal populations diminished on farmland compared to nature. The food that would otherwise be theirs, is ours. It is also unsurprising to find decreased soil organic matter on farmland compared to nature—with soil organic matter levels being heavily driven by plant biomass inputs (Janzen et al. 2022) and agriculture redistributing a large fraction of local plant biomass to humans off farm. We can certainly do our best to conserve soil organic matter levels and the associated soil health in cropped soils, but they will never be equal to those of native ecosystems absent a massive redistribution of biomass from outside sources.

Agriculture exports nutrients as well as energy

Just as agriculture exports energy, it also exports nutrients. In natural systems, nutrients are largely conserved in place while in cropping systems, harvest moves nutrients to somewhere off farm. These exported nutrients must eventually be replaced if agriculture is to continue at that location. No matter how nutrient efficient we make cropping systems, they will always require nutrient inputs to continue producing crops. Of course, the soil can be mined of nutrients for a while, longer with deep fertile soil, shorter without. But in the long-term, extracting nutrients from the soil without replacing them is not sustainable; yields will eventually falter. Long-term research has shown that relying only on soil nutrient sources means adjusting to much lower yields.

Cropping systems differ from natural systems by design

Ecologists and agroecologists often criticize cropping systems for failing to work like natural systems (Cabral and Sumberg, 2022), but this criticism is misguided. Agriculture was developed for the express purpose of working differently than natural systems. If natural systems could have fed and clothed over 8 billion people, we surely could have saved ourselves the trouble of farming.

In order to feed us, cropping systems must harvest and export crop NPP, which means that cropping systems have to function differently than natural systems There is no way around this. Just imagine if we started exporting half of the NPP from natural systems. You would see decreases in soil organic matter and wildlife just like you do in agriculture. While we should continue to work to minimize environmental harm from crop production, this should be done in view of the reality of the tradeoffs that come with food production.

Given the similarities of natural and cropping systems – plants growing in soils, needing nutrients, water, and sunlight – you’d think we could look at how natural systems work to improve cropping systems. However, the differences in the energy flow in each system and where it goes means they will always function differently. It was the ecologist J.J. Ewel (1999) who noted that “the more that is harvested, the more we deviate from nature’s model.” and that’s the fundamental tradeoff in agriculture: the deviation from nature to produce food.

Editing and contributions from Angela Florence.

References

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Cabral, L., and J. Sumberg. 2022. The use of epic narratives in promoting ‘natural agriculture.’ Outlook Agric 51(1): 129–136. doi: 10.1177/00307270221077708

Cherlet, M., C. Hutchinson, J. Reynolds, J. Hill, S. Sommer, et al., editors. 2018. Net Primary Production. World Atlas of Desertification. European Union, Luxembourg

DeLucia, E.H., N. Gomez-Casanovas, J.A. Greenberg, T.W. Hudiburg, I.B. Kantola, et al. 2014. The Theoretical Limit to Plant Productivity. Environ. Sci. Technol. 48(16): 9471–9477. doi: 10.1021/es502348e.

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Janzen, H., K.J. van Groenigen, D.S. Powlson, T. Schwinghamer, and J.W. van Groenigen. 2022. Net Primary Production constraints are crucial to realistically project soil organic carbon sequestration. Response to Minasny et al. Geoderma 424: 115974. doi: 10.1016/j.geoderma.2022.115974.

Monfreda, C., N. Ramankutty, and J.A. Foley. 2008. Farming the planet: 2. Geographic distribution of crop areas, yields, physiological types, and net primary production in the year 2000. Global Biogeochemical Cycles 22(1). doi: 10.1029/2007GB002947.

Prince, S.D., J. Haskett, M. Steininger, H. Strand, and R. Wright. 2001. Net Primary Production of U.s. Midwest Croplands from Agricultural Harvest Yield Data. Ecological Applications 11(4): 1194–1205. doi: 10.1890/1051-0761(2001)011[1194:NPPOUS]2.0.CO;2.

Reichle, D.E. 2023. Chapter 9 – Ecosystem productivity. In: Reichle, D.E., editor, The Global Carbon Cycle and Climate Change (Second Edition). Elsevier. p. 197–232

Whittaker, R.H., and G.E. Likens. 1975. Primary productivity. Ecological Studies 14: 305–328.