USC Dana and David Dornsife College of Letters, Arts & Sciences > Blog

April 22, 2012

California Agriculture and the Nitrogen Cycle

Filed under: California Agriculture,Nitrogen Cycle — dginsbur @ 9:23 pm

Agriculture is one of the largest and most important industries in California, supplying half of all the produce for the nation. According to the California Department of Food and Agriculture, agriculture generates almost $100 billion for the state’s economy. The strength of the agricultural industry was built on the diversity of crops from almonds and broccoli to grapes and squash that can be grown in the mild climate and fertile soil available in California.

The nitrogen cycle is key to making sure that crops continue to grow so that the agricultural industry can maintain the same level of crop yields and keep on feeding the country. Nitrogen is used by plants not only in nucleic acids and proteins, but also specifically with chlorophyll, which is key to photosynthesis, the process that provides plants with food to continue growing. However, plants cannot access atmospheric nitrogen in the form of N2—the nitrogen gas needs to be converted or fixed into a different form that is accessible to plants.

Therefore, the current problem that farms are facing is that not enough nitrogen is being naturally fixed to promote plant growth and fulfill the demand for produce. Farms in California are beginning to rely more and more heavily on nitrogen fertilizers to boost plant productivity. However, this input of manufactured nitrogen has many effects on the nitrogen cycle. Many plants do not absorb all of the nitrogen from the fertilizer, leaving excess nitrogen in the soil, which causes an imbalance to the ecosystem. This nitrogen can also be deposited in bodies of water through runoff, causing eutrophication due to its role as a limiting nutrient. As agriculture in California has risen to be major industry, more synthetic fertilizers are being used to grow crops, and the excess nitrogen is becoming an important problem.

Fortunately, this issue has been recognized by organizations like the Environmental Protection Agency and the California Department of Food and Agriculture. In 2010, the Agricultural Sustainability Institute at UC Davis sponsored the California Nitrogen Assessment, an attempt to assess California’s current agricultural needs in regards to nitrogen and to discuss possible solutions. In the information and progress report, they stated, “Our researchers are working to establish a baseline of credible knowledge about nitrogen, which includes comprehensive accounting of nitrogen flows, agricultural practices, and the policies that shape these practices. They will also assess the quality of information and knowledge about these issues.”

Some solutions to be considered are utilizing a crop-rotation system with nitrogen-fixing crops, or developing crops that use endosymbiotic nitrogen-fixing bacteria to reduce the amount of synthetic, nitrogen-rich fertilizers used. Another option would be genetically modifying crops to be more nitrogen efficient, but there are many controversies surrounding such practices.

In terms of recent developments, the CDFA posted a press release stating “The California State Board of Food and Agriculture will discuss a variety of topics related to the nitrogen cycle and the proactive work by California farmers and ranchers on the issue at its upcoming meeting on March 6th [2012] in Sacramento.” While they have yet to release a summary of the meeting, the conference shows that there is still state interest in regulating the nitrogen cycle in regards to agriculture. As the agriculture industry continues to grow, it is important to be aware of the nitrogen cycle and the impact humans have on it.


Harriet Arnold and Divya Rao are undergraduates in the USC Dornsife College of Letters, Arts and Sciences.

Can California agriculture help increase carbon sequestration?

The carbon cycle is currently out of balance. Humans have introduced too much carbon dioxide to the atmosphere from burning fossil fuels, causing climate change and changing weather patterns. We have the technology to do work with nature to sequester more carbon. Agricultural land accounts for 455 million acres of the total land area in the US of 1.9 billion acres. Unfortunately, since the time that we have settled the land, the soil organic content has dropped to about less than one-fourth of what it once was.

Humans could sequester organic carbon into soil if we operated farms and ranches with practices that increase and maintain the organic material in the soil. For example, conventional farm practices that include improper tillage and overuse of chemical fertilizers result in about 20,000 pounds of carbon dioxide. We can help control the CO2 released by adding organic material to the soil. Practices in which soil is mulched and rarely tilled result in a dramatic decrease in the loss of carbon dioxide from the soil. Tilling the soil upsets soil life and exposes it to sunlight and oxidation, releasing large amounts of CO2. In the natural environment, the carbon-based roots and other soil life are rarely exposed or destroyed. Such oxidation naturally takes place, but the natural process is much slower, so plants can capture the CO2 and reprocess it instead of letting it into the atmosphere.

California is a huge venue for carbon storage potential, as much of California’s agriculture is perennial. Perennial crop residue is more readily decomposed than annual residues, and perennials store carbon within the woody biomass of trees and vines. Further, with the increase in agricultural yields, the biomass returned to the soils has increased, promoting sequestration. Rice farmers have also contributed to sequestration efforts. Instead of burning the fields after harvest, most of the crop residue is now returned to the soil. Through similar small efforts, California agriculture can greatly increase its agricultural carbon sequestration.

Although there has not been significant research into vineyards as carbon sequestration resources, they hold high potential. Permanent cover cropping has been shown to increase soil organic matter when used instead of bare fallow rotations. Growing cover crops, however, can be negatively impacted by one light tillage annually. Further research is needed to understand the ability of different cover crops to increase soil carbon in vineyards. Still, there have not been many studies of vineyards and carbon sequestration. Vineyard specific studies are needed to understand the effects of vineyard management practices on carbon storage.

California could almost double carbon sequestration by adapting conservation tillage practices and returning prunings to the soil. This assumes that area for perennial agriculture continues to expand, and that the biomass of crops continues to grow. As of 2002, California’s agriculture was not sequestering through conservation tillage although the practice is commonly cited as sequestering carbon by reducing soil respiration. Due to the low erosion potential of the land and the high intensity multicropping, California agriculture has not widely adopted conservation tillage. If further research were done on adapting conservation tillage to California agriculture, we could help restore balance to the carbon cycle.


Christopher Miranda is an undergraduate in the USC Dana and David Dornsife College of Letters, Arts and Sciences.

February 27, 2012

Which Came First: Soil Conservation or Sustainable Agriculture?

Agriculture requires fertile soils and is therefore dependent on a high level of soil biodiversity. However, agriculture itself has a major influence on biodiversity. For sustainable farming, a farmer should manage his soil’s health, ensuring that the soil will support crops for years to come. The FoodandAgricultureOrganization has historically encouraged scientists and farmers to share research and experiences for the benefit of agricultural development programs and farmers. As soil is fundamental to agriculture, it is also fundamental to human health and food security. It is important that we conserve soil biodiversity and the manage soil for the value of its ecosystem services.

One common agricultural practice, the use of fertilizer, is advantageous to the soil biota. For example, mineral fertilizers can increase the abundance of nematodes. However, because soil biodiversity is very sensitive to the changes in soil pH and the concentration pore water salts, using fertilizer might decrease the soil biodiversity. It is important to use the appropriate amount of fertilizer to avoid damage to the soil organisms.

Pesticides are also commonly used, and can affect soil biota. Soil organisms can be exposed to applied pesticides, so it’s important that the pesticides don’t harm the soil organisms. Testing has led to the development of regulations to ensure that when used properly, pesticides will not cause unacceptable harm to the soil organisms. When planning for fertilizer and pesticide use, a farmer can work towards improving soil biodiversity. By using an appropriate amount of fertilizer and pesticides, the farmer can stimulate plant and soil organism growth while decreasing the risk towards soil organisms.

The farmer can use several physical techniques to manage his soil. The first is planting his crops. By providing plant cover for the soil, the farmer protects his soil and the organisms with in his soil from wind or water erosion. Further, cultivation of row crops such as sugar beet, maize, potato and vegetables provides only partial soil coverage and protection, leaving the land vulnerable to erosion. Large field areas are often devoid of any morphological structures, such as hedges, that could potentially mitigate erosion from wind or water. The farmer might also reduce or even stop tilling the fields. Intense mechanical soil treatment that disturbs the soil pore system is a common cause of erosion. Reduction may improve soil structure, increasing water capacity, and decreasing erosion. The consequence of the erosion is usually the loss of humus and nutrients from the upper soil, leading to reduced fertility.

As such, agricultural practices and following natural processes can have tremendous influences on soil and soil biodiversity. To maintain adequate food supply, and reach sustainable agriculture, conservation of soil is the most important factor in today’s agriculture business. Farmers can conserve soil biodiversity by using contemporary agricultural techniques that cause fewer disturbances to the soil than traditional techniques. Although soil analysis may be an extra cost to production, the benefits would outweigh the cost. With analysis and proper planning, the farmer will be able to enjoy his soil for a lifetime. Through effective soil management, the farmer can avoid stripping the land of nutrients.

As the world’s population grows and its food needs increase, we must work to relieve population pressure on food supply. Soil biodiversity is the key factor for sustainable agriculture, and thus the practices to conserve soil biodiversity are important. As the soil biodiversity and agriculture are the basis of human food supply, we need to take action to preserve our soils.


Wonho Jung and Christopher Miranda are undergraduates in the USC Dornsife College of Letters, Arts and Sciences.

October 31, 2011

Where would California agriculture be without the carbon cycle?

The amount of global carbon outputs have increased with the industrial revolution and rising need for agriculture, brought about by a rapid population growth. Agriculture over the years has become a major contributor to the global carbon cycle, especially as it now accounts for approximately 14% of global land use. It tends to deplete soil carbon because land used for agriculture has a lower net primary productivity than land that is kept in it’s natural state with undisturbed soil and processes.

In order to better understand the ways in which we are emitting carbon and effecting the carbon cycle, one must better understand the cycle itself. Carbon is absorbed by plants during photosynthesis in the form of carbon dioxide, along with sunlight and water. Plants then produce glucose and oxygen. When plants decompose, the carbon is transferred to the soil, where it is stored along with mineral carbon. Additionally, when animals eat plants, the carbon is passed from one to the other. Therefore this carbon can be released into the atmosphere during the respiration of plants and animals, the burning of fossil fuels, or, in the case of farming, when soil is tilled (Soil Science and Management 5th Edition, Plaster).

California’s agriculture tends to differ from that in the rest of the US in that it grows a large amount of perennial crops and specialty crops such as vegetables, nuts and fruits. California’s climate makes it the ideal provider of specialized high value crops to the rest of America, and much of its economy has been built on this industry.  Perennial orchards and vineyards account for roughly a third of agricultural land (Kroodsma and Field). Since California has long hot summers, irrigated soils go through distinctive wet and dry cycles between water applications, which impacts the amount of dissolved organic carbon in soils. For these reasons, it’s evident that the carbon cycle is particularly vital in California industries and agriculture (Kroodsma and Fields).

California has smaller levels of carbon sequestration, most likely largely due to warmer temperatures, irrigation, and a strong lack of conservation tillage. This practice disturbs the soil less, but is not practiced as much in California because there is less of a risk of erosion than in the rest of the US. However, carbon storage in California is improved by the growth of perennial agriculture. It agitates the soil less and decomposes faster than annual plants.

There are practices, both farming and industrial, that can be put into effect in California’s agriculture to reduce carbon emissions. Farmers can implement increased use of conservation tillage to leave soil undisturbed and reduce emissions. They can also return pruning of plants to the soil as mulch. It has been proven that if soil is mulched, rarely tilled and has plants growing, loss of CO2 is decreased. Waste wood from orchards and vineyards can be used in biomass power-plants, both to reduce waste and the burning of fossil fuels. California has already begun decreasing field burnings, as that releases CO2 (Kroodsma and Fields). These practices implemented together would reduce carbon emissions while simultaneously improving crop yields and productivity.

About the authors: Lily Phillips and Ariana Verdu are working towards their bachelor degrees in the USC Dornsife College of Letters, Arts and Sciences.