Spring 2015 Colloquium: Guihua Chen

Mitigation of N2O emissions using conservation tillage in vegetable fields transitioning to organic productions

Figure 1. Conventionally tilled field. Photo credit: Guihua Chen

Is global warming still happening? As succinctly stated by Entomology Post-Doc Guihua Chen: yes, obviously. The real question is what can we do about it.


Nitrous oxide, or N2O, is one of the most important non-CO2 (carbon dioxide) greenhouse gases (Reay et al. 2012).  N2O emission has been a topic of increasing controversy, particularly in the discussion of biofuels, as agricultural production is one of its primary anthropogenic sources (Crutzen et al. 2008). Although N2O is a natural byproduct of the nitrogen cycle, many agricultural practices lead to increased N2O emissions. For example, the addition of fertilizer adds more nitrogen into the system, and conventional tillage may increase the transport rate of N2O from the soil (Chatskikh and Olesen 2007). What management practices, then, could a farmer use to reduce N2O emissions from their crop systems? Dr. Chen’s research centers on this question, and how it relates specifically to organic farming techniques.

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Figure 2. Black plastic mulch field. Photo credit: Guihua Chen

Dr. Chen investigated the effect of four different tillage practices on N2O emissions while using organic vegetable production methods: conventional tillage, conventional tillage with plastic mulch on rows, conservation strip-till, and conservation no-till. Conventional tillage involves tilling the entire field, incorporating crop residue and fertilizers with soils (Figure 1). As the name suggests, in plastic mulching, after tillage a plastic mulch (here black plastic) is placed over the row where the crops will be planted, but the interrows are left uncovered (Figure 2). In strip-till, only the row where the crop will be planted is tilled, and the rest of the field is left undisturbed (Figure 3); no-till involves no soil disturbance (Figure 4). For more on these tillage practices in organic vegetable production, see the UMD extension publication located here. Each of the four treatments was run for three years (2012-2014) at the Central Maryland Research and Education Center Farm in Upper Marlboro, MD. Just as different crops may be planted in a field in different years, eggplants were planted in the study fields in 2012 and 2014, and sweet corn was planted in 2013. Fertilizers provide nitrogen to crops and may alter the amount of N2O emissions, so organic fertilizers (chicken manure and blood meal) were applied at the same rate for all treatments. Dr. Chen measured N2O emissions from the soil as well as soil temperature, moisture, and mineral nitrogen in both the rows and interrows before and after all field operations and rainfall events as these events can affect the nitrogen cycle in the soils leading to changes in N2O emissions.

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Figure 3. Strip-tilled field. Photo credit: Guihua Chen

Soil temperature and moisture were both higher in the rows of the plastic mulch treatment during the early growing season, but no significant differences among treatments were seen later in the growing season as the plants grew, shading and utilizing the water and nutrients in the soil. Nitrous oxide emissions were highest early in the growing season, from late May to June when soil temperature was highest and water was less quickly taken up by the young plants. 

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Figure 4. No-till field. Photo credit: Guihua Chen

Later in the growing season, emissions were low and similar across treatments.  Annual N2O emissions were generally highest in the plastic mulch and lowest in the no-till treatments across years. Crop yield was largely similar among treatments in all years, signifying that different tillage practices did not seem to affect the amount of eggplant or sweet corn that grew in the fields even with differing emissions that could indicate differences in soil conditions including nitrogen availability. Similarly, scaled to yield N2O emissions showed consistently higher values in the plastic mulch treatment for both eggplant and corn. No-till and strip-till consistently showed the lowest values. Based upon the differences in emissions, Dr. Chen concluded both mineral nitrogen and moisture appeared to be highly important factors controlling the amount of N2O emissions that occurred across treatments and years.  Warmer soil temperature in the plastic mulch treatment was also responsible for higher N2O emissions during the early growing season. Changes in these factors due to mulching and tillage practices can therefore have an impact on the amount of N2O emissions from a field.

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So how can farmers (both backyard and commercial) reduce the N2O emissions from their fields? Dr. Chen’s recommendations focus on moderation and limiting usage of conventional tillage practices. If plastic mulch is needed for weed control, choosing a color other than black that does not warm the soil as much may help reduce N2O emissions. Reducing tillage also appears to reduce emissions, and practicing conservation tillage may have several additional benefits. Dr. Chen also recommended avoiding over-irrigation and over-fertilization, and if possible split-application of fertilizers. Split-application of fertilizer applies the same amount of fertilizer in two smaller applications rather than one large application. Applying smaller quantities allows the plants to absorb the nitrogen before it contributes to N2O emissions. With enough implementation of these suggestions, we may be able to significantly reduce the amount of N2O emissions from agricultural systems.

If interested in learning more about Dr. Chen’s research on N2O emissions in agricultural fields, be on the lookout for a publication in the near future!

References

Chatskikh, D. and J. E. Olesen. 2007. Soil tillage enhanced CO2 and N2O emissions from loamy sand soil under spring barley. Soil and Tillage Research 97: 5-18.

Crutzen, P.J., A.R. Mosier, K.A. Smith, & W. Winiwarter. 2008. N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmospheric Chemistry and Physics 8: 389-395.

Reay, D.S., E.A. Davidson, K.A. Smith, P. Smith, J.M. Melillo, F. Dentener, & P.J. Crutzen. 2012. Global agriculture and nitrous oxide emissions. Nature Climate Change 2: 410-416.

Becca Eckert is a Ph.D. student working with Bill Lamp. She is interested in the interactions between and amongst algae, macroinvertebrates, and nutrients in streams.

Becca Wilson is a Ph.D. student in Bill Lamp’s lab. She is researching the distribution patterns of nuisance black flies in western Maryland and their impact on human quality of life.