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Research Update: Nitrogen Management for Container Plant Production

Note: This is part of a Research Update series that highlights projects funded by the California Department of Food and Agriculture (CDFA) Fertilizer Research and Education Program (FREP) annual grant program.

Project Title: A System Nitrogen Balance for Container Plant Production

Project Leaders:  Lorence R. Oki and Bruno J.L. Pitton (University of California, Davis)

Project Status: Complete

Overview: This FREP-funded project measured nitrogen (N) input and output from a woody ornamental plant production system to understand the fate of incorporated controlled-release and surface-applied fertilizer.

Background: A large proportion of groundwater in the Central Valley is currently experiencing nitrate contamination because of inefficient irrigation and fertilization. The Central Valley Regional Water Quality Control Board requires growers to complete an Irrigation and Nitrogen Management Plan (INMP) annually to document N inputs and outputs from production areas. Existing knowledge gaps prevent wholesale container plant growers from accurately filling out the INMP Worksheet. To assist growers in filling out the INMP Worksheet, a system N balance was developed for a woody ornamental nursery production system.

Approach: A woody ornamental was grown for 81 days in containers filled with Douglas fir bark soilless substrate. The experiment consisted of four lined and four unlined experimental beds for runoff capture. Nitrogen inputs measured included: substrate with controlled-release fertilizer and roots incorporated, irrigation water, and surface-applied fertilizer (Figure 1). Harvested product N outputs included: the substrate, including roots and remaining fertilizer, and plant shoots at end of the production cycle (Figure 1). Additional outputs included gaseous N emitted from the substrate and aqueous N that was leached from the substrate or applied as irrigation water and flowed off the growing beds during the production cycle (Figure 1).

Fig. 1. Diagram of nitrogen (N) inputs and outputs measured to develop the
nursery system N balance. Inputs include substrate N (fertilizer, roots, and
media), surface-applied fertilizer, and irrigation water N. Outputs include N in
runoff water, shoot uptake, remaining in substrate, and nitrous oxide gas from
substrate (N2O-N) and growing bed soil (N2+ N2O-N).

Results:

The finished plant ready for wholesale retained 62% of total input N (Table 1) with 5% of input N partitioned in the plant shoot while the substrate retained 57% of the total input N (Table 1). Having a reserve of N in the growing substrate ensures that the plants will continue to be aesthetically appealing at the time of purchase by the consumer. Nitrogen losses in bed runoff represented 6.5% of the total input N (Table 1) and could be captured for reuse. Nitrogen that infiltrated into the soil below the growing bed amounted to 2.4% of total input N. Although nearly 23 kg N ha-1 infiltrated during the woody ornamental production cycle of 81 days, total nursery acreage in California is small compared to that of other horticultural, which limits the overall impact. Total N gas emission from the growing bed soil was approximately 1% of total N infiltration below the growing bed (Table 1). Nearly 28% of total input N was unaccounted for (Table 1) and is attributed to complete denitrification in soilless substrate.

Table 1. Nitrogen balance for a container plant production system at a nursery. The first row below each column header is the mass of nitrogen and the second row is the amount of nitrogen as a percentage of total applied nitrogen.

Conclusion: The N balance indicates that applying N fertilizer that exceeds plant demand results in leaching and runoff losses as well as greenhouse gas emissions. A small fraction of applied N infiltrated the soil and INMP calculations would overestimate this. Outreach efforts should continue to be made to inform growers of best nutrient management practices to improve nutrient use efficiency.

To learn about more findings and details of this study, visit the recently published article by the project leaders.

Reference:

Pitton, B. J., Oki, L. R., Sisneroz, J., & Evans, R. Y. (2022). A nursery system nitrogen balance for production of a containerized woody ornamental plant. Scientia Horticulturae291, 110569.

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Visit the FREP Grant Program webpage for information about the annual, competitive grant that funds research and education promoting the agronomically safe and environmentally sound use of fertilizer in California. Visit the FREP Research & Project Database for details about current and completed FREP-funded projects, as well as a searchable database that aims to make the research available, understandable, and convenient for growers.


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