Yun and Ro (2009) conducted an experiment using Chinese cabbage plants to determine whether the amount of 15N within the plant tissues can be used to indicate the overuse of compost in soil. Compost is an important inexpensive alternative to chemical fertilizers, although the overuse of compost can lead to the same environmental impacts that overuse of N fertilizers have. The amount of nutrients in compost varies depending upon the source, therefore the concentrations of N and P need to be determined before application to soils in order to avoid overuse. Four different amounts of compost were used in the experiment and three areas of plant tissue and the soil were analyzed for 15N content. They determined the amount of 15N found in the plants along with the soil nitrate content can be used to tell if the level of compost application was too high.— Maria Harwood
Yun, S., Ro, H., 2009. Natural 15N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost. Soil Biology and Biochemistry 41, 1541–1547.
Yun and Ro used potted Chinese cabbage plants grown for 42 days in soil amended with a compost made of pig manure mixed with sawdust. Four different rates of compost were applied to the soil; 0, 500, 1000, 1500 mg N/kg. The outer, middle, and inner leaves of the Chinese cabbage plants were analyzed for N content to determine whether the source of N found in the plants was from the compost or the indigenous N soil content. This distinction is key to figuring out if 15N can be used as an indicator of the amount of compost used or if it is only showing the N usage from the soil.
Compost application increased the amount of dry mass accumulation of the cabbage with the lowest application rate, but successive increasing amounts of compost produced no increase in dry mass. Although the amount of dry mass did not increase with added compost, the uptake of N by the cabbage plants did increase in proportion to the amount of compost. The increasing compost amounts also produced greater amounts of 15N found in the cabbage for all applications except the lowest. The inner, younger leaves of the cabbage showed a strong correlation of the 15N levels being derived from the compost, therefore the authors determined the 15N levels found in plants can be used as an indicator of the amount of compost being applied to the soil.