Gross N transfer rates in field soils measured by ¹⁵N-pool dilution

Abstract

A micro-plot ¹⁵N-tracer experiment was established in three different soils of a long-term soil fertility field experiment. The nutrient-poor loam sand has been subjected to various treatments over the years and this has resulted in different organic C (0.35% – 0.86%), microbial biomass (38.3 – 100.0 µg C _mic g⁻¹ soil), clay and fine silt contents. Using the ¹⁵N-pool dilution technique, we assessed gross N-transfer rates in the field. Gross N mineralization rates varied strongly among the three plots and ranged between 0.4 and 4.2 µg N g⁻¹ soil d⁻¹. Gross nitrification rates were estimated to be between 0 and 2.1 µg N g⁻¹ soil d⁻¹. No correlation between gross N mineralization rates and the organic matter content of the soils was established. However, gross nitrate consumption rates increased with increasing soil C content. The ¹⁵N-pool dilution technique was successfully used to measure gross N transfer rates directly in the field.

Keywords:

• Gross N turnover
• ¹⁵N-pool dilution
• N immobilization
• microbial biomass
• long-term field experiment
• N consumption

Acknowledgements

We sincerely thank Michael Baumecker and Stefanie Schmidt for their assistance in the field, Birgit Wernitz for microbial laboratory work, and Susanne Remus for help with the isotopic analysis. We would also like to thank Dr. Ruth Willmott for her English-language editing.