Microbially-mediated gross N transfer rates in field soils in relation to physico-chemical transfer processes

Abstract

Measurements of gross N transfer in soils have as yet not distinguished between biological or physico-chemical processes. Here, we present a new approach that allows microbially-mediated gross N transfer rates to be estimated in undisturbed soils without adding ¹⁵N. It is based on the assumption that in undisturbed soil, the soil microbial growth rate is equal to its death rate. To assess the contribution of biological versus physico-chemical N transfer processes, we combined the new approach with the ¹⁵N-pool dilution technique. The relationship between both processes varied with soil C and fine particle contents. Nearly equal rates were observed within the carbon-poor soil (0.35% C_org, low fine particle content), whereas up to 2.5 times higher physico-chemical than biological N transfer rates were measured within the carbon-enriched soil (0.86% C_org, higher fine particle content). Furthermore, microbially-mediated gross N transfer rates increased three-fold after N fertilization compared to the unfertilized control.

Keywords:

• Gross N transfer rates
• ¹⁵N-pool dilution
• N immobilization
• microbial biomass
• specific growth rate
• doubling time
• long-term field experiment
• N consumption rate

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.