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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Soil Respiratory Quotient Determined via Barometric Process Separation Combined with Nitrogen-15 Labeling

^a Dep. of Plant Ecology, Univ. of Giessen, Heinrich-Buff-Ring 26-32, 35392 Giessen, Germany
^b Soil, Plant and Ecological Sciences Division, Soil and Physical Sciences Group, P.O. Box 84, Lincoln Univ., Canterbury, New Zealand
^c Agriculture, Food and Environmental Science Division, The Dep. of Agriculture and Rural Development, Newforge Lane, Belfast BT9 5PX, Northern Ireland, UK

^* Corresponding author (Christoph.Mueller{at}bot2.bio.uni-giessen.de)

The barometric process separation (BaPS) and ¹⁵N dilution techniques were used to determine gross nitrification rates on the same soil cores from an old grassland soil. The BaPS-technique separates the O₂ consumption into that from nitrification and that from soil organic matter (SOM) respiration. The most sensitive parameter for the calculations via the BaPS technique is the respiratory quotient (RQ = CO₂/O₂) for SOM turnover (RQ_SOM). Combining both methods (BaPS–¹⁵N) allowed the determination of the RQ_SOM. The RQ value determined in such a way is adjusted for the influence of nitrification and denitrification, which are both characterized by totally different RQ values. The results for the grassland soil showed that 6 to 10% of O₂ was consumed by nitrification when incubated at 20°C and 0.49 g H₂O g^–1 soil. A set of BaPS measurements with the same soil at various temperature and moisture contents showed that up to 49% of the total O₂ consumption was due to nitrification. The calculated RQ_SOM values via the BaPS–¹⁵N technique presented here are more closely associated with the overall SOM turnover than the usual net RQ reported in the literature. Furthermore, the RQ_SOM value provides an overall indication of the decomposability and chemical characteristics of the respired organic material. Hence, it has the potential to serve as a single state index for SOM quality and therefore be a useful index for SOM turnover models based on substrate quality.

Abbreviations: aut/(aut + het), ratio of autotrophic to total nitrification • BaPS, barometric process separation • BaPS–¹⁵N, combined barometric process separation–¹⁵N labeling technique • RN-ratio, total mole CO₂ produced per mole of N nitrified • RQ, respiratory quotient • RQ_net, total mole O₂ uptake per mole CO₂ respired (CO₂/O₂) • RQ_SOM, RQ related to SOM turnover • SOM, soil organic matter

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