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

utilization patterns (Dilly, 2001), the microbial community structure (e.g., proportions of bacteria to fungi) The barometric process separation (BaPS) and 15N dilution tech(Klein, 1977) and abiotic factors such as soil moisture– niques were used to determine gross nitrification rates on the same soil cores from an old grassland soil. The BaPS-technique separates temperature relationships (Klein, 1977; Scagel and Anthe O2 consumption into that from nitrification and that from soil dersen, 1997). organic matter (SOM) respiration. The most sensitive parameter for In all previous studies the net RQ (RQnet) is measured the calculations via the BaPS technique is the respiratory quotient rather than the exact RQ resulting from SOM turnover (RQ CO2/ O2) for SOM turnover (RQSOM). Combining both (RQSOM) alone. The RQnet integrates the effects of all methods (BaPS–15N) allowed the determination of the RQSOM. The microbial transformations that consume O2 and conRQ value determined in such a way is adjusted for the influence of sume or produce CO2. For instance, denitrification utinitrification and denitrification, which are both characterized by tolizing NO 3 instead of O2 increases the overall RQnet tally different RQ values. The results for the grassland soil showed when occurring in anaerobic microsites within the surthat 6 to 10% of O2 was consumed by nitrification when incubated rounding aerobic soil. Autotrophic nitrification, howat 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 ever, decreases the RQnet because it takes up O2 and that up to 49% of the total O2 consumption was due to nitrification. CO2 according to a ratio of 1.68 moles of O2 to 0.23 The calculated RQSOM values via the BaPS–15N technique presented moles of CO2 (Ingwersen et al., 1999). In particular the here are more closely associated with the overall SOM turnover than effect of nitrification in lowering the RQ value might the usual net RQ reported in the literature. Furthermore, the RQSOM be considerable but in most cases it is not taken into value provides an overall indication of the decomposability and chemiaccount (Dilly, 2001). cal characteristics of the respired organic material. Hence, it has the To investigate the relationship between nitrification potential to serve as a single state index for SOM quality and therefore and RQ values, both the gross nitrification rates as well be a useful index for SOM turnover models based on substrate quality. as the O2 and CO2 dynamics have to be determined. The BaPS technique for determination of gross nitrification rates combines all of these requirements and is therefore T RQ in soil is defined as the ratio of mole O2 ideally suited for such an analysis (Ingwersen et al., uptake per mole CO2 respired ( CO2/ O2). In aero1999; UMS, 2003). The BaPS technique requires specifibic soils the measured RQ value is usually in the range cations of soil and microbial parameters that are readily of 0.6 to 1.0 (Andersen and Scagel, 1997; Klein, 1977). available (e.g., pH, soil moisture) but also more specific The RQ value is frequently higher than 1.0 (Dilly, 2001) parameters that are usually not known such as: (a) the under anaerobic conditions, when alternative electron ratio of N2O/N2 during denitrification, (b) the ratio of acceptors are available (e.g., NO 3 ) or shortly after subautotrophic/heterotrophic nitrification, and (c) the RQ strate (e.g., glucose) addition stimulating microbial during soil respiration (default RQ value used in the growth. In general, the RQ decreases as the C/O ratio calculations is 1.0). Sensitivity analyses showed that Pain the components being metabolized increases (e.g., rameters a and b have only a minor influence, but small mineralization of humic acids has a RQ of 0.909; Dilly, changes in the RQ value have a large effect on the 2001). A RQ value of 1.0 is obtained only under condicalculated gross nitrification rates (Breuer et al., 2002; tions when the respired C substrate has a chemical comIngwersen et al., 1999). Therefore, a cross-comparison position comparable with that of glucose (Dilly, 2001). of the BaPS results with an independent method for Therefore, the RQ value of metabolized SOM in soils gross nitrification rate determination is needed. In addiprovides an indication of the substrate utilization. The tion, such a cross-comparison would provide a basis for observed temporal variations in RQ values under field accurate setting of the Parameters a through c needed conditions are associated with variations in substrate for the BaPS calculations. The most widely used method to determine gross nitrification rates in soil is the 15N C. Müller, M.K. Abbasi, C. Kammann, and H.-J. Jäger, Dep. of Plant dilution technique (Stark, 2000). Furthermore, specific Ecology, Univ. of Giessen, Heinrich-Buff-Ring 26-32, 35392 Giessen, 15N labeling techniques exist to derive ratios of autotroGermany; T.J. Clough and R.R. Sherlock, Soil, Plant and Ecological Sciences Division, Soil and Physical Sciences Group, P.O. Box 84, phic/heterotrophic nitrification (Müller et al., 2004) and Lincoln Univ., Canterbury, New Zealand; R.J. Stevens, Agriculture, ratios of N2O/N2 (Stevens and Laughlin, 1998) in soil. Food and Environmental Science Division, The Dep. of Agriculture and Rural Development, Newforge Lane, Belfast BT9 5PX, Northern Abbreviations: aut/(aut het), ratio of autotrophic to total nitrificaIreland, UK. Received 10 Nov. 2003. *Corresponding author (Christoph. tion; BaPS, barometric process separation; BaPS–15N, combined [email protected]). metric process separation–15N labeling technique; RN-ratio, total mole CO2 produced per mole of N nitrified; RQ, respiratory quotient; RQnet, Published in Soil Sci. Soc. Am. J. 68:1610–1615 (2004).  Soil Science Society of America total mole O2 uptake per mole CO2 respired ( CO2/ O2); RQSOM, RQ related to SOM turnover; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA