High abundance of ammonia-oxidizing archaea in acidified subtropical forest soils in southern China after long-term N deposition.

Nitrification has been believed to be performed only by autotrophic ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) until the recent discovery of ammonia-oxidizing archaea (AOA). Meanwhile, it has been questioned whether AOB are significantly responsible for NH(3) oxidation in acidic forest soils. Here, we investigated nitrifying communities and their activity in highly acidified soils of three subtropical forests in southern China that had received chronic high atmospheric N deposition. Nitrifying communities were analyzed using PCR- and culture (most probable number)-based approaches. Nitrification activity was analyzed by measuring gross soil nitrification rates using a (15) N isotope dilution technique. AOB were not detected in the three forest soils: neither via PCR of 16S rRNA and ammonia monooxygenase (amoA) genes nor via culture-based approaches. In contrast, an extraordinary abundance of the putative archaeal amoA was detected (3.2 × 10(8) -1.2 × 10(9)  g soil(-1) ). Moreover, this abundance was correlated with gross soil nitrification rates. This indicates that amoA-possessing archaea rather than bacteria were predominantly responsible for nitrification of the soils. Furthermore, sequences of the genus Nitrospira, a dominant group of soil NOB, were detected. Thus, nitrification of acidified subtropical forest soils in southern China could be performed by a combination of AOA and NOB.

Analytical techniques for quantifying (15)N/(14)N of nitrate, nitrite, total dissolved nitrogen and ammonium in environmental samples using a gas chromatograph equipped with a quadrupole mass spectrometer.

The measurement of (15)N concentrations in environmental samples requires sophisticated pretreatment devices and expensive isotope-ratio mass spectrometry (IRMS). This report describes the use of a gas chromatograph equipped with a quadrupole-type mass spectrometer (GC/MS) to measure (15)N concentrations of ammonium, nitrate, nitrite, and total dissolved nitrogen (TDN) in distilled water, a 2 M KCl solution and a 0.5 M K(2)SO(4) solution. The system measures nitrous oxide (N(2)O) that is ultimately converted from the target N compound, requiring no special apparatus such as a purge-and-trap pretreatment device. It uses a denitrifier lacking N(2)O reductase, which produces N(2)O from nitrate. Persulfate oxidation was applied to convert TDN to nitrate, while additional pretreatment with ammonia diffusion was required for ammonium prior to the persulfate oxidation. Up to 100 samples can be measured daily using the system. We can generally run (15)N measurements with only 1-10 mL of sample for each chemical species of N, a volume 1/10-1/100 times smaller than the amount necessary for conventional methods. Our method is useful for measuring (15)N with GC/MS, offering greater convenience than IRMS.