Environmental Engineering Reference
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(4.75 cm i.d.) was inserted 17 cm into the soil and immediately extracted, encasing
the soil core. Two centimeter of the encased soil were removed from the bottom of
the cylinder and replaced with a nylon resin. The bottom of the cylinder was then
covered with sturdy nylon cloth to prevent root entry and the cylinder assembly was
reinserted into the original hole. They found that incubation periods greater than
60 days resulted in resulted in loss of inorganic N from resins, and recommended
28- to 40-day incubations. However, they conducted their study in uplands; loss of
inorganic N from resins would probably be greater under saturated conditions. They
also cautioned against compacting the soil below the cylinder during installation as
that can impede water movement through the soil core.
7.5.5 Denitrification
7.5.5.1 Overview
Several approaches have been used to estimate denitrification, including in situ
measurements using natural N isotopic abundances (Søvik and Mørkved 2008 ), in
situ measurement of N 2 O evolution without an C 2 H 2 block (Jordan et al. 2007 ;Whalen
2000 ; Wray and Bayley 2007 ), ex situ measurement of N 2 and-or N 2 O evolution from
intact cores without an C 2 H 2 block (Horwath et al. 1998 ;WrayandBayley 2007 ),
ex situ measurement of N 2 O evolution from intact cores using an C 2 H 2 block (Bohlen
and Gathumbi 2007 ; Horwath et al. 1998 ;Huntetal. 2003 ), ex situ measurement
using intact cores coupled with isotopic approaches (Racchetti et al. 2011 ;R¨ckauf
et al. 2004 ), and ex situ measurement of denitrifying enzyme activity (DEA) in
homogenized soil with or without using an C 2 H 2 block and optimized incubation
conditions (Bruland et al. 2009 ;Hunteta . 2003 , 2007 ; Jordan et al. 2007 ;
Sirivedhin and Gray 2006 ). The methods perhaps best suited to routine estimations,
and the two considered here, are (1) ex situ measurement of N 2 O evolution from intact
cores using an C 2 H 2 block, and (2) ex situ measurement of DEA in homogenized
soil with using an C 2 H 2 block and optimized incubation conditions.
Using C 2 H 2 block eliminates problems with measuring N 2 evolution against high
atmospheric background levels of the gas or the necessity of using a non-N 2 -
containing atmosphere during incubations. Acetylene blocks the conversion of N 2 O
to N 2 , thus simplifying analyses and providing a more sensitive assay. Intact cores can
arguably give more realistic estimates of denitrification than does DEA, but are
typically more variable both spatially and temporally. In comparison with using intact
cores, estimates of denitrification based on DEA using homogenized soil are generally
less variable but may give unrealistically high values, especially if the usual practice of
optimizing incubation conditions is used (i.e., anaerobic atmosphere, presence of
excess C and NO 3 ). It is for this reason that the values obtained are commonly
considered potential rates of denitrification and are viewed as representing primarily
maximum relative rather than absolute estimates. Thus, the investigator will need to
decide which of the two approaches best meets the needs of the situation at hand.
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