Agriculture Reference
In-Depth Information
More subtle patterns in the potential activity of C vs. N cycling en-
zymes were apparent along the second RDA axis. Microbes regulate ex-
tracellular enzyme production to acquire limiting nutrients, so changes in
enzyme activities may refl ect patterns of microbial nutrient limitations
and hence nutrient availability (Allison et al., 2007, Allison et al., 2011,
Sinsabaugh et al., 2008 and Burns et al., 2013). The strong association
among soil inorganic N, particularly NO
3
−
, and the activities of C-cycling
enzymes (β-glucosidase, α-galactosidase) and a C/N-cycling enzyme
(β-glucosaminidase) suggest a shift toward increased C acquisition as N
becomes readily available. Other studies have shown increased activity of
cellulases (i.e. enzymes that catalyze degradation of cellulose, including
β-glucosidase and α-galactosidase in this study) in response to N fertiliza-
tion (Bandick and Dick, 1999, Sinsabaugh et al., 2005 and Piotrowska and
Wilczewski, 2012). Reduced activity of enzymes involved directly in N
mineralization (e.g. urease and amidase) with higher inorganic N avail-
ability has also been shown in agricultural systems (Dick et al., 1988 and
Bandick and Dick, 1999) and agree with our results of reduced potential
activity of l-asparaginase and aspartase in several fi elds with higher NO
3
−
(e.g. fi eld 4). Higher levels of soil NO
3
−
were typically found in fi elds with
intermediate levels of soil C and N in conjunction with application of a
labile N source (e.g. seabird guano), which was likely rapidly mineralized
and nitrifi ed.
In contrast, greater potential activity of two N-cycling enzymes (l-
asparaginase and aspartase) but lower activity of C-cycling enzymes oc-
curred in fi elds with higher soil C and N where composted green waste
was applied as a primary organic matter source. In such situations, an
abundant supply of diverse C sources may have resulted in N limitation
for the microbial community and hence, greater production of enzymes
to mineralize N. The high concentrations of EOC and EON and the low
concentrations of soil NH
4
+
and NO
3
−
in fi elds 10, 11, 12, and 13 sup-
port this hypothesis. EOC and EON are comprised of a diverse array of
organic molecules, including free amino acids (Yu et al., 2002 and Paul
and Williams, 2005) that would include substrates for l-asparaginase and
aspartase (Frankenberger and Tabatabai, 1991 and Senwo and Tabatabai,
1996). Furthermore, we hypothesize that rapid microbial and plant uptake
