Agriculture Reference
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rived from active microorganisms and the stabilized pool in clay-humus
complexes (Tabatabai, 1994 and Burns et al., 2013), plays a major role in
the depolymerization of structurally diverse polymeric macromolecules,
which is considered the rate-limiting step in decomposition and nutrient
mineralization potential of soil (Schimel and Bennett, 2004).
Organic management increases overall enzyme activity (Mäder et al.,
2002, García-Ruiz et al., 2008 and Moeskops et al., 2010), but activities
of specifi c enzymes may change depending on the composition of the
amendments and the relative availability of nutrients, as well as other fac-
tors, such as soil type and its unique characteristics, e.g. pH and texture
(Acosta-Martínez et al., 2007, Sinsabaugh et al., 2008 and Štursová and
Baldrian, 2010). Given the relatively constrained C:N:P ratios of micro-
bial biomass (Cleveland and Liptzin, 2007), enzymatic activity might be
expected to enhance the availability of the most limiting nutrients in order
to meet microbial metabolic demands (Sinsabaugh et al., 2008 and Al-
lison et al., 2011). For instance, in grassland and forest soils, long-term
N fertilization increased the activity of soil enzymes involved in labile C
breakdown (Ajwa et al., 1999, Saiya-Cork et al., 2002 and Tiemann and
Billings, 2010) with similar trends in conventionally-managed agricultural
soils (Bandick and Dick, 1999 and Piotrowska and Wilczewski, 2012).
Properties of SOM and organic amendments may also infl uence micro-
bial community composition and in turn, microbial activity and associated
ecosystem processes (Fraterrigo et al., 2006 and Reed and Martiny, 2013).
Increases in the fungal:bacterial ratio have been linked to increases in soil
C and the C:N ratio across landscapes (Fierer et al., 2009 and de Vries et
al., 2012) and in response to organic management (Bossio et al., 1998) as
well as various organic amendments, such as conifer-based compost (Ber-
nard et al., 2012) and vetch cover-cropping (Carrera et al., 2007). Other
studies have shown increases in phospholipid fatty acid biomarkers for ar-
buscular mycorrhizal fungi (AMF) in response to composted green waste
as well as long-term organic management (Bossio et al., 1998, Moeskops
et al., 2010 and Moeskops et al., 2012). While management that supports
fungal communities has been suggested as a means of increasing agroeco-
system N retention and other functions (de Vries and Bardgett, 2012 and
Jackson et al., 2012), changes in microbial community composition may
be relatively constrained in agricultural landscapes with a legacy of inten-
