178
(Wardle et al. 2004). Most short-term decomposition studies indicate that leaf N
increases leaf litter decay (Cornwell et al. 2008). However, as decomposition progresses, leaf N may negatively affect the latter stages of decomposition, possibly due
to interactions with lignified substrates (Berg 2014; discussed in brief below).
Carbon quality (lignin) The second most abundant natural polymer following cellulose is lignin, a complex phenolic polymer that wraps in and out of the structural
polysaccharides in cell walls (Cadisch and Giller 1997). Due to its central roles in
both aboveground biomass and belowground decomposition, lignin has been targeted as an important plant trait for RS techniques (Wessman et al. 1988; Serbin
et al. 2014; Serbin and Townsend, Chap. 3). While lignin is a polyphenolic compound comprised of linked phenols (Horner et al. 1988), it is considered separate
from other polyphenols because lignin is a primary structural component, whereas
other polyphenols are a subset of secondary metabolites not directly involved
with plant growth. The structure role of lignin and its low solubility also merit
distinction from other polyphenolics when considering belowground processes
(Hättenschwiler and Vitousek 2000). Lignin concentrations are negatively correlated with decomposition rates (Meentemeyer 1978; Melillo et al. 1982; Horner
et al. 1988). The recalcitrant nature of lignin is due, in part, to its irregular structure
and low energy yield, which largely limits its degradation to white-rot fungus
members of Basidiomycota (Chapin et al. 2002).
The interaction of N and lignin during decomposition is not straightforward
because N limits the early stages of decomposition, whereas lignin limits the latter
stages of decomposition (Burns et al. 2013). Newly senesced leaves are composed
largely of polysaccharides of holocellulose and lignin. High N availability will stimulate holocellulose decomposition in the early stages of decomposition but will then
retard lignin decomposition in later stages of decomposition leading to lignified soil
organic matter (SOM), potentially due to white-rot fungi favoring low N conditions
(Berg 2014). The degradation of lignin is often a rate-limiting step during the later
stages of decomposition because it protects cell wall polysaccharides physically and
chemically (Talbot et al. 2012). Despite the changing roles that leaf N and lignin
have over the course of decomposition, litter quality metrics such as C:N and lignin:
N can explain variation in decomposition, with decomposition rates increasing with
N in the early stages, but decreasing with N in the later stages, and decreasing with
lignin (Fanin and Bertrand 2016).
While lignin almost universally retards decomposition, there is a large amount of
variation within lignin compounds based on the proportion of specific monomers
that varies across major plant groups (Thevenot et al. 2010). Angiosperm lignin
tends to degrade more quickly than does gymnosperm lignin due to the specific
identities of constituting moieties of lignin in each species (Higuchi 2006). The
compact nature of gymnosperm lignin subunits is thought to protect them from
enzymatic degradation (Hatakka and Hammel 2010). Functional measurements of
lignin are often made via either acid digestion or thioglycolic acid methods that can
then be used to calibrate spectroscopic methods (Brinkmann et al. 2002; Schweiger
et al. 2018).
M. Madritch et al.
(Wardle et al. 2004). Most short-term decomposition studies indicate that leaf N
increases leaf litter decay (Cornwell et al. 2008). However, as decomposition progresses, leaf N may negatively affect the latter stages of decomposition, possibly due
to interactions with lignified substrates (Berg 2014; discussed in brief below).
Carbon quality (lignin) The second most abundant natural polymer following cellulose is lignin, a complex phenolic polymer that wraps in and out of the structural
polysaccharides in cell walls (Cadisch and Giller 1997). Due to its central roles in
both aboveground biomass and belowground decomposition, lignin has been targeted as an important plant trait for RS techniques (Wessman et al. 1988; Serbin
et al. 2014; Serbin and Townsend, Chap. 3). While lignin is a polyphenolic compound comprised of linked phenols (Horner et al. 1988), it is considered separate
from other polyphenols because lignin is a primary structural component, whereas
other polyphenols are a subset of secondary metabolites not directly involved
with plant growth. The structure role of lignin and its low solubility also merit
distinction from other polyphenolics when considering belowground processes
(Hättenschwiler and Vitousek 2000). Lignin concentrations are negatively correlated with decomposition rates (Meentemeyer 1978; Melillo et al. 1982; Horner
et al. 1988). The recalcitrant nature of lignin is due, in part, to its irregular structure
and low energy yield, which largely limits its degradation to white-rot fungus
members of Basidiomycota (Chapin et al. 2002).
The interaction of N and lignin during decomposition is not straightforward
because N limits the early stages of decomposition, whereas lignin limits the latter
stages of decomposition (Burns et al. 2013). Newly senesced leaves are composed
largely of polysaccharides of holocellulose and lignin. High N availability will stimulate holocellulose decomposition in the early stages of decomposition but will then
retard lignin decomposition in later stages of decomposition leading to lignified soil
organic matter (SOM), potentially due to white-rot fungi favoring low N conditions
(Berg 2014). The degradation of lignin is often a rate-limiting step during the later
stages of decomposition because it protects cell wall polysaccharides physically and
chemically (Talbot et al. 2012). Despite the changing roles that leaf N and lignin
have over the course of decomposition, litter quality metrics such as C:N and lignin:
N can explain variation in decomposition, with decomposition rates increasing with
N in the early stages, but decreasing with N in the later stages, and decreasing with
lignin (Fanin and Bertrand 2016).
While lignin almost universally retards decomposition, there is a large amount of
variation within lignin compounds based on the proportion of specific monomers
that varies across major plant groups (Thevenot et al. 2010). Angiosperm lignin
tends to degrade more quickly than does gymnosperm lignin due to the specific
identities of constituting moieties of lignin in each species (Higuchi 2006). The
compact nature of gymnosperm lignin subunits is thought to protect them from
enzymatic degradation (Hatakka and Hammel 2010). Functional measurements of
lignin are often made via either acid digestion or thioglycolic acid methods that can
then be used to calibrate spectroscopic methods (Brinkmann et al. 2002; Schweiger
et al. 2018).
M. Madritch et al.