In recent years, clim ate change im pacts and G H G em ission s from different form s
o f land use have em erged as key factors shaping agricultural p o licies w orld w ide.
W hereas agriculture is one o f the largest sources o f anthropogenic GHG
em ission s, it also provides several p ossib ilities for GHG m itigation (IPCC 2 001).
S p ecifically, the con version o f surplus agricultural land to bioenergy crops
provides great potential for C O2 m itigation across Europe (Sm ith et al. 2 000). B iom ass energy is clo se to ‘carbon neutral’, that is to say, it produces energy
w h ile only releasin g C to the atm osphere that has been captured during the
grow in g cy c le o f the plant, rather than em itting C that has been locked aw ay from
the atm osphere in fossil reserves for m illion s o f years. B ioenergy crops can take
m any form s and can be converted to a number o f different products. M any crop
sp ecies are m ultipurpose in that they can be used to produce m ore than one type
o f energy product (S im s et a l., 2 006).
In recent years, M iscan th u s {M iscan th us x gig a n teu s G reef and D eu .) has
received m uch attention as a potential bioenergy crop (S tyles e t al., 2 0 0 7 ). This
perennial rhizom atous grass, w hich is native to East A sian tropical and
subtropical regions, has a considerable biom ass production potential even under
tem perate clim atic con d itions (L ew an dow sk i e t al., 2 000). C lifton -B row n e t al.
(2 0 0 4 ) estim ated peak annual y ie ld s across Europe ranging from 13 t ha'' in
Finland and S w ed en to 2 5 .8 t ha'' in B elgiu m . By both d isplacin g C released
electricity production cou ld m itigate 9% o f the total European C em issions in 1990 (C lifton -B row n e t a l., 2004).
Previous research on M iscan thu s primarily focu sed on m anagem ent and
eco n o m ics in relation to establishm ent and productivity, harvest and storage, and com bustion feasib ility (Jorgensen e t a l , 2000). Even though soil C represents the
largest C p ool in m ost agro-ecosystem s (IPCC, 2 0 0 1 ), research on SOC dynam ics under M iscan th u s plantation remains scarce (H ansen e t a l., 2004). In particular,
little is know n about C dynam ics b elow the top soil layer. A sse ssin g the effect o f agricultural practices on C stocks in top soil alone m ay lead to erroneous co n clu sion s regarding so il C storage potential (Baker et al., 2007). T o determine
the C O2 m itigation potential o f M iscanthus, w e therefore need to m easure its effect on soil C dynam ics through the w hole soil profile.
Several features o f M iscan thu s ’ p h ysiology and the agricultural practices
associated w ith its cultivation su ggest a large potential for soil C sequestration. Firstly, perennial plants such as M iscan thu s allocate high proportions o f the
assim ilated C below ground as a C reservoir for growth in spring (K uzyakov & D om anski, 2 0 0 0 ). S econ d ly, w hen M iscan thu s is planted on former arable land,
the ab sen ce o f soil tillage results in less aeration, low er d ecom p osition rates and increased soil C stabilization (B eu ch , 1999; C lifton-B row n e t ah, 2 007). Thirdly,
as M iscan th u s system s are typically harvested after aboveground biom ass has
sen esced , stem s and leaves accum ulate on the soil surface as pre-harvest lo sses,
cau sin g high soil C input rates o f aboveground biom ass (B eu ch , 1999). Finally, M iscan th u s system s typically receive little or no N fertilizer, leading to increased
soil C : N ratios and a slower decomposition o f plant residue (Schneckenberger & Kuzyakov, 2007).
To accurately determine the potential for soil C sequestration following land use changes, we need to know the fate o f new soil C input. Most organic matter enters the soil as readily recognizable plant litter and is mineralized within months (Christensen, 2001). A small portion, however, may be stabilized through interactions with mineral surfaces for periods up to thousands o f years (Six et aL, 2004; Lehmann et a l , 2007). In many soils, such as Mollisols and Alfisols, strong feedbacks exist between SOM stabilization and aggregate turnover (Jastrow & Miller, 1998; Six et al., 2004). In these soils, the deposition and transformation of SOM is a dominant aggregate stabilizing mechanism. Soil aggregate structure is usually hierarchical (Tisdall & Oades, 1982; Oades & Waters, 1991) with primary particles and silt-sized aggregates (<50 [xm diameter) bound together to form microaggregates (50-250 |xm diameter). These primary and secondary structures, in turn, are bound into macroaggregates (>250 |im diameter).
Current evidence suggests that microaggregates are formed inside macroaggregates, and that factors increasing macroaggregate turnover decrease the formation and stabilization o f microaggregates (Angers et a l , 1997; Gale et a l , 2000; Six et a l , 2000, 2004). However, microaggregates, and smaller aggregated units, are generally more stable and less susceptible to disturbance than macroaggregates (Tisdall & Oades, 1982; Dexter, 1988; McCarthy et a l, 2008). Soil C storage following land use change has previously been attributed to changing C contents o f microaggregates within macroaggregates (Six et a l.
2 0 0 4 ). For these reasons, soil physical fractionation forms a u seful tool to evaluate changes in soil C and SOM dynam ics.
Isotopic lab elling o f C allow s the tracing o f n ew ly sequestered C into SOM p ools (B alesdent e t al., 1987). U sin g a sim p le isotopic dilution m odel, differen ces in ’^C-signature betw een C3 and C4 plants have been used to trace
n ew ly sequestered C (e.g. Jastrow e t a l., 1996; C ollin s e t a l., 1999). This is particularly the case w hen C4 plantation are grow n on so ils w hich have retained a
predom inantly C3 sign al, associated w ith a C3 vegetation history. The
com bination o f SO M fractionation techn iqu es with '^C natural abundance analyses offers an elegan t approach to in vestigate sm all shifts on soil C stores that w ould be sign ifican t in the lon g term, but that m ight not be detected by conventional m eth od ologies in the short term (D el Galdo e t a l , 2 003).
In this study, w e report on soil C sequestration beneath a 14 years old Irish
M iscan th u s plantation established on form er arable land. M easurem ents o f the '^C/'^C ratios in the so il dow n to 60 cm depths in M iscan th u s plots and in nearby references plots with a C3 plant history allow ed us to estim ate the fraction o f SOC
derived from M iscan th u s as w ell as ch an ges in overall SOC storage.