The greatest extent of biodegradation among the fiber composite
materials tested was the composted cow manure (35%). This
was unexpected since low carbon conversion rates were anticipated
for the composted cow manure since it had previously been
biologically degraded. After undergoing a composting cycle, much
of the carbon contained in the cow manure was expected to be
stable and humified [26,40]. However, much less extents of
degradation were observed for uncomposted composites produced
from rice hulls, from peat fiber pot and coconut coir. For these
materials, the extent of degradation in soil ranged from 14 to 21%
(Fig. 3). These materials have been used as natural composites due
to their low price and structural strength [41,42]. Approximately
46% of coconut coir is lignin [43] as is 21e40% of rice hulls [44]
which may have limited their biodegradation.