Research on Soil Organic Carbon and Nitrogen Decomposition in Fecal Manure of Cattle Fed Browse/Maize Silages

Soil organic carbon (C) and nitrogen (N) decomposition in fecal manure of cattle fed browses of
Calliandra (Calliandra calothyrsus), Gliricidia (Gliricidia sepium) and Leucaena (Leucaena
leucocephala) browse/maize silage mixtures and maize (Zea mays) silage alone when applied to the
soil were investigated in a pot experiment in comparison to the corresponding silages fed. Maize
silage alone had the lowest N and a larger C: N ratio, making it a poor quality compost when applied
to the soil, but compared to the browse/maize silage mixtures it had the highest level of soluble N as
non-protein nitrogen (NPN) which makes much of its N available for soil microbial decomposition of its
organic C. Calliandra browse/maize silage mixture had the highest level of fiber-bound N (ADFN),
which reduces N availability for soil microbial decomposition of its organic C in spite of its high N
content and a narrower C: N ratio. Fecal manure from maize silage alone had a lower level of N and a
wider C: N ratio than fecal manure from the other silages fed which would affect its decomposition in
the soil, but it had the lowest level of ADFN and much of its N is made available for soil microbial
decomposition of its organic C. Soil samples after 12 weeks of the experiment showed that Calliandra
browse/maize silage mixture maintained the highest level of C in the soil, while maize silage alone
maintained the lowest level. Also soils treated with fecal manure from the other browse/maize silage
mixtures maintained higher levels of C than fecal manure from maize silage alone. Organic C levels
were lowest at 8 weeks of the experiment for all treatments and rose to the original levels at 12 weeks
which could have been as a result of biotic and hydrologic factors coupled with soil aggregation.
Decomposition of organic N followed a similar trend as organic C. The two elements are linked in both
plant inputs in the soil and in the eventual soil humic substances. The soils treated with browse/maize
silage mixtures maintained C: N ratios that were similar to that of the control soil and higher than
those of the fecal manure treatments. Thus, in spite of the added silage materials to the soil, rapid
decomposition of organic C could not occur to reflect benefits of adding the silage materials to the
soil. Thus, fecal manure, particularly from feeding animals on browse/forage diets is more beneficial in
the soil as it would decompose more readily releasing the plant nutrients they contain.