Land application of digestate - The Biogas: Science, Production and Applications - page 316

higher than the control plots. The data presented no significant effect
between soil amendments on bacteria numbers in the drainage tile water.
Although the digestate applied to the plots was 2-log lower in bacterial
concentrations than the raw manure, these lower bacterial counts in
digestate did not translate to lower bacterial counts in drainage water from
plots treated with the two manure types. This may be due to the heavy clay
soil type in the plots, which slowed down the migration of bacteria through
the 1m of soil before reaching the drainage tiles, thereby masking the fact
that fewer bacteria were actually applied to the plots receiving digestate.
Bacterial migration in sandy soils may prove to be quite different.
It has been established that the land application of manure and digestate
can impact bacteria concentrations in drainage water. However, further
studies are needed to determine how lower bacteria concentrations in
digestate will impact bacterial transport through soils and, ultimately, the
contamination of drainage waters.

13.4 Conclusion and future trends

The land application of digestate when compared with raw manure or
inorganic fertilizer has shown some differences with respect to environ-
mental and agronomic impacts.

. Increased NH 3 volatilization in soils amended with digestate or raw

manure has been observed. Some studies have reported no observed
difference between soils treated with raw manure and digestate-amended
soils. Ammonia volatilization is highly dependent on climatic condi-
tions, pH and soil infiltration, thereby making data variable. Further
field studies are needed to better demonstrate the long-term effect of
land application of digestate on NH 3 volatilization under various
climatic and soil conditions. Immediate incorporation or injection
should greatly reduce NH 3 volatilization.

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. Nitrous oxide emissions from digestate-treated plots have been reported

to be similar, and sometimes lower, than raw manured plots. It was
suggested the C:N ratio and digestible C may have been too low to
stimulate denitrification. However, N 2 O emission studies using digestate
are limited and further studies are needed to better understand gas
emission rates under various climatic and soil conditions.

. Odour emissions were significantly lower with the application of

digestate rather than manure or inorganic fertilizer, which demonstrates
that anaerobic digestion is an important tool in reducing odours.

. The effects on crop yields were variable, from no effect to significant

increases in crop yield. Further field studies are needed to better