152
The added step of producing mushrooms from straw compost could theoretically
reduce N 2 O emissions further by increasing N immobilization through mushroom
nutrient uptake, although this has not been established. Studies do suggest that infield emissions of CH 4 can be substantially mitigated by incorporating spent mushroom compost to the field in place of fresh rice straw. One study in the Philippines
estimated CH 4 emissions from mushroom production at only 73 g CH 4 t
−1
of straw
(dry weight) compared to the IPCC default emission factor of 4 kg CH 4 t
−1
for straw
manure compost (Truc 2011). Arai et al. (2015) also found that the total GWP in
straw-mushroom cultivation is 12.5% lower than straw burning.
9.3.2 Biochar
Like compost, biochar can mitigate the CH 4 emissions associated with fresh straw
incorporation by providing an off-field use for straw. The total mitigation potential
of biochar, however, extends beyond compost due to its ability to improve sequestration by converting straw to a more stabilized form of C (Yin et al. 2014). Studies
on C cycling of crop residue suggest that incorporation and composting lose 80–90%
of the initial carbon as CO 2 during decomposition in the first 5–10 years. In contrast,
about 50% of the carbon can be captured as stable SOC when residue is converted
to biochar (Lehmann et al. 2006)
Biochar blended with manure/straw compost has also been shown to substantially reduce N losses during the composting process due to its effect on nutrient
sorption. Like straw, biochar can increase the adsorption of N and prevent NH 3 volatilization and this effect from biochar can be many times greater than that of straw
due its high adsorption capacity or CEC. Studies on compost showed total N losses
could be reduced by 52% with the addition of biochar (Steiner et al. 2010).
When biochar is returned to the field, its effects on total GHGEs; however, are
mixed—possibly due to the variable quality of biochar products and dynamic conditions of soil. A meta-analysis of 61 studies on biochar of various feedstocks showed
that GHGEs in paddy rice were: −5% for CO 2 , −20% for N 2 O, but +19% for CH 4
(P < 0.05) with the addition of biochar (Song et al. 2016). Conversely, another metaanalysis of 42 studies showed that biochar reduced CH 4 in acidic soils (Jeffrey et al.
2016). A CH 4 reduction along with a 50–70% reduction in the total C footprint for
rice production was also reported in a life cycle assessment study comparing openfield straw burning to straw biochar (Mohammadi et al. 2016). A meta-analysis of
29 studies comparing biochar effects among cropping systems showed that biochar
reduced GHGI (yield-scaled emissions) by 41% in upland soils and 17% in paddy
soils (Liu et al. 2019).
J. Allen et al.
The added step of producing mushrooms from straw compost could theoretically
reduce N 2 O emissions further by increasing N immobilization through mushroom
nutrient uptake, although this has not been established. Studies do suggest that infield emissions of CH 4 can be substantially mitigated by incorporating spent mushroom compost to the field in place of fresh rice straw. One study in the Philippines
estimated CH 4 emissions from mushroom production at only 73 g CH 4 t
−1
of straw
(dry weight) compared to the IPCC default emission factor of 4 kg CH 4 t
−1
for straw
manure compost (Truc 2011). Arai et al. (2015) also found that the total GWP in
straw-mushroom cultivation is 12.5% lower than straw burning.
9.3.2 Biochar
Like compost, biochar can mitigate the CH 4 emissions associated with fresh straw
incorporation by providing an off-field use for straw. The total mitigation potential
of biochar, however, extends beyond compost due to its ability to improve sequestration by converting straw to a more stabilized form of C (Yin et al. 2014). Studies
on C cycling of crop residue suggest that incorporation and composting lose 80–90%
of the initial carbon as CO 2 during decomposition in the first 5–10 years. In contrast,
about 50% of the carbon can be captured as stable SOC when residue is converted
to biochar (Lehmann et al. 2006)
Biochar blended with manure/straw compost has also been shown to substantially reduce N losses during the composting process due to its effect on nutrient
sorption. Like straw, biochar can increase the adsorption of N and prevent NH 3 volatilization and this effect from biochar can be many times greater than that of straw
due its high adsorption capacity or CEC. Studies on compost showed total N losses
could be reduced by 52% with the addition of biochar (Steiner et al. 2010).
When biochar is returned to the field, its effects on total GHGEs; however, are
mixed—possibly due to the variable quality of biochar products and dynamic conditions of soil. A meta-analysis of 61 studies on biochar of various feedstocks showed
that GHGEs in paddy rice were: −5% for CO 2 , −20% for N 2 O, but +19% for CH 4
(P < 0.05) with the addition of biochar (Song et al. 2016). Conversely, another metaanalysis of 42 studies showed that biochar reduced CH 4 in acidic soils (Jeffrey et al.
2016). A CH 4 reduction along with a 50–70% reduction in the total C footprint for
rice production was also reported in a life cycle assessment study comparing openfield straw burning to straw biochar (Mohammadi et al. 2016). A meta-analysis of
29 studies comparing biochar effects among cropping systems showed that biochar
reduced GHGI (yield-scaled emissions) by 41% in upland soils and 17% in paddy
soils (Liu et al. 2019).
J. Allen et al.