4. DISCUSSION
4.1 CO 2 flux and its controlling factors in
degraded cropland soils of southern India
The average CO2 efflux rate in the C treatment was 15.2
mg CO2-C m−2 h−1,
and this value was in line with our previous study conducted in the same
field (20.5 mg CO2-C m−2h−1; Seki et al., 2019). These values were relatively
small when compared with those in the other studies in similar tropical
ecosystems, such as 46.0 mg CO2-C m−2h−1 in cropland of Tanzania with 13.8 g C
kg−1 of soil (Sugihara et al., 2012), and 63.1 mg
CO2-C m−2 h−1 in
bare land of Brazil with 12.2 g C kg−1 of soil (La
Scala et al., 2000). The low CO2 efflux rate in this
study might be explained by the low C content of the degraded cropland
soil in our study site (SOC; 3.2 g kg−1), compared
with those in the above studies that varied from 12.2 to 13.8 g C
kg−1 of soil.
In agreement with previous studies in dry tropical areas (Kim et al.,
2015), the CO2 efflux rate was positively correlated
with soil moisture. Therefore, the low annual CO2 flux
in the first cultivation period was likely because of the low rainfall
during this cultivation period of the first year.
4.2 Impact of land management on CO 2 flux,
C budget, and associated microbial responses
Biochar application did not affect CO2 flux and
microbial dynamics, although it increased the soil moisture throughout
the experimental period. Increased soil moisture with biochar
application indicates that biochar application improved the soil water
holding capacity because of its high porosity (Jeffery et al., 2011),
and this is consistent with other studies with similar soil texture (Liu
et al., 2016) and/or similar biochar application amount (Karhu et al.,
2011). Previous research showed higher SOC or biochar decomposition with
biochar application, caused by (1) improved soil water holding capacity
(Jeffery et al., 2011), (2) degradation of the easily decomposable
fraction in biochar (Keith et al., 2011), and (3) increased MBC (Lehmann
et al., 2011). In our study, like the CO2 flux, MBC did
not increase with biochar application. This is possibly because (1) soil
microbes could not promptly respond to increased soil moisture because
of the small amount of decomposable substrate in SOC poor soil of
southern India (Sugihara et al., 2014), or (2) the increase in soil
moisture was not enough to stimulate the microbial growth and/or
activity. The biochar application significantly increased surface SOC
stock, creating a positive C budget (Fig. 3 and Table S3), in agreement
with many other studies which have reported C sequestration by biochar
addition (Agegnehu et al., 2015; El-Naggar et al., 2018). These results
show that biochar application would be a sustainable and effective
option to prevent or recover the soil degradation by increasing SOC
stock in this area.
FYM application significantly increased the CO2 efflux
rate (Table 3), resulting in 1.6 Mg C ha−1 27
month−1 larger CO2 flux in the M
treatment than in the C treatment. Many studies have reported that
manure application clearly increased soil respiration because of easily
decomposable C addition (Lai et al., 2017). Larger CO2flux with FYM application was associated with increased microbial
responses, i.e., both increased MBC and qCO2, in all
cultivation periods (Table 3) (Lian et al., 2016). Additionally, FYM
application significantly increased the surface SOC stock by 2.0 Mg C
ha−1 over the experiment (Fig. 3 and Table S3). These
results suggest that 1.1 Mg C ha−1 FYM application
every year would maintain and improve the SOC storage in this area. This
is in agreement with our previous study (Seki et al., 2019) and other
studies that estimated the necessary amount of C addition for sustaining
SOC levels based on the fluctuations of soil C stock in India (Kundu et
al., 2001; Datta et al., 2018).
In the current study, combined application of biochar and FYM did not
stimulate MBC and qCO2, resulting in no clear difference
in CO2 flux between the M and BM treatments throughout
most of the experimental period, in contrast to our hypothesis. Only for
the first few months after both products’ applications were the
CO2 efflux rate lower in the BM treatment than in the M
treatment, resulting in 0.3 Mg C ha−1 smaller
cumulative CO2 flux in the BM treatment over the 27
months. Zavalloni et al. (2011) also observed an inhibitory effect of
biochar and plant residue application on residue decomposition. The
difference in this period might have been caused by ca. 30% lower
qCO2 in the BM treatment than in the M treatment,
although MBC did not change. Lehmann et al. (2011) speculated that the
possible mechanism of low OM decomposition observed with biochar
addition was because of changes in the enzyme activity and/or microbial
community composition, while the physical protection provided by biochar
could also be involved (Zimmerman et al., 2011; Hernandez-Soriano et
al., 2016). Based on our calculation of the possible amount of absorbed
DOC derived from applied FYM to biochar, in another equilibration
experiment, ca. 1500 mg C kg−1 FYM could be absorbed
on biochar, which was equivalent to only ca. 20 kg C
ha−1 in this study (data not shown). This implies that
sorption of FYM-derived DOC to biochar can only account for a limited
part of the difference between the M and BM treatments in this study
(Mukherjee & Zimmerman, 2013). Therefore, another factor might also
contribute to the inhibitory effect of biochar and FYM application on
microbial activity. Further studies are required to elucidate the
mechanism involved in the effect of the combined application on
decreased microbial activity just after combined application, to develop
effective C management in this area.
Finally, we found that the combined application of biochar and FYM
increased SOC stock after 27 months, resulting in the largest SOC
increment in the BM treatment (8.9 Mg C ha−1; Table
S3). The rate of C-input retention in soil (SOC increment per C input as
biochar and/or FYM (Kan et al., 2020)) in the BM treatment (ca. 0.78)
was relatively higher than that in the B (ca. 0.74) and M (ca. 0.63)
treatments, indicating that combined application of biochar and FYM
would be more efficient to sequester C than an individual application of
either amendment to soils (Jien et al., 2015). Hence, our results
suggest that combined application of biochar and FYM would be an
effective way to achieve sustainable SOC management for preventing land
degradation both in terms of C output and C sequestration, in the
tropical degraded cropland soils of southern India.