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.