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INTRODUCTION
Proper soil organic carbon (SOC) management is essential to prevent land degradation and mitigate climate change in the world (Lal, 2004; Minasny et al., 2017). Accurate evaluation of carbon dioxide (CO2) flux is vital to develop effective SOC management strategies (Lehmann & Kleber, 2015). Changes in annual CO2 flux could substantially alter the pool size of SOC (Moinet et al., 2016). Soils in dry tropical areas retain low SOC, and soil fertility is correspondingly low (Powlson et al., 2016) because of the small amount of fresh litter return to the soil and fast decomposition of litter and SOC under tropical climate conditions. Therefore, it is critically important to estimate annual CO2 flux to conduct sustainable SOC management in degraded soils of tropical agroecosystems.
Biochar, made by biomass pyrolysis with low/no oxygen, has become globally popular to increase soil C stocks because of its high resistance to microbial decomposition (Lehmann et al., 2011; Al-Wabel et al., 2018). Recent research found that biochar application increased soil C decomposition by increasing soil water holding capacity (Jeffery et al., 2011) and/or soil microbial biomass C (MBC) (Thies & Rillig, 2012), while other studies found that it decreased soil C decomposition because of reduced soil microbial activity (Li et al., 2018), and/or the sorption of SOM to biochar (Zimmerman et al., 2011). To assess accurate CO2 fluxes following biochar application, the controlling factors need to be evaluated, i.e., environmental factors containing soil moisture and temperature (Kim et al., 2015) and microbial factors such as MBC and metabolic quotient (Schmidt et al., 2011). Many studies have been conducted on the impact of biochar addition on soil respiration (Senbayram et al., 2019), soil C sequestration (El-Naggar et al., 2018) , and associated microbial responses (Gul et al., 2015), though these studies have mainly been conducted under controlled conditions. While these studies are important, they do not integrate all the biotic and abiotic factors impacting in situ CO2 fluxes, such as moisture and temperature fluctuations. Zhou et al. (2017) reviewed the literature from 2001 to 2015 focusing on soil respiration and/or MBC with biochar addition to croplands, and they found that 26 studies investigated both soil respiration and MBC, nine of which were conducted in the field. Moreover, most studies of biochar addition were conducted in acidic soils because biochar addition can ameliorate soil acidity (Hernandez-Soriano et al., 2016). Therefore, there is limited research on the impact of biochar application on in situCO2 flux and associated microbial responses in tropical alkaline soils, although they are globally distributed and are subject to the critical problem of land degradation such as low SOC accumulation (Tavakkoli et al., 2015).
Tropical alkaline soils in India are mostly degraded and characterized by low soil C stock due to the long-term use of excessive cultivation and removal of crop residue, especially in croplands (Lal, 2004b). Srinivasarao et al. (2009) investigated soil C stocks at 21 locations under different land uses in India and found low soil C contents (<5 g kg−1), which was less than the threshold level of SOC for crop production in the tropics (1.1 %) (Aune & Lal, 1997). Traditionally, most Indian farmers make farmyard manure (FYM) from livestock excreta and soil, which is applied to the soil to maintain soil C level and soil fertility (Srinivasarao et al., 2014). However, a decline in the availability of FYM because of its utility for other domestic purposes such as fuel, and replacement of manure with chemical fertilizers, have reduced SOC stocks over decades (Indoria et al., 2018). Therefore, alternative C management strategies such as biochar could enhance soil C stocks. Hamer et al (2004) revealed that combined biochar and organic substrate application stimulated biochar decomposition, resulting from increased MBC, in a 26-day incubation experiment in Germany. In contrast, Zavalloni et al. (2011) found that fresh OM decomposition was decreased with combined biochar and plant residue application because of physical protection by biochar, i.e., substrate sorption to the biochar surface and pores, in an 84-day incubation experiment using Cambisols. These contradictory results make it difficult to evaluate whether the combined application of biochar and FYM increase or decrease soil respiration and/or SOC stock in tropical alkaline soils, especially under field conditions.
The objectives of this study were to evaluate the impact of land management (biochar and manure application) on in situCO2 fluxes, associated microbial responses (i.e., MBC and qCO2), and C budget in tropical alkaline degraded cropland soils of southern India. We hypothesized that biochar and FYM combined application would stimulate microbial growth and activity, causing increased OC decomposition and high CO2 flux in tropical alkaline cropland soil (Awad et al., 2013). To verify this hypothesis, we conducted a 27-month field experiment with three cropping periods and evaluated the CO2 efflux rate with environmental factors, MBC, qCO2, and SOC stock under different land management.