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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.