Rafeza BEGUMMohammad M.R.JAHANGIR∗Mohammad JAHIRUDDINMohammad Rafiqul ISLAMShaikh M.BOKHTIAR and Khandakar R.ISLAM
1Department of Soil Science,Bangladesh Agricultural University,Mymensingh 2202(Bangladesh)
2Soil Resource Development Institute,Dhaka 1215(Bangladesh)
3South Asian Association for Regional Cooperation(SAARC)Agriculture Center,Dhaka 1215(Bangladesh)
4The Ohio State University,Columbus OH 43235(USA)
ABSTRACT Soil total organic carbon(TOC)is a composite indicator of soil quality with implications for crop production and the regulation of soil ecosystem services.Research reports on the dynamics of TOC as a consequence of soil management practices in subtropical climatic conditions,where microbial carbon(C)loss is high,are very limited.The objective of our study was to evaluate the impact of seven years of continuous tillage and residue management on soil TOC dynamics(quantitative and qualitative)with respect to lability and stratification under an annual wheat-mung bean-rice cropping sequence.Composite soil samples were collected at 0–15 and 15–30 cm depths from a three-replicate split-plot experiment with tillage treatment as the main plots and crop residue levels as the sub-plots.The tillage treatments included conventional tillage(CT)and strip tillage(ST).Residue levels were high residue level(HR),30%of the plant height,and low residue level(LR),15%.In addition to TOC,soil samples were analyzed for particulate organic C(POC),permanganate oxidizable C(POXC),basal respiration(BR),specific maintenance respiration rate(qCO2),microbial biomass C(MBC),potentially mineralizable C(PMC),and TOC lability and management indices.The ST treatment significantly increased the TOC and labile C pools at both depths compared with the CT treatment,with the effect being more pronounced in the surface layer.The HR treatment increased TOC and labile C pools compared with the LR treatment.The ST+HR treatment showed significant increases in MBC,metabolic quotient(qR),C pool index(CPI),C lability index(CLI),and C management index(CMI),indicating improved and efficient soil biological activities in such systems compared with the CT treatment.Similarly,the stratification values,a measure of soil quality improvement,for POC and MBC were>2,indicating improved soil quality in the ST+HR treatment compared with the CT treatment.The ST+HR treatment not only significantly increased the contents of TOC pools,but also their stocks.The CMI was correlated with qCO2,BR,and MBC,suggesting that these are sensitive indicators of early changes in TOC.The qCO2 was significantly higher in the CT+LR treatment and negatively correlated with MBC and CMI,indicating a biologically stressed soil condition in this treatment.Our findings highlight that medium-term reduced tillage with HR management has profound consequences on soil TOC quality and dynamics as mediated by alterations in labile C pools.
KeyWords:basal respiration,carbon management index,carbon stratification,particulate organic carbon,permanganate oxidizable carbon,total organic carbon
The content,distribution,and composition of soil total organic carbon(TOC)are important indicators of soil health and crop productivity(Coulteret al.,2009;Hartemink and McSweeney,2014;Liet al.,2018).As TOC is a major driver of key soil ecosystem functions,quantitative and qualitative changes in TOC can have great impacts on the functional components of terrestrial ecosystems(Liet al.,2018).However,TOC composition and lability are extremely complex due to the nature of the diverse organic inputs and their different rates and stages of decomposition,physicochemical reactions,and biophysical stability in soil(Sundermeieret al.,2011;Chenuet al.,2015).As it takes several years to detect consistent changes in bulk TOC,greater emphasis was given to smaller TOC labile pools as early indicators of management-induced changes within a short period of time(Stockmannet al.,2013;Benbiet al.,2015;Kravchenkoet al.,2015;Chenet al.,2016).Medium-to long-term experiments are necessary to assess relative changes in TOC and its labile pools,especially where soil and crop management treatments are involved.
The TOC pools that are considered labile are active carbon(C)or permanganate oxidizable C(POXC),total microbial biomass C(MBC),potentially mineralizable C(PMC),light fraction,hot-water soluble C,anthrone reactive C,and particulate organic C(POC)(Islam and Weil,2000;Ghimireet al.,2012;Liet al.,2012).In addition,C-based quotients and indices,such as metabolic quotient(qR),specific maintenance respiration rate(qCO2),C pool index(CPI),C lability index(CLI),and C management index(CMI)derived from TOC,soil microbial biomass,basal respiration(BR),and POXC have also been suggested as sensitive and early indicators of TOC lability(Blairet al.,1995;Andersen and Sparling,1997;Islam and Weil,2000;Islamet al.,2021).
The conventional farming practices of seasonal plowing,monocropping,and crop residue removal are often associated with TOC loss,causing soil quality degradation(Haddawayet al.,2017).Alternative management practices such as conservation agriculture(CA)that cause minimal disturbance to the soil ecosystem extend ground cover with living and organic mulching.In addition,C and nitrogen(N)inputs from diversified crops through crop rotations can help restore the quantitative and qualitative aspects of TOC to sustain and improve soil quality(Liet al.,2018).However,the effects of CA on TOC accumulation or degradation are still under debate.Previous research argued that the stratification of TOC in soil can balance total TOC sequestration in both conventional agriculture and CA systems(Christopheret al.,2009;Paustianet al.,2016).In addition,labile TOC pools may respond more sensitively to soil management changes than TOC(Yanet al.,2007;Benbiet al.,2015).However,it is not well documented which among the TOC pools are the most suitable and consistent indicators for detecting and assessing early changes in TOC in a rice(Oryza sativa)-based triple cropping sequence.
Our hypothesis was that continuous strip tillage(ST)with increased crop residue retention would increase the concentrations,stocks,and lability of TOC pools under a diverse cropping sequence in subtropical agroecosystems.The objectives of our research were to i)measure the effects of soil tillage and residue retention levels on the sequestration of labile C and TOC pools;ii)evaluate the impacts of soil different tillage and residue retention levels on TOC stocks and lability;and iii)determine the stratification of TOC pools under wheat(Triticum aestivum)-mung bean(Vigna radiata)-rice rotation over time.
Experimental site
An experiment was conducted at the Bangladesh Agricultural University(BAU)Experimental Farm(24◦43.407′N,90◦26.22′E),Mymensingh,Bangladesh.The climate is subtropical monsoon with a mean annual temperature of 26◦C,average annual rainfall of 1 800 mm,and relative humidity between 65%and 96%.The soil is a non-calcareous dark grey floodplain soil(Old Brahmaputra Floodplain)(FAO,1988)or Aeric Haplaquept in United States Department of Agriculture(USDA)Soil Taxonomy,having a silt loam texture.
Experimental design
The experimental field had been managed for many years under a conventionally plowed system.In 2012,the experiment was commenced with two soil disturbance levels,ST and conventional tillage(CT),and two residue retention levels,low(LR)and high(HR).The LR and HR levels were obtained by retention of 15%and 30%,respectively,of the crop height(bottom part)for both rice and wheat,while 100% of mung bean was returned to all plots.In the CT treatments,soils were plowed repeatedly in each season(i.e.,three seasons per year)up to 15 cm depth and crop residues were incorporated into the soils by these repeated ploughings.In contrast,soil was kept undisturbed in the ST treatments,with the exception of a 3-cm furrow for seedling transplanting or seed planting between plant rows.The furrows were made with a versatile multi-crop planter and crop residues were left on the soil surface as standing stubble.A split-plot design was adopted,with three replications for each treatment combination.Tillage treatments were assigned to the main plots and residue retentions to sub-plots.The size of each plot was 7 m×7 m,with 50-cm buffers between plots.
Cultural practices
Land preparation for the wheat-mung bean-rice cropping sequence began with the planting of wheat in the third week of November,followed by the planting of mung bean in the last week of March,and the transplanting of rice seedlings(transplanted Aman rice)in the last week of July.Seeding rates were 120,30,and 35 kg ha−1for wheat(cv.BARI Gom-30),mung bean(cv.BINA mung-8),and rice(cv.BRRI dhan-9),respectively.Wheat grew from the last week of November to mid-March(rabi season),followed by mung bean from early April to late June(pre-monsoon/pre-kharif season),and transplanted Aman rice as a rainfed crop from early July to November(monsoon/kharif season).Nitrogen fertilizer(urea)was applied at a rate of 100,20,and 80 kg N ha−1for wheat,mung bean,and rice,respectively,in an annual sequence for each year.Nitrogen was applied in three equal splits at 0,25,and 50 d after transplanting for wheat,as a single starter for mung bean,and in three splits at 3(50%),30(25%at the tillering stage),and 50 d after transplanting(25% at the panicle initiation stage)for rice.Phosphorus(P)as triple super phosphate,potassium(K)as muriate of potash,sulfur(S)as gypsum,zinc(Zn),and boron(B)were applied at(per hectare)20 kg P,60 kg K,10 kg S,2 kg Zn,and 1.5 kg B for wheat,20 kg P,30 kg K,and 10 kg S for mung bean,and 10 kg P,30 kg K,10 kg S,and 2 kg Zn for rice.
Nonselective herbicide glyphosate(Roundup®)was sprayed over the field at 1.85 kg ha−12–3 d before the transplanting of rice seedlings or the planting of wheat and mung bean.In addition,Pretilachlor(Superhit®,post emergence herbicide)was applied at 450 g ha−15–7 d after transplanting rice seedlings in the ST treatments.Insecticide Brifar 5G was applied 50 d after the planting of wheat,diazinon was sprayed three times for mung bean(36,48,and 59 d after planting),and the insecticides Brifer 5G and Cidial 5G were applied to control insects for rice.Irrigation was provided twice for wheat,once during the crown root initiation and again at the flowering stage.The rice fields were irrigated one day before the final land preparation.
Soil sampling and analysis
After harvesting the 21st crop(rice)of the sequence in November 2018(winter),five soil cores were randomly collected from each plot at the 0–15 and 15–30 cm depths using a 10 cm-diameter auger,composited,and stored in sealable plastic bags.From each field-moist composite soil sample,approximately 400 g was taken and divided into two portions.One portion was sieved through a 2-mm mesh to remove visible organic residues and analyzed for MBC and associated biological properties.The other portion was air-dried under shade at room temperature(ca.25◦C)for two weeks,sieved through a 2-mm mesh,and analyzed for selected physical and chemical properties.Soil MBC(mg kg−1)was determined by the CHCl3fumigation-extraction method(Wuet al.,1990)and calculated using the following equation:
where EOC is the amount of extracted organic C from the CHCl3-fumigated soil minus the organic C extracted from non-fumigated soil(mg kg−1)and kEOC is the extraction efficiency(0.45).
Soil BR was measured by thein vitrosteady static incubation method(Chenget al.,2013).The CO2-C evolved during a 10-d incubation was absorbed with 0.5 mol L−1NaOH and titrated with 0.1 mol L−1HCl to the phenolphthalein endpoint.Specific maintenance respiration(qCO2),as a measure of the labile C used for microbial catabolism,was calculated by dividing BR with MBC(Islam and Weil,2000).The qR was calculated as a ratio of MBC over TOC(Anderson and Domsch,1993).The PMC pool was determined byin vitrostatic incubation of the field-moist soil.Briefly,20 g soil was transferred to a 25-mL glass beaker,adjusted to 70% water-filled porosity,and incubated in a 1-L mason jar along with a glass vial containing 10 mL distilled water to maintain a moist atmosphere and a plastic vial containing 20 mL 0.5 mol L−1NaOH for absorption of CO2.The jars were sealed airtight and incubated in the dark at 25±1◦C for 10 d.The CO2released from the soil and absorbed by the NaOH was titrated with standard 0.5 mol L−1HCl solution to calculate the PMC pool after dividing with TOC.
The TOC was analyzed by the wet oxidation method(Black,1965).The POXC pool,as a measure of AC,was determined upon mild oxidation of air-dried soil with a neutral KMnO4solution(Weilet al.,2003).Particulate organic matter(POM)-associated C(i.e.,POC)was determined using the method described by Cambardella and Elliott(1992).Briefly,POM was separated from soil by adding 240 mL 0.5%sodium hexametaphosphate solution to 80 g air-dried soil,followed by shaking for 18 h and then wet-sieving the soil slurry through a 53-µm mesh under running water(Cambardella and Elliot,1992;Cambardellaet al.,2001).Sand particles and POM remaining on the sieve were then oven-dried at 105◦C for 48 h,weighed,burned in a muffle furnace at 460◦C for 16 h,and weighed again in a manner consistent with the loss on ignition procedure.Soil bulk density was determined by the standard core sampler method(McKenzieet al.,2004).Antecedent soil moisture content was measured by the gravimetric method(Black,1965).
Soil C stocks and stratification
The TOC stratification ratio,which is low and seldom≥2,is considered as an indicator of soil quality improvement due to management practices(Franzluebbers,2002b).Soil TOC and labile C stocks at different depths and within the profile were calculated by multiplying their concentrations with the sampling depth interval and concurrently measured bulk density values.The stratification ratio of TOC pools was calculated by dividing their concentrations at each depth by the concentrations of the respective C pools at the deeper soil depth(15–30 cm)in the CT treatment,as a control(Franzluebbers,2002a).
Soil C labilityand management indices
Using the measured TOC and POXC data,the CMI was calculated as follows(Blairet al.,1995;Islamet al.,2021):
where CPI and CLIwere calculated as follows:
where control soil is the soil in CT+LR and CL refers to the lability of C,calculated as follows:
where labile(or active)C was considered the portion of TOC that was measured as the POXC pool(mg kg−1),and the non-labile C was calculated by subtracting POXC from TOC(mg kg−1).The calculated CMI values were normalized(nCMI,%)by dividing them with the highest CMI value in the database to a relative scale between 0 and 100%,considering higher nCMI values as better indicators of TOC sequestration and lability in response to management practices.
Statistical analysis
Three-way analysis of variance(ANOVA)procedures of SAS(SAS Institute,1995)were used to separate and compare the simple and interactive effects of tillage,residue management,and soil depth on dependent variables.The tillage,residue management,and soil depth were considered fixed variables.For all statistical analyses,significant simple and interactive effects of independent variables on dependent variables were separated by the least significant difference test atP<0.05,unless otherwise mentioned.Due to lack of significant differences,the results on three-way interaction(tillage×residue management×soil depth)are not shown.Moreover,the profile-wise stocks of C were pooled across soil depths.Regression and correlation analyses were performed using SigmaPlot®.
Effects of tillage and residue retention on soil C pools
After seven consecutive years of CA practices,MBC increased by 34%,while qCO2decreased by 23% in ST as compared with CT(Table I).The MBC and associated biological properties(except qCO2)were not significantly different between HR and LR.The qCO2was significantly lower(by 9%)under HR than under LR.Irrespective of tillage and residue retention,MBC and qR decreased with soil depth.The qCO2,in contrast,was significantly higher(by 24%)in sub-surface soil(15–30 cm)when compared with the surface soil(0–15 cm).Tillage×residue retention significantly affected MBC and qR,while both tillage×soil depth and residue×soil depth significantly influenced PMC.
The means of TOC,POC,and POXC differed significantly between tillage treatments and residue levels(Table II).The TOC,POC,and POXC were increased by 15%,22.4%,and 22%,respectively,in ST compared with CT and by 21.1%,20.9%,and 40.9%,respectively,in HR compared with LR.While POM and the POXC/TOC ratio were not significantly influenced by tillage×residue retention,POC,in contrast,was significantly influenced by tillage×residue rentention.Both POC and POXC decreased significantly with soil depth.The tillage×depth interaction significantly influenced TOC and POXC,whereas the residue×depth inertaction significantly influenced TOC,POXC,and POXC/TOC ratio.
TABLE IMicrobial biomass carbon(MBC),metabolic quotient(qR),basal respiration(BR),specific maintenance respiration rate(qCO2),and potentially mineralizable carbon(PMC)at different soil depths of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of three-way analysis of variance(ANOVA)of the simple and interactive effects of tillage,residue retention,and soil depth on these parameters
Effects of tillage and residue retention on soil C stocks
The mean stocks of POC,POXC,and MBC were 23%,21%,and 36%,respectively,significantly higher in ST than in CT(Table III).On the contrary,the stocks of TOC,POM,and PMC were not significantly different between ST and CT.The HR treatment significantly increased the TOC and POXC stocks by 24%and 40%,respectively,compared with the LR treatment.The stocks of TOC,POM,and PMC were not significantly different between soil depths,whereas the POC,POXC,and MBC stocks decreased significantly with soil depth.The interactive effects of tillage×depth and residue×depth were significant on TOC and POXC stocks,while the interactive effect of tillage×residue on MBCstocks were significant.The ST+HR treatment showed the highest POXC and MBC stocks of 1.38 and 0.47 Mg ha−1,respectively.
TABLE IITotal organic carbon(TOC),particulate organic matter(POM),particulate organic carbon(POC),permanganate oxidizable carbon(POXC),and the POXC/TOC ratio at different soil depths of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of three-way analysis of variance(ANOVA)of the simple and interactive effects of tillage,residue retention,and soil depth on these parameters
TABLE IIIStocks of total organic carbon(TOC),particulate organic matter(POM),particulate organic carbon(POC),permanganate oxidizable carbon(POXC),microbial biomass carbon(MBC),and potentially mineralizable carbon(PMC)at different soil depths of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of three-way analysis of variance(ANOVA)of the simple and interactive effects of tillage,residue retention,and soil depth on these parameters
When pooled across soil depths,the profilewise TOC,POC,POXC,and MBC stocks increased significantly in ST and HR(Table IV).The stocks of POC,POXC,and MBC in ST were higher by 23%,20.69%,and 35.71%,respectively,compared with CT.In contrast,the POM and PMC stocks were not significantly different between CT and ST.Likewise,HR had significantly higher stocks of TOC and other labile C pools except POM and PMC.The TOC,POC,POXC,and MBC stocks were 24%,15%,43%,and 20%,respectively,higher in HR than in LR.The effect of tillage×residue was significant for only MBC stock,the highest value of which(0.84 Mg ha−1)was found in ST+HR.
Effects of tillage and residue retention on soil C stratification
The effect of tillage and residue retention on the stratification of TOC pools was significant for POC,POXC,and MBC but not for TOC and POM(Table V).While the effect of tillage on PMC was not significant,the effect of residue was significant.The stratification ratios of POC,POXC,and MBC were significantly higher in the surface soil.The interactive effects of tillage×depth and residue×depth weresignificant for TOC,with the highest TOC in the surface soil of ST+HR.The stratification ratios for all C pools,except POC and MBC,were<2(Table V).
TABLE IVStocks of total organic carbon(TOC),particulate organic matter(POM),particulate organic carbon(POC),permanganate oxidizable carbon(POXC),microbial biomass carbon(MBC),and potentially mineralizable carbon(PMC)in the soil profiles of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of analysis of variance(ANOVA)of the simple and interactive effects of tillage and residue retention on these parameters
TABLE VStratification values of total organic carbon(TOC),particulate organic matter(POM),particulate organic carbon(POC),permanganate oxidizable carbon(POXC),microbial biomass carbon(MBC),and potentially mineralizable carbon(PMC)at different soil depths of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of three-way analysis of variance(ANOVA)of the simple and interactive effects of tillage,residue retention,and soil depth on these parameters
Effects of tillage and residue retention on soil C labilityand management
Tillage had significant impacts on CPI,CMI,and nCMI(Table VI).However,the CL and CLIdid not differ significantly between CT and ST.Soils in ST had higher CPI values(by 14%)than those in CT.Likewise,nCMI was significantly higher in ST than in CT(by 22.3%).Crop residue retention significantly influenced the TOC lability and management indices except CL.Regarding soil depth,surface soils(0–15 cm)had significantly higher CL,CLI,CMI,and nCMI than subsurface soils,but CPI was not significantly different between the two soil layers.In contrast,tillage and residue retention had no significant interactions on the TOC lability and management parameters.The interactive effect of tillage×depth was significant for all TOC lability and management indices except CLI.The interactive effect of residue×depth was significant for all TOC lability and management indices.
Relationships between soil biological properties,C pools and C lability
Positive linear relationships were observed between TOC pools,including POC,POXC,MBC,and BR(Fig.1).The TOC accounted for 24% of the variation in POC(R2=0.24,P<0.05),49%variation in POXC(R2=0.49,P<0.05),34%variation in MBC(R2=0.34,P<0.05),and 32% variation in BR rates(R2=0.32,P<0.05).Likewise,POC showed significant positive relationships with POXC(R2=0.26,P<0.05)and MBC(R2=0.66,P<0.01)(Fig.2),explaining 26%and 66%of their individual variation,respectively.Moreover,MBC showed positive linear relationships with POXC(R2=0.28,P<0.05)and CMI(R2=0.35,P<0.05).While there was a strong negative,non-linear relationship between MBC and qCO2(R2=0.90,P<0.01)(Fig.3),the postive linear relationship between MBC and BR was moderate(R2=0.29,P<0.05).By contrast,CMI had a positive linear relationship with BR(R2=0.16,P<0.05)but a negative linear relationship with qCO2(R2=0.16,P<0.05).
Fig.1 Relationships between soil total organic carbon(TOC)and particulate organic carbon(POC),permanganate oxidizable carbon(POXC),microbial biomass carbon(MBC),and basal respiration(BR)in a seven-year field experiment with tillage and residue retention treatments in Mymensingh,Bangladesh from 2012 to 2018.POM=particulate organic matter.
Fig.2 Relationships between soil particulate organic carbon(POC)and permanganate oxidizable carbon(POXC),POC and microbial biomass carbon(MBC),POXC and MBC,and carbon management index(CMI)and MBC in a seven-year field experiment with tillage and residue retention treatments in Mymensingh,Bangladesh from 2012 to 2018.
Fig.3 Relationships between soil microbial biomass carbon(MBC)and basal respiration(BR)and specific maintenance respiration rate(qCO2)and between carbon management index(CMI)and BR and qCO2 in a seven-year field experiment with tillage and residue retention treatments in Mymensingh,Bangladesh from 2012 to 2018.
Effects of tillage and residue retention on soil organic C pools
The significant differences in the depth distribution of TOC pools between the tillage and residue retention treatments are related to the intensity of physical disturbance,the amount,placement,and mixing of crop residues,and chemical inputs(Moraru and Rusu,2012;Tripathiet al.,2014).In ST,the unfragmented residues(without incorporation into the soils)and less disturbed soils may have shifted the microbial composition from bacteria-to fungi-dominated systems(Khan,1996;Moraru and Rusu,2012).The MBC is a labile pool of TOC,includes microbial cells and their metabolites,and is sensitive to management-induced changes.The ST,as one of CA practices,supported a large MBC pool.This was due to less frequent soil disturbance and greater availability of the labile C pools,which together created suitable conditions for higher anabolism compared with the CT system.Our results are in line with that of Bausenweinet al.(2008),whoreported a greater MBC pool in ST than in CT.Likewise,other studies also suggested that an increase in the MBC pool occurs in the surface soils in ST due to the surface placement of C-enriched residues and the undisturbed and partially anaerobic soils(Douet al.,2008).In ST,surface retention of residues of three different crops in the sequence over the course of a year sequestered more organic C from diverse crops different in quality and quantity and restricted TOC oxidation(Jinet al.,2010;Alamet al.,2018).Conversely,qCO2decreased in ST,suggesting higher microbial efficiency(anabolism)due to the greater availability of labile C and the more energy-efficient fungi-based food web compared with CT.Residue retention provides a steady source of unfragmented C substrates,supporting a more fungi-based population with high C-use efficiency.Conversely,CT causes an intimate soil-fragmented residue mixing,resulting in a temporary flourish of microbial activity and thus an accelerated loss of TOC as CO2emissions(Balotaet al.,2003).It is likely that the comparatively disturbed soils had low availability of labile C,resulting in a bacteria-dominated food web that consumed more C-based energy to survive under CT or low-input LR systems than under ST.
TABLE VICarbon pool index(CPI)and carbon lability and management indicesa)at different soil depths of the tillage and residue retention treatments in a seven-year field experiment in Mymensingh,Bangladesh from 2012 to 2018 and summary of three-way analysis of variance(ANOVA)of the simple and interactive effects of tillage,residue retention,and soil depth on these parameters
The MBC pool was smaller in CT that spent more energy in catabolism and resulted in a higher qCO2to survive under such a highly disturbed ecosystem.Liet al.(2012)and Zhuet al.(2014)argued that CT increases soil aeration and supports greater aerobic bacterial activity,which accelerates crop residue and native TOC decomposition,resulting in a net loss of TOC or lower TOC accumulation.The POC,mainly comprised of incompletely decomposed plant residues and microbial metabolites,is an important source of C and energy for soil microbes(Yanget al.,2009).The higher POC in ST corresponded to the synergistic effect of a larger MBC pool and a higher C-use efficiency in ST compared with CT.The accumulation of POC in soil aggregates helps to protect the POC against microbial decomposition and makes it possible for macroaggregate formation and occlusion of POC therein(Sixet al.,2002;Sleutelet al.,2007).A higher TOC stock in HR could be attributed to a cumulative effect of greater amount of annual diverse residue recycling,efficient mineralization,and greater accumulation and physical protection of C in the soil(Al-Kaisiet al.,2005).
Effects of tillage and residue retention on soil C stocks
Significant variations in profile-wise distribution of TOC stocks may be attributed to the variable TOC concentrations and bulk density values with soil depth in response to management practices.Even though the POC,POXC,and MBC stocks were higher in ST than in CT,together they formed a smaller but distinct pool of TOC,resulting in no significant differences in bulk TOC stocks between the two tillage systems.Several studies have reported that reduced tillage or no tillage increased TOC content in the topsoil(0–15 cm),while other studies observed the opposite(Bakeret al.,2007;Christopheret al.,2009).As the bulk density values of ST and CT were similar,the higher TOC stock(30 Mg ha−1)in the former compared with that in the latter(26 Mg ha−1)was due to difference in TOC concentration.A lack of consistent difference in TOC stock between ST and CT suggests that ST regimes may require a longer time to sequester C under tropical agroecosystems.Our results agree and disagree with other studies to various degrees(Follettet al.,2013;Paustianet al.,2016;Yuet al.,2020).In general,in the subtropical climatic conditions,higher catabolic activities of opportunistic microbes accelerate the loss of labile TOC pools or restrict TOC accumulation(Ontl and Schulte,2012).Increment in TOC stock by crop residue retention was in line with previous studies(Luoet al.,2010;Liet al.,2018).
Davidson and Ackerman(1993)suggested that it is important to account for TOC storage in the top 30 cm of soil because up to 40%of TOC in this layer is lost due to historic plowing activities compared with native and uncultivatedlands.In our study,the TOC,POC,POXC,and PMC storages in the 0–30 cm profile were significantly higher in ST than in CT,suggesting that under ST,soil is a sink for TOC.In addition,residue retention of 30% of the plant height can be equally effective for significant increase of TOC storage in the 0–30 cm layer(Table IV).Périéand Ouimet(2008)suggested that sampling depth often affected the results because of the possible change in TOC pools and variations in TOC movement due to tillage practices.It is worth noting that TOC stocks were not significantly different between CT and ST at a single depth but were significantly different in the 0–30 cm profile,suggesting that to evaluate the overall effects of any management practice,it is necessary to optimize sampling depth at least up to 30 cm.
Effects of tillage and residue retention on soil C stratification
In our study,the stratification values of POC and MBC indicated that management practices involving conversion of CT to ST with HR management improve soil quality over time.These results agree with those of Morenoet al.(2006)and Franzluebbers(2002b)that stratification of TOC occurs over time,and it is usually greater in undisturbed soils than in disturbed soils.However,stratification of TOC pools may increase over time under conservation management practices(de M Sáet al.,2001).As indicated by the CPI values(Table VI),the stratification values of TOC pools would increase with ST and HR management,that is,soil quality would improve over time with ST and HR management.Even though the stratification values of TOC pools were low(ca.1),the values of labile C pools were>1,suggesting that ST and HR management improved soil quality by accumulating TOC in the more labile pools.Among the TOC pools,POC and MBC can be considered as the most sensitive labile C pools for early indication of soil quality changes in response to management practices.
Effects of tillage and residue retention on soil C labilityand management
The TOC pools have different lability and turnover rates,and the labile pool is easily decomposed for microbial metabolism.The labile TOC pool plays a vital role in regulating soil biological activity and soil aggregate formation(Sixet al.,2004).It responds rapidly to changes in soil management practices because of its rapid turnover due to preferential utilization for microbial metabolsim(Franzluebbers and Stuedemann,2002).Therefore,labile C pool can be a sensitive and consistent indicator of early changes in the contents,composition,and quality of TOC(Haynes and Beare,1996).The CPI indicates whether there are any changes in TOC content due to changes in management practices.The higher CPI values in ST and HR suggest that the integration of these practices significantly increased TOC accumulation over time.Likewise,the tillage×depth,as well as residue×depth,had significant effect on TOC content,which suggests that TOC accumulated at both depths in ST+HR.The CL and CLIwere similar across tillage and residue retention treatments,suggesting a temporal accumulation of TOC due to increases in both labile and stable C pools.Higher rates of labile C accumulation were found at surface soils under the combined treatment of tillage and residue retention.
The nCMI is considered an indicator of TOC dynamics and provides an integrated measure for the quantity and quality of TOC(Haynes and Beare,1996).Soils with higher nCMI values are considered to be better managed(Diekowet al.,2005).The tillage×residue interaction had a significant effect on the depth distribution of nCMI,suggesting that sustainable management practices increased TOC content compared with CT.The nCMI values,>50%for the surface soils in ST+HR,indicate increments in TOC content due to CA practices.
Relationships among soil biological properties,C pools,and C lability
The significant positive relationships among the TOC pools,microbial properties,and TOC lability were evidence of the impact of ST and HR management practices.The parameters BR and MBC might be better indicators of management effetcs on TOC sequestration,as they showed strong positive relationships with CMI.All the labile C pools were positively correlated with TOC,and POXC showed a higherR2value than the other pools,suggesting that POXC is a better indicator of early changes in TOC content.The negative relationships of qCO2with CMI and MBC indicate that qCO2increases when the soil is under bio-physical stress or has insufficient labile C to support microbial anabolism because of management practices that deteriorate soil quality(Islam and Weil,2000;Sundermeieret al.,2011).
After seven years of wheat-mung bean-rice rotation,ST coupled with HR management resulted in markedly higher soil TOC,labile C pools,and C lability and management indices than CT with HR or LR.The higher qCO2and lower MBC in CT+LR indicate a stressed soil biological condition,while the lower qCO2 and higher MBC observed in ST+HR suggest an efficient biological soil condition.In ST+HR,soil C accumulated at both 0–15 and 15–30 cm depths.In addition,the increased TOC,POC,POXC,CPI,and nCMI at both depths in ST+HR indicate improvements in soil chemical and biological properties and thereby soil quality.To fully understand the effect of reduced tillage on soil quality parameters,the depth distribution of TOC andlabile C pools(i.e.,C stratification)should be considered when estimating C stock in soils.Our results suggest that ST coupled with HR could be a suitable management strategy to restore or improve soil quality in subtropical rice-based agroecosystems.
This research was financially supported by South Asian Association of Regional Cooperation(SAARC)Agriculture Ph.D.Scholarship 2017(SAC No.611/17/143).The authors are grateful to Soil Resource Development Institute(SRDI),Dhaka,Bangladesh for granting leave to the first author for Ph.D.study.Help and cooperation from the laboratory and field staffs of the Department of Soil Science,Bangladesh Agricultural University(BAU)are gratefully acknowledged.
