Applied Soil Ecology 85 (2014) 86–93

Biosolids amendment dramatically increases sequestration of cropresidue-carbon in agricultural soils in western Illinois

Guanglong Tian a,b,*, Chih-Yu Chiu c, Alan J. Franzluebbers d, Olawale O. Oladeji a,Thomas C. Granato a, Albert E. Cox a

a Environmental Monitoring and Research Division, Monitoring and Research Department, Metropolitan Water Reclamation District of Greater Chicago(MWRD), Lue-Hing R&D Complex, 6001 W. Pershing Road, Cicero, IL 60804, USAbDepartment of Civil, Architectural and Environmental Engineering, Illinois Institute of Technology, 3201 South Dearborn Street, Chicago, IL 60616, USAcBiodiversity Research Center, Academia Sinica, Taipei 11529, TaiwandUSDA-Agricultural Research Service, 3218 Williams Hall, NCSU Campus Box 7619, Raleigh, NC 27695, USA

A R T I C L E I N F O

Article history:Received in revised form 29 August 2014Accepted 1 September 2014Available online xxx

Keywords:Carbon sinkMicrobial metabolic quotientMicrobial stressMidwest

A B S T R A C T

In agricultural soils, a large portion of C in crop residues (i.e., non-harvested plant parts left in the field) isannually lost to atmosphere due to the low C use metabolism of soil microorganisms adapting to theenvironmental stress (moisture stress and substrate C and N imbalance). In this study, we tested thehypothesis that amending soil with biosolids (treated sewage sludge with high stable organic matter andlow C:N ratio) can improve the C metabolism of microorganisms in agricultural soils through alleviationof microbial stress, leading to increased sequestration of crop residue-C in agricultural soils. Biosolidswere applied at a mean annual rate of 4.2 kg m�2 (dry weight) to eight agricultural fields (biosolids-amended) for 13 years (1972–1984) in western Illinois. Four agricultural fields (unamended) receivedchemical fertilizer as control. We measured the sequestration rate of crop residue-C in the soils over thespan of 34 years (1972–2006) using a 13C technique. We found dramatically greater sequestration rate ofcrop residue-C in biosolids-amended soil (32.5 �1.7% of total crop residue-C) versus unamended soil(11.8 � 1.6%). Soil microbial metabolic quotient was significantly lower in biosolids-amended than inunamended fields, indicating that biosolid-amendment reduced soil microbial stress and improvedmicrobial C metabolism. The study concludes use of a soil amendment with high stable C and low C:N is avalid approach to transform agricultural soils from current C-neutral status to a C sink. Biosolidsrepresent a good choice of such soil amendments.

ã 2014 Elsevier B.V. All rights reserved.

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1. Introduction

Of the 2 Pg of crop residue-C generated annually worldwide inagricultural soils (Lal 2005), only 10–20% (Kimble et al., 2002) issequestered in soil as organic matter. This low sequestration ratenot only lessens the replenishment of lost soil organic carbon (SOC)but also limits the capacity of agricultural soils for offsetting globalCO2 emission. Sequestration of crop residue-C in soil requiresmicrobial decomposition and stabilization of transformed organicmatter in soil. When crop residues are left in soil, about 70% C isconsumed by microorganisms annually (Buyanovsky and Wagner,1987), and that C is then partitioned as maintenance respirationCO2-C, microbial biomass-C (cell growth), and metabolite-C(Collins et al., 1997). Although association of undecomposed cropresidues and microbial products with clay minerals and soil

* Corresponding author. Tel.: +1 708 588 4054; fax: +1 708 780 6706.E-mail address: guanglong.tian@mwrd.org (G. Tian).

http://dx.doi.org/10.1016/j.apsoil.2014.09.0010929-1393/ã 2014 Elsevier B.V. All rights reserved.

particles influences transformation of crop residue-C into SOC(Paustian et al., 2000; Six et al., 2002), microbial growth efficiencyin utilizing the C source affects C sequestration in soils (Schimel,2013; Wieder et al., 2013). Manipulation of the soil microbialcommunity in agricultural soils that alters the respiration/growthbalance in favor of microbial growth could increase the sequestra-tion rate of crop residue-C in agricultural soils. Yet there are noreports of such a manipulation to improve sequestration of cropresidue-C in agricultural soils.

One possible reason for lower sequestration of crop residue-C inagricultural soils than in native soils (Buyanovsky et al., 1987) isdeterioration of the agricultural soil environment for micro-organisms. Conversion of native to agricultural systems leads toloss of dense vegetation that normally protects the soil fromintense fluctuations in moisture (Tian et al.,1997), and reduction inthe input of substrates with balanced C:N ratio to soil micro-organisms. To tackle moisture stress in agricultural soils, micro-organisms allocate more resources to produce osmolytes, whichreduce their internal water potential to avoid dehydrating (Harris,

G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93 87

1981; Schimel et al., 2007). When soil is rewetted, microorganismshave to dispose osmolytes to avoid cell burst (Kieft et al., 1987).Production and disposal of protective molecules against soilmoistures stress in agricultural soils causes more crop residue-C tobe partitioned to maintenance respiration CO2-C (Buyanovskyet al.,1987). Because N is transferred to grain, most of crop residues(e.g., wheat straw, corn stover) are depleted in N, and often have avery high C:N ratio (e.g., 50). The C:N ratio in soil microbial biomassis low (8.6 � 0.3) (Cleveland and Liptzin, 2007) and does notchange dramatically with type of substrates (Kallenbach andGrandy, 2011). Even though additional C from the substrate isneeded for the maintenance respiration CO2-C as microorganismsconsume the substrate, the optimum substrate C:N ratio formicroorganisms involved in residue decomposition is still about 20(Mooshammer et al., 2014). In utilizing crop residues of high C:Nratios in agricultural soils, microorganisms must “throw out” someC to the atmosphere as extra respiration to maintain low and stableC:N ratio in their biomass. This can reduce the C utilizationefficiency in agricultural soils.

In acclimating to environmentally stressed conditions inagricultural soils, microorganisms tend to physiologically changefrom growth to survival with relatively high respiration and lowgrowth, though there may be a response from communitycomposition (Schimel et al., 2007). Such microbial acclimationcan generally cause more crop residue-C in agricultural soils to bepartitioned to CO2and less to microbial biomass. Thus, low microbialC metabolism(low biomass and high respiration) in agricultural soilscould be the predominant reason for the low soil CO2 sequestration.We hypothesize a soil amendment, which can effectively increasesoil water holding capacity (WHC) and decrease substrate C:N ratio,can improve microbial C metabolism in agricultural soils, leading toincreased sequestration of crop residue-C.

Organic amendments (e.g., animal manure, green manure,farmyard manure, compost) used in the past (Kallenbach andGrandy, 2011) might have contributed in relieving agriculturalsoil’s microbial moisture stress through improving soil waterholding and/or decreasing the substrate C:N ratio. However, mostof the soil amendments tested before could have one or anotherlimitation in relieving microbial stress in agricultural soils. Organicamendments with low C:N (e.g., green manure) can disappear fastin soils because low C:N is often associated with high proportion ofeasily decomposable C. Organic amendments like compost havesome stable C, but still do not have a very long residence time insoil for a persistent effect. Mean residence time of biosolids in soilis estimated at 20 years (Tian et al., 2009) due to the highproportion of stable C. Organic matter in biosolids is mainlyderived from microbially dominated-activated sludge, and thiscauses low C:N ratio of biosolids. Simmons (2003) reported thatbiosolids have twice as high WHC as soils.

Our objectives were therefore to determine the rate ofsequestration of crop residue-C in soil after biosolids amendment,and look into the correlation between crop residue-C sequestrationand alleviation of environmental stress to soil microorganisms inagricultural soils. We used a 13C technique, which can separate theC sources, to determine the crop residue-C in soil. We extended themicrobial metabolic quotient concept to assess the alleviation ofmicrobial stress and restoration of microbial physiology inagricultural soils by biosolids amendment.

2. Materials and methods

2.1. Site description and treatments

The study was conducted at Fulton County in western Illinois, atypical temperate zone climate with annual mean air temperatureof 10.4 �C and annual precipitation of 1013 mm. We conducted the

study in 12 agricultural fields in one watershed: four fields asunamended group and eight fields as biosolids-amended group.The soil for the study was predominantly Alfisol (Clarksdale soilseries: Fine smectitic, mesic Udollic Endoaqualf) with a smallproportion as Entisol (Orthents soil series: Coarse-silty, mixed,superactive, nonacid, mesic Aquic Udifluvent) and Mollisol (Ipavasoil series: Fine, smectitic, mesic Aquic Argiudoll) (USDA-NRCS,1997). Field size averaged 17 ha for unamended group and 19 ha forbiosolids-amended group. The initial soils of the two field groupshad the same mean soil pH (6.8). Soil texture in both field groupswas silt loam (USDA-NRCS, 1997). Prior to the study, meanconcentration of SOC was nearly the same in the two field groups:1.06% for the unamended field group and 1.10% for thebiosolids-amended field group. In principle, two groups of fieldswere managed in the same manner over the study period. Discingas tillage was used for seedbed preparation in all fields. Fields werecropped in rotation with corn (C4 crop), wheat and soybean(C3 crops), and occasionally rye (C3) as a cover crop. Crop residueswere retained in the field. Although fields did not have the samecrops every year, all fields were predominantly cropped in annualcorn/soybean rotation pattern over the study period. Rye wasplanted in a few years when application of biosolids was not earlyenough to allow planting of corn/soybean. Rye biomass wasincorporated into soil before cropping in subsequent years.

From 1972 to 1984, biosolids-amended fields receivedlagoon-aged liquid biosolids with mean annual rate of 4.2 kg m�2

(dry weight), while unamended fields were treated conventionallywith chemical fertilizer at agronomic rate: 300 for N, 100 for P, and100 for K in kg ha�1 yr�1. The majority of liquid biosolidsapplication (1975–1984) was performed through directing bio-solids slurry to between the disc blades by a liquid fertilizermanifold and surface-incorporated by discing. A small portion ofliquid biosolids application (1972–1974) was done by sprayingbiosolids slurry to the fields by a traveling sprinkler and withoutincorporation. From 1985–2005, both biosolids-amended andunamended fields were treated conventionally with chemicalfertilizer at agronomic rate: 300 for N, 100 for P, and 100 for K inkg ha�1 yr�1, and no organic matter applied.

Biosolids were produced in Chicago by anaerobically digestingwastewater treatment sludge for at least 15 days at 35 �C to meetminimum criteria as biosolids. Anaerobically digested sludge wasshipped to the site and stored in a holding basin from several monthsto years before being applied to fields. On a dry matter basis, volatilesolids (approximate organic matter), total N and C:N ratio in liquidbiosolids averaged 44%, 4.9%, and 5.2, respectively (Tian et al., 2009).The analysis of heavy metals in biosolids used in the fields can befound in Tian et al. (2006). Lagooning at the study site contributed tostabilization of organic matter in biosolids, which mainly occurredduring the 0.5 year of storage (Lukicheva et al., 2012).

2.2. Soil sampling and analysis

Soil samples were collected at two depths (0–15 and 15–30 cm)in 2006, and one depth (0–15 cm) in 1972. In sampling, each fieldwas divided into two halves, and about 20–40 cores (depending onfield size) were taken to make one composite sample in each half.Sampling cores were distributed randomly to the entire samplingarea. Soil bulk density was measured in 2005 using a 7.6 cm (diam.)stainless steel ring.

Soil samples were air-dried and then ground to pass a 2-mmsieve. Soil samples for organic C determination were furtherground to <0.063 mm size. Soil pH was measured in a 1:1 water:soil mixture (Thomas, 1996). Concentration of SOC was measuredusing dry combustion (Nelson and Sommers, 1996).

The 13C isotope was determined by a PDZ Europa ANCA-GSLelemental analyzer interfaced with a PDZ Europa 20–20 isotope-

Table 1Crop harvest index, root:shoot ratio and carbon concentration from literature andused in the calculationa.

Crop Corn Soybean Wheat

Harvest indexb 0.47 0.31 0.39Root:shoot ratio 0.85 0.82 0.88C concentration of shoot (%) 40.9 40.5 37.0C concentration of root (%) 26.1 26.2 29.6

a From Buyanovsky and Wagner (1986), except for Crookston et al. (1991) for cornand soybean harvest index and Hucl and Baker (1987) for wheat harvest index.

b Harvest index: fraction of grain yield as total above ground biomass.

88 G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93

ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Inorganic Cwas removed by acidification of soil samples with 12 M HCl beforeisotope analysis. The d13C was calculated as follows:

d13C ðmÞ ¼ ð Rsample

RStandard� 1Þ1000

where Rsample and Rstandard are the 13C/12C ratio for sample andstandard, respectively. The standard was the carbonate ofBelemnitella americana fossil skeleton (Peedee formation, SouthCarolina) with 13C/12C ratio = 0.0112372.

Microbial metabolic quotient (qCO2) in soil was defined as theratio of microbial maintenance respiration rate to microbialbiomass C (Anderson and Domsch, 1990). We measured soilmicrobial biomass C (SMBC) and maintenance respiration insamples of 2006, 22 years after the last biosolids application. Soilwas moistened and pre-incubated for 10 days to allow microbialactivity to stabilize before measuring microbial maintenancerespiration and biomass. The procedure consisted of placingduplicate 20–65 g (depending on SOC level) soil samples into60-mL glass jars, wetting to 50% water-filled pore space, andplacing duplicates into a 1-L canning jar along with 10 mL of 1 MNaOH to trap CO2, and a vial of water to maintain humidity(Franzluebbers et al., 1999). Samples were incubated at 25 �1 �C.The CO2-C trapped was determined by the titration of remainingNaOH with 1 M HCl in the presence of excess BaCl2 to aphenolphthalein endpoint. Soil microbial maintenance respira-tion or basal respiration was calculated as the rate of CO2-Cevolved from 10–24 days of incubation (Franzluebbers et al.,2000). The SMBC was measured using the fumigation–incubationmethod after 10 days of pre-incubation, in which one of the soilsamples was fumigated with CHCl3 under vacuum, and vaporsremoved after 24 h. Fumigated samples were placed in a separatecanning jar along with vials of alkali and water for incubation at25 �C for 10 days. The SMBC was calculated as the quantity of Cmineralized during 10 days of incubation following fumigationsamples divided by an efficiency factor of 0.41 (Voroney and Paul,1984; Franzluebbers et al., 1999).

It is reasonable to assume that in a given condition, moisturestress of an agricultural soil to microorganisms tends to be low whensoil WHC is high. Soil WHC was therefore measured to find out ifbiosolids application could have potentially reduced microbialmoisture stress. Soil WHC was determined using a procedureadapted from Cassel and Nielsen (1986). The bulk of soil sampleswere collected from the same fields. The soil was hand-crushed toimprove the homogeneity during the air-drying. Ten kg air-dried soilwas placed in a 100 cm long by 15 cm wide by 5 cm deep metal traywith the cheesecloth on the bottom. Soil was packed using a woodendamper to the bulk density of 1.3. Reverse osmosis water was addedgradually until the tray started draining water from the bottom. Thetray was covered with plastic sheet and left to stand forapproximately 24 h. Soil samples were taken from the tray andwater content determined by oven-drying at 105 �C.

2.3. Data processing and statistical test

We calculated crop residue mass from grain yields andpublished harvest indices and root:shoot ratios (Prince et al.,2001). Crop residue mass was converted into crop residue-C usingthe C concentration in shoot and root. Harvest index, root:shootratio, and C concentration in biomass were obtained from theliterature (Table 1). Rye biomass was not recorded, but weassumed 0.61 kg m�2 shoot dry matter production under fertilizerin Illinois (Ruffo et al., 2004) with 42% C in rye shoot (Ruffo andBollero, 2003).

Because fields were not planted with pure C3 or C4 crops,weighted mean d13C of crop residues in a field was calculated

based on relative proportion of biomass C of each crop (Diels et al.,2004). The d13C value used was �12.5m for C4 (corn residue) (Paratet al., 2007), and �27.3m for C3 (soybean, wheat, and rye residues)(Zach et al., 2006).

The total SOC stock was calculated using organic C concentra-tion, bulk density and soil depth. In 1972, soil bulk density datawas only available as the mean (1.29 Mg m�3) of non-minedagricultural soils (Peterson et al., 1979). We calculated the soilbulk density for individual fields in 1972 using the SOCconcentration in 1972 and SOC-bulk density relationship estab-lished by 2005 sampling. The mean of calculated bulk density ofall fields in 1972 was comparable to mean of non-minedagricultural soils before biosolids application reported inPeterson et al. (1979). We used the “equal soil mass” approach(Ellert et al., 2001) in calculating total SOC stock. Our calculationof total SOC stock started with a depth of 30 cm (0–15 and 15–30 cm) for biosolids-amended fields in 2006. The plow layerdepth of an agricultural soil is generally 15 cm. Inbiosolids-amended soil, biosolids earth and remaining organicmatter can increase plow layer depth to more than 15 cm, thus the30 cm depth of soil sampling in 2006 included only a fraction of15–30 cm depth of initial soil. Biosolids-added depth wasmodeled yearly using soil bulk density, biosolids bulk densityand annual biosolids application rate in our previous study (Tianet al., 2009). Mean biosolids-added soil depth was 6.5 � 0.8 cm,close to 7.9 cm estimated by Granato et al. (2004) usingcumulative biosolids loading rate (54.3 kg m�2) and biosolidsbulk density (0.69 Mg m�3). Therefore, initial soil depth prior tobiosolids application that corresponds to the 2006 0–30 cm depthin biosolids-amended fields should be 23.5 cm (0–15 and 15–23.5 cm). Thus, initial total SOC stock in biosolids-amended fieldsin 1972 was calculated based on 23.5 cm soil depth (0–15 cm) and(15–23.5 cm). Because the initial 23.5 cm soil depth in thebiosolids-amended fields contained soil mass weight of 0.32 Mgm�2, soil C stock in unamended fields for both 1972 and 2006 wascalculated based on a soil depth that contained 0.32 Mg m�2 ofsoil mass. Soil samples of 15–30 cm depth in 1972 were not taken,thus we estimated C concentration in this soil depth in 1972 usingC ratio of 15–30 cm to 0–15 cm depths in 2006 in unamendedfields.

The crop residue-C stock in 2006 soil (CCr) was calculated usingthe formula below, which we have developed (see SupplementaryMaterial) using d13C theory as described in Balesdent et al. (1987),Diels et al. (2004) and Parat et al. (2007):

CCr ¼ COlde�ktdOld þ ðT � COlde�ktÞdBS � TdSoil

dBS � dCr

In which T = total SOC stock in 2006 (kg C m�2); COld = initial (old)soil C stock (kg C m�2); dSoil = soil d13C in 2006 (m); dOld = initial(old) soil d13C (m); dBS = biosolids d13C (m); dCr = weighted meand13C of all crop residues (m); k = first order exponential decay ofinitial (old) soil C (y�1); and t = time (y).

Table

2Mea

nva

lues

�stan

darderror(SE)

oftotalso

ilorga

nic

C(SOC)stoc

k,d1

3Cin

soilan

dinputmaterials,a

ndso

urce-fraction

of20

06SO

Cstoc

k.

Trea

tmen

tTo

talSO

Cstoc

kad1

3C

2006

SOCstoc

kby

sourced

1972

2006

1972

soil

Bioso

lids

Cropresidues

c20

06so

ilOld

CBioso

lids-C

Cropresidue-C

––––-kgC

m�2–––––

–––––––––––––––––m–––––––––––––––-

––––––––––-kgCm

�2–––––––––

Unam

ended

( m1)

3.14

�0.83

3.59

�0.53

-21.5�

0.3

NRb

-19.1�

1.2

-20.3�1.0

2.11

�0.49

01.48

�0.03

Bioso

lids-am

ended

( m2)

3.24

�0.37

10.3

�0.5

-21.0�0.3

-25.6�0.6

-18.5�

0.4

-21.6�0.1

2.08

�0.21

3.59

�0.28

4.60

�0.27

H0:m

1=m

2Pva

lue

>0.05

<0.001

>0.05

NR

>0.05

<0.05

>0.05

NR

<0.001

aIn

0–30

cmdep

thin

bios

olids-am

ended

soil(2006

),an

din

areviseddep

thin

bios

olids-am

ended

soil(197

2)an

dunam

ended

soil(197

2an

d20

06)co

ntainingtheso

ilmasseq

uivalen

tto

initialsoilm

ass(0.32Mgm

�2)in

the0–

30cm

dep

thof

bios

olids-am

ended

soil(2006

).bNot

releva

ntas

bios

olidswerenot

applied

inunam

ended

fields.

cW

eigh

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allC3an

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s.dPa

rtitioningof

totalSO

Cstoc

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variou

sso

urces

was

conducted

basedon

the

13dap

proachas

described

inMaterialan

dMethod

san

dSu

pplemen

talMaterials.

G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93 89

During sequestration of crop residue-C in soil, old soil C issusceptible to decomposition as CO2. Therefore, the balancebetween crop residue-C in soil and loss of old soil C during thesame period is referred to as net soil C sequestration (NSCS).Annual rate of NSCS (kg C m�2 y�1) from 1972 to 2006 wascalculated as:

NSCS ¼ Ccr � COld � COldRemð Þ34

In which CCr = crop residue-C stock in 2006 soil (kg C m�);COld = initial (old) soil C stock (kg C m�2); COldRem = remaining ofinitial (old) soil C stock in 2006 (kg C m�2). Biosolids-C remainingwas not considered as soil C sequestration as it comes from theamendment applied.

We applied a two-tailed unpaired t test to evaluate thesignificance of differences between biosolids-amended soil andunamended soil for each variable. In conducting the statistical test,fields were considered as replicates, thus there were 8 replicatesfor biosolids-amended treatment and 4 replicates for unamended(control). Data of a field was the mean of two halves. The level ofsignificance was set at P = 0.05. When appropriate, a standard error(SE) was provided.

3. Results

3.1. Total SOC

In the 0.32 Mg m�2 standard soil mass, total SOC stock at thebeginning of the study did not show significant difference betweenthe two treatments: 3.14 kg C m�2 in unamended soil and 3.24 kgC m�2 in amended soil (Table 2). In biosolids-amended soil, totalSOC stock increased sharply in 2006 as compared to 1972, while itincreased only slightly in unamended soil. Thus, in 2006, almost3-times more total SOC stock was observed in thebiosolids-amended soil than in unamended soil in the same massof initial soil (Table 2).

3.2. Partitioning of total SOC and sequestration of crop residue-C insoil

Total SOC in the biosolids-amended treatment in 2006 consistedof old C (the remaining of initial-old SOC), biosolids C (SOC derivedfrom biosolids) and crop residue-C (the SOC derived from cropresidues). For unamended soil (no biosolids), there were only twosources in total SOC, i.e., old SOC and crop residue-C. Throughanalysis of d13C in input materials and soil, the proportion of each Csource in total SOC can be determined (Balesdent et al., 1987; Dielset al., 2004; Parat et al., 2007). Initial soil d13C was similar in bothtreatments with �21.5m for unamended treatment and -21.0m forbiosolids-amended treatment (Table 2). As compared to initial SOC,crop residue-C was relatively enriched with 13C (less negative d13C),and biosolids C relatively depleted with 13C (more negative d13C).The depleted 13C in biosolids, which was also reported in otherstudies (Parat et al., 2007; Gerzabek et al., 2001), was due topreferential degradation of labile fractions during biosolidsstabilization processes (anaerobic digestion and lagoon-aging),leading to relative enrichment of lignin-derived carbon (Harten-stein, 1981), which is depleted in 13C (Benner et al., 1987). Inunamended soil, d13C was enriched by 1.2m in 2006 as compared to1972. In contrast, d13C of biosolids-amended soil was depleted by0.6m during the same period, leading to significantly lower (morenegative) d13C in biosolids-amended soil than unamended soil.Using the d13C data and the approach we developed (seeSupplementary Material), total SOC was partitioned into threerelevant sources (Table 2).

Table 3Mean values � standard error (SE) of crop residue-C input and rate of sequestration of crop residue-C in agricultural soils from 1972 to 2006.

Treatment Total crop residue-Cinput to soila

Percentage of crop residue-C input sequestered in soilb

kg C m�2 %Unamended (m1) 12.6 � 0.5 11.8 � 1.6Biosolids-amended (m2) 14.3 � 0.1 32.5 � 1.7H0: m1 = m2

P value< 0.05 < 0.001

a Crop residue-C input was calculated using grain yields from 1972 to 2005 and harvest indices (fraction of grain yield as total above ground biomass) and root:shoot ratio(Prince et al., 2001).

b Calculated as: crop residue-C in 2006 soil/total crop residue-C input to soil *100.

90 G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93

In 2006, stock of old SOC was the same in both unamended andamended soil (Table 2). Biosolids-C stock in biosolids-amended soilwas 35% of total SOC stock in 2006. The biosolids amendment tripledcropresidue-Cstockin2006soil(Table2).Ascomparedtounamended,biosolids amendment increased production of crop residue-C by only14%, however, biosolids application resulted in the increase in cropresidue-C input sequestered in soil by 2.8-fold (Table 3).

3.3. Net soil C sequestration (NSCS)

The annual rate of NSCS was near-zero without amendment, butsignificantly positive with biosolids amendment (Fig. 1). Further-more, the mean NSCS value of biosolids-amended soil was greaterthan the global mean NSCS value of soil managed by no-tillage (Westand Post, 2002), which is considered a key soil managementtechnology for soil C sequestration in agricultural soils.

3.4. Microbial metabolic quotient (qCO2) and soil water holdingcapacity (WHC)

Amending soil with biosolids increased both SMBC andmicrobial maintenance respiration, however, such effect wasgreater on SMBC than microbial maintenance respiration (Table 4).The qCO2 was lower in biosolids-amended soil than unamendedsoil. Biosolids-amended soil had greater WHC than unamendedsoil (Table 4).

Fig. 1. Annual rate of net soil C sequestration (NSCS) from 1972 to 2006 inagricultural soils as affected by biosolids application from 1972 to1984. Biosolids-amended soil received the same chemical fertilizer application from 1985 to2005 as unamended soil. The reduced tillage (between conventional tillage and no-tillage) was used in study fields. Bar represents SE. The reference level of NSCSunder no-tillage with 0–30 cm soil depth was reported by West and Post (2002).

4. Discussion

Significantly greater total SOC stock in biosolids-amended thanunamended soil indicated that biosolids amendment was veryeffective in increasing SOC in agricultural soils. In comparison withnumerous long-term trials conducted in North America (Paul et al.,1997) and in many other parts of the world (Diacono andMontemurro, 2010), we conclude that no other soil amendmentslike biosolids have displayed such high effectiveness in restoringthe lost organic C in agricultural soils. Soil organic matter isgenerally composed of four major C groups: alkyl, O-alkyl, aromaticand carboxyl, and SOC accumulation is often correlated with theabundance of the alkyl-C group (Baldock et al., 1997). Biosolids-C isdominated by alkyl-C (Rowell et al., 2001), and the application ofbiosolids has been shown to increase alkyl-C of soil humicsubstances (Chiu and Tian, 2011). Thus, the effectiveness we foundof biosolids in restoring SOC could be partially due to its dominantcomposition of recalcitrant alkyl-C.

A more important reason for the remarkable increase in SOCafter biosolids amendment was greater crop residue-C in soil(Table 2). Greater crop residue-C in biosolids-amended thanunamended soil could have simply been due to greater input(production) of crop residue-C in the former than the latter, andnot greater sequestration efficiency of crop residue-C. Observa-tion of greater sequestration rate of crop residue-C in biosolids-amended than unamended soil (Table 3) suggests that greatercrop residue-C sequestration in biosolids-amended soil be due toa greater fraction of crop residue-C being sequestered, and not anincrease in input of crop residue-C by biosolids. Low sequestra-tion rate of crop residue-C in unamended soil implies that inagricultural soils, a substantial fraction of C in crop residues(about 90%) is lost directly to the atmosphere without cyclingthrough soil organic matter, a “short-circuit” in the C cycle (Austinand Vivanco, 2006). Amending agricultural soils with biosolidscan reduce such short-circuit of the C cycle.

We hypothesized that the greater fraction of crop residue-Cbeing sequestered in soil was due to alleviation of microbial stressand restoration of microbial C metabolism with biosolids amend-ment. As an indictor for maturity of an ecosystem, low qCO2 isassociated with high maturity of an ecosystem (Insam andHaselwandter, 1989; Anderson and Domsch, 2010). When micro-organisms are environmentally stressed, they tend to allocate moreC resource to respiration (Schimel et al., 2007), so less C is availablefor microbial growth. Thus, under environmentally stressedconditions, agricultural soils tend to demonstrate high microbialmaintenance respiration and low SMBC, leading to high qCO2 (poorC metabolism). As the qCO2 and soil microbial stress are related, wecan, therefore, introduce qCO2 as an indicator of microbial stressalleviation in agricultural soils. A reduction in qCO2 after biosolidsamendment suggests that the soil amendment had alleviatedmicrobial stress in the agricultural soils. Further decrease in qCO2

may be possible with greater biosolids application rate (Tian et al.,

Table 4Mean values � standard error (SE) of soil microbial biomass C (SMBC), microbial maintenance respiration, microbial metabolic quotient (qCO2), and water holding capacity in2006a.

Treatment SMBC Microbial maintenance respiration qCO2 Soil water holding capacity

mg C kg�2 mg CO2-C kg�2 d�1 g CO2-C kg SMBC�1 d�1 % (w/w)Unamended (m1) 310 � 21 10.0 � 0.6 32.8 � 1.1 66.6 � 1.2Biosolids-amended (m2) 628 � 19 13.6 � 0.5 22.1 � 0.9 50.7 � 2.8H0: m1 = m2 P value < 0.001 < 0.001 < 0.001 < 0.01

a In 0–30 cm depth in biosolids-amended soil, and in a revised depth in unamended soil containing the soil mass equivalent to initial soil mass (0.32 Mg m�2) in the 0–30 cmdepth of biosolids-amended soil.

G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93 91

2013). Besides reducing C:N ratio of substrates in agricultural soils,biosolids amendment could have reduced moisture stress tomicroorganisms in agricultural soils as evidenced by greater soilWHC in biosolids-amended than unamended soil (Table 4). Furtherstudies are necessary to evaluate if possible changes in communitycomposition such as soil fungi/bacteria ratio, enzyme activities, andsoil porosity under biosolids might have contributed to lower qCO2.

Even though intensive tillage (moldboard/chisel plowing) wasreplaced by reduced tillage (discing) since the 1980s in this study,net soil C sequestration (NSCS) in unamended soil was nearly zero,indicating great difficulty for agricultural soils to become a C sinkunder current management (Luo et al., 2010; Powlson et al., 2011).While high rates of chemical N fertilization application inagricultural fields could have increased the loss of native SOC(Khan et al., 2007), the low sequestration rate of crop residue-C forunamended soil as found in our study may have explained whythere was no sign of SOC increase in US Midwest for the pastdecades (Russell et al., 2005; Huggins et al., 2007). Remarkablyhigh NSCS in biosolids-amended soil due to greater sequestrationof crop residue-C suggests that biosolids amendment can helppromote agricultural soils to become a C sink. Amending soils toimprove microbial C metabolism is needed to transform agricul-tural soils from being C-neutral to a C sink in addition to the changefrom intensive tillage to reduced tillage.

5. Conclusions

Biosolids application to agricultural soils can be an effectiveway to restore lost C from historical cultivation. Amending soilwith biosolids can increase sequestration of crop residue-C inagricultural soils. The results from this study offer an ecologicalway for stakeholders in the Midwest to rebuild SOC stock so thatproductivity and sustainability of ecosystems will be greatlyimproved. Our findings also have an important implication foroffsetting CO2 emissions through C sequestration by agriculturalsoils. Biosolids amendment can change agricultural soils fromprevalent C-neutral to a C sink to offset global CO2 emissions.Substantially greater sequestration of crop residue-C in agricul-tural soils amended with biosolids was related to improvement inmicrobial C metabolism as a result of alleviation of stress to soilmicroorganisms. Our findings suggest environmentally stressedsoil microorganisms in agricultural soils have low microbial C useefficiency, and thus could not effectively sequester crop residue-C.Alleviation of microbial stress in agricultural soils with applicationof high stable C and low C:N materials to modify microbialphysiology may be a prerequisite for enhanced soil C sequestration.

Supplemental material

Process to derive a formula for calculating crop residue-C in soil

For the calculation of crop residue-C accumulated in soil from1972 to 2006, the total soil C stock (T) in 2006 was partitioned into

remaining of initial (old) C (COldRem), biosolids C (CBS), and cropresidue-C (CCr) with corresponding d13C as dOld, dBS, and dCr. Theother source for SOC could be that fixed by microorganisms(Miltner et al., 2004), however, its size is too small to beappreciable for the high total SOC in this study. The d13C of soilsample (dSoil) in 2006 can therefore be expressed as theweighted mean of the delta values of the above three Csources.

So, we have:

COldRemdOld þ CBSdBS þ CCrdCrT

¼ dSoil (1)

COldRem+ CBS + CCr = T (2) (2)

The decomposition of old C in soil is known to follow the first orderexponential decay model (Jenny, 1980), so:

COldRem= COlde�kt (3) (3)

In which COld is total initial (old) soil C, k first order exponentialdecay of initial (old) soil C, and t the time. Since there was nobiosolids C in the unamended soil, Eqs. (1) and (2) can be simplifiedas:

COldRemdOld þ CCrdCr

T¼ dSoil (4)

COldRem+ CCr = T (5) (5)

By solving Eqs. (4) and (5), we get:

COldRem ¼ 1 � dSoil � dOlddSoil � dOld

� �T (6)

After obtaining COldRem by Eq. (6), we can calculate k in unamendedsoil using Eq. (3) as follow:

k ¼ LnCOld

COldRem=t (7)

Assuming biosolids application did not change the decay rate ofinitial (old) soil C, the k obtained through Eq. (7) can be applied tothe biosolids-amended soil. Then, through solving Eqs. (1)–(3)together, the crop residue-C stock (CCr) in soil accumulated from1972 to 2006 could be calculated as:

CCr ¼ COlde�ktdOld þ ðT � COlde

�ktÞdBS � TdSoildBS � dCr

(8)

Acknowledgments

We thank Richard Doucett and Melanie Caron at the ColoradoPlateau Stable Isotope Laboratory, Northern Arizona University, for13C analysis, and Steven Knapp for microbial analysis, and NickLafary, Richard Adams, and Mina Patel for technical assistance. We

92 G. Tian et al. / Applied Soil Ecology 85 (2014) 86–93

appreciate contribution of Lucas Tian in the development of theconcept of the paper and the interpretation of data duringpreparation of the manuscript. We are grateful to anonymousreviewers, Shiping Deng, Heribert Insam, and Dave Coleman fortheir constructive comments on the manuscript.

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