for review only - university of toronto t-space · 2019. 8. 13. · for review only 2 dryland...

For Review OnlyAggregate size distribution and associated carbon and

nitrogen in mulched winter wheat and spring corn

Journal: Canadian Journal of Soil Science

Manuscript ID CJSS-2019-0015.R1

Manuscript Type: Article

Date Submitted by the Author: 15-May-2019

Complete List of Authors: Fu, Xin; Northwest UniversityWang, Jun; Northwest UniversitySainju, Upendra; USDA-ARS, Northern Plains Agricultural Research LaboratoryLiu, WenZhao; Chinese Academy of Sciences, Institute of Soil and Water Conservation

Keywords: mulching,, Aggregation, soil organic carbon, soil total nitrogen, dryland crops

Is the invited manuscript for consideration in a Special

Issue?:Not applicable (regular submission)

https://mc.manuscriptcentral.com/cjss-pubs

Canadian Journal of Soil Science

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Aggregate size distribution and associated carbon and

nitrogen in mulched winter wheat and spring corn

Xin Fu1, 2, Jun Wang1, Upendra M. Sainju3, Wenzhao Liu1

___________________________________________________________________________

1 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of

Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources,

Yangling, Shaanxi 712100, PR China

2 Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity,

College of Urban and Environmental Science, Northwest University, Xi’an, Shaanxi 710127,

China

3 USDA, Agricultural Research Service, Northern Plains Agricultural Research Lab., 1500

North Central Avenue, Sidney, Montana 59270, USA;

3 Corresponding author. E-mail address: [email protected]

Abstract: The influence of surface mulching on soil aggregation and associated carbon (C)

and nitrogen (N) varies by mulching materials and crop types. The six-year effect of straw

mulching (SM), plastic film mulching (PM), and no mulching (CK) on soil aggregation and

associated C and N concentrations at 0-20 and 20-40 cm soil layers were studied under

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mailto:[email protected]

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dryland winter wheat (Triticum aestivum L.) and spring corn (Zea mays L.) in the Loess

Plateau of China. Regardless of crop types, aggregate proportion was greater in

macroaggregates (2.00-10.00 mm), but lower in microaggregates (<0.50 mm) with mulching

than without in both soil layers. The mean weight diameter (MWD) of aggregates was greater

with SM and PM than CK. Compared to CK and PM, SM increased soil organic C (SOC) and

total N (STN) concentrations in both macroaggregates and bulk soil at 0-20 cm. Aggregate

proportion and soil C and N concentrations at both depths were more pronounced in winter

wheat than spring corn. The recovery rates of bulk soil SOC and STN in aggregates varied

from 94 to 107%. Straw and plastic film mulching enhanced soil aggregation compared to no

mulching. Straw mulching was more effective in increasing SOC and STN concentrations at

the surface layer in dryland winter wheat and spring corn.

Key words: mulching, aggregation, soil organic carbon, soil total nitrogen, dryland crops

Introduction

Soil aggregation reduces erosion, improves water filtration capacity, and enhances crop

root growth compared to no aggregation (Tisdall and Oades 1982; Jastrow 1998). Increased

root growth improves soil aggregation by enmeshing soil particles together and by increasing

microbial biomass that produce polymers which act as binding agents for aggregates (Tisdall

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and Oades 1982; Jastrow 1998). Aggregates protect the mineralization of soil organic C and N

by reducing access to the substrates for microbes (Wright and Hons. 2005). Sequestration of

C and N in the soil due to improved management practices occur in aggregates (Sainju et al.

2009; Udom et al. 2016). The rate of aggregate formation and aggregate turnover influence

retention of C and N in aggregates (Tisdall and Oades 1982; Six et al. 2000).

Carbon and N sequestrations have been considered as potential mechanisms to mitigate

the emissions of atmospheric greenhouse gases from agricultural systems (Sainju et al. 2009)

and play crucial roles in sustaining soil fertility (Luo et al. 2015). Storage and mineralization

of C and N in aggregates differ by size classes. Macroaggregates (>0.25 mm) are the main

sites where C and N from fresh crop residues are incorporated, which upon degradation, form

microaggregates (<0.25 mm) (Jastrow 1998). Because macroaggregates are composed of

microaggregates and organic binding agents, soil organic C (SOC) and total N (STN)

concentrations are higher in macroaggregates than microaggregates (Elliott 1986). In contrast,

Udom et al. (2016) reported higher SOC concentration in microaggregates due to their greater

stability than macroaggregates. Because of reduced exposure to microbial decay,

aggregate-protected C and N pools are more labile than unprotected pools (Beare et al. 1994).

Straw mulching can increase soil aggregation by enhancing C and N sequestration

compared to no mulching (Mulumba et al. 2008). Mbah et al. (2010) found that plastic film

mulching increased soil aggregation and total porosity and decreased bulk density compared

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to no mulching. Others (Ogbonna and Obi 2007) reported no or negative effect of mulching

on water-stable aggregates. Short-term plastic film mulching favored the proportion of

macroaggregates, but long-term mulching reduced aggregation by increasing the

mineralization of soil organic matter due to enhanced microbial activity as a result of

increased soil temperature and water content (Li et al. 2007). Tian et al. (2013) observed that

plastic film mulching had little effect on soil aggregation and SOC. The effect of mulching on

soil aggregation and C and N storage in aggregates vary by soil type, management practices,

climatic condition, and land use (Jordán et al. 2010).

In the last decades, surface mulching with straw or plastic film has been widely adopted

to conserve soil water and improve crop production in dryland cropping systems (Kahlon et

al. 2013; Zhang et al. 2015). Mulching can influence soil temperature and water content,

which in turn, affect microbial activity, root growth, and soil aggregation (Balesdent et al.

2000). Enhanced microbial activity due to increased soil temperature and water content from

mulching can increase the production of fungal hyphae and polymers that bind soil particles

together to form aggregates (Mulumba and Lal. 2008; Mbah et al. 2010). Studies across the

globe have shown that straw mulching can increase soil aggregation and C and N

sequestration by enhancing fresh organic matter and C and N inputs compared to no mulching

(Mulumba and Lal 2008; Kahlon et al. 2013).

In the Loess Plateau of China, aboveground crop residues (stems and leaves) are usually

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removed from the soil for hay and fuel uses after grain harvest, resulting in reduced organic

matter content and increased exposure of the soil to erosion (Zhang et al. 2015). Straw and

plastic film mulches are applied at the soil surface to cover soil, conserve soil water and

reduce evaporation and soil erosion, especially in dryland cropping systems (Kahlon et al.

2013). Introduction of plastic film (a biodegradable material) mulching in late 1970 in China

substantially increased dryland crop yields by enhancing soil water conservation (Wang et al.

2018). As a result, the use of plastic film mulching in dryland crop production has increased

from 319 to 1245 megatons from 1991 to 2011 (Chen et al. 2017). Although plastic materials

can result in environmental problems, thin plastic film is biodegradable and recommended by

the Government of China to enhance dryland crop yields to feed the growing population

(Chen et al. 2017). Information is lacking about the effect of straw and plastic mulching on

soil aggregation and associated C and N in winter wheat and spring corn, which are the major

dryland crops in China.

We studied the effect of 6 yr of straw and plastic film mulching on soil aggregation and

SOC and STN concentrations in aggregates compared to no mulching in winter wheat and

spring corn in the Loess Plateau of China. The objectives of this study were to (1) examine

soil water-stable aggregates and associated SOC and STN concentrations with and without

mulching in winter wheat and spring corn and (2) determine a mulching practice that enhance

soil aggregation and C and N concentrations in dryland cropping systems. We hypothesized

that straw and plastic film mulching would improve soil aggregation and associated SOC and

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STN concentrations compared to no mulching.

Materials and Methods

Experimental site, treatments, and crop management

Two experiments in winter wheat and corn were conducted separately from 2008 to 2014

at the Changwu Agro-Ecological Station in the Loess Plateau (107° 44.70′ E, 35° 12.79′ N)

in Shaanxi Province of China. The mean annual air temperature is 9.1oC and the annual

precipitation is 580 mm. The mean frost-free period is 194 days and the open pan evaporation

1440 mm. The soil was a Heilutu silt loam and classified as a Calcarid Regosol, with 35 g kg-1

sand, 656 g kg-1 silt, 309 g kg-1 clay, 1.30 Mg m-3 bulk density, 8.30 pH, 9.10 g kg-1 SOC, and

1.10 g kg-1 STN at the 0-20 cm depth at the initiation of the experiment in 2008.

For both winter wheat and spring corn experiments, three treatments that included no

mulching (CK), straw mulching (SM), and plastic film mulching (PM), were arranged in a

randomized complete block design with three replications for a total of nine plots. The plot

size was 6.7 m wide by 10.0 m long. Plots were spaced 0.5 m apart and replications separated

by a 1 m strip. In winter wheat, SM included a surface mulch of 10-15 cm long wheat straw

(5-10 cm thick) applied at a rate of 9 Mg ha-1. The PM included covering the soil surface with

a thin plastic film (1 mm thick), with edges covered by soil particles at the plot boundary.

Winter wheat was planted in late September and harvested in late June of the next year. In

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spring corn, SM included a surface mulching of corn straw (2 m long and 10 cm thick)

applied at 9 Mg ha-1 and PM included plastic film mulching similar to that in winter wheat.

Spring corn was planted in mid-April and harvested in late September in each year.

Both winter wheat and spring corn were planted by hand under conventional tillage,

which consisted of hand tractor-drawn plows to a depth of 10 cm at planting. Nitrogen

fertilizer as urea (46% N) at 120 kg N ha-1 and P fertilizer as calcium superphosphate (20% P)

at 60 kg P ha-1 were broadcast and then incorporated to a depth of 20 cm using a rotary tiller

before planting. Potassium fertilizer was not applied because of high K content (about 130 mg

K kg-1 at the 0-20 cm soil depth) in the soil according to the soil test. Immediately after

fertilization and tillage, wheat was planted by hand at 2.23 million plants ha-1 with a row

spacing of 20 cm. Wheat straw was applied as mulch just after planting in SM plots. In PM

plots, plastic film was applied, followed by planting in drilled holes. In spring corn, corn was

planted at 40,000 plants ha-1 at a 60 cm row spacing using mulches similar to those in wheat.

Weeds were controlled by hand weeding and pesticides applied as needed. Both winter wheat

and spring corn were grown under rainfed condition. After crop harvest, left-over straw and

plastic film mulches were removed from the soil.

In winter wheat, total aboveground biomass (grains, stems, and leaves) was harvested

manually in late June 2009-2014 by cutting all plants at 2 cm above the ground. Grains and

biomass (stems and leaves) were separated and oven dried at 70oC for 3 d to determine the dry

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matter yield, from which grain and total aboveground biomass yields were calculated. In

spring corn, corn was harvested in late September, 2009-2014. Grains and biomass were

separated and oven dried at 70oC for 3 d, from which grain and total aboveground biomass

yields were determined similar to those in winter wheat. Belowground root biomass residues

of winter wheat and spring corn returned to the soil were estimated by multiplying total

aboveground biomass by 0.33 and 0.35, respectively (Bolinder et al. 1999; Campbell and de

Jong 2001).

Soil sampling and analysis

Soil samples were collected from each plot after winter wheat harvest in June 2014 and

after spring corn harvest in September 2014. Samples (about 1.5 kg) were collected with a

spade from 0-20 and 20-40 cm depths from five places within a plot and composited by depth.

Field-moist soils were passed through a 10-mm sieve by gently breaking clods by hand.

Visible crop residue and coarse fractions were removed. Soil samples were air dried and

stored in plastic bags for further analysis. Each sample was divided into two parts for

analysis: one part for the bulk soil and other for separating aggregates.

Water-stable aggregates were separated by using the wet-sieving method (Kemper &

Rosenau 1986). Fifty grams soil (<10 mm) was placed at the top sieve connected to a nest of

sieves (20 cm diameter and 5 cm high) with sizes of 5.00, 2.00, 1.00, 0.50, and 0.25 mm at the

bottom and presoaked in distilled water for 5 min. The nest of sieves was oscillated vertically

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in water in a bucket for 30 times with 3 cm amplitude using a mechanical shaker. Aggregates

deposited in each sieve were collected and oven dried at 50oC for 3 d. Aggregates removed

from 5.00, 2.00, 1.00, 0.50, and 0.25 mm sieves represented 10.00-5.00, 5.00-2.00, 2.00-1.00,

1.00-0.50, and 0.50-0.25 mm size classes, respectively, and aggregates that passed through the

0.25 mm sieve represented the <0.25 mm size class. The proportion of water-stable

aggregates in each size class and the mean-weight diameter (MWD) of aggregates were

determined as described by method (Kemper & Rosenau 1986).The SOC and STN

concentrations (g kg-1) in the bulk soil and aggregates were determined using a high induction

furnace C and N analyzer (Euro Vector EA3000) after grinding particles to <0.5 mm and

pretreating with 6 mole L-1 HCl to remove inorganic C.

The contributions of SOC and STN in ith aggregate size to the bulk soil (SOCPi and

STNPi, %) was calculated as (Wang et al. 2018).

SOCPi (or STNPi) = Pi×SOCCi (or STNCi)/SOCC (or STNC)

Where Pi is the proportion of ith aggregate size in the bulk soil, SOCCi and STNCi are

SOC and STN concentrations in ith aggregate size, and SOCC and STNC are SOC and STN

concentrations in the bulk soil, respectively.

The recovery rates of bulk soil SOC and STN in aggregates (SOCR and STNR) was

calculated as (Yang et al. 2007)

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SOCR (or STNR) = ∑6𝑖 = 1𝑆𝑂𝐶𝑃𝑖 (𝑜𝑟 𝑆𝑇𝑁𝑃𝑖)

Where SOCPi and STNPi are contributions of SOC and STN in ith aggregate size to the

bulk soil.

Data analysis

Data for crop grain and biomass yields, soil aggregation, and SOC and STN

concentrations were tested for normality using the Kolmogorov-Smirnov test (P > 0.05). Data

for crop grain and biomass yields from 2009 to 2014 and soil aggregate parameters in 2014

were analyzed using the SAS-MIXED model (Littell et al. 2006). For grain and biomass

yields, mulching, year, and their interaction were considered as fixed effects and replication

as the random effect. For soil aggregation and SOC and STN concentrations, mulching was

considered as the main plot treatment and aggregate-size class as the split-plot treatment for

data analysis. Fixed effects were mulching, aggregate-size class, and mulching ×

aggregate-size class. Random effects were replication and replication × mulching. Means

were separated using the least square means test (Littell et al. 2006) when treatments and

interactions were significant. Pearson correlation analysis was used to determine the

relationships between crop parameters, soil macroaggregate proportion, and MWD.

Differences between treatments were reported significant at P ≤ 0.05.

Results

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Soil aggregation and crop yield

Mulching treatment influenced aggregate proportion and MWD in both 0-20 and 20-40

cm soil layers (Table 1). In winter wheat, aggregate proportion was greater with SM and PM

than CK in the 2.00-10.00 mm size class, but the trend reversed in the <0.50 mm class at 0-20

cm. At 20-40 cm, aggregate proportion was greater with SM than CK and PM in the

2.00-10.00 mm class, but was greater with PM than CK and SM in the <0.25 mm class. In

spring corn, aggregate proportion was similarly greater with SM and PM than CK at 0-20 cm

and greater with SM than PM and CK at 20-40 cm in the 5.00-10.00 mm class, but the trend

reversed in the <0.25 mm class. In the 0.50-1.00 mm class, aggregate proportion varied with

mulch treatments at 0-20 and 20-40 cm. The mean-weight diameter (MWD) of aggregates

was greater with SM and PM than CK at 0-20 cm and greater with SM than CK and PM at

20-40 cm in both winter wheat and spring corn.

In winter wheat and spring corn, crop grain yield, total aboveground biomass, and the

estimated root residue returned to the soil varied with mulching, years, and the interaction of

mulching × year (Table 2). Averaged across years, winter wheat grain yield was greater with

CK than SM and PM, but total aboveground biomass and the estimated root residue returned

to the soil were greater with SM and PM than CK. Spring corn grain yield was greater with

PM than SM and CK and total aboveground biomass and the estimated root residue returned

to the soil was greater with SM and PM than CK. The estimated root residue returned to the

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soil correlated positively with macroaggregate proportion and MWD at 0-20 cm in both

winter wheat and spring corn (Table 3).

Soil carbon and nitrogen concentrations

The SOC and STN concentrations in the bulk soil varied with treatments and soil depths

in winter wheat and spring corn (Fig. 1). In winter wheat, SOC and STN concentrations were

greater with SM than CK and PM at 0-20 cm and 20-40 cm. In spring corn, similar results

were obtained for SOC and STN concentrations with treatments and depths, except for the

SOC concentration at 20-40 cm where SOC was greater with SM than PM and CK.

Aggregate-associated carbon and nitrogen concentrations

The SOC concentration varied with aggregate size classes and treatments (Table 4). In

winter wheat, SOC concentration at 0-20 cm was greater with SM than CK and PM in all

aggregate-size classes, except in the 1.00-2.00 mm and <0.25 mm classes. The SOC

concentration was greater with SM than CK in the 1.00-2.00 mm class and not different

among treatments in the <0.25 mm class. At 20-40 cm, SOC concentration was greater with

SM than CK and PM in 2.00-5.00 and 0.25-0.50 mm classes. In spring corn, SOC

concentration was greater with SM than CK and PM in all aggregate-size classes at 0-20 cm,

except in the <0.25 mm size class. In this class, SOC concentration was greater with SM than

PM. At 20-40 cm, SOC concentration was greater with CK and SM than PM in the

1.00-2.00 and <0.50 mm classes. Averaged across treatments, SOC concentration in the <0.25

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mm class was lower than other classes in both winter wheat and spring corn at both depths,

except in the 0.25-0.50 mm class at 0-20 cm in spring corn.

Similar to SOC, STN concentration was greater with SM than CK and PM in all

aggregate-size classes at 0-20 cm, except in the <0.25 mm class in winter wheat (Table 5).

The STN concentration was greater with SM than CK and PM at the 2.00-10.00 mm class at

20-40 cm. In spring corn, STN concentration was also greater with SM than CK and PM in all

aggregate-size classes at 0-20 cm, except in the 5.00-10.00 mm class where STN

concentration was greater with SM than PM and in the 2.00-5.00 mm class where STN

concentration was greater with SM than CK. At 20-40 cm, STN concentration was greater

with SM than CK and PM in the >2.00 mm class. Averaged across treatments, STN

concentration was lower in the <0.25 mm class than other classes for both winter wheat and

spring corn at 0-20 cm, except in the 2.00-10.00 mm class for spring corn.

The contributions of bulk soil SOC and STN concentrations in large macroaggregates

(2.00-10.00 mm) were 25-39% and 23-36%, respectively, in winter wheat and 28-39% and

26-39%, respectively, in spring corn (Table 6). The contributions of SOC and STN

concentrations were greater with SM and PM than CK at 0-20 cm. At 0-20 and 20-40 cm, the

contributions of bulk soil SOC and STN concentrations in microaggregates (<0.25 mm) were

greater in spring corn than winter wheat for all treatments. The recovery rates of bulk soil

SOC and STN concentrations in aggregates varied from 94 to 107%.

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Discussion

Soil aggregation is affected by mulching methods due to different binding agents and

microbial polysaccharides (Tian et al. 2013). The enmeshing action of roots and

rhizodeposition can enhance soil aggregation (Jastrow et al. 1998; Zhang et al. 2015).

Previous studies showed that surface mulching increased soil water content and root growth

compared to no mulching (Li et al., 2007; Zhang et al. 2015). Increased root growth due to

mulching after 6 years and favorable action of roots on aggregation appeared to increase

aggregate proportion in large aggregates (2.00-10.00 mm size class) with SM and PM than

CK in both winter wheat and spring corn. This is supported by increased estimated root

residue returned to the soil with SM and PM compared to CK (Table 2) and significant

positive correlations between root residue, macroaggregate proportion, and MWD at 0-20 cm

(Table 3). Several researchers (Mulumba and Lal 2008; Mbah et al. 2010) have also reported

increased soil aggregation with straw and plastic film mulching compared to no mulching.

Enhanced soil microbial biomass and activity due to increased soil temperature and water

content resulting from straw and plastic film mulching have been linked to greater soil

aggregation as a result of increased production of polymers (Mulumba and Lal 2008).

Increased soil water content with straw and plastic film mulching compared to no mulching

have been reported in our previous studies (Wang et al. 2018). Although increased soil water

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content can reduce aggregation through slaking of aggregates, greater root growth due to

mulching may have enhanced the formation of water-stable aggregates. The increased

particulate organic matter inside macroaggregates due to enhanced root growth acts as

substrate for microorganisms, thereby favoring aggregate stability (Kahlon et al. 2013; Jordán

et al. 2010). This probably resulted in increased proportions of aggregates in macroaggregates

with SM and PM than CK, but the reverse trend in small macroaggregates and

microaggregates (<0.50 mm class). Because soil aggregates <0.84 mm class are prone to wind

erosion in dryland cropping systems (López et al. 2007), mulching can also reduce soil

erosion by enhancing the formation of large aggregates compared to no mulching.

The greater aggregate proportion in the 2.00-10.00 mm class than other classes with SM

and PM, but the reverse trend with CK in microaggregates suggests that large

macroaggregates contributed the greatest proportion of water-stable aggregates with mulching

in winter wheat compared to spring corn. It may be possible that reduced microbial activity

due to lower soil temperature and water content reduced soil organic matter mineralization in

the winter, which stabilized aggregation in winter wheat. In contrast, increased mineralization

of soil organic matter due to enhanced microbial activity, followed by increased raindrop

impact of the monsoon rain in the summer may have disrupted large aggregates into small

aggregates in spring corn, regardless of mulching practices. Aggregate stability was even

greater with SM than PM and CK in the subsoil layer, as shown by greater MWD with SM at

20-40 cm in both winter wheat and spring corn (Table 1).

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Aggregate proportion was greater in the <0.25 mm class than other classes in spring corn

and more so than in winter wheat (Table 1). The proportion of macroaggregates ranged from

65 to 81% in winter wheat and spring corn, which were consistent with those found by

Caesar-Tonthat et al. (2010) in the northern Great Plains, USA. Mendes et al. (1999),

however, found no significant difference in aggregate proportion among aggregate-size

classes in a cover crop mulching system in the western USA, while Schutter and Dick (2002)

reported a greater aggregate proportion in macroaggregates (>0.25 mm classes) than

microaggregates (<0.25 mm classes). Li et al. (2007) found that plastic film mulching

enhanced aggregate proportion in macroaggregates compared to microaggregates in rice in

central China.

The reasons for greater winter wheat grain yield with CK than SM and PM, but the

reverse trend with total aboveground biomass (Table 2) were not clear. The likely scenario

could be reduced soil N availability due to enhanced N immobilization from application of

straw and plastic film materials with increased C/N ratio of the mulching residue, thereby

reducing grain yield, or increased N uptake by biomass compared to grain due to mulching in

the winter. This was not the case in spring corn where SM and PM increased both grain and

total aboveground biomass compared to CK. Differences in soil N availability due to N

mineralization as a result of variations in soil temperature and water content in the winter and

summer due to mulching may have resulted in different trends in grain yield and total biomass

in winter wheat and spring corn. It is likely that higher soil temperature and water content

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enhanced soil N availability and spring corn yield in the summer compared to lower N

availability and winter wheat yield in the winter.

The greater SOC and STN concentrations with SM than CK and PM in the bulk soil

(Fig. 1) were likely a result of increased C and N inputs for six years from wheat and corn

straw used for mulching and increased root growth. Addition of crop residue C and N can

increase SOC and STN (Sainju et al. 2009). It is likely that most of the straw used for

mulching decomposed during wheat and corn growing seasons, some of which converted to

SOC and STN, and little was removed after grain harvest (Mulumba and Lal 2008). In

contrast, increased mineralization of organic matter due to enhanced soil temperature and

water content may have reduced SOC and STN concentrations with PM compared to SM.

Several researchers (Mbah et al. 2010; Zhang et al. 2015) have also reported reduced SOC

and STN with PM than SM due to increased mineralization. As most of the plastic film mulch

was removed after crop harvest, it is possible that C and N inputs from increased root residue

was not significant to alter SOC and STN concentrations with PM compared to CK. Other

researchers (Li et al. 2007) have found that plastic film mulching can disturb the original

balance between abiotic and biotic factors and SOC may be exhausted by plastic film

mulching over the long term.

The distribution of SOC and STN concentrations in aggregates strongly affect their

concentrations in the bulk soil (Li et al. 2007). Application of straw mulching increased SOC

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and STN concentrations in different aggregate sizes, particularly at the 0-20 cm depth

compared to other treatments (Tables 4 and 5). However, the effect was more pronounced in

macroaggregates than microaggregates, which was consistent with the results found in other

studies (Liu et al. 2015). It was shown that macroaggregate-associated SOC is mostly labile

and younger with faster turnover rates than microaggregate-associated SOC (Du et al. 2013).

Increased C and N inputs from straw mulching probably increased SOC and STN

concentrations in macroaggregates more than microaggregates. Plastic film mulching had

varied effect on SOC and STN concentrations in aggregates, probably due to the differences

in mineralization rates of organic matter within aggregates.

The greater SOC and STN concentrations in macroaggregates than microaggregates,

regardless of crop types, found in this study were similar to those reported by various

researchers (Sainju et al. 2009; Sodhi et al. 2009). Microaggregates contain older and

humified organic matter, while macroaggregates contain C and N-rich fresh crop residue,

roots, fungal hyphae, and polysaccharides, which result in higher SOC and STN

concentrations in macroaggregates (Elliott 1986). Binding of soil mineral particles and

microaggregates within macroaggregates by fungal hyphae and polysaccharides can also

increase SOC and STN concentrations in macroaggregates (Sodhi et al. 2009).

Previous studies (Zhang et al. 2015) have reported that soil macroaggregate proportion

and aggregate stability were strongly correlated with SOC. In this study, macroaggregate

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proportion was related to SOC in both winter wheat (R2=0.458, P=0.045) and spring corn

(R2=0.860, P=0.001) at 0-20 cm, but not at 20-40 cm. This finding is consistent with the

studies of Zhang et al. (2015) who suggested that labile C and N fractions, such as particulate

organic C and N and microbial biomass C and N, contribute to soil aggregation.

The greater distribution (23-39%) of bulk soil SOC and STN in large macroaggregates

(2.00-10.00 mm) suggests that most of the bulk soil C and N resided in macroaggregates and

that macroaggregates are the major sites for C and N sequestration in the soil, regardless of

mulching practices and crop types. The high recovery rates of bulk soil SOC and STN

concentrations in aggregates in all treatments indicated that water-soluble C and N fractions

lost during aggregate separation constituted small portions of SOC and STN and most of bulk

soil SOC and STN were recovered in aggregates in the wet sieving method.

Conclusions

Six years of straw and plastic film mulching increased aggregate proportion compared to

no mulching in large macroaggregates (2.00-10.00 mm class), but the trend reversed in small

aggregates (<0.50 mm class) at the 0-20 cm depth in both winter wheat and spring corn. As a

result, aggregate stability increased with SM and PM at 0-20 cm and with SM at 20-40 cm.

Compared to CK, SM increased SOC and STN concentrations in macroaggregates and the

bulk soil, but PM had varied effect. Most of the soil C and N concentrations occurred in

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macroaggregates. Regardless of crop type, straw mulching can increase soil aggregation and

C and N concentrations compared to no mulching, especially at the surface layer. Although

plastic film mulching increased soil aggregation compared to no mulching, straw mulching

can enhance both soil aggregation and C and N sequestrations and the effect was pronounced

more in winter wheat than spring corn.

Acknowledgments

We appreciate the funding for this study by National Natural Science Foundation of China

(Grant No. 31570440, 31270484), the CAS “Light of West China” Program and the

Outstanding Doctoral Dissertation Cultivation Project of Northwest University (YYB17017).

References

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dynamics to physical protection and tillage. Soil Tillage Res. 53: 215-230.

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Table 1. Proportion of water-stable soil aggregates (mean ± standard deviation) at 0-20 and 20-40 cm depths as affected by mulching in winter wheat and spring corn.

Aggregate proportion (%) in different size classes Soil layer Mulchinga

5.00-10.00 mm 2.00-5.00 mm 1.00-2.00 mm 0.50-1.00 mm 0.25-0.50 mm <0.25 mm MWD (mm)b)

Winter wheat

CK 10.±2.22 bcBd 13.0±1.59bB 12.0±3.65aB 14.7±1.19bB 25.1±5.77aA 25.2±0.50aA 1.62±0.21b

SM 13.9±1.69aBC 20.5±0.62aA 11.4±2.99aC 18.3±1.07aA 17.3±3.18bAB 18.6±1.67bA 2.13±0.15a0-20 cm

PM 15.0±0.91aB 21.3±1.64aA 11.7±1.21aC 16.1±0.19bB 15.1±0.15bB 20.8±3.21bA 2.22±0.13a

CK 15.7±1.21bBC 19.3±0.67aAB 8.7±0.12aD 13.8±4.23aC 21.8±2.75aA 20.7±2.70bA 2.20±0.078b

SM 20.2±1.08aA 18.1±1.18aAB 9.2±1.03aC 16.0±0.71aB 15.8±2.58bB 20.7±1.33bA 2.50±0.066a

20-40 cm

PM 16.0±1.30bCD 19.9±2.79aAB 9.0±1.55aE 12.3±1.06aDE 19.3±2.77abBC 23.5±2.42aA 2.23±0.19b

Spring corn

CK 13.0±1.47bC 14.8±2.59bBC 8.2±1.23aD 11.8±1.19abCD 17.7±2.92aB 34.5±2.30aA 1.77±0.017b

SM 19.9±1.36aB 17.8±0.90abBC 7.9±0.35aE 13.3±2.29aD 15.5±1.15abCD 25.6±1.48cA 2.39±0.12a

0-20 cm

PM 18.3±1.17aB 20.3±0.31aB 8.7±0.99aD 9.1±1.58bD 13.3±0.63bC 30.3±1.61bA 2.37±0.073a

CK 13.5±1.09bD 19.7±1.22aB 6.0±1.32aE 18.8±1.79aBC 15.7±1.58aCD 26.3±3.85bA 2.02±0.055b

SM 16.5±2.02aC 20.3±2.89aB 7.5±0.97aD 10.9±0.16bD 15.1±2.62aC 29.7±1.80bA 2.24±0.10a

20-40 cm

PM 12.9±0.042bC 14.5±0.78bC 8.3±1.62aD 12.9±0.46bC 16.7±1.75aB 34.7±1.23aA 1.80±0.046ca Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. b Mean weight diameter of aggregates.

c Numbers followed by different lowercase letters within a column (aggregate-size class) between mulching types in a crop are significantly

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different at P = 0.05 by the least square means test. d Numbers followed by different uppercase letters within a row (mulching type) between aggregate-size classes in a crop are significantly different at P = 0.05 by the least square means test.

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Table 2. Effect of straw and plastic film mulching on grain yield, total aboveground biomass, and estimated root residue returned to the soil from 2009 to 2014 in winter wheat and spring corn.

Winter wheat Spring corn

Mulching aGrain yield （Mg ha-1）

Total aboveground biomass (Mg ha-1)

Estimated root residue returned to the soil (Mg ha-1)b

Grain yield (Mg ha-1）

Total aboveground biomass (Mg ha-1)

Estimated root residue returned to the soil at 0-30 cm (Mg ha-1)b

CK 5.85a c 13.6c 4.49 7.70b 17.1b 5.99

SM 5.06b 15.7a 5.18 7.74b 18.6a 6.51

PM 5.14b 14.4b 4.75 9.86a 18.8a 6.58

Significance

Mulching (M) *** *** *** *** *** ***

Year (Y) *** *** *** *** *** ***

M×Y *** *** *** *** *** ***

*** Significant at P ≤ 0.001.a Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. b Estimated root residue was calculated as 33% of the aboveground biomass (grain + straw) in winter wheat and 35% in spring corn (Campbell

& de Jong 2001; Bolinder et al. 1999).. c Numbers followed by different lowercase letters within a column (aggregate-size class) between mulching types in a crop are significantly

different at P = 0.05 by the least square means test.

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Table 3. Correlation (r) between soil aggregation and the estimated root residue returned to the soil from winter wheat and corn (n=9).

Crop Soil layer Parameter Estimated root residue returned to the soil

Winter wheat 0-20 cm Macroaggregatea proportion 0.759*

MWDb 0.673*

20-40 cm Macroaggregate proportion -0.032

MWD 0.716*

Spring corn 0-20 cm Macroaggregate proportion 0.748*

MWD 0.883**

20-40 cm Macroaggregate proportion -0.588

MWD -0.134

* Significant at P ≤ 0.05.

** Significant at P ≤ 0.01.a Macroaggregate (5.00-10.00 mm size class).b Mean weight diameter of aggregates.

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Table 4. Soil organic carbon (SOC) concentration in aggregates (mean ± standard deviation) at 0-20 and 20-40 cm depths as affected by mulching in winter wheat and spring corn.

Crop Soil layer Mulchinga SOC concentration (g kg-1) in aggregates___________________________________________________________________________________________ 5.00-10.00 mm 2.00-5.00 mm 1.00-2.00 mm 0.50-1.00 mm 0.25-0.50 mm <0.25 mm

Winter wheat 0-20 cm CK 11.2±0.06 cbAc 10.8±0.08bAB 10.7±0.40bB 11.0±0.31bAB 10.6±0.05bB 9.65±0.03aC

SM 12.1±0.31aA 11.7±0.43aA 12.1±0.19aA 12.0±0.43aA 12.1±0.11aA 10.1±0.30aB

PM 10.2±0.18bB 11.1±0.06bA 11.4±0.79abA 11.1±0.15bA 10.9±0.09bA 9.79±0.25aB

Mean 11.2A 11.2A 11.4A 11.4A 11.2A 9.85B

20-40 cm CK 9.48±0.42abA 8.88±0.42bB 9.3±0.19aAB 9.60±0.18abA 9.0±0.30bAB 7.87±0.10aC

SM 9.76±0.30aA 9.68±0.08aA 9.41±0.25aA 9.72±0.22aA 9.57±0.28aA 8.14±0.29aB

PM 8.84±0.38bAB 8.31±0.46bBC 9.35±0.40aA 9.32±0.07bA 9.06±0.11bA 7.99±0.07aC

Mean 9.36AB 8.96C 9.38AB 9.55A 9.23B 8.00D

Spring corn 0-20 cm CK 10.7±0.17bAB 10.4±0.40bBC 11.2±0.54bA 11.1±0.28bA 10.6±0.19bAB 9.84±0.15abC

SM 11.6±0.25aBC 11.7±0.11aB 12.8±0.25aA 12.1±0.20aB 11.1±0.42aC 10.5±0.43aD

PM 10.9±0.05bA 10.9±0.06bAB 10.5±0.11cB 10.9±0.25bA 10.5±0.24bB 9.46±0.31bC

Mean 11.1A 11.0A 11.5A 11.4A 10.7AB 9.93B

20-40 cm CK 9.62±0.17aC 10.1±0.15aAB 9.74±0.38aBC 10.2±0.13aA 9.45±0.04aC 8.74±0.22aD

SM 9.76±0.28aBC 10.5±0.24aA 9.95±0.29aBC 10.2±0.18aAB 9.49±0.41aCD 9.09±0.12aD

PM 9.93±0.35aA 9.95±0.35aA 8.93±0.42bB 9.96±0.47aA 8.24±0.10bC 8.26±0.24bC

Mean 9.77B 10.2A 9.54C 10.1A 9.06D 8.70E

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a Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. b Numbers followed by different lowercase letters within a column (aggregate-size class) between mulching types in a crop are significantly

different at P = 0.05 by the least square means test. c Numbers followed by different uppercase letters within a row (mulching type) between aggregate-size classes in a crop are significantly different at P = 0.05 by the least square means test.

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Table 5. Soil total nitrogen (STN) concentration (g kg-1) (mean ± standard deviation) in aggregates at 0-20 and 20-40 cm depths as affected by mulching in winter wheat and spring corn

STN concentration in aggregates (g kg-1)Soil layer Mulchinga 5.00-10.00 mm 2.00-5.00 mm 1.00-2.00 mm 0.50-1.00 mm 0.25-0.50 mm <0.25 mm

Winter wheat

CK 1.30±0.013 bbAc 1.30±0.025bA 1.29±0.020cA 1.28±0.058bA 1.31±0.058bA 1.23±0.025bB

SM 1.45±0.033aA 1.45±0.030aA 1.44±0.022aA 1.43±0.027aA 1.43±0.023aA 1.31±0.049aB

PM 1.29±0.025bA 1.31±0.012bA 1.36±0.044bA 1.29±0.021bA 1.31±0.021bA 1.29±0.086abA

0-20 cm

Mean 1.35A 1.35A 1.36A 1.33A 1.35A 1.28B

CK 1.18±0.029bA 1.08±0.057bC 1.14±0.040aABC 1.12±0.023bABC 1.15±0.066bAB 1.10±0.044aBC

SM 1.31±0.052aA 1.18±0.046aB 1.17±0.038aB 1.20±0.017aB 1.20±0.032aB 1.09±0.031aC

PM 1.16±0.027bAB 1.08±0.040bCD 1.14±0.039aBC 1.15±0.038abAB 1.21±0.031aA 1.06±0.024aC

20-40 cm

Mean 1.22A 1.11C 1.15B 1.16B 1.19AB 1.08C

Spring corn

CK 1.34±0.018abAB 1.25±0.098bBC 1.37±0.032bA 1.31±0.005bABC 1.30±0.045bABC 1.21±0.068bC

SM 1.35±0.048aBC 1.39±0.033aABC 1.43±0.080aAB 1.46±0.023aA 1.41±0.070aABC 1.33±0.040aC

PM 1.28±0.023bA 1.33±0.048abA 1.32±0.053bA 1.31±0.045bA 1.30±0.022bA 1.18±0.078bB

0-20 cm

Mean 1.32AB 1.32AB 1.37A 1.36A 1.34A 1.24B

CK 1.18±0.058cC 1.18±0.026bC 1.26±0.038abB 1.34±0.015aA 1.26±0.078aB 1.19±0.015aBC

SM 1.28±0.058aA 1.33±0.029aA 1.32±0.062aA 1.33±0.121aA 1.26±0.049aA 1.25±0.047aA

20-40 cm

PM 1.22±0.021bA 1.20±0.045bA 1.19±0.032bAB 1.23±0.030aA 1.14±0.005bB 1.20±0.015aAB

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Mean 1.23B 1.24B 1.26AB 1.30A 1.22B 1.21Ba Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. b Numbers followed by different lowercase letters within a column (aggregate-size class) between mulching types in a crop are significantly

different at P = 0.05 by the least square means test. c Numbers followed by different uppercase letters within a row (mulching type) between aggregate-size classes in a crop are significantly different at P = 0.05 by the least square means test.

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Table 6. Percent contributions of bulk soil organic carbon (SOC) and total nitrogen (STN) in aggregates (mean ± standard deviation) at 0-20 and 20-40 cm soil depths in winter wheat and spring corn.

Percent contribution (%)Parameter Soil layer Mulchinga

5.00-10.00 mm 2.00-5.00 mm 1.00-2.00 mm 0.50-1.00 mm 0.25-0.50 mm <0.25 mmRecovery rate (%)

Winter wheat

CK 10.8±2.30 b b 13.7±1.79c 12.3±3.51a 15.6±1.71a 25.9±5.99a 23.6±0.63a 102.1±1.10b

SM 13.6±1.61ab 19.6±1.20b 11.3±3.09a 17.8±0.73a 17.0±2.99b 15.2±1.00b 94.5±2.31c

0-20 cm

PM 15.3±1.01a 23.5±2.39a 13.4±1.74a 17.8±0.29a 16.4±0.46b 20.3±2.62a 106.6±2.25a

CK 16.7±0.55b 19.3±1.72a 9.14±0.29a 15.0±4.38a 22.2±2.59a 18.3±3.47ab 100.7±3.92a

SM 20.2±1.10a 17.9±1.74a 8.85±1.33a 16.0±0.34a 15.5±1.93b 17.2±2.33b 95.7±3.93a

20-40 cm

PM 16.1±0.99b 18.8±2.46a 9.58±1.91a 13.0±1.70a 19.8±3.42ab 21.3±2.89a 98.5±2.37a

Spring corn

CK 13.3±1.45b 14.8±3.10b 8.81±1.44a 12.5±0.59ab 18.1±3.04a 32.6±3.21a 100.2±3.18a

SM 19.5±1.35a 17.7±0.72ab 8.69±0.45a 13.7±2.74a 14.6±0.88ab 22.7±1.60b 96.9±1.57a

0-20 cm

PM 18.6±1.07a 20.5±0.32a 8.56±1.10a 9.19±1.36b 13.0±0.44b 26.6±1.15b 96.4±0.97a

CK 13.5±1.56b 20.5±1.37a 6.05±1.25a 19.8±1.62a 15.3±1.14a 23.7±3.75b 98.8±1.64a

SM 16.6±1.59a 22.0±3.34a 7.70±0.84a 11.5±0.38c 14.7±2.44a 27.9±1.36ab 100.3±0.65a

SOC

20-40 cm

PM 14.2±0.56ab 16.0±0.42b 8.16±1.40a 14.2±0.52b 15.3±1.50a 31.7±1.19a 99.5±1.43a

STN Winter wheat

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0-20 cm CK 10.0±2.35b 13.0±1.53b 11.9±3.49a 14.5±1.23b 25.4±3.87a 23.9±0.67a 98.8±2.34a

SM 13.9±1.37a 20.5±1.22a 11.3±2.90a 18.1±1.45a 17.1±3.67b 16.8±1.53b 97.7±3.41a

PM 14.2±0.94a 20.5±2.19a 11.7±1.23a 15.2±0.39b 14.4±0.13b 19.8±3.71ab 95.8±3.57a

20-40 cm CK 16.2±0.55b 18.3±1.72a 8.72±0.29a 13.7±4.38a 22.1±2.59a 20.0±3.47a 99.1±3.10a

SM 20.1±1.10a 16.2±1.74a 8.22±1.33a 14.6±0.34a 14.3±1.93b 17.2±2.33a 90.5±3.79a

PM 16.1±0.99b 18.6±2.46a 8.89±1.91a 12.3±1.70a 20.2±3.42a 21.6±2.89a 97.6±6.28a

Spring corn

0-20 cm CK 13.9±1.91b 14.9±3.45b 8.98±1.36a 12.4±1.01a 18.6±3.90a 33.4±3.08a 102.2±1.51a

SM 18.6±2.43a 17.1±1.96ab 7.91±0.86a 13.4±2.14a 15.1±0.28ab 23.6±3.10b 95.6±6.70a

PM 17.9±0.50a 20.7±1.91a 8.87±1.48a 9.05±0.97b 13.3±1.43b 27.4±3.92ab 97.3±7.20a

20-40 cm CK 12.9±1.13b 18.7±1.02a 6.12±1.40a 20.2±1.65a 15.9±1.32a 25.2±3.81b 99.1±1.45a

SM 15.9±1.75a 20.4±2.70a 7.46±0.69a 10.9±0.96b 14.3±2.22a 28.1±2.10ab 97.2±1.07a

PM 12.4±0.22b 13.8±1.14b 7.77±1.36a 12.5±0.36b 15.1±1.41a 32.7±1.25a 94.2±0.60ba Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. b Numbers followed by different lowercase letters within a column (aggregate-size class) between mulching types in a crop are significantly

different at P = 0.05 by the least square means test.

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Figure Legend

Fig.1. Effect of mulching on soil organic C and total N concentrations (g kg-1) in the bulk soil at 0-20 and 20-40 cm depths in winter wheat and spring corn. Mulching treatments are CK, no mulching; PM, plastic film mulching; and SM, straw mulching. Data represent the average of three replicates and error bars represent standard deviations. Different lowercase letters above the bars indicate significant differences (P < 0.05) among mulching types at a soil depth.

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Fig.1.

183x163mm (300 x 300 DPI)

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