Effects of tillage on CO2 fluxes in a typical karst calcareous soil

Geoderma 337 (2019) 191–201

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Effects of tillage on CO2 fluxes in a typical karst calcareous soil a,b,c,1

Dan Xiao , Yingying Ye ⁎ Kelin Wanga,b,

a,b,c,1

, Shuangshuang Xiao

a,b,d

, Wei Zhang

a,b,⁎

T a,b

, Xunyang He

,

a

Key Laboratory of Agro-ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences, Huanjiang 547100, China University of Chinese Academy of Sciences, Beijing 100039, China d Key Laboratory of Environment Change and Resources Use in Beibu Gulf (Guangxi Teachers Education University), Ministry of Education, Nanning 530001, China b c

A R T I C LE I N FO

A B S T R A C T

Handling Editor: Junhong Bai

It is widely believed that soil disturbance by tillage was the primary cause of historical soil organic carbon (SOC) loss and high carbon dioxide (CO2) emission levels in the calcareous karst region. Nevertheless, the mechanisms underlying CO2 fluxes resulting from the soil property changes caused by tillage are poorly understood. A oneyear simulation experiment using different tillage frequencies was conducted to quantify the impacts of tillage and soil property changes on CO2 flux. Treatments included conservation tillage (T0), semiannual tillage (T1), tillage every four months (T2), bimonthly tillage (T3), and monthly tillage (T4). The effects of tillage on soil CO2 flux had a strong seasonal pattern. CO2 fluxes in higher tillage frequencies of T3 and T4 were significantly higher than those of other treatments in spring, summer, and autumn. For viable tillage management, CO2 fluxes in T1 and T2 were significantly higher than those in T0 in spring and autumn. No significant differences in CO2 flux were found among treatments in winter. Tillage also had a significant influence on soil biogeochemical properties. Aggregate stability, dissolved organic carbon (DOC), and microbial biomass carbon (MBC) significantly decreased in T2, T3, and T4, whereas SOC significantly decreased just in T3 and T4 after 1 year. A structural equation model analysis showed that the annual cumulative soil CO2 flux was directly affected by annual changes in SOC (ΔSOC), DOC (ΔDOC), and MBC (ΔMBC). Tillage frequency directly influenced annual changes in large macroaggregates (ΔAG) and ΔMBC. These results indicated that tillage practice indirectly lowered SOC by reducing large macroaggregates and microbial biomass, which in turn, enhanced CO2 flux. Our results suggested that tillage disturbance in the karst soil significantly increased SOC loss through enhanced CO2 flux compared with that in the non-karst soil in a similar climate. In contrast, reducing or eliminating tillage in the wet-hot season could lower CO2 flux rate by minimizing large macroaggregate disturbance and, by extension, microbial access to mobile carbon sources.

Keywords: Karst soils Tillage frequency CO2 fluxes Soil properties

1. Introduction Soil respiration is a major CO2 emission source and adds 100 Pg C yr−1 to the atmosphere; thus, there is increasing interest in suppressing CO2 emission in order to minimize the potential impacts on the global climate (Liu et al., 2015a). Changes in land use, land cover, and agricultural practices contribute to about 20% of the global annual CO2 emission (IPCC, 2001), and intensive efforts have been made to determine the drivers of soil respiration (Reynolds et al., 2015). Soil tillage is a key determinant of soil structure quality and aggregate stability and has altered soils significantly by decreasing the amount of

organic matter (Wick et al., 2016). When soil is subjected to tillage disturbance, CO2 emission into the atmosphere is affected (Aslam et al., 2000; Chatskikh and Olesen, 2007). Several studies have observed greater CO2 flux under conventional tillage compared with conservation agricultural practices, such as no tillage, due to the stimulatory effects of tillage on soil structural properties (Alvarez et al., 2001; Dao, 1998; Rochette and Angers, 1999). A shift from no tillage to conventional tillage management can release soil-bound carbon into the atmosphere as CO2 (Fuß et al., 2011). Tillage may disrupt aggregates and make occluded particulate organic material available to microorganisms. SOC is less protected by macroaggregates in conventional tillage

⁎

Corresponding authors at: Key Laboratory of Agro-ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China. E-mail addresses: [email protected] (W. Zhang), [email protected] (K. Wang). 1 These authors contributed equally to the manuscript. https://doi.org/10.1016/j.geoderma.2018.09.024 Received 10 October 2017; Received in revised form 11 September 2018; Accepted 12 September 2018 0016-7061/ © 2018 Published by Elsevier B.V.

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disturbance, low nutrient levels, and lack of self-adjustment (Liu et al., 2015b). In the past few decades, severe anthropogenic disturbances and cultivation have seriously damaged large portions of the karst region in southwest China (Wen et al., 2016). Soil physicochemical properties were significantly altered by tillage (Zhang et al., 2007). Chen et al. (2012) found that SOC was higher in karst than non-karst ecosystems but that cultivation-induced SOC losses occurred more rapidly in calcareous soils than in other types of soils. Soil organic matter decreased by 19–42% in the first 2 years after tillage (Zhang et al., 2013). Increasing tillage intensity and frequency may improve crop yield, alter soil properties, and increase CO2 flux (Sainju et al., 2006). Nevertheless, the effects of tillage practices on soil properties and CO2 fluxes have seldom been comprehensively investigated (Li et al., 2013). We hypothesized that the effects of tillage on CO2 fluxes in a typical karst calcareous soil with high SOC content, clay, and Ca2+ were different from those of other soils with low nutrients, clay, and Ca2+. Compared with no tillage, tilling at various frequencies was expected to easily induce CO2 emissions due to disruption of the physical protection of SOM by occlusion in aggregates, along with changes soil properties, such as SOC, DOC, and MBC, which would then lead to reduce carbon storage. A simulation experiment was conducted in which various tillage frequencies were tested in a typical karst area to quantify their effects on soil properties and CO2 flux. The objectives of this study were to (1) investigate seasonal and annual CO2 fluxes under different tillage frequencies; (2) determine the effects of tillage frequency on soil properties; and (3) demonstrate the relationships among tillage frequency, aggregate stability, ΔMBC, ΔSOC, ΔDOC, and annual cumulative CO2 flux.

than in soils without tillage (Bossuyt et al., 2002; Oorts et al., 2006). Tillage can also degrade soil structure and alter soil temperature and water content, both of which are closely associated with CO2 flux (Liang et al., 2007). Furthermore, tillage increases soil pore volume and surface roughness, accelerating the oxidation of soil organic matter and driving up the rates of soil respiration and CO2 flux (Kainiemi et al., 2016; Usowicz et al., 2013). The magnitude of soil CO2 loss due to tillage is strongly correlated with the frequency and intensity of soil disturbance (Al-Kaisi and Yin, 2005). Unlike conventional methods, long-term lack of tillage or reduced tillage systems decreases soil disturbance and CO2 fluxes, thereby increasing soil C storage (Al-Kaisi and Yin, 2005; Alvarez et al., 2001). CO2 evolution from soils is controlled by both biotic and abiotic factors, all of which may be altered by tillage practices (Ussiri and Lal, 2009). Biotic factors include microbial type, mass, and activity. Abiotic factors consist of soil and air temperatures, moisture levels, quantity and quality of soil organic matter (SOM), the CO2 concentration gradient between soil and atmosphere, pore size distribution, and nutrient availability (Jarecki and Lal, 2006; Liang et al., 2016; Templer et al., 2012). MBC responds quickly to soil management changes and has an important impact on soil CO2 production. It is a sensitive indicator of organic matter content and, by extension, soil quality (Powlson et al., 1987; Wardle et al., 1999). MBC is also closely correlated with SOM and aggregates that stimulate organic carbon turnover in soils (Han et al., 2010; Nyamadzawo et al., 2009). Soil microbial biomass increases with organic matter content (Patino-Zuniga et al., 2009). Tillage changes the microbial community structure by reducing microbial niches and water retention or by increasing physical soil disturbance (Madejon et al., 2007). Moreover, higher CO2 fluxes under tillage may be due to metabolic and compositional shifts in the microbial community and the decline in soil carbon (Jacinthe and Lal, 2005). Consequently, detailed knowledge of CO2 fluxes affected by biotic and abiotic factors is crucial. Tillage can also profoundly affect soil structural properties, including aggregation (Paustian et al., 2000). Tillage breaks up large aggregates and generates microaggregates with larger surface areas and the labile organic matter associated with them. These microaggregates are readily accessible to microbial attack (Iqbal et al., 2010), which causes rapid mineralization, a decline in organic matter, and an increase in CO2 flux (Sainju et al., 2006). DOC is used as an indicator of carbon availability to soil microorganisms (Boyer and Groffman, 1996). It is an active and labile component of SOC (Burford and Bremner, 1975). The microbial community only has access to soluble carbon. Therefore, it is the concentration, composition and supply rate of carbon substrates to the soil solution that determine soil respiration (van Hees et al., 2005). Organic matter decomposition leads to soil carbon loss in the form of CO2 emissions (Mancinelli et al., 2010). Many field experiments have shown that the CO2 fluxes are significantly and positively correlated with soil organic carbon (Liu et al., 2014), soil temperature, and moisture level (Cartwright and Hui, 2014; Wan et al., 2007). Chatskikh and Olesen (2007) reported that the highest CO2 fluxes are observed during periods with increasing temperature and relatively high soil water content. Soil moisture directly affects microbial activities and indirectly influences soil physicochemical properties (Raich and Schlesinger, 1992). The magnitude of soil disturbance causes a rapid loss of soil carbon pool, leading to low soil biological activity and affecting aggregates (Blanco-Canqui and Lal, 2004). Therefore, changing the disturbance intensity caused by tillage modifies soil CO2 emissions (Sanchez et al., 2003). Many studies have reported the CO2 emission rates associated with different tillage practices in different regions (Alvaro-Fuentes et al., 2007; Lopez-Garrido et al., 2009); however, few studies have described this in karst ecosystems. Karst is widespread globally and covers about 15% of Earth's land surface. One of the world's largest karst regions is located in the southwest China, it occupies ~540,000 km2 (Pan et al., 2016; Qi et al., 2013). This region is characterized by an extremely fragile geological background, reduced environmental capacity, reduced resistance to

2. Materials and methods 2.1. Experimental description A field experiment was set up in the Huanjiang Observation and Research Station for Karst Ecosystems (107°51′–108°43′E, 24°44′–25°33′N), Chinese Academy of Sciences (CAS), Guangxi Province, China. The local climate is subtropical monsoon with an average annual temperature of 18.5 °C and a mean annual precipitation of ~1380 mm. Rainfall occurred mainly from April to September (~1100 mm). The climate was relatively dry from October to March (~230 mm). The soil in this region was calcareous, developed from a dolostone base, and was characterized by high clay and Ca2+. Soil texture was clay-loam (clay: 30.9%, silt: 41.5%, sand: 27.6%) (Yang et al., 2016). Total calcium in the karst soil (7.13 g kg−1) was significantly higher than in red soil (0.2 g kg−1), which was found in the same experimental area (Hu et al., 2012). Shrubs were the primary vegetation before the tillage experiment. This area experienced severe deforestation from 1958 to the mid-1980s and has been under natural restoration for almost 30 years (Yang et al., 2016). In January 2014, twenty 2 × 2 m plots were set up at the experimental site. The experiment was conducted using a completely randomized block design with four replicates per treatment. To simulate the cultivation environment without planting corn (which would remove nutrients from and add litter to the soil), we used plastic plants to mimic corn crown shade and prevent raindrop splash erosion. In order to simulate the real cover environment, the artificial plants were placed in the field for double planting season according to the real cropping time. The first round of cropping was from February to May, and the second round of cropping was from June to September. There were two, four, and six leaves placed in the artificial plant in the jointing stage, bell stage and milk stage, respectively. Each artificial plant was ~1.5 m high and its crown was ~1 m2. Each plot had four plants. All plots were surrounded by polyvinyl chloride (PVC) boards to prevent soil water and nutrients from being transferred among plots. Each PVC board was 2 m long and 0.5 m wide. About 0.35 m of each board was inserted into the soil, leaving 0.15 m protruding above the ground. Starting in June 192

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Table 1 Selected properties of the experimental soil. pH

6.40 ± 0.08

SOC (mg g−1)

37.99 ± 4.00

TN (g kg−1)

2.71 ± 0.28

DOC (mg kg−1)

105.57 ± 7.86

Ca2+ (g kg−1)

MBC (mg kg−1)

558.36 ± 160.72

2.97 ± 0.26

Aggregate size (%) > 1 mm

0.25–1 mm

< 0.25 mm

91.60 ± 0.61

7.34 ± 0.27

1.06 ± 0.35

SOC, TN, DOC, MBC, and Ca2+ indicate soil organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon, and exchangeable calcium, respectively.

method was used to measure soil MBC (Dong et al., 2009; Vance et al., 1987). A 100-g soil sample was taken from each plot and divided into two equal parts to be used as a control and a fumigated subsample. After 7 days of incubation at 25 °C in a vacuum drier, each 50-g subsample was placed in an 80-mL beaker. The subsample to be treated was fumigated with ethanol-free chloroform. Each subsample was then subjected to extraction with 100 mL 0.5 M K2SO4, shaken for 30 min, filtered, cooled to −18 °C, and analyzed. Extractable DOC in the K2SO4 suspensions was measured using a total carbon analyzer (Model TOC500; Shimadzu Corp., Kyoto, Japan). SOC was determined by the Walkley-Black wet-chemical oxidation method (Nelson and Sommers, 1996). Total nitrogen (TN) was determined with an Element AutoAnalyzer (Vario MAX CN; Elementar, Hanau, Germany). Exchangeable calcium (Ca2+) was displaced via compulsive exchange in 1 mol L−1 ammonium acetate at pH 7.0 and analyzed using an Agilent 720 ICPOES (Agilent, Santa Clara, CA, USA) (Hendershot et al., 2006). The undisturbed soil sample was gently broken up, passed through an 8-mm sieve and air-dried. Samples weighing ~500 g were separated into three aggregate sizes (> 1 mm [large macroaggregate], 0.25–1 mm [small macroaggregate], and < 0.25 mm [microaggregate]) by sieving through a sieve nest (1 and 0.25 mm) over a dry-sieving instrument and mechanically shaken (amplitude = 1.5 mm) for 2 min. The aggregates were then collected from each sieve (Dorodnikov et al., 2009; Nie et al., 2014).

2014, each of the plots received one of the following treatments: (1) conservation tillage (T0); (2) semiannual tillage (T1); (3) tillage every four months (T2); (4) bimonthly tillage (T3); or monthly tillage (T4). The depth of tillage was 15 cm with chisels by hand. Soil samples were collected in June 2014, December 2014, and June 2015. One undisturbed soil sample was taken in separate pits (15 cm height × 20 cm length × 20 cm width) after the residue was removed. According to the “S” type, surface soils (0–10 cm in depth) were collected from five random locations within the plot and blended to make a uniform composite soil. Visible plant residues and roots were manually removed from the samples. In the laboratory, the surface soil samples were sieved through a 2-mm mesh then divided into two lots. One of them was stored at 4 °C for MBC and DOC analyses, and the other was kept dry at room temperature and used for chemical tests (Table 1). 2.2. Temperature and soil moisture measurements Soil temperature at 5 cm depth was measured using a digital thermometer (iButton Model DS1923; Maxim Integrated, Shanghai, China). Moisture content at 10 cm depth was determined by collecting soil samples monthly before tillage at 0–10 cm depth using a stainless steel corer 5 cm in diameter. Five cores were randomly taken from each plot and blended to produce a uniform composite soil. Soil moisture (g of water per 100 g dry soil) was measured by oven-drying the soil for 12 h at 105 °C (Liu et al., 2014). Daily precipitation and air temperature were obtained from a weather station at the Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences.

2.5. Statistical analyses The gas flux was estimated using the following equation from (Liu et al., 2014):

2.3. CO2 flux measurements and calculations

F = ρ·

Between August 2014 and July 2015, gas samples were collected five times per month at two-day intervals during the first week and once at the end of the month after tillage (1st, 3rd, 5th, 7th, and 30th day) between 9:00 a.m. and 10:00 a.m. Diurnal studies have demonstrated that greenhouse gas flux measured at this time of day is representative of daily average flux values (Cosentino et al., 2012). A static chamber was installed on each plot at the start of the experiment (June 2014) and consisted of a ring 25 cm in diameter permanently embedded in the soil at a depth of 5 cm. During flux measurements, a chamber top 35 cm in height was attached to the ring. When the lid was set on the base, the groove was filled with water to a depth of 5 cm, creating an airtight seal. Before each sampling, a 100-mL syringe was flushed 3–4 times with chamber gas to mix the headspace (Zhang et al., 2012). Gas samples were collected from the center of each plot with a 60-mL gastight syringe 0, 15, 30, 45, and 60 min after chamber closure. The samples were analyzed for CO2 within 24 h using gas chromatography (GC) (Agilent 7890A; Agilent Co., Santa Clara, CA, USA). Gas flux was calculated from the linear regression of concentration versus time. All coefficients of determination (R2) of the linear regression were > 0.98 (p < 0.001).

V ∆c 273 · · A ∆t 273 + T

where F is the CO2 flux (mg m−2 h−1), ρ is the gas density of CO2 (ρ = P / RT [g m−3]), P is the air pressure, T is the air temperature inside the chamber (°C), R is the gas constant, V (m3) and A (m2) are the volume and bottom surface area of the sampling chamber, respectively, and Δc/Δt is the change in gas concentration (c) inside the chamber per unit time (t) during the sampling period (m3 m−3 h−1). Cumulative soil CO2 flux was determined from the accumulated emission rates between every two consecutive measurement days as follows (Li et al., 2013):

CE =

n

∑i =1 ⎡ ⎣

Fi + Fi + 1 ⎤·t·24·10−2 2 ⎦

where CE is the monthly cumulative soil CO2 flux (kg ha−2 mo−1), Fi and Fi+1 are the measured fluxes of two consecutive sampling days (mg m−2 h−1), t is the number of days between two adjacent measurements, and n is the total number of measurements performed. All statistical analyses were performed using SPSS v. 18.0. Repeated-measures ANOVA was performed on the soil CO2 flux, temperature, and moisture level among the treatments between August 2014 and July 2015. The effects of soil temperature (ST) and soil moisture (SM) on CO2 flux were evaluated by linear regression models. Multiple regression equations were used to relate CO2 flux to both soil temperature and soil moisture (Ranucci et al., 2010). A structural equation modeling (SEM) framework was applied to investigate the

2.4. Measurement of soil properties Soil pH (1:2.5 soil/water ratio) was measured using a Metter-Toledo 320 pH meter (Liu, 1996). The chloroform fumigation-extraction 193

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lower and statistically similar among the five treatments during winter. In contrast, by summertime, they were significantly lower under T0 than under T3 and T4. In July, T4 emitted more CO2 than T0, with one large peak occurring early in the month. By July, T4 had a maximum flux rate of 166.41 mg m−2 h−1, whereas the T0 flux was only 122.41 mg m−2 h−1 (Fig. 3A). The cumulative CO2 fluxes from August 2014 to July 2015 were 7.02, 7.56, 7.59, 8.68, and 9.15 Mg ha−1 yr−1 for T0, T1, T2, T3, and T4, respectively. The cumulative CO2 fluxes were highest in the summertime, but no significant differences were observed in the winter. The cumulative CO2 fluxes in the high tillage disturbance frequencies of T3 and T4 were significantly higher than other tillage frequencies in the spring, summer and autumn. For viable tillage management, T1 and T2 were significantly higher than T0 in spring and autumn (Fig. 3B).

direct and indirect effects of tillage times and annual changes in large macroaggregates (> 1 mm; ΔAG), SOC (ΔSOC), DOC (ΔDOC), and MBC (ΔMBC) on annual cumulative CO2 flux. The values of ΔAG, ΔSOC, ΔDOC and ΔMBC were the reduction of before tillage compared to after 1 year of tillage. p-values, x2, goodness-of-fit indices (GFIs), and root mean square errors of approximation (RMSEA) were used to evaluate the structural equation model fit (Hu et al., 2014). The SEM was run with Amos v. 17.0 (Smallwaters Corp., Chicago, IL, USA). To ensure the data met the assumptions of normality in the SEM, tillage times, ΔAG, ΔSOC, ΔDOC, and ΔMBC were subjected to standardized transformation. The least significant difference between any two means was calculated using Student's t-tests, and differences with p values of < 0.05 were considered significant. 3. Results

3.3.3. Driving factors for CO2 fluxes Linear regression of the data set across all treatments over time showed that CO2 flux was positively and linearly correlated with soil temperature (p < 0.05) and moisture level (p < 0.05). Adjusted R2 values indicated that the soil temperature and moisture level explained up to 71.89% and 61.24% of the variations in CO2 fluxes, respectively (Fig. 4). Soil temperature, moisture, and their interaction significantly influenced CO2 fluxes (Table 4). Moreover, CO2 flux was significantly affected by tillage frequency (p < 0.001), month (p < 0.001), and the interactions between these two factors (p < 0.01) (Table 2). SEM analysis generated a model which fit our hypothesis reasonably well (x2 = 6.02, df = 6, P = 0.42, GFI = 0.91, RMSEA < 0.01) and explained 92.0% of the total variance in the annual cumulative CO2 fluxes (Fig. 5). Details of the path coefficient x for the causal models are displayed in Table 3. The model indicated that tillage times were positively correlated with ΔAG and ΔMBC, and ΔAG and ΔMBC were positively correlated with ΔDOC and ΔSOC. Cumulative CO2 fluxes were directly affected by ΔMBC, ΔSOC, and ΔDOC. ΔSOC made the largest direct contribution to the annual cumulative CO2 fluxes. Tillage times had the largest indirect effect on annual cumulative CO2 fluxes.

3.1. Temperature and soil moisture Annual during sampling year precipitation was 1119.9 mm, similar to the average annual rainfall (1380 mm). Precipitation patterns showed very strong seasonality with 78% of rainfall occurring during the rainy season (May to September). The mean annual air temperature was 20 °C and the monthly average temperatures ranged from 11 °C (January 2015) to 27 °C (June 2015) (Fig. 1A). Soil temperatures at 5 cm and moisture levels at 0–10 cm exhibited clear seasonal patterns in all plots (Fig. 1B, C). The soil was comparatively warmer and wetter between April and September, and cooler and dryer between December and February. The mean soil temperature between August 2014 and July 2015 was 20.53 ± 0.23 °C (Fig. 1B). The mean soil moisture content was 27.57 ± 0.37% across all plots. Soil moistures were fairly higher during the rainy season than those during the dry season (Fig. 1C). 3.2. Tillage effect on soil physicochemical properties Soil MBC showed clear seasonal variations. It was lower in the cold and dry months (e.g., December) than in hot and humid month (e.g., June) for all tillage treatments. MBC did not significantly differ among treatments in winter (Fig. 2B). MBC was significantly affected by tillage frequency, decreasing with increasing disturbance. After 1 year, MBC decreased significantly in T2, T3, and T4; however there were no significant changes in MBC change T0 or T1 (Fig. 2B). Frequent tillage in T2, T3, and T4 significantly decreased large macroaggregates and increased small macroaggregates relative to T0 (Fig. 2A). SOC significantly decreased by 9.26 and 10.84 g kg−1 in T3 and T4, respectively. In contrast, SOC did not significantly change in T0, T1, or T2 (Fig. 2C). The seasonal pattern of DOC resembled that of MBC. DOC was lowest in winter (December 2014), and there were no significant differences among treatments. In T2, T3, and T4, DOC decreased significantly, declining by 25.06, 34.04, and 32.49 mg kg−1, respectively (Fig. 2D).

4. Discussion 4.1. Effects of tillage frequency on CO2 fluxes Tillage affects CO2 flux by altering several biological and physicochemical processes and the interactions among them (Oorts et al., 2007). CO2 fluxes are indicators of the effect of tillage frequency on the soil ecosystem and are closely correlated with microbial turnover and the accessibility of organic matter to microbes and extracellular enzymes (Paustian et al., 2000). In the present study, the annual cumulative CO2 fluxes ranked in the following order with T4 > T3 > T2 > T1 > T0 (Fig. 3B). This result suggested the occurrence of higher CO2 fluxes along with increasing tillage disturbance frequency. In the present study, T0, T1, and T2 were common tillage managements in real field conditions, and the annual cumulative CO2 fluxes were significantly higher in T1 and T2 than in T0 (Fig. 3B). Similar results were found by Bauer et al. (2006), who showed that conventional tillage significantly increased CO2 emissions compared with no tillage management. Tillage loosens the soil and enhances surface soil macroporosity (Salinas-Garcia et al., 1997), which would accelerate microbial activity, cause a rapid loss of organic matter, and enhance CO2 flux (Bayer et al., 2001; Regina and Alakukku, 2010). Moreover, tillage disturbance during warm, wet field conditions can influence soil structure (Rubol et al., 2012), and, by extension, most production and transport processes, including diffusion rates (Petersen et al., 2005). Soil emission rates can vary with tillage frequency because tillage directly affects diffusion and indirectly influences soil pore oxygen content (Liptzin et al., 2011; Rubol et al., 2013). Conservation tillage with no tillage or reduced tillage frequency benefits organic matter accumulation by reducing damage to the soil structure (Pinheiro et al.,

3.3. CO2 fluxes 3.3.1. Seasonal patterns of CO2 flux The highest CO2 fluxes occurred in the hot and wet season (May to August), and the lowest occurred in winter (December to February) (Figs. 1, 3A). Regardless of tillage management, > 39.02% of the total cumulative CO2 flux occurred during summer. The seasonal order of CO2 flux magnitudes was as follows: summer > spring > autumn > winter (Fig. 3B). 3.3.2. Tillage frequency effects on CO2 fluxes Average CO2 fluxes were 80.41 ± 3.39, 87.64 ± 3.27, 87.59 ± 4.50, 99.59 ± 2.95, and 105.96 ± 1.76 mg m−2 h−1 for T0, T1, T2, T3, and T4, respectively (Fig. 3A). CO2 fluxes were generally 194

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Fig. 1. Seasonal daily air temperature and precipitation patterns (A), soil temperature at 5 cm depth (B), and soil moisture content at 0–10 cm depth (C) measured in the different tillage treatments from August 2014 to July 2015. T0, T1, T2, T3 and T4 represent no tillage, semiannual tillage, tillage every four months, bimonthly tillage, and monthly tillage, respectively. Bars show one standard error of the mean (n = 4).

2004). Soil CO2 fluxes can be reduced by using reasonable tillage frequencies which optimize water and nutrient utilization while favoring net biomass production. In this way, the carbon balance is shifted

towards accumulation in the soil (Paustian et al., 2000). For viable tillage management, the annual cumulative CO2 fluxes in T0, T1, and T2 were 7.02, 7.56, and 7.59 Mg ha−1 yr−1, respectively 195

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Fig. 2. Changes in aggregate size from June 2014 to June 2015 (A), MBC (B), SOC (C), and DOC (D) were sampled in June 2014, December 2014, and June 2015. T0, T1, T2, T3, and T4 represent no tillage, semiannual tillage, tillage every four months, bimonthly tillage, and monthly tillage, respectively. We separated soil into three fractions (> 1 mm, large macroaggregate; 0.25–1 mm, small macroaggregate; < 0.25 mm, microaggregate). Different uppercase letters in Fig. 2B, C, and D indicate significant differences between treatments at the same sampling time, and differences between treatments at the same aggregate size in Fig. 2A (p < 0.05); different lowercase letters indicate significant differences at different sampling time for the same treatment (p < 0.05). Bars show one standard error of the mean (n = 4).

and no tillage in sandy soil, possibly due to the low clay content, which is essential for the physical protection of SOC by mineral association (Feller and Beare, 1997) and aggregate formation (Six et al., 2002a). These results suggested that karst soils were sensitive to tillage disturbance, and that lack of tillage or reduced tillage disturbance may be used to maintain a relatively good physical structure and high organic matter levels, thereby limiting CO2 fluxes and increasing carbon storage in the calcareous soils of the karst region (Roldan et al., 2005).

(Fig. 3B). Ussiri and Lal (2009) reported that annual CO2 emissions calculated in the conventional tillage were 6.2–6.6 Mg ha−1 yr−1 in Crosby silt-loam soil. Omonode et al. (2007) also reported higher CO2 emissions under conventional tillage (5.9–6.2 Mg ha−1 yr−1) than that without tillage (5.7 Mg ha−1 yr−1) from Indiana with Chalmers silty clay loam soils. These results suggested that CO2 emissions were significantly higher in karst soils than in other soils under similar tillage management. The calcareous soil in the karst region has a higher calcium and clay content than that in the non-karst region, and the microaggregate SOM can be immobilized when Ca2+ acts as a cationic bridge (Briedis et al., 2012). Ca2+ ions bind with radicals in organic matter, covering it with a surface layer of CaCO3 and inhibiting its decomposition (Chouliaras and Jacquin, 1976). The surface layer of CaCO3 deposited in organic matter was disrupted when the soil was subjected to tillage disturbance, thereby enhancing SOC release from aggregates and increasing CO2 emission (Chouliaras and Jacquin, 1976). In addition, calcareous soil has a greater clay component than other soils because its parent material is carbonate rock, which is precipitated in a clean sea environment (Cao et al., 2003). The high clay content of soil provides potential for the stabilization of SOC by association of organic materials with clay minerals and the formation and stabilization of organic materials within aggregates (Six et al., 2002b). The stabilization of SOC is reduced within aggregates and clay-protected C is lost after tillage disturbance, thereby increasing CO2 emissions (Chivenge et al., 2007). However, Chivenge et al. (2007) also found that there were no differences in SOC between tillage disturbance

4.2. Temporal variations in CO2 fluxes In the present study, the CO2 fluxes for all treatments followed a similar seasonal pattern. CO2 fluxes in the hot-wet season (May to September) were significantly higher than in the cold-dry season (November to March) (Figs. 1, 3A). This may be due to the observation that the hot-wet environment benefits microbial decomposition of organic carbon, which would enhance CO2 flux (Al-Kaisi and Yin, 2005). Rewetting of relatively dry soil due to rainfall increased CO2 flux by increasing microbial activities and C mineralization (Morell et al., 2010). Tillage frequency and season (month) had a significant interaction effect on CO2 flux (Table 2). However, there was no significant difference between different tillage frequency and CO2 flux in winter. Vanhala et al. (2007) found that the carbon mineralization rate depended on the amount and type of available organic C and on soil characteristics. Therefore, CO2 fluxes were lowest in wintertime when reduced soil temperature and moisture levels were not conducive to 196

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Fig. 4. Relationships between soil CO2 fluxes, soil temperature (5 cm depth), and soil moisture content (10 cm depth) for all treatments (A, B) from August 2014 to July 2015. Gas samples, soil temperature, and soil moisture were obtained simultaneously (n = 240).

Fig. 3. Seasonal variations in CO2 emission rate (A) and annual cumulative CO2 fluxes (B) for the different tillage treatments during the experimental period. T0, T1, T2, T3, and T4 represent no tillage, semiannual tillage, tillage every four months, bimonthly tillage, and monthly tillage, respectively. Asterisk (*) indicates significant differences in the different tillage treatments at the same time period (p < 0.05). Letters accompanying each bar represent significant differences between the different treatments at the same season and annual cumulative CO2 fluxes (p < 0.05). Error bars represent one standard deviation (n = 4).

Table 2 Responses of CO2 flux to tillage frequency, sample time, and their interaction in two-way ANOVA. Parameters

CO2 fluxes DF

Tillage frequency Month Tillage frequency × month

microbial activity. At this time, most microorganisms are dormant or even non-viable (Wang et al., 2003). In the present study, MBC and DOC were significantly lower in winter than summer and were not significantly different among treatments. The soil temperature is an important factor regulating seasonal variations in MBC, which, in turn, controls soil CO2 emissions (Iqbal et al., 2010). Moreover, the strong positive correlations of soil temperature and moisture with CO2 flux may explain the significant influences of these parameters on CO2 flux (Zhang et al., 2008). Multiple regression equations suggested that soil temperature, moisture, and their interaction had significant effects on soil CO2 flux and that soil temperature had greater effects than soil moisture in the total tillage treatments (Table 4). Soil temperature is the primarily factor governing seasonal variations in soil CO2 emission by affecting soil enzymes and activities of microorganisms; however, these effects dependent on suitable moisture content (Cartwright and Hui, 2014; Ussiri and Lal, 2009). It suggested that the very high variability of CO2 fluxes at high soil temperature was probably due to the variability of soil moisture during the hot-wet season. On the contrary, the lower variability of CO2 fluxes at low soil temperature was due to the lower moisture levels of the dry-cold season. In contrast, highest CO2 flux was found in summer. CO2 flux in

⁎ ⁎⁎

⁎

4 11 44

F

p ⁎⁎

79.12 655.52⁎⁎ 2.77⁎⁎

< 0.001 < 0.001 < 0.001

p < 0.05. p < 0.01.

higher tillage frequency with T3 and T4 was significantly higher than others in rainy season. However, conventional viable tillage disturbance with T1 and T2 results in slightly higher CO2 flux than that of conservation tillage with T0 in summer (Fig. 3B). Mueller et al. (2003) reported that soil water content for optimum workability/tillage in clay and loam soils ranged from 20% to 26%. In the present study, soil moisture was above the workable range during the rainy season. Wet soil will result in lower strength of the aggregates, and more soil failure will be initiated (Cadena-Zapata et al., 2002). Thus, increasing the tillage intensity may result in more soil shattering, which could increase oxygen diffusion rates, microbial activity, and organic matter decomposition rates (Davidson and Janssens, 2006; Skopp et al., 1990). In contrast, when the soil is drier, the aggregates are stronger, and it is difficult to reduce their size (Cadena-Zapata et al., 2002). This implies that the effects of T3 and T4 on CO2 fluxes are due not only to the 197

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Fig. 5. Structural equation model (SEM) showing the direct- and indirect effects of tillage times, annual changes in large macroaggregates, SOC, DOC, and MBC upon annual cumulative CO2 fluxes after 1 year of tillage. The width of the arrows indicates the strength of the standardized path coefficient. The solid lines indicate positive path coefficients, the dashed lines indicate negative path coefficients, and the R2 values represent the proportion of the variance explained for each endogenous variable (n = 20).

significantly (Fig. 2A), whereas ΔAG directly affected ΔDOC and ΔMBC (Fig. 5). Large macroaggregates were broken up by tillage disturbance (Cates et al., 2016), because the transient binding agents they contain, such as mycelia, are sensitive to mechanical perturbations (Wang et al., 2016). A higher tillage frequency produces microaggregates that have higher surface areas than large macroaggregates and their available SOM is more susceptible to microbial attack, which can then metabolize them and indirectly affect CO2 fluxes (Christensen and Sorensen, 1985; Curtin et al., 2000; Six et al., 2000). More specifically, tillage disturbance causes a loss of C-rich macroaggregates and an increase of Cdepleted microaggregates (Six et al., 2000). Six et al. (2002a) found that C stabilization is greater within free microaggregates than within macroaggregates, suggesting that tillage-disrupted macroaggregates have a negative effect on the accumulation of SOC. For viable tillage management, MBC and DOC decreased significantly in T2, but there was no significant difference between T0 and T1 after 1 year of tillage (Fig. 2B, D). These results may be explained by the observation that tillage disturbance contributes to the disruption of a suitable environment and significantly decreases soil MBC (Pabst et al., 2013). Frequent tillage generally results in a decrease in MBC (Alvarez and Alvarez, 2000). Xiao et al. (2017) found that MBC is significantly decreased when natural soil systems are disturbed in karst soil. DOC has been used as an indicator of the availability of carbon to soil microorganisms and could be considered as an early indicator for SOC changes under different soil management strategies in the short-term (Boyer and Groffman, 1996; Laik et al., 2009). All dissolved substances are assumed to be labile and rapidly utilized, although their consumption rates depend on changes in SOC (Burford and Bremner, 1975). Our results supported previous findings that higher tillage disturbance gradients induced significant decreases in the DOC compared with lack of tillage and that this labile organic matter was more easily affected by tillage disturbance than SOC (Cookson et al., 2008; Malhi et al., 2006). Above all, conventional tillage of T2 eventually reduced large macroaggregates, MBC, and DOC compared with less disturbance (e.g., in T0 and T1), which would increase CO2 emissions in the karst soil. Similarly, Nyamadzawo et al. (2009) found that conventional tillage significantly crushed macroaggregates compared with no tillage in the Alfisols, and Liu et al. (2014) found that conventional tillage was also likely to decrease MBC and DOC in silt loam soil. These findings suggested that aggregates, MBC, and DOC, whether in karst soil or other soils, are sensitive to tillage disturbance. Overall, tillage disturbance enhanced CO2 emissions through directly decreasing large macroaggregates, thereby reducing organic matter and microbial biomass. In the present study, ΔSOC was identified as the main direct contributor to annual cumulative CO2 fluxes, whereas tillage frequency had the most important indirect effect on this parameter. Higher CO2 fluxes

Table 3 Direct, indirect, and total effects of environmental factors on annual cumulative CO2 fluxes determined using a structural equation model (SEM). Observed outcomes (λ)

CO2 fluxes

Direct Indirect Total

TT

ΔAG

ΔSOC

ΔMBC

ΔDOC

0 0.88 0.88

0 0.41 0.41

0.47 0.07 0.54

0.37 0.48 0.86

0.16 0 0.16

TT, tillage times; ΔAG, annual change in aggregates; ΔSOC, annual change in soil organic carbon; ΔMBC, annual change in microbial biomass carbon; ΔDOC, annual change in dissolved organic carbon. Table 4 Multiple linear regression models between soil CO2 emission (FCO2) and soil temperature (ST) and soil moisture (SM) for studied treatments. Tillage treatment

Multiple linear regression (FCO2 = A + B × ST + C × SM + D × ST × SM)

R2

Total⁎

A = −197.50 ± 45.80⁎⁎⁎ B = 8.66 ± 1.76⁎⁎⁎ C = 8.37 ± 1.96⁎⁎⁎ D = −0.20 ± 0.07⁎⁎

0.78

R2: determination coefficient; A: mg CO2 m2 h−1; B: mg CO2 m2 h−1(°C)−1; C: mg CO2 m2 h−1(%)−1; D: mg CO2 m2 h−1 (°C)−1 (%)−1. ⁎ p < 0.05. ⁎⁎ p < 0.01. ⁎⁎⁎ p < 0.001.

tillage frequency but also to higher soil structure changes under tillage disturbance due to higher soil moisture in the rainy season. Consequently, wintertime tillage, then, is an effective method for reducing CO2 emissions by decreasing microbial decomposition rates (Freibauer et al., 2004). Limiting the rainy season with high tillage intensity in agricultural fields is another net CO2 flux reduction strategy; however the efficacy of this approach may depend on soil temperature and moisture conditions (Ogle et al., 2005).

4.3. Soil biogeochemical properties driving the observed differences in CO2 fluxes Gas fluxes may change with different tillage frequencies because of variations in soil properties (Liu et al., 2014). In the present study, annual cumulative CO2 fluxes were directly affected by ΔSOC, ΔDOC, and ΔMBC, and were indirectly influenced by tillage frequency and ΔAG (Fig. 5). Large macroaggregates in T2, T3, and T4 decreased 198

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Acknowledgements

occur with higher tillage frequencies through decreased SOC content. SOC decreased by 27.84% in higher tillage frequency and decreased by 13.34% and 17.74% in the viable tillage management of T1 and T2 after 1 year of manual tillage, respectively. Riezebos and Loerts (1998) reported that mechanically tilled fields have a more rapid decline in organic matter than manually tilled fields because of heavy disruption of the soil structure. However, Cookson et al. (2008) found that total carbon at 0–5 cm was decreased only by 24.16% after 6 years mechanical tillage, suggesting that karst soils showed reduced resistance to disturbance. Additionally, Chen et al. (2012) observed that SOC in the same study area with red soil decreased 37.5% after scrubland reclaiming of farmland for > 20 years. These findings may be explained by differences in parent materials in the karst and non-karst areas. Shang and Tiessen (2003) found that calcareous soils had exceptionally high SOC contents (29–87 g kg−1) and that red soils contained half as much SOC as calcareous soils in Yucatan. When the karst soil was subjected to tillage disturbance, SOC loss could be easily enhanced by increasing CO2 emissions. This indicated that CO2 flux was much higher in the calcareous soils than in red soils through enhancement of SOC loss after disturbance of cultivation in a similar climate, particularly with a higher tillage frequency. ΔSOC values in the viable tillage of T1 and T2 were slightly higher than those of T0, but significantly higher with CO2 flux. The effects of short-term tillage on soil C dynamics and soil aggregates are complex and often variable. SOC in conventional tillage in the short-term was relatively stable, but increased frequency and intensity of soil disturbance would significantly decrease SOC (AlKaisi and Yin, 2005). CO2 emission is the primary mechanism of soil C loss (Parkin and Kaspar, 2003). Higher CO2 emission in the short-term tillage experiment may be governed by soil structural pore changes due to changes in the microbial community population and its activity (AlKaisi and Yin, 2005). In summary, developing viable conservation agriculture practices to decrease CO2 flux in the karst regions should prioritize the reduction of tillage disturbance to protect SOC in large macroaggregates. In addition, we should focus on maintenance of C inputs (e.g., residue retention) to improve SOC content.

The study was supported by the National Key Research and Development Program (2016YFC0502400), the National Key Basic Research Program of China (2015CB452703), and the National Natural Science Foundation of China (31670529). References Al-Kaisi, M.M., Yin, X.H., 2005. Tillage and crop residue effects on soil carbon and carbon dioxide emission in corn-soybean rotations. J. Environ. Qual. 34, 437–445. Alvarez, C.R., Alvarez, R., 2000. Short-term effects of tillage systems on active soil microbial biomass. Biol. Fertil. Soils 31, 157–161. Alvarez, R., Alvarez, C.R., Lorenzo, G., 2001. Carbon dioxide fluxes following tillage from a mollisol in the Argentine Rolling Pampa. Eur. J. Soil Biol. 37, 161–166. Alvaro-Fuentes, J., Cantero-Martinez, C., Lopez, M.V., Arrue, J.L., 2007. Soil carbon dioxide fluxes following tillage in semiarid Mediterranean agroecosystems. Soil Tillage Res. 96, 331–341. Aslam, T., Choudhary, M.A., Saggar, S., 2000. 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5. Conclusions In this study, we evaluated the effects of tillage frequency on soil CO2 flux dynamics. Soil CO2 flux rates increase with tillage frequency. CO2 flux was significantly higher in the wet-hot season than in the dry season due to higher soil temperature and moisture. Therefore, reduced- or no tillage is recommended in the karst area, particularly during the rainy season. Karst soils were sensitive to tillage disturbance and led to higher SOC loss compared with red soil in the same region. Our study demonstrated the occurrence of higher CO2 flux in the karst soil compared with that in other soils after tillage disturbance. Aggregate stability, DOC, and MBC decreased significantly under T2, T3 and T4 after 1 year of tillage, whereas SOC just decreased significantly under T3 and T4. According to the SEM analysis, tillage directly affected ΔAG and ΔMBC, whereas ΔSOC, ΔDOC, and ΔMBC directly influenced the annual cumulative CO2 fluxes. Tillage had the largest indirect effect on annual cumulative CO2 fluxes, whereas ΔSOC had the strongest direct influence on this parameter. These results indicated that soil management in the karst region should focus on protection of soil large macroaggregates and SOC instead of reduction of tillage disturbance. The results of this work suggested that CO2 flux was easily enhanced by increasing tillage disturbance through disruption of large soil macroaggregates and changes in microbial biomass in karst soils. Further studies are needed examine the relationships between CO2 flux and SOC in different aggregate sizes after application of various tillage disturbance intensities.

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