- Email: [email protected]
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
Journal of Integrative Agriculture 2014, 13(3): 615-623
March 2014
RESEARCH ARTICLE
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols YOU Meng-yang1, 2, YUAN Ya-ru1, 3, LI Lu-jun1, XU Yan-li1 and HAN Xiao-zeng1 1
Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, P.R.China 2 University of Chinese Academy of Sciences, Beijing 100049, P.R.China 3 College of Geographical Science, Harbin Normal University, Harbin 150025, P.R.China
Abstract Long-term continuous cropping of soybean (Glycine max), spring wheat (Triticum aesativum) and maize (Zea mays) is widely practiced by local farmers in northeast China. A field experiment (started in 1991) was used to investigate the differences in soil carbon dioxide (CO2) emissions under continuous cropping of the three major crops and to evaluate the relationships between CO2 fluxes and soil temperature and moisture for Mollisols in northeast China. Soil CO2 emissions were measured using a closed-chamber method during the growing season in 2011. No remarkable differences in soil organic carbon were found among the cropping systems (P>0.05). However, significant differences in CO2 emissions from soils were observed among the three cropping systems (P<0.05). Over the course of the entire growing season, cumulative soil CO2 emissions under different cropping systems were in the following order: continuous maize ((829±10) g CO2 m-2)>continuous wheat ((629±22) g CO2 m-2)>continuous soybean ((474±30) g CO2 m-2). Soil temperature explained 42-65% of the seasonal variations in soil CO2 flux, with a Q10 between 1.63 and 2.31; water-filled pore space explained 25-47% of the seasonal variations in soil CO2 flux. A multiple regression model including both soil temperature (T, °C) and water-filled pore space (W, %), log(f)=a+bT log(W), was established, accounting for 51-66% of the seasonal variations in soil CO2 flux. The results suggest that soil CO2 emissions and their Q10 values under a continuous cropping system largely depend on crop types in Mollisols of Northeast China. Key words: CO2 flux, monocultures, soil organic carbon, temperature sensitivity, water-filled pore space
INTRODUCTION Maintenance of soil organic carbon (SOC) is critical not only to maintain soil fertility and further crop yield but also for environmental sustainability in agroecosystems (Lal 2004; Singh et al. 2009). Soil organic carbon can be either a source or a sink of atmospheric carbon dioxide (CO2), and its function in carbon cycling plays a crucial role in global climate
change (Lal 2004). Thus, the sequestration of carbon into soils has been of increasing concern, and serves as one of the strategies to offset anthropogenic CO2 (Ussiri and Lal 2009; Conant et al. 2011). However, the change in SOC occurs slowly and is relatively small in comparison with the vast background of SOC (Varvel 2006; Purakayastha et al. 2008; Lou et al. 2011a); therefore, measuring soil CO2 emissions has been widely performed to evaluate the change in SOC in response to agricultural management practices (Jabro
Received 9 October, 2013 Accepted 18 December, 2013 Correspondence HAN Xiao-zeng, Tel: +86-451-86602940, E-mail: [email protected]; LI Lu-jun, Tel: +86-451-86601048, Fax: +86-451-86603736, E-mail: lilujun@ iga.ac.cn
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd. doi: 10.1016/S2095-3119(13)60719-4
YOU Meng-yang et al.
616
et al. 2008; Wang et al. 2013). Agricultural management practice has been suggested as an important factor controlling soil carbon in agroecosystems (Sombrero and de Benito 2010; Lou et al. 2011b; Scheer et al. 2011). As one of the most common agricultural practices, crop rotation plays a significant role in impacting soil carbon (Lal 2004). Previous studies have suggested that increases in carbon sequestration can be achieved by changing from continuous monoculture to a crop rotation system (West and Post 2002; Jarecki and Lal 2003; González-Sánchez et al. 2012). In Northeast China, however, longterm continuous cropping has been widely adopted by local farmers due to the relatively low costs of continuous monoculture in field management. Moreover, information about soil CO2 emissions and the effect of continuous monocultures on the extensive Mollisols in Northeast China is limited (Kou et al. 2012). In addition to management practices, soil micro-climate, such as soil temperature and moisture, can influence soil CO2 emissions (Bauchmann 2000; Bauer et al. 2008; Fiener et al. 2012; Xue et al. 2012). However, models of the relationships between soil micro-climate factors and soil CO2 fluxes are still inexact, particularly for the relationship between soil moisture and CO2 fluxes. The disparities among trials may be collectively caused by soil types (Gao et al. 2011), modeling equations and parameters (Davidson et al. 2000; Jia et al. 2006), and methods for measuring or modeling CO2 emissions (Herbst et al. 2009; de Bortoli Teixeira et al. 2011; Emran et al. 2012). In addition, soil temperature and moisture likely have a confounding effect on soil CO2 fluxes (Davidson et al. 1998; Iqbal et al. 2008; Almagro et al. 2009). Because approaching a new equilibrium is slow after changed environmental conditions, the effects of cropping regimes on soil carbon dynamic should be studied through long-term field experiments. Therefore, using a long-term continuous cropping experiment, we investigated the impacts of continuous monocultures of three main crops (soybean, Glycine
max; spring wheat, Triticum aesativum; and maize, Zea mays) in Northeast China on SOC content and soil CO2 emissions. The specific objectives were to estimate soil CO2 emissions as influenced by different cropping regimes and to quantify the seasonal variations in soil CO2 fluxes in relation to soil temperature and moisture.
RESULTS Soil properties, grain yield, and biomass No significant differences in SOC content were observed among the three continuous cropping systems (P>0.05; Table 1). Slight but significantly lower total nitrogen (TN) content and soil pH values were observed under the continuous maize cropping system compared with the soybean monoculture (P<0.05; Table 1). The continuous maize monoculture had the highest biomass of straw, root, and crop grain among the three cropping systems. Compared with the soybean monoculture, the root biomass under the cropping systems of continuous wheat and maize was significantly higher by 235 and 100%, respectively (both P<0.05; Table 1).
Soil temperature and moisture Soil temperature varied from 4.7 to 35.3°C during the course of the sampling period, with averages of 22.2 and 18.9°C at depths of 5 and 10 cm, respectively (Fig. 1-A and B). Soil temperature at both the 5 and 10 cm depths reached a maximum on June 24 (Fig. 1- A and B). There was no significant difference in the seasonal averages of soil temperature at the 5 and 10 cm depths among treatments (Fig. 1-A and B). Soil moisture ranged from 20 to 60% of water-filled pore space (WFPS) during the whole sampling period, with an average of 42% WFPS (Fig. 1-C). A significantly
Table 1 Soil properties, crop yield and biomass at harvest under different continuous cropping systems Treatment Soybean Wheat Maize
SOC (mg g-1) 30.53 (0.11) a 30.19 (0.26) a 29.45 (0.23) a
TN (mg g-1) 2.23 (0.01) a 2.28 (0.01) a 2.14 (0.02) b
pH 5.47 (0.11) a 5.40 (0.03) ab 5.09 (0.09) b
Grain (g m-2) 153 (12) b 172 (14) b 659 (42) a
Straw (g m-2) 145 (7) c 281 (13) b 434 (41) a
Root (g m-2) 17 (1) c 34 (7) b 57 (6) a
Values in parentheses are SE (n=3). Letters following the means in columns indicate a significant different at P<0.05.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
higher seasonal average of soil WFPS was observed under the continuous maize cropping system compared with the soybean or wheat monoculture (P<0.05; Fig. 1-C). Soybean
Wheat
Maize
30 20 10 5
a
a
25
15
T5 (°C)
T5 (°C)
25
20
a
15 10
0
Soybean Wheat Maize
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2 27
40 35
B
25 20
25
15
T10 (°C)
T10 (°C)
30
10 5
20
a
a
60
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2
C
40
20 10 0
60 WFPS (%)
WFPS (%)
50
30
40
b
b
a
20 0
Wheat
Maize
600 480 360 BF
SF
240 120 ay n un un un ul ul ul ul ug ug ug ug ep ep ep ep ep -M 4-Ju10-J 17-J 24-J 1-J 8-J 15-J 23-J 6-A 12-A19-A26-A 2-S 9-S 16-S 23-S 30-S
27
Soybean Wheat Maize
27
70
Soybean
A
0
15 10
0
a
720
Soybean Wheat
Maize
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2 27
Date (d-mon)
Fig. 1 Seasonal variations and means of soil temperature and moisture. A, soil temperature at 5 cm (T5). B, soil temperature at 10 cm (T10). C, soil water-filled pore space (WFPS) in the 5 cm soil layer. Vertical bars denote the standard error of the mean (n=3). The same as below. Lowercase letters in insets indicate significant difference in seasonal averages among treatments at P<0.05.
Soil CO2 emissions Seasonal variations in soil CO2 fluxes are summarized in Fig. 2-A. Soil CO2 fluxes increased gradually from the beginning of the experiment, reached maximum levels between July 15 and August 6, and then declined
Date (d-mon) Cumulative CO2 emission (g CO2 m-2)
35
A
gradually until the end of September (Fig. 2-A). The mean soil CO 2 flux in the continuous maize cropping system was higher by 23 and 42% compared with those in the continuous wheat and soybean monocultures, respectively (both P<0.05). Cumulative CO 2 emissions during the entire sampling season in the continuous soybean, wheat, and maize systems were estimated to be (474±30), (629±22), and (829±10) g CO 2 m -2 , respectively (Fig. 2-B). Soils in the continuous maize and wheat monoculture emitted 75 and 33% more CO2 than the soybean monoculture (P<0.05; Fig. 2-B).
Soil CO2 flux (mg CO2 m-2 h-1)
40
617
900
B
720 540 360 180 0
20
40 60 80 100 120 Days after emergence (d)
140
Fig. 2 Seasonal variations of soil CO2 fluxes (A) andcumulative CO2 emissions (B) under different continuous cropping systems. Arrows denote the application time of basal (BF) and supplemental (SF) fertilizers for maize.
Relations between soil CO2 fluxes and soil micro-climate factors Regression analyses showed a poor relationship between soil CO2 flux and soil temperature during the full sampling periods (Table 2). The relationship fit well using an exponential model only for samples at a depth of 10 cm under wheat monoculture (R2=0.23, P<0.05, n=18; Table 2). However, excluding the initial five data sets greatly improved the relationships
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
YOU Meng-yang et al.
618
between soil CO2 fluxes and soil temperature for both layers under the three monoculture systems (Table 2). The exponential model explained 42-65% of the seasonal variations in soil CO2 fluxes (P<0.05, n=13), and the temperature sensitivity (Q10) ranged between 1.63 and 2.31 (Table 2). After the first five samples were excluded, the relationship between soil CO2 fluxes and soil WFPS could also be fitted by an exponential model with an improved R2, which explained 25-47% of the seasonal variations in soil CO 2 fluxes (P<0.10; Table 3). Additionally, a log-transformed multiple regression model, log(f)=a+bTlog(W), including both soil temperature and soil WFPS, accounted for 51-66% of the seasonal variations in soil CO2 fluxes for the three cropping systems (P<0.01; Table 3).
DISCUSSION Effects of long-term cropping regimes on SOC and CO2 emissions The lack of significant differences in the SOC content among the three continuous cropping systems in the present study (Table 1) is inconsistent with another study on the effects of long-term cropping regimes on soil carbon (Kou et al. 2012). Kou et al. (2012) observed a higher increase in SOC levels in the topsoil under continuous maize monoculture compared with continuous soybean, suggesting a greater potential
of carbon sequestration under the continuous maize monoculture. The amount of carbon sequestration is dependent on the level of applied organic amendments (Diacono and Montemurro 2010) and on the amount of carbon which is already in the soil (Six et al. 2002). Thus, the disparities between the results might partially originate from the differences in the applied fertilizers. Organic manure combined with inorganic fertilizer was applied in the study by Kou et al. (2012), whereas the plots in our present study were treated with inorganic fertilizer alone. On the other hand, the negligible differences in SOC among the cropping systems could be partially attributed to the relatively minor change in SOC induced by cropping regimes compared to the large carbon background of the Mollisols. Therefore, it is necessary to measure soil CO2 emissions in order to evaluate carbon loss in response to cropping regimes. Carbon dioxide emission from soils is the product of root and soil microbial respiration (Kuzyakov 2006). Although root respiration is an important contributor to total soil CO2 emissions in situ (Raich and Tufekcioglu 2000), there are large discrepancies regarding the contribution of roots to total soil respiration in agroecosystems (Raich and Mora 2005; Singh et al. 2009). The root contribution to soil CO2 emissions was estimated to be as high as 48% or as low as 12% of the total (Raich and Tufekcioglu 2000; Kuzyakov and Larionova 2005). The disparities between the trials may be collectively caused by environmental
Table 2 Relationship between soil CO2 flux (f) and soil temperature (T) at depths of 5 and 10 cm under different continuous cropping systems Treatment Soybean Wheat Maize
Depth (cm) 5 10 5 10 5 10
Total experiment period (May 27-Sep. 30, 2011, n=18) Q10 Equation R2 0.02 0.08 0.16 f=76.427exp(0.0408T) 0.23* 1.50 0.00 0.03 -
From July 1 to the end of the experiment (July 1-Sep. 30, 2011, n=13) Equation R2 f=58.713exp(0.0488T) 0.42* f=62.821exp(0.0517T) 0.46* f=46.836exp(0.0672T) 0.62** f=54.901exp(0.0706T) 0.65*** f=53.568exp(0.0837T) 0.56** f=62.828exp(0.0822T) 0.56**
Q10 1.63 1.68 1.96 2.03 2.31 2.27
* ** , , and *** indicate significance at P<0.05, 0.01, and 0.001, respectively. The same as below. -, the regression equation was not established due to low R2.
Table 3 The relationship between soil CO2 flux (f) and soil water-filled pore space (W, WFPS) and the combination of soil temperature (T) and WFPS under different continuous cropping systems (n=13) Treatment Soybean Wheat Maize
With soil WFPS f=22.758exp(0.0464W) f=15.810exp(0.0562W) f=17.013exp(0.0598W)
(R2) 0.47** 0.25† 0.47**
With T combined with soil WFPS log(f)= 1.7546+0.0136T log(W) log(f)= 1.6596+0.0180T log(W) log(f)= 1.7391+0.0215T log(W)
(R2) 0.51** 0.66*** 0.63**
† indicates significance at P=0.05-0.10 level.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
conditions (notably air and/or soil temperature and soil moisture) (Ding et al. 2007; Almagro et al. 2009), inherent soil fertility, land management, and crop type (Rudrappa et al. 2006; Sugihara et al. 2012). In our study, the influence of long-term cropping on soil CO2 emissions was dependent upon crop types. Cumulative soil CO2 emissions under different cropping systems followed the order of maize>wheat>soybean (Fig. 2-B). A previous study conducted in Central Iowa showed that the increased soil CO 2 emission from continuous maize plots was due to increased crop residue incorporation compared to other cropping systems (Wilson and Al-Kaisi 2008). In the present study, straw was removed from fields after harvest for all treatments, and there was no crop straw incorporation into the soils. Thus, the residue incorporation into the soil was mainly from root biomass. The significant positive correlation between cumulative CO2 emissions and root biomass, but not SOC (data not shown), combined with the significant difference in root biomass among treatments (Table 1), indicated that the observed higher CO 2 emissions from the continuous maize cropping plots (Fig. 2-B) can be mainly attributed to the autotrophic respiration of the roots compared to the other two cropping systems. In addition to root respiration, soil microorganisms are considered to be another important contributor to soil CO 2 emissions due to their key roles in transforming native and/or exogenous organic matter (Kuzyakov 2006; Singh et al. 2009). The presence of roots can greatly affect the rate of soil organic matter decomposition in the rhizosphere by changing microbial activity, which is known as a “rhizosphere priming effect”, representing the interaction between the growing roots and decomposition of soil organic matter (Kuzyakov 2006). In the present study, although there was no significant difference in SOC content among the cropping systems (P>0.05), a remarkable difference in root biomass was observed (P<0.05; Table 1). Therefore, a positive priming effect induced by the higher root biomass in the continuous maize cropping system might cause a higher rate of decomposition of “old” carbon and thus higher heterotrophic soil respiration compared with the continuous soybean and wheat systems.
619
Effects of soil temperature and moisture on CO2 emissions Generally, soil temperature and moisture have been identified as the main factors influencing the seasonal dynamics of soil respiration in short time scales (Bauchmann 2000; Xu and Qi 2001; Ding et al. 2007), and their relationship is typically described using an exponential equation model (Davidson et al. 1998; Ding et al. 2010). However, the present study found a poor relationship between soil CO 2 flux and soil temperature when using the complete sampling data (Table 2). After the initial five data sets were excluded, we found an improved fit for the relationship using an exponential equation between soil temperature and soil CO2 fluxes that could model their relationships under all treatments with greatly improved R 2 values (42-65%; Table 2). Similar phenomena were reported in our previous study (Li et al. 2013a, b) and in other studies (Ding et al. 2010; Fuß et al. 2011). We argued that soil disturbances (such as ploughing or fertilization at the seedling stage) might have affected soil CO2 fluxes and further modified the influence of soil temperature on CO 2 fluxes (Li et al. 2013a). Additionally, root growth may have influenced the relationship between soil CO2 fluxes and soil temperature because soil respiration has been shown to correlate with fine root production (Lee and Jose 2003). The period we excluded from our calculations contained mostly heterotrophic respiration; thus, the improved relationship between soil temperature and CO2 fluxes indicated that soil temperature may be more closely associated with autotrophic respiration than with heterotrophic respiration. The Q10 values of soil respiration ranged between 1.63 for soybean and 2.31 for maize at the 5-cm depth in the present study (Table 2). The different Q 10 values for different cropping systems suggest that the crop-specific effects on the estimation of temperature sensitivity should be taken into account when modeling the carbon cycle, as most carbon models assume a constant Q 10 under long-term continuous cropping systems, regardless of the crop type (Tian et al. 1999; Graf et al. 2008). In addition, it is noteworthy that temperature measurements within the
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
YOU Meng-yang et al.
620
upper 10 cm are likely to lead to an underestimation of temperature sensitivity (Graf et al. 2008). Thus, the Q10 values of soil respiration at a depth of 10 cm might be more accurate than those at a depth of 5 cm. Similarly, the relationship between soil CO2 fluxes and soil WFPS was also fitted with an exponential model after exclusion of the first five samples (Table 3). We also attributed this result to the root growth or soil disturbance due to ploughing or the application of chemical fertilizer. Furthermore, the multiple regression model including both soil temperature and WFPS (Table 3) revealed a collective impact of soil temperature and moisture on soil CO2 emissions from the Mollisols.
CONCLUSION No remarkable differences in SOC were observed after 20 yr of continuous maize, soybean, and wheat cropping. Although there were no differences in SOC, significant differences in soil CO 2 emissions were observed among the three cropping systems. The influences of soil temperature and WFPS on soil CO2 fluxes could be described well using an exponential equation model. The Q 10 values ranged between 1.63 for soybean and 2.31 for maize at a depth of 5 cm depth. A log-transformed multiple regression model including both soil temperature and moisture accounted for 51-66% of the seasonal variations in soil CO2 fluxes. Our results suggest that soil CO2 emissions and Q10 values from continuous cropping systems largely depend on crop types in Mollisols of northeast China.
MATERIALS AND METHODS Site description and experimental design The field experiment was carried out at the Hailun National Experiment Station of Agroecosystem of the Chinese Academy of Sciences, located in Hailun County, Heilongjiang Province (47°26´N, 126°38´E), in the growing season between late spring and early autumn in 2011. The region has a typical temperate continental monsoon climate, characterized by hot summers and cold winters. The mean annual temperature was 1.5°C, and the lowest and highest mean monthly values were detected in January (-23°C) and
in July (21°C), respectively. The mean annual precipitation was 550 mm, more than 80% of which occurred from May to September. The frost-free period was approximately 120 d. The soil type is loamy loess and is classified as a Mollisol (Typic Hapludoll). A long-term continuous cropping experiment was established in a completely randomized block design in 1991. The experiment included three replicates of three treatments: continuous monocultures of soybean, wheat, and maize. Each block covered 77 m2, with 10 rows in each block, and was separated by a 0.7-m buffer strip. The fertilizer applications were as follows: 27 kg N ha-1 and 69 kg P ha-1 for soybean; 92 kg N ha-1 and 39 kg P ha-1 for wheat; 131 kg N ha-1 and 69 kg P ha-1 for maize. Fertilizers N and P were applied in the form of urea and diammonium phosphate, respectively. For urea application, two splits were adopted, with the treatments receiving N fertilizer as the basal (May 10) and supplementary (June 22) fertilizers at a ratio of 1:1 for continuous maize cropping. The crop straw was removed after harvest under all treatments, following the common practice of local farmers. The soil was manually tilled to a depth of 15 to 20 cm after harvest.
Soil CO2 flux measurement Soil CO2 emission was sampled using a closed-chamber followed by gas chromatography. Three chambers (0.7 m× 0.2 m×0.25 m) were laid within each treatment. Air samples were manually collected using a 20-mL syringe from a septum installed at the top of the chamber at 0, 10, 20, and 30 min after closure, and then injected into preevacuated 18-mL vials and taken to the laboratory for analysis. Gas sampling was performed between 9:00 and 11:00 a.m. once per week, except on July 30th due to crop lodging resulting from heavy rainfall. Carbon dioxide was collected from the whole soil including rhizosphere and bulk soil (i.e., rhizosphere respiration and native soil respiration). Sample CO2 concentration was determined with a gas chromatograph (GC-2010, Shimadzu Corp., Japan) equipped with a flame ionization detector. The increased rate of CO2 concentration vs. time in the chamber air was calculated by linear regression analysis. All linear regression values of R2 were larger than 0.90. Carbon dioxide flux was computed from the rate of the change in chamber CO 2 concentration vs. time, chamber volume V and soil surface area A using the following formula: (1) f=ρ×Δc/Δt×V/A×273/(273+T) Where, f is the soil CO2 flux in mg CO2 m-2 h-1, ρ is the CO2 density under standard conditions, Δc/Δt is the linear change in CO2 concentration inside the chamber and T is the temperature inside the chamber. Cumulative soil CO2 emission (g CO2 m-2) during the entire sampling season was calculated by summing the products of the (averaged) two neighboring fluxes, multiplied by their interval time. The air temperature inside the chamber was recorded with
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
an Hg thermometer. Soil temperatures at vertical depths of 5- and 10-cm were measured in situ using a geothermometer simultaneous to the gas sampling. Field soil water content was determined by a gravimetric method. Soil bulk density was also gravimetrically determined from undisturbed soils within 1 m of the field chambers. Soil WFPS was used as the moisture variable in all analyses, as this property integrates porosity and moisture variables (Franzluebbers 1999). Values of the gravimetric soil moisture were converted to soil WFPS values using the following equation: (2) WFPS (%)=(θg×sbd×100%)/[1-(sbd/spd)] Where, θg is gravimetric soil moisture (kg water kg-1 soil), sbd is soil bulk density (Mg m-3 soil) and spd is soil particle density.
Soil sampling and analyses, and biomass determination Soil samples were collected in early October 2011. Eight soil samples (0-20 cm) were randomly collected within each block and then mixed thoroughly to form a composite for analyses of SOC, TN, and soil pH. Air-dried soil samples (<2 mm) were used to determine soil pH in a 1:2.5 (w/v) soil to water ratio. Soil organic C and TN concentrations were analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). Soybean and maize were planted in early May and harvested in late September; wheat was directly sown in April and harvested in July, 2011. All aboveground crop materials within a plot area of 2 m×1.4 m were cut in each block. Root biomass (0-20 cm) was measured in the same location used for aboveground sampling. Roots were separated from soils by picking and washing. All crop samples (grain, straw and root) were oven-dried at 70°C for 48 h and then weighed.
Calculations The response of soil CO 2 flux to soil temperature was described by an exponential function: (3) f=aebT Where, f is the soil CO 2 flux (mg CO 2 m -2 h -1) at soil temperature T, and a and b are regression coefficients. The temperature sensitivity (Q10), defined as the difference in soil respiration rate over a 10°C increment, was calculated by the following first-order exponential function: (4) Q10=e10b
Statistical analyses Homogeneity of variance and normality tests were carried out on gas and biomass data. A one-way analysis of variance (ANOVA) following the least significant difference (LSD) was used to test the differences in SOC content, biomass, and cumulative CO2 emissions among treatments. Regression
621
analysis was used to determine the relationships between soil environmental parameters (soil temperature, soil moisture, and soil temperature combined with soil moisture) and soil CO2 fluxes. All statistical analyses were conducted using SPSS 13.0 for Windows (SPSS Inc, Chicago, IL).
Acknowledgements This work was supported by the Key Research Program of the Chinese Academy of Sciences (KZZD-EW-TZ-16-02), the Foundation for Young Talents of the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (DLSYQ13001), and the National Natural Science Foundation of China (41101283). We thank Ms. Sun Guidan for laboratory assistance and Ms. Gao Weili and Wen Xiuling for participation in the field sampling.
References Almagro M, Loez J, Querejeta J L, Martinez-Mena M. 2009. Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem. Soil Biology & Biochemistry, 41, 594-605. Bauchmann N. 2000. Biotic and abiotic factors controlling soil respiration rates in Picea abies stands. Soil Biology & Biochemistry, 32, 1625-1635. Bauer J, Herbst M, Huisman J A, Weihermüller L, Vereecken H. 2008. Sensitivity of simulated soil heterotrophic respiration to temperature and moisture reduction functions. Geoderma, 145, 17-27. de Bortoli Teixeira D, Panosso A R, Cerri C E P, Pereira G T, La Scala N. 2011. Soil CO2 emission estimated by different interpolation techniques. Plant and Soil, 345, 187-194. Conant R T, Ogle S M, Paul E A, Paustian K. 2011. Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation. Frontiers in Ecology and the Environment, 9, 169-173. Davidson E A, Belk E, Boone R D. 1998. Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest. Global Change Biology, 4, 217-227. Davidson E A, Verchot L V, Cattanio J H, Ackerman I L, Carvalho J E M. 2000. Effects of soil water content on soil respiration in forests and cattle pastures of eastern Amazonia. Biogeochemistry, 48, 53-69. Diacono M, Montemurro F. 2010. Long-term effects of organic amendments on soil fertility. A review. Agronomy for Sustainable Development, 30, 401-422. Ding W, Cai Y, Cai Z. 2007. Soil respiration under maize crops: effects of water, temperature, and nitrogen fertilizaton. Soil Science Society of America Journal, 71, 944-951. Ding W, Yu H, Cai Z, Han F, Xu Z. 2010. Responses of soil respiration to N fertilization in a loamy soil under maize cultivation. Geoderma, 155, 381-389.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
622
Emran M, Gispert M, Pardini G. 2012. Comparing measurement methods of carbon dioxide fluxes in a soil sequence under land use and cover change in North Eastern Spain. Geoderma, 170, 176-185. Fiener P, Dlugoß V, Korres W, Schneider K. 2012. Spatial variability of soil respiration in a small agricultural watershed - Are patterns of soil redistribution important? Catena, 94, 3-16. Franzluebbers A J. 1999. Microbial activity in response to water-filled pore space of variably eroded southern Piedmont soils. Applied Soil Ecology, 11, 91-101. Fuß R, Ruth B, Schilling R, Scherb H, Munch J C. 2011. Pulse emissions of N 2 O and CO 2 from an arable field depending on fertilization and tillage practice. Agriculture, Ecosystems & Environment, 144, 61-68. Gao J, Ouyang H, Lei G, Xu X, Zhang M. 2011. Effects of temperature, soil moisture, soil type and their interactions on soil carbon mineralization in Zoigê alpine wetland, Qinghai-Tibet Plateau. Chinese Geographical Science, 21, 27-35. González-Sánchez E J, Ordóñez-Fernández R, CarbonellBojollo R, Veroz-González O, Gil-Ribes J A. 2012. Meta-analysis on atmospheric carbon capture in Spain through the use of conservation agriculture. Soil & Tillage Research, 122, 52-60. Graf A, Weihermüller L, Huisman J A, Herbst M, Bauer J, Vereecken H. 2008. Measurement depth effects on the apparent temperature sensitivity of soil respiration in field studies. Biogeosciences, 5, 1175-1188. Herbst M, Prolingheuer N, Graf A, Huisman J A, Weihermüller L, Vanderborght J. 2009. Characterization and understanding of bare soil respiration spatial variability at plot scale. Vadose Zone Journal, 8, 762771. Iqbal J, Hu R, Du L, Lu L, Lin S, Chen T, Ruan L. 2008. Differences in soil CO 2 flux between different land use types in mid-subtropical China. Soil Biology & Biochemistry, 40, 2324-2333. Jabro J D, Sainju U, Stevens W B, Evans R G. 2008. Carbon dioxide flux as affected by tillage and irrigation in soil converted from perennial forages to annual crops. Journal of Environmental Management, 88, 1478-1484. Jarecki M K, Lal R. 2003. Crop management for soil carbon sequestration. Critical Reviews in Plant Sciences, 22, 471-502. Jia B, Zhou G, Wang Y, Wang F, Wang X. 2006. Effects of temperature and soil water-content on soil respiration of grazed and ungrazed Leymus chinensis steppes, Inner Mongolia. Journal of Arid Environments, 67, 60-76. Kou T J, Zhu P, Huang S, Peng X X, Song Z W, Deng A X, Gao H J, Peng C, Zhang W J. 2012. Effects of longterm cropping regimes on soil carbon sequestration and aggregate composition in rainfed farmland of Northeast China. Soil & Tillage Research, 118, 132-138. Kuzyakov Y. 2006. Sources of CO 2 efflux from soil and review of partitioning methods. Soil Biology &
YOU Meng-yang et al.
Biochemistry, 38, 425-448. Kuzyakov Y, Larionova A A. 2005. Review of methods and results of separation of root and rhizomicrobial respiration. Journal of Plant Nutrition and Soil Science, 168, 503-520. Lal R. 2004. Soil carbon sequestration impacts on global climate change and food security. Science, 304, 16231626. Lee K H, Jose S. 2003. Soil respiration, fine root production, and microbial biomass in cottonwood and loblolly pine plantations along a nitrogen fertilization gradient. Forest Ecology and Management, 185, 263-273. Li L J, You M Y, Shi H A, Ding X L, Qiao Y F, Han X Z. 2013a. Soil CO2 emissions from a cultivated Mollisol: Effects of organic amendments, soil temperature, and moisture. European Journal of Soil Biology, 55, 83-90. Li L J, You M Y, Shi H A, Han X Z. 2013b. Tillage effects on SOC and CO 2 emissions of Mollisols. Journal of Food, Agriculture & Environment, 11, 340-345. Lou Y, Wang J, Liang W. 2011a. Impacts of 22-year organic and inorganic N managements on soil organic C fractions in a maize field, northeast China. Catena, 87, 386-390. Lou Y, Xu M, Wang W, Sun X, Liang C. 2011b. Soil organic carbon fractions and management index after 20 yr of manure and fertilizer application for greenhouse vegetables. Soil Use & Management, 27, 163-169. Purakayastha T J, Rudrappa L, Singh D, Swarup A, Bhadraray S. 2008. Long-term impact of fertilizers on soil organic carbon pools and sequestration rates in maize-wheat-cowpea cropping system. Geoderma, 144, 370-378. Raich J W, Mora G. 2005. Estimating root plus rhizosphere contributions to soil respiration in annual croplands. Soil Science Society of America Journal, 69, 634-639. Raich J W, Tufekcioglu A. 2000. Vegetation and soil respiration: correlation and controls. Biogeochemistry, 48, 71-90. Rudrappa L, Purakayastha T J, Singh D, Bhadraray S. 2006. Long-term manuring and fertilization effects on soil organic carbon pools in a Typic Haplustept of semi-arid sub-tropical India. Soil & Tillage Research, 88, 180-192. Scheer C, Grace P R, Rowlings D W, Kimber S, Zwieten L V. 2011. Effect of biochar amendment on the soilatmosphere exchange of greenhouse gases from an intensive subtropical pasture in northern New South Wales, Australia. Plant and Soil, 345, 47-58. Singh K P, Ghoshal N, Singh S. 2009. Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality. Applied Soil Ecology, 42, 243-253. Six J, Conant R T, Paul E A, Paustian K. 2002. Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils. Plant and Soil, 241, 155-176. Sombrero A, de Benito A. 2010. Carbon accumulation in
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
soil. Ten-year study of conservation tillage and crop rotation in a semi-arid area of Castile-Leon, Spain. Soil & Tillage Research, 107, 64-70. Sugihara S, Funakawa S, Kilasara M, Kosaki T. 2012. Effects of land management on CO2 flux and soil C stock in two Tanzanian croplands with contrasting soil texture. Soil Biology & Biochemistry, 46, 1-9. Tian H, Melillo J M, Kicklighter D W, McGuire A D, Helfrich J. 1999. The sensitivity of terrestrial carbon storage to historical climate variability and atmospheric CO2 in the United States. Tellus B, 51, 414-452. Ussiri D A N, Lal R. 2009. Long-term tillage effects on soil carbon storage and carbon dioxide emissions in continuous corn cropping system from an alfisol in Ohio. Soil & Tillage Research, 104, 39-47. Varvel G E. 2006. Soil organic carbon changes in diversified rotations of the western corn belt. Soil Science Society of America Journal, 70, 426-433. Wang W, Liao Y C, Guo Q. 2013. Seasonal and annual
623
variations of CO2 fluxes in rain-fed winter wheat agroecosystem of loess plateau, China. Journal of Integrative Agriculture, 12, 147-158. West T O, Post W M. 2002. Soil oganic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Science Society of America Journal, 66, 1930-1946. Wilson H M, Al-Kaisi M M. 2008. Crop rotation and nitrogen fertilization effect on soil CO2 emissions in central Iowa. Applied Soil Ecology, 39, 264-270. Xu M, Qi Y. 2001. Soil-surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California. Global Change Biology, 7, 667-677. Xue Y D, Yang P L, Luo Y P, Li Y K, Ren S M, Su Y P, Niu Y T. 2012. Characteristics and driven factors of nitrous oxide and carbon dioxide emissions in soil irrigated with treated wastewater. Journal of Integrative Agriculture, 11, 1354-1364. (Managing editor SUN Lu-juan)
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
March 2014
RESEARCH ARTICLE
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols YOU Meng-yang1, 2, YUAN Ya-ru1, 3, LI Lu-jun1, XU Yan-li1 and HAN Xiao-zeng1 1
Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, P.R.China 2 University of Chinese Academy of Sciences, Beijing 100049, P.R.China 3 College of Geographical Science, Harbin Normal University, Harbin 150025, P.R.China
Abstract Long-term continuous cropping of soybean (Glycine max), spring wheat (Triticum aesativum) and maize (Zea mays) is widely practiced by local farmers in northeast China. A field experiment (started in 1991) was used to investigate the differences in soil carbon dioxide (CO2) emissions under continuous cropping of the three major crops and to evaluate the relationships between CO2 fluxes and soil temperature and moisture for Mollisols in northeast China. Soil CO2 emissions were measured using a closed-chamber method during the growing season in 2011. No remarkable differences in soil organic carbon were found among the cropping systems (P>0.05). However, significant differences in CO2 emissions from soils were observed among the three cropping systems (P<0.05). Over the course of the entire growing season, cumulative soil CO2 emissions under different cropping systems were in the following order: continuous maize ((829±10) g CO2 m-2)>continuous wheat ((629±22) g CO2 m-2)>continuous soybean ((474±30) g CO2 m-2). Soil temperature explained 42-65% of the seasonal variations in soil CO2 flux, with a Q10 between 1.63 and 2.31; water-filled pore space explained 25-47% of the seasonal variations in soil CO2 flux. A multiple regression model including both soil temperature (T, °C) and water-filled pore space (W, %), log(f)=a+bT log(W), was established, accounting for 51-66% of the seasonal variations in soil CO2 flux. The results suggest that soil CO2 emissions and their Q10 values under a continuous cropping system largely depend on crop types in Mollisols of Northeast China. Key words: CO2 flux, monocultures, soil organic carbon, temperature sensitivity, water-filled pore space
INTRODUCTION Maintenance of soil organic carbon (SOC) is critical not only to maintain soil fertility and further crop yield but also for environmental sustainability in agroecosystems (Lal 2004; Singh et al. 2009). Soil organic carbon can be either a source or a sink of atmospheric carbon dioxide (CO2), and its function in carbon cycling plays a crucial role in global climate
change (Lal 2004). Thus, the sequestration of carbon into soils has been of increasing concern, and serves as one of the strategies to offset anthropogenic CO2 (Ussiri and Lal 2009; Conant et al. 2011). However, the change in SOC occurs slowly and is relatively small in comparison with the vast background of SOC (Varvel 2006; Purakayastha et al. 2008; Lou et al. 2011a); therefore, measuring soil CO2 emissions has been widely performed to evaluate the change in SOC in response to agricultural management practices (Jabro
Received 9 October, 2013 Accepted 18 December, 2013 Correspondence HAN Xiao-zeng, Tel: +86-451-86602940, E-mail: [email protected]; LI Lu-jun, Tel: +86-451-86601048, Fax: +86-451-86603736, E-mail: lilujun@ iga.ac.cn
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd. doi: 10.1016/S2095-3119(13)60719-4
YOU Meng-yang et al.
616
et al. 2008; Wang et al. 2013). Agricultural management practice has been suggested as an important factor controlling soil carbon in agroecosystems (Sombrero and de Benito 2010; Lou et al. 2011b; Scheer et al. 2011). As one of the most common agricultural practices, crop rotation plays a significant role in impacting soil carbon (Lal 2004). Previous studies have suggested that increases in carbon sequestration can be achieved by changing from continuous monoculture to a crop rotation system (West and Post 2002; Jarecki and Lal 2003; González-Sánchez et al. 2012). In Northeast China, however, longterm continuous cropping has been widely adopted by local farmers due to the relatively low costs of continuous monoculture in field management. Moreover, information about soil CO2 emissions and the effect of continuous monocultures on the extensive Mollisols in Northeast China is limited (Kou et al. 2012). In addition to management practices, soil micro-climate, such as soil temperature and moisture, can influence soil CO2 emissions (Bauchmann 2000; Bauer et al. 2008; Fiener et al. 2012; Xue et al. 2012). However, models of the relationships between soil micro-climate factors and soil CO2 fluxes are still inexact, particularly for the relationship between soil moisture and CO2 fluxes. The disparities among trials may be collectively caused by soil types (Gao et al. 2011), modeling equations and parameters (Davidson et al. 2000; Jia et al. 2006), and methods for measuring or modeling CO2 emissions (Herbst et al. 2009; de Bortoli Teixeira et al. 2011; Emran et al. 2012). In addition, soil temperature and moisture likely have a confounding effect on soil CO2 fluxes (Davidson et al. 1998; Iqbal et al. 2008; Almagro et al. 2009). Because approaching a new equilibrium is slow after changed environmental conditions, the effects of cropping regimes on soil carbon dynamic should be studied through long-term field experiments. Therefore, using a long-term continuous cropping experiment, we investigated the impacts of continuous monocultures of three main crops (soybean, Glycine
max; spring wheat, Triticum aesativum; and maize, Zea mays) in Northeast China on SOC content and soil CO2 emissions. The specific objectives were to estimate soil CO2 emissions as influenced by different cropping regimes and to quantify the seasonal variations in soil CO2 fluxes in relation to soil temperature and moisture.
RESULTS Soil properties, grain yield, and biomass No significant differences in SOC content were observed among the three continuous cropping systems (P>0.05; Table 1). Slight but significantly lower total nitrogen (TN) content and soil pH values were observed under the continuous maize cropping system compared with the soybean monoculture (P<0.05; Table 1). The continuous maize monoculture had the highest biomass of straw, root, and crop grain among the three cropping systems. Compared with the soybean monoculture, the root biomass under the cropping systems of continuous wheat and maize was significantly higher by 235 and 100%, respectively (both P<0.05; Table 1).
Soil temperature and moisture Soil temperature varied from 4.7 to 35.3°C during the course of the sampling period, with averages of 22.2 and 18.9°C at depths of 5 and 10 cm, respectively (Fig. 1-A and B). Soil temperature at both the 5 and 10 cm depths reached a maximum on June 24 (Fig. 1- A and B). There was no significant difference in the seasonal averages of soil temperature at the 5 and 10 cm depths among treatments (Fig. 1-A and B). Soil moisture ranged from 20 to 60% of water-filled pore space (WFPS) during the whole sampling period, with an average of 42% WFPS (Fig. 1-C). A significantly
Table 1 Soil properties, crop yield and biomass at harvest under different continuous cropping systems Treatment Soybean Wheat Maize
SOC (mg g-1) 30.53 (0.11) a 30.19 (0.26) a 29.45 (0.23) a
TN (mg g-1) 2.23 (0.01) a 2.28 (0.01) a 2.14 (0.02) b
pH 5.47 (0.11) a 5.40 (0.03) ab 5.09 (0.09) b
Grain (g m-2) 153 (12) b 172 (14) b 659 (42) a
Straw (g m-2) 145 (7) c 281 (13) b 434 (41) a
Root (g m-2) 17 (1) c 34 (7) b 57 (6) a
Values in parentheses are SE (n=3). Letters following the means in columns indicate a significant different at P<0.05.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
higher seasonal average of soil WFPS was observed under the continuous maize cropping system compared with the soybean or wheat monoculture (P<0.05; Fig. 1-C). Soybean
Wheat
Maize
30 20 10 5
a
a
25
15
T5 (°C)
T5 (°C)
25
20
a
15 10
0
Soybean Wheat Maize
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2 27
40 35
B
25 20
25
15
T10 (°C)
T10 (°C)
30
10 5
20
a
a
60
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2
C
40
20 10 0
60 WFPS (%)
WFPS (%)
50
30
40
b
b
a
20 0
Wheat
Maize
600 480 360 BF
SF
240 120 ay n un un un ul ul ul ul ug ug ug ug ep ep ep ep ep -M 4-Ju10-J 17-J 24-J 1-J 8-J 15-J 23-J 6-A 12-A19-A26-A 2-S 9-S 16-S 23-S 30-S
27
Soybean Wheat Maize
27
70
Soybean
A
0
15 10
0
a
720
Soybean Wheat
Maize
ay un un un un ul ul Jul Jul ug ug ug ug ep ep ep ep ep -M 4-J 10-J 17-J 24-J 1-J 8-J 15- 23- 6-A 2-A 9-A 6-A 2-S 9-S 16-S 23-S 30-S 1 1 2 27
Date (d-mon)
Fig. 1 Seasonal variations and means of soil temperature and moisture. A, soil temperature at 5 cm (T5). B, soil temperature at 10 cm (T10). C, soil water-filled pore space (WFPS) in the 5 cm soil layer. Vertical bars denote the standard error of the mean (n=3). The same as below. Lowercase letters in insets indicate significant difference in seasonal averages among treatments at P<0.05.
Soil CO2 emissions Seasonal variations in soil CO2 fluxes are summarized in Fig. 2-A. Soil CO2 fluxes increased gradually from the beginning of the experiment, reached maximum levels between July 15 and August 6, and then declined
Date (d-mon) Cumulative CO2 emission (g CO2 m-2)
35
A
gradually until the end of September (Fig. 2-A). The mean soil CO 2 flux in the continuous maize cropping system was higher by 23 and 42% compared with those in the continuous wheat and soybean monocultures, respectively (both P<0.05). Cumulative CO 2 emissions during the entire sampling season in the continuous soybean, wheat, and maize systems were estimated to be (474±30), (629±22), and (829±10) g CO 2 m -2 , respectively (Fig. 2-B). Soils in the continuous maize and wheat monoculture emitted 75 and 33% more CO2 than the soybean monoculture (P<0.05; Fig. 2-B).
Soil CO2 flux (mg CO2 m-2 h-1)
40
617
900
B
720 540 360 180 0
20
40 60 80 100 120 Days after emergence (d)
140
Fig. 2 Seasonal variations of soil CO2 fluxes (A) andcumulative CO2 emissions (B) under different continuous cropping systems. Arrows denote the application time of basal (BF) and supplemental (SF) fertilizers for maize.
Relations between soil CO2 fluxes and soil micro-climate factors Regression analyses showed a poor relationship between soil CO2 flux and soil temperature during the full sampling periods (Table 2). The relationship fit well using an exponential model only for samples at a depth of 10 cm under wheat monoculture (R2=0.23, P<0.05, n=18; Table 2). However, excluding the initial five data sets greatly improved the relationships
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
YOU Meng-yang et al.
618
between soil CO2 fluxes and soil temperature for both layers under the three monoculture systems (Table 2). The exponential model explained 42-65% of the seasonal variations in soil CO2 fluxes (P<0.05, n=13), and the temperature sensitivity (Q10) ranged between 1.63 and 2.31 (Table 2). After the first five samples were excluded, the relationship between soil CO2 fluxes and soil WFPS could also be fitted by an exponential model with an improved R2, which explained 25-47% of the seasonal variations in soil CO 2 fluxes (P<0.10; Table 3). Additionally, a log-transformed multiple regression model, log(f)=a+bTlog(W), including both soil temperature and soil WFPS, accounted for 51-66% of the seasonal variations in soil CO2 fluxes for the three cropping systems (P<0.01; Table 3).
DISCUSSION Effects of long-term cropping regimes on SOC and CO2 emissions The lack of significant differences in the SOC content among the three continuous cropping systems in the present study (Table 1) is inconsistent with another study on the effects of long-term cropping regimes on soil carbon (Kou et al. 2012). Kou et al. (2012) observed a higher increase in SOC levels in the topsoil under continuous maize monoculture compared with continuous soybean, suggesting a greater potential
of carbon sequestration under the continuous maize monoculture. The amount of carbon sequestration is dependent on the level of applied organic amendments (Diacono and Montemurro 2010) and on the amount of carbon which is already in the soil (Six et al. 2002). Thus, the disparities between the results might partially originate from the differences in the applied fertilizers. Organic manure combined with inorganic fertilizer was applied in the study by Kou et al. (2012), whereas the plots in our present study were treated with inorganic fertilizer alone. On the other hand, the negligible differences in SOC among the cropping systems could be partially attributed to the relatively minor change in SOC induced by cropping regimes compared to the large carbon background of the Mollisols. Therefore, it is necessary to measure soil CO2 emissions in order to evaluate carbon loss in response to cropping regimes. Carbon dioxide emission from soils is the product of root and soil microbial respiration (Kuzyakov 2006). Although root respiration is an important contributor to total soil CO2 emissions in situ (Raich and Tufekcioglu 2000), there are large discrepancies regarding the contribution of roots to total soil respiration in agroecosystems (Raich and Mora 2005; Singh et al. 2009). The root contribution to soil CO2 emissions was estimated to be as high as 48% or as low as 12% of the total (Raich and Tufekcioglu 2000; Kuzyakov and Larionova 2005). The disparities between the trials may be collectively caused by environmental
Table 2 Relationship between soil CO2 flux (f) and soil temperature (T) at depths of 5 and 10 cm under different continuous cropping systems Treatment Soybean Wheat Maize
Depth (cm) 5 10 5 10 5 10
Total experiment period (May 27-Sep. 30, 2011, n=18) Q10 Equation R2 0.02 0.08 0.16 f=76.427exp(0.0408T) 0.23* 1.50 0.00 0.03 -
From July 1 to the end of the experiment (July 1-Sep. 30, 2011, n=13) Equation R2 f=58.713exp(0.0488T) 0.42* f=62.821exp(0.0517T) 0.46* f=46.836exp(0.0672T) 0.62** f=54.901exp(0.0706T) 0.65*** f=53.568exp(0.0837T) 0.56** f=62.828exp(0.0822T) 0.56**
Q10 1.63 1.68 1.96 2.03 2.31 2.27
* ** , , and *** indicate significance at P<0.05, 0.01, and 0.001, respectively. The same as below. -, the regression equation was not established due to low R2.
Table 3 The relationship between soil CO2 flux (f) and soil water-filled pore space (W, WFPS) and the combination of soil temperature (T) and WFPS under different continuous cropping systems (n=13) Treatment Soybean Wheat Maize
With soil WFPS f=22.758exp(0.0464W) f=15.810exp(0.0562W) f=17.013exp(0.0598W)
(R2) 0.47** 0.25† 0.47**
With T combined with soil WFPS log(f)= 1.7546+0.0136T log(W) log(f)= 1.6596+0.0180T log(W) log(f)= 1.7391+0.0215T log(W)
(R2) 0.51** 0.66*** 0.63**
† indicates significance at P=0.05-0.10 level.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
conditions (notably air and/or soil temperature and soil moisture) (Ding et al. 2007; Almagro et al. 2009), inherent soil fertility, land management, and crop type (Rudrappa et al. 2006; Sugihara et al. 2012). In our study, the influence of long-term cropping on soil CO2 emissions was dependent upon crop types. Cumulative soil CO2 emissions under different cropping systems followed the order of maize>wheat>soybean (Fig. 2-B). A previous study conducted in Central Iowa showed that the increased soil CO 2 emission from continuous maize plots was due to increased crop residue incorporation compared to other cropping systems (Wilson and Al-Kaisi 2008). In the present study, straw was removed from fields after harvest for all treatments, and there was no crop straw incorporation into the soils. Thus, the residue incorporation into the soil was mainly from root biomass. The significant positive correlation between cumulative CO2 emissions and root biomass, but not SOC (data not shown), combined with the significant difference in root biomass among treatments (Table 1), indicated that the observed higher CO 2 emissions from the continuous maize cropping plots (Fig. 2-B) can be mainly attributed to the autotrophic respiration of the roots compared to the other two cropping systems. In addition to root respiration, soil microorganisms are considered to be another important contributor to soil CO 2 emissions due to their key roles in transforming native and/or exogenous organic matter (Kuzyakov 2006; Singh et al. 2009). The presence of roots can greatly affect the rate of soil organic matter decomposition in the rhizosphere by changing microbial activity, which is known as a “rhizosphere priming effect”, representing the interaction between the growing roots and decomposition of soil organic matter (Kuzyakov 2006). In the present study, although there was no significant difference in SOC content among the cropping systems (P>0.05), a remarkable difference in root biomass was observed (P<0.05; Table 1). Therefore, a positive priming effect induced by the higher root biomass in the continuous maize cropping system might cause a higher rate of decomposition of “old” carbon and thus higher heterotrophic soil respiration compared with the continuous soybean and wheat systems.
619
Effects of soil temperature and moisture on CO2 emissions Generally, soil temperature and moisture have been identified as the main factors influencing the seasonal dynamics of soil respiration in short time scales (Bauchmann 2000; Xu and Qi 2001; Ding et al. 2007), and their relationship is typically described using an exponential equation model (Davidson et al. 1998; Ding et al. 2010). However, the present study found a poor relationship between soil CO 2 flux and soil temperature when using the complete sampling data (Table 2). After the initial five data sets were excluded, we found an improved fit for the relationship using an exponential equation between soil temperature and soil CO2 fluxes that could model their relationships under all treatments with greatly improved R 2 values (42-65%; Table 2). Similar phenomena were reported in our previous study (Li et al. 2013a, b) and in other studies (Ding et al. 2010; Fuß et al. 2011). We argued that soil disturbances (such as ploughing or fertilization at the seedling stage) might have affected soil CO2 fluxes and further modified the influence of soil temperature on CO 2 fluxes (Li et al. 2013a). Additionally, root growth may have influenced the relationship between soil CO2 fluxes and soil temperature because soil respiration has been shown to correlate with fine root production (Lee and Jose 2003). The period we excluded from our calculations contained mostly heterotrophic respiration; thus, the improved relationship between soil temperature and CO2 fluxes indicated that soil temperature may be more closely associated with autotrophic respiration than with heterotrophic respiration. The Q10 values of soil respiration ranged between 1.63 for soybean and 2.31 for maize at the 5-cm depth in the present study (Table 2). The different Q 10 values for different cropping systems suggest that the crop-specific effects on the estimation of temperature sensitivity should be taken into account when modeling the carbon cycle, as most carbon models assume a constant Q 10 under long-term continuous cropping systems, regardless of the crop type (Tian et al. 1999; Graf et al. 2008). In addition, it is noteworthy that temperature measurements within the
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
YOU Meng-yang et al.
620
upper 10 cm are likely to lead to an underestimation of temperature sensitivity (Graf et al. 2008). Thus, the Q10 values of soil respiration at a depth of 10 cm might be more accurate than those at a depth of 5 cm. Similarly, the relationship between soil CO2 fluxes and soil WFPS was also fitted with an exponential model after exclusion of the first five samples (Table 3). We also attributed this result to the root growth or soil disturbance due to ploughing or the application of chemical fertilizer. Furthermore, the multiple regression model including both soil temperature and WFPS (Table 3) revealed a collective impact of soil temperature and moisture on soil CO2 emissions from the Mollisols.
CONCLUSION No remarkable differences in SOC were observed after 20 yr of continuous maize, soybean, and wheat cropping. Although there were no differences in SOC, significant differences in soil CO 2 emissions were observed among the three cropping systems. The influences of soil temperature and WFPS on soil CO2 fluxes could be described well using an exponential equation model. The Q 10 values ranged between 1.63 for soybean and 2.31 for maize at a depth of 5 cm depth. A log-transformed multiple regression model including both soil temperature and moisture accounted for 51-66% of the seasonal variations in soil CO2 fluxes. Our results suggest that soil CO2 emissions and Q10 values from continuous cropping systems largely depend on crop types in Mollisols of northeast China.
MATERIALS AND METHODS Site description and experimental design The field experiment was carried out at the Hailun National Experiment Station of Agroecosystem of the Chinese Academy of Sciences, located in Hailun County, Heilongjiang Province (47°26´N, 126°38´E), in the growing season between late spring and early autumn in 2011. The region has a typical temperate continental monsoon climate, characterized by hot summers and cold winters. The mean annual temperature was 1.5°C, and the lowest and highest mean monthly values were detected in January (-23°C) and
in July (21°C), respectively. The mean annual precipitation was 550 mm, more than 80% of which occurred from May to September. The frost-free period was approximately 120 d. The soil type is loamy loess and is classified as a Mollisol (Typic Hapludoll). A long-term continuous cropping experiment was established in a completely randomized block design in 1991. The experiment included three replicates of three treatments: continuous monocultures of soybean, wheat, and maize. Each block covered 77 m2, with 10 rows in each block, and was separated by a 0.7-m buffer strip. The fertilizer applications were as follows: 27 kg N ha-1 and 69 kg P ha-1 for soybean; 92 kg N ha-1 and 39 kg P ha-1 for wheat; 131 kg N ha-1 and 69 kg P ha-1 for maize. Fertilizers N and P were applied in the form of urea and diammonium phosphate, respectively. For urea application, two splits were adopted, with the treatments receiving N fertilizer as the basal (May 10) and supplementary (June 22) fertilizers at a ratio of 1:1 for continuous maize cropping. The crop straw was removed after harvest under all treatments, following the common practice of local farmers. The soil was manually tilled to a depth of 15 to 20 cm after harvest.
Soil CO2 flux measurement Soil CO2 emission was sampled using a closed-chamber followed by gas chromatography. Three chambers (0.7 m× 0.2 m×0.25 m) were laid within each treatment. Air samples were manually collected using a 20-mL syringe from a septum installed at the top of the chamber at 0, 10, 20, and 30 min after closure, and then injected into preevacuated 18-mL vials and taken to the laboratory for analysis. Gas sampling was performed between 9:00 and 11:00 a.m. once per week, except on July 30th due to crop lodging resulting from heavy rainfall. Carbon dioxide was collected from the whole soil including rhizosphere and bulk soil (i.e., rhizosphere respiration and native soil respiration). Sample CO2 concentration was determined with a gas chromatograph (GC-2010, Shimadzu Corp., Japan) equipped with a flame ionization detector. The increased rate of CO2 concentration vs. time in the chamber air was calculated by linear regression analysis. All linear regression values of R2 were larger than 0.90. Carbon dioxide flux was computed from the rate of the change in chamber CO 2 concentration vs. time, chamber volume V and soil surface area A using the following formula: (1) f=ρ×Δc/Δt×V/A×273/(273+T) Where, f is the soil CO2 flux in mg CO2 m-2 h-1, ρ is the CO2 density under standard conditions, Δc/Δt is the linear change in CO2 concentration inside the chamber and T is the temperature inside the chamber. Cumulative soil CO2 emission (g CO2 m-2) during the entire sampling season was calculated by summing the products of the (averaged) two neighboring fluxes, multiplied by their interval time. The air temperature inside the chamber was recorded with
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
an Hg thermometer. Soil temperatures at vertical depths of 5- and 10-cm were measured in situ using a geothermometer simultaneous to the gas sampling. Field soil water content was determined by a gravimetric method. Soil bulk density was also gravimetrically determined from undisturbed soils within 1 m of the field chambers. Soil WFPS was used as the moisture variable in all analyses, as this property integrates porosity and moisture variables (Franzluebbers 1999). Values of the gravimetric soil moisture were converted to soil WFPS values using the following equation: (2) WFPS (%)=(θg×sbd×100%)/[1-(sbd/spd)] Where, θg is gravimetric soil moisture (kg water kg-1 soil), sbd is soil bulk density (Mg m-3 soil) and spd is soil particle density.
Soil sampling and analyses, and biomass determination Soil samples were collected in early October 2011. Eight soil samples (0-20 cm) were randomly collected within each block and then mixed thoroughly to form a composite for analyses of SOC, TN, and soil pH. Air-dried soil samples (<2 mm) were used to determine soil pH in a 1:2.5 (w/v) soil to water ratio. Soil organic C and TN concentrations were analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). Soybean and maize were planted in early May and harvested in late September; wheat was directly sown in April and harvested in July, 2011. All aboveground crop materials within a plot area of 2 m×1.4 m were cut in each block. Root biomass (0-20 cm) was measured in the same location used for aboveground sampling. Roots were separated from soils by picking and washing. All crop samples (grain, straw and root) were oven-dried at 70°C for 48 h and then weighed.
Calculations The response of soil CO 2 flux to soil temperature was described by an exponential function: (3) f=aebT Where, f is the soil CO 2 flux (mg CO 2 m -2 h -1) at soil temperature T, and a and b are regression coefficients. The temperature sensitivity (Q10), defined as the difference in soil respiration rate over a 10°C increment, was calculated by the following first-order exponential function: (4) Q10=e10b
Statistical analyses Homogeneity of variance and normality tests were carried out on gas and biomass data. A one-way analysis of variance (ANOVA) following the least significant difference (LSD) was used to test the differences in SOC content, biomass, and cumulative CO2 emissions among treatments. Regression
621
analysis was used to determine the relationships between soil environmental parameters (soil temperature, soil moisture, and soil temperature combined with soil moisture) and soil CO2 fluxes. All statistical analyses were conducted using SPSS 13.0 for Windows (SPSS Inc, Chicago, IL).
Acknowledgements This work was supported by the Key Research Program of the Chinese Academy of Sciences (KZZD-EW-TZ-16-02), the Foundation for Young Talents of the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (DLSYQ13001), and the National Natural Science Foundation of China (41101283). We thank Ms. Sun Guidan for laboratory assistance and Ms. Gao Weili and Wen Xiuling for participation in the field sampling.
References Almagro M, Loez J, Querejeta J L, Martinez-Mena M. 2009. Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem. Soil Biology & Biochemistry, 41, 594-605. Bauchmann N. 2000. Biotic and abiotic factors controlling soil respiration rates in Picea abies stands. Soil Biology & Biochemistry, 32, 1625-1635. Bauer J, Herbst M, Huisman J A, Weihermüller L, Vereecken H. 2008. Sensitivity of simulated soil heterotrophic respiration to temperature and moisture reduction functions. Geoderma, 145, 17-27. de Bortoli Teixeira D, Panosso A R, Cerri C E P, Pereira G T, La Scala N. 2011. Soil CO2 emission estimated by different interpolation techniques. Plant and Soil, 345, 187-194. Conant R T, Ogle S M, Paul E A, Paustian K. 2011. Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation. Frontiers in Ecology and the Environment, 9, 169-173. Davidson E A, Belk E, Boone R D. 1998. Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest. Global Change Biology, 4, 217-227. Davidson E A, Verchot L V, Cattanio J H, Ackerman I L, Carvalho J E M. 2000. Effects of soil water content on soil respiration in forests and cattle pastures of eastern Amazonia. Biogeochemistry, 48, 53-69. Diacono M, Montemurro F. 2010. Long-term effects of organic amendments on soil fertility. A review. Agronomy for Sustainable Development, 30, 401-422. Ding W, Cai Y, Cai Z. 2007. Soil respiration under maize crops: effects of water, temperature, and nitrogen fertilizaton. Soil Science Society of America Journal, 71, 944-951. Ding W, Yu H, Cai Z, Han F, Xu Z. 2010. Responses of soil respiration to N fertilization in a loamy soil under maize cultivation. Geoderma, 155, 381-389.
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
622
Emran M, Gispert M, Pardini G. 2012. Comparing measurement methods of carbon dioxide fluxes in a soil sequence under land use and cover change in North Eastern Spain. Geoderma, 170, 176-185. Fiener P, Dlugoß V, Korres W, Schneider K. 2012. Spatial variability of soil respiration in a small agricultural watershed - Are patterns of soil redistribution important? Catena, 94, 3-16. Franzluebbers A J. 1999. Microbial activity in response to water-filled pore space of variably eroded southern Piedmont soils. Applied Soil Ecology, 11, 91-101. Fuß R, Ruth B, Schilling R, Scherb H, Munch J C. 2011. Pulse emissions of N 2 O and CO 2 from an arable field depending on fertilization and tillage practice. Agriculture, Ecosystems & Environment, 144, 61-68. Gao J, Ouyang H, Lei G, Xu X, Zhang M. 2011. Effects of temperature, soil moisture, soil type and their interactions on soil carbon mineralization in Zoigê alpine wetland, Qinghai-Tibet Plateau. Chinese Geographical Science, 21, 27-35. González-Sánchez E J, Ordóñez-Fernández R, CarbonellBojollo R, Veroz-González O, Gil-Ribes J A. 2012. Meta-analysis on atmospheric carbon capture in Spain through the use of conservation agriculture. Soil & Tillage Research, 122, 52-60. Graf A, Weihermüller L, Huisman J A, Herbst M, Bauer J, Vereecken H. 2008. Measurement depth effects on the apparent temperature sensitivity of soil respiration in field studies. Biogeosciences, 5, 1175-1188. Herbst M, Prolingheuer N, Graf A, Huisman J A, Weihermüller L, Vanderborght J. 2009. Characterization and understanding of bare soil respiration spatial variability at plot scale. Vadose Zone Journal, 8, 762771. Iqbal J, Hu R, Du L, Lu L, Lin S, Chen T, Ruan L. 2008. Differences in soil CO 2 flux between different land use types in mid-subtropical China. Soil Biology & Biochemistry, 40, 2324-2333. Jabro J D, Sainju U, Stevens W B, Evans R G. 2008. Carbon dioxide flux as affected by tillage and irrigation in soil converted from perennial forages to annual crops. Journal of Environmental Management, 88, 1478-1484. Jarecki M K, Lal R. 2003. Crop management for soil carbon sequestration. Critical Reviews in Plant Sciences, 22, 471-502. Jia B, Zhou G, Wang Y, Wang F, Wang X. 2006. Effects of temperature and soil water-content on soil respiration of grazed and ungrazed Leymus chinensis steppes, Inner Mongolia. Journal of Arid Environments, 67, 60-76. Kou T J, Zhu P, Huang S, Peng X X, Song Z W, Deng A X, Gao H J, Peng C, Zhang W J. 2012. Effects of longterm cropping regimes on soil carbon sequestration and aggregate composition in rainfed farmland of Northeast China. Soil & Tillage Research, 118, 132-138. Kuzyakov Y. 2006. Sources of CO 2 efflux from soil and review of partitioning methods. Soil Biology &
YOU Meng-yang et al.
Biochemistry, 38, 425-448. Kuzyakov Y, Larionova A A. 2005. Review of methods and results of separation of root and rhizomicrobial respiration. Journal of Plant Nutrition and Soil Science, 168, 503-520. Lal R. 2004. Soil carbon sequestration impacts on global climate change and food security. Science, 304, 16231626. Lee K H, Jose S. 2003. Soil respiration, fine root production, and microbial biomass in cottonwood and loblolly pine plantations along a nitrogen fertilization gradient. Forest Ecology and Management, 185, 263-273. Li L J, You M Y, Shi H A, Ding X L, Qiao Y F, Han X Z. 2013a. Soil CO2 emissions from a cultivated Mollisol: Effects of organic amendments, soil temperature, and moisture. European Journal of Soil Biology, 55, 83-90. Li L J, You M Y, Shi H A, Han X Z. 2013b. Tillage effects on SOC and CO 2 emissions of Mollisols. Journal of Food, Agriculture & Environment, 11, 340-345. Lou Y, Wang J, Liang W. 2011a. Impacts of 22-year organic and inorganic N managements on soil organic C fractions in a maize field, northeast China. Catena, 87, 386-390. Lou Y, Xu M, Wang W, Sun X, Liang C. 2011b. Soil organic carbon fractions and management index after 20 yr of manure and fertilizer application for greenhouse vegetables. Soil Use & Management, 27, 163-169. Purakayastha T J, Rudrappa L, Singh D, Swarup A, Bhadraray S. 2008. Long-term impact of fertilizers on soil organic carbon pools and sequestration rates in maize-wheat-cowpea cropping system. Geoderma, 144, 370-378. Raich J W, Mora G. 2005. Estimating root plus rhizosphere contributions to soil respiration in annual croplands. Soil Science Society of America Journal, 69, 634-639. Raich J W, Tufekcioglu A. 2000. Vegetation and soil respiration: correlation and controls. Biogeochemistry, 48, 71-90. Rudrappa L, Purakayastha T J, Singh D, Bhadraray S. 2006. Long-term manuring and fertilization effects on soil organic carbon pools in a Typic Haplustept of semi-arid sub-tropical India. Soil & Tillage Research, 88, 180-192. Scheer C, Grace P R, Rowlings D W, Kimber S, Zwieten L V. 2011. Effect of biochar amendment on the soilatmosphere exchange of greenhouse gases from an intensive subtropical pasture in northern New South Wales, Australia. Plant and Soil, 345, 47-58. Singh K P, Ghoshal N, Singh S. 2009. Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality. Applied Soil Ecology, 42, 243-253. Six J, Conant R T, Paul E A, Paustian K. 2002. Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils. Plant and Soil, 241, 155-176. Sombrero A, de Benito A. 2010. Carbon accumulation in
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Soil CO2 Emissions as Affected by 20-Year Continuous Cropping in Mollisols
soil. Ten-year study of conservation tillage and crop rotation in a semi-arid area of Castile-Leon, Spain. Soil & Tillage Research, 107, 64-70. Sugihara S, Funakawa S, Kilasara M, Kosaki T. 2012. Effects of land management on CO2 flux and soil C stock in two Tanzanian croplands with contrasting soil texture. Soil Biology & Biochemistry, 46, 1-9. Tian H, Melillo J M, Kicklighter D W, McGuire A D, Helfrich J. 1999. The sensitivity of terrestrial carbon storage to historical climate variability and atmospheric CO2 in the United States. Tellus B, 51, 414-452. Ussiri D A N, Lal R. 2009. Long-term tillage effects on soil carbon storage and carbon dioxide emissions in continuous corn cropping system from an alfisol in Ohio. Soil & Tillage Research, 104, 39-47. Varvel G E. 2006. Soil organic carbon changes in diversified rotations of the western corn belt. Soil Science Society of America Journal, 70, 426-433. Wang W, Liao Y C, Guo Q. 2013. Seasonal and annual
623
variations of CO2 fluxes in rain-fed winter wheat agroecosystem of loess plateau, China. Journal of Integrative Agriculture, 12, 147-158. West T O, Post W M. 2002. Soil oganic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Science Society of America Journal, 66, 1930-1946. Wilson H M, Al-Kaisi M M. 2008. Crop rotation and nitrogen fertilization effect on soil CO2 emissions in central Iowa. Applied Soil Ecology, 39, 264-270. Xu M, Qi Y. 2001. Soil-surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California. Global Change Biology, 7, 667-677. Xue Y D, Yang P L, Luo Y P, Li Y K, Ren S M, Su Y P, Niu Y T. 2012. Characteristics and driven factors of nitrous oxide and carbon dioxide emissions in soil irrigated with treated wastewater. Journal of Integrative Agriculture, 11, 1354-1364. (Managing editor SUN Lu-juan)
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.