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Soil CO2 emissions from a cultivated Mollisol: Effects of organic amendments, soil temperature, and moisture
European Journal of Soil Biology 55 (2013) 83e90
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European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Soil CO2 emissions from a cultivated Mollisol: Effects of organic amendments, soil temperature, and moisture Lu-Jun Li a, *,1, Meng-Yang You a,1, Hong-Ai Shi a, b, Xue-Li Ding a, Yun-Fa Qiao a, Xiao-Zeng Han a, * a b
Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, China Northeast Agricultural University, Harbin 150030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2012 Received in revised form 25 December 2012 Accepted 31 December 2012 Available online 10 January 2013 Handling editor: Yakov Kuzyakov
A field experiment was conducted to examine the influences of long-term applications of maize straw and organic manure on carbon dioxide (CO2) emissions from a cultivated Mollisol in northeast China and to evaluate the responses of soil CO2 fluxes to temperature and moisture. Soil CO2 flux was measured using closed chamber and gas chromatograph techniques. Our results indicated that the application of organic amendments combined with fertilizer nitrogen, phosphorus and potassium (NPK) accelerated soil CO2 emissions during the maize growing season, whereas NPK fertilization alone did not impact cumulative CO2 emissions. Cumulative CO2 emissions were higher from soils amended with pig manure relative to those with maize residue. Cumulative CO2 emissions during the growing season were 988 and 1130 g CO2 m2 under applications of 7500 and 22,500 kg ha1 pig manure combined with NPK, respectively, which were 42 and 63% higher than the emissions from the control (694 g CO2 m2). The applications of 2250 and 4500 kg ha1 maize straw combined with NPK marginally increased soil CO2 emissions by 23 and 28% compared with the control, respectively. A log-transformed multiple regression model including both soil temperature and moisture explained 50e88% of the seasonal variation in soil CO2 fluxes. Cumulative soil CO2 emissions were affected more by applied treatments than by soil temperature and moisture. Our results suggest that the magnitude of the impact of soil amendments on CO2 emissions from Mollisols primarily depends on the type of organic amendments applied, whereas the application rate has limited impacts. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: Mollisols Soil organic amendment Soil respiration Temperature sensitivity Water-filled pore space
1. Introduction Increased atmospheric carbon dioxide (CO2) has been considered a major contributor to global warming as well as climatic change [1,2]. Although arable soil has been identified as one of the main CO2 sources in agroecosystems due to inappropriate management practices, it can also serve as a net sink for atmospheric CO2 through appropriate agricultural management [3,4]. Organic amendments (e.g., straw and organic manure) have been widely used in agroecosystems due to their positive roles in soil fertility improvement and climate change mitigation via soil carbon sequestration [3,5,6]. Previous studies have shown various responses of soil CO2 emissions to applications of organic
* Corresponding authors. Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 138 Haping Rd, Harbin 150081, China. Tel.: þ86 451 8660 2940; fax: þ86 451 8660 3736. E-mail addresses: [email protected] (L.-J. Li), [email protected] (X.-Z. Han). 1 These authors contributed equally to this work. 1164-5563/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ejsobi.2012.12.009
amendments [7,8]. The amount of soil CO2 emissions is dependent on many factors, primarily the type and level of applied organic amendments [9], as well as the quantity of carbon already in the soil [10]. In fact, soil management, plant cover and soil nutrient status can not only alter soil respiration, but also change the temperature sensitivity of this process [11]. Thus, the overall response of soil CO2 emissions to organic amendments is a complex process and remains uncertain. Soil temperature and moisture have been identified as the most important environmental factors influencing soil CO2 emissions [12,13]. The temperature effect is commonly described as an exponential equation [12], whereas the effects of soil moisture are not always consistent [14]. The lack of consensus among trials could result from the collective impacts of differences in soil types [15], experiment duration and methods of CO2 emission measurement [16], added to the confounded effects of soil temperature and moisture [17,18]. Therefore, the roles of soil temperature and moisture in soil CO2 emissions are still unclear. Mollisols in northeast China are characterized by a high carbon content and fine texture. Due to intensive cultivation and
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low organic material return, a remarkable decline in soil organic carbon (SOC) in croplands has occurred over the last several decades in the Mollisols. To improve the fine texture and increase the SOC content of these soils, organic amendments (e.g. crop straw and organic manure) have been widely applied in the region. However, field observations on the responses of soil CO2 emissions to long-term organic material incorporations into high carbon soils are still limited. The objectives of this study were to examine the influences of long-term applications of organic amendments combined with chemical fertilizers on CO2 emissions from Mollisols in China and to clarify the responses of soil CO2 fluxes to soil temperature and moisture. We hypothesized that soil CO2 emissions might be enhanced by long-term applications of organic amendments due to the incorporation of exogenous carbon. 2. Materials and methods 2.1. Site description The experimental site was located at the Hailun National Experiment Station of Agroecosystems of the Chinese Academy of Sciences (47 260 N, 126 380 E). The mean annual temperature was 1.5 C, and the lowest and highest mean monthly values were 23 C in January and 21 C in July, respectively. The mean annual precipitation was 550 mm, more than 80% of which occurred from MayeSeptember. The frost-free period is approximately 120 days. The soil is a loamy loess and classified as Typic Hapludoll. 2.2. Experimental design The field site is on a flat plain and was a native prairie before the land was cleared for cropping more than 100 years ago. A long-term field experiment was established in a randomized block design in 1990 based on four treatments with three replicates: no fertilizer (Control), chemical fertilizer made up of nitrogen, phosphorus and potassium (NPK), NPK fertilizer plus maize straw (NPK þ MS1), and NPK fertilizer plus double the maize straw of NPK þ MS1 (NPK þ MS2). In 2001, we added two additional treatments with three replicates: NPK fertilizer plus pig manure (NPK þ OM1) and NPK fertilizer plus triple the pig manure of NPK þ OM1 (NPK þ OM2). Each replicate covered a surface area of 12 5.6 m2 and was separated from other replicates by a 0.7-m buffer strip. A maizeesoybeanewheat crop rotation was established, and the crop was maize in 2011. Maize was sown on May 8 and harvested on September 26, 2011. To increase soil temperature to stimulate maize germination in early spring, the field was split into ridges (approximately 10 cm in height) and furrows using a ridge plow. The distance between adjacent ridges was 70 cm. The depth of tillage was approximately 20 cm. The fertilizer applications were as follows: 1) 120 kg N ha1, 60 kg phosphorus pentoxide (P2O5) ha1, and 30 kg potassium oxide (K2O) ha1 for maize; 2) 20.25 kg N ha1, 51.75 kg P2O5 ha1, and 30 kg K2O ha1 for soybeans; and 3) 75 kg N ha1, 60 kg P2O5 ha1, and 30 kg K2O ha1 for wheat. The maize straw and organic manure were applied as follows (dry-weight basis): 2250 kg ha1 in NPK þ MS1, 4500 kg ha1 in NPK þ MS2, 7500 kg ha1 in NPK þ OM1, and 22,500 kg ha1 in NPK þ OM2 for the 3 year rotation. Urea was split into two applications for the treatments receiving N fertilizer, the basal and supplementary fertilizers, with a ratio of 1:2 for maize. The pig manure was composted before application. Straw and organic manure applications were performed in the previous year after crop harvest. The maize straw contained 411 mg organic C g1 and 6.7 mg total N (TN) g1; the pig manure contained 265 mg organic C g1 and 31 mg TN g1.
2.3. Measurement of CO2 fluxes Soil CO2 flux was measured using closed chamber and gas chromatograph techniques during the maize growing season. Gas sampling was initiated on May 27, 2011 at the time of emergence and ended on September 30, 2011 with the maize harvest. During the maize growing season, gas sampling was conducted once per week between 9:00 and 11:00 am, the optimal sampling time to represent the average daily CO2 efflux in this region [19]. The CO2 flux was not measured on July 30 due to heavy rain. Four gas samples were collected at 0, 10, 20, and 30 min after closure of the chamber from a septum installed at the top of the closed chamber (0.7 m 0.2 m 0.25 m) using a 20 ml gas-tight syringe. The collected gas samples were immediately transferred to pre-evacuated vials and transported to the laboratory for analysis. The collected gases included the CO2 from both rhizosphere respiration and native soil respiration. Carbon dioxide concentration was determined using a gas chromatograph (GC-2010, Shimadzu Corp., Japan) equipped with a flame ionization detector (FID) using an 80/100 mesh Chromosorb 102 column. Carbon dioxide flux was calculated from the change in CO2 concentration in the chamber vs. closure time using the following formula:
f ¼ r Dc=Dt V=A 273=ð273 þ TÞ
(1)
where f is the CO2 flux (mg CO2 m2 h1), r is the CO2 density under standard conditions (mg m3), Dc/Dt is the change in CO2 concentration in the chamber (m3 m3 h1), V is the chamber volume (m3), A is the soil surface area (m2), and T is the air temperature inside the chamber ( C). Cumulative soil CO2 emission (g CO2 m2) was calculated by summing the average production of two neighboring fluxes, multiplied by the collection interval time. 2.4. Soil sampling and analyses and measurement of micro-climate factors Soil samples (0e20 cm) were collected within each block in early October 2011. Eight soil samples were randomly collected and then mixed thoroughly to form a composite. After the visible roots, fauna and organic debris were removed by hand, soil samples were sieved (<2 mm) and air-dried. Air-dried soil samples (<2 mm) were used to determine soil pH from a 1:2.5 (w/v) mixture of soil and water. A portion of the air-dried samples was ground (<0.25 mm) prior to the SOC and TN analyses. Soil organic C and TN contents were analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). Soil bulk density was measured using the core method. The air temperature inside the chamber was measured with a mercury thermometer; soil temperatures at 5 and 10 cm depths were determined in situ simultaneously using a geothermometer. Field soil moisture was gravimetrically determined from undisturbed soils within 1 m of the field chambers for the calculation of soil water-filled pore space (WFPS). The maize crop was harvested on September 26, 2011. All aboveground crop materials within a plot area of 2 1.4 m were cut. The harvested materials were separated into grain and straw, the latter including stems and leaves. Root biomass (0e20 cm) was quantified at the same location used for aboveground sampling. Roots were separated from soils by hand picking and washing with water. All crop samples (grain, straw and root) were oven-dried at 70 C for 48 h and then weighed to determine the biomass. 2.5. Calculation and statistical analyses The accepted significance level was a ¼ 0.05. Homogeneity of variance and normality tests were conducted on the gas and biomass
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data at the 5% level. A one-way analysis of variance (ANOVA) with least significant difference was used to test the differences in maize yield, biomass, soil property parameters, and cumulative CO2 emissions among the six treatments at P ¼ 0.05. The relationships between straw biomass, maize yield, root biomass, and SOC concentration and cumulative CO2 emissions were examined using linear regression. Nonlinear regression analyses were used to determine the relationships between soil temperature, moisture, and soil temperature combined with moisture and soil CO2 fluxes. The response of soil CO2 fluxes to soil temperature was described by an exponential function:
f ¼ a expðbTÞ;
(2)
where f is the soil CO2 flux at soil temperature T and a and b are regression coefficients. The temperature sensitivity (Q10), defined as a multiplier of soil CO2 flux for a 10 C increase in soil temperature, was calculated as follows: Q10 ¼ exp (10b). To clarify the effects of soil moisture on soil CO2 flux, we removed the possible confounding effect of soil temperature on soil CO2 flux by standardizing soil CO2 flux to a soil temperature of 10 C using the following equation:
f ¼ f10 expðlnQ10 ðT T10 Þ=10Þ;
(3)
where f is the soil CO2 flux measured in the field, f10 is the soil CO2 flux at 10 C (T10), Q10 is the temperature sensitivity listed in Table 2, and T is the soil temperature measured in the field at a 5 cm depth. 3. Results 3.1. Soil properties, crop yield and biomass The NPK þ OM2 fertilization increased SOC content by 12% compared with the control (P < 0.05, Table 1). However, the SOC contents in treatments NPK, NPK þ MS1, NPK þ MS2, and NPK þ OM1 were not significantly different from that in the control (P > 0.05; Table 1). Total N content significantly increased by 24% over the control under the NPK þ OM2 treatment (Table 1). The pH value under the NPK þ OM2 treatment was higher than that under NPK þ MS1 (P < 0.05; Table 1). Crop grain, straw, and root biomasses in the control plots were significantly lower than in all other treatments (P < 0.05; Table 1). Compared with the control, the applications of NPK, NPK þ MS1, NPK þ MS2, NPK þ OM1, and NPK þ OM2 significantly increased straw biomass by 45, 46, 62, 11, and 20%, respectively and increased root biomass by 50, 139, 138, 110, and 126%, respectively (all P < 0.05; Table 1). 3.2. Soil temperature and moisture Soil temperature varied from 4 to 37 C during the maize growing season, with averages of 21 C and 18 C at 5 and 10 cm
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depths, respectively (Fig. 1a, b). Soil temperatures at both 5 and 10 cm depths reached maxima on June 24 (Fig. 1a, b) when the soil WFPS was lower than at most other sampling times (Fig. 1c). Mean soil temperatures during the growing season were 19.5 and 19.3 C at a 5 cm depth (Fig. 1a) and 17.4 and 17.5 C at a 10 cm depth for NPK þ OM1 and NPK þ OM2, respectively (Fig. 1b), which were lower values than those in the control treatment (5 cm: 22 C, 10 cm: 19.7 C). Soil moisture ranged from 25 to 51% WFPS during the maize growing season and was higher on average in the control than in the other treatments (Fig. 1c). Soil moisture was significantly correlated with cumulative precipitation between two neighboring measurement times (r2 ¼ 0.54e0.63, n ¼ 18, P < 0.001). 3.3. Seasonal variations in soil CO2 fluxes and cumulative CO2 emissions Soil CO2 fluxes, regardless of organic or chemical fertilizer application, increased gradually from the experiment’s beginning in May and reached a maximum on August 6 (Fig. 2a). Soil CO2 fluxes then declined gradually until the harvest at the end of September. The mean soil CO2 flux during the maize growing season in the control treatment was 215 mg CO2 m2 h1, which was significantly lower than those in the treatments of organic manure applications (NPK þ OM1, 310 mg CO2 m2 h1; NPK þ OM2: 355 mg CO2 m2 h1; P < 0.05). Based on soil CO2 fluxes, cumulative CO2 emissions during the maize growing season in the control, NPK, NPK þ MS1, NPK þ MS2, NPK þ OM1, and NPK þ OM2 treatments were estimated to be 694, 678, 853, 889, 988, and 1130 g CO2 m2 (Fig. 2b). Cumulative CO2 emissions in the NPK þ OM1 and NPK þ OM2 treatments were significantly higher than that in the control by 42 and 63%, respectively (both P < 0.05); the NPK þ MS1 and NPK þ MS2 treatments marginally increased CO2 emissions by 23 and 28%, respectively (P < 0.1). In contrast, NPK fertilization alone did not significantly impact cumulative CO2 emission (P > 0.05; Fig. 2b). In addition, there was no significant difference in soil CO2 emissions between NPK þ OM1 and NPK þ OM2 or between NPK þ MS1 and NPK þ MS2 (both P > 0.05; Fig. 2b). 3.4. Impacts of micro-climate factors on soil CO2 fluxes During the maize growing season, an exponential model explained 26e34% of the seasonal variations in soil CO2 fluxes in only four out of the six treatments at a 10 cm depth (Table 2). Using only the data from the elongation to the harvest stage, however, we found improved relationships between soil temperature and CO2 fluxes (R2 ¼ 0.43e0.91, P < 0.05; Table 2). The temperature sensitivity (Q10) of soil CO2 fluxes during the growing season ranged between 1.87 and 3.00 (Table 2). Correlation analysis showed poor relationships between soil CO2 fluxes and WFPS during the growing season (Table 3). After excluding the masking influence of soil temperature, we found greatly improved relationships between
Table 1 Soil properties, maize yield and biomass under different fertilization treatments. The values are the means (n ¼ 3) with SE. Treatment a
Control NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
SOC (g kg1) b
28.20(0.17)b 27.66(0.09)b 29.89(0.26)ab 28.75(0.10)ab 28.57(0.47)ab 31.56(0.74)a
TN (g kg1)
pH
Grain (kg ha1)
Straw (kg ha1)
Root (kg ha1)
2.06(0.03)b 2.06(0.02)b 2.21(0.11)b 2.14(0.05)b 2.18(0.08)b 2.55(0.08)a
5.91(0.02)ab 5.92(0.05)ab 5.69(0.02)b 5.76(0.17)ab 5.76(0.12)ab 6.01(0.04)a
5797(344)d 8384(146)b 8449(103)b 9394(312)a 6446(124)c 6981(120)c
3922(180)d 6348(341)c 7455(458)b 7177(181)bc 8173(455)ab 8700(195)a
517(30)c 775(42)b 1238(111)a 1232(98)a 1090(162)a 1166(125)a
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure. b The different letters in each column indicate the significant difference at P < 0.05.
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Fig. 2. Seasonal variations of soil CO2 fluxes (a) and cumulative CO2 emissions (b) from a Mollisol during a maize growing season. The vertical bars denote the standard error of the mean (n ¼ 3). The arrows denote the application time of basal (BF) and supplemental (SF) fertilizers. A one-way ANOVA with LSD post hoc test was performed to examine the differences in cumulative CO2 emissions among treatments. The different small letters indicate the significant difference in cumulative CO2 emissions among treatments at P < 0.05. Control, no fertilizer; NPK, chemical fertilizer NPK; NPK þ MS1, NPK plus maize straw; NPK þ MS2, NPK plus twice as much maize straw; NPK þ OM1, NPK plus pig manure; and NPK þ OM2, NPK plus three time the pig manure.
soil CO2 fluxes and WFPS in the 5 cm layer (Table 3), which explained 30e60% of the seasonal variations in soil CO2 flux. Furthermore, a log-transformed multiple regression model including both soil temperature and moisture [log(f) ¼ a þ b T log(W)] accounted for 50e88% of the seasonal variation in soil CO2 fluxes (Table 4). Cumulative CO2 emissions were significantly correlated with the harvested straw and root biomass (r2 ¼ 0.62, n ¼ 18, P < 0.001 and r2 ¼ 0.46, n ¼ 18, P ¼ 0.002, respectively; Fig. 3a). A significant correlation was also found between cumulative CO2 emissions and SOC (r2 ¼ 0.40, n ¼ 18, P ¼ 0.005; Fig. 3b). 4. Discussion 4.1. Effect of long-term organic amendment applications on soil CO2 emissions Consistent with our hypothesis, enhanced soil CO2 emissions were observed under organic amendment treatments (NPK þ MS1, NPK þ MS2, NPK þ OM1, NPK þ OM2) in comparison with the unfertilized control in our study. Several studies have shown that a long-term return of straw or addition of organic manure to soil can result in substantial increases in the soil respiration rate [8,20]. The total CO2 flux from soils can be divided into two components: rhizosphere-derived CO2 (rhizosphere respiration) and soil organic matter (SOM)-derived CO2 in root-free soil (basal respiration) [21]. Rhizosphere respiration reflects both the metabolic activities of the living root tissue and SOM decomposition in the rhizosphere, and both processes are proportional to root biomass [22]. Using a rootexclusion technique, a previous study observed that rhizosphere respiration comprised the majority of soil respiration in croplands, accounting for 67e80% of the overall CO2 emission from the Mollisols [23], and root respiration is an important contributor to rhizosphere respiration in situ [24]. Although the present study was not capable of distinguishing the root respiration component of total soil CO2 emissions, the increased root biomass with organic amendment applications (Table 1) might have partly enhanced cumulative CO2 emissions as reflected by the positive correlation
between CO2 emissions and root biomass (r2 ¼ 0.46, n ¼ 18, P ¼ 0.002; Fig. 3a). Aside from autotrophic root respiration, another important source of total CO2 flux is heterotrophic respiration from soil microorganisms and macrofauna [21,25]. As the contribution of soil macrofauna to total CO2 fluxes from soils is usually only a few percent [26], most of the CO2 derived from heterotrophic respiration is respired by soil microorganisms. Similar to an earlier observation [23], we found that soil CO2 emissions were significantly correlated to SOC (r2 ¼ 0.40, n ¼ 18, P ¼ 0.005; Fig. 3b). Soil CO2 emission related to microbial respiration is derived from basal respiration and CO2 production as a consequence of a priming effect [21]: organic manure and straw applications can greatly increase the amount of substrates for soil microorganisms and also alter soil microbial activity, subsequently accelerating SOM decomposition [27]. Therefore, the increased CO2 emissions from the organic amendment treatments in our study may be partly attributed to the positive priming effect of substrate inputs compared to the control. In addition to the sources affecting soil CO2 emissions described above, rhizomicrobial respiration and the decomposition of added organic amendments were also biogenic sources of soil CO2 emissions. However, due to high turnover rates and thus low residence times in soils [21], rhizomicrobial respiration and amendment decomposition probably make limited contributions to total CO2 emissions. More cumulative CO2 was emitted from soils amended with pig manure relative to those with maize residue (Fig. 2b), indicating that the manure amendment led to an enhancement of microbial activity in comparison with the maize residue. Microbial activity can be influenced by soluble carbon and nitrogen compounds in soils [28]. The levels of soluble carbon and nitrogen compounds in soils can be increased by consecutive applications of manure [29], whereas they can be decreased by maize residue applications due to the assimilation of available carbon and immobilization of available nitrogen by microorganisms [30]. Manure applications thus could provide more soluble carbon and nitrogen for microbial activity than maize residue. In addition, microbial activity and the related decomposition rate of organic amendments are partly
Fig. 1. Seasonal variations in soil temperature at 5 (a) and 10 cm (b) depths, soil water-filled pore space (WFPS) in the 5 cm layer (c), and daily precipitation and air temperature data (d) from a meteorological station at the field experiment site during a maize growing season. The vertical bars denote the standard error of the mean (n ¼ 3) for Fig. 1a, b, c. Control, no fertilizer; NPK, chemical fertilizer NPK; NPK þ MS1, NPK plus maize straw; NPK þ MS2, NPK plus twice as much maize straw; NPK þ OM1, NPK plus pig manure; and NPK þ OM2, NPK plus three time the pig manure.
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Table 2 Relationships between soil CO2 flux (f) and soil temperature (T) at 5 and 10 cm depths during the maize growing season. Treatment
Control
a
NPK
Depth Entire maize growing period From elongation to harvest (cm) (n ¼ 18) only (n ¼ 13) (May 27eSeptember 30, (July 1eSeptember 30, 2011) 2011) Equation
R2
Q10b Equation
R2
Q10
5
e
0
e
0.43*
1.87
10
e
0.06 e
5
e
0.17 e
f ¼ 93.234 exp(0.0412T) e
0.26* 1.51
f ¼ 88.912 exp(0.0530T) e
0.33* 1.70
f ¼ 79.630 exp(0.0571T) e
0.27* 1.77
f ¼ 101.244 exp(0.0566T) e
0.34* 1.76
e
0.13 e
10 NPK þ MS1
5 10
NPK þ MS2
5 10
NPK þ OM1
5 10
NPK þ OM2
5 10
0.14 e
0.10 e
0.20 e
0.04 e
f ¼ 57.703 exp(0.0627T) f ¼ 54.911 exp(0.0711T) f ¼ 51.821 exp(0.0677T) f ¼ 62.770 exp(0.0684T) f ¼ 44.806 exp(0.0793T) f ¼ 53.671 exp(0.0832T) f ¼ 34.516 exp(0.1030T) f ¼ 37.764 exp(0.1097T) f ¼ 65.664 exp(0.0820T) f ¼ 68.940 exp(0.0866T) f ¼ 65.412 exp(0.0935T) f ¼ 67.284 exp(0.0984T)
0.58** 2.04 0.72*** 1.97 0.81*** 1.98 0.58** 2.21 0.72*** 2.30 0.82*** 2.80 0.91*** 3.00 0.70*** 2.27 0.76*** 2.38 0.74*** 2.55 0.79*** 2.68
Correlation significance levels: *P < 0.05, **P < 0.01 and ***P < 0.001. a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure. b Q10: temperature sensitivity.
related to organic amendment quality, in which the C/N ratio has been shown to be a good predictor of the decomposition of organic amendments applied to soils [28]. The pig manure, with a low C/N ratio (w9), could decompose more easily than the maize residue with a higher C/N ratio (w61). Therefore, pig manure most likely provided more easily degradable and potentially more soluble carbon for microbial activity than the maize residue and thus led to greater soil CO2 emissions. Our results suggest that the magnitude of the impact of soil amendments on soil CO2 emissions primarily depends on the type of organic amendments applied to the test soil, whereas the rate of application has limited effects on cumulative Table 3 Relationships between soil CO2 flux (f) and soil WFPS (W) in the 5 cm layer during the maize growing season (n ¼ 13). Treatment
Equation
R2
P
Excluding the influence of soil temperature Equation
Control
a
NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
f ¼ 10.005W 188.059 f ¼ 6.815W 32.567 f ¼ 16.631W 309.019 f ¼ 12.650W 134.627 f ¼ 12.657W 123.650 f ¼ 16.018W 163.948
0.31 0.048 f ¼ 17.777 exp(0.0423W) 0.20 0.127 f ¼ 39.393 exp(0.0246W) 0.46 0.011 f ¼ 15.485 exp(0.0524W) 0.20 0.121 f ¼ 26.696 exp(0.0369W) 0.20 0.128 f ¼ 40.125 exp(0.0312W) 0.19 0.142 f ¼ 54.677 exp(0.0309W)
R2
P
0.39 0.023 0.41 0.019 0.60 0.002 0.53 0.005 0.35 0.033 0.30 0.054
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure.
Table 4 Relationships between soil CO2 flux (f) and soil temperature (T) at a 5 cm depth and soil WFPS (W) in the 5 cm layer during the maize growing season (n ¼ 13). Treatment a
Control NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
Equation log(f) log(f) log(f) log(f) log(f) log(f)
¼ ¼ ¼ ¼ ¼ ¼
1.732 1.709 1.628 1.538 1.812 1.819
þ þ þ þ þ þ
0.018 0.019 0.023 0.029 0.023 0.026
T T T T T T
log(W) log(W) log(W) log(W) log(W) log(W)
R2
P
0.50 0.79 0.68 0.88 0.75 0.79
0.007 <0.001 <0.001 <0.001 <0.001 <0.001
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure.
soil CO2 emissions as shown by the absence of a remarkable difference in CO2 emissions under different application rates of a given organic amendment (Fig. 2b). Despite the increased soil CO2 emissions, the combined use of organic amendments and chemical fertilizers improved or at least maintained the SOC content of the studied Mollisols (Table 1). This indicates that the carbon inputs to the soils were larger than or at least in equilibrium with the carbon losses from soils induced by the organic amendment applications. 4.2. Effects of soil temperature and moisture on the temporal variation of soil CO2 fluxes The temporal variation in soil CO2 fluxes is commonly related to soil temperature, moisture or both [12,31,32]. In the present study, however, only 26e34% of the seasonal variations in soil CO2 fluxes could be explained by soil temperature with an exponential equation under certain treatments (Table 2), which implied that other factors were affecting the CO2 fluxes. In our study, enhanced CO2 fluxes were observed after the applications of both basal and supplemental fertilizers (Fig. 2a). Moreover, an exponential equation described the relationship between soil temperature and CO2 fluxes well under all treatments when we restricted the analysis to the period from the elongation stage to the harvest time: R2 values were also greatly improved (0.43e0.91) given this time restriction, accompanied by increases in Q10 values (1.87e3.00; Table 2). Thus, we believe that a disturbance, most likely related to plowing or fertilization, affected soil CO2 emissions at the seedling stage and, further, partly modified the influence of soil temperature on CO2 fluxes, as shown by a previous study [33]. Previous studies have shown that soil moisture status can also influence soil respiration [14,34]. However, the present study found a poor relationship between soil CO2 fluxes and WFPS. There could be several reasons for this result. The narrow range of soil WFPS (25e51%, Fig. 1c) in the studied field might have resulted in the observed weak influence on soil CO2 fluxes [35]. Soil temperature was also an important factor regulating the effects of moisture on CO2 fluxes [36]. Previous researchers have noted that the effects of soil moisture on CO2 fluxes are partly obscured by soil temperature, because soil moisture and temperature usually change simultaneously [34,37]. In the present study, we found improved relationships (R2 ¼ 0.30e0.60) between soil CO2 fluxes and WFPS after the masking influence of soil temperature was excluded (Table 3). Furthermore, the logtransformed multiple regression model including both soil temperature and moisture was much better able to explain the seasonal variations in soil CO2 fluxes than the regression model with moisture alone, with or without excluding the influence of temperature (Tables 3 and 4). Therefore, our results showed that there was a significant interdependence between soil temperature and moisture in their effects on soil CO2 fluxes in the studied Mollisols.
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Fig. 3. Relationships between cumulative CO2 emissions during the maize growing season and maize biomass at harvest (a) and soil organic carbon concentration (b) (n ¼ 18).
When a separate analysis was conducted for each treatment individually, the seasonal variations in soil CO2 flux rates could be positively explained by the seasonal variations in soil temperature and moisture. For all treatments together, however, we observed that the treatments with lower mean soil temperatures and moisture had higher CO2 emissions than others. These results, combined with the positive relationships between soil temperature, moisture and CO2 fluxes (Table 2), indicated that cumulative CO2 emissions during the growing season were affected more by the applied amendments than by soil temperature and moisture in the cultivated Mollisols. It is noteworthy that the closed chamber system is known to underestimate soil CO2 fluxes by approximately 10% due to the effective volume being larger than the volume of the chamber itself [38]. Thus, a further study comparing a closed- and an openchamber methodology is needed to obtain a calibration factor for the Mollisols. 5. Conclusion Applications of organic amendments combined with NPK accelerated CO2 emissions from soils, whereas NPK fertilization alone did not significantly impact cumulative CO2 emissions. More cumulative CO2 was emitted from soils amended with pig manure relative to those with maize residue. The log-transformed multiple regression model log(f) ¼ a þ b T log(W) including soil temperature and moisture accounted for 50e88% of the season variation in soil CO2 fluxes. Cumulative soil CO2 emissions during the growing season were affected more by the applied amendments than by soil temperature and moisture in the cultivated Mollisols in northeast China. Our results suggest that for the Mollisols, the magnitude of the impact of soil amendments on soil CO2 emissions depends primarily on the type of organic amendments applied, whereas the application rate has limited impacts. Acknowledgments This work was funded by the Strategic Priority Research ProgramClimate Change: Carbon Budget and Related Issues of the Chinese Academy of Sciences (no. XDA05050501), the National Key Basic Research Program of China (no. 2011CB100506), the Natural Science Foundation of China (no. 41101283 and 41101282), and the Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (no. 2011ZKHT01). We thank Gui-Dan Sun for laboratory assistance and Wei-Li Gao
and Xiu-Ling Wen for their participation in the field sampling. We also thank three anonymous reviewers for their helpful comments and suggestions, which improved the manuscript.
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Contents lists available at SciVerse ScienceDirect
European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Soil CO2 emissions from a cultivated Mollisol: Effects of organic amendments, soil temperature, and moisture Lu-Jun Li a, *,1, Meng-Yang You a,1, Hong-Ai Shi a, b, Xue-Li Ding a, Yun-Fa Qiao a, Xiao-Zeng Han a, * a b
Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, China Northeast Agricultural University, Harbin 150030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2012 Received in revised form 25 December 2012 Accepted 31 December 2012 Available online 10 January 2013 Handling editor: Yakov Kuzyakov
A field experiment was conducted to examine the influences of long-term applications of maize straw and organic manure on carbon dioxide (CO2) emissions from a cultivated Mollisol in northeast China and to evaluate the responses of soil CO2 fluxes to temperature and moisture. Soil CO2 flux was measured using closed chamber and gas chromatograph techniques. Our results indicated that the application of organic amendments combined with fertilizer nitrogen, phosphorus and potassium (NPK) accelerated soil CO2 emissions during the maize growing season, whereas NPK fertilization alone did not impact cumulative CO2 emissions. Cumulative CO2 emissions were higher from soils amended with pig manure relative to those with maize residue. Cumulative CO2 emissions during the growing season were 988 and 1130 g CO2 m2 under applications of 7500 and 22,500 kg ha1 pig manure combined with NPK, respectively, which were 42 and 63% higher than the emissions from the control (694 g CO2 m2). The applications of 2250 and 4500 kg ha1 maize straw combined with NPK marginally increased soil CO2 emissions by 23 and 28% compared with the control, respectively. A log-transformed multiple regression model including both soil temperature and moisture explained 50e88% of the seasonal variation in soil CO2 fluxes. Cumulative soil CO2 emissions were affected more by applied treatments than by soil temperature and moisture. Our results suggest that the magnitude of the impact of soil amendments on CO2 emissions from Mollisols primarily depends on the type of organic amendments applied, whereas the application rate has limited impacts. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: Mollisols Soil organic amendment Soil respiration Temperature sensitivity Water-filled pore space
1. Introduction Increased atmospheric carbon dioxide (CO2) has been considered a major contributor to global warming as well as climatic change [1,2]. Although arable soil has been identified as one of the main CO2 sources in agroecosystems due to inappropriate management practices, it can also serve as a net sink for atmospheric CO2 through appropriate agricultural management [3,4]. Organic amendments (e.g., straw and organic manure) have been widely used in agroecosystems due to their positive roles in soil fertility improvement and climate change mitigation via soil carbon sequestration [3,5,6]. Previous studies have shown various responses of soil CO2 emissions to applications of organic
* Corresponding authors. Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 138 Haping Rd, Harbin 150081, China. Tel.: þ86 451 8660 2940; fax: þ86 451 8660 3736. E-mail addresses: [email protected] (L.-J. Li), [email protected] (X.-Z. Han). 1 These authors contributed equally to this work. 1164-5563/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ejsobi.2012.12.009
amendments [7,8]. The amount of soil CO2 emissions is dependent on many factors, primarily the type and level of applied organic amendments [9], as well as the quantity of carbon already in the soil [10]. In fact, soil management, plant cover and soil nutrient status can not only alter soil respiration, but also change the temperature sensitivity of this process [11]. Thus, the overall response of soil CO2 emissions to organic amendments is a complex process and remains uncertain. Soil temperature and moisture have been identified as the most important environmental factors influencing soil CO2 emissions [12,13]. The temperature effect is commonly described as an exponential equation [12], whereas the effects of soil moisture are not always consistent [14]. The lack of consensus among trials could result from the collective impacts of differences in soil types [15], experiment duration and methods of CO2 emission measurement [16], added to the confounded effects of soil temperature and moisture [17,18]. Therefore, the roles of soil temperature and moisture in soil CO2 emissions are still unclear. Mollisols in northeast China are characterized by a high carbon content and fine texture. Due to intensive cultivation and
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low organic material return, a remarkable decline in soil organic carbon (SOC) in croplands has occurred over the last several decades in the Mollisols. To improve the fine texture and increase the SOC content of these soils, organic amendments (e.g. crop straw and organic manure) have been widely applied in the region. However, field observations on the responses of soil CO2 emissions to long-term organic material incorporations into high carbon soils are still limited. The objectives of this study were to examine the influences of long-term applications of organic amendments combined with chemical fertilizers on CO2 emissions from Mollisols in China and to clarify the responses of soil CO2 fluxes to soil temperature and moisture. We hypothesized that soil CO2 emissions might be enhanced by long-term applications of organic amendments due to the incorporation of exogenous carbon. 2. Materials and methods 2.1. Site description The experimental site was located at the Hailun National Experiment Station of Agroecosystems of the Chinese Academy of Sciences (47 260 N, 126 380 E). The mean annual temperature was 1.5 C, and the lowest and highest mean monthly values were 23 C in January and 21 C in July, respectively. The mean annual precipitation was 550 mm, more than 80% of which occurred from MayeSeptember. The frost-free period is approximately 120 days. The soil is a loamy loess and classified as Typic Hapludoll. 2.2. Experimental design The field site is on a flat plain and was a native prairie before the land was cleared for cropping more than 100 years ago. A long-term field experiment was established in a randomized block design in 1990 based on four treatments with three replicates: no fertilizer (Control), chemical fertilizer made up of nitrogen, phosphorus and potassium (NPK), NPK fertilizer plus maize straw (NPK þ MS1), and NPK fertilizer plus double the maize straw of NPK þ MS1 (NPK þ MS2). In 2001, we added two additional treatments with three replicates: NPK fertilizer plus pig manure (NPK þ OM1) and NPK fertilizer plus triple the pig manure of NPK þ OM1 (NPK þ OM2). Each replicate covered a surface area of 12 5.6 m2 and was separated from other replicates by a 0.7-m buffer strip. A maizeesoybeanewheat crop rotation was established, and the crop was maize in 2011. Maize was sown on May 8 and harvested on September 26, 2011. To increase soil temperature to stimulate maize germination in early spring, the field was split into ridges (approximately 10 cm in height) and furrows using a ridge plow. The distance between adjacent ridges was 70 cm. The depth of tillage was approximately 20 cm. The fertilizer applications were as follows: 1) 120 kg N ha1, 60 kg phosphorus pentoxide (P2O5) ha1, and 30 kg potassium oxide (K2O) ha1 for maize; 2) 20.25 kg N ha1, 51.75 kg P2O5 ha1, and 30 kg K2O ha1 for soybeans; and 3) 75 kg N ha1, 60 kg P2O5 ha1, and 30 kg K2O ha1 for wheat. The maize straw and organic manure were applied as follows (dry-weight basis): 2250 kg ha1 in NPK þ MS1, 4500 kg ha1 in NPK þ MS2, 7500 kg ha1 in NPK þ OM1, and 22,500 kg ha1 in NPK þ OM2 for the 3 year rotation. Urea was split into two applications for the treatments receiving N fertilizer, the basal and supplementary fertilizers, with a ratio of 1:2 for maize. The pig manure was composted before application. Straw and organic manure applications were performed in the previous year after crop harvest. The maize straw contained 411 mg organic C g1 and 6.7 mg total N (TN) g1; the pig manure contained 265 mg organic C g1 and 31 mg TN g1.
2.3. Measurement of CO2 fluxes Soil CO2 flux was measured using closed chamber and gas chromatograph techniques during the maize growing season. Gas sampling was initiated on May 27, 2011 at the time of emergence and ended on September 30, 2011 with the maize harvest. During the maize growing season, gas sampling was conducted once per week between 9:00 and 11:00 am, the optimal sampling time to represent the average daily CO2 efflux in this region [19]. The CO2 flux was not measured on July 30 due to heavy rain. Four gas samples were collected at 0, 10, 20, and 30 min after closure of the chamber from a septum installed at the top of the closed chamber (0.7 m 0.2 m 0.25 m) using a 20 ml gas-tight syringe. The collected gas samples were immediately transferred to pre-evacuated vials and transported to the laboratory for analysis. The collected gases included the CO2 from both rhizosphere respiration and native soil respiration. Carbon dioxide concentration was determined using a gas chromatograph (GC-2010, Shimadzu Corp., Japan) equipped with a flame ionization detector (FID) using an 80/100 mesh Chromosorb 102 column. Carbon dioxide flux was calculated from the change in CO2 concentration in the chamber vs. closure time using the following formula:
f ¼ r Dc=Dt V=A 273=ð273 þ TÞ
(1)
where f is the CO2 flux (mg CO2 m2 h1), r is the CO2 density under standard conditions (mg m3), Dc/Dt is the change in CO2 concentration in the chamber (m3 m3 h1), V is the chamber volume (m3), A is the soil surface area (m2), and T is the air temperature inside the chamber ( C). Cumulative soil CO2 emission (g CO2 m2) was calculated by summing the average production of two neighboring fluxes, multiplied by the collection interval time. 2.4. Soil sampling and analyses and measurement of micro-climate factors Soil samples (0e20 cm) were collected within each block in early October 2011. Eight soil samples were randomly collected and then mixed thoroughly to form a composite. After the visible roots, fauna and organic debris were removed by hand, soil samples were sieved (<2 mm) and air-dried. Air-dried soil samples (<2 mm) were used to determine soil pH from a 1:2.5 (w/v) mixture of soil and water. A portion of the air-dried samples was ground (<0.25 mm) prior to the SOC and TN analyses. Soil organic C and TN contents were analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). Soil bulk density was measured using the core method. The air temperature inside the chamber was measured with a mercury thermometer; soil temperatures at 5 and 10 cm depths were determined in situ simultaneously using a geothermometer. Field soil moisture was gravimetrically determined from undisturbed soils within 1 m of the field chambers for the calculation of soil water-filled pore space (WFPS). The maize crop was harvested on September 26, 2011. All aboveground crop materials within a plot area of 2 1.4 m were cut. The harvested materials were separated into grain and straw, the latter including stems and leaves. Root biomass (0e20 cm) was quantified at the same location used for aboveground sampling. Roots were separated from soils by hand picking and washing with water. All crop samples (grain, straw and root) were oven-dried at 70 C for 48 h and then weighed to determine the biomass. 2.5. Calculation and statistical analyses The accepted significance level was a ¼ 0.05. Homogeneity of variance and normality tests were conducted on the gas and biomass
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data at the 5% level. A one-way analysis of variance (ANOVA) with least significant difference was used to test the differences in maize yield, biomass, soil property parameters, and cumulative CO2 emissions among the six treatments at P ¼ 0.05. The relationships between straw biomass, maize yield, root biomass, and SOC concentration and cumulative CO2 emissions were examined using linear regression. Nonlinear regression analyses were used to determine the relationships between soil temperature, moisture, and soil temperature combined with moisture and soil CO2 fluxes. The response of soil CO2 fluxes to soil temperature was described by an exponential function:
f ¼ a expðbTÞ;
(2)
where f is the soil CO2 flux at soil temperature T and a and b are regression coefficients. The temperature sensitivity (Q10), defined as a multiplier of soil CO2 flux for a 10 C increase in soil temperature, was calculated as follows: Q10 ¼ exp (10b). To clarify the effects of soil moisture on soil CO2 flux, we removed the possible confounding effect of soil temperature on soil CO2 flux by standardizing soil CO2 flux to a soil temperature of 10 C using the following equation:
f ¼ f10 expðlnQ10 ðT T10 Þ=10Þ;
(3)
where f is the soil CO2 flux measured in the field, f10 is the soil CO2 flux at 10 C (T10), Q10 is the temperature sensitivity listed in Table 2, and T is the soil temperature measured in the field at a 5 cm depth. 3. Results 3.1. Soil properties, crop yield and biomass The NPK þ OM2 fertilization increased SOC content by 12% compared with the control (P < 0.05, Table 1). However, the SOC contents in treatments NPK, NPK þ MS1, NPK þ MS2, and NPK þ OM1 were not significantly different from that in the control (P > 0.05; Table 1). Total N content significantly increased by 24% over the control under the NPK þ OM2 treatment (Table 1). The pH value under the NPK þ OM2 treatment was higher than that under NPK þ MS1 (P < 0.05; Table 1). Crop grain, straw, and root biomasses in the control plots were significantly lower than in all other treatments (P < 0.05; Table 1). Compared with the control, the applications of NPK, NPK þ MS1, NPK þ MS2, NPK þ OM1, and NPK þ OM2 significantly increased straw biomass by 45, 46, 62, 11, and 20%, respectively and increased root biomass by 50, 139, 138, 110, and 126%, respectively (all P < 0.05; Table 1). 3.2. Soil temperature and moisture Soil temperature varied from 4 to 37 C during the maize growing season, with averages of 21 C and 18 C at 5 and 10 cm
85
depths, respectively (Fig. 1a, b). Soil temperatures at both 5 and 10 cm depths reached maxima on June 24 (Fig. 1a, b) when the soil WFPS was lower than at most other sampling times (Fig. 1c). Mean soil temperatures during the growing season were 19.5 and 19.3 C at a 5 cm depth (Fig. 1a) and 17.4 and 17.5 C at a 10 cm depth for NPK þ OM1 and NPK þ OM2, respectively (Fig. 1b), which were lower values than those in the control treatment (5 cm: 22 C, 10 cm: 19.7 C). Soil moisture ranged from 25 to 51% WFPS during the maize growing season and was higher on average in the control than in the other treatments (Fig. 1c). Soil moisture was significantly correlated with cumulative precipitation between two neighboring measurement times (r2 ¼ 0.54e0.63, n ¼ 18, P < 0.001). 3.3. Seasonal variations in soil CO2 fluxes and cumulative CO2 emissions Soil CO2 fluxes, regardless of organic or chemical fertilizer application, increased gradually from the experiment’s beginning in May and reached a maximum on August 6 (Fig. 2a). Soil CO2 fluxes then declined gradually until the harvest at the end of September. The mean soil CO2 flux during the maize growing season in the control treatment was 215 mg CO2 m2 h1, which was significantly lower than those in the treatments of organic manure applications (NPK þ OM1, 310 mg CO2 m2 h1; NPK þ OM2: 355 mg CO2 m2 h1; P < 0.05). Based on soil CO2 fluxes, cumulative CO2 emissions during the maize growing season in the control, NPK, NPK þ MS1, NPK þ MS2, NPK þ OM1, and NPK þ OM2 treatments were estimated to be 694, 678, 853, 889, 988, and 1130 g CO2 m2 (Fig. 2b). Cumulative CO2 emissions in the NPK þ OM1 and NPK þ OM2 treatments were significantly higher than that in the control by 42 and 63%, respectively (both P < 0.05); the NPK þ MS1 and NPK þ MS2 treatments marginally increased CO2 emissions by 23 and 28%, respectively (P < 0.1). In contrast, NPK fertilization alone did not significantly impact cumulative CO2 emission (P > 0.05; Fig. 2b). In addition, there was no significant difference in soil CO2 emissions between NPK þ OM1 and NPK þ OM2 or between NPK þ MS1 and NPK þ MS2 (both P > 0.05; Fig. 2b). 3.4. Impacts of micro-climate factors on soil CO2 fluxes During the maize growing season, an exponential model explained 26e34% of the seasonal variations in soil CO2 fluxes in only four out of the six treatments at a 10 cm depth (Table 2). Using only the data from the elongation to the harvest stage, however, we found improved relationships between soil temperature and CO2 fluxes (R2 ¼ 0.43e0.91, P < 0.05; Table 2). The temperature sensitivity (Q10) of soil CO2 fluxes during the growing season ranged between 1.87 and 3.00 (Table 2). Correlation analysis showed poor relationships between soil CO2 fluxes and WFPS during the growing season (Table 3). After excluding the masking influence of soil temperature, we found greatly improved relationships between
Table 1 Soil properties, maize yield and biomass under different fertilization treatments. The values are the means (n ¼ 3) with SE. Treatment a
Control NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
SOC (g kg1) b
28.20(0.17)b 27.66(0.09)b 29.89(0.26)ab 28.75(0.10)ab 28.57(0.47)ab 31.56(0.74)a
TN (g kg1)
pH
Grain (kg ha1)
Straw (kg ha1)
Root (kg ha1)
2.06(0.03)b 2.06(0.02)b 2.21(0.11)b 2.14(0.05)b 2.18(0.08)b 2.55(0.08)a
5.91(0.02)ab 5.92(0.05)ab 5.69(0.02)b 5.76(0.17)ab 5.76(0.12)ab 6.01(0.04)a
5797(344)d 8384(146)b 8449(103)b 9394(312)a 6446(124)c 6981(120)c
3922(180)d 6348(341)c 7455(458)b 7177(181)bc 8173(455)ab 8700(195)a
517(30)c 775(42)b 1238(111)a 1232(98)a 1090(162)a 1166(125)a
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure. b The different letters in each column indicate the significant difference at P < 0.05.
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Fig. 2. Seasonal variations of soil CO2 fluxes (a) and cumulative CO2 emissions (b) from a Mollisol during a maize growing season. The vertical bars denote the standard error of the mean (n ¼ 3). The arrows denote the application time of basal (BF) and supplemental (SF) fertilizers. A one-way ANOVA with LSD post hoc test was performed to examine the differences in cumulative CO2 emissions among treatments. The different small letters indicate the significant difference in cumulative CO2 emissions among treatments at P < 0.05. Control, no fertilizer; NPK, chemical fertilizer NPK; NPK þ MS1, NPK plus maize straw; NPK þ MS2, NPK plus twice as much maize straw; NPK þ OM1, NPK plus pig manure; and NPK þ OM2, NPK plus three time the pig manure.
soil CO2 fluxes and WFPS in the 5 cm layer (Table 3), which explained 30e60% of the seasonal variations in soil CO2 flux. Furthermore, a log-transformed multiple regression model including both soil temperature and moisture [log(f) ¼ a þ b T log(W)] accounted for 50e88% of the seasonal variation in soil CO2 fluxes (Table 4). Cumulative CO2 emissions were significantly correlated with the harvested straw and root biomass (r2 ¼ 0.62, n ¼ 18, P < 0.001 and r2 ¼ 0.46, n ¼ 18, P ¼ 0.002, respectively; Fig. 3a). A significant correlation was also found between cumulative CO2 emissions and SOC (r2 ¼ 0.40, n ¼ 18, P ¼ 0.005; Fig. 3b). 4. Discussion 4.1. Effect of long-term organic amendment applications on soil CO2 emissions Consistent with our hypothesis, enhanced soil CO2 emissions were observed under organic amendment treatments (NPK þ MS1, NPK þ MS2, NPK þ OM1, NPK þ OM2) in comparison with the unfertilized control in our study. Several studies have shown that a long-term return of straw or addition of organic manure to soil can result in substantial increases in the soil respiration rate [8,20]. The total CO2 flux from soils can be divided into two components: rhizosphere-derived CO2 (rhizosphere respiration) and soil organic matter (SOM)-derived CO2 in root-free soil (basal respiration) [21]. Rhizosphere respiration reflects both the metabolic activities of the living root tissue and SOM decomposition in the rhizosphere, and both processes are proportional to root biomass [22]. Using a rootexclusion technique, a previous study observed that rhizosphere respiration comprised the majority of soil respiration in croplands, accounting for 67e80% of the overall CO2 emission from the Mollisols [23], and root respiration is an important contributor to rhizosphere respiration in situ [24]. Although the present study was not capable of distinguishing the root respiration component of total soil CO2 emissions, the increased root biomass with organic amendment applications (Table 1) might have partly enhanced cumulative CO2 emissions as reflected by the positive correlation
between CO2 emissions and root biomass (r2 ¼ 0.46, n ¼ 18, P ¼ 0.002; Fig. 3a). Aside from autotrophic root respiration, another important source of total CO2 flux is heterotrophic respiration from soil microorganisms and macrofauna [21,25]. As the contribution of soil macrofauna to total CO2 fluxes from soils is usually only a few percent [26], most of the CO2 derived from heterotrophic respiration is respired by soil microorganisms. Similar to an earlier observation [23], we found that soil CO2 emissions were significantly correlated to SOC (r2 ¼ 0.40, n ¼ 18, P ¼ 0.005; Fig. 3b). Soil CO2 emission related to microbial respiration is derived from basal respiration and CO2 production as a consequence of a priming effect [21]: organic manure and straw applications can greatly increase the amount of substrates for soil microorganisms and also alter soil microbial activity, subsequently accelerating SOM decomposition [27]. Therefore, the increased CO2 emissions from the organic amendment treatments in our study may be partly attributed to the positive priming effect of substrate inputs compared to the control. In addition to the sources affecting soil CO2 emissions described above, rhizomicrobial respiration and the decomposition of added organic amendments were also biogenic sources of soil CO2 emissions. However, due to high turnover rates and thus low residence times in soils [21], rhizomicrobial respiration and amendment decomposition probably make limited contributions to total CO2 emissions. More cumulative CO2 was emitted from soils amended with pig manure relative to those with maize residue (Fig. 2b), indicating that the manure amendment led to an enhancement of microbial activity in comparison with the maize residue. Microbial activity can be influenced by soluble carbon and nitrogen compounds in soils [28]. The levels of soluble carbon and nitrogen compounds in soils can be increased by consecutive applications of manure [29], whereas they can be decreased by maize residue applications due to the assimilation of available carbon and immobilization of available nitrogen by microorganisms [30]. Manure applications thus could provide more soluble carbon and nitrogen for microbial activity than maize residue. In addition, microbial activity and the related decomposition rate of organic amendments are partly
Fig. 1. Seasonal variations in soil temperature at 5 (a) and 10 cm (b) depths, soil water-filled pore space (WFPS) in the 5 cm layer (c), and daily precipitation and air temperature data (d) from a meteorological station at the field experiment site during a maize growing season. The vertical bars denote the standard error of the mean (n ¼ 3) for Fig. 1a, b, c. Control, no fertilizer; NPK, chemical fertilizer NPK; NPK þ MS1, NPK plus maize straw; NPK þ MS2, NPK plus twice as much maize straw; NPK þ OM1, NPK plus pig manure; and NPK þ OM2, NPK plus three time the pig manure.
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Table 2 Relationships between soil CO2 flux (f) and soil temperature (T) at 5 and 10 cm depths during the maize growing season. Treatment
Control
a
NPK
Depth Entire maize growing period From elongation to harvest (cm) (n ¼ 18) only (n ¼ 13) (May 27eSeptember 30, (July 1eSeptember 30, 2011) 2011) Equation
R2
Q10b Equation
R2
Q10
5
e
0
e
0.43*
1.87
10
e
0.06 e
5
e
0.17 e
f ¼ 93.234 exp(0.0412T) e
0.26* 1.51
f ¼ 88.912 exp(0.0530T) e
0.33* 1.70
f ¼ 79.630 exp(0.0571T) e
0.27* 1.77
f ¼ 101.244 exp(0.0566T) e
0.34* 1.76
e
0.13 e
10 NPK þ MS1
5 10
NPK þ MS2
5 10
NPK þ OM1
5 10
NPK þ OM2
5 10
0.14 e
0.10 e
0.20 e
0.04 e
f ¼ 57.703 exp(0.0627T) f ¼ 54.911 exp(0.0711T) f ¼ 51.821 exp(0.0677T) f ¼ 62.770 exp(0.0684T) f ¼ 44.806 exp(0.0793T) f ¼ 53.671 exp(0.0832T) f ¼ 34.516 exp(0.1030T) f ¼ 37.764 exp(0.1097T) f ¼ 65.664 exp(0.0820T) f ¼ 68.940 exp(0.0866T) f ¼ 65.412 exp(0.0935T) f ¼ 67.284 exp(0.0984T)
0.58** 2.04 0.72*** 1.97 0.81*** 1.98 0.58** 2.21 0.72*** 2.30 0.82*** 2.80 0.91*** 3.00 0.70*** 2.27 0.76*** 2.38 0.74*** 2.55 0.79*** 2.68
Correlation significance levels: *P < 0.05, **P < 0.01 and ***P < 0.001. a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure. b Q10: temperature sensitivity.
related to organic amendment quality, in which the C/N ratio has been shown to be a good predictor of the decomposition of organic amendments applied to soils [28]. The pig manure, with a low C/N ratio (w9), could decompose more easily than the maize residue with a higher C/N ratio (w61). Therefore, pig manure most likely provided more easily degradable and potentially more soluble carbon for microbial activity than the maize residue and thus led to greater soil CO2 emissions. Our results suggest that the magnitude of the impact of soil amendments on soil CO2 emissions primarily depends on the type of organic amendments applied to the test soil, whereas the rate of application has limited effects on cumulative Table 3 Relationships between soil CO2 flux (f) and soil WFPS (W) in the 5 cm layer during the maize growing season (n ¼ 13). Treatment
Equation
R2
P
Excluding the influence of soil temperature Equation
Control
a
NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
f ¼ 10.005W 188.059 f ¼ 6.815W 32.567 f ¼ 16.631W 309.019 f ¼ 12.650W 134.627 f ¼ 12.657W 123.650 f ¼ 16.018W 163.948
0.31 0.048 f ¼ 17.777 exp(0.0423W) 0.20 0.127 f ¼ 39.393 exp(0.0246W) 0.46 0.011 f ¼ 15.485 exp(0.0524W) 0.20 0.121 f ¼ 26.696 exp(0.0369W) 0.20 0.128 f ¼ 40.125 exp(0.0312W) 0.19 0.142 f ¼ 54.677 exp(0.0309W)
R2
P
0.39 0.023 0.41 0.019 0.60 0.002 0.53 0.005 0.35 0.033 0.30 0.054
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure.
Table 4 Relationships between soil CO2 flux (f) and soil temperature (T) at a 5 cm depth and soil WFPS (W) in the 5 cm layer during the maize growing season (n ¼ 13). Treatment a
Control NPK NPK þ MS1 NPK þ MS2 NPK þ OM1 NPK þ OM2
Equation log(f) log(f) log(f) log(f) log(f) log(f)
¼ ¼ ¼ ¼ ¼ ¼
1.732 1.709 1.628 1.538 1.812 1.819
þ þ þ þ þ þ
0.018 0.019 0.023 0.029 0.023 0.026
T T T T T T
log(W) log(W) log(W) log(W) log(W) log(W)
R2
P
0.50 0.79 0.68 0.88 0.75 0.79
0.007 <0.001 <0.001 <0.001 <0.001 <0.001
a Control: no fertilizer, NPK: chemical fertilizer NPK, NPK þ MS1: NPK plus maize straw, NPK þ MS2: NPK plus twice as much maize straw, NPK þ OM1: NPK plus pig manure, and NPK þ OM2: NPK plus three times the pig manure.
soil CO2 emissions as shown by the absence of a remarkable difference in CO2 emissions under different application rates of a given organic amendment (Fig. 2b). Despite the increased soil CO2 emissions, the combined use of organic amendments and chemical fertilizers improved or at least maintained the SOC content of the studied Mollisols (Table 1). This indicates that the carbon inputs to the soils were larger than or at least in equilibrium with the carbon losses from soils induced by the organic amendment applications. 4.2. Effects of soil temperature and moisture on the temporal variation of soil CO2 fluxes The temporal variation in soil CO2 fluxes is commonly related to soil temperature, moisture or both [12,31,32]. In the present study, however, only 26e34% of the seasonal variations in soil CO2 fluxes could be explained by soil temperature with an exponential equation under certain treatments (Table 2), which implied that other factors were affecting the CO2 fluxes. In our study, enhanced CO2 fluxes were observed after the applications of both basal and supplemental fertilizers (Fig. 2a). Moreover, an exponential equation described the relationship between soil temperature and CO2 fluxes well under all treatments when we restricted the analysis to the period from the elongation stage to the harvest time: R2 values were also greatly improved (0.43e0.91) given this time restriction, accompanied by increases in Q10 values (1.87e3.00; Table 2). Thus, we believe that a disturbance, most likely related to plowing or fertilization, affected soil CO2 emissions at the seedling stage and, further, partly modified the influence of soil temperature on CO2 fluxes, as shown by a previous study [33]. Previous studies have shown that soil moisture status can also influence soil respiration [14,34]. However, the present study found a poor relationship between soil CO2 fluxes and WFPS. There could be several reasons for this result. The narrow range of soil WFPS (25e51%, Fig. 1c) in the studied field might have resulted in the observed weak influence on soil CO2 fluxes [35]. Soil temperature was also an important factor regulating the effects of moisture on CO2 fluxes [36]. Previous researchers have noted that the effects of soil moisture on CO2 fluxes are partly obscured by soil temperature, because soil moisture and temperature usually change simultaneously [34,37]. In the present study, we found improved relationships (R2 ¼ 0.30e0.60) between soil CO2 fluxes and WFPS after the masking influence of soil temperature was excluded (Table 3). Furthermore, the logtransformed multiple regression model including both soil temperature and moisture was much better able to explain the seasonal variations in soil CO2 fluxes than the regression model with moisture alone, with or without excluding the influence of temperature (Tables 3 and 4). Therefore, our results showed that there was a significant interdependence between soil temperature and moisture in their effects on soil CO2 fluxes in the studied Mollisols.
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Fig. 3. Relationships between cumulative CO2 emissions during the maize growing season and maize biomass at harvest (a) and soil organic carbon concentration (b) (n ¼ 18).
When a separate analysis was conducted for each treatment individually, the seasonal variations in soil CO2 flux rates could be positively explained by the seasonal variations in soil temperature and moisture. For all treatments together, however, we observed that the treatments with lower mean soil temperatures and moisture had higher CO2 emissions than others. These results, combined with the positive relationships between soil temperature, moisture and CO2 fluxes (Table 2), indicated that cumulative CO2 emissions during the growing season were affected more by the applied amendments than by soil temperature and moisture in the cultivated Mollisols. It is noteworthy that the closed chamber system is known to underestimate soil CO2 fluxes by approximately 10% due to the effective volume being larger than the volume of the chamber itself [38]. Thus, a further study comparing a closed- and an openchamber methodology is needed to obtain a calibration factor for the Mollisols. 5. Conclusion Applications of organic amendments combined with NPK accelerated CO2 emissions from soils, whereas NPK fertilization alone did not significantly impact cumulative CO2 emissions. More cumulative CO2 was emitted from soils amended with pig manure relative to those with maize residue. The log-transformed multiple regression model log(f) ¼ a þ b T log(W) including soil temperature and moisture accounted for 50e88% of the season variation in soil CO2 fluxes. Cumulative soil CO2 emissions during the growing season were affected more by the applied amendments than by soil temperature and moisture in the cultivated Mollisols in northeast China. Our results suggest that for the Mollisols, the magnitude of the impact of soil amendments on soil CO2 emissions depends primarily on the type of organic amendments applied, whereas the application rate has limited impacts. Acknowledgments This work was funded by the Strategic Priority Research ProgramClimate Change: Carbon Budget and Related Issues of the Chinese Academy of Sciences (no. XDA05050501), the National Key Basic Research Program of China (no. 2011CB100506), the Natural Science Foundation of China (no. 41101283 and 41101282), and the Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (no. 2011ZKHT01). We thank Gui-Dan Sun for laboratory assistance and Wei-Li Gao
and Xiu-Ling Wen for their participation in the field sampling. We also thank three anonymous reviewers for their helpful comments and suggestions, which improved the manuscript.
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