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Published online 16 May 2007
Published in Soil Sci Soc Am J 71:944-951 (2007)
DOI: 10.2136/sssaj2006.0160
© 2007 Soil Science Society of America
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SOIL BIOLOGY & BIOCHEMISTRY

Soil Respiration under Maize Crops: Effects of Water, Temperature, and Nitrogen Fertilization

Weixin Dinga,*, Yan Caia, Zucong Caia, Kazuyuki Yagib and Xunhua Zhengc

a State Key Lab. of Soil and Sustainable Agriculture, Inst. of Soil Science Chinese Academy of Sciences, Nanjing 210008, China
b National Inst. for Agro-Environmental Sciences, Tsukuba, Ibaraki 305-8604, Japan
c Inst. of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

* Corresponding author (wxding{at}mail.issas.ac.cn).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To evaluate the response of soil respiration to soil moisture, temperature, and N fertilization, and estimate the contribution of soil and rhizosphere respiration to total soil CO2 emissions, a field experiment was conducted in the Fengqiu State Key Agro-Ecological Experimental Station, Henan, China. The experiment included four treatments: bare soil fertilized with 150 kg N ha–1 (CK), and maize (Zea mays L.)-cropped soils amended with 0 (N0), 150 (N150), and 250 (N250) kg N ha–1. Mean seasonal soil CO2 emissions in the CK, N0, N150, and N250 treatments were estimated to be 294, 598, 541, and 539 g C m–2, respectively. The seasonal soil CO2 fluxes were significantly affected by soil temperature, with the change in the rate of flux for each 10°C increase in temperature (Q10) of 1.90 to 2.88, but not by soil moisture. Nitrogen fertilization resulted in a 10.5% reduction in soil CO2 flux; however, it did not significantly increase the maize aboveground biomass but did increase maize yield. Soil respiration measurement using the root-exclusion technique indicated that soils fertilized with 150 kg N ha–1 contributed 54% of the total soil CO2 emission, or 8% of soil organic C down to a depth of 40 cm. An amount of C equivalent to 26% of the net assimilated C in harvested above- and belowground plant biomass was returned to the atmosphere by rhizosphere respiration.

Abbreviations: SOC, soil organic carbon • WFPS, water-filled pore space


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Carbon dioxide emission from soils is an important component of the global C cycle and may be connected with global climate change, which extensive evidence suggests is associated with the increasing atmospheric CO2 concentration (Intergovernmental Panel on Climate Change, 2001). Globally, soils contain 1500 to 1600 Pg C, second only to the C storage in the deep oceans and twice as much as in the atmosphere (Raich and Potter, 1995; Schimel, 1995). Accordingly, a small variation in the turnover intensity of soil organic carbon (SOC) could result in a large change in the CO2 concentration in the atmosphere (Riley et al., 2005). These small variations in SOC are difficult to measure directly in the field, however, due to the high spatial variability and very small relative changes in SOC content during a single vegetative growth season. Hence, soil CO2 emission is commonly measured to investigate short-term SOC turnover.

Soil respiration returns the part of photosynthesis-sequestered C to the atmosphere. The release of CO2 from soils accounts for about 25% of the total annual exchange of C between the atmosphere and terrestrial sources (Post et al., 1990), and is estimated to be 75 Pg C (Schlesinger and Andrews, 2000). Therefore, understanding the factors that control soil respiration is of particular importance to land use and management, since certain measures can be taken to enable lands to sequester atmospheric C (Townsend et al., 1996; Nadelhoffer et al., 1999; Bowden et al., 2004).

Nitrogen addition to soil has been shown to have different effects on soil CO2 emission. Some studies (Liljeroth et al., 1990; Pregitzer et al., 2000; Burton et al., 2002; Bowden et al., 2004) have shown that N input increased soil respiration, and suggested that the stimulatory effects of N loading on ecosystems might reduce ecosystem C storage (Aber et al., 1993; Cao and Woodward, 1998). Conversely, N fertilization has also been observed to reduce organic C decomposition and suppress soil respiration, resulting in an increase in SOC (Bowden et al., 2000; Burton et al., 2002; Foereid et al., 2004). Townsend et al. (1996) and Nadelhoffer et al. (1999) proposed that N fertilization in northern temperate zones could lead to an annual increase in soil C storage by 0.3 to 0.5 Pg C. With increasing rates of anthropogenic N deposition and application of fertilizers, there is a strong need to understand the links between N inputs and soil respiration.

Most of the studies reporting N loading effects on soil respiration have focused on forest soils (Nadelhoffer et al., 1999; Pregitzer et al., 2000; Burton et al., 2002; Bowden et al., 2004; Foereid et al., 2004). Few studies have examined the partitioning of plant shoot C and soil respiration in arable soils because such an evaluation is difficult to complete in an active crop field (Rochette and Flanagan, 1997; Rochette et al., 1999; Kuzyakov et al., 2001, 2002). The purpose of this study was to evaluate the effects of soil moisture, temperature, and N addition on soil respiration by measuring CO2 emissions in an arable sandy loam soil during maize cultivation.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
The field experiment was conducted in a well-drained field at the Fengqiu State Key Agro-Ecological Experimental Station, Fengqiu County, Henan Province, China (35°00' N, 114°24' E), a region typical of the North China Plain covering 310000 km2. The area has a semiarid and subhumid monsoon climate. The 30-yr mean annual temperature is 13.9°C, and the lowest and highest mean monthly values were –1.0°C in January and 27.2°C in July, respectively. The mean precipitation was 615 mm, two-thirds of which fell between June and September. The soil, derived from alluvial sediments of the Yellow River and classified as an Aquic Inceptisol, has a sandy loam texture and an average pH in water of 8.65. The soil contained 7.28 g organic C kg–1, 0.57 g total N kg–1, 10.27 mg NO3––N kg–1, and 0.81 mg NH4+–N kg–1 in June 2004, before initiation of the experiment.

Experimental Design
The experiment included four treatments: bare soil fertilized with 150 kg N ha–1 to serve as a control (CK), and soils fertilized with 0 (N0), 150 (N150), and 250 kg N ha–1 (N250) and cropped with maize. There were three replicates in each treatment, which were laid out in a randomized block design, with each plot measuring 4 by 9 m. Calcium superphosphate (75 kg P2O5 ha–1) and K2SO4 (150 kg K2O ha–1) were applied as basal fertilizers on 7 June 2004 (Day 159), whereas N fertilizer was applied in the form of urea and split into two applications as the basal and supplementary fertilizer, with a ratio of 4:6. The prescribed application rate of N fertilizer, 150 kg N ha–1, was similar to the locally recommended doses for cereal crops. Before sowing, all mixed fertilizers were evenly broadcast onto the soil surface by hand and immediately tilled into the plowed soil (0–20-cm depth). To reduce volatilization of NH3 in the alkaline soil, supplementary fertilizer urea was also surface applied by hand and then incorporated into the plowed layer with 20 mm of irrigation water on 27 July 2004 (Day 209). Maize was directly sown into each plot by hand on 8 June 2004 (Day 160) except in the gas measurement region. In this region, a cylindrical polyvinyl chloride (PVC) plastic tube (length 10 cm, 10-cm outer diameter in the bottom half and 10-cm inner diameter in the upper half) was inserted approximately 5 cm into the soil and three maize seeds were sown into the tube. The distances between rows and hills were 70 and 30 cm, respectively. After 2 wk, the seedlings were thinned to approximately 48000 ha–1 and one seedling was left in the center of each tube. The mature maize was harvested on 20 Sept. 2004 (Day 264). We found that the PVC tube did not greatly affect maize root growth and distribution in the soil. The herbicide was sprayed about 20 d after sowing, and visible weeds were pulled out by hand during the maize growth season.

Gas Sampling for Carbon Dioxide Flux Measurements
A close-chamber method was used to determine fluxes of CO2 (Fig. 1). Immediately after sowing, a stainless steel rectangular chamber base (70 by 30 cm) was inserted into the soil to a depth of approximately 5 cm around the PVC tube in the row at the center of each plot. To collect gas samples, a separate PVC pipe (length 35 cm, 10-cm outer diameter) was placed into the existing PVC tube. Silicone grease was used to create an airtight seal between the pipe and tube. At the upper end of the PVC pipe was an airtight rubber seal. A specially designed stainless steel rectangular chamber (70 by 30 by 30 cm) with a 10-cm-diameter center opening (for the PVC pipe) was fitted atop the base by inserting the flange of the chamber into the water trough at the upper end of the chamber base. The chamber consisted of two separate parts that were combined using the hinge and airtight rubber seal, and covered outside with plastic foam to minimize solar heating and reduce the fluctuation of chamber temperature. The chamber was equipped with two internal battery-operated fans to mix the air in the chamber. The chamber was also equipped with a small, silicon-sealed vent for sampling and another vent for measuring chamber temperature. Samples were taken twice weekly between 0900 and 1200 h during the maize growth season to minimize diel variation in flux patterns. Each time, four samples of the chamber air were manually extracted into 50-mL syringes at 0, 10, 20, and 30 min after closure, injected into pre-evacuated vials fitted with butyl rubber stoppers, and taken to our laboratory for analysis. The air temperature inside the chamber was measured with a Hg thermometer, and soil temperatures at 5-, 10-, and 15-cm depths were measured with a digital thermometer (Model 2455, Yokogawa Electric Corp., Tokyo). After sampling, the PVC pipe was removed so that plants could grow normally. When the maize height was >70 cm, however, the PVC pipe was left connected to the PVC tube so as to reduce chamber-deployment time.


Figure 1
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  		Fig. 1. Diagram of a specially designed chamber for sampling gases: 1, PVC tube; 2, PVC pipe; 3, silicone grease; 4, stainless steel chamber base; 5, stainless steel chamber; 6, plastic foam; 7, fan; 8, thermometer; 9, silicon septum; 10, rubber seal; and 11, maize.

 
Carbon dioxide concentration was determined with a gas chromatograph (GC-14B, Shimadzu Corp., Kyoto, Japan) equipped with a thermal conductivity detector operated at 60°C. Separation was done using a 177/149-µm (80/100 mesh) Chromosorb 102 column (Advanced Minerals, Santa Barbara, CA) at 60°C. The temperature of the injecting port was 100°C. The carrier gas (H2) flow rate was 80 mL min–1. Carbon dioxide gas standards were supplied by BOC Inc. (Shanghai, China). The rate of CO2 increase vs. time in the chamber air was calculated by linear regression analysis with an averaged temperature in the chamber.

Soil Sampling and Analysis
Soil bulk density was measured by the core method (Culley, 1993). Soil samples were collected from 0 to 20 and 20 to 40 cm for analysis. Soil pH was measured from soil–water suspensions (1:2.5 v/v). Total soil C and N concentrations were determined with a Series II CHNS/O 2400 Analyzer (PerkinElmer, Waltham, MA). The soil NH4+–N and NO3––N concentrations were determined by extraction with 2 M KCl and colorimetric analysis with a segmented flow analyzer (Skalar Analytical, Breda, the Netherlands). Soil moisture was measured directly using time domain reflectometry probes positioned vertically through the soil. Soil water-filled pore space (WFPS) was calculated as follows:

Formula
where total soil porosity = {1 – [soil bulk density (g cm–3)/2.65 g cm–3]}, with 2.65 g cm–3 being the assumed particle density of the soil.

Crop Harvest
The maize seeds and shoots were separated manually and then sampled. The samples were washed with tap water, rinsed with distilled water, dried at 70°C for 72 h, weighed, and ground for analysis of C and N concentrations. The concentrations of C and N in plant tissue were determined in the same manner as soil contents.

Statistical Analysis
Statistical analysis was done using the SPSS software package for Windows (SPSS, Chicago, IL) and Microsoft Excel for Windows 2000. Statistically significant differences were identified using ANOVA and LSD calculations at P = 0.05. Correlation and nonlinear regression analyses were used to test the relationships between soil temperature, soil moisture, and CO2 flux as follows:

Formula
where T is the soil temperature (°C), and ß0 and ß1 are regression coefficients. The Q10 values were calculated using the following equation:

Formula

Under the assumption that the difference between CO2 fluxes from the maize-cropped soil (Rt) and from the bare soil (Rs) was equal to the contribution of plant roots to the total CO2 flux, the rhizosphere respiration (Rrh) was calculated as follows:

Formula


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Biomass and Its Nitrogen Concentrations
Plants developed normally within the PVC tube, which was inserted into the soil about 5 cm, and aboveground (grain plus straw) biomass within the tube was not significantly different from that in the plot (Fig. 2), indicating that the use of the PVC tubes did not significantly affect maize growth and aboveground dry production. Grain yield and total aboveground biomass in the unfertilized soil were 6548 and 14985 kg ha–1, respectively. Grain yield was significantly lower than in the soils fertilized with either 150 or 250 kg N ha–1, but total aboveground biomass was not significantly different among the three treatments (Table 1). Nitrogen concentrations in the maize grain and straw increased with the amount of fertilizer N applied and was significantly lower in the N0 treatment than in the N150 and N250 treatments.


Figure 2
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  		Fig. 2. Comparison of mean maize aboveground biomass between the PVC tubes and the plots among the three treatments (N0 = no N fertilizer, N150 = 150 kg N ha–1, and N250 = 250 kg N ha–1. Vertical bars denote the standard error of the averages (n = 3).

 

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  		Table 1. Maize biomass and its nutrient concentrations as affected by N fertilization at 0 (N0), 150 (N150), or 250 kg ha–1 (N250) in an intensively cultivated soil.

 
Carbon Dioxide Emission
Seasonal variations of soil CO2 fluxes are summarized in Fig. 3. Although the flux in the CK treatment fluctuated overall due to variations in soil temperature, at the seedling stage it was generally higher than at the other stages. By contrast, the fluxes in soils cropped with maize, regardless of N fertilization, increased gradually with plant age, reached a maximum at Days 211 to 217, the late elongation stage, and then sharply decreased at Day 231 when maize transited from the nutritive growth stage to the reproductive growth stage. The fluxes increased again at the early filling stage and then decreased gradually until harvest. The average CO2 flux during the experimental period in the CK treatment was 106 mg C m–2 h–1, significantly lower than those in other treatments. This overall difference in CO2 flux was a result of differences from the elongation stage to harvest (Days 182–263). In contrast to the maize planting, N fertilization in the tested soil slightly reduced CO2 fluxes by 10.5%, a statistically insignificant amount. Based on soil CO2 fluxes, the mean cumulative soil CO2 emissions during the maize growth season in the CK, N0, N150, and N250 treatments were estimated to be 294, 598, 541, and 539 g C ha–1, respectively (Table 2). These were calculated by summing the products of the (averaged) two neighboring measurement fluxes multiplied by their interval time.


Figure 3
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  		Fig. 3. Seasonal pattern of soil CO2 fluxes in an arable loamy sand soil fertilized with no (N0), 150 (N150), or 250 kg N ha–1 (N250) cropped with maize or bare soil fertilized at 150 kg N ha–1 (CK). Vertical bars indicate the standard error of the averages (n = 3). Means labeled with different letters among the four treatments on the same measurement day are significantly different at P = 0.05.

 

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  		Table 2. Cumulative CO2 emission during the maize growth season and at the different growth stages among four treatments: N fertilization at 0 (N0), 150 (N150), or 250 kg ha–1 (N250) cropped with maize or bare soil fertilized at 150 kg N ha–1 (CK).

 
During the seedling stage, no significant difference in the cumulative CO2 emission was observed among the four treatments, and N fertilization did not affect CO2 emission. Cumulative CO2 emission at the elongation stage in the N0 treatment was significantly higher, however, than in the N150 and N250 treatments.

Soil Temperature, Moisture Content, and Precipitation
Soil moisture content (WFPS) varied from 20.3 to 73.8% during the growth season, but no significant differences were observed among treatments (Fig. 4a). The cumulative precipitation between two measurement times was significantly correlated with soil WFPS (R = 0.437–0.479, n = 30, P < 0.05) because maize was mainly rainfed and no more than 20 mm of irrigation water was applied after the application of urea as the supplementary fertilizer (Fig. 5). Soil temperature ranged between 19 and 34°C during the growth season, and was, on average, 1°C higher in the CK treatment than in other treatments (Fig. 4b). Temperature increased gradually, reaching a maximum around Day 181 when soil WFPS was lowest, and then declining with a greater degree of fluctuation. This pattern was different from that of air temperature measured at the meteorological station 100 m from the experimental site. There was a negative correlation between soil temperature and WFPS (R = –0.361 to –0.515, n = 30, P < 0.05), suggesting that soil moisture may have partially modified the soil temperature pattern during the growth season. Soil temperature at the 10-cm depth was generally lower than at the 5- and 15-cm depths, especially at the early stage, implying that the diel variation of soil temperature mainly occurred in the top 15 cm of soil.


Figure 4
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  		Fig. 4. Seasonal variation of soil moisture and temperature during the maize growth season under four treatments: no (N0), 150 (N150), or 250 kg N ha–1 (N250) cropped with maize or bare soil fertilized at 150 kg N ha–1 (CK). Vertical bars indicate the standard error of the averages (n = 12).

 

Figure 5
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  		Fig. 5. Dynamics of precipitation and air temperature at a meteorological station about 100 m away from the field experimental site during the growth season.

 
During the growth season, there was a significant positive relationship between soil temperature and CO2 flux. An exponential model explained about 36% of the seasonal variation of CO2 flux in the CK treatment (Table 3). Using data except for the seedling stage, however, we hypothesized that plowing disturbance might affect CO2 flux, and thus improved the relationship. Correlation analysis did not indicate a significant relationship between CO2 flux and soil moisture content.


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  		Table 3. Relationships between soil CO2 flux and soil temperature (T) measured at 5-, 10-, and 15-cm depths, as shown by the change in the rate of CO2 flux for each 10°C increase in soil temperature (Q10), soil water-filled pore space (WFPS), and cumulative precipitation, grouped according to treatment.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effect of Soil Moisture and Temperature on Carbon Dioxide Emission
Microbial C cycling is linked to soil water status (Parkin et al., 1996). Linn and Doran (1984) reported a close relationship between microbial respiration and soil moisture content, with a peak of activity at 60% WFPS. Doran et al. (1990) reported an optimum WFPS of 55% for coarse-textured soils; however, other studies have indicated that fluxes exhibit significantly negative relationships with soil moisture due to poor gas diffusion in surface soils and the reduction in the activity of obligate aerobic microbes caused by excessive water above the optimum WFPS (Rochette et al., 1991; Bowden et al., 2004). In the present study, soil moisture varied from 20 to 74% WFPS, but no significant relationship was found between CO2 flux and soil WFPS (Table 3). Bajracharya et al. (2000) and Mielnick and Dugas (2000) reported similar findings. Several explanations have been proposed to describe this poor relationship. Chantigny et al. (1999) regarded soil WFPS in the range of 45 to 60% too narrow to have a strong influence on soil respiration under field conditions. Rochette et al. (1991) suggested that in a cool and wet climate, soil moisture seldom fell to levels that could affect soil respiration. They also proposed that soil temperature and the presence of plants were more important regulators of soil respiration. These explanations, however, inadequately describe the current results. We found that the highest soil temperature occurred at the lowest WFPS, and that there was a significant decrease in soil temperature with increasing soil WFPS (R = –0.361 to –0.515, n = 30, P < 0.05), which was further correlated with cumulative precipitation (R = 0.437–0.479, n = 30, P < 0.05). We therefore suggest that soil moisture may affect soil respiration indirectly by modifying soil temperature, and both may be interdependent in their effects on soil respiration (Wildung et al., 1975). Further study seems required to evaluate the interaction of soil temperature and moisture on CO2 emission.

There are still uncertainties associated with modeling the strong temperature dependence of soil respiration. Under some circumstances, the good relationships between CO2 flux and soil temperature could be established using linear or sinusoidal regressions (Raich and Schlesinger, 1992; Sun et al., 2005). Some researchers (Lloyd and Taylor, 1994; Thierron and Laudelout, 1996) strongly recommended an Arrhenius equation because it results in evenly distributed residual variances across the entire temperature range. Bauchmann (2000), however, found that an exponential equation could more accurately explain the observed relationships. In our study, the temperature sensitivity of soil CO2 fluxes during the growth season varied between 1.90 and 2.88 according to the exponential equation, well within the range of 2.0 to 3.9 generally given for bulk soil respiration (Raich and Schlesinger, 1992; Davidson et al., 1998); however, we found that only 18 to 25% of the seasonal variation in soil CO2 fluxes could be explained by soil temperature, indicating that other factors were affecting soil respiration. When data were restricted to the period from the elongation stage to the harvest, the CO2 flux–soil temperature relationship greatly improved, with increasing R2 and Q10 values (Table 3). It is noteworthy that soil disturbance due to plowing resulted in more CO2 release at the seedling stage (Rochette et al., 1999; Cardon et al., 2001), which in turn partly obscured the influence of soil temperature on soil respiration.

Using the literature data, we estimated a Q10 value of 2.65, which is similar to the value of 2.4 calculated by Raich and Schlesinger (1992). When the masking effect of other factors was lowered and even excluded, however, Q10 values in the CK treatment were estimated to be 3.6 to 4.3 (Table 2). These are approximately the same as the values (3.5–4.2) measured in bulk soils by Boone et al. (1998) and Janssens and Pilegaard (2003), but were <4.6 for root respiration (Boone et al., 1998) and 4.6 to 6.8 in the N0 treatment, where the absence of N fertilization stimulated the allocation of photosynthetic C to below ground. Brooks et al. (2005) and Eliasson et al. (2005) determined that soil respiration in some soils was strongly suppressed by the shortage of labile organic C. Winkler et al. (1996) attributed low Q10 values (1.7–1.9) to low availability of labile substrates in soils. Accordingly, predictions of soil respiration will be possibly improved if Q10 values generally used in models could vary with both soil temperature and labile organic C in soils, rather than being held as a constant.

Effect of Nitrogen Fertilization on Soil Respiration
The estimated average soil CO2 emission in the N0 treatment during the maize growth season was 598 g C m–2, which was higher than, but insignificantly different from, those in soils fertilized with 150 and 250 kg N ha–1 (Table 2). We found that the difference mainly occurred at the maize elongation stage rather than at the filling and ripening stages. These data suggest that N fertilization in the tested soil partly suppressed soil respiration, assuming plant biomass in the unfertilized soil was not significantly decreased (Table 1). This possibility is supported by other research (Liljeroth et al., 1990; Haynes and Gower, 1995; Knapp et al., 1998; Cardon et al., 2001; Bowden et al., 2004).

Haynes and Gower (1995), Knapp et al. (1998), Cardon et al. (2001), and Bowden et al. (2004) found that N fertilization reduced soil respiration. Foereid et al. (2004) demonstrated that the reduction of soil respiration by the addition of N was due to the suppression of the respiration of native soil organic C. Several possible mechanisms have been suggested. First, a pH decrease, probably caused by nitrification, may inhibit microbial activity (Fog, 1988; Kuzyakov et al., 2000). Second, soil microbial populations may have been adversely affected by the increase in solute concentration (DeForest et al., 2004). Third, high N levels in N-abundant soils repressed the synthesis and activity of certain enzymes (Carreiro et al., 2000). In the present study, we found that, in the absence of N addition to this type of soil, the N demand of growing plants could be roughly met through decomposing SOC, even if the N concentration in plants was significantly lowered (Table 1). Hence, higher root or microbial respiration resulted in slightly more soil CO2 emissions in unfertilized than in N-fertilized but N-unlimited soils. It is not clear based on the present study, however, whether the response of soil respiration to N fertilization is temporary or not, and whether there exists differences in soil respiration between N-fertilized and unfertilized treatments following maize harvest and on an annual basis. A further year-round field experiment is required to evaluate the influence of N fertilization on soil respiration.

Contribution of Rhizosphere Respiration to Soil Respiration
The estimate of rhizosphere respiration showed that the plant contribution to belowground CO2 production in the soil fertilized with 150 kg N ha–1 was below 20% at the seedling stage until approximately Day 185 (Fig. 6). At that time, the aboveground biomass began to exponentially increase, and rhizosphere respiration varied from 30 to 70%, close to the 15 to 60% measured using 14C labeling by (Kuzyakov et al. (2001, 2002), but higher than the values reported by Rochette et al. (1999), who observed that the maximum contribution of rhizosphere respiration to total respiration was 45%. The rates of oxidation of SOC (Rs) were highest at the seedling stage and tended to decrease as the season progressed, which is in accordance with measurements using the isotopic technique (Rochette et al., 1999). Losses of SOC were estimated to be 2.94 Mg C ha–1 during the maize growth season, which represents 8% of the total SOC to a depth of 40 cm (36.4 Mg ha–1), slightly less than the range of 3.15 to 3.80 Mg C ha–1 yr–1 measured in various temperate zone soils in Germany (Dorr and Munnich, 1987). An input of 7.35 Mg dry matter ha–1 season–1 of plant residues with 40% C would be needed to maintain soil C balance. This is roughly double the total maize root biomass (3.93 Mg ha–1) left in the field, but close to the total amount (7.86 Mg ha–1) of maize root biomass plus organic metabolites excreted by roots during the maize growth season and left in the soil at harvest. This estimate is calculated based on the assumption that a quantity of extra C (besides roots) was produced that was equal to the amount of root biomass C at the harvest and the root/shoot ratio was 0.2 (Bolinder et al., 1999). Total cumulative C loss by rhizosphere respiration was 2.47 Mg C ha–1, which was equivalent to 26% of the maize net assimilation calculated from total aboveground dry phytomass (7.85 Mg C ha–1), well within the range of 10 to 35% reported by Rochette and Flanagan (1997). Compared with data measured using the 13C isotopic technique (Rochette et al., 1999), however, the magnitude of Rs in this study was slightly low (2.94 vs. 3.92 Mg C ha–1 or 8 vs. 6%). One possible explanation is that low SOC content and the negative priming effect of N fertilization might reduce the decomposition of SOC.


Figure 6
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  		Fig. 6. Contribution of maize rhizosphere respiration (Rrh) to total soil respiration (Rt) during the maize growth season measured using the root-exclusion technique.

 
It should be noticed that in the present study, a new closed-chamber system (Fig. 1) was used to cover the maize roots and to evaluate the contribution of rhizosphere respiration. When counting the rate of CO2 fluxes, we found that the linear regression relationship (R2) of CO2 concentration increase vs. time in the chamber was always >0.95 and variability of the mean CO2 fluxes in the three plots was low (Fig. 3), indicating that the data presented here was reliable; however, the non-steady-state non-through-flow chambers are known to underestimate CO2 fluxes generally by an average of about 15% (Pumpanen et al., 2004) and the chamber partly rather than completely covered all maize roots. Therefore, data in the present study may underestimate the CO2 fluxes and contribution of the rhizosphere respiration to total CO2 emission.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Soil CO2 fluxes exhibited a seasonal pattern that was more similar to soil temperature than soil moisture. An exponential equation best described the relationships between soil temperature and CO2 flux. However, plowing prior to sowing disturbed the soil and confounded the evaluation of relationships, resulting in lower R2 and Q10 values. When the masking effect caused by other factors was minimized, R2 values were improved and accompanied by an increase in Q10 values.

Mean seasonal soil CO2 fluxes in the CK, N0, N150, and N250 treatments were estimated to be 294, 598, 541, and 539 g C m–2, respectively. Nitrogen fertilization resulted in a 10.5% reduction in soil CO2 flux. Soil respiration measurement using the root-exclusion technique indicated that soils fertilized with 150 kg N ha–1 contributed 54% of the total soil CO2 emission, or 8% of soil organic C down to a depth of 40 cm. An amount of C equivalent to 26% of the net assimilated C in harvested above- and belowground plant biomass was returned to the atmosphere by rhizosphere respiration.


   ACKNOWLEDGMENTS
 
This project was supported by the Natural Science Foundation of China (40621001), the Chinese Academy of Sciences (KZCXZ-YW-407), and the National Basic Research Program of China (2005CB121101). We thank the Hundred Talents program of the Chinese Academy of Sciences for W.X. Ding.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication April 14, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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