Journal of Environmental Quality 31:634-641 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Plant and Environment Interactions
Simulating Interactive Effects of Symbiotic Nitrogen Fixation, Carbon Dioxide Elevation, and Climatic Change on Legume Growth
Mei Yua,
Q. Gao*,a and
M. J. Shafferb
a MOE Key Lab of Environmental Change and Natural Disasters, Institute of Resources Science, Beijing Normal Univ., Beijing 100875, China
b USDA-ARS, Great Plains Systems Research, P.O. Box E, Fort Collins, CO 80522
* Corresponding author (gaoq{at}bnu.edu.cn, qgao{at}163bj.com)
Received for publication January 29, 2001.
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ABSTRACT
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The underlying mechanisms of interaction between the symbiotic nitrogen-fixation process and main physiological processes, such as assimilation, nutrient allocation, and structural growth, as well as effects of nitrogen fixation on plant responses to global change, are important and still open to more investigation. Appropriate models have not been adequately developed. A dynamic ecophysiological model was developed in this study for a legume plant [Glycine max (L.) Merr.] growing in northern China. The model synthesized symbiotic nitrogen fixation and the main physiological processes under variable atmospheric CO2 concentration and climatic conditions, and emphasized the interactive effects of these processes on seasonal biomass dynamics of the plant. Experimental measurements of ecophysiological quantities obtained in a CO2 enrichment experiment on soybean plants, were used to parameterize and validate the model. The results indicated that the model simulated the experiments with reasonable accuracy. The R2 values between simulations and observations are 0.94, 0.95, and 0.86 for total biomass, green biomass, and nodule biomass, respectively. The simulations for various combinations of atmospheric CO2 concentration, precipitation, and temperature, with or without nitrogen fixation, showed that increasing atmospheric CO2 concentration, precipitation, and efficiency of nitrogen fixation all have positive effects on biomass accumulation. On the other hand, an increased temperature induced lower rates of biomass accumulation under semi-arid conditions. In general, factors with positive effects on plant growth tended to promote each other in the simulation range, except the relationship between CO2 concentration and climatic factors. Because of the enhanced water use efficiency with a higher CO2 concentration, more significant effects of CO2 concentration were associated with a worse (dryer and warmer in this study) climate.
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INTRODUCTION
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ATMOSPHERIC CO2 CONCENTRATION has been increasing since the industrial revolution and is expected to reach twice pre-industrial levels by the middle of this century (Rogers and Dahlman, 1993; Keeling et al., 1989). Many studies have investigated the effects of elevated atmospheric CO2 concentration and the induced climatic change on terrestrial ecosystems and the feedback of the ecosystems to the global environment (Rogers and Dahlman, 1993; Bazzaz, 1990; Shugart, 1990; Kimball, 1983; Kimball et al., 1993; Lemon, 1983; Leadley and Drake, 1993; Vogel and Curtis, 1995). Among various issues of global change studies, responses of individual plants to environmental change are primary and essential in order to explain the macroscopic responses of terrestrial ecosystems at scales of communities, landscapes, and biomes in light of the fundamental underlying physiological mechanisms. Dahlman (1993) and Malanson (1993) reviewed studies of plant responses to doubled CO2 concentration, and concluded that combined effects of increased CO2 and induced climatic change on plants deserved more attention.
Legume plants, being very important in regions with poor nutrient supply because of their capability for symbiotic nitrogen fixation, have been shown to be sensitive to global change in a number of experimental studies (Bai et al., 1996). In particular, biomass of root nodules of legume plants, an indicator of symbiotic nitrogen fixation intensity, increased significantly with atmospheric CO2 concentration. However, the underlying mechanisms of interactions between the process of symbiotic nitrogen fixation and the main physiological processes, such as assimilation, carbon and nutrient allocation, and structural growth, as well as the effects of nitrogen fixation on plant responses to global change, are still open to more investigation.
Experimental quantification is in general difficult for the combined effects of increased atmospheric CO2 concentration interacting with climatic change on plants, as an experiment of this kind would be too complicated and would take too long to accomplish. Consequently, modeling plant responses to global change is more appealing, and models of a number of plant species based on short-term experiments have been reported in the literature (Agren et al., 1991; Nonhebel, 1993; Dahlman, 1985). In the field of agricultural crop sciences, a number of models for soybean, a legume plant, were developed (Acock and Trent, 1991; Acock et al., 1997; Salado-Navarro et al., 1986a,b; Wann and Raper, 1979; Wilkerson et al., 1983). With the common objective to predict the food yield, these models have very detailed and complicated descriptions regarding reproductive growth and seed production. However, the interactions between symbiotic nitrogen and the main physiological processes, and the combined effects of nitrogen fixation and climatic change, more or less, have been ignored in these models.
Our objective was to develop a simple, dynamic ecophysiological model for legumes in semiarid northern China, in order to investigate the effects of climatic change on the essential traits of these plants, as well as the effects of nitrogen fixation on plant responses to global change. Symbiotic nitrogen fixation was incorporated with physiological processes, such as photosynthesis, respiration, and partitioning of assimilation. Instead of stressing the aspect of reproductive growth, like most soybean models for agriculture, major emphases were given to the dynamics of plant and root nodule biomass under various environmental scenarios. The model was formulated so that it can be easily adapted for other wild legume plants. Data from an open-top chamber (OTC) experiment on soybean were used to parameterize the model. The model was run to simulate dynamic responses of plant biomass to variations of atmospheric CO2 concentration and climate, with or without symbiotic nitrogen fixation.
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MATERIALS AND METHODS
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Model Description
Our model simulates interactions of plant biomass accumulation, soil water dynamics, and symbiotic nitrogen fixation. Hence it includes as system state variables the total plant biomass Mt, soil moisture content W, defined as volumetric water content multiplied by surface soil thickness, and biomass of root nodules Mb, which is an indicator of intensity of nitrogen fixation. Major assumptions and treatments used in model formulation are as follows.
Plant biomass Mt was divided into a green component Mg (leaves) and a nongreen component Mn (other organs), thus total biomass M is the sum of Mg and Mn. An important feature of the model is its treatment of the interactions between carbon assimilation and symbiotic nitrogen fixation. The accumulating rate of total biomass, including Mg and Mn, was assumed to be proportional to net assimilation rate of leaves (photosynthesis minus respiration), and to be a monotonically increasing function of root nodule biomass Mb. On the other hand, the rate of accumulation of root nodule biomass was considered to be proportional to existing green biomass. The rate of biomass disappearance (death) was assumed to be proportional to existing biomass, and the proportionality is a function of time so that death occurs mostly in the late growing season. The rate of disappearance of root nodule biomass was assumed to be proportional to the existing root nodule biomass.
The daily increases and/or decreases in total biomass 
Mt, green biomass Mg, and root nodule biomass Mb of a plant, and the moisture content W of the soil in which the plant grows, were defined as follows:
where k is a parameter signifying the effect of symbiotic nitrogen fixation on structural growth, so that a zero value of k corresponds to plants without symbiotic nitrogen fixation; h and t are time in hours and days, respectively; Pn,h(W,T,C,I) defines hourly net photosynthesis by 1 g of green biomass as a function of soil moisture content W, ambient temperature T, atmospheric CO2 concentration C, and photosynthetic active radiation I; Rd,h denotes hourly dark respiration of 1 g biomass; 
(t) is the proportion of assimilation allocated to green biomass; ß is a constant describing the dependence of root nodule growth on green biomass; dg(t) and dn(t) are the specific death rates of green and nongreen biomass, respectively; Rain denotes daily precipitation; and Ep and Ea(W, Ep) are potential and actual daily evapotranspiration, respectively.
The hourly net photosynthesis Pn,h was treated as a Michaelis–Menten function of both atmospheric CO2 concentration C and photosynthetically active radiation I, an asymmetric unimodal function of air temperature T, and a linear function of soil water content W, within the simulation ranges (France and Thornley, 1984; Gao and Zhang, 1997), that is:
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where T(T) and W(W) are two functions describing the dependence of assimilation on temperature and soil water, respectively, and both range from 0 to 1; 
is photo efficiency,
or the slope of the curve of Pn,h versus I, when I approaches 0 and T(T) and W(W) is equal to 1; and
and is the slope of the curve of Pn,h versus C at light saturation and optimal water and temperature condition, thus it describes CO2 transmission efficiency. Both and 
depend on CO2 concentration and the plant phenology (Yu et al., 1997). The function describing the effect of temperature on photosynthesis, T(T), takes the following form:
 | [3] |
where bi, ci, di, i = 1, 2 are constants; and Tmin, Topt, and Tmax are respectively the minimum, optimum, and maximum temperature for photosynthesis. These parameters were obtained from previous physiological measurements by means of statistical methods (Yang and Miao, 1983). The term W(W) was assumed to be a linear function of soil moisture content:
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where Wp, Wopt, and Ws are the wilting point, optimum soil water content, and field capacity, respectively.
Dark respiration Rd,h was assumed to consist of maintenance respiration Rm,h and growth respiration Rg,h. Maintenance respiration was assumed to increase with temperature, but decrease with atmospheric CO2 concentration. Growth respiration, however, was considered to be proportional to net growth rate but insensitive to temperature and CO2 concentration (Amthor, 1991; Bunce, 1995). Specifically:
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where rm,h is the coefficient of maintenance respiration and declines with increasing CO2 concentration C; and fTR
= Q
is the temperature effect on maintenance respiration. In this study, Q and T0 were set to 2 and to 25°C respectively, and rm,h was set to 19.6 and 11.2 nmol CO2 g-1 dry wt. s-1 for contemporary and doubled atmospheric CO2 concentration, respectively (Bruce, 1995; Kishitani and Shibles, 1986). Growth respiration Rg,h was given by:
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where rg,h is the coefficient of growth respiration set to 4.959 x 106 nmol CO2 g-1 dry wt. (Bruce, 1995; Kishitani and Shibles, 1986).
Partitioning between green and nongreen biomass was regulated by plant phenology so that most assimilation products were distributed to green biomass at the early stage of plant growth, but to nongreen biomass at late growing season. Thus, the partitioning for green biomass (t) took the following form:
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where 
1, 2, 
1, and 2 are constants tuned to fit the experimental measurements on green and nongreen biomass in this study. The death proportions of green biomass dg(t) and nongreen biomass dn(t) are given by:
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 | [9] |
where ts and te are the starting and ending days of the growing season. The terms Dg and Dn are the death rates for green and nongreen biomass, respectively. Equations [8] and [9] imply that the death rates were negligible at the early part of the growing season.
Soil moisture balance in one day within a depth of approximately 0 to 1 m was calculated as daily precipitation minus daily evapotranspiration, with runoff occurring at saturation of soil moisture. Actual daily evapotranspiration Ea was calculated as the potential value Ep multiplied by the degree of soil moisture saturation:
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The daily potential evapotranspiration was assumed to be equivalent to daily pan evaporation measured by meteorological stations. Because of the lack of radiation measurements, Penman's model was modified to describe the dependence of daily potential evapotranspiration Ep on daily mean solar height, daily sunshine fraction, daily mean wind velocity, and deficit of water vapor pressure (Gao and Zhang, 1997):
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where s is the slope of the curve of saturated vapor vs. temperature; is a constant depending slightly on temperature; S(t) is the daily sunshine fraction; es is the daily mean saturated vapor pressure depending on temperature; RH is the daily mean relative humidity; Rs(hs) denotes daily radiation proportional to daily mean solar height hs; and fw(VW) indicates the effect of wind velocity on evapotranspiration. All the constants in Rs(hs) and fw(VW) were estimated from pan evaporation measurements using nonlinear regression (Yu et al., 1997).
The model was driven by a weather generator that calculates daily precipitation, daily mean temperature, wind velocity, relative humidity, and daily sunshine fraction (cloudiness), all randomly distributed around their respective monthly mean values (Yu et al., 1997). Hourly quantities were then derived from the statistics of daily courses of these quantities for different seasons.
Our model resembles many existing ecophysiological models for individual plants in terms of assimilation, allocation, respiration, and death. However, the integration of these physiological processes with symbiotic nitrogen fixation made the model useful to investigate the effects of nitrogen fixation and the interactions between nitrogen fixation and other physiological processes on ecosystem dynamics under variable driving climatic factors.
Experiment and Model Parameterization
A soybean variety, Jindou No. 1, was planted in 60 pots of 20 cm in diameter and 15 cm in height, on 20 June 1994, with each pot containing five plants. The soil conditions (classified as cinnamon soil according to Russian taxonomic system, sandy clay loam in texture, with pH around 8.4 and organic matter around 1.8%) were approximately homogeneous across all pots. On 30 June, 20 pots were moved into an open-top chamber maintained at 700 µmol mol-1 of CO2 (micromole CO2 per mole of air), and 20 pots were moved into another chamber with a 350 µmol mol-1 CO2 concentration, leaving 20 pots outside the chambers as controls. The pots were kept in and outside the chambers for sampling measurement during the rest of the growing season until 17 September, when the plants were harvested. The pots were irrigated during the growing period to ensure that the soil water was not a limiting factor for plant growth. Hourly measurements on instantaneous leaf temperature, air temperature, ambient CO2 concentration, photon flux, net photosynthesis rate, transpiration rate, and relative humidity were done by means of a LI-6200 photosynthesis analyzer (LI-COR, Lincoln, NE) on 30 June, 10 July, 20 July, 30 July, 10 August, 20 August, 30 August, 10 September, and 17 September. On each of these scheduled days, 20 plant leaves of each treatment and the control were randomly sampled and subjected to the above measurements. In addition, 10 plants in two pots from each treatment and/or control were taken out of the pots to measure the dry weights of different parts of the plants, such as roots, stems, green leaves, yellow leaves, and root nodules. A more complete description of the experiment was given by Gao et al. (1995).
Monthly average weather data from 1951 to 1980 and daily weather data from 1981 to 1983 from the Beijing Meteorological Station were used to parameterize the submodel of the weather generator so that the statistical monthly means of these quantities were maintained for the present climate (Yu et al., 1997). Since we had 10 replicates for the biomass observations, we divided them into two groups. One group of observations on five plants was used for parameterization, leaving the data for the remaining five plants for model validation. While many parameters of the plant ecophysiological model were taken or derived from existing literature, partitioning parameters 1, 1, 1, and 2, nitrogen fixation coefficient k, root nodule growth coefficient ß, and death coefficients Dn and Dg were adjusted to fit observed biomass data of the parameterization group. To validate the model, initial biomass values were set to those of the first open-top chamber experimental observation on 30 June. Soil water content, however, was set to optimal value during the whole growth period to simulate the well-watered experimental condition. The simulated biomasses were then compared with the validation measurements.
Simulation Layout
To simulate responses of soybean biomass to elevated CO2 concentration and the possible induced climatic change, we introduced perturbations into atmospheric CO2 concentration, precipitation, and temperature. Specifically, monthly precipitation Pm and monthly mean temperature Tm were either set to their present values (Pm = 0, Tm = 0), or increased by 10% and 2°C (Pm/Pm = 10%, Tm = 2°C), respectively. Similarly, two levels of atmospheric CO2 concentrations, 350 and 700 µmol mol-1, were used (C/C = 0 or C/C = 1). Hence a factorial design with three variables, Pm/Pm, Tm, and C/C, each set to two levels, constituted eight driving scenarios. The effects of symbiotic nitrogen fixation on plant biomass dynamics were obtained by comparing outputs of the model with nitrogen fixation to those without nitrogen fixation, the latter of which was realized by setting k and ß to zero in Eq. [1]. The time step of integration and initial values for biomass used for all simulation runs were the same as those used in parameterization and validation. However, the initial value of soil moisture content was set to 40% (volumetric content), a value that leads to approximately soil water equilibrium for one year under present climatic conditions.
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RESULTS AND DISCUSSION
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Model Validation: Comparison between Simulation and the Experimental Data
The simulation results of the soybean experiment containing symbiotic nitrogen fixation were compared with the validation data in Fig. 1
. The simulated plant total and green biomass for both CO2 levels, and the simulated root nodule biomass for 700 µmol mol-1 CO2, followed trends and magnitudes of the observations (Fig. 1a,b,d). There was an overprediction of root nodule biomass for the 350 µmol mol-1 CO2 concentration (Fig. 1c), and underpredictions of total biomass for the late growing season (Fig. 1a,b). However, paired t tests between simulation and observation yielded t statistics of -1.45, -0.34, and 1.38 for total, green, and root nodule biomass, respectively at the 350 µmol mol-1 CO2 level. The corresponding t statistics for the 700 µmol mol-1 CO2 level were -1.26, -0.38, and 0.41. Comparing these t statistics to table values t8,0.05 = 2.306 and t3,0.05 = 3.182, we can conclude that our simulations were not significantly different from observations. Furthermore, regressions of predicted biomasses against the observed values gave the R2 of 0.94, 0.95, and 0.86 for total, green, and nodule biomasses, respectively. And the t test indicated that the intercepts and slopes of the regression equations were not significantly different from 0 or 1, respectively. Figure 1 indicated that green biomass reached its maximum approximately 10 days earlier than the total biomass, and also started to decline earlier than the total biomass, because most assimilation products were distributed to green organs earlier in the growing season, and to nongreen ones in the later portions of the growing season. Doubling atmospheric CO2 concentration induced 65, 44, and 36% increases in the peaks of total, green, and root nodule biomass, respectively.
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Fig. 1. Comparison between observed (dots) and simulated (lines) total biomass Mt and green biomass Mg (a, b), and root nodule biomass Mb (c, d), under contemporary and doubled CO2 concentration.
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Interactive Effects of Nitrogen Fixation and Carbon Dioxide Enrichment on Plant Biomass Dynamics
Figure 2
illustrates the simulated total and green biomass, with and without nitrogen fixation, under the two levels of CO2. Simulated peak values of total and green biomass are shown in Table 1. It can be seen that both CO2 and symbiotic nitrogen fixation have positive effects on plant growth. The model with nitrogen fixation (C0N1) yielded 20% more total biomass and 13% more green biomass than the model without nitrogen fixation (C0N0) under the present CO2 concentration. In comparison, doubling CO2 concentration amplified the effects of symbiotic nitrogen fixation. A 30% increase in maximum total biomass and a 19% increase in maximum green biomass with nitrogen fixation were obtained for doubled CO2 concentration. Figure 2 also indicates that symbiotic nitrogen fixation tended to promote the responses of plant growth to enrichment of the atmospheric CO2. With nitrogen fixation, the total and green biomass under doubled CO2 concentration were respectively 65 and 44% larger than those under the present CO2 concentration, compared with only 52 and 37% induced increases in these quantities without nitrogen fixation.
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Fig. 2. Simulated responses of total biomass Mt (g) and green biomass Mg (g) to elevated CO2 at well-watered conditions, with and without symbiotic nitrogen fixation. C0/C1, current/doubled CO2 concentration; N0/N1, without/with symbiotic nitrogen fixation.
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Table 1. Simulated peak values of total biomass Mt and green biomass Mg in response to elevated CO2 and capability of symbiotic nitrogen fixation (under the experimental climate conditions).
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The mutual promotion of nitrogen fixation and CO2 enrichment might be related to the capability of the plants to keep a relatively constant C to N ratio in order to maintain their physiological and structural functions. With more assimilation under an enriched CO2 environment, plants need more nutrients from root systems in order to convert assimilation products into other functional and structural materials. On the other hand, nitrogen fixation provides more nutrients for structural growth, so that more leaf area is available for photosynthesis. At the same time more available nutrients from nitrogen fixation can "consume" more assimilation products by means of more active metabolic activities to allow more efficient transportation of assimilation products, which in turn can facilitate carbon assimilation in the sites of photosynthesis.
Interactive Effects of Elevated Carbon Dioxide, Climatic Change, and Nitrogen Fixation
Shown in Fig. 3a
is the predicted total biomass under altered climatic and CO2 concentration, with and without symbiotic nitrogen fixation. Predicted peak values of total biomass are listed in Table 2.
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Fig. 3. Simulated responses of total plant biomass to climate change and CO2 elevation. (a) Contemporary CO2, varied climate, with symbiotic nitrogen fixation; (b) contemporary CO2 concentration, variable climate, with and without symbiotic nitrogen fixation; (c) variable CO2 and climate, with symbiotic nitrogen fixation. T0/T2, monthly mean temperature unchanged/increased by 2°C; P0/P1, monthly precipitation unchanged/increased by 10%; C0/C1, current/doubled CO2 concentration; N0/N1, without/with symbiotic nitrogen fixation.
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Table 2. Simulated peak values of total biomass Mt in response to elevated CO2 and climatic change, with and without symbiotic nitrogen fixation.
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Figure 3a illustrates the predicted responses of total biomass to climatic change under current CO2 concentration with symbiotic nitrogen fixation. The figure indicates that in the semi-arid region around Beijing, increasing precipitation has a positive effect, while increasing temperature has a negative effect, on plant growth. Increasing precipitation by 10% alone led to a 49% increase in maximum total biomass (T0P1C0N1 and T0P0C0N1), while increasing temperature by 2°C alone caused a 31% decrease in total biomass (T2P-0C0N1 and T0P0C0N1). Furthermore, the positive effect of precipitation increase is much weaker for increased temperature than at the present temperature, with only a 20% increase in maximum total biomass for precipitation increase at increased temperature (T2P-1C0N1 and T2P0C0N1). This was partially because of the declined efficiency of precipitation caused by a larger evapotranspiration associated with a higher temperature. The effect of temperature on plant biomass was stronger for increased precipitation. Increasing temperature by 2°C led to a 44% decrease in total biomass (T2P1C0N1 and T0P1C0N1) at increased precipitation, in comparison with a 31% decrease induced by increasing temperature at present precipitation (T2P0C0N1 and T0P0C0N1). This might have something to do with more available water to evaporate and transpire with increased precipitation, hence the stronger negative effect of temperature increase with increased precipitation.
Figure 3b illustrates the predicted responses of total biomass on the combined effects of climatic change and symbiotic nitrogen fixation under current atmospheric CO2 concentration. Symbiotic nitrogen fixation seemed to have a stronger effect on plant growth under the better climatic condition (wet and cool in this study) with more precipitation and less evapotranspiration. For example, nitrogen fixation brought about a 22% increase in the peak total biomass when precipitation was increased by 10% (T0P1C0N1 and T0P1C0N0), but only induced a 14% increase under the present climate conditions (T0P0C0N1 and T0P0C0N0). The effects of climatic change were more significant for plants with symbiotic nitrogen fixation than those without nitrogen fixation. For instance, increasing precipitation by 10% led to a 49% increase in peak total biomass with nitrogen fixation, compared with the corresponding 39% increase without nitrogen fixation.
The interactions between climate variation and nitrogen fixation can be explained as follows. With symbiotic nitrogen fixation, nitrogen is less likely to be a limiting factor for plant growth, so that variations of other factors, including climatic variables, tend to have stronger effects. Similarly, a better climatic condition released an environmental constraint on plant growth to some extent, and thus made the effects of nitrogen fixation more evident.
Figure 3c shows the predicted responses of total biomass to combined effects of elevated CO2 and climatic change on plant growth with symbiotic nitrogen fixation. It is interesting to see that the positive effect of CO2 enrichment was stronger with a worse (dryer and warmer) climate condition. Doubling CO2 concentration led to a 57% increase in peak total biomass at the contemporary climate (T0P0C1N1 and T0P0C0N1), in comparison with the corresponding 88% increase for increased temperature (T2P0C1N1 and T2P0C0N1). The model behavior conformed to many previous experiments (Idso et al., 1987; Drake and Leadley, 1991; Lawlor and Mitchell, 1991; Leadley and Drake, 1993; Pettersson and McDonald, 1992; Long et al., 1993). The CO2 enrichment increased photosynthesis rate, and reduced transpiration rate (Eamus, 1991; Tyree and Alexander, 1993; Sionit et al., 1980), because more influx of CO2 blocks the stomata passage of H2O outflux. As a result, water-use efficiency increased sharply. The suppression of actual evapotranspiration by CO2 enrichment was likely to be more evident under a higher temperature.
Figure 3c also shows that the effects of variations of climatic factors tended to be weaker under increased CO2 concentration. For example, increasing monthly mean temperature by 2°C caused a 31% decrease in total biomass under contemporary CO2 concentration (T2P0C0N1 and T0P0C0N1), but only an 18% decrease in total biomass was simulated with doubled CO2 concentration (T2P0C1N1 and T0P0C1N1). This might be partially due to the enhanced water use efficiency under enriched CO2, which renders a plant more resistant to the climatic change.
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SUMMARY AND CONCLUSIONS
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A dynamic ecophysiological model of a legume plant was developed to incorporate the effects of symbiotic nitrogen fixation with the main physiological processes and seasonal biomass dynamics subject to variable atmospheric CO2 concentration and climatic conditions. Reasonable agreement between experiment and model simulation of plant biomass and root nodule biomass was achieved. The R2 values were 0.94, 0.95, and 0.86 for total biomass, green biomass, and nodule biomass, respectively. Interactions among symbiotic nitrogen fixation, CO2, precipitation, and temperature were investigated by means of a number of prescribed simulation runs of the model. We conclude the following from the simulation results:
(i) In general, increases in atmospheric CO2 concentration, precipitation, and nitrogen-fixing capability all have positive effects, while increasing temperature has a negative effect, on plant biomass production for semi-arid climate conditions.
(ii) CO2 enrichment and symbiotic nitrogen fixation tend to promote each other's effect on plant growth in the simulated ranges.
(iii) In a semi-arid region, a stronger negative effect on biomass production with increasing temperature is usually associated with higher precipitation. On the other hand, a larger increase in plant biomass with increasing precipitation is obtained with lower temperature.
(iv) The effects of climate change become more significant for plants with symbiotic nitrogen fixation, and the effect of nitrogen fixation tends to be stronger under a better climatic condition with a lower temperature but a larger precipitation.
(v) A higher atmospheric CO2 concentration inhibits the effects of climatic change. Under doubled CO2 concentration, an increase in temperature causes less reduction in plant biomass, and an increase in precipitation causes a smaller increase in plant biomass, than those at the present CO2 concentration. On the other hand, when temperature is higher and precipitation is less, the increases in plant biomass brought about by CO2 enrichment are greater than those with lower temperature and more precipitation.
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ACKNOWLEDGMENTS
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This research was jointly supported by Ministry of Science and Technology under Grant no. G2000018605 and the Natural Science Foundation of China under Grant no. 39725006.
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INTRODUCTION
