水蚀风蚀交错区土壤—紫花苜蓿系统中水热气运移的试验与模拟研究
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摘要
在陕北水蚀风蚀交错区,自20世纪90年代以来开展了退耕还林(草)等一系列植被恢复和建设措施,以遏制当地严重的水土流失。干旱半干旱地区植被重建恢复过程中,土壤水分是其主要限制性因子,且由此引起的土壤碳排放的环境效应成为当前需要研究的重要科学问题。本研究围绕典型植被根系生长和耗水规律,动态模拟土壤水分消耗过程,并在土壤水分研究的基础上,结合其它环境影响因素,对该区植被恢复过程中的土壤碳通量变化进行研究,为该区国家退耕还林(草)所引起的生态环境效应提供科学依据。为此,选取典型牧草紫花苜蓿为供试植物,利用连续两年的土柱栽培试验,研究了不同水分条件下耐旱植物的根系生长动态过程和耗水规律,测定了土壤水分剖面变化以验证了经验根系分布模式的适用性;结合野外田间试验,测定了不同土地利用方式下的土壤呼吸及其环境驱动因子,采用经验性模型估算了植物生长季节里土壤碳排放动态变化,并探讨了该区典型植被生长条件下的土壤水热气运动的动态模拟。结果表明:
(1)紫花苜蓿细根平均根径为0.20 mm左右,比根长为274 mm/mg,细根根长、干重、表面积和体积之间呈极显著线性关系,其细根根长y (m)和细根干重x (g)关系为:。紫花苜蓿根系生长迅速,在适宜水分条件下,第一年生长期内根系平均下扎速率1.03 cm/d。建立了反映植物实际根长分布情况数据拟合方法:即通过建立紫花苜蓿归一化累计根长与土壤深度的函数拟合关系,对拟合的函数求导可得其根长密度与土壤深度的关系,从而得到连续的根长密度分布剖面。种植2年紫花苜蓿根系生长时间可分为三个阶段,在第二年的分枝现蕾期根系生长速率最快;株高生长最快在6月中旬开始到到7月下旬的开花期,成熟期株高达到稳定值;叶面积在第一年生长期内呈单峰曲线变化趋势,在开花成熟期达到最大值。水分对主根、细根生物量均有显著的影响,高水处理下的株高和叶面积大于低水处理。y =274.18x
(2)水分胁迫对于紫花苜蓿细根干重的累积速率无影响,但对其地上产量和主根生物量的累积速率影响较为明显。每年生长期平均蒸腾速率不仅和水分条件有关,还和苜蓿的生长年限密切相关。高低两种水分处理下的生物产量蒸腾系数差异不显著(P >0.05),蒸腾水的转化率对于同一时期的同一种植物而言几乎是固定的,这主要是针对生物产量而言;对于经济产量来说,适宜供水情形下蒸腾系数则会显著低于亏缺供水情形。不考虑水分条件,紫花苜蓿生物量和耗水量之间存在极显著线性相关关系。水分胁迫对于地下部分的影响要弱于对地上部分产量的影响,根系的迅速生长可以吸收更多的土壤水分,但是田间也会造成土壤的迅速干燥化。在苜蓿开花-成熟生育阶段,所测定的苜蓿蒸腾量与采用气象数据计算的参考作物蒸散量均呈极显著线性相关。
(3)两种经验根系分布模型(Prasad与Hoffman和van Genuchten)可近似反映紫花苜蓿根系实际分布状态,进一步通过土壤水分动态模拟验证得出这两种经验根系分布模式适用于根系分布的描述,且参数简单、获取方便,具有一定的实用性。拟合的根系分布、Prasad分布与Hoffman和van Genuchten分布与36 cm以下的的根长密度实测值较为吻合,Raats根系分布与实测值及其它分布模式则差别较大。在缺乏根系测定资料情况下,可通过经验根系模式确定其分布。
(4)5种土地利用类型土壤呼吸速率季节性变化均呈现单峰型曲线,与气温变化趋势一致,其7、8月份土壤呼吸速率均显著高于其它月份(P<0.05);生长季节土壤CO_2平均释放速率顺序为:长芒草地>苜蓿地>柠条地>农地>沙柳地,草地在生长前期和旺盛期土壤呼吸强度均显著高于农地和灌木林地,退耕还林还草显著提高了土壤呼吸。除沙柳地和苜蓿地以外,在土壤呼吸与所有温度指标的关系中,与10 cm深度的土壤温度相关性最好,且除沙柳地外,其它4种土地利用类型均与之达到显著相关;农地土壤呼吸对温度的响应最敏感(Q10值为2.20),除沙柳地(Q10值为1.48)外,其它4种土地利用类型Q10值均在2.0左右,接近于全球Q10的平均水平;通过Van’t Hoff模型估算,2007年植物整个生长季节(5～10月份),5种土地利用类型的土壤呼吸量从高到低依次为:苜蓿地259 gC·m~(-2),长芒草地236 gC·m~(-2),柠条地226 gC·m~(-2),农地170 gC·m~(-2),沙柳地94 gC·m~(-2);水分对农地和沙柳地的土壤呼吸影响不大;长芒草地、柠条地和苜蓿地土壤呼吸的双变量模型关系显著(P<0.05),比相应的单变量模型更好地解释了土壤呼吸变异。
5)采用HYDRUS-1D中集成的SOILCO2模型可以为该区土壤CO_2通量的预测提供可靠的模型方法。在植物生长季节,田间苜蓿地土壤水热气运动模拟显示:土壤水分剖面动态变化和土壤温度动态变化和实测数据相差不大;土壤CO_2通量实测值和模拟值在苜蓿整个生长期内有相同的变化趋势:即在苜蓿生长前期(5月初分枝期以前)较低,生长旺盛期(7月份开花期)达到最高,而后随着生育期延长(8月份成熟期以后),逐渐降低;土壤温度对土壤CO_2通量的影响要比土壤水分大;7月份,土壤温度和土壤水分均达到较为适宜条件,且植物处于旺盛生长状态,根系和微生物活动旺盛,土壤呼吸处于季节高峰期。建立了土壤呼吸与土壤温度和水分的经验模型,两种模型估算土壤CO_2通量的结果差别不大,分别为618 gC·m~(-2)和605 gC·m~(-2)。其土壤呼吸模拟值的季节变化趋势基本一致,但在生长前期和生长后期,经验函数模拟结果偏高,在生长旺盛期200 d左右,SOILCO2模拟结果日变化幅度则要比经验函数的模拟结果日变化幅度大。利用模型进行土壤呼吸的模拟计算可以为不同土地利用方式下碳循环过程的评估提供简便方法。
综上所述,植被恢复过程中,通过对植被生长、耗水、土壤水分和土壤温度等因素动态变化的研究,可为该区植被生长条件下土壤水分、土壤温度、土壤气体等物理过程的动态模拟奠定基础。从提高模拟的准确度来看,还需进一步修改、校正与完善已有的根系吸水模型,深入研究根系伸展和发育规律,考虑土壤养分和孔隙度等因素对CO_2产生和运移的机理性影响。
In the water-wind erosion crisscross region of the Loess Plateau, the conversion of croplands to frorest and grasslands has been implemented to control soil erosion since 1990s. As vegetation is rehabilitating in arid and semiarid area, soil water is one of main restrictive factors, and the consequent CO_2 exchange between soil and atmosphere is one important concern of the resulting eco-environmental effects. As mentioned above, studies about soil water and soil CO_2 movement in this area should receive more attention. This research was trying to preliminanily verify the process of root growth and water consumption of local representative vegetation, to estimate soil CO_2 flux using emprical and mechanical models, so as to provide scientific bases for evaluation of eco-environmental effects caused by vegetation restoration. In this research, soil column experiment with growing alfalfa (Medicago Sativa L.) was conducted to obtain root distribution, soil water contents and transpiration rates for two growing years. Empirical root distribution functions were verified with measured data. Employing the derived function from measured root length data and the different empirical root distribution functions in the root water uptake model, which was incorporated in the Hydrus-1D model, we tested empirical root distribution functions and its effects on the soil water simulation. Soil respiration rates under five landuse patterns were measured by a closed-chamber IRGA method during the growing season in 2007. Differences in soil respiration under the five land use patterns and the relationships between soil respiration and soil temperature and soil water contents were analyzed. Soil respiration, soil water and soil temperature in alfalfa land were measured simutaneously during the growing season of 2008, and were compared with simulated results obtained by HYDRUS-1D.
The main results are as follows:
(1)The average diameter of alfalfa fine root is about 0.20 mm, specific root length is about 274 mm/mg. The relationships between the fine root length, dry weight, surface area, and volume were significantly related and with linear forms. The relation between fine root length y (m) and fine root dry weight x (g) can be expressed as: . A fitting procedure, which can be used to derive the distribution function of fine root, was implemented and the fitting results showed its effectiveness. Fine roots of alfalfa in 2 growth years could be divided into 3 growth stages, and its peak biomass accumulation rate is the branching stage in the second year. The increase of the plant height was fastest in the flowering period, which was between the middle of June and end of July. In the maturity the plant height was stable. The leaf area of the plant showed a single peak curve in the first growing season, and reached the largest value in the maturity. The soil water content had very siginicant effect on the biomass of the above-ground part and main roots. The plant height and leaf area at high water treatment was larger than that at the low one. y =274.18x
(2)Soil water stress had little effect on fine root biomass, but resulted in significant decreases in both above-ground and main root biomasses. The average transpiration not only was related with soil water content, but also with growth years. Soil water treatment had no effect on transpiration coefficient of biomass (P>0.05). Conversion of transpiration to biomass was almost stable at the same period for the same plant, but as for economic yield, the transpiration coefficient under high soil water was smaller than that under deficit one significantly. Biomass had linear corrlation with water consumption significantly no matter how the water levels. Soil water stress had more effects on aboveground biomass than underground biomass. During flowering-mature period,measured transpiration significantly correlated the calculated data using meteorological data.
(3)The root distribution models of Prasad (1988) and Hoffman & van Genuchten (1983) fitted measured data well especially below 36 cm of depth under a non-stressed condition, and Raats (1974) root distribution model was not as good as the other two models in terms of fitting the measured data. The distribution models resulted in nearly identical soil water content distributions, and its average root mean square error was below 3.5 %.
(4)Seasonal changes of soil respiration showed single peak curves, and that of air temperature followed a similar trend, which had the highest values in July and August. The order of average soil respiration rates under the five land use types during the growing seasons was: bunge needlegrass land > alfalfa land > Korshinsk Peashrub land > cropland > sand willow land. The soil respiration rate of the grassland was significantly higher than those of the crop and shrub lands. Except for sand willow and alfalfa lands, soil respiration rates were better correlated with soil temperature at 10 cm depth than with other temperatures at other depths. According to Q10 values (temperature sensitive index), soil respiration under cropland was the most sensitive to temperature (Q10=2.20) and Q10 of the other land use patterns, except for sand willow land, were around 2.0, which is close to the global average Q10 value. Estimating the soil respiration flux using the Van′t Hoff model gave CO_2 efflux values for alfalfa land, bunge needlegrass land, Korshinsk Peashrub land, crop land, and sand willow land, during the growth period, of 259 gC·m~(-2), 236 gC·m~(-2), 226 gC·m~(-2), 170 gC·m~(-2), and 94 gC·m~(-2), respectively. Soil moisture did not significantly affect soil respiration under crop and sand willow lands. A two-variable (soil temperature and soil moisture) soil respiration model best explained the variance of soil respiration under alfalfa land, bunge needlegrass land, and Korshinsk Peashrub land.
(5)Using the code of HYDRUS-1D, related simulations can be done. The simulating results on water, heat, and CO_2 movement in alfalfa fields during growing season showed that the predicted soil water and temperature were in good agreement with the observed values; the predicted CO_2 flux had the same trend to the observed data in the whole growing season. The soil CO_2 flux was low during the first stage, increased to the highest in the second stage and dropped gradually in the last stage. In addition, the results showed that soil temperature had greater effect on soil CO_2 flux than soil water. In July, both plant growth and microorganism activities were vigorous because of comfortable soil temperature and water conditions and thus the soil respiration reached the maximum. A empirical model was derived for simulate soil CO_2 flux. By model validations, both of the derived relationship and SOILCO2 model were not poor models for simulation of soil respiraton. The derived empirical model is probably site specific but is very simple and easy to implement. SOILCO2 were capable of satisfactorily describing the diurnal and seasonal variations in soil temperature and soil water content. Using these two models, there was about 600 g C m~(-2) emissions in the growth season, from Apr 1st. to Nov. 3rd of 2008. Our study showed that the process-based soil CO_2 flux model SOILCO2 could be applied to simulate CO_2 flux from alfalfa land in norther Loess Plateau of China together with soil temperature and soil water content even with mostly default parameters. This study suggests that more datails about the transport parameters of soil water, heat and carbon dioxide should be determined to estimate the CO_2 flux more accurately.
As the results mentioned above, in the process of vegetation rehabilitation from cropland to forest and grassland, the experiment on root growth, plant water consumption, soil water and soil temperature change, could help to simulate soil water, heat, and CO_2 movement. In order to improve the accuracy of simulation, current root-water-uptake models need improvement and take the effects of soil nutrient and porosity on the production and movement of soil CO_2 into account.
(1)紫花苜蓿细根平均根径为0.20 mm左右,比根长为274 mm/mg,细根根长、干重、表面积和体积之间呈极显著线性关系,其细根根长y (m)和细根干重x (g)关系为:。紫花苜蓿根系生长迅速,在适宜水分条件下,第一年生长期内根系平均下扎速率1.03 cm/d。建立了反映植物实际根长分布情况数据拟合方法:即通过建立紫花苜蓿归一化累计根长与土壤深度的函数拟合关系,对拟合的函数求导可得其根长密度与土壤深度的关系,从而得到连续的根长密度分布剖面。种植2年紫花苜蓿根系生长时间可分为三个阶段,在第二年的分枝现蕾期根系生长速率最快;株高生长最快在6月中旬开始到到7月下旬的开花期,成熟期株高达到稳定值;叶面积在第一年生长期内呈单峰曲线变化趋势,在开花成熟期达到最大值。水分对主根、细根生物量均有显著的影响,高水处理下的株高和叶面积大于低水处理。y =274.18x
(2)水分胁迫对于紫花苜蓿细根干重的累积速率无影响,但对其地上产量和主根生物量的累积速率影响较为明显。每年生长期平均蒸腾速率不仅和水分条件有关,还和苜蓿的生长年限密切相关。高低两种水分处理下的生物产量蒸腾系数差异不显著(P >0.05),蒸腾水的转化率对于同一时期的同一种植物而言几乎是固定的,这主要是针对生物产量而言;对于经济产量来说,适宜供水情形下蒸腾系数则会显著低于亏缺供水情形。不考虑水分条件,紫花苜蓿生物量和耗水量之间存在极显著线性相关关系。水分胁迫对于地下部分的影响要弱于对地上部分产量的影响,根系的迅速生长可以吸收更多的土壤水分,但是田间也会造成土壤的迅速干燥化。在苜蓿开花-成熟生育阶段,所测定的苜蓿蒸腾量与采用气象数据计算的参考作物蒸散量均呈极显著线性相关。
(3)两种经验根系分布模型(Prasad与Hoffman和van Genuchten)可近似反映紫花苜蓿根系实际分布状态,进一步通过土壤水分动态模拟验证得出这两种经验根系分布模式适用于根系分布的描述,且参数简单、获取方便,具有一定的实用性。拟合的根系分布、Prasad分布与Hoffman和van Genuchten分布与36 cm以下的的根长密度实测值较为吻合,Raats根系分布与实测值及其它分布模式则差别较大。在缺乏根系测定资料情况下,可通过经验根系模式确定其分布。
(4)5种土地利用类型土壤呼吸速率季节性变化均呈现单峰型曲线,与气温变化趋势一致,其7、8月份土壤呼吸速率均显著高于其它月份(P<0.05);生长季节土壤CO_2平均释放速率顺序为:长芒草地>苜蓿地>柠条地>农地>沙柳地,草地在生长前期和旺盛期土壤呼吸强度均显著高于农地和灌木林地,退耕还林还草显著提高了土壤呼吸。除沙柳地和苜蓿地以外,在土壤呼吸与所有温度指标的关系中,与10 cm深度的土壤温度相关性最好,且除沙柳地外,其它4种土地利用类型均与之达到显著相关;农地土壤呼吸对温度的响应最敏感(Q10值为2.20),除沙柳地(Q10值为1.48)外,其它4种土地利用类型Q10值均在2.0左右,接近于全球Q10的平均水平;通过Van’t Hoff模型估算,2007年植物整个生长季节(5～10月份),5种土地利用类型的土壤呼吸量从高到低依次为:苜蓿地259 gC·m~(-2),长芒草地236 gC·m~(-2),柠条地226 gC·m~(-2),农地170 gC·m~(-2),沙柳地94 gC·m~(-2);水分对农地和沙柳地的土壤呼吸影响不大;长芒草地、柠条地和苜蓿地土壤呼吸的双变量模型关系显著(P<0.05),比相应的单变量模型更好地解释了土壤呼吸变异。
5)采用HYDRUS-1D中集成的SOILCO2模型可以为该区土壤CO_2通量的预测提供可靠的模型方法。在植物生长季节,田间苜蓿地土壤水热气运动模拟显示:土壤水分剖面动态变化和土壤温度动态变化和实测数据相差不大;土壤CO_2通量实测值和模拟值在苜蓿整个生长期内有相同的变化趋势:即在苜蓿生长前期(5月初分枝期以前)较低,生长旺盛期(7月份开花期)达到最高,而后随着生育期延长(8月份成熟期以后),逐渐降低;土壤温度对土壤CO_2通量的影响要比土壤水分大;7月份,土壤温度和土壤水分均达到较为适宜条件,且植物处于旺盛生长状态,根系和微生物活动旺盛,土壤呼吸处于季节高峰期。建立了土壤呼吸与土壤温度和水分的经验模型,两种模型估算土壤CO_2通量的结果差别不大,分别为618 gC·m~(-2)和605 gC·m~(-2)。其土壤呼吸模拟值的季节变化趋势基本一致,但在生长前期和生长后期,经验函数模拟结果偏高,在生长旺盛期200 d左右,SOILCO2模拟结果日变化幅度则要比经验函数的模拟结果日变化幅度大。利用模型进行土壤呼吸的模拟计算可以为不同土地利用方式下碳循环过程的评估提供简便方法。
综上所述,植被恢复过程中,通过对植被生长、耗水、土壤水分和土壤温度等因素动态变化的研究,可为该区植被生长条件下土壤水分、土壤温度、土壤气体等物理过程的动态模拟奠定基础。从提高模拟的准确度来看,还需进一步修改、校正与完善已有的根系吸水模型,深入研究根系伸展和发育规律,考虑土壤养分和孔隙度等因素对CO_2产生和运移的机理性影响。
In the water-wind erosion crisscross region of the Loess Plateau, the conversion of croplands to frorest and grasslands has been implemented to control soil erosion since 1990s. As vegetation is rehabilitating in arid and semiarid area, soil water is one of main restrictive factors, and the consequent CO_2 exchange between soil and atmosphere is one important concern of the resulting eco-environmental effects. As mentioned above, studies about soil water and soil CO_2 movement in this area should receive more attention. This research was trying to preliminanily verify the process of root growth and water consumption of local representative vegetation, to estimate soil CO_2 flux using emprical and mechanical models, so as to provide scientific bases for evaluation of eco-environmental effects caused by vegetation restoration. In this research, soil column experiment with growing alfalfa (Medicago Sativa L.) was conducted to obtain root distribution, soil water contents and transpiration rates for two growing years. Empirical root distribution functions were verified with measured data. Employing the derived function from measured root length data and the different empirical root distribution functions in the root water uptake model, which was incorporated in the Hydrus-1D model, we tested empirical root distribution functions and its effects on the soil water simulation. Soil respiration rates under five landuse patterns were measured by a closed-chamber IRGA method during the growing season in 2007. Differences in soil respiration under the five land use patterns and the relationships between soil respiration and soil temperature and soil water contents were analyzed. Soil respiration, soil water and soil temperature in alfalfa land were measured simutaneously during the growing season of 2008, and were compared with simulated results obtained by HYDRUS-1D.
The main results are as follows:
(1)The average diameter of alfalfa fine root is about 0.20 mm, specific root length is about 274 mm/mg. The relationships between the fine root length, dry weight, surface area, and volume were significantly related and with linear forms. The relation between fine root length y (m) and fine root dry weight x (g) can be expressed as: . A fitting procedure, which can be used to derive the distribution function of fine root, was implemented and the fitting results showed its effectiveness. Fine roots of alfalfa in 2 growth years could be divided into 3 growth stages, and its peak biomass accumulation rate is the branching stage in the second year. The increase of the plant height was fastest in the flowering period, which was between the middle of June and end of July. In the maturity the plant height was stable. The leaf area of the plant showed a single peak curve in the first growing season, and reached the largest value in the maturity. The soil water content had very siginicant effect on the biomass of the above-ground part and main roots. The plant height and leaf area at high water treatment was larger than that at the low one. y =274.18x
(2)Soil water stress had little effect on fine root biomass, but resulted in significant decreases in both above-ground and main root biomasses. The average transpiration not only was related with soil water content, but also with growth years. Soil water treatment had no effect on transpiration coefficient of biomass (P>0.05). Conversion of transpiration to biomass was almost stable at the same period for the same plant, but as for economic yield, the transpiration coefficient under high soil water was smaller than that under deficit one significantly. Biomass had linear corrlation with water consumption significantly no matter how the water levels. Soil water stress had more effects on aboveground biomass than underground biomass. During flowering-mature period,measured transpiration significantly correlated the calculated data using meteorological data.
(3)The root distribution models of Prasad (1988) and Hoffman & van Genuchten (1983) fitted measured data well especially below 36 cm of depth under a non-stressed condition, and Raats (1974) root distribution model was not as good as the other two models in terms of fitting the measured data. The distribution models resulted in nearly identical soil water content distributions, and its average root mean square error was below 3.5 %.
(4)Seasonal changes of soil respiration showed single peak curves, and that of air temperature followed a similar trend, which had the highest values in July and August. The order of average soil respiration rates under the five land use types during the growing seasons was: bunge needlegrass land > alfalfa land > Korshinsk Peashrub land > cropland > sand willow land. The soil respiration rate of the grassland was significantly higher than those of the crop and shrub lands. Except for sand willow and alfalfa lands, soil respiration rates were better correlated with soil temperature at 10 cm depth than with other temperatures at other depths. According to Q10 values (temperature sensitive index), soil respiration under cropland was the most sensitive to temperature (Q10=2.20) and Q10 of the other land use patterns, except for sand willow land, were around 2.0, which is close to the global average Q10 value. Estimating the soil respiration flux using the Van′t Hoff model gave CO_2 efflux values for alfalfa land, bunge needlegrass land, Korshinsk Peashrub land, crop land, and sand willow land, during the growth period, of 259 gC·m~(-2), 236 gC·m~(-2), 226 gC·m~(-2), 170 gC·m~(-2), and 94 gC·m~(-2), respectively. Soil moisture did not significantly affect soil respiration under crop and sand willow lands. A two-variable (soil temperature and soil moisture) soil respiration model best explained the variance of soil respiration under alfalfa land, bunge needlegrass land, and Korshinsk Peashrub land.
(5)Using the code of HYDRUS-1D, related simulations can be done. The simulating results on water, heat, and CO_2 movement in alfalfa fields during growing season showed that the predicted soil water and temperature were in good agreement with the observed values; the predicted CO_2 flux had the same trend to the observed data in the whole growing season. The soil CO_2 flux was low during the first stage, increased to the highest in the second stage and dropped gradually in the last stage. In addition, the results showed that soil temperature had greater effect on soil CO_2 flux than soil water. In July, both plant growth and microorganism activities were vigorous because of comfortable soil temperature and water conditions and thus the soil respiration reached the maximum. A empirical model was derived for simulate soil CO_2 flux. By model validations, both of the derived relationship and SOILCO2 model were not poor models for simulation of soil respiraton. The derived empirical model is probably site specific but is very simple and easy to implement. SOILCO2 were capable of satisfactorily describing the diurnal and seasonal variations in soil temperature and soil water content. Using these two models, there was about 600 g C m~(-2) emissions in the growth season, from Apr 1st. to Nov. 3rd of 2008. Our study showed that the process-based soil CO_2 flux model SOILCO2 could be applied to simulate CO_2 flux from alfalfa land in norther Loess Plateau of China together with soil temperature and soil water content even with mostly default parameters. This study suggests that more datails about the transport parameters of soil water, heat and carbon dioxide should be determined to estimate the CO_2 flux more accurately.
As the results mentioned above, in the process of vegetation rehabilitation from cropland to forest and grassland, the experiment on root growth, plant water consumption, soil water and soil temperature change, could help to simulate soil water, heat, and CO_2 movement. In order to improve the accuracy of simulation, current root-water-uptake models need improvement and take the effects of soil nutrient and porosity on the production and movement of soil CO_2 into account.
引文
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