Study On Water, Heat And Co2 Movement Of A Soil-alfalfa System In Wind-water Erosion Crisscross Region-measuring And Modelling

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 CO₂ exchange between soil and atmosphere is one important concern of the resulting eco-environmental effects. As mentioned above, studies about soil water and soil CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ flux had the same trend to the observed data in the whole growing season. The soil CO₂ 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 CO₂ 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 CO₂ 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 CO₂ flux model SOILCO2 could be applied to simulate CO₂ 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 CO₂ 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 CO₂ 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 CO₂ into account.