Mechanism Of The Water Transportation Response To Chemical Regulation In Dryland Soil-root Systems And Its Simulation

Chemical regulation technology boasts the advantages of easy operation, low monetary investment, and rapid results, which make it an important farming technology in dryland agriculture. The regulating effect of chemicals on water transportation in the soil and crop system comes from its typical effects (e.g. reducing soil surface crust, enhancing soil water holding capacity, or reducing leaf stomatal conductance), which enhance the crops’effective utilization of water. Compared to the application of a single chemical, the application of multiple chemicals more powerfully promote the crops’effective utilization of water. This method has great potential for relieving the water resources stress caused by drought. Based on field cultivation trials, soil culture tests, and the soil column experiment, the combined application mode of multiple chemicals was explored, as was the mechanism of multi-chemical coordinated regulation on water transportation in the soil-root system. A soil water transport model and a root water uptake model were proposed based on Richard’s equation under the application of chemicals; these were used for modeling water transportation in a real field and were proved to be reliable. The main results include:(1) We researched the combination mode of three typical chemicals (PAM, SAP, and FA) under rain-fed and fertilizer controlling conditions through three years’(from 2012 to 2014) of field cultivation trials and found no significant effect of PAM on maize yield, but significant effects for SAP, FA, and fertilizer; in addition, a significant interaction can be found between SAP and FA and SAP and fertilizer. The regression model results showed that the predicted maximum mass of maize is 1.36 kg/m2 when the application of SAP with 93.38 kg/km2, the application of FA with 23.03 kg/km2and the application of fertilizer with 376.88 kg/km2 kg/km2.(2) The effect of SAP on soil hydraulic parameters and water retention characteristics is uncertain and varies with time, so we introduced timet into the soil hydraulic parameters D(θ) and K(θ) to reflect this typical effect; Very importantly, we considered the time t for SAP application as the same time t for the infiltration experiment; this established a soil water transport model based on Richard’s equation with time-varying soil hydraulic parameters D(θ,t) and K(θ,t); The idea for establishing model and the numerical solution method in this paper provided the feasible condition for establishing the water transport model with root uptake source-sink term under chemicals application.(3) We proposed an inverse method for calculating the source sink term (root water uptake) of Richard’s equation with time-varying soil hydraulic parameters. Several sets of numerical experiments were used to test its stability. The results showed that the water uptake interval, soil hydraulic parameters, test error, and instrument precision obviously influenced its stability and time step; however, layered soil and boundary conditions were not influenced. The suitable condition for this inverse method is a water uptake interval from 5d to 17d, a time step from 1000 to 10000, test error controlled within 0.9, instrument precision controlled within 0.03, and soil surface evaporation controlled within 0.6.(4) Application of SAP can enhance the root water uptake capability, but FA may weak the root water uptake capability by decreasing the leaf stomatal conductance. We compared the effect of chemicals on root length density and root nitrogen mass density to find chemicals paly a big role in increasing the maize root length density at early stage and gradually weak with the growth of maize, however, the contrary phenomenon can be found when the effect of chemicals on root nitrogen mass density, and showed a gradually improvement with the growth of maize and more stability. That made the root nitrogen mass density more suitable in reflecting distribution of root water uptake value in soil profile.(5) Combined application of SAP and FA can effectively reduce the "midday depression of photosynthesis," which enhances the maize’s photosynthesis and reduces transpiration. After comprehensively analyzing the relationship between maize root growth, leaf stomatal conductance, and endogenous hormones, we found that root length density, root nitrogen mass density, and leaf stomatal conductance were not only controlled by single endogenous hormones but also by a variety of endogenous hormones. On the one hand, the application of SAP and FA controlled the distribution of ABA and ABA/(ZR+GA+IAA) in crops and then to control maize root water uptake, on the other hand, which controlled the distribution of ZR and ZR/ABA in crops and then to control leaf stomatal conductance.(6) The results for modeling root water uptake using models based on root length density and root nitrogen mass density showed that, compared to the model based on root length density, the model based on root nitrogen mass density was more suitable for modeling water uptake by maize roots under chemical application, which is consistent with what we get from the theoretic studying results. Some obvious modeling errors between the simulated value and "measured value" can be found in the soil layer where SAP is applied, owing to the fact that root length density was reduced by SAP application. The modeling results from the field experiment showed that the proposed water transport model with a water uptake source sink term can be effectively used for reflecting the effects of chemicals on water transportation in soil and root systems.