Transport And Use Of PAR And Water In Maize/Soybean Strip Intercropping System

Intercropping is an effective way of using several kinds of natural resources more effectively and improving yield per unit of cultivated land. In recent years, natural resources, such as arable land and water, are becoming more and more limited with quick social development and rapid growth of industry and population, which constitutes a serious threat on food supply security in China. Therefore, it is necessary meaningful to study the radiation interception and utilization by intercropping canopies, the soil evaporation, plant transpiration, and root water uptake dynamics in intercropping systems for optimizing components, improving management and rising resources use efficiency and output of intercropping systems.Field experiments were conducted at Shangqiu Agro-ecosystem Experimental Station in 2006-2008 years to study the maize/soybean intercropping system, a widely practiced intercropping model in Northern China. The field experiment comprised of four treatments with four replications. Four treatments were set as: sole maize (SM), sole soybean (SSB), maize/soybean 1:3 intercropping (three rows of soybean by every one row of maize, I₁₃) and maize/soybean 2:3 intercropping (three rows of soybean by every two rows of maize, I₂₃). Light environment characteristics, soil evaporation, plant transpiration, root distribution, soil water movement and resources utilization efficiency in the four different cropping systems were investigated and simulated with field experimental data got in three years. The main results were as following:(1) About the PAR radiation interception and distribution in intercropping systemsIn earlier growing stage, photosynthetically active radiation (PAR) transmittance at the bottom of edge row of soybean strip adjacent to maize row in I₁₃ and I₂₃ treatments was higher than that of inner row of soybean strip, while it was in adverse for maize strip in I₂₃ treatment. In later growing stage, PAR transmittance at the bottom of the intercropping canopies did not vary significantly, and the averaged PAR transmittance was less than 7%. The averaged extinction coefficient (K) of treatment SM, SSB, I₁₃ and I₂₃ over three growing seasons was 0.46, 0.59, 0.51 and 0.50, respectively.(2) About soil evaporation and plant transpirationIn earlier growing stage (LAI<1), there were little differences for soil evaporation at different locations in I₁₃ and I₂₃ treatments. With development of crop canopy (after LAI≥1), the differences among soil evaporation rates at different locations became more and more significant. Soil evaporation rate was mainly influenced by water content in surface layer of soil and leaf area index (LAI). The relative soil evaporation intensity (E/ET0) showed a good positive exponential growth relation with water content in 0-5 cm layer of soil, and a good negative exponential growth relation with LAI.The diurnal variation of maize and soybean plant sap flow in I₂₃ treatment fitted a single-peak curve in sunny day and multi-peak curve in cloudy day, respectively. Sap flow was influenced by many environmental factors, but showed a closest relationship with solar radiation. The correlations between daily sap flow of maize or soybean and environmental factors was extremely significant. During observation period of sap flow (June 1st– June 30th , 2008), mean diurnal transpiration value for maize plant (1.44 mm d-1) in I₂₃ treatment was 1.8 time of that for soybean plant (0.79 mm d-1), and maize transpiration and soybean transpiration contributed 64% and 36% to the total transpiration in I₂₃ treatment, respectively.It is very important to determine correctively relevant resistances for modeling soil evaporation and plant transpiration in intercropping systems with ERIN model. In maize/soybean 2:3 intercropping system (I₂₃ treatment), surface resistance of substrate soil was 300 sm^-1, aerodynamic resistance r^a_a and r^s_a was set as 4.50 and 42.41 sm^-1, boundary layer resistances as 12.19 and 38.89 sm^-1 for maize and soybean canopies, and stomatal resistance as 81.32 and 64.92 sm^-1 for maize and soybean leaves, respectively. Results indicated that simulated maize or soybean transpiration value was higher than measured value, and calculated soil evaporation value 94.67% of measured value in I₂₃ treatment. Statistics analysis showed that there was a significant correlation relationship between measured values and simulated values with a correlation coefficient of greater than 0.8. Simulated results showed ERIN model in relative excellent for estimating soil evaporation and plant transpiration of intercropping systems, and for determining transpiration contribution of each component in an intercropping system.(3) About root distribution and water uptakeRoot growth and distribution of intercropping system was investigated with washing out, numbering, sampling and measuring growing roots from a thin practical soil profile on site. Results indicated that developed depth for intercropped maize was greater than that for intercropped soybean in I₂₃ treatment under full irrigation. In maize/soybean 2:3 intercropping system (I₂₃ treatment), maize roots not only developped horizontally in the soil zone just under the maize strip, but also extended in adjacent soil zone and can reach the position just vertical under the middle row of the soybean strip. For soybean, however, most roots developped horizontally in the soil zone just under the soybean strip. The greatest horizontal extension distance for maize and soybean roots appeared in 16²2 cm soil layer from surface, relative to the existing of a plough pan and clay intercalation.Root length density (RLD) of maize and soybean in I₂₃ treatment were primarily distributed in surface soil layer of 0³0 cm and in soil zone under crop rows. An exponential model fitted very well to the 2D distribution of root length density for I₂₃ treatment. In maize-soybean 2:3 intercropping system (I₂₃ treatment), the ranges of soil water content change were in the order: maize zone>soybean zone>middle zone. This order indicated that each crop preferentially absorbed soil water from soil zone just under its strip, and then from adjacent mixed zone in intercropping system.It was very difficult to determine accurately root density distribution in intercropping system, because of mixed root distribution of different crops. A two-dimensional root distribution and water uptake model for intercropped crops was developed with some easily acquired parameters. Soil water transport in intercropping system (I₂₃ treatment) was simulated with HYDRUS software package and the two-dimensional root water uptake model. Results showed that the model was suitable for simulating soil water transport in intercropping system.(4) About biomass accumulation and yieldBiomass accumulation of maize and soybean was fitted with Logistic equation for both the intercropping and monocultures, and correlation reached very significant level (P<0.01). Mean above-ground biomass accumulations and grain yield of each intercropped maize plant were higher than that of each sole maize plant because of edge effect. There was no significant difference for biomass and yield between I₁₃ and I₂₃ treatments. However, there was little difference between intercropped soybean plant and sole soybean plant. Comparing with sole maize, the intercropped maize had higher translocation rate of biomass. Translocation rate of biomass of the sole soybean was slightly higher than that of the intercropped soybean. There was no significant difference for translocation rate of biomass of soybean in I₁₃ and I₂₃ treatments.Grain maize yield for I₁₃ and I₂₃ treatments was 83% and 95% of that for SM treatment, respectively, which indicated that the grain yield increase caused by edge-effect was not enough to compensate the grain yield lose caused by maize occupied area decrease. Grain soybean yield for treatment I₁₃ and I₂₃ was 82% and 76% of that for SSB treatment, respectively, wihch showed that the shading effects of maize to soybean in maize/soybean intercropping system. The total yield of maize/soybean intercropping system, however, was significantly higher than the yield of each sole cropping, which showed that the advances of intercropping in increasing biomass and yield for per unit of cultivated land.(5) About resource use efficiencyPAR interception and utilization by maize/soybean intercropping canopy were evaluated with the fraction of radiation intercepted (F), radiation use efficiency (RUE) and harvest index (HI). The averaged maize HI for treatment SM, I₁₃ and I₂₃ over three growing seasons was 0.42, 0.45 and 0.44, respectively; the averaged soybean HI for treatment SSB, I₁₃ and I₂₃ was 0.40, 0.35 and 0.36, respectively. Maize RUE of treatment I₁₃ and I₂₃ was 3.14 and 3.13 g MJ^-1, slightly less than that of treatment SM (3.18 g MJ^-1). Soybean RUE of treatment I₁₃ and I₂₃ was 1.65 and 1.63 g MJ^-1, slightly higher than that of treatment SSB (1.55 g MJ^-1). RUE of treatment I₁₃ (2.82 g MJ^-1) was slightly higher than that of treatment I₂₃ (2.78 g MJ^-1). RUE of treatment I₁₃ and I₂₃ was less than that of treatment SM by 11% and 13%, but higher than that of treatment SSB by 82% and 79%, respectively. The results indicated that intercropping may increase grain yield per unit of land by more efficient radiation utilization.Mean evapotranspiration (ETc) of I₁₃ treatment (495.84 mm) was slightly lower than that of treatment I₂₃ (506.16 mm) over three growing seasons. ETc of I₁₃ treatment was 15% and 6% higher than that of SM and SSB treatments, respectively. ETc of I₂₃ treatment was 17% and 9% higher than that of SM and SSB treatments, respectively. Water use efficiency (WUE) of I₁₃ and I₂₃ treatments (I₁₃:20.24 kg ha^-1 mm^-1;I₂₃:21.97 kg ha^-1 mm^-1) was slightly lower than that of SM treatment (22.67 kg ha^-1 mm^-1), but significantly higher than that of SSB treatment (5.07 kg ha^-1 mm^-1). Land equivalent ratio (LER) of I₁₃ and I₂₃ treatments was 1.65 and 1.71, which derived that 65% and 71% more land in the monocultures was required than that in the intercropping for producing same grain, and that maize/soybean intercropping has significant advantage of improving yield and land use efficiency.