| Wheat/maize intercropping is long established cereal/cereal intercropping systems in the major grain production areas of northwest China. Previous studies have proved that root of wheat could gain competitve advantage and limit the lateral spread of maize roots. There is little reaserch which has investigated whether resource competition or other factors lead to the root interactions in wheat/maize intercropping. The aim of the present study was to investigate the effect of different nitrogen application rates, irrigation times or wheat sowing date on interspecific interactions in wheat/maize intercropping. Three field experiments were conducted in2012and2013. The experiments1and2focused on the root growth and distribution in different resource availability (nitrogen application rates or irrigation times) and the relationship between the responses of crop root plasticity to mineral N and water availabilities in soil profiles and the interspecific competition between intercropped wheat and maize under field conditions. The experiment3was targeted to determine the productivity and interspecific competition at co-growth stages with three sowing dates of wheat and the relationship between sowing date of wheat and relative competitive ability in the intercropping. The main results were as follows:(1) Wheat intercropped with maize can adjust root length density and root distribution according to different soil mineral N concentrations. Under N deficient conditions (basal nitrogen fertilizer0kg N ha-1), intercropped wheat had the root length density (RLD) of wheat was increased, and spread laterally up to45cm away from the nearest wheat row. Under N sufficient condition, the roots of intercropped wheat spread laterally just only25cm away from the wheat row, and the RLD was significantly decreased accordingly. The results highlight there was significant morphological plasticity for wheat roots with variable soil N availability. The RLD of intercropped maize roots was negatively correlated with soil mineral N concentrations under intercropped maize plants (own area), but not under wheat plants. The roots of intercropped maize, especially in the top soil, were limited laterally by associated wheat at all N treatments. In addition, the roots of wheat were more sensitive to the mineral nitrogen availability than those of maize did, indicating that wheat roots had higher morphological plasticity in response to soil inorganic N status than maize did. Hence higher morphological plasticity may provide greater competitive ability of intercropped wheat under diverse soil resource conditions in wheat/maize intercropping.(2) The average nitrogen uptake rate per unit root length (NUR1) of intercropped wheat in all growth stage was significant increased by55-375%compared to sole-cropped wheat, while NUR1of intercropped maize in0-69days of co-growth period was15-58%lower by than those in corresponding sole-cropped maize. The physiological differences in root absorption capacity between maize and wheat are another mechanism behind the more competition ability of intercropped wheat than that of maize. Soil mineral nitrogen concentration was one of causes in increase of NUR1, which were increased by41-165%or44-46%with increasing topdressing nitrogen application rates for intercropped wheat or maize, respectively. Therefore, intercropped wheat acquires soil mineral nitrogen not only under wheat plants, but also under the maize plants. This was probably the main reason for the competition advantage of wheat NUR1in wheat/maize intercropping.(3) When increasing nitrogen topdressing at elongation or tasseling stages, the RLD of intercropped maize after wheat harvest declined significantly by13-50%compared with conventional nitrogen application. In the meantime the roots of intercropped maize could extend up to the soil soil under the maize and wheat plant (10-80cm) with the decreasing nitrogen application. The dynamic root growth and distribution contributed to the recovery growth of intercropped maize after wheat harvest. The root distribution of intercropped maize responses to the availability and distribution of soil nitrate was controlled by nitrogen concentration of shoot tissues. Moreover, the NUR1of intercropped maize was1.97times as greater as that of monoculture, which was the other mechanism resulted in the recovery growth of intercropped maize after wheat harvest.(4) The plasticity of wheat and maize roots in response to soil water variability were found at co-growth stages in wheat/maize intercropping. Four times of irrigation at co-growth stages decreased the lateral growth of intercropped wheat roots which extended horizonally only25cm away from the nearest wheat row. The RLDs of intercropped wheat and maize under4times irrigation during co-growth stage were decreased by64%and10%compared with those only2times of irrigation, respectively. The roots of intercropped maize spread laterally only20cm from row of maize plants under soil water deficiency (2times of irrigation during co-growth stage), but up to50cm under the increased water supply (4times of irrigation), which also led to increase RLD of maize significantly. The results indicated that plasticity in both wheat and maize roots in response to soil water status was displayed in co-growth period. Moreover, wheat roots were more sensitive to the soil water availability than the roots of maize, which further suggested that wheat had greater plasticity of roots than maize plant.(5) The competitive ability of wheat was greater than that of maize although two crops were sown at same date, where aggressivity of wheat relative to maize (Awm) was0.16-1.85. When wheat was sown earlier one month than maize did, the Awm of wheat relative to maize was increased by28%and36%, respectively, in2012and2013, compared to when two crops were sown at the same date. When wheat and maize were sown simultaneously, intercropped wheat overyielded by18%and22%, and intercropped maize overyielded by28%and24%in2012and2013, respectively. However, compare with solo-cropping maize, the biomass accumulation of intercropped maize in seeding and elongation stage was significantly decreased by14%and16%, respectively, and dramatically increased after wheat harvest. |
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