NO₃^--N Concentration In Rhizosphere Soil Solution And Leaching Characteriristics In Typical Crops

The study on typical crop root area and different rhizosphere area in the field and NO3--N concentration at different rhizosphere area are lacking, and the technique of assay root NO3--N concentration in situ is still not mature. This study was conducted for the two typical cropping systems of greenhouse and wheat/corn in North China Plain. Using measures of shaking up soil and soil solution in soil multi-layers to determinate NO3--N concentration of rhizosphere soil solution, it explored crop rhizosphere and the non-rhizosphere soil solution nitrogen change at different growth stages. The study also took comparison analysis on the NO3--N concentration of soil solution at different root part during maximum efficiency period, cleared the dynamics and spatial variation regularity under different nitrogen concentrationin of soil solution. Moreover, it analysised the leaching characteriristics of nitrogen in deep soil to explore the greenhouse vegetables leaching mechanism, providing theoretical basis for reducing the leaching of nitrate loss and prevention of groundwater pollution. The main results are as follows:(1)Topsoil solution of corn was influenced by nitrogen input. The rhizosphere was significantly higher than non-rhizosphere in the trilobites period and large bugle stage, indicating that the root absorption of NO3--N was weak, the nitrogen was enriched into root top; the nitrogen rhizosphere was lower than non-rhizosphere in the other stages, indicating that the root absorption of was high, movement to top root of NO3--N was lower. At the large bugle stage the rhizosphere concentration of soil profile was higher than non-rhizosphere, fluctuating with the range of 0.98-5.34 mmol/L and 0.51-0.67 mmol/L. The rhizosphere and non-rhizosphere soil solution concentration of soil profile were both lower 2 mmol/L at pollination and tasselling; rhizosphere soil solution concentration of soil profile was lower than non-rhizosphere, fluctuating with the range of 0.62-1.37 mmol/L and 0.89-2.35 mmol/L respectively at the adult stages. Along with the soil depth increasing when the corn was at the nitrogen maximum efficiency period, NO3--N concentration of root area trended to decrease. The trend of NO3--N concentration of soil solution at different rhizosphere part was: rhizosphere>root area>non-rhizosphere.(2)With the nitrogen source added into the wheat topsoil solution, the rhizosphere was higher than non-rhizosphere at the seeding, elongation and flowering stage, reaching significant level at the seeding and flowering stage, indicating that the root absorption of NO3--N was lower; the rhizosphere at the other stages was significantly lower than non-rhizosphere, indicating that the root absorption of NO3--N was significantly higher than the nitrogen movement and resulting in the nitrogen enrichment in the non-rhizosphere. From the total soil profile, rhizosphere nitrogen concentration was fluctuated with the range of 2.14-6.57 mmol/L and higher than rhizosphere. The rhizosphere concentration of soil profile 0-60 cm at flowering stage was higher than the non-rhizosphere. The rhizosphere and non-rhizosphere concentration was gradually reducing below soil profile 60 cm, and varied slightly. The rhizosphere concentration at adult stage was lower than non-rhizosphere, fluctuating with the range of 0.60-1.39 mmol/L and 1.02-3.08 mmol/L.(3)The tomatoes topsoil solution rhizosphere nitrogen concentration at the seedling and fruit-set period, harvest time 1, 2 and adult stages was significantly higher than non-rhizosphere except blooming time,indicating that the root absorption of NO3--N was lower. The rhizosphere nitrogen of all soil profile at fruit-set period was higher than non rhizosphere, fluctuating with the range of 2.46-7.48 mmol/L and 2.41-6.44 mmol/L respectively; as the tomatoes frequently were fustigating rhizosphere nitrogen, the rhizosphere nitrogen concentration of root area at the harvest time 1 gradually increased with the range of 2.50-15.71 mmol/L and 2.55-7.50 mmol/L; the rhizosphere nitrogen of soil profile 0-100 cm at adult stages was higher than non-rhizosphere, fluctuating with the range of 2.54-14.61 mmol/L. Along with the soil depth increasing when tomatoes were at the nitrogen maximum efficiency period, NO3--N concentration of root area trended to decrease orderly. The NO3--N concentration trend of soil solution at different rhizosphere part was: The lateral root rhizosphere was the first, main root rhizosphere area was the second, the root area was third and the last was non-rhizosphere. The rhizosphere nitrogen of harvest time 1 was higher than at fruit-set period, when there was deep leaching risk.(4)During water filled time wheat/corn root was depended low affinity transportation system, there were nutriment leaching risks. During most of crop growth and development period high affinity transportation system run and with the increasing of soil depth, rhizosphere relied mainly by high affinity transportation system, and the leaching risk was lower; tomato was depended low affinity transportation system during the whole crop growth and development period. If the root area kept higher NO3--N concentration, nutriment was easily leaching out of root area, which leading to groundwater pollution.(5) Under two cropping systems, topsoil solution concentration in greenhouse was significantly higher than in the farmland and leaching depth reached 10 m; NO3--N concentration of wheat/corn profile soil solution mainly centered at 0-3 m depth and there was not significantly accumulation below 3 m. The risk of greenhouse soil to groundwater was greatly higher than farmland, and the deeper the easily polluted by nitrates.