Background In older to solve the problems of low level of water and fertilizer management of vegetable facilities and imperfect irrigation and fertilization system in China, field experiments were conducted in a solar-heated greenhouse to study the effects of different fertigation strategies on root growth and distribution of nitrate-nitrogen from a surface point source of urea.

Methods Four fertigation strategies with different orders in which water and nutrients was applied for an irrigation event were that T1(1/5N-4/5W), T2(1/5W-1/5N-3/5W), T3(2/5W-1/5N-2/5W), and T4(3/5W-1/5N-1/5W), for example, the 1/5W-1/5N-3/5W represented irrigating water at the first 1/5 of the fertigation time, applying nitrogen at the second 1/5 of the fertigation time, and washing pipeline with water at the last 1/5 of the fertigation time, the interpretation of other treatments were similar. Cauliflower was taken as the research object in the experiment, and each treatment was triplicate. Soil moisture was determined once a week during caulifower growth. Yield was recorded for each harvest, and the quality, root length density of cauliflower and soil inorganic nitrogen was determined at the end of the experiment.

Results 1) Fertigation strategies had no significant effects on yield, quality and nitrogen accumulation of cauliflower. The treatments of applying the nitrogen in the later stage (T3 and T4) had higher yield by comparing the treatments of applying the nitrogen at the beginning stage (T1 and T2). The total nitrogen accumulation of cauliflower showed a trend of decreasing first and then increasing by delaying the nitrogen application to later stage, the nitrogen accumulation in T4 reached the highest of 50.23 mg/kg. 2) By comparing each treatment in the distribution of inorganic nitrogen during per-planting and post-harvest, we found that the content of inorganic nitrogen in soil before planting was high in surface and low in deep layer and the distribution of inorganic nitrogen in post-harvest soil profile was more uniform. Comparing other treatment, nitrate leaching was more likely to occur in T1 and T4. 3) The measurements of root distribution showed that fertigation at the beginning of the irrigation cycle promoted longitudinal growth of cauliflower's roots, and fertigation at the end of the irrigation cycle promoted lateral growth of roots. Contrast with other treatments, T3 had the largest root length density (2.54 cm/cm³) and the widest root distribution. 4) A significant linear relationship between the content of soil nitrate-nitrogen and the root length was found, and the root with diameter < 2 mm had better fitting effect than other indexes of root length (R²=0.811 8).

Conclusions Considering yield, quality, nitrogen use efficiency, root length density and the nitrogen content in soil, the strategy of T3, i.e., applying water at first for 2/5 of the total irrigation time, then applying fertilizer for 1/5 of the total irrigation time, followed by applying water for the last 2/5 of the total irrigation time, was suggested.