Cutting nitrogen leaching in greenhouse soil by water-nitrogen partially decoupled drip fertigation

It is challenging by popular water-nitrogen coupled drip fertigation (CF) to control dissolved nitrogen (DN) leaching in greenhouse soils planted with cucumber-represented vegetables that often need sufficient and frequent irrigation to ensure yields. This study proposed water-nitrogen partially decoupled drip fertigation (DF) as a substitute for CF to cut DN leaching from cucumber-planted greenhouse soil without cutting water and N input. DF performance in comparison with CF in soil DN leaching control was evaluated both theoretically based on presumed criteria and experimentally in a greenhouse loam soil without and with cucumbers. DF had a good potential in reducing DN leaching under moderate water leaching (15 %-25 %) when the ratio of irrigation and the ratio of N fertilization in water-more-dissolved nitrogen-less (WM) and water-less-dissolved nitrogen-more (WL) subzone soil specially formed under DF were respectively set at 2.0–3.0 and 0.6–0.8. Experimental results showed that the reduction effect of DF was limited on water leaching (<10 %) but significant on DN leaching, especially with cucumbers. The cumulative leaching loss of DN dominated by nitrate N (>75 %) from seven leaching events during the water demand-high fruiting period under moderate water leaching (averagely around 15 %) was 41.7 % lower (p < 0.05) under DF than under CF, predominantly (78.3 %) via lowering its availability in leachate. Moreover, DF increased the cucumber yield somewhat with retention of significantly more nitrate N mainly in WL subzone soil. The presence of cucumber improved DF performance mainly by absorbing more water from WM subzone soil than from WL subzone soil to weaken lateral diffusion of water and nitrate-dominated DN between WM and WL subzone soil, lowering DN level in leachate mainly derived from WM subzone soil while enhancing its level mainly in WL subzone soil. Hence, DF may replace CF to cut N leaching without cutting irrigation in greenhouse soils cultivated with cucumber-represented vegetables needing sufficient and frequent irrigation.