Organic farming decreases nitrate leaching but increases dissolved organic nitrogen leaching in greenhouse vegetable production systems

Background and aims Organic farming has been viewed as a sustainable practice that can maintain yields and improve soil quality in greenhouse vegetable production. However, little attention has been given to the leaching of dissolved organic nitrogen (DON) due to excessive application of organic manure. The objectives of this study were to compare the characteristics of dissolved inorganic nitrogen (DIN) and DON leaching under different greenhouse vegetable production systems. Methods A 2-year lysimeter field experiment was conducted with three different farming systems in North China, i.e. conventional (CON, 70% chemical fertilizer +30% organic manure), integrated (INT, 50% chemical fertilizer +50% organic manure) and organic (ORG, 100% organic manure). Results The results indicated that vegetable yields under the ORG system were normally larger than the CON or INT. The amount of water drainage and dissolved total N (DTN, DIN + DON) leaching differed during two cropping seasons, with spring and summer seasons giving greater leaching than autumn and winter seasons. The total DIN leaching throughout four seasons of vegetable production under ORG was 176 kg N ha(-1), which was much lower than that of CON (327 kg N ha(-1)) and INT (269 kg N ha(-1)). The DTN leaching under ORG (485 kg N ha(-1)) was also less than that of CON (528 kg N ha(-1)) and INT (521 kg N ha(-1)). However, the total DON leaching under the ORG was 309 kg N ha(-1), far higher than the amounts for CON (202 kg N ha(-1)) and INT (252 kg N ha(-1)). The average DON leaching ratio (DON/DTN) under the ORG system was 63.7%, which was also much higher than that of the CON (38%) and INT (48%) systems. The increase in the proportion of organic manure in fertilizer types and over-irrigation were the main reasons for the increase in DTN leaching. Structural equation modeling analysis showed that the soil bulk density, saturated hydraulic conductivity and saturated soil water content were the most important factors influencing the DTN leaching. Conclusions While the ORG system has many advantages, irrigation and fertilizer scheduling should be optimized under the ORG system in order to minimize DON leaching.