Effect of vegetable cropping system on total nitrogen, phosphorus and COD in farmland leachate

At present, China has the largest land area under greenhouse vegetable cultivation in the world. Greenhouse production has frequently been associated with excessive fertilizer use and high proportion of organic fertilizer input. These factors have induced changes in soil nutrient content and physicochemical properties, and enhanced leaching of soil nitrogen and phosphorus. However, comparative investigations of soil nutrient losses due to leaching in conventional greenhouse vegetable cropping systems have been inconclusive. Thus this study investigated long-term field trials of three different greenhouse vegetable cropping systems in terms of total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD) leaching. The experiment, started in March 2002, was carried out in three side-by-side greenhouses with three cropping systems-(1) organic system (ORG), in accordance with IFOAM Basic Standards by using only compost and physical and biological control; (2) integrated system (INT), with lower inputs of agrochemicals and compost; (3) conventional system (CON), with chemical fertilizers and composts applied in accordance with local practices. The application rates of compost and chemical fertilizer in the INT system was half of those in the ORG and CON systems. While pest control in ORG included physical control and biological fungicide, sulfur fumigation was used to control plant diseases. In the INT and CON systems, insecticides and fungicides were used to control insects and diseases. Each treatment was applied to the entire greenhouse. Chemical fertilizer was in the form of urea, calcium superphosphate and potassium chloride at the ratio of 3:5:4. Compost composed of cow manure, dry chicken manure and straw with VT microbial agent. All three systems were under the same irrigation scheme and 13 times of flood irrigation was used during the two seasons in 2014 in each cropping system, with single-event irrigation amount of 975 m³·hm^-2. Leaching soil water at about 1 m deep was collected using lysimeter within 4-6 days after each irrigation and then analyzed for TN, TP and COD. The objective was to build the theoretical base for the development of sustainable agricultural systems. Results showed that the amount of TN in the leachate in ORG during the two seasons was 137.02 kg·hm^-2, respectively 12.0% and 25.9% lower than that in INT and CON cropping systems. The amount of TP in the leachate collected under ORG was 18.23 kg·hm^-2, which was respectively 51.2% and 119.9% higher than that in INT and CON systems. The COD in the leachate under ORG cropping system was 856.99 kg·hm^-2, which was also 32.4% and 3.1% higher than that in INT and CON systems, respectively. The trends of variation in TN, TP and COD in leachates under three systems were significantly different. The TN in leachate in prophase was constantly high, with an obvious peak after top dressing, which then decreased rapidly before it maintained a low level in June. The loss of TP via leaching changed smoothly, with an increasing trend followed by a decreasing and eventually reaching a peak level in June or July. The COD in leachate slightly decreased initially to the minimum point before top dressing and then gradually increased in May. In summary, it was concluded that ORG system reduced nitrogen loss via leaching, but significantly increased the risk of leaching of phosphorus and COD compared with INT and CON systems.