The potential importance of soil denitrification as a major N loss pathway in intensive greenhouse vegetable production systems

Background About 30 % of vegetables in China are produced in intensively managed greenhouses comprising flood irrigation and extreme rates of nitrogen fertilizers. Little is known about denitrification N losses. Methods Soil denitrification rates were measured by the acetylene inhibition technique applied to anaerobically incubated soil samples. Four different greenhouse management systems were differentiated: Conventional flood irrigation and over-fertilization (CIF, 800 kg N ha−1, 460 mm); CIF plus straw incorporation (CIF+S, 889 kg N ha−1, 460 mm); Drip fertigation with reduced fertilizer application rates (DIF, 314 kg N ha−1, 190 mm); DIF plus straw incorporation (DIF+S, 403 kg N ha−1, 190 mm). Soil denitrification was measured on nine sampling dates during the growing season (Feb 2019-May 2019) for the top-/ subsoil (0 – 20/ 20- 40 cm) and on three sampling dates for deep soils (40-60/ 80-100 cm). Data was used to constrain N-input-output balances of the different vegetable production systems. Results Rates of denitrification were at least one magnitude higher in topsoil than in sub- and deep soils. Total seasonal denitrification N losses for the 0 – 40 cm soil layer ranged from 76 (DIF) to 422 kg N ha−1 (CIF+S). Straw addition stimulated soil denitrification in top- and subsoil, but not in deep soil layers. Integrating our denitrification data (0-100 cm) with additional data on N leaching, N2O emissions, plant N uptake, and NH3 volatilization showed, that on average 50 % of added N fertilizers are lost due to denitrification. Conclusions Denitrification is likely the dominant environmental N loss pathway in greenhouse vegetable production systems. Reducing irrigation and fertilizer application rates while incorporating straw in soils allows the reduction of accumulated nitrate.

Entomoponics: combining insect rearing and greenhouse vegetable production – a case study with Tenebrio molitor and high-wire cucumber cultivation

Insect production is generally a monoculture where insects are kept in an enclosed environment with a stable climate to maximise production. To maintain these conditions air treatment is necessary, which results in high operational costs. Combining insect rearing with hydroponic greenhouse cultivation (HGC) of fruit vegetables might offer an opportunity for cost reduction. Fruit vegetables generally require more elevated air temperature, while leaving enough space under the substrate supporting gutters to allow insect rearing. In this study the feasibility of combining both production systems was evaluated with mealworms (Tenebrio molitor) and cucumber HGC serving as model species. The influence of the greenhouse climate was assessed by rearing mealworms simultaneous at two locations (a climate room and a cucumber HGC). Furthermore, pruning waste and aesthetically declined fruits could serve as a feed for insects. This was tested by comparing 4 different wet feeds (whole and mashed cucumber pruning, tomatoes and agar-agar). Larval growth was monitored and at harvest the mealworm yield was compared among treatments. Mealworm growth in the greenhouse was on average 8.1% slower than growth in a climate room even though the average ambient temperature in the greenhouse was lower and more variable (22.1±3.30 °C standard deviation compared to 27.0±0.34 °C). Moreover, the results showed that the tested HGC residues can be used as wet feed given that mashed cucumber pruning gave similar results as agar-agar (control) and tomatoes even outperformed the control significantly in terms of growth. ‘Entomoponics’ is introduced as the name for the combination of insect production and HGC of vegetables as a way to create added value in unused heated space inside a greenhouse and valorise greenhouse residues.