Soil nitrogen mineralisation dynamics under long-term different irrigation methods in greenhouses

Irrigation is the only source of soil water in greenhouse cultivation. The objective of this study is to illustrate the differences in soil organic nitrogen mineralisation caused by irrigation volume and frequency, and provide theoretical bases for the process of nitrogen transformation and nitrogen fertiliser application under different irrigation methods. The soil nitrogen mineralisation process was studied by the long-term intermittent leaching aerobic incubation method which was used by Standford and Smith in 1972. Parameters of soil nitrogen mineralisation potential (N-0), the mineralisation rate constant (k) and half-life period (t(1/2)) in different soil layers were studied in greenhouse tomato cultivation for seven years, irrigated in three ways: subsurface irrigation (SI), drip irrigation (DI) and furrow irrigation (FI). The results showed that the first-order reaction kinetic model N-t = N-0[1 - exp(-kt)] can be used to predict the soil organic nitrogen mineralisation characteristics in a greenhouse. There were five soil layers (0-10, 10-20, 20-30, 30-40 and 40-50 cm) in this study. In 0-20 cm soil layers, the N-0 value of DI was highest, then FI, and SI was lowest. Apart from 10 to 20 cm soil layer, the k values of SI and DI were higher than for FI in other soil layers, while the t(1/2) value of FI was highest, then DI, and t(1/2) of SI was the lowest. The main reasons were that large amounts of water in FI would lead to increased soil bulk density and reduced soil temperature. DI with property ratio of the temperature and moisture was beneficial to organic nitrogen mineralisation. For SI, due to the overall upward movement of water for many years, nitrate accumulated and the pH value decreased in 0-20 cm soil layers. DI could increase the proportion of N-0 value to the soil's total nitrogen. Therefore, according to the results of this study, DI gives the best effect on soil fertility, followed by SI and then by FI.