Objectives Acidolysable ammonium N (AN) and acidolysable amino acid N (AAN) are the dominated forms of soil organic nitrogen, which play key roles in the processes of soil nitrogen metabolism and nitrogen supply for plant growth. The effect of long-term irrigation and nitrogen fertilization on the AN and AAN contents was investigated, to evaluate soil nitrogen supply ability and provide reference for scientific water and nutrient management in greenhouse.

Methods A five-year’s tomato field experiment was conducted in the greenhouse of Shenyang Agricultural University. The field was mulched with plastic film and drip irrigation pipes were loaded under the film. The treatment included three nitrogen rates of 75 kg/hm² (N₁), 300 kg/hm² (N₂) and 525 kg/hm² (N₃), and three irrigation rate, in which the irrigation amounts were controlled in soil water tension ranges of 25–6 kPa (W₁), 35–6 kPa (W₂) and 45–6 kPa (W₃). The yield and yield components of tomato were investigated in August 2016. The contents of soil organic nitrogen fractions, total nitrogen (TN) and organic carbon (SOC) in 0–10 cm, 10–20 cm and 20–30 cm soil depths were determined in the fallow period (September, 2016).

Results The proportions of acidolysable N (AHN) in 0–10, 10–20 and 20–30 cm deep of soil were 66.0%, 64.6% and 55.2%, respectively. The contents of TN, SOC and all soil organic nitrogen fractions, except acidolysable amino sugar N (ASN), decreased with the increasing of soil depths, and the differences of contents among the three soil depths were significant at 5% level. The content and the proportion of each fraction in the AHN was in order of AAN, AN > acidolysable unknown N (UN) > ASN. Under the same N application rate, the contents of AN in 0–30 cm soil depths and the contents of AAN in 0–20 cm soil depths were both the highest in the irrigation treatment of W₂. Moreover, the contents of AAN in 0–10 and 10–20 cm soil depths were also the highest in the W₂N₁ treatment (35 kPa + 75 kg/hm²). The single effect of irrigation and nitrogen rate on tomato yield and yield components were extremely significant (P < 0.01), and their interaction was also significant (P < 0.05). AN content during the fallow period had a significant negative correlation with tomato yield. There were no significant differences in tomato yield among the treatments of W₁N₂ (25 kPa + 300 kg/hm²), W₂N₁ (35 kPa + 75 kg/hm²) and W₁N₁ (25 kPa + 75 kg/hm²).

Conclusions Irrigation and nitrogen fertilization significantly influence the contents of total nitrogen, acidolysable nitrogen, acidolysable amino acid nitrogen and acidolysable ammonium nitrogen in the soil (P < 0.01), but not on soil organic carbon contents. Significant interaction of irrigation and fertilization is existed at the same time. In the view of water-saving and nitrogen-reducing with high tomato yield, keeping soil water suction in range of 35–6 kPa, and applying N of 75 kg/hm² is the optimum combination of irrigation and nitrogen fertilization in tomato production inside greenhouse.