Effects of irrigation and nitrogen fertilization on greenhouse soil organic nitrogen fractions and soil-soluble nitrogen pools

Soil organic nitrogen fractions and soil-soluble nitrogen pools are important indicators of soil fertility and plant nutrition requirements. However, little is known about soil organic nitrogen fractions and their correlations with soil mineral nitrogen (SMN) and soluble organic nitrogen (SON). A five-year field experiment was conducted to determine the effects of lower irrigation limits (25, 35, and 45 kPa) and nitrogen fertilization (75, 300, and 525 kg ha(-1)) rates on greenhouse soil organic nitrogen fractions and soil-soluble nitrogen pools. The results indicated that the soil organic nitrogen fractions and soil-soluble nitrogen pools were significantly affected by the irrigation and nitrogen fertilization rates and their interactions at various greenhouse soil depths (P < 0.01). The soil organic nitrogen occurred mainly in the form of acid-hydrolyzable N (AHN). The soil organic nitrogen fractions other than the amino sugar N (ASN) and the soil-soluble nitrogen pools (SMN and SON) decreased with increasing soil depth. Our findings suggest that ammonium N (AN), amino acid N (AAN), and SMN may predict the soil nitrogen supply potential in greenhouse agroecosystems. Soil total nitrogen (N-total), soil organic carbon (SOC), and nitrate nitrogen (NO3--N) appeared to be the major factors explaining the soil organic nitrogen fractions. Considering water saving, nitrogen reduction, and soil nitrogen supply optimization, it was concluded that the best schedules to use in the greenhouse were I2N1 and I2N2 treatments.