[Soil soluble organic nitrogen in different horticultural production systems].

By using ultra-centrifugal technique and KC1 extraction, the contents of soil soluble organic nitrogen (SON), dissolved organic nitrogen (DON), and free amino acid (FAA) in three horticultural production systems, i.e., organic (OS), transitional (TS), and conventional (CS), were determined, and their correlations with some other soil properties were investigated. The results showed that SON content in the three production systems was in the order of OS > CS > TS, and the ratios of SON to soil total soluble N (TSN) and total N (TN) in OS, CS, and TS were 42.9%, 24.5%, and 18.4%, and 11.1%, 11.9%, and 7.4%, respectively. The DON content in OS, TS and CS was 4. 38, 1.68 and 3. 26 mg x kg(-1), occupying 47.9% , 22.1% and 26.1% of TSN, and 2.9%, 2.3% and 3.6% of TN, respectively. FAA content occupied 1%-3% of TSN and 2%-10% of SON, and decreased in the order of CS > TS > OS. Correlation analysis showed that TSN, SON and FAA were significantly correlated with soil total N, total carbon, NO3(-)-N and NH4(+)-N. Different management measures of horticultural production system changed the SON content and its characters.