Large accumulation of mineral nitrogen (N) in agric ultural eco systems is concerned to have potential risks to environment. This s tudy was aimed toward better understanding on the mechanisms in the accumulation and movement of mineral N in cropping soils in semi arid regions, based on a l ong term field experiment established in 1984 to monitor semi arid agro syste ms in the Loess Plateau. The annual means of rainfalls at the experimental site were 580 mm (about 70% received between July and Oct.) and 9 3℃, respectively. Between May, 2000 and April, 2001, the contents of water, NO 3 N, NH 4 N in soil profiles (0～300 cm) were monthly determined in 10 selected treatments (in triplicates), including permanent fallow (F), continuous winter wheat (W) witho ut fertilizer and with N (at annually 120 kgN/hm 2 as urea), NP (at annually 26 kgP/hm 2 as super phosphate), farmyard manure (FYM, at annually 75 t/hm 2), N+FYM, NP+FYM, and continuous legumes (L) without fertilizer and with NP+FYM. Al l fertilizers were applied at the time of sowing. For each plots, 3 soil scores were taken from 0～300 cm soil profiles, cut into 20cm sections, then mixed to g ive a combined sample for each 20 cm of soil layers. Water content was measured by drying soil sample at 105℃ to a constant weight. For measuring NO 3 N and NH 4 N, two portions from a soil sample were extracted in 1mol KCl, and the ex tractants were analyzed by a dual channel nitrogen analyzer system (FIAstar 500 0, FOSS). Data obtained showed that for the fallow treatment, water content fluctuated sig nificantly with seasons in the top layer (0～20 cm) of the soil profile but litt le (17%～20%) in layers between 20 and 300 cm. For cropping treatments, water co ntent in soil profiles decreased largely during the season when crop grew vigoro usly (from April to June), then recovered after crops were harvested and rainfal ls increased (as determined from Aug. to Feb.). However, the scales of the chang es decreased as the depth increased. In general, water content in the same soil layers was 2%～6% smaller under cropping systems than under fallow, with the sma l lest values generally found under continuous legume (L, L NP FYM).
For all the treatments investigated, NH 4 N content in soil profiles was fairly small and constant (4～7 mgN/kg soil). Under the fallow treatment, NO 3 N content in the soil profile remained relati vely constant (6～10 mgN/kg soil), with the exception of some fluctuations in th e top layer (varying from 10 mgN/kg soil in April and June to 22 mgN/kg soil in Aug. and Feb.). However, NO 3 N content and seasonal changes in soil profiles varied widely with different cropping and fertilizer practices. In general, seas onal changes in NO 3 N content in the soil profiles under cropping treatments occurred in similar patterns as to water content (decreased significantly from A pril to June, recovered by Aug.). Under cropping treatments without fertilizer ( W and L), NO 3 N content in the top layer (2～12 mgN/kg soil, dependent on the time determined) was remarkably smaller than that under the fallow treatment, a nd decreased further to a minimal level (< 2 mgN/kg soil) in soil layers below 1 00 cm. For continuous wheat with FYM or P+FYM fertilizers (but no chemical N), N O 3 N content varied largely with the time determined (large increases in Aug. ) in 0～40 cm layers, then decreased to a minimal level (also < 2 mgN/kg soil) b y 60～100 cm deep in the soil profiles. A large amount of NO 3 N (1065 kgN/hm 2, accounted to 55% of total N applied during 14 years since the experiment wa s established) has accumulated in the soil profile under continuous wheat with N fertilizer alone. In difference from all of the other treatments where the high est NO 3 N content was present in the upper layers (20～140 cm, dependent on t he types of crops and fertilizers used), the soil profile under continuous wheat with N fertilizer (W N) had two NO 3 N pea