Characteristics of nitrogen forms in the surface sediments of southwestern Nansha Trough, South China Sea

The area of the southwestern Nansha Trough is one of the most productive areas of the southern South China Sea. It is a typical semi-deep sea area of transition from shoal to abyssal zone. To understand distributions and roles of nitrogen forms involved in biogeochemical cycling in this area, contents of nitrogen in four extractable forms: nitrogen in ion exchangeable form (IEF-N), nitrogen in weak acid extractable form (WAEF-N), nitrogen in strong alkali extractable form (SAEF-N) and nitrogen in strong oxidation extractable form (SOEF-N), as well as in total nitrogen content (TN) in surface sediments were determined from samples collected from the cruise in April-May 1999. The study area was divided into three regions (A, B and C) in terms of clay sediment (< 4 mu m) content at < 40%, 40%-60% and > 60%, respectively. Generally, region C was the richest in the nitrogen of all forms and region A the poorest, indicating that the finer the grain size is, the richer the contents of various nitrogen are. The burial efficiency of total nitrogen in surface sediments was 28.79%, indicating that more than 70% of nitrogen had been released and participated in biogeochemical recycling through sediment-water interface.

Two new species of Saxifraga (Saxifragaceae) from Southwestern China

Two new species, Saxifraga xiaozhongdianensis J. T. Pan and S. ludingensis J. T. Pan, from the Saxifragaceae in China are described and illustrated. Of these, S. xiaozhongdianensis is endemic to Zhongdian, Yunnan, and is related to S. brachyphylla Franchet. It differs from S. brachyphylla in the sepals adaxially dark brown glandular-villose and the petals basally subauriculate. Saxifraga ludingensis occurs in Luding, Sichuan, and is very similar to S. egregioides J. T. Pan and S. stellariifolia Franchet. It differs from S. egregioides in the stems brown glandular-villose, the cauline leaves adaxially brown glandular-villose, the sepals spreading in anthesis and abaxially brown glandular-villose, and the petals 5-veined. It differs from S. stellariifolia in the leaves cordate and abaxially glabrous, the sepals abaxially brown glandular-villose, and the petals 4-callose and 5-veined. Saxifraga xiaozhongdianensis and S. ludingeasis are apparently endemic to western China and belong to Saxifraga sect. Ciliatae Haworth, emend. J. T. Pan.