Abstract: By using a customized manual rainwater sampler made of polyvinyl chloride plastic, the molar ratio of NH₄⁺-N/NO₃^--N and the natural ¹⁵N abundance of NH₄⁺ (δ¹⁵NH₄⁺) in rainwater was monitored all year round from June 2003 to July 2005 at three observation sites (Changshu, Nanjing, and Hangzhou) in the Yangtze River Delta. The results indicated that at the three sites, the NH₄⁺-N/NO₃^--N ratio and the δ¹⁵NH₄⁺ value in rainwater had the similar seasonal variation trend, being more obvious in Changshu (rural monitoring type) site than in Nanjing (urban monitoring type) and Hangzhou (urban-rural monitoring type) sites. The NH₄⁺-N/NO₃^--N ratio peaked from early June to early August, declined gradually afterwards, and reached the bottom in winter; while the δ¹⁵NH₄⁺ value was negative from late June to midAugust, turned positive from late August to mid or late November, became negative again when winter dominated from December to March, but turned positive again in next May and negative again in next July. These seasonal variation patterns of NH₄⁺-N/NO₃^--N ratio and δ¹⁵NH₄⁺ value were found in relation to the application of chemical nitrogen fertilizers during different crop growth periods, and also, the alternation of seasons and the NH₃ volatilization from other NH₃ emission sources (including excrements of human and animals, nitrogen-polluted water bodies, and organic nitrogen sources, etc.), which could be taken as an indicator of defining the sources and form composition of NH₄⁺ in atmospheric wet deposition and the intensity of various terrestrial NH₃ emission sources.

Key words: biological soil crust, erosion region, acetylene reduction assay (ARA), nitrogenase activity, environmental factor.