Long-term effects of urea and green manure on rice yields and nitrogen balance

Data from a 14-yr double-crop rice (Oryza sativa L.) experiment allowed com parison of the long-term effects of N fertilizer from different sources (ur ea and in situ grown azolla [Azolla microphylla Kaulf.] and sesbania [Sesba nia rostrata Bremek. & Oberm.]) on IV balances, soil N pools (both total an d available), and yields. Although data show that plant-available N was mai ntained over time in both wet (WS) and dry seasons (DS), yields declined si gnificantly, indicating a decline in physiological IV use efficiency. The y ield declines were generally similar regardless of N source in both seasons . The WS decline averaged 150 kg ha(-1) yr(-1) in the three added-N treatme nts, while the DS decline averaged 185 kg ha(-1) yr(-1). After 27 crops, th e cumulative positive N balance was estimated at 1244, 348, 646, and 1039 k g N ha(-1) in control, urea, sesbania, and azolla treatments, respectively. There was no significant change in soil total N content in the control and urea treatments, whereas it increased to 344 to 541 kg after 27 crops in t he sesbania and azolla treatments. Conservation of the soil N status and po sitive N balance, in spite of the high amounts of N removed through grain a nd straw, reflect the N contribution (13-46 kg ha(-1) crop(-1)) from nonsym biotic N-2 fixation. In addition, sesbania and azolla were estimated to add approximate to 57 to 64 kg ha(-1) crop(-1) through symbiotic N-2 fixation. These results demonstrate that in rice-rice cropping systems biological N fixation plays a vital role in replenishing the soil N pool. However, conti nuous application of green manure N (GM-N) did not increase crop N availabi lity, perhaps because of the presence of a recalcitrant soil organic matter fraction. Residual effects on rice grain yield and N uptake were observed only with GM-N sources.