NETWORK ANALYSIS OF NITROGEN CYCLING IN TEMPERATE, WETLAND RICEFIELDS

Nitrogen is considered an important limiting element in ricefields. Th e importance of fertilizer as a source of nitrogen for rice and denitr ification as a pathway of nitrogen varies among ricefields depending o n agricultural practices. Ecological network analysis was applied to r icefields of the Ebro Delta (N. E. Spain) in order to establish (1) th e fate of nitrogen sources to the fields (2) the importance of and int eraction between aquatic and benthic processing of nitrogen cycling an d (3) the importance of internal recycling and the role of straw burie d in sediment at the end of the cultivation period. Fertilizer is the major source of nitrogen during the first month of the cultivation per iod. During the initial (INI) phase rice incorporates the equivalent o f 92.8% of the nitrogen added. Only about half of the fertilizer added is incorporated during the second addition. Aquatic compartments are highly dependent upon sediment nitrogen as a source. The loss of phyto plankton between submersed (SUB) and emergent (EME) phases can be part ially explained by a decrease of nitrogen flux from sediment to water. An important dependence of rice upon straw was observed: about 20% of the nitrogen comes from straw. High Finn Cycling Index (FCI) values o ccur at the beginning of the cultivation period but the highest FCI va lue was for the end phase according to the high nutrient loads to rice fields. The indirect relationship between FCI and the number of cycles can be explained by a correlation based on the removal of phytoplankt on during the latter phases.