Marine benthic macrophytes were examined as possible nitrogen (N) sources for agriculture in Greece in terms of N mineralization and available standing stocks. Net N mineralization patterns were determined over a 30-week incubation period under aerobic conditions. Ulva sp., Cystoseira barbata C. Agardh, and Posidonia oceanica (L.) Delile storm-cast material (SC) and harvested leaves (L) were incorporated into acidic, slightly acidic after liming, and alkaline soils and incubated at 35°C. From the initial stage until the 12th week of incubation, ammonification was higher than nitrification in most cases. The decreasing order of mineralized N originating from macrophytes was: Ulva sp. > P. oceanica (SC) > C. barbata > P. oceanica (L). Rates of net N mineralization of the macrophytes were the highest in the alkaline soil, where-as the lowest values were recorded in the strongly acidic soil. Application of Ca into the acidic soil revealed a considerable increase of N mineralization. Immobilization and ammonification were observed after incorporation of Ulva sp. into the acidic soil. As Ulva sp. proved to be the only potential alternative N source for arable crops, its standing stocks in two geographically different Greek lagoons, the Vassova Lagoon (N Greece) and Papas Lagoon (S Greece) were estimated. It was estimated that 2 ha of Ulva could supply adequate N to cultivate approximately 1 ha cotton. Posidonia and Cystoseira did not show potential as an alternative N source for agriculture and especially in plant nutrition.