Abstract
Supplying nitrogen (N) to crops by incorporating N 2 -fixing green manures into soil can improve soil functioning and increase soil carbon storage. However, as with N-fertiliser use, excess mineral N from decomposed green manures can be lost via leaching as NO 3 − and emitted as the greenhouse gas N 2 O. To improve nitrogen use efficiency (NUE) we propose supplying leaf matter harvested from permanent stands of N 2 -fixing plants grown ex-situ as perennial mobile green manures (PMGMs), thus allowing flexible timing of additions to synchronise with N demand. To assess the effectivity of PMGM use, we monitored crop N uptake, biomass production and soil mineral N from fertilisation by three potential PMGM species, Alnus glutinosa (alder), Gunnera tinctoria (gunnera), and Ulex europaeus (gorse), compared to the conventional green manure Trifolium pratense (red clover) and NH 4 NO 3 fertiliser. Effectivity of N fertilisation by PMGMs increased with the duration of their use. In the first season of a field experiment, crop N uptake after addition of 200 kg N ha −1 within gunnera was equivalent to that from only 14 kg NH 4 NO 3 -N ha −1 , but after two yearly harvests with annual gunnera additions it increased to 70 kg NH 4 NO 3 -N ha −1 year −1 equivalent. In a 1-year pot experiment, PMGMs resulted in equal or higher crop biomass than clover-fed crops, with lower soil NO 3 − (≤ 25 mg N L −1 soil solution) than in clover-treated soil (≥ 130 mg N L −1 ). We conclude that PMGMs have potential to increase NUE over that of traditional green manures and discuss the feasibility and possible agro-ecological benefits of PMGM-fed systems.