Potential to improve nitrogen use efficiency (NUE) by use of perennial mobile green manures

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 NO3- and emitted as the greenhouse gas N2O. 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 NH4NO3 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 NH4NO3-N ha(-1), but after two yearly harvests with annual gunnera additions it increased to 70 kg NH4NO3-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 NO3- (<= 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.