Department of Plant Sciences

Tree legume fallows capture and ‘pump-up’ leached nitrate and recycle N through leaf litter. This study compared subsoil nitrate under continuous maize (Zea mays), natural weed fallow, sesbania (Sesbania sesban) fallow and bare fallow. Results showed that across systems, NO3-N in the top 30 cm ranged from 6.9 to 11 kg N ha-1. While NO3-N increased to 28 kg ha-1 in maize and 86 kg ha-1 in bare fallow at 50-100 cm, it remained below 10 kg N ha-1 in sesbania and weed fallow. Ammonium N ranged between 1-5 kg N ha-1 at 0-50 cm and between 5-10 kg N ha-1 at lower depths in all LUS. At the topsoil (0-30 cm), weed fallow had higher root length density of 32,000 m m-3 compared to 5,970 m m-3 in maize and 15,800 m m-3 in sesbania. Root length density at 105-150 cm was 958, 1746, and 1479 m m-3 in maize, weed, and sesbania, respectively. The root biomass to 150 cm was 10.9 tonnes ha-1 in sesbania, 0.91 tonnes ha-1 in weeds and 0.78 tonnes ha-1 in maize. Shoot P content ranged from 7-11 kg ha-1 for all LUS, while 248 kg N ha-1 in sesbania shoots was 3-fold and 5-fold higher than continuous maize and weed fallow. Compared to continuous maize and weed fallows, sesbania reduced subsoil nitrate. Incorporating deep rooting plants into the cropping system can substantially reduce subsoil nitrate, and the N contribution by legumes would be greater than that of weed biomass or maize stover.