Productivity and Nitrogen Dynamics under Extended Organic Potato Rotations in Atlantic Canada

Derek Lynch¹, Mehdi Sharifi², David Burton² and Andrew Hammermeister³

²Dept. of Plant and Animal Sciences, Nova Scotia Agricultural College (NSAC), Truro, Nova Scotia, Canada  ²Dept. of Environmental Science, NSAC, Truro, NS; ³Organic Agriculture Centre of Canada, (NSAC) Truro, NS

The impact of green manure type and frequency with or without organic amendment or fertilizer on tuber yields, potato N uptake (PNU), greenhouse gas (GHG) emissions and overwinter N losses were evaluated under various extended (5y) organic potato (Solanum tuberosum L.) rotations at NSAC from 2005-2010. Three pre-potato sequences included: C1(oats underseeded with red clover-red clover), C3 (carrots-oats/pea/vetch mixture (OPV)) and C4 (beans followed by buckwheat-OPV). Soil fertility treatments, applied to potatoes only included: non-amended (control), supplemented with inorganic P and N fertilizer (FERT), municipal food waste compost (MSW) (7 Mg DM ha^-1), or composted paper mill biosolids (PMB) (10 Mg DM ha^-1).

Rotation sequences did not affect potato tuber yields in 2008 and 2010. However PNU was increased by C1 in both years. Yields for control and PMB were not different (average 31 Mg ha^-1). Tuber yields were greatest for the FERT treatment averaging 38 Mg ha^-1 followed by MSW at 34 Mg ha^-1. Potato N uptake (PNU) ranged from 96 to 149 kg ha^-1 and was greatest for FERT. Crop N use efficiency (ranging from 30-38 kg N uptake per 10 t of yield) decreased as N input increased (C1 vs C3/C4; FERT vs MSW/PMB).

In 2008, higher soil mineral N (SMN) (125 vs. 85 kg N ha^-1) and mineralizable N were measured prior to potatoes in C1 compared with C3 and C4 with more than 80% of the seasonal decrease in SMN attributed to PNU. About 30 kg N ha^-1 SMN was estimated lost from the root zone (0- 30 cm) over winter (2008-2009) across all treatments. N₂O emissions under potatoes ranged from 1.30 to 0.28 kg N₂O-N ha^-1 for C1 and C3 sequences. In control potato sub-plots, emissions were 16% lower than under FERT (0.82 vs. 0.96 kg N₂O-N ha^-1) but two times greater than under red clover green manure. Data on soil mineral N and GHG emissions from the 2010 season will also be presented.