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Since 2006, the impact of green manure type and frequency, with or without organic amendment or fertilizer, on soil quality, greenhouse gas emissions and overwinter N losses are being evaluated under four 5yr vegetable rotations at NSAC. Three pre-potato (Solanum tuberosum L.) sequences include: 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 include: nonamended potatoes (control), supplemented with P and N fertilizer (FERT), municipal food waste compost (MSW), or composted paper mill biosolids (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 potato N uptake. Spring soil N supply rate measured in-situ by PRSTM ion exchange membranes was 58% greater in C1 compared with C3. Particulate organic C averaged 21% of total soil C but was unaffected by treatment. Amendment treatments but not rotations influenced microbial biomass and microbial quotient, with 50% higher values measured for MSW compared with FERT and PMB. About 30 kg N ha-1 was SMN estimated lost from the root zone (0- 30 cm) over winter (2008-2009) and was unaffected by treatments. N2O emissions under potatoes ranged from 1.30 to 0.28 kg N2O-N ha-1 for C1 and C3 sequences. In control subplots, emissions were 31% lower than under the FERT treatment (1.31 vs. 1.72 kg N2O-N ha-1) but double that obtained for a standing red clover crop. Additional data for 2009 will also be presented.
© 2012, Organic Agriculture Centre of Canada (OACC)