INPUT LEVEL, CROPPING DIVERSITY AND CROP SPECIES EFFECTS ON NITRATE-N AND EXTRACTABLE P IN SOIL AFTER 8 YEARS

摘要

A field experiment was established in 1995 on a Dark Brown loam soil at Scott, Saskatchewan, Canada to determine the influence of input level, cropping diversity and crop species on the distribution of nitrate-N and extractable P in the soil profile. The treatments included three input levels (organic - ORG, reduced - RED and high - HIGH), three cropping diversities (low diversity - LOW, diversified annual grains - DAG, and diversified annual and perennials - DAP) and six crop phases [fallow (tillage-fallow or chem-fallow), green manure (lentil -Lens culinaris Medicus or sweet clover - Melilotus officinalis (L.) Lam), spring wheat (Triticum aestivum L.), canola (Brassica napus L. and B. rapa L), mustard (Brassica juncea L.), fall rye (Secede cereale L.), field pea (Pisum sativum L.), spring barley (Hordeum vulgare L.), flax (Linum usitatissimum L.), oats (Avena sativa L.) and bromegrass (Bromus inermis Leyss)-alfalfa (Medicago sativa Leyss) mixture hay]. The second 6-year rotation cycle started in 2001 growing season. Soil was sampled from the 0-15, 15-30, 30-60 and 60-90 cm depths in each crop phase in October, 2002. Accumulation and distribution of nitrate-N in soil varied with input level, cropping diversity and crop species. Amount of nitrate-N in soil was usually greater at HIGH input than at ORG or RED input, especially under LOW cropping diversity. There was some downward movement of nitrate-N in the soil profile. In LOW cropping diversity, nitrate-N in soil was highest after GM/F2 and lowest after mustard. In DAG cropping diversity, nitrate-N in soil was highest after wheat in ORG and RED inputs, after flax in HIGH input, and lowest after mustard in ORG input and after barley in RED and HIGH inputs. In DAP cropping diversity, nitrate-N in soil was highest after mustard (followed very closely by barley) in ORG input and after wheat in RED and HIGH inputs, and lowest after hay in all input levels. In some instances, soil under ORG input had greater nitrate-N than the soil under RED input. This was most likely because of relatively lower extractable P in soil for optimum crop growth under ORG input compared to RED input. HIGH or RED inputs tended to have greater extractable P in the 0-15 cm layer than ORG input, but cropping diversity had no effect on extractable P in soil. In conclusion, combination of HIGH input and LOW cropping diversity had higher nitrate-N in soil than the corresponding treatments. The findings suggest that at this site there is little potential for bringing P from subsoil to the surface by using taproot crops, because the subsoil is extremely low in available P. This also suggests that if surface and sub-soil are low in available P or other nutrients, it may not be possible to sustain high crop yields under organic farming systems without using external nutrient sources.