Effect of Different Methods of Crop Rotation and Fertilization on Canola Traits and Soil Microbial Activity (Report‪)‬

Introduction Winter oilseed rape (Brassica napus L.) is the dominant oilseed crop in northwest region of Iran and is often grown in short term rotations. In Iran, winter oilseed rape is generally cultivated in a crop rotation including chickpea (Cicer aritenium L.) or winter wheat (Triticum aestivum L.). Introducing a good rotation for canola can lead to increase grain yield and improve sustainability of canola farms. Maintenance and management of soil fertility is the core of development of sustainable food production systems (Doran et al., 1988). Applying green manures is considered as a good management practice in all agricultural production systems because of increasing sustainability cropping system through reducing soil erosion, improving soil physical properties, increasing soil organic matter and fertility levels (Tejada et al., 2008). To be sustainable, organic farming needs to be self-sufficient in nitrogen (N) through the fixation of atmospheric di-nitrogen ([N.sub.2]) by legumes (Berry et al., 2002), recycling of crop residues (green manures) (Elfstrand et al., 2007) and the application of farmyard manure (FYM), compost and biofertilizer (Rokhzadi and Toashih, 2011). Proper use of N sources is, therefore, required to optimize the economic yield (Mason and Brennan, 1998) and to minimize the potential for environmental pollution (Aufhammer et al., 1994). While a substantial amount of N is provided by conversion of previous crop residues and soil organic matter into soluble soil N, additional mineral N is a prerequisite for high yields. Several investigations have confirmed that canola following cereal crops yielded substantially lower compared to growing after legume (Christen and Sieling, 1995; Sieling et al., 1997). Compared to cereals, canola requires a higher amount of nutrients. Moreover, available nitrogen and sulfur frequently limits seed yield of canola. Fertilization is one of the soil and crop management practices, which exert a great influence on soil and grain quality (Chander et al., 1998). Compost and FYM are organic sources of nutrients, which increase soil organic matter and enhance soil quality. It is well known that organic amendments have a number of benefits for soil biological properties. Enzyme activities have been indicated as soil properties suitable for use in the evaluation of the degree of soil alteration in both natural and agro-ecosystems. Soil microbial properties have a strong correlation with soil health. Some researchers have already suggested the favorable effects of organic manure application and crop rotation on soil enzyme activities (Chander et al., 1998; Kandeler et al., 1999). The study of combining organic manures in crop rotation is a great potential value to organic agriculture in order to avoid applying chemical fertilizers. The present research is going to introduce a sustainable soil fertility system and to evaluate the combined effects of organic manure such as green manure, compost, and FYM on canola production and microbiological soil quality indicators in crop rotations.