Effect of Nitrogen Rate and Diverse Cropping Systems on Applied Nitrogen Fertilizer Recovery in a Long-Term No-till Cropping System.

Our understanding of the economic and ecological benefits of crop diversification in no-till dryland crop production systems is evolving in the northern Great Plains.  Wheat-based monocultures, typically in crop-fallow rotation, have dominated Montana cropping systems for decades; however, in recent years, interest in diversified crop rotations, continuous cropping, and alternative crops has increased.  The long-term effects of these emergent systems on soil fertility and nitrogen use efficiency (NUE) remain largely unknown as there are few long-term studies comparing NUE in diversified systems to wheat monocultures.  Our goal was to identify the effects of N rate and crop rotation on partial N balance or fertilizer N recovery (FNR; N removed in grain divided by N fertilizer applied) in the most recent four years of a 12 year study conducted near Bozeman, Montana. In 2000, five no-till, four year crop rotation systems were established to evaluate crop yield and quality.  Three of these systems have been treated with two N rates (full and ½) since 2004.  The FNR at the full N rate ranged from 43% (continuous wheat) to 68% (a system with sweet clover green manure) with an average of 52%.  Three of four diverse rotations containing legumes and oilseeds had higher recoveries than continuous wheat.  At the low N rate, FNR averaged 84%, almost twice as high as for high N input systems. This may lead to the conclusion that lower fertilizer rates result in the most efficient system; however, lower N rates were also generally associated with lower yield.  These results suggest that increasing crop diversity may improve NUE, but results will need to be coupled with crop yield, quality, soil nutrient data and net returns to fully understand the effects of crop diversification in this region.