Impact of Dairy Manure and Crop Rotations on Nitrogen Mass Balance in the Southern Idaho Calcareous Soils.

Dairy manure is commonly applied to fields in Southern Idaho, yet little is known about how the nitrogen (N) is stored in and removed from these cropping systems. The objective of this study was to determine the mass balance of N in a sprinkler irrigated calcareous silt loam soil in response to cropping rotations and different rates of dairy manure application. The study was a randomized complete block design with the following treatments: a control (no nutrient source applied), fertilizer, and three rates (17.3, 34.8, and 52.0 Mg ha^-1; dry weight basis) of manure that were fall-applied either every year or every other year from 2012 to 2015. Dairy manure was analyzed each year for total N at the time of application. Urea and mono-ammonium phosphate fertilizers were applied at varying rates to fertilizer and selected manure treatments, following University of Idaho recommendations. The crop rotation was spring malt barley (Hordeum vulgare L.) in 2013, sugar beet (Beta vulgaris L.) in 2014, and hard red spring wheat (Triticum aestivum) in 2015. Treatment plots were subsampled (above-ground biomass for barley and wheat; beet roots for sugar beets) prior to harvest and analyzed for dry matter biomass and total N content to estimate plant N removal for each crop. Soil samples were collected in the spring of 2016 at the 0-30 and 30-60 cm depths and analyzed for total N. Total N balances were calculated by summing the N additions in manure and fertilizer and calculating N removal with the crop and N remaining in the soil. Gaseous losses of N were not determined in this study. Nitrogen use efficiency (NUE) was calculated as the amount removed in the crop vs. that applied. Effects of treatment and crop rotation on NUE will be discussed.