Quantifying the Nitrogen Contribution from Alfalfa to Soil and Two Succeeding Crops Using Nitrogen‐15

Abstract

With the current interest in sustainable agricultural systems, the use of legumes in crop rotations to provide N to subsequent crops is increasing. The objective of this study was to quantify the N contribution from different alfalfa (Medicago sativa L.) plant parts to a subsequent corn (Zea mays L.) crop, various soil fractions, and a 2nd yr spring barley (Hordeum vulgare L.) crop. The study was conducted at two field locations in Michigan, on a Capac loam (fineloamy, mixed, mesic, Aerie Ochraqualf) in East Lansing (EL) and on an Oshtemo sandy loam (coarse‐loamy, mixed, mesic, Typic Hapludalf) at the Kellogg Biological Station (KBS) in Hickory Corners. Alfalfa shoots and roots/crowns labeled with ¹⁵N were applied separately to microplots in Fall 1985 and Spring 1986 at a rate equivalent to 112 kg N ha⁻¹. Corn was harvested and soil was sampled from all microplots in Fall 1986 and analyzed for ¹⁵N. Corn recovered 17 and 25% of the alfalfa‐^15N applied to the loam and sandy loam soils at ELand KBS, respectively. Alfalfa‐^15N remaining in soil averaged 46% of the initial input for both locations. Most (96%) of the alfalfa‐^15N remaining in soil was recovered in the organic fraction, with microbial biomass accounting for 18% of this recovery. More ¹⁵N was recovered by corn and in soil from alfalfa shoots than roots/crowns at both locations, and from spring‐incorporated than fall‐incorporated plant material on the loam soil. Only 1% of the alfalfa‐^15N from the original application was recovered by a 2nd yr spring barley crop at both locations