Nitrogen fixation by annual legume green manures in a semi-arid cropping system

Abstract

There is renewed interest within agriculture to improve and sustain soil fertility. Legume crops can provide an alternative nitrogen (N) input to a cropping system through biological N fixation. The contribution of N from legume cropping systems depends on the quantity of N fixed and soil processes that influence soil N availability. The primary objectives of this project were 1) to evaluate the effect of planting and termination time on biomass production and N fixation by two legume green manure crops; and 2) to investigate the role of soil P availability on legume growth and N fixation. A two-yr dryland field study was conducted with three treatments: legume (field pea and lentil), planting time (spring and summer [2010 only]), and termination time (flower, intermediate [2009 only], and pod). Two methods, ¹⁵ N natural abundance and N difference, were used to quantify N fixation. In 2009, N fixed by spring-planted lentil was higher by pod than flower (P=0.03). There was no difference in N fixed by spring-planted pea among termination times, likely because of reduced precipitation during the middle of the growing season. In 2010, both spring-planted crops fixed more N by pod than flower (P<0.01) and more N was fixed by spring-planted crops than summer-planted crops (P<0.01). A greenhouse study was conducted in an unsterilized, low P soil (8 mg kg-soil -¹) with three treatments: legume crop (field pea or lentil), P fertilizer (0, 4, or 8 mg P kg -¹), and arbuscular mycorrhizae fungus (AMF) inoculum (AMF-, AMF+). Shoot biomass was sampled at flower, and N fixation was estimated with ¹⁵ N natural abundance method. Fertilization increased biomass yield and tissue N and P uptake for both crops (P<0.01) and increased N fixed by pea (P<0.01). Inoculation with AMF had little effect on measured parameters; however, there was an increase in pea biomass and N uptake in the AMF+ versus AMF- treatments at the 4P rate. Several variables that affect N fixation in semi-arid cropping systems were identified in this project, however further research assessing the effect of other soil and environmental conditions on N fixation and the cycling of fixed N in an agroecosystem is needed.