Legume identity and timing of incorporation affect corn and soil responses to green manure

Legume green manure management options such as timing of incorporation and choice of legume species, as well as environmental conditions, may affect nitrogen (N) availability to a succeeding crop. In this study, conducted in 2000 and 2001 in northeastern Iowa, oat (Avena sativa L.) was sown with red clover (Trifolium pretense L.), with alfalfa (Medicago sativa L.), or sown alone. Oat residue and legumes were incorporated in the fall of the seeding year or in the following spring, before producing corn (Zea mays L.). The objectives of this research were to test the effects of timing of incorporation and legume species in green manure 1) on corn grain yield and corn N indices, and 2) on soil mineralizable N through three 28-day laboratory incubations (Field Experiment I) and eleven 14-day field incubations (Field Experiment II). The timing of incorporation was observed to have little effect on corn response variables. In general, spring-incorporation increased the biomass N concentration [N] and N content of the incorporated biomass. Presumably this was due to new growth in the early spring, particularly with the oat/legume treatments. Overall, corn responses to both legume treatments were similar to each other, but the oat/red clover treatment appeared to have an advantage in biomass N production and in fertilizer replacement value, with a mean FRV of 101 kg N ha−1. The soil studies indicated that fall-incorporated biomass had decomposed and mineralized over the winter months. The lack of incorporation time differences both in the corn variables and in the net N mineralization studies indicated that N within the top 15-20 cm remained relatively stable and available to the succeeding corn crop. The incorporation of oat/legume biomass provided material that increased the soil N mineralization potential presumably through increased organic matter, which provided a pool of organic N for transformation to plant-available N. Although most of the benefit appeared to be associated with legume-N effects, increasing the diversity of a cropping system with green manures provided additional benefits such as weed suppression, particularly with the oat/red clover treatment