In greenhouse experiments, we examined the N transferred to intercropped corn from N-15-labeled alfalfa shoot residue and intact roots in an undisturbed soil system in response to two different suppression treatments and complete killing of alfalfa. The alfalfa treatments included complete killing (glyphosate only), glyphosate injury + cutting, and cutting only, with alfalfa shoot residue returned to the soil surface in all three treatments. Corn was planted in each pot following application of the treatments. When alfalfa was suppressed by glyphosate injury + cutting, corn had recovered 12% of the alfalfa N by 8 weeks of growth, but with cutting only, N recovery by corn was reduced to 4.0%. The complete-kill treatment resulted in 8% recovery by corn of alfalfa N. In all treatments, most of the alfalfa-N remained in the soil organic pool. A second experiment tested a cutting only treatment with N-15-labeled alfalfa residue returned to the soil surface. The N-15-labeled alfalfa residue contributed 4.1% of N to corn during the 8-week growth cycle. Twice as much N-15 was found in the active microbial biomass pool in the two treatments with live intercropped plants compared to the monoculture treatments with complete killing (non-intercropped) and the control treatment of alfalfa regrowth only. An analysis of the change in the N-15 content of the undisturbed alfalfa roots from just before the suppression until 8 weeks later suggested that approximately 80% of the root N-15 was lost from the plant suppressed by cutting. This corresponds to 28% of the total N released from the alfalfa. The results suggest that the degree of legume suppression was a key factor in the availability of legume N to the second crop. When the two species were intercropped, more of the N available from legume residues went to plant uptake and microbial biomass and was not stabilized as quickly in the soil organic pool. Appropriate management schemes must be designed to increase N availability to the second crop without yield reduction. These studies suggest severe suppression is necessary; if successful, more of the N can be maintained in active pools.
