Nitrogen availability from alfalfa suppressed or killed for no-till production

There is interest in intercropping perennial legumes with corn (Zea mays L.) under no-tillage soil management. Evaluation of N availability by measuring plant N uptake in field research trials with such systems is often complicated by competition for water. We monitored soil inorganic N (ammonium and nitrate) levels at 14-day intervals for 42 days at 0-10 cm and 10-20 cm soil depths after an alfalfa (Medicago sativa L.) sod was subjected to four different suppression treatments: A) cut and remove, b) cut and return, c) above-ground kill with paraquat, and d) complete kill with glyphosate. The initial (14 day) release of N was similar in all treatments where residues were left or returned, but alfalfa regrowth immobilized much of the N mineralized in all treatments, except where alfalfa was killed. The greatest quantity of soil inorganic N was found on day 28 where alfalfa was killed, equal to nearly 72% of the N contained in the alfalfa topgrowth. Soil nitrate N concentrations, averaged to a depth of 20 cm and over the 28 and 42 day sampling times, were 6, 9, 13, and 28 mg N/kg for cut and remove, cut and return, paraquat and glyphosate treatments, respectively. Only where alfalfa was killed did soil nitrate concentrations reach the levels established in recently published work as indicative of no further need for fertilizer N (21-25 mg N/kg). These results suggest that a perennial alfalfa intercrop will compete with corn for available soil N. Measures of inorganic soil N might now permit the evaluation of N adequacy for corn in various intercrop systems.