Tree legume fallows capture and ‘pump-up’ leached nitrate and recycle N through leaf litter. This study compared subsoil nitrate under continuous maize (Zea mays), natural weed fallow, sesbania (Sesbania sesban) fallow and bare fallow. Results showed that across systems, NO3-N in the top 30 cm ranged from 6.9 to 11 kg N ha-1. While NO3-N increased to 28 kg ha-1 in maize and 86 kg ha-1 in bare fallow at 50-100 cm, it remained below 10 kg N ha-1 in sesbania and weed fallow. Ammonium N ranged between 1-5 kg N ha-1 at 0-50 cm and between 5-10 kg N ha-1 at lower depths in all LUS. At the topsoil (0-30 cm), weed fallow had higher root length density of 32,000 m m-3 compared to 5,970 m m-3 in maize and 15,800 m m-3 in sesbania. Root length density at 105-150 cm was 958, 1746, and 1479 m m-3 in maize, weed, and sesbania, respectively. The root biomass to 150 cm was 10.9 tonnes ha-1 in sesbania, 0.91 tonnes ha-1 in weeds and 0.78 tonnes ha-1 in maize. Shoot P content ranged from 7-11 kg ha-1 for all LUS, while 248 kg N ha-1 in sesbania shoots was 3-fold and 5-fold higher than continuous maize and weed fallow. Compared to continuous maize and weed fallows, sesbania reduced subsoil nitrate. Incorporating deep rooting plants into the cropping system can substantially reduce subsoil nitrate, and the N contribution by legumes would be greater than that of weed biomass or maize stover.

Switchgrass (Panicum virgatum L.) has been targeted for cellulosic ethanol production. Our objective was to evaluate effects of location, harvest system, and N fertilizer rates on switchgrass biomass yield and N, P, and K removal. Randomized complete block experiments with four replications were established on one-year old stands of ‘Alamo’ switchgrass at two Oklahoma locations in 2008. Harvest system and N rate interactions affected total annual yield. Biomass yields ranged from 9020 to 10530 kg/ha across harvest systems when no N was applied. With application of 179 kg N/ha, biomass yields averaged 10715, 13912, and 16516 kg/ha when harvested at seed maturity (October), after a killing frost (December), and twice per year at boot stage (July) and after a killing frost, respectively. Nutrient removal tended to increase with N fertilization and was generally twice as great for each nutrient within the two-cut system relative to the one-cut systems. When 179 kg N/ha was applied, N removal was 198, 82, and 122 kg N/ha when cut twice, cut once at seed maturity, and cut once after frost, respectively. Phosphorus removal was 22, 12, and 11 kg/ha among these systems, respectively. Corresponding K removal was 203, 62, and 25 kg/ha. Applying N and harvesting once after frost ensures both high biomass production and reduces soil nutrient mining. Total biomass harvest, however, was greatest under the two-cut system, enabling a potential use of switchgrass early in the season for forage and availability of regrowth for bioenergy purposes.

Switchgrass (Panicum virgatum L.) has been targeted for cellulosic ethanol production. Our objective was to evaluate effects of P and K fertilization on switchgrass biomass yields. Experiments were established in switchgrass (cv. Alamo) fields planted at three Oklahoma locations in 2007. Interactions of N and P fertilizer rates were evaluated on two sites that had low soil test P (6 to 9 kg/ha). Interactions of N and K fertilizer rates and harvest system were evaluated on a site that had low soil test K (134 kg/ha). In the N and P rate experiments, biomass yields during establishment were unaffected by treatments, averaging 2145 kg/ha in 2007. During 2008, main effects of location, N, and P rate were significant. Application of 134 kg N/ha increased yields from 7945 to 9967 kg/ha. Applying 101 kg P2O5/ha increased yields from 8528 to 10242 kg/ha. In the harvest system, N, and K rate experiment, biomass yields were unaffected by treatments in 2007 and 2008, averaging 4603 and 16145 kg/ha, respectively. Cutting biomass twice per year as opposed to once a year, however, removed more N,P, and K. Uptake of N, P, and K averaged 216, 24, 209 kg/ha within the two-cut system and 133, 13, and 36 kg/ha, within the one-cut system, respectively. Although low soil test P and K did not strongly limit biomass yields of these young stands, continued biomass harvesting and removal without nutrient replacement has the potential to mine soil of nutrients and constrain biomass yields as stands age.