Rotation and nitrogen management impact of no-till dryland agroecosystems

Dryland no-till management allows increased soil water storage efficiency and production potential. Greater production, will increase crop N requirements. Changes in soil N processes under no-till greatly impact crop N management. Two sites were used in eastern Colorado (1) Sterling (Weld loam, Aridic Argiustoll) and (2) Stratton (Keith clay loam, Pachic Argiustoll). The objectives were (1) determine N requirements of wheat (Triticum aestivum L.)-fallow (WF) and wheat-corn (Zea mays L.)-fallow (WCF) rotations, (2) compare production and N-use-efficiency of N source, placement and timing methods (NSP) for crops within these rotations, (3) determine core numbers needed per experimental unit and the effect of core placement and ion exchange capacity on soil net N mineralization via the ion exchange resin/soil core method (IER/SCM), and (4) examine the impact of increasing cropping intensity and previous applied N rates on in situ soil net N mineralization.; Wheat yields did not decrease as cropping intensity increased at adequate N rates. Wheat yield response to N rate was greater in WCF vs. WF. Yield increases from 68 to 101 kg N ha{dollar}sp{lcub}-1{rcub}{dollar} were not large enough to warrant the higher rate. Annual production of grain biomass per kg of total N uptake in WCF was 80% more than WF at comparable N rates. Average annual grain yields were affected by NSP treatments, but differences were small.; The IER/SCM can be successfully adapted to measure in situ soil net N mineralization in dryland no-till agroecosystems. Placement of cores relative to old corn rows affected mineralization, therefore random placement is recommended. Statistical evaluation of this procedure revealed that a precision of 1.5 mg kg{dollar}sp{lcub}-1{rcub}{dollar} and an alpha level of 0.20 required 5 cores/experimental unit.; Increased cropping intensity decreased total net N mineralization by one-half (22 kg ha{dollar}sp{lcub}-1{rcub}{dollar} in WCF vs. 43 kg ha{dollar}sp{lcub}-1{rcub}{dollar} in WF) due to higher residue levels. Increasing N rate from previously applied fertilizer increased average net daily and total net N mineralization. For every 1 kg ha{dollar}sp{lcub}-1{rcub}{dollar} of previous annually applied N, 0.20 kg ha{dollar}sp{lcub}-1{rcub}{dollar} of total N are mineralized.