C ROP SEQUENCE AND TILLAGE should be employed so that N mineralization is synchronized with subsequent crop N uptake. Improving N utilization reduces potential NO3 leaching into the groundwater from agricultural fields. The amount of NO 3 leaching out of the root zone is a function of the NO3 concentration in the root zone and the water flux through this zone.Large amounts of N are mineralized after alfalfa is killed, especially the first year. Fox and Piekielek (1988) reported that 70, 20, and 10 % of the contribution from alfalfa was available the first, second, and third years, respectively. Varco et al. (1991) measured 31.5 mg kg -1 extractable NO3-N in the upper 0.20 m of soil 28 d after alfalfa was killed. Calculations by Peterson and Russelle (1991) for the Corn Belt demonstrated that the N fertilizer applied to corn could be reduced by 8% if growers would properly credit the extra N mineralized following alfalfa. In south-central Idaho, 2 yr of bean are commonly grown following alfalfa, which means a legume following another legume that does not need fertilizer N. Robbins and Carter (1980) determined that 85 to 95 kg NO 3-N ha-1 yr I moved below the root zone when bean followed Abbreviations: BT-BT, conventional-tilled bean grown in 1990 and 1991; CNT-WNT, no-till silage corn grown in 1990, and no-till winter wheat grown in 1990-1991; CT-WT, conventional-tilled silage grown in 1990 and conventional-tilled winter wheat grown in 1990-1991.

The potential for NO3-N leaching after alfalfa (Medicago saliva L.) in irrigated crop production depends on cropping sequence and tillage practices. A 2-yr field experiment in south-central Idaho compared the NO3-N leached following alfalfa of a conventional tillage beanbean (Phaseolus vulgaris L.) rotation with a silage corn (Zea mays L.)-winter wheat (Trilicum aeslivum L.) rotation in a conventional tillage or no-till system. Nitrate leaching was determined by: (i) sampling the soil solution below the root zone (1.2 and 1.5 m) using ceramic-tipped samplers and calculating the N movement from the water balance, and (ii) measuring the change in soil NO 3-N at 1.35 to 4.5 m from soil samples taken in the fall and spring to 4.5 m. During the second growing season, average soil solution NO 3-N concentrations (below the root zone) were 28, 4, and 10 mg L-' for the bean-bean, corn-wheat no-till, and corn-wheat tilled treatments, respectively. The soil NO3 -N in 1.35 to 3.3 m at the end of the study was 80 kg N ha -1 higher for the bean-bean treatment than for the corn-wheat treatments. The NO3 -N that moved below 1.35 m during the 2 yr was 53 kg ha -1 higher for the bean-bean than for the corn-wheat treatments. The soil NO 3-N in the 1.35 to 3.3 m depth after 2 yr was 21 kg ha -' higher for the corn-wheat under conventional tillage than under the no-till system.

The objective of this research was to evaluate field decomposition of 23 cereal grain straws. Straw samples of ‘Fieldwin’ and ‘Owens’ soft white spring wheats (Triticum aestivum L.); ‘Borah’ and ‘McKay’ hard red spring wheat; ‘Nugaines’, ‘Daws’, ‘Lewjain’, ‘Stephens’, and ‘Hill 81’ soft white winter wheats; ‘Weston’, ‘Manning’, ‘Neeley’ hard red winter wheats; ‘Advance’, ‘Steptoe’, ‘Lud’, ‘Pirolene’, and ‘Klages’ spring barleys (Hordeum vulgare L.); ‘Kamiak’, ‘Boyer’, and ‘Schuyler’ winter barleys; Waid durham wheat (Triticum durum Desf.); and ‘Flora’ and ‘Palouse’ triticales (Triticale hexaploide Lart.); were buried in field plots. The straw bags were removed at intervals during 1 yr and analyzed for the amount of decomposition, and C and N contents. Final decomposition ranged from 54% for the Borah wheat straw to 75% for the Lud barley straw with about half of the straws decomposing 64 or 65%. The hard red spring wheat and triticale straws decomposed the slowest and two spring barley straws decomposed most rapidly, but no completely systematic differences were observed for different genera of straws. The original straw N concentrations ranged from 2.2 to 12.5 g/kg. Nine straws lost N during the incubation period from September to November while eight increased in N concentration during this time. In the next three incubation periods, from November to the following October, nearly every straw sample increased in N concentration. The total N weight in the straw increased to the May sampling and then decreased thereafter. This pattern indicates that the straw was releasing N as it was mineralized and became available for utilization by the growing corn crop. Carbon/N ratios ranged widely in the initial straw samples, becoming more nearly uniform as the decomposition process developed. The C/N ratios remained fairly high avg 41 for the final sampling even though the straw in every case had released N into the soil during the last sampling period. Regression analysis of total decomposition and change in weight of N in the wheat straw samples for the first 67‐d incubation in the field showed a linear relationship with decomposition during the year, decreasing with increasing N loss in the early stage.

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