Nitrogen Mineralization Responses to Cropping, Tillage, and Nitrogen Rate in the Northern Great Plains

Nitrogen-mineralization rates are needed to accurately determine N fertilization requirements to meet plant needs while minimizing environmental contamination. A spring wheat (Triticum aestivum L.)-fallow (SW-F) system was compared with a spring wheat-winter wheat-sunflower (Helianthus annuus L.) (SW-WW-SF) system on a Temvik-Wilton silt loam (finc-silty, mixed Tj pic and Pachic Haploborolls) at three N rates (0, 22, and 45 kg ha" for SW-F and 34,67, and 101 kg ha" for SW-WW-SF) under conventional, minimum, and no-tillage. After 10 yr, soil samples were incubated to determine Nmineralization rates. Cropping intensity, N rate, and tillage intensity interacted to affect N-mineralization rates. Within the SW-F system N-mineralization rates in 0to 0.05-m depth were 8.2 ± 0.8 kg ha" wk" in the fallow phase vs. 5.0 ± 0.7 kg ha" wk" in the crop phase under conventional tillage and were 6.2 ± 0.3 kg ha" wk" under minimum and no-tillage in both phases. The N-mineralization rates were 2.3 ± 0.4 kg ha" wk" in 0.05to 0.15-m depth soils of the SW-F system. In spring wheat, N-mineralization rates in 0to 0.05-m depth soil were 9.9 ± 0.8 kg ha" wk" in the SW-WW-SF system vs. 5.6 ± 0.4 kg ha" wk" in the SW-F system and in the 0.05to 0.15-m depth were 3.6 ± 0.1 kg ha" wk" in the SW-WW-SF system vs. 2.4 ± 0.2 kg ha" wk" in the SW-F system Within the SW-WW-SF system, N-mineralization rates in the 0to 0.05-m soil layer were 6.8 ± 0.5 kg ha" wk" under winter wheat vs. 9.9 ± 0.8 kg ha" wk" under spring wheat and 9.2 ± 0.6 kg ha" wk" under sunflower. In the 0.05to 0.15-m soil layer, N-mineralization rates were 3.3 ± 1.0 kg ha" wk". More intensive cropping and conservation tillage increased N-mineralization rates in this soil and may ameliorate the decline in N fertility associated with crop-fallow systems. C has been the traditional crop production system for much of the semiarid Great Plains. More intensive cropping combined with conservation tillage has been shown to be an economical alternative (Norwood and Dhuyvetter, 1993; Dhuyvetter et al., 1996). Dhuyvetter et al. (1996) reported that economic risk decreased and net returns increased when more intensive cropping systems were used even though production costs increased. In addition, cropping rotations provide producers with management options that can be used to break weed, insect, and disease cycles (Holtzer et al., 1996); make more efficient use of available soil water (Norwood, 1994); control saline seep development (Black et al., 1981); and add stability and diversity to the farm operation. In spite of the many potential benefits associated with annual cropping, crop-fallow is still used in the Northern Great Plains. Implementing a more intensive cropping system requires adjusting Brian J. Wienhold, USDA-ARS, Soil and Water Conservation Research Unit, Lincoln, NE 68583-0934. Ardell D. Halvorson, USDAARS, Soil-Plant-Nutrient Research Lab., Ft. Collins, CO 80522. USDA-ARS, Northern Plains Area, is an equal opportunity employer/ affirmative action employer and all agency services are available without discrimination. Received 25 Aug. 1997. * Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 63:192-196 (1999). management practices, such as fertilizer and pesticide inputs, to meet crop needs and control pests under the new set of conditions. Uncertainty about how conservation tillage and more intensive cropping affect soil fertility and pest populations may play a role in the reluctance of some producers to abandon crop-fallow systems. In the semiarid Northern Great Plains, water is the factor most limiting crop production. Conservation tillage increases moisture availability, allowing the crop to more efficiently use soil nutrients (Fox and Bandel, 1986). The nutrient that most commonly limits crop growth and is applied by producers in the largest amount is N. Fertilizer rates are usually based on a yield goal, with credits given for residual soil inorganic N, previous crop N inputs, and a sampling date adjustment (North Dakota State University Extension Service, 1997). Residual inorganic N can be estimated through sampling and laboratory analysis. A yield goal can be based on historical performance of the field or crop performance on similar soils in the area. Sampling date adjustments attempt to account for N mineralized from the time of sampling to the time of fertilizer application. Previous crop N inputs are assessed to account for N made available to the crop as previous crop residue decomposes and is mineralized. These last two variables are the most difficult to quantify and incorporate into the fertilizer rate calculation. Understanding the effect of management practices on N-mineralization rates should improve fertilizer management. Knowledge of the soil's ability to supply the crop with N is needed to ensure that N applied as fertilizer is optimized so that crop needs are met and the potential for surface and groundwater contamination is minimized. Cropping intensity, tillage intensity, and N applications are three management practices that have been shown to affect N-mineralization rates (Campbell et al., 1991; Franzluebbers et al., 1994a). In 1984, a study was initiated near Mandan, ND to compare the performance of a 3-yr annual cropping system (one crop each year) with a crop-fallow (crop every other year). There were three tillage intensities and three N rates within each cropping system. This study provided an opportunity to compare the effects of a number of management practices on crop yields and soil properties after several cycles of the contrasting cropping systems (Black and Tanaka, 1997). The objective of the work reported here was to compare N-mineralization rates after these various management practices had been in place for 10 yr. MATERIALS AND METHODS