Long-term cropping system effects on mineralizable nitrogen in soil

Increasing use of N fertilizer for crop production necessitates more precise estimates of N provided by the soil in order to prevent under- or over-fertilization and their adverse effect on plant nutrition and environmental quality. Better laboratory tests and models of N mineralization are needed to better estimate fertilizer need. Long-term changes in N mineralization potential may also identify changes in soil quality that relate to favorable sustainable agricultural practices. We collected samples from the 0-20 cm soil zone of treatments in five long-term (30-60 y old) experiments to determine the effect of crop rotation, tillage, fertilizer and residue management on N mineralized during aerobic incubation, and compared results with N mineralized under field conditions where possible. Soil samples were incubated in glass bottles at 25 degrees C and -0.02 MPa for 0, 7, 14, 28 and 49 d. Soil N mineralization from wheat (Triticum aestivum L.)-summerflow, wheat-pea and from wheat-wheat crop rotations were 32, 42 and 51% of that mineralized from non-cultivated pasture soil. Nitrogen mineralized, as a fraction of the total N present, increased with increasing N application, reduction in tillage intensity and higher frequency of cropping. Stubble-mulch soils mineralized 10-20% more N than did moldboard-plowed soils. The fraction of total N mineralized increased with increasing soil organic N content, indicating that organic N added through recent crop management practices is more labile than N in the native soil matrix. Nitrogen mineralization in situ increased linearly as a function of past N fertilizer application, which implies that a substantial portion of previously-applied N may be recovered slowly over time in subsequent crops and that fertilizer N needed for optimum crop yield may not be increasing as rapidly as expected. Nitrogen mineralized during laboratory incubation also increased linearly with increasing N application, but the rate of change differed significantly from that for in situ N mineralization (0.0042 vs 0.0112 kg ha(-1) per kg of applied N). The difference in N mineralization rate between laboratory and in situ experiments is not easily explained; perhaps soil processing for incubation altered physical access to organic N pools or caused a shift in microbial communities in soil. The difference in N mineralization rates implies that laboratory incubations do not accurately reflect N mineralization in the field, and strongly suggests that laboratory estimates of N mineralization be interpreted with care. Previous N fertilization, tillage and cropping patterns all affect N mineralization potential, and must be taken into consideration when estimating N fertilizer needs.