Sanchez, J. E. 2000. Managing cropping systems to enhance the active soil nitrogen pool and control its mineralization. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Sustainable nitrogen (N) management requires practices that enhance soil fertility and minimize N loss. This study explores the effects of substrate diversity and living roots on N mineralization, and the influence of management on the seasonal pattern of soil nitrate (NO3 ) levels. In part 1 a “diverse system”, consisting of a corn ( Zea mays L.)-corn-soybean ( Glycine max L.)-wheat ( Triticum aestivum L.) rotation with cover crops and fertilized with composted dairy manure was compared to a corn monoculture with conventional fertilizers. The specific objectives were to determine if a diverse system would induce higher mineralization rates than would a continuous corn monoculture, and to determine whether these two systems differ in their ability to mineralize added substrate. Net mineralized N in the diverse system was 90 and 40% higher than that of the monoculture at 70 and 150 days of laboratory incubations respectively. A comparable response was found in situ where a 70% higher net mineralization was observed at 70 days. The active N pool increased over time, but the ability of soil organisms to break down additional substrate did not change as a result of diversity. In part 2, soil from the diverse system was used to determine if living corn roots influence N mineralization. Corn roots increased the mineral-N supplying capacity of soil from the diverse system by more than 50%. This increase appears to be caused by a stimulation of N mineralization, but a previous increase in the active N pool seems to be necessary significant stimulation can be realized. In part 3, soil NO 3 levels were measured throughout six years from April through December in the previously described rotation and continuous corn both receiving compost or fertilizer. The objectives were to determine if NO3 fluctuation patterns could be used as a suitable indicator of soil fertility changes within or between seasons, and to determine the leaching potential resulting from the management strategies. Seasonal soil NO 3 levels were sensitive to management. Soil NO 3 levels were generally higher in plots receiving fertilizer than those receiving compost. The exceptions were the cover cropped 1 st year corn and all soybean plots. Soil NO3 levels responded not only to fertilizer applications but also to an increased active N pool. The leaching potential in fertilized corn was always significantly higher than in corn receiving compost. In soybean and wheat, no difference between fertility sources was observed. A comparison between leaching potential and NO3 levels in the following spring revealed that fertilized corn may be susceptible to higher leaching losses. The seasonal NO3 patterns were sensitive indicators of fertility status. Fertility can be increased by application of conventional fertilizer and also by soil conditioning practices that increases the mineralizable N pool size. Farming practices enhancing substrate diversity can dramatically increase the mineralizable N pool size. The use of rotations, cover crops, and organic amendments from animal sources enhanced the mineralizable N pool. This in combination with a crop specially selected for its ability to stimulate N mineralization has the potential to increase yields, decrease fertilizer requirements, and thus have a beneficial impact on soil and water quality.

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