Long-term cropping system and nitrogen management effects on soil properties

The sustainability of current crop production systems, and their impact on soil properties related to sustainability, is in question. The effects of several Iowa long-term cropping and fertilization systems on soil organic carbon (SOC) and selected physical properties were examined. At three sites, crop rotations had been continuous for 12, 34, and 36 yr. Treatments included crop management variations and/or nitrogen (N) fertilization rates, initiated on previously cropped soils. Effects on SOC levels only were examined on a 75-yr continuous corn site, with N, phosphorus, and potassium fertilization treatments established in 1952. Soils were primarily Typic and Aquic Hapludolls and Typic Haplaquolls. Composite samples were collected from all sites for the SOC analysis. Near-surface core samples were collected from the rotation studies at several times for analysis of bulk density, pore size fractions, moment analysis of pore size distribution, water retention characteristic, and hydraulic conductivity. All cropped soils had significantly lower SOC levels than adjacent fence row samples. Crop rotations that included forage legumes and small grains maintained higher SOC levels than other systems. Corn (Zea mays) silage and no N fertilizer treatments had the lowest SOC levels. Continuous corn management without amendments resulted in SOC losses. Fertilizer N, initial SOC, residue additions, and previous management practices significantly affected SOC levels. Increases in SOC levels under N fertilization were a function of increased biomass production under those systems. Data from the continuous corn experiment were used to test the predictive ability of an SOC dynamics model. Cropping altered physical properties from those found in adjacent fence rows. Significant treatment differences occurred with all methods of soil physical property analysis. Sampling time was the most common factor affecting physical properties, yet direction of change with time was not consistent. Cropping systems also affected physical properties, but the effect was not consistent among sampling times. Nitrogen fertilization had no effect on soil physical properties.