| Production of acidity (H+) during nitrification determines the role of the nitrogen cycle in soil acidification processes. Barak et al. (1997) showed how excessive N fertilization induces acidification and loss of Ca and Mg in soils. For this study, continuous corn (CS1), corn-soy (CS2), and corn-alfalfa (CS4) rotations were selected from the Wisconsin Integrated Cropping Systems Trial (WICST). The main objective was to quantify and compare the acid/base balance of these rotations, and to elucidate the mechanisms by which N-fixing crops induce soil acidification at the WICST.; Total acidity inputs to soil in CS1, CS2, and CS4 averaged 17.5, 10.7, and 13.0 kmol H+/ha/yr. Net acidity inputs in CS1 and CS2 averaged 4.5 and 4.7 kmol H+/ha/yr with aboveground biomass harvest, while in CS4 they averaged 7.6 kmol H+/ha/yr. However, at the WICST crop residues are returned to the soil and, in such case, net acidity inputs averaged 0.8, 1.3 and 6.5 kmol H+/ha/yr for CS1, CS2, and CS4, respectively.; Acidity inputs to soil by N-fixing crops was through nitrification of biologically-fixed N remaining in the soil after harvest. The acid/base balance of the soy crop (CS2) averaged 3.3 kmol H+/ha/yr when only beans were harvested. In the second and third years of the alfalfa crop (CS4) the acid base balance averaged 9.2 and 15.4 kmol H+/ha/yr, respectively. Although most N-fixing crops excrete acidity (H+) due to excess cation uptake, at the WICST this occurred only in the third year of the alfalfa crop and averaged 0.8 kmol H+/ha/yr.; Use of "best management practices" at the WICST significantly reduced the acidity inputs to soil by CS1, CS2, and CS4. Use of pre-plant nitrate test credits reduced acidity inputs from ammoniacal fertilizers in corn crops of CS1 and CS2. Manure applications limited H+ excretion by soy and alfalfa in CS2 and CS4 and added alkalinity to the soil in the form of organic anions. Finally, return of crop residues to soil after harvest produced significant reductions in the net acidity inputs to soil by CS1, CS2, and CS4. |
