Influence of Cropping Intensity and Nitrogen Fertilizer Rates on In Situ Nitrogen Mineralization

span of years also greatly affects C and N dynamics. Wood et al. (1990) measured greater potentially minerCycling of N through an agroecosystem can be managed more alizable C and N in surface soils of a wheat–corn–millet– effectively if effects of N management and cropping sequence on soil fallow (WCMF), 4-yr rotation than a WF, 2-yr rotation. N microbial processes are understood. Effects of cropping intensity and N fertilizer rate on net soil N mineralization were studied as well These results were found after a relatively short period as their correlation with precipitation, air temperature and soil water (3.5 yr), occurring after conversion from conventional content. Net soil N mineralization was measured by incubating undisto no-till management. Wood et al. (1990) concluded turbed soil cores (15-cm depth) containing anion and cation exchange that mineralization differences between the two rotaresins at their bottoms. Cores were incubated during each of five time tions resulted from greater surface organic C concentraperiods (3–4 wk each) during the fallow phase (mid-April to midtions under WCMF compared with WF. These differSeptember) of two no-till cropping systems, wheat (Triticum aestivum ences were related to cumulative plant residue additions L.)–fallow (WF) and wheat–corn (Zea mays L.)– fallow (WCF). Past (9.01 vs. 7.04 Mg ha21 for WCMF and WF, respectively) N fertilizer applications were over four rates with total amounts apduring the 3.5 yr of no-till management. They also proplied during the previous 6 yr of 0, 95, 190, and 286 kg N ha21 in WF posed that the most active surface soil organic C and N and 0, 134, 269, and 403 kg N ha21 in WCF. Soils were an Aridic Paleustoll at Sterling and an Aridic Argiustoll at Stratton in eastern fractions are particularly sensitive to change in cultural Colorado. Total net N mineralization in WCF was half that in WF practices, and that both are increased by greater crop(22 vs. 43 kg N ha21; 2-site average), probably due to greater immobiliping intensity under no-till management. zation as evidenced by nearly three times greater accumulation of Fertilizer N additions continue to affect soil C and N crop residue on the soil surface after 6 yr of no-till management. dynamics after the crop is harvested, since only a portion Greater conservation of applied N and soil N can be expected in the of the added N is taken up by the plants (Clay and more intensive WCF system. Total mineralized N increased with N Clapp, 1990). Greater N mineralization was found in rate by ≈0.2 kg ha21 for each kg ha21 of previously applied N. Precipitapreviously fertilized plots vs. unfertilized plots in cotton tion in combination with air temperature and their interaction term (Gossypium hirsutum L.) cultivation (Hadas et al., gave the best prediction of average daily N mineralization at both sites. 1989). Applied N fertilizer also interacts with crop residues to enhance soil N mineralization. Singh and Singh (1994) measured greater increases in N mineralization C and fertilizer management practices rate with a fertilizer plus wheat straw treatment than have a cumulative effect on nutrient cycling and with separately applied fertilizer or straw. availability over time. The long-term effects may be Soil moisture and temperature are two other factors additive and/or synergistic. Traditional tillage managethat substantially affect N mineralization. Increases in ment in the Great Plains has resulted in a steady deboth soil moisture and temperature generally increase crease in soil organic C and N contents (Haas et al., SOM decomposition within the ranges typical of most 1957; Stewart et al., 1983). Losses of the original soil soils. For soil moisture this range lies between 210 to organic matter (SOM) have been estimated to be as 250 kPa, and for temperatures between 0 and 458C high as 60% after 50 yr of cultivation. Relatively short(Haynes, 1986). Repeated drying and wetting cycles can term (,20 yr) effects of tillage also have been found to have a stimulatory effect on cumulative N mineralizareduce potentially mineralizable C and N (Woods and tion relative to constant moisture conditions (Van Schuman, 1988) and to restrict the soil’s ability to immoSchreven, 1968), while fluctuating temperatures generbilize and conserve mineral N (Follett and Schimel, ally inhibit microbial activity (Haynes, 1986). Changes 1989). Conversely, reducing tillage has resulted in in both soil moisture and temperature have a greater greater potentially mineralizable C and N where the effect on mineralization than their individual additive soil microbial population was less C-limited compared with a tilled system (Follett and Schimel, 1989). effects alone. Cassman and Munns (1980) found signifiRecent research has suggested the existence of an cantly greater mineralization over suboptimal moisture active SOM fraction that is strongly affected by current levels at 308C compared with lower temperatures than management practices (Bonde and Rosswall, 1987). that which would be expected by strictly additive effects. Cropping intensity or number of crops grown over a The objectives of this study were to (i) measure the influence of cropping intensity on net soil N mineralization, (ii) quantify the effect of N fertilizer rate on net R.L. Kolberg, USDA-ARS, 1500 N. Central Ave., Sidney, MT 59270; soil N mineralization, and (iii) correlate precipitation, D.G. Westfall and G.A. Peterson, Dep. of Soil and Crop Sci., Colorado State Univ., Ft. Collins, CO 80523. Contribution from the Dep. of Soil and Crop Sci., Colorado State Univ. Received 16 Dec. 1996. Abbreviations: SOM, soil organic matter; SWC, soil water content; *Corresponding author ([email protected]). WCF, wheat–corn–fallow; WCMF, wheat–corn–millet–fallow; WF, wheat–fallow. Published in Soil Sci. Soc. Am. J. 63:129–134 (1999).