Ssj50058 249..255

High grain production of corn (Zea mays L.) can be maintained by adding inorganic N fertilizer, and also by using crop rotations that include alfalfa (Medicago sativa L.), but the relative impact of these management practices on soil quality is uncertain. We examined the effects on soil of N fertilization rate (0, 90, 180, 270 kg ha, corn phase only) in four cropping systems: CC, continuous corn; CS, corn–soybean [Glycine max (L.) Merr.]; CCOA, corn–corn–oat (Avena sativa L.)– alfalfa; and corn–oat–alfalfa–alfalfa (COAA). The 23and 48-yr-old experimental sites, situated in northeast (Nashua) and north central (Kanawha) Iowa, were in a replicated split-plot design and managed with conventional tillage. At Nashua, we measured available N, potential net N mineralization and microbial biomass C (MBC) throughout the growing season; all were significantly higher in the CCOA system. At both sites, post-harvest N stocks, and soil organic C (SOC) concentrations were significantly higher in systems containing alfalfa. Grain yield was most strongly correlated with soil N properties. At Nashua, N fertilizer additions resulted in significantly lower soil pH (0to 15-cm depth) and lower exchangeable Ca, Mg, and K and cation exchange capacity (CEC) in the CC and CCOA systems. In an undisturbed prairie reference site for Nashua, low available N, low pH, and high CEC suggested a strong influence of the vegetation on nutrient cycling. In terms of management of soil fertility, inclusion of alfalfa in the rotation differed fundamentally from addition of N fertilizer because high yield was maintained with fewer adverse effects on soil quality. CONVENTIONAL MANAGEMENT PRACTICES can differ significantly in their impacts on soil C sequestration (Studdert and Echeverrı́a, 2000; West and Post, 2002; Russell et al., 2005). These differences in C sequestration are expected to have broad implications for long-term soil fertility, soil quality, and the impact of management on the surrounding environment. Soil C quantity and quality influence the capacity of soil to store nutrients, and to release nutrients for crop growth during decomposition and mineralization (Lal, 2002; Horwath et al., 2002). Soil organic C influences water quality by regulating the release of nutrients to the ground water (Lal et al., 2004), and air quality by regulating emissions of greenhouse gases such as CO2 and N2O to the atmosphere (IPCC, 1997). Although soil productivity can be enhanced both through N fertilization and by growing legumes in rotation, the two management strategies may have different impacts on nutrient cycling and soil quality. Nevertheless, both can increase availableN (Taet al., 1986;Liebig et al., 2002;Mayer et al., 2003), and thereby increase yields. An important advantage of using symbiotic N-fixing crops in the context of a complex cropping system is that it increasesthediversityofsubstratesavailable forNmineralization,andtherebypromotesamoresustainableNsupply (Sanchez et al., 2001). Legumes can also provide improved soil structure, erosion protection, and greater biological diversity (Jensen andHauggaard-Nielsen, 2003). Achieving the optimal N addition, to maximize yield while minimizing environmental impact, is an important goal of best management practices. Excess N additions can negatively influence soil properties. For example, long-term N fertilization has been demonstrated to decrease levels of exchangeable Ca, Mg, and K, and cation exchange capacity (CEC) (Barak et al., 1997; Liu et al., 1997), and the reversibility of these changes is unknown. Ammoniacal fertilizers also cause acidification of soil (Bouman et al., 1995; Barak et al., 1997; Liebig and Doran, 1999; Gajda et al., 2000), but the rate of acidification depends on the crop system (Liebig et al., 2002) and soil type. Recent research has highlighted the adverse impact of excessive N fertilizer use in agricultural land on aquatic ecosystems (Turner and Rabalais, 2003). Furthermore, N fertilization may contribute to production of greenhouse gases such as N2O (Robertson et al., 2000). For cropping systems using biological N fixation, N losses via volatilization, N2O emission, and NO3 leaching may be lower during precropping and cropping, but post-harvest losses may be greater relative to cropping systems that rely on N fertilization (Jensen and Hauggaard-Nielsen, 2003). Effects of management on soil properties are difficult to evaluate because spatial and temporal variability in soil properties can easily mask the impact of management. We utilized two well-managed, replicated, and long-term experiments in northern Iowa to address the effects of N fertilization and cropping system on indices of soil quality. We have previously reported soil properties that related to C sequestration for these sites, including soil C stocks, potential mineralization of C (PMC), particulate organic C (POC), and resistant C (Russell et al., 2005). In this study, our objectives were to quantify the effect of N fertilization and cropping systems on bulk density (rb), available N, potential net N mineralization (PMN), MBC, SOC concentrations, exchangeable cations, CEC, and pH. A.E. Russell, Dep. of Natural Resource Ecology & Management, Iowa State Univ., Ames, IA 50011; D.A. Laird, USDA-ARS, National Soil Tilth Lab., Ames, IA 50011; A.P. Mallarino, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. Contribution from the National Soil Tilth Laboratory. Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product listed by the USDA. Received 23 Feb. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:249–255 (2006). Soil Biology and Biochemistry doi:10.2136/sssaj2005.0058 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CC, continuous corn; CCOA, corn–corn–oats–alfalfa; CEC, cation exchange capacity; COAA, corn-oats-alfalfa-alfalfa; CS, corn-soy; MBC, microbial biomass C; MSD, minimum significant difference by Tukey’s multiple comparison test; POC, particulate organic C; SOC, soil organic C; rb, bulk density. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 249 Published online January 6, 2006