Soil tillage can affect the formation and stability of soil aggregates. The disruption of soil structure weakens soil aggregates to be susceptible to the external forces of water, wind, and traffic instantaneously, and over time. The choice of tillage system or land management changes the soil physical condition and soil organic matter content, which is an essential factor in building soil aggregates. This study was conducted to investigate the effects of different tillage systems on the rate of decay of different sizes of soil aggregate fractions and other associated properties over time as subjected to a continuous wetting process. This research was conducted on a long-term tillage study, established in 2002 at the Iowa State University Agronomy Research Farm near Ames, Iowa. The soil association in this study is Clarion-Nicollet-Webster (Clarion [fine-loamy, mixed, mesic, Typic Hapluduolls], Nicollet [fine-loamy, mixed, mesic, Aquic Hapluduolls], and Webster [fine-loam, mixed, mesic, Typic Endoaquolls]). The experimental design was a randomized complete block design with four replications. Main plot treatments were five tillage systems: moldboard-plow, chisel-plow, deep-rip, strip-till, and no-till. The cropping system was corn (Zea mays L.)-soybean (Glycine max L.) rotation. Wet aggregate stability was measured using the Wet Sieving Apparatus (Eijkelkamp, Agrisearch Equipment. Art no. 08.13). Soil organic carbon (SOC) and soil total nitrogen (N) were analyzed by dry combustion using CHN Analyzer (TruSpec CHN Version 2.5x). Results show no-till with the highest carbon (C) content and the highest macro-and microaggregate stability over time. The findings also show a strong relationship between the increase in SOC content and the stability of macro-and microaggregate under continuous wetting process. Furthermore, the findings suggest that aggregate stability and moisture content are highly correlated with SOC content, and the rate of decay of both aggregate sizes (macro and micro) is highly influenced by the intensity of tillage. The implication of this research is the importance of no-till not only in increasing the stability of micro-and macroaggregates and SOC storage, but also in its effect on increasing the stability of all aggregate fractions in continuous wet conditions for extended periods of time.

Corn yield (Zea mays L.) and economic return with diff erent tillage systems and crop rotations are highly infl uenced by regional soil and climate conditions. Th is study was conducted at seven locations in Iowa from 2003 to 2013. Th e experiment design was split-plot with tillage as the main factor, which included fi ve tillage systems (no-tillage, NT; strip-tillage, ST; chisel plow, CP; deep rip, DR; and moldboard plow, MP).Th ree crop rotations of corn-soybean (Glycine max L.), C-S; corn-corn-soybean, C-C-S; and corn-corn, C-C were subplots in a completely randomized block design in four replications. Th e objectives were to: (i) investigate seasonal variability in corn yield as aff ected by tillage and crop rotation, (ii) identify appropriate tillage for each crop rotation and location, and (iii) evaluate the magnitude of crop rotation eff ect on corn yield. Corn yields varied from 2.5 to 15.8 Mg ha -1 with no detectable increase over time. Th e results showed northern locations have yield of 1.9 Mg ha -1 and economic return of US$329 ha -1 advantage over southern locations. Yield and economic returns for the three rotations were as follow: C-S > C-C-S > C-C. Yield and economic penalty were greater with NT than conventional tillage in the northern locations (poorly-drained soils) than locations with well-drained soils. Th e corn yield penalty associated with C-C was location specifi c and varied from 11 to 28%. Th e fi ndings suggest a location specifi c adoption of tillage and crop rotation for achieving optimum yield.

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Conservation tillage systems present a challenge for integrating an efficient fertilizer program for corn (Zea mays L.) production in the U.S. Midwest and elsewhere. The objective of this study was to evaluate corn response to three tillage systems (no-tillage, strip-tillage, and chisel plow) and four N rates (0, 85, 170, and 250 kg N ha 21 ) of liquid swine manure and commercial fertilizer in a corn-soybean [Glycine max (L.) Merr.] rotation. A 3-yr study from 2002 to 2004 was conducted on a Kenyon soil (fine-loamy, mixed, mesic Typic Hapludoll) at the Iowa State University Northeast Research and Demonstration Farm near Nashua, IA. The experimental design was a randomized complete block with split plots. Tillage and N rates were randomly assigned as main plot and subplot treatments, respectively. Corn grain yield and aboveground biomass response to different tillage systems were not significantly different for all N rates of both N sources; however, the biomass yield at different growth stages with different N sources, tillage, and N rates were inconsistent. There were no interaction effects of tillage and N rates on yields, except in a few cases. The 3-yr average of maximum (MNR) and economic optimal (EONR) N rates (182 and 174 kg N ha 21 , respectively) across all tillage systems with liquid manure produced identical maximum (MGY) and economic optimum (EOGY) grain yields of 11.7 Mg ha 21 . In contrast, the MNR and EONR (176 and 144 kg N ha 21 , respectively) with commercial fertilizer N produced a MGYand an EOGYof 11.2 and 11.1 Mg ha 21 , respectively.

Understanding tillage, N, and P interactions can contribute to improved N and P utilization and crop response. This study examined the interaction effects of tillage and N rate of two N sources on N and P uptake by corn (Zea mays L.). The study was conducted on Kenyon loam (fine loamy, mixed, mesic Typic Hapludolls) soil at the Iowa State University Northeast Research and Demonstration Farm near Nashua, IA. A randomized complete block design with a split-plot arrangement in three replications was used for the two separate N source experiments. The tillage systems consist of no-tillage (NT), strip-tillage (ST), and chisel plow (CP) as main plots. Within each tillage four N rates (0, 85, 170, and 250 kg N ha 21 ) were assigned as subplots for each N source of manure (total N) and fertilizer (anhydrous ammonia) in a corn-soybean [Glycine max (L.) Merr.] rotation. Tillage and increase in N rate beyond 85 kg ha 21 had no effect on corn grain yield with both N sources. Tillage 3 N rate had a significant effect on plant N and P uptake, especially at early growth stages with both N sources. Recovery percentage of applied N across all tillage systems and N rates was 40% and 27% for manure and fertilizer sources, respectively, at the 12th-leaf growth stage of corn (V12). Plant N and P uptake at V12 growth stage was 44% and 37%, respectively, across tillage systems, N sources, and N rates. The findings indicate that the N rate and seasonal variability have more influence on plant N and P uptake than does the tillage system.