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Methods to determine crop water use may be difficult to implement on numerous research plots. The purpose of this study was to calculate Priestley-Taylor evapotranspiration (ET) from measured input and to assess uncertainty. Input data included net radiation, air temperature, barometric pressure, green ground cover, total ground cover, the ratio of crop to total green cover, soil texture class, and clay content for each soil layer, and periodic water content measurements with depth (daily for soil surface). The relationship between seasonal water use and yield was similar to literature values for alfalfa (Medicago sativa L.) and oat (Avena sativa L.) dry matter as well as for soybean [Glycine max (L.) Merr.] grain yield, but the calculated water use was lower (-80%) than literature values for corn (Zea mays L.) grain yield. Monthly trends in water use reflected the seasonal crop growth. In summary, the proposed technique was useful across multiple plots and cropping treatments, and was more reliable than a water balance approach in areas with lateral and upward flow due to periodic high water tables. [ABSTRACT FROM AUTHOR]

Leaf area index (LAI) is important in crop models, but determination of LAI can be challenging for crops and mixtures of crops. The purpose of this study was to determine LAI indirectly from ground cover (determined from photographic images) and canopy height using defined relationships based on crop coefficients. Images of plant canopies were obtained to estimate the green cover fraction. An equation was derived to determine LAI from ground cover and plant height, and the technique was tested on different crops. For three of the measurement dates, the data were compared to LAI measurements obtained with a LAI-2200 plant canopy analyzer. The trend was the same for the two LAI procedures, but there was some scatter. The photographic technique was adequate for LAI determination. [ABSTRACT FROM AUTHOR]

Farmers ate concerned that soil compaction will be a problem in the first few years after conversion to strict no-till. The objective of this study was to determine if the changes in depth-incremental soil bulk density during transition to no-till were greater than density changes due to natural variation. We sampled six locations from a no-till field and six locations from a disk field of a soybean [Glycine max (L.) Merr.]-corn (Zea mays L.) rotation and took 12 samples at each of the 12 locations. The soil types sampled were Clarion (fine-loamy, mixed, superactive, mesic, Typic Hapludoll), Nicollet (fine-loamy, mixed, superactive, mesic, Aquic Hapludoll), and Canisteo (fine-loamy, mixed, superactive, calcareous, mesic, Typic Endoaquoll) soils. We divided each 300-mm soil sample into 15 depth-increments, and pooled the 12 samples for each location for each 20-mm depth-increment. Bulk densities ranged from 0.9 to 1.3 Mg [m.sup.-3] in the top 0 to 20 mm, and 1.2 to 1.6 Mg [m.sup.-3] for the depth-increments below 20 mm. Comparing the first and last sampling dates for the no-till field, no significant changes in bulk density were detected for any depth-increment; however, for the disk field, increases in significant bulk density were detected in 10 out of 15 depth-increments. Natural causes probably contributed to the bulk density changes that occurred over time in both fields. None of these bulk densities were high enough to restrict root growth. We concluded that concern about soil compaction under no-till management is inappropriate for these structured soils.

Decomposition and mineralization of cover crop roots needs to be understood to determine if N taken up by cover crops is mineralized during main crop growth. Two experiments were conducted in a controlled environment to measure decomposition of oat (Avena sativa L. `Ogle') and rye (Secale cereale L. `Rymin') root residues and to examine its effect on soil N mineralization. In the first, oat and rye roots were mixed with soil and in the second, roots were grown in situ. At 7, 14, 28, 56, 84, and 112 d after the start of decomposition, denitrification, soil [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and soil [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] were measured to determine net mineralized N. Soil respiration and C and N contained in roots and coarse soil organic matter were measured to determine decomposition. All treatments in both experiments showed an increase in net mineralized N during the first 56 d. After 56 d, net mineralized N in the control remained relatively constant, whereas mineral N continued to accumulate in the treatments with root residues. Net N mineralization of the rye and oat root treatments did not differ. Roots mixed with soil had high respiration rates during the first 3 d and there were no differences between oat and rye root treatments. In the roots in situ experiment, however, respiration peaked for oat roots at Day 12 and for rye roots at Day 33. The oat treatment also had less C and N remaining in roots and coarse organic matter throughout the experiment. Even though oat roots decomposed faster than the rye roots, we predict that [is less than] 55% of the N contained in the roots of a spring-killed oat or rye cover crop will become available to the following crop.

It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no-till study was conducted to examine changes in free and aggregate-associated POM during the decomposition of in situ [.sup.14]C-labeled roots during a 1-yr incubation in a loess-derived silt loam. Two water pre-treatments (capillary-wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250-2000 [micro]m) were enriched in iPOM-[.sup.14]C on Day 0 which suggested that many of these aggregates formed around cores of new, root-derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250-2000 [micro]m) and a large increase in frPOM-[.sup.14]C on Day 0. The amount of [.sup.14]C released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root-derived iPOM-[.sup.14]C in large microaggregates (53-250 [micro]m) that were released when unstable macroaggregates ([is greater than] 250 [micro]m) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong evidence that, in no-till, aggregate formation and stabilization processes are directly related to the decomposition of root-residue and the dynamics of POM C in the soil.

No-till practices have the potential to increase soil organic C, but little is known about the relative contribution of surface residue and roots to soil organic C accumulation. In a simulated no-till experiment, we studied the fate of [.sup.14]C-labeled surface residue and in situ roots during a 1-yr incubation. Soil samples collected during the incubation were chemically dispersed and separated into five particle size and density fractions. The organic C, [.sup.14]C, and total N content of each fraction was determined. Alkali traps were used to measure [.sup.14]C losses due to respiration. After 360 d, 66% of the [.sup.14]C contained in the surface residue on Day 0 had been respired as [.sup.14][CO.sub.2],11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root-derived [.sup.14]C in the soil was evolved as [.sup.14]CO.sub.2] and 42% remained in the soil. The large (500-2000 [micro]m) and small (53-500 [micro]m) particulate organic matter (POM) fractions together contained 11 to 16% of the initial root-derived [.sup.14]C in the soil. In contrast, POM contained only 1 to 3% of the inital surface residue-derived [.sup.14]C. These data show clear differences in the partitioning of surface residue- and root-derived C during decomposition and imply that the beneficial effects of no-till on soil organic C accrual are primarily due to the increased retention of root-derived C in the soil.

Stable macroaggregates are enriched in new C relative to unstable macroaggregates, but the origin and form of this new C is not known. Under simulated no-till (NT) conditions, we used a [.sup.14]C label to monitor changes in the concentration of new surface residue- and root-derived C in aggregates of different size and stability during a 1-yr incubation. Two water pretreatments (capillary-wetted and slaked) were applied to the soil samples collected during the incubation. The samples were then wet sieved to obtain five aggregate size classes. Densiometric separations were used to isolate free and released particulate organic matter (frPOM) and intraaggregate POM (iPOM). Root-derived [.sup.14]C was distributed differently in the soil compared to surface residue-derived [.sup.14]C. A comparison of the two water pretreatments indicated that root-derived aggregate-[.sup.14]C and iPOM-[.sup.14]C concentrations were significantly higher in stable (slaking-resistant) small macroaggregates (250-2000 [micro]m) relative to those in the capillary-wetted pretreatment. In contrast, there were no significant differences in the amount of surface residue-derived aggregate-[.sup.14]C or iPOM-[.sup.14]C in small macroaggregates (250-2000 [micro]m) between the two pretreatments. We conclude that in relatively undisturbed systems like no-till, new root-derived iPOM-C is more important than surface residue-derived C in the stabilization of small macroaggregates (250-2000 [micro]m).

Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] farmers in the upper Midwest are showing increasing interest in winter cover crops. The effects of winter cover crops on soil quality in this region, however, have not been investigated extensively. The objective of this experiment was to determine the effects of a cereal rye (Secale cereale L.) winter cover crop after more than 9 yr in a corn silage-soybean rotation. Four cereal rye winter cover crop treatments were established in 2001 : no cover crop, rye after soybean, rye after silage, and rye after both. Soil organic matter (SOM), particulate organic matter (POM), and potentially mineralizable N (PMN) were measured in 2010 and 2011 for two depth layers (0-5 and 5-10 cm) in both the corn silage and soybean phases of the rotation. In the 0- to 5-cm depth layer, a rye cover crop grown after both main crops had 15% greater SOM, 44% greater POM, and 38% greater PMN than the treatment with no cover crops. In general, the treatments that had a rye cover crop after both crops or after corn silage had a positive effect on the soil quality indicators relative to treatments without a cover crop or a cover crop only after soybean. Apparently, a rye cover crop grown only after soybean did not add enough residues to the soil to cause measureable changes in SOM, POM, or PMN. In general, rye cover crop effects were most pronounced in the top 5 cm of soil. [ABSTRACT FROM AUTHOR]