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Widespread implementation of precision agriculture practices requires low-cost, high-quality data such as soil organic C (SOC) content, but SOC mapping currently requires expensive sample collection and analysis techniques. Soils higher in organic C appear darker than surrounding soils in aerial imagery after tillage, although this difference is only relative without knowledge of the range of SOC. This ran could be estimated from Soil Survey Geographic (SSURGO) database. To verify this, the SSURGO database was used to estimate the SOC range at three sites in central Iowa. Soil organic C content across each field was then linearly interpolated within the SSURGO-estimated range for each field using the brightness values at each pixel in the aerial photograph as a scaling factor. Measured SOC data from the three sites ranged from 3.4 to 50 g kg-1 and the R² and RMSE values between the measured and estimated SOC concentrations ranged from 0.60 to 0.82 and 3.5 to 7.6 g kg-1 respectively. Limited thresholding of the brightest and darkest pixels improved the accuracy and precision of SOC estimates over raw imagery. These results imply that aerial imagery supplemented by SSURGO-estimated SOC ranges can provide georeferenced SOC estimates suitable for site-specific recommendations and analysis without field sampling. [ABSTRACT FROM AUTHOR]

Soil respiration is an important component of the carbon dynamics of terrestrial ecosystems. Many factors exert controls on soil respiration, including temperature, soil water content, organic matter, soil texture, and plant root activity. This study was conducted to quantify soil respiration in the Walnut Creek watershed in central Iowa, and to investigate the factors controlling this process. Six agricultural fields were identified for this investigation: three of the fields were cropped with soybean [Glycine max (L.) Merr.] and three were cropped with corn (Zea mays L.). Within each field, soil respiration was measured at nine locations, with each location corresponding to one of three general landscape positions (summit, side slope, and depression). Soil respiration was measured using a portable vented chamber connected to an infrared gas analyzer. Soil samples were collected at each location for the measurement of soil water content, pH, texture, microbial biomass, and respiration potential. Field respiration rates did not show a significant landscape effect. However, there was a significant crop effect, with respiration from cornfields averaging 37.5 g CO2 m-2 day-1 versus an average respiration of 13.1 g CO2 m-2 day-1 in soybean fields. In contrast, laboratory measurements of soil respiration potential, which did not include plant roots, showed a significant landscape effect and an insignificant cropping system effect. Similar relationships were observed for soil organic C and microbial biomass. Additional analyses indicate that corn roots may be more important than soybean roots in their contribution to surface CO2 flux, and that root respiration masked landscape effects on total soil respiration. Also, the failure to account for soil respiration may lead to biased estimates of net primary production measured by eddy covariance. [ABSTRACT FROM AUTHOR]

Fertilizer N that leaches from the root zone is not available for uptake by plants and can be transported to surface waters by subsurface drains, where it is considered a contaminant. Localized soil compaction and doming (LCD) to divert precipitation or irrigation water around the zone of fertilizer injection may be one way to reduce nitrate (NO−3) leaching from agricultural fields. This study was designed to determine whether anion tracer transport through soil profiles to subsurface drains could be delayed by modified application methods, specifically (i) injection using a conventional knife shank; (ii) broadcast application; and (iii) injection and covering using an LCD injector. Three fluorobenzoate tracers, which move similarly to NO−3 through soil, were applied to eight lysimeters at 63 kg ha−1 using the three application treatments. Each treatment contained an individual tracer. Two rainfall intensity treatments were imposed, four lysimeters per rainfall intensity treatment. All lysimeters received 50 mm of simulated rain 3 d after chemical application. A storm of this magnitude occurs once in 5 yr in Iowa. There were no differences in cumulative tracer leaching between the different rainfall intensity treatments. At the end of 6 mo, 5% of the tracer applied by a conventional knife shank leached to the subsurface drains, 4% applied as broadcast leached, and 1% applied by the LCD leached. At 18 mo, corresponding values were 25, 17, and 13%. These results suggested that soil modification in the fertilizer injection zone was a simple and effective way to reduce anion leaching. [ABSTRACT FROM AUTHOR]

The application of site-specific conservation practices requires knowledge of spatial patterns in fields. This study evaluated two methods of delineating soil erosion patterns in a central Iowa field. First, soil erosion rates of individual grid based samples were estimated using soil displacement of cesium-137 (137Cs). Second, tillage and water erosion were estimated using the topography-driven Water and Tillage Erosion Model (WATEM). The tillage erosion map showed soil loss in convex shoulder positions and soil accumulation in concave footslope and toeslope landscape positions. Alternately, water erosion was associated with slope severity and slope length on backslopes. When the tillage and water erosion map patterns were combined a good correlation with the 137Cs soil erosion pattern was graphically and statistically exhibited. Study results suggest that tillage erosion be included with water and wind erosion estimates when developing spatial maps that reflect a field's erosion history. Spatial maps depicting a field's erosion history and the primary processes affecting erosion could be used for site-specific implementation of conservation practices such as cover crops, organic matter additions, and no-till, which could be targeted at specific erosion processes. [ABSTRACT FROM AUTHOR]

Harvesting corn stover removes N from the fields, but its effect on subsurface drainage and other N losses is uncertain. We used the Root Zone Water Quality Model (RZWQM) to examine N losses with 0 (NRR) or 50% (RR) corn residue removal within a corn and soybean rotation over a 10-yr period. In general, all simulations used the same pre-plant or post-emergence N fertilizer rate (200 kg ha -1  yr -1 ). Simulated annual corn yields averaged 10.7 Mg ha -1 for the post emergence applications (NRRpost and RRpost), and 9.5 and 9.4 Mg ha -1  yr -1 for NRRpre and RRpre. Average total N input during corn years was 19.3 kg N ha -1 greater for NRRpre compared to RRpre due to additional N in surface residues, but drainage N loss was only 1.1 kg N ha -1  yr -1 greater for NRRpre. Post-emergence N application with no residue removal (NRRpost) reduced average drainage N loss by 16.5 kg ha -1  yr -1 compared to pre-plant N fertilization (NRRpre). The farm-gate net energy ratio was greatest for RRpost and lowest for NRRpre (14.1 and 10.4 MJ output per MJ input) while greenhouse gas intensity was lowest for RRpost and highest for NRRpre (11.7 and 17.3 g CO 2 -eq. MJ -1 output). Similar to published studies, the simulations showed little difference in N 2 O emissions between scenarios, decreased microbial immobilization for RR compared to NRR, and small soil carbon changes over the 10-yr simulation. In contrast to several previous modeling studies, the crop yield and N lost to drain flow were nearly the same between NRR and RR without supplemental N applied to replace N removed with corn stover. These results are important to optimizing the energy and nitrogen budgets associated with corn stover harvest and for developing a sustainable bioenergy industry., (Copyright © 2019 Elsevier B.V. All rights reserved.)