Rex A. Omonode

Spatial dependence and relationships of electrical conductivity to soil organic matter, phosphorus, and potassium

Apparent electrical conductivity (ECa) can be an indirect indicator of soil nutrient concentrations, and strong relationships between ECa and nutrients may help delineate fertility management zones. The objectives of this study were to determine the relationships among ECa, organic matter (OM), P, and K and to assess the spatial dependency and correlation components of such relationships. Spatial correlations and relationships of ECa with OM, P, and K were determined using geostatistical and regression analyses. ECa was measured in parallel transects 4.6 m apart in adjacent Indiana (United States) fields with multiple soil series in 2001, 2002, and 2003. Electrical conductivity, OM, P and K were strongly spatially dependent. Spatial correlations between ECa and these variables were cyclic: positive at distances of <40 m, but negative at distances <70 m. Organic matter, P, and K showed relatively low but significant correlations with ECa in all the 3 years of study. Maximum correlations were obtained with OM, P, and K (r = 0.55, 0.50, 0.53, respectively) when soil moisture content was relatively higher in 2003. Data separation by soil series improved correlations between ECa and soil chemical attributes when the soil was a Mollisol, but not for an Alfisol. Regression models showed that ECa was significantly related to the variables but that ECa failed to detect concentration changes and explained <7% of the variability of these nutrients. The results suggested that ECa has limited usefulness in estimating soil chemical properties or in defining possible management zones for OM, P, and K in these soils.

Achieving Lower Nitrogen Balance and Higher Nitrogen Recovery Efficiency Reduces Nitrous Oxide Emissions in North America's Maize Cropping Systems

Few studies have assessed the common, yet unproven, hypothesis that an increase of plant nitrogen (N) uptake and/or recovery efficiency (NRE) will reduce nitrous oxide (N2O) emission during crop production. Understanding the relationships between N2O emissions and crop N uptake and use efficiency parameters can help inform crop N management recommendations for both efficiency and environmental goals. Analyses were conducted to determine which of several commonly used crop N uptake-derived parameters related most strongly to growing season N2O emissions under varying N management practices in North American maize systems. Nitrogen uptake-derived variables included total aboveground N uptake (TNU), grain N uptake (GNU), N recovery efficiency (NRE), net N balance (NNB) in relation to GNU [NNB(GNU)] and TNU [NNB(TNU)], and surplus N (SN). The relationship between N2O and N application rate was sigmoidal with relatively small emissions for N rates <130 kg ha−1, and a sharp increase for N rates from 130 to 220 kg ha−1; on average, N2O increased linearly by about 5 g N per kg of N applied for rates up to 220 kg ha−1. Fairly strong and significant negative relationships existed between N2O and NRE when management focused on N application rate (r2 = 0.52) or rate and timing combinations (r2 = 0.65). For every percentage point increase, N2O decreased by 13 g N ha−1 in response to N rates, and by 20 g N ha−1 for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships (R2 = 0.73–0.77) existed between N2O and NNB(TNU), NNB(GNU), and SN, regardless of rate and timing of N application; on average N2O emission increased by about 5, 7, and 8 g N, respectively, per kg increase of NNB(GNU), NNB(TNU), and SN. Neither N source nor placement influenced the relationship between N2O and NRE. Overall, our analysis indicated that a careful selection of appropriate N rate applied at the right time can both increase NRE and reduce N2O. However, N2O reduction benefits of optimum N rate-by-timing practices were achieved most consistently with management systems that reduced NNB through an increase of grain N removal or total plant N uptake relative to the total fertilizer N applied to maize. Future research assessing crop or N management effects on N2O should include N uptake parameter measurements to better understand N2O emission relationships to plant NRE and N uptake.