Crop diversity effects on soil organic matter and nitrate retention in surface and subsoils

Much of the available soil organic carbon (SOC) is in subsoil, yet few studies have evaluated how crop rotation affects SOC below the plow layer. This project looks at whether crop rotations with greater belowground C inputs would increase SOC stocks by delivering C to subsoil with relatively low SOC levels

Long-term fate of nitrate fertilizer in agricultural soils is not necessarily related to nitrate leaching from agricultural soils

Accounting for the fate of inorganic N fertilizer in agricultural systems is critical to sustainable production. Sebilo et al. (1) provide a unique long-term record of 15NO3 fertilizer fate that demonstrates N molecules from a discrete fertilizer application are transferred to soil organic matter (SOM) and subsequently mineralized over the course of ca.100 years during which they contribute to NO3 leaching. The authors conclude “attempts to reduce agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizers”. Further, they suggest a recent decrease in anthropogenic N inputs to the Mississippi River Basin, without a concomitant decrease in riverine NO3 loads, is consistent with their conclusio

Research Shows Extra Cover Crop Growth Prior to Soybeans Provides Benefits

This study, funded by the United Soybean Board (USB), was conducted to understand the potential for cover crops to perform in a corn and soybean rotation, and to collect data on the performance of cover crops in those rotations in relation to the timing of termination

Physicochemical Organic Matter Stabilization across a Restored Grassland Chronosequence

In reconstructed grasslands, soil organic matter (SOM) is the largest CO2 and reactive N sink but SOM gains after reconstruction rarely achieve precultivation levels. Through a chronosequence of reconstructed grasslands 1 to 21 yr after establishment, we explored which physicochemical mechanisms protect accumulated soil organic C (SOC) and N from mineralization. After 21 yr, total SOC and soil N concentrations increased by 32 and 23%. The SOC concentration was within 5% of a new equilibrium but was 64% of a never‐cultivated remnant. Chemically stabilized C and N pools on free silt and clay surfaces increased with time. Coarse particulate organic matter C increased with time but accounted for \u3c12% of SOC. Microaggregate‐stabilized SOM did not change. The positive linear relationship between total SOC and free silt and clay C indicates that 21 yr after establishment, reconstructions have unsatisfied capacity for further SOM storage, despite proximity to a new SOC equilibrium. The accumulated C and N associated with free silt and clay suggest that ammonium oxalateextractable Fe (AmOx‐Fe) and polyvalent cation concentrations could be correlated with total C and N stocks. These promote SOM stabilization and were possibly affected by human activity before reconstruction. However, AmOx‐Fe and polyvalent cation concentrations were not associated with total SOC or soil N and could not explain the slowing SOM accumulation. Regardless of time since reconstruction, AmOx‐Fe was highly concentrated on microaggregate surfaces compared with other fractions and was positively associated with microaggregate C and N, suggesting a link between Fe and microaggregate stabilization

Land use and hydrologic flowpaths interact to affect dissolved organic matter and nitrate dynamics

The transport and transformation of dissolved organic matter (DOM) and dissolved inorganic nitrogen (DIN) through the soil profile impact down-gradient ecosystems and are increasingly recognized as important factors affecting the balance between accumulation and mineralization of subsoil organic matter. Using zero tension and tension lysimeters at three soil depths (20, 40, 60 cm) in paired forest and maize/soybean land uses, we compared dissolved organic C (DOC), dissolved organic N (DON) and DIN concentrations as well as DOM properties including hydrophilic-C (HPI-C), UV absorption (SUVA254), humification index and C/N ratio. Soil moisture data collected at lysimeter locations suggest zero tension lysimeters sampled relatively rapid hydrologic flowpaths that included downward saturated flow through the soil matrix and/or rapid macropore flow that is not in equilibrium with bulk soil solution whereas tension lysimeters sampled relatively immobile soil matrix solution during unsaturated conditions. The effect of land use on DOC and DON concentrations was largely limited to the most shallow (20 cm) sampling depth where DOC concentrations were greater in the forest (only zero tension lysimeters) and DON concentrations were greater in the cropland (both lysimeter types). In contrast to DOC and DON concentrations, the effect of land use on DOM properties persisted to the deepest sampling depth (60 cm), suggesting that DOM in the cropland was more decomposed regardless of lysimeter type. DOC concentrations and DOM properties differed between lysimeter types only in the forest at 20 cm where soil solutions collected with zero tension lysimeters had greater DOC concentrations, greater SUVA254, greater humification index and lower HPI-C. Our data highlight the importance of considering DOM quality in addition to DOC quantity, and indicate long-term cultivation reduced the delivery of relatively less decomposed DOM to all soil depths

Accuracy and precision of no instrument is guaranteed

Photoacoustic infrared spectroscopy (PAS) is increasingly used for measurement of N2O and CO2 fluxes at the soil surface. However, PAS calibration is complex. Water vapor, CO2, and temperature interfere with accurate N2O measurement. To accurately measure N2O, PAS calibrations must compensate for these interferences. Our article, ‘Evaluation of photoacoustic infrared spectroscopy for the simultaneous measurement of N2O and CO2 gas concentrations and fluxes at the soil surface’ (Iqbal et al., 2013), compared PAS and gas chromatography (GC) analytical procedures. Results demonstrated that PAS can measure N2O concentrations (ca. 0.5–3.0 ppm) and fluxes (ca. 0.5–5.0 ppm min−1) with accuracy and precision similar to GC without interferences from H2O vapor or CO2 concentrations typically encountered in static flux chambers at the soil surface

What drives corn yield stability in the context of climate variability?

The links between nitrogen fertilizer rates and varying crop rotation schemes are examined in this project. The role that organic matter inputs play in supporting corn-soybean rotations also was investigated

Evaluation of photoacoustic infrared spectroscopy for simultaneous measurement of N2O and CO2 gas concentrations and fluxes at the soil surface

Simultaneous measurement of N2O and CO2 flux at the soil surface with photoacoustic infrared spectroscopy (PAS) is gaining popularity due to portability, low maintenance, and ease-of-operation. However, the ability of PAS to measure N2O with accuracy and precision similar to gas chromatography (GC) is uncertain due to overlap in N2O, CO2, and H2O absorbance spectra combined with the large range in analyte concentrations. We tested the ability of six PAS units to simultaneously measure N2O and CO2 gas concentrations and fluxes with accuracy and precision similar to two GC units. We also evaluated H2O vapor and CO2 interferences with N2O measurement. The accuracy and precision of standard gas concentration measurements with PAS and GC were similar. High water vapor (~26 600 ppm) and CO2 concentrations (~4500 ppm) did not interfere with N2O measurement across the concentration range typically observed in static flux chambers at the soil surface (~0.5–3.0 ppm N2O). On average, N2O fluxes measured with the six PAS were 4.7% higher than one GC and 9.9% lower than the second GC

Climate Warming Trends in the U.S. Midwest Using Four Thermal Models

Thermal time (TT) is an agro-climate index widely established and used in predicting plant development based on temperature. This index is a powerful tool for measuring multi-faceted changes in temperature occurring from climate change. In the present study, TT was calculated for the entire frost-free period and individual spring, summer, and fall seasons using growing degree day (GDD), general thermal index (GTI), crop heat unit (CHU), and heat stress degree day (HSDD) models for 1054 counties across 12 Midwest states on a daily basis from 1950 to 2017. The temporal trend for each county was fit with a linear regression model for percent change per year. During the frost-free period, warming occurred in 260 to 489 counties with 0.06 to 0.34% gain per year dependent on model and county selected. Warming has occurred in northern and eastern counties primarily from gains in the fall season and partially from the spring. These TT gains are from additional calendar days from an expanded frost-free period and secondarily from a change in maximum temperature (fall only). Heat stress (\u3e30°C) during the frost-free period has decreased for 212 counties in the west-central region. Overall, the CHU model detected the most counties warming and had the lowest error particularly compared to the GDD model. Compared to 1950, some counties showed up to 1.2-fold increase in frost-free TT and are projected to 1.8-fold by end of the 21st century. Current warming trends are related to projected TT trends such that adaptation planning can be guided by the trajectory from the past 68 yr

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

Aims High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Results Peak maize N uptake rates averaged 4.4 kg N ha−1 d−1, while gross ammonification rates over the 0–80 cm depth averaged 23 kg N ha−1 d−1. Gross ammonification was highly stratified, with 63% occurring in the 0–20 cm depth and 37% in the 20–80 cm depth. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions Gross ammonification rate was 3.4 to 4.5 times greater than peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand, and that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment