Mitigation of global warming potential and greenhouse gas intensity in arable soil with green manure as source of nitrogen.

This study was conducted to determine the effect of different green manure treatments on net GWP and GHGI in upland soil. Barley (B), hairy vetch (HV), and a barley/hairy vetch mixture (BHV) were sown on an upland soil on November 4, 2017 and October 24, 2018. The aboveground biomass of these green manures was incorporated into soil on June 1, 2018 and May 8, 2019. In addition, a fallow treatment (F) was installed as the control. Maize was transplanted as the subsequent crop after incorporation of green manures. Green manuring significantly affected CO2 and N2O emission, but not CH4. Average cumulative soil respiration across years with HV and BHV were 37.0 Mg CO2 ha-1 yr-1 and 35.8 Mg CO2 ha-1 yr-1, respectively and significantly higher than those with under F and B (32.7 Mg CO2 ha-1 yr-1 and 33.0 Mg CO2 ha-1 yr-1, respectively). Cumulative N2O emissions across years with F and HV were 6.29 kg N2O ha-1 yr-1 and 5.44 kg N2O ha-1 yr-1, respectively and significantly higher than those with B and BHV (4.26 kg N2O ha-1 yr-1 and 4.42 kg N2O ha-1 yr-1, respectively). The net ecosystem carbon budget for HV (-0.5 Mg C ha-1 yr-1) was the greatest among the treatments (F; -1.61 Mg C ha-1 yr-1, B; -3.98 Mg C ha-1 yr-1, and BHV; -0.91 Mg C ha-1 yr-1) because of its high biomass yields and the yield of maize after incorporation of HV. There was no significant difference of GHGI among F, HV, and BHV. Incorporation of HV or BHV could reduce net CO2 emissions per unit of maize grain production as well as F.

Cadmium phytoavailability from 1976 through 2016: Changes in soil amended with phosphate fertilizer and compost.

This study aimed to determine cadmium (Cd) accumulation in arable soil, changes in Cd extractability and relevant soil properties, and Cd uptake by rice plants after long-term (50 years) application of phosphate (P) fertilizer and compost. A long-term field experiment was performed with rice crops from 1967 to 2016. Treatments included nitrogen and potassium fertilization (NK), nitrogen, phosphate, and potassium fertilization (NPK), nitrogen, phosphate, and potassium fertilization with compost application (NPK + compost), and control. Total Cd concentration in soil amended with NPK and NPK + compost continuously increased from 110 µg kg-1 up to 232 µg kg-1 from 1976 to 2016 but remained unchanged in control soil and soil amended with only NK. Plant-available Cd concentration in soil increased with year for all treatments, likely as a result of relevant changes in soil chemical properties. Cd concentrations in rice harvested in 2017 treated with NPK or NPK + compost were 212 µg Cd kg-1 and 223 µg Cd kg-1, respectively. These values exceed the maximum permissible level (200 µg Cd kg-1) established by the Ministry of Food and Drug Safety of Korea.

Intercropping kura clover with prairie cordgrass mitigates soil greenhouse gas fluxes.

Prairie cordgrass (PCG) (Spartina pectinata Link) has a high tolerance to soil salinity and waterlogging, therefore, it can thrive on marginal lands. Optimizing the nitrogen (N) input is crucial to achieving desirable biomass production of PCG without negatively impacting the environment. Thus, this study was based on the hypothesis that the use of legumes such as kura clover (Trifolium ambiguum M. Bieb.) (KC) as an intercrop with PCG can provide extra N to the crop reducing the additional N fertilizer and mitigating soil surface greenhouse gas (GHG) emissions. Specific objective of the study was to assess the impact of PCG managed with different N rates [0 kg N ha-1 (PCG-0N), 75 kg N ha-1 (PCG-75N), 150 kg N ha-1 (PCG-150N), and 225 kg N ha-1 (PCG-255N)], and PCG intercropped with KC (PCG-KC) on GHG fluxes and biomass yield. The experimental site was established in 2010 in South Dakota under a marginally yielding cropland. The GHG fluxes were measured from 2014 through 2018 growing seasons using the static chamber. Net global warming potential (GWP) was calculated. Data showed that cumulative CH4 and CO2 fluxes were similar for all the treatments over the study period. However, the PCG-KC, PCG-0N, and PCG-75N recorded lower cumulative N2O fluxes (384, 402, and 499 g N ha-1, respectively) than the PCG-150N (644 g N ha-1) and PCG-255N (697 g N ha-1). The PCG-KC produced 85% and 39% higher yield than the PCG-0N in 2016 and 2017, respectively, and similar yield to the other treatments (PCG-75N, PCG-150N, and PCG-255N) in these years. Net GWP was 52% lower for the PCG-KC (112.38 kg CO2-eq ha-1) compared to the PCG-225N (227.78 kg CO2-eq ha-1), but similar to other treatments. Soil total N was 15%% and 13% higher under PCG-KC (3.7 g kg-1) than that under PCG-0N (3.2 g kg-1) and PCG-75N (3.3 g kg-1), respectively. This study concludes that intercropping prairie cordgrass with kura clover can enhance biomass yield and reduce fertilizer-derived N2O emissions and net global warming potential.

Adsorption and precipitation of cadmium affected by chemical form and addition rate of phosphate in soils having different levels of cadmium.

Although a number of studies have examined cadmium (Cd) immobilization by phosphate (P) in soils, determining the exact mechanism of Cd immobilization in various conditions, including P chemical form, P rate, and inherent Cd concentrations in soil has not been examined. The objective of this study was to determine changes in Cd immobilization through adsorption and precipitation in different conditions. Arable soils were spiked with inorganic Cd to give a total Cd concentration of 10, 100, and 1000 mg Cd kg-1. K2HPO4 (DPP) and KH2PO4 (MPP) were selected and mixed with the pretreated arable soil at the rates of 0, 800, 1600 and 3200 mg P kg-1. In soils with 10 and 100 mg Cd kg-1, both P materials similarly increased negative charge of soil and decreased extractable Cd concentration. Using X-ray diffraction (XRD), a diffraction peak revealing the presence of Cd(H2PO4)2 was observed in soil with 1000 mg Cd kg-1 and 3200 mg P kg-1 soil. In addition, X-ray photoelectron spectroscopy (XPS) analysis and modeling for saturation index for Cd minerals proved that formation of CdCO3 and Cd3(PO4)2 occurred in soil having 1000 mg Cd kg-1 and addition of both DPP and MPP. Immobilization of Cd might be attributed to Cd adsorption in soil with relatively low Cd levels (<100 mg kg-1). Precipitation of Cd(H2PO4)2, CdCO3, and Cd-phosphate might be a dominant mechanism to immobilize Cd, besides Cd adsorption, in soil with relatively high Cd levels (1000 mg kg-1).

How Does Oyster Shell Immobilize Cadmium?

The exact mechanism of cadmium (Cd) immobilization by oyster shell (OS) has not been reported. The effect of OS on Cd immobilization and the exact mechanism should be known before applying remediation technology using OS to Cd contaminated soils. Therefore, the objective of this study was to elucidate the mechanism of Cd immobilization by OS. Three grams of OS (< 0.84 mm) was reacted with 30 mL of 0-3.56 mg Cd L-1 solution at 25 °C for 48 h. Cadmium adsorption increased with increasing initial concentration of Cd in solution. The X-ray diffraction patterns clearly demonstrated that precipitation of CdCO3 did not take place in suspensions of OS after reacting with up to 3.56 mol Cd L-1. Interestingly, we found formation of Ca0.67Cd0.33CO3 crystalline in suspension of OS after reacting with maximum initial Cd concentrations. Precipitation and chemisorption might contribute to Cd immobilization together. However, we feel confident that chemisorption is the major mechanism by which Cd immobilization occurs with OS. In conclusion, OS could be an effective bioadsorbent to immobilize Cd through formation of geochemically stable Cd mineral.

Modeling the impacts of temperature and precipitation changes on soil CO2 fluxes from a Switchgrass stand recently converted from cropland.

Switchgrass (Panicum virgatum L.) is a perennial C4 grass native to North America and successfully adapted to diverse environmental conditions. It offers the potential to reduce soil surface carbon dioxide (CO2) fluxes and mitigate climate change. However, information on how these CO2 fluxes respond to changing climate is still lacking. In this study, CO2 fluxes were monitored continuously from 2011 through 2014 using high frequency measurements from Switchgrass land seeded in 2008 on an experimental site that has been previously used for soybean (Glycine max L.) in South Dakota, USA. DAYCENT, a process-based model, was used to simulate CO2 fluxes. An improved methodology CPTE [Combining Parameter estimation (PEST) with "Trial and Error" method] was used to calibrate DAYCENT. The calibrated DAYCENT model was used for simulating future CO2 emissions based on different climate change scenarios. This study showed that: (i) the measured soil CO2 fluxes from Switchgrass land were higher for 2012 which was a drought year, and these fluxes when simulated using DAYCENT for long-term (2015-2070) provided a pattern of polynomial curve; (ii) the simulated CO2 fluxes provided different patterns with temperature and precipitation changes in a long-term, (iii) the future CO2 fluxes from Switchgrass land under different changing climate scenarios were not significantly different, therefore, it can be concluded that Switchgrass grown for longer durations could reduce changes in CO2 fluxes from soil as a result of temperature and precipitation changes to some extent.

Evaluating the impacts of landscape positions and nitrogen fertilizer rates on dissolved organic carbon on switchgrass land seeded on marginally yielding cropland.

Dissolved organic carbon (DOC) through leaching into the soils is another mechanism of net C loss. It plays an important role in impacting the environment and impacted by soil and crop management practices. However, little is known about the impacts of landscape positions and nitrogen (N) fertilizer rates on DOC leaching in switchgrass (Panicum virgatum L.). This experimental design included three N fertilizer rates [0 (low); 56 (medium); 112 (high) kg N ha(-1)] and three landscape positions (shoulder, backslope and footslope). Daily average DOC contents at backslope were significantly lower than that at shoulder and footslope. The DOC contents from the plots that received medium N rate were also significantly lower than the plots that received low N rates. The interactions of landscape and N rates on DOC contents were different in every year from 2009 to 2014, however, no significant consistent trend of DOC contents was observed over time. Annual average DOC contents from the plots managed with low N rate were higher than those with high N rate. These contents at the footslope were higher than that at the shoulder position. Data show that there is a moderate positive relationship between the total average DOC contents and the total average switchgrass biomass yields. Overall, the DOC contents from leachate in the switchgrass land were significantly influenced by landscape positions and N rates. The N fertilization reduced DOC leaching contents in switchgrass field. The switchgrass could retain soil and environment sustainability to some extent. These findings will assist in understanding the mechanism of changes in DOC contents with various parameters in the natural environment and crop management systems. However, use of long-term data might help to better assess the effects of above factors on DOC leaching contents and loss in the switchgrass field in the future.

Effect of Phosphate Addition on Cadmium Precipitation and Adsorption in Contaminated Arable Soil with a Low Concentration of Cadmium.

The objectives of this study were to determine (1) the phosphorus (P) level required to induce cadmium (Cd) precipitation in a contaminated arable soil with low concentrations of Cd and (2) the primary mechanism of Cd immobilization at different P levels. Phosphorus was added at levels of 0 800, 1600, and 16,000 mg P kg(-1) to a soil containing 5.57 mg Cd kg(-1). The concentration of 1 M NH4OAc extractable Cd decreased significantly with P levels up to 1600 mg kg(-1) due to an increase in soil pH and negative charge of soil (p<0.001). A further decrease in 1 M NH4OAc extractable Cd concentration was noted when P was increased to 16,000 mg P kg(-1) and may have been the result of Cd precipitation. This study suggest that adding P at high levels may help in the formation of geochemically stable Cd minerals in soil containing low levels of this heavy metal.

Soil pH effect on phosphate induced cadmium precipitation in Arable soil.

The objective of this study was to determine soil pH conditions that allow cadmium (Cd) to precipitate as Cd minerals in phosphate (P) amended soil. Cadmium immobilization could be attributed primarily to Cd adsorption due to increase in pH and negative charge. Soil pH might not affect Cd precipitation as Cd3(PO4)2 by direct reaction of Cd and P in the studied soil, even when soil pH increased up to 9.0. However, Cd might precipitate as CdCO3 with increasing pH up to 9.0 in P untreated soil and up to 8.0 in P treated soil depending on CO2 level.