Optimizing calcium materials for minimizing arsenate phytoavailability in upland arable soil based on geochemical analysis.

This study aimed to evaluate the reducing effects of calcite and phosphogypsum on arsenate [As(V)] availability to plants and elucidate the mechanisms of As(V) immobilization. The concentration of available As(V) to plants in upland arable soils with 1% calcite and phosphogypsum decreased to 17.4% and 36.9%, respectively, compared to the control. As(V) phytoavailability depends on the soil pH and calcium materials. The process of stabilizing As(V) (F3; anion exchange) with phosphogypsum is faster and easier compared to that with calcite (F4; bind to carbonate), but it results in a less stable form. New Ca-As(V) minerals (Ca52(HAsO4)x(AsO4)∙yH2O, Ca5H2x(AsO4)∙yH2O, or Ca32(AsO4)∙10 H2O) were identified in X-ray diffraction (XRD) patterns with calcite treatment. Precipitation, the primary mechanism induced by calcite, was activated at a soil pH above 8.0. Based on the deconvolution of calcium and sulfur X-ray photoelectron spectroscopy spectra and the peak shift in the XRD pattern in phosphogypsum, the substitution in which SO42- is exchanged with HAsO42- is the primary mechanism for As(V) immobilization. Substitution induced by phosphogypsum is a suitable reaction in upland arable soils, the predominant form of As(V) in the soil, with a pH range of 5-7.

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.