Biochar counteracts nitrification inhibitor DMPP-mediated negative effect on spinach (Spinacia oleracea L.) growth.

The use of nitrification inhibitors (NIs) such as 3,4-dimethylpyrazole phosphate (DMPP) has been suggested to diminish agricultural soil nitrate (NO3-) loss and increase nitrogen (N) use efficiency (NUE). However, the yield of ammonium (NH4+)-sensitive plants such as spinach (Spinacia oleracea L.) may be adversely affected by the application of NIs at high N levels and, on the other hand, the efficiency of the NIs may also be affected by soil amendments such as biochar. These two issues are still not adequately addressed. The aim of this study was to evaluate the effect of different N levels including DMPP or not in a calcareous soil with and without amendment of wheat straw biochar on spinach yield, NUE, nitrate concentration of spinach leaf, activity of enzymes nitrate reductase (NR) and nitrite reductase (NiR), and soil ammonium (NH4+) and NO3- concentration under greenhouse conditions. This experiment was carried out with different N rates factor at seven levels (un-fertilized, N0; fertilized with 50 mg N kg-1 soil, N50; fertilized with 75 mg N kg-1 soil, N75; fertilized with 100 mg N kg-1 soil, N100; fertilized with N50 + DMPP; fertilized with N75 + DMPP; and fertilized with N100 + DMPP) and biochar (BC) factor at two levels (0, 0%BC; and 2% (w/w), 2%BC) with six replications over a 56-day cultivation period of spinach. Results showed that the application of DMPP had no significant effect on the yield of spinach plant at low and medium levels of N (50 and 75 mg N kg-1 soil), but decreased the yield of this plant at the higher level of N (100 mg N kg-1 soil). However, application of BC decreased the negative effect of DMPP on spinach yield as the yield in spinach plants fertilized with N75 + DMPP and N100 + DMPP significantly increased. Both application of DMPP and addition of BC to soil decreased leaf NO3- concentration by 29.2% and 16.3% compared to control, respectively. Biochar compared to control decreased NR activity by 46.3%. With increasing N rate, NR and NiR activities increased, but DMPP decreased the activities of both enzymes. Biochar reduced the efficiency of DMPP as soil NH4+ concentration was higher in the treatments containing DMPP without BC at 56 days after planting. Biochar and DMPP could increase the quality of spinach plant through decreasing the leaf NO3- concentration. In general, wheat straw biochar counteracted DMPP-mediated negative effect on growth of spinach plant at high level of N by decreasing the efficiency of this inhibitor. These results provide the useful information for managing the application rate of N fertilizers including DMPP in biochar-amended soil.

Adsorption and desorption processes of boron in calcareous soils.

Boron (B) availability is regulated by its equilibrium concentration that in turn is buffered by adsorption and desorption reactions. Ionic strength, pH, OM content, and the type and amount of minerals are the major factors affecting B sorption reactions. To evaluate the influence of calcium carbonate equivalent (CCE) and ionic strength on B chemical behavior, its adsorption and desorption isotherms were measured in eight calcareous soils differed in CCE (0-85%). Adsorption and desorption data were described by the Langmuir and the Linear adsorption equations, respectively. No statistically significant relation was found between model parameters and soil properties. However, in comparison, soils with higher reactive particles (clay and OM) and higher pH adsorbed more boron. Removing CCE from a soil sample (CCE=18%) lowered B adsorption maximum by 35%. In contrast, increasing electrolyte concentration (0.01 M NaCl) to 0.1 and 0.5M caused to increase B adsorption maximum by 30% and 75%, respectively. At the equi-molar concentration, CaCl(2) increased B adsorption stronger than NaCl. The positive effect of ionic strength was attributed to a better screening of surface charges and compaction of double layer thickness. Desorption data were deviated from adsorption isotherms only at equilibrium concentrations smaller than 2 mM. Analysis of boron solution speciation and adsorption-desorption data revealed that B is mainly adsorbed as spectroscopically proved outer-sphere complex in the studied soil samples. The experimental data and model prediction could be used to manage B bio-availability and to optimize remediation processes in calcareous soils.