Browsing by Person "Wanke, Daniel"

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Increasing soil phosphate availability and phosphate fertilizer efficiency
(2023) Wanke, Daniel; Müller, Torsten
Phosphate is an important nutrient for agricultural animal and plant production. However, phosphate resources are limited, and unevenly distributed worldwide. Furthermore, accessing phosphate in the soil solution is complicated by the fact that soil phosphate solution concentrations generally remain low because the majority of soil phosphate is adsorbed to soil particles. Therefore, to efficiently utilize P, it is important to have an optimal fertilizer strategy where plants receive only the amount of phosphate needed to be adequately fertilized while minimizing soil phosphate accumulation. To determine precise fertilizer recommendations, phosphate extraction and detection methods are needed which indicate a high correlation between extracted soil phosphate and phosphate plant uptake. During the last decades, inductively coupled plasma optical emission spectroscopy (ICP OES) began to be used for phosphate detection more frequently in labs and, to some extent, replaced colorimetric phosphate detection. As a consequence, the question then arose whether ICP-OES and colorimetry can be used interchangeably for fertilizer recommendations. To answer this question, the Mehlich 3 extraction method was used to investigate differences between the colorimetric phosphate detection (Col-P) and phosphate detection with ICP-OES (ICP-P). It was found that the differences were great enough to state that Col-P and ICP-OES cannot be used interchangeably in Mehlich 3 extracts. Since Germany primarily uses the CAL extraction method, and not Mehlich 3, we chose to investigate whether there would be differences in results when using CAL extracts. The main objective of the first paper in this thesis was to investigate potential differences between Col-P and ICP-P for CAL extracts and compare the results to widely used phosphate extraction methods. Activated charcoal (AC) was used to explore whether the resulting differences could be reduced between both detection methods by the potential removal of organic phosphate from the solutions. The results of this experiment showed that among all tested extraction methods (CAL, Water, Olsen and Mehlich 3), CAL detected a low concentration of soil molybdate unreactive phosphate (MURP), and, simultaneously, a very strong correlation between ICP-P and Col-P with and without AC (-AC: RS = 0.990 and +AC: RS = 0.996). The regression line was nearly parallel to the 1:1 line, thus, making a conversion factor calculation for a broad spectrum of soils feasible. Although the correlation was less strong (-AC: RS = 0.85) between Col-P and ICP-P in Olsen extracts without an AC treatment, the addition of AC increased the correlation (+AC: RS = 0.99). Therefore, adding AC may be obligatory if a conversion factor needs to be calculated for Olsen extracts. Liquid 31P-NMR is predominantly used to differentiate between different organic phosphate (Porg) species. In addition, we used Raman spectroscopy to test if it can be used for the detection of Pi species in different soil matrices. For this purpose, Ca(H2PO4)2⋅H2O, CaHPO4 and β Ca3(PO4)2 were added to Luvos® healing earth, loess (C-horizon) and Filder loam (Ap horizon). All Pi species were detected using Raman spectroscopy which demonstrated its feasibility as an instrument to investigate different Pi species in soil, and which is expected to be useful in gaining further knowledge about the soil phosphate cycle. In the next experiment, we investigated whether the CAL extraction method is also reliable to characterize plant phosphate availability in organic agriculture. Organic farmers have frequently reported sufficient yield levels despite low or even very low soil CAL-P concentrations and, as a result, questioned the applicability of the acidic CAL method as a method for fertilizer recommendations for organic agriculture. This raised the question whether an alkaline extraction method such as Olsen would show more reliable results due to the extraction of labile Porg species. Therefore, phosphate of 22 soil samples of organic farms were extracted with different extractants (Water, CAL, Olsen, Mehlich 3, Bray P1, Bray P2, NaOH + EDTA) and phosphate was detected with ICP-OES and colorimetry. Using Olsen extracts, showed a correlation between plant offtake and Olsen-P that was very strong (RS = 0.94) compared to the very acidic extractants. Nevertheless, the correlation between plant offtake and CAL-P was also strong (RS = 0.91) despite the low extraction of Porg. Therefore, it was concluded that CAL can be used in organic agriculture for fertilizer recommendations. Furthermore, the comparison of CAL-P extracted and determined at the University of Hohenheim compared to CAL-P provided by farmers showed large discrepancies at times. Those differences may be explained by soil heterogeneity or the use of different phosphate detection methods; therefore, we recommended that soil samples be taken on a regular basis to increase the precision of the fertilizer strategy. Soil Porg may be an important phosphate source for plants. Therefore, in the fourth paper 31P NMR was used to gain an insight into how long-term compost application influence Porg in soil. Additionally, we measured phosphate turnover indicators such as phosphatase activity and microbial biomass P. Especially high compost application (treatment 400) increased the soil phosphatase activity and the inorganic phosphate (Pi) (CAL-P and Olsen P) in comparison to the control. Furthermore, 31P-NMR analysis showed that phospholipid-P and DNA-P in the treatment 400 increased due to compost application while all phosphomonoesters decreased compared to the control. These results suggest that organic fertilization in the form of compost increases mineralization processes which, in turn, reduces the phosphomonoesters.