Recovery of 15 N Labelled Urea as Affected by Fixation of Ammonium by Clay Minerals

Availability of Nitrogen in Soil for Irrigated Cotton Following Application of Urea and 3,4-Dimethylpyrazole Phosphate-Coated Urea in Concentrated Bands

Low nitrogen (N) fertilizer use efficiency for irrigated cotton has been attributed to the limited ability of tap roots to access N from concentrated subsurface bands, or the preferential root uptake of microbially-mineralized dissolved organic N. This work investigated how applying high-rate banded urea affects the availability of N in soil and the capacity of cotton roots to take up N. Soil was analyzed for water-extractable total dissolved N and inorganic N species after urea or urea coated with 3,4-dimethylpyrazole phosphate (DMPP) was applied at concentrations of 261, 455, 461, and 597 mg N kg^-1 of (air-dry) soil (mean bulk density: 1.01 g cm^-3). A mass balance was used to compare N applied as fertilizer and in unfertilized soil (supplied N) with the N recovered from soil within the cylinders (recovered N) at five plant growth phases. Root uptake was estimated by comparing ammonium-N (NH₄-N) and nitrate-N (NO₃-N) in soil sampled from within cylinders with soil sampled from immediately outside. Recovered N was up to 100% above supplied N within 30 days of applying urea above 261 mg N kg^-1 of soil. Significantly lower NO₃-N in soil sampled from immediately outside the cylinders suggests urea application stimulates cotton root uptake. The use of DMPP-coated urea prolonged high NH₄-N in soil and inhibited the mineralization of released organic N. These results imply the release of previously sequestered soil organic N within 30 days of applying concentrated urea enhances the availability of NO₃-N in the rhizosphere, reducing N fertilizer use efficiency.

Effects of a nitrification inhibitor on nitrogen species in the soil and the yield and phosphorus uptake of maize

Phosphorus (P) resource availability is declining and the efficiency of applied nutrients in agricultural soils is becoming increasingly important. This is especially true for P fertilizers from recycled materials, which often have low plant availability. Specific co-fertilization with ammonium can enhance P plant availability in soils amended with these P fertilizers, and thus the yield of plants. To investigate this effect, we performed a pot experiment with maize in slightly acidic soil (pH 6.9) with one water-soluble (triple superphosphate [TSP]) and two water-insoluble (sewage sludge-based and hyperphosphate [Hyp]) P fertilizers and an ammonium sulfate nitrate with or without a nitrification inhibitor (NI). The dry matter yield of maize was significantly increased by the NI with the Hyp (from 14.7 to 21.5 g/pot) and TSP (from 40.0 to 45.4 g/pot) treatments. Furthermore, P uptake was slightly increased in all three P treatments with the NI, but not significantly. Olsen-P extraction and P K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy showed that apatite-P of the water-insoluble P fertilizers mobilized during the plant growth period. In addition, novel nitrogen (N) K-edge micro-XANES spectroscopy and the Mogilevkina method showed that the application of an NI increased the fixation of ammonium in detectable hot spots in the soil. Thus, the delay in the nitrification process by the NI and the possible slow-release of temporarily fixed ammonium in the soil resulted in a high amount of plant available ammonium in the soil solution. This development probably decreases the rhizosphere pH due to release of H⁺ by plants during ammonium uptake, which mobilizes phosphorus in the amended soil and increases the dry matter yield of maize. This is especially important for water-insoluble apatite-based P fertilizers (conventional and recycled), which tend to have poor plant availability.

Soil Properties for Predicting Soil Mineral Nitrogen Dynamics Throughout a Wheat Growing Cycle in Calcareous Soils

A better understanding of the capacity of soils to supply nitrogen (N) to wheat can enhance fertilizer recommendations. The aim of this study was to assess the soil mineral N (Nmin) dynamics throughout the wheat growing season in crucial stages for the plant yield and grain protein content (GPC). To this aim, we evaluated the utility of different soil properties analyzed before sowing: (i) commonly used soil physicochemical properties, (ii) potentially mineralizable N or No (aerobic incubation), and (iii) different extraction methods for estimating No. A greenhouse experiment was established using samples from 16 field soils from northern Spain. Wheat N uptake and soil Nmin concentrations were determined at following growing stages (GS): sowing, GS30, GS37, GS60, harvest, post-harvest, and pre-sowing. Pearson’s correlation analysis of the soil properties, aerobic incubations and chemical extractions with the soil Nmin dynamics and N uptake, yield and GPC was performed. In addition, correlations were performed between Nmin and the N uptake, yield, and GPC. The dynamics of soil Nmin throughout the cropping season were variable, and thus, the crop N necessities were variable. The soil Nmin values in the early wheat growth stages were well correlated with the yield, and in the late stages, they were well correlated with GPC. N0 was correlated with the late N uptake and GPC. However, the chemical methods that avoid the long periods required for N0 determinations were not correlated with the N uptake in the late wheat growth stages or GPC. Conversely, clay was positively correlated with the late Nmin values and GPC. Chemical methods were unable to estimate the available soil N in the later stages of the growing cycle. Consequently, as incubation methods are too laborious for their widespread use, further research must be conducted.