Evaluation of Phosphorus in the Soil Particulate Fraction as an Estimator of Phosphorus Mineralization
The objective of this study is to compare the P-PF content of different soils under different management practices to the P mineralization rate measured by isotopic dilution experiments.
IPNI-2014-USA-GA28
01 Mar 2014
Project Description
Introduction
Phosphorus (P) is a macronutrient involved in the energetic metabolism and in the synthesis of cellular structures of superior plants (Raghothama et al., 2005). Consequently, the availability of P controls the growth and development of all crops. It is estimated that 5,700 million hectares of soils worldwide do not have the adequate P content required to achieve optimum crop production (Batjes, 1997). To alleviate this deficiency, inorganic P fertilizers are usually used at rates based on availability tests such as those developed by Bray and Kurtz (1945), Mehlich (1984) and Olsen et al. (1954). However, all of these techniques consider only the inorganic available P pool, without taking into account the organic P fraction which can represent up to 80 % of the total P, and can become plant available through the process of mineralization. Therefore, the measurement or estimation of P mineralization is highly important to improve P fertilizer diagnostic methods.
An inconvenience that arises is that the quantification of P mineralization can be severely hampered by the high reactivity of the PO43- ions released from the organic pool to the soil solution, which are rapidly adsorbed by the soil mineral matrix. This difficulty reduces the amount of existent methods to measure mineralization when compared to other nutrients such as nitrogen (N) and sulfur (S). All of the reliable methods to quantify P mineralization are based on isotopic dilution experiments, which are expensive and time consuming.
In the last few years, the content of P in the particulate fraction of the soil (˃53μm) (P-PF) has been proposed as an estimator of P mineralization. Ciampitti et al. (2011) proved that the content of P in the soil particulate fraction is related to the content of P in unfertilized corn plants. In other words, these authors showed that there is a relationship between P-PF and the capacity of the soil organic matter to provide P during the crop cycle. As this determination is simple and economical, P-PF appears to be a promising tool to estimate P mineralization. However, the evidence that this fraction is associated with P mineralization is empirical, and it should be validated by comparison with results from isotopic dilution experiments.
Objective
The objective of this study is to compare the P-PF content of different soils under different management practices to the P mineralization rate measured by isotopic dilution experiments.
Materials and Methods
This research will be conducted using composite soil samples (0 to 20 cm) collected in 2012 from plots receiving long-term applications of poultry litter or inorganic fertilizer in Delaware, Iowa and Georgia (Table 1). After collection, the samples were air-dried, passed through a 2-mm sieve, and stored at room temperature.
Soil samples will be physically fractionated using the method proposed by Cambardella and Elliott (1992), but replacing the 0.05 M sodium hexametaphosphate [(NaPO3)6] solution with a 1 M sodium chloride (NaCl) solution as dispersing agent to avoid the addition of P. The fraction that does not pass through the 53-μm sieve will be considered as PF, and will be analyzed for total P by wet digestion.
The isotopic dilution experiment will consist of a 100-min batch experiment to quantify the isotopic dilution caused by physicochemical processes, and a 31-day incubation experiment of rewetted soil samples labeled with 33P to determined the isotopic dilution caused by mineralization and physicochemical processes (Oehl et al., 2001). The samples will be inoculated with 1 mL of a soil solution obtained from fresh soil samples in order to restore the biological activity of dried samples. The difference between the change in the specific activity of P determined in the batch experiment and the one determined in the incubation experiment will be considered as the potential of the soil to mineralize P.
Table 1. Selected soil properties from the sampled experimental sites.
| Soil | C (%) | pH | %Clay | % Sand | Mehlich P (mg kg-1) |
| Delaware 1 | 1.24 | 6.06 | 11.8 | 82.0 | 38.8 |
| Delaware 2 | 1.16 | 6.00 | 14.5 | 16.2 | 98.6 |
| Iowa | 2.30 | 5.46 | 29.2 | 42.0 | 16.5 |
| EAT | 2.64 | 5.46 | 39.9 | 43.6 | 5.5 |
The results from the P-PF and isotopic dilution analysis will be analyzed by ANOVA with site and fertilization treatments (inorganic versus poultry litter) as main factors. Effects will be considered significant at p<0.05. Regression analysis will be performed to compare the results from P-FP with those from the isotopic dilution experiment.
Deliverables
The results from this investigation will be published as a research paper in an important soil science journal, and also as an extension article in Better Crops Journal.
If P-FP proves to be an efficient estimator of P mineralization, then new research lines will be open to calibrate the results from this analysis to the response of crops to P fertilization.
Project duration: 09/01/2014 to 01/30/2015
References
Batjes, N. 1997. A world dataset of derived soil properties by FAO–UNESCO soil unit for global modelling. Soil use and management 13: 9-16.
Bray, R.H. and L. Kurtz. 1945. Determination of total, organic, and available forms of phosphorus in soils. Soil science 59: 39-46.
Cambardella, C. and E. Elliott. 1992. Particulate soil organic-matter changes across a grassland cultivation sequence. Soil Sci. Soc. Am. J. 56: 777-783.
Ciampitti, I.A., F.O. García, L.I. Picone and G. Rubio. 2011. Soil carbon and phosphorus pools in field crop rotations in Pampean soils of Argentina. Soil Sci. Soc. Am. J. 75: 616-625.
Mehlich, A. 1984. Mehlich 3 soil test extractant: A modification of Mehlich 2 extractant. Commun Soil Sci Plan 15: 1409-1416.
Oehl, F., A. Oberson, S. Sinaj and E. Frossard. 2001. Organic phosphorus mineralization studies using isotopic dilution techniques. Soil Sci. Soc. Am. J. 65: 780-787.
Olsen, S.R., C. Cole, F.S. Watanabe and L. Dean. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonateUS Department of Agriculture Washington, DC.
Raghothama, K.G., J. Sims and A. Sharpley. 2005. Phosphorus and plant nutrition: an overview. Phosphorus: agriculture and the environment: 355-378.