Abstract
The use of chemical methods to assess the soil organic nitrogen (N) potentially available to plants is not a common practice in Brazil. However, associated with others, this tool might improve efficiency in the use of waste and nitrogen fertilizers. In our study, chemical methods were tested to assess potentially available soil N in samples of 17 representative soils of the western plateau of the state of Sao Paulo (10 Oxisols and 7 Ultisols). Available soil N was extracted from the collected soil samples using moderate (ISNT-Illinois Soil Nitrogen Test) and mild (hot water and heated solutions of 2 mol L-1 KCl and 0.01 mol L-1 CaCl2) extraction methods. The levels of potentially available N obtained from these chemical methods were correlated with dry matter (DM) and N uptake (Nup) by corn plants grown in pots in a greenhouse experiment carried out with the same 17 soil samples. The ISNT method showed the highest available N extraction capacity, whereas hot water showed the lowest capacity, followed closely by the hot 0.01 mol L-1 CaCl2 solution. Despite the differences among the quantities of available N extracted, the methods correlated with each other and with DM and Nup, but the values from the ISNT method showed the lowest correlation with plant variables (rDM = 0.67** and rNup = 0.81**). Procedures of extraction with water or 0.01 mol L-1 CaCl2 heated for 16 h, and 2 mol L-1 KCl heated for 4 h, resulted in similar correlation values (r) with plant DM and Nup. Thus, water (rDM = 0.77** and rNup = 0.90**) and 0.01 mol L-1 CaCl2 (rDM = 0.82** and rNup = 0.93**) heated for 16 h can be recommended as the best options for N extraction.considering the possibility for predicting N availability, lower generation of waste, and lower cost of analysis.
Highlights
The quantity of soil organic N mineralized in a specific period of time may be estimated using short-term methods of soil incubation under aerobic or anaerobic conditions (Keeney and Bremner, 1966), or long-term methods under aerobic conditions (Stanford and Smith, 1972)
The use of N-uptake by plants as reference values of soil N availability to evaluate the efficacy of chemical methods is still a classic procedure, mainly because the presence of plants alters the rate of organic N mineralization in the soil
According to measurements taken by Herman et al (2006), 9.2 mg kg-1 d-1 of mineral N was produced in the rhizosphere soil, whereas in the non-rhizosphere soil, the mineralization rate was 1 mg kg-1 d-1
Summary
The quantity of soil organic N mineralized in a specific period of time may be estimated using short-term methods of soil incubation under aerobic or anaerobic conditions (Keeney and Bremner, 1966), or long-term methods under aerobic conditions (Stanford and Smith, 1972). The long-term method of Stanford and Smith (1972) under aerobic and controlled temperature and soil moisture conditions is the most used This method allows estimation of potentially available N (N0) in soils, which may be used as reference values in studies of chemical extractants for soil organic N. This method might result in overestimated values of N0 for plants due to sample handling and ideal temperature and moisture conditions during soil incubation (Sistani et al, 2008; Yagi et al, 2009). According to measurements taken by Herman et al (2006), 9.2 mg kg-1 d-1 of mineral N was produced in the rhizosphere soil, whereas in the non-rhizosphere soil, the mineralization rate was 1 mg kg-1 d-1
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