Abstract
Abstract. Soil organic N is largely composed of inherently biologically labile proteinaceous N and its persistence in soil is mainly explained by stabilization through binding to minerals and other soil organic matter (SOM) components at varying strengths. In order to separate kinetically different soil N fractions we hypothesize that an approach which isolates soil N fractions on the basis of bonding strength is required, rather than employing chemical agents or physical methods. We developed a sequential subcritical water extraction (SCWE) procedure at 100, 150 and 200 °C to isolate SOM fractions. We assessed these SCWE N fractions as predictors for aerobic and anaerobic N mineralization measured from 25 paddy soil cores in incubations. SCWE organic carbon (SCWE OC) and N (SCWE N) increased exponentially with the increase of temperature and N was extracted preferentially over OC. The efficiency of SCWE and the selectivity towards N were both lower in soils with increasingly reactive clay mineralogy. Stepwise linear regression found no relations between the SCWE fractions and the anaerobic N mineralization rate but instead with pH and a model parameter describing the temperature dependency of SCWE extraction. Both were linked to texture, mineralogy and content of pedogenic oxides, which suggests an indirect relation between anaerobic NH4+ release and these edaphic soil factors. N mineralization appeared to be largely decoupled from SOM quantity and quality. From the present study on young paddy soils low in pedogenic oxides and with high fixed NH4+ content we cannot infer the performance of SCWE to isolate bio-available N in more developed upland soils. There may be potential to separate kinetically different SOM pools from upland soils because 1° for aerobic N mineralization at 100–150 °C SCWE N was the best predictor; and 2° SCWE selectively extracted N over C and this preference depended on the mineralogical composition. Hence N fractions differing in bonding strength with minerals or SOM might be isolated at different temperatures, and specifically this association has frequently been found a prominent stabilization mechanism of N in temperate region cropland soils.
Highlights
Mineralization of soil organic nitrogen (N) remains a significant source of mineral N in paddy rice cultivation despite the widespread use of inorganic N fertilizers and organic N sources
This study shows that subcritical water extraction (SCWE) can be used to preferentially extract N from soil and that the quantity of extracted N increases exponentially with the increase of temperature
At 200 ◦C probably mostly clay fraction mineral bound OM is released by SCWE as seen from the negative relation between the extracts’ C : N ratio and extraction temperature
Summary
Mineralization of soil organic nitrogen (N) remains a significant source of mineral N in paddy rice cultivation despite the widespread use of inorganic N fertilizers and organic N sources. Successful indices to predict paddy soil’s N supplying capacity include N uptake by plants (Sahrawat, 1983a; Nayyar et al, 2006; Curtin and McCallum, 2004) or measurement of increases in soil inorganic N upon incubation (Augus et al, 1994; Mikha et al, 2006). Such “reference” biological methods are time-consuming and expensive with little significance for in-time fertilizer recommendation.
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