Saturday, 15 July 2006

Evaluation of Nitrogen Status in Japanese Agricultural Soils.

Shuji Sano, Laboratory of Soil Science, Graduate School of Agricuture, Kyoto Univ, Sakyo, Kyoto, Japan, Junta Yanai, Kyoto Prefectural Univ, Shimogamo Hangi-cho 1-5, Sakyo, Kyoto, 606-8522, Japan, and Takashi Kosaki, Kyoto Univ, Sakyo, Kyoto, Japan.

The Nitrogen (N) status of Japanese agricultural soils was characterized according to form and availability with reference to land use, soil type and land management. Plow layer soil samples (paddy (80) and upland (67)) were collected from agricultural fields all over Japan. To investigate an effect of continuous application of chemical fertilizer and Organic Matter (OM) on soil N fractions, soils from the field of agricultural experiment station under long-term experiment (control plot, NPK plot, NPK + OM plot, etc.) were also collected (one paddy field (non-volcanic soils) and two upland fields (volcanic soils and non-volcanic soils)). Soil N was separated into four fractions; inorganic extractable-N (Iex-N) determined by extracting with 2 mol L-1 KCl solution, fixed NH4+-N (Ifix-N), organic mineralizable-N (Omin-N) as potentially mineralizable N (N0) and organic stable-N (Osta-N) as the difference between total-N and above mentioned three fractions. Various extraction methods of labile organic-N (OrgN), such as extraction with autoclave, phosphate buffer extraction etc., were also examined. The labile N fraction (Iex-N and Omin-N) was strongly affected by the land use, whereas the stable N fraction (Ifix-N and Osta-N) was mainly determined by the soil type (Sano et al. (2004) Soil Sci. Plant Nutr., 50(4), 501-510). For example, Omin-N was higher in paddy soils (200 mg kg-1) than in upland soils (108 mg kg-1) reflecting the relatively high input of OM through not only routine farm management (e.g. rice straw input) but also natural depositions (e.g. green algae) in paddy. The effect of soil type (volcanic soils and non-volcanic soils) on Omin-N was not observed, whereas that on Osta-N was observed. The Osta-N content was higher in volcanic soils (3639 mg kg-1) than in non-volcanic soils (1632 mg kg-1). Since the 48.1% of variance of the Osta-N content was explained by the Alo content by stepwise multiple regression analysis, non-crystalline minerals rather than other factors (e.g. clay content) contributed to the content of Osta-N. Extractable OrgN most related to Omin-N depended on land use and soil type (Sano et al. (2006) Soil Sci. Plant Nutr., 52(1), in press). In paddy soils, OrgN determined by autoclave extraction showed closest relationship with Omin-N (r2=0.70** for non-volcanic soils) whereas OrgN determined by phosphate buffer extraction showed closest relationship with Omin-N in upland soils (r2=0.66** for non-volcanic soils and r2=0.72** for volcanic soils). This result suggested that the status of labile organic N varied among soils under different land use (paddy and upland). Application of mineral N fertilizer and OM affected the soil N fractions, and the tendency was different between paddy soils and upland soils. Namely, by application of mineral N fertilizer, Iex-N did not increase in paddy soils probably due to the relatively high potential of N supplying capacity from soil whereas it increased in upland soils (1.6-2.0 times lager than control plot). By application of OM (rice straw compost), organic fractions (Omin-N and Osta-N) increased in both paddy and upland. However the proportion of Omin-N to total organic-N of paddy soils was changed only slightly (13.2% and 14.8% for NPK and NPK + OM, respectively). On the other hand, that of upland soils was changed drastically (3.1% and 5.4% for NPK and NPK + OM of volcanic soils, and 6.1% and 9.9% for NPK and NPK + OM of non-volcanic soils, respectively). The low value of the proportion of Omin-N to total-N in volcanic upland soils was due to high content of total-N (nearly equal to Osta-N). This result indicated the difference in the quality of OM between paddy soils and upland soils. In conclusion, the fractionation method used in this study was suitable to characterize soil N status, and the characteristics of soil N distribution in Japanese agricultural land where paddy fields and volcanic ash soils were widely distributed were affected by land use, soil type and land management. This study provides useful information for the estimation of amount of soil N related to N dynamics in agricultural fields.

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