Labile pools of organic matter and microbial biomass in the surface soils under shifting cultivation in northern Thailand

Abstract

In order to analyze the N mineralization process under shifting cultivation in northern Thailand, labile pools of soil organic matter were studied, which were considered to be the factors contributing to the N mineralization process.Organic C, (organic + NH. +)-N, and hexose-C were extracted from fresh soils in the surface 0-5 cm layers with a 0.5 M K 2 SO. solution at 110°C in an autoclave (fraction A) or at room temperature with a reciprocal shaker (fraction B), and analyzed as labile pools of organic matter. In the traditional shifting cultivation system, the content of organic C in fraction A in the fallow fields for 8 to 15 y was 3,710 mg kg-l while that in the fallow fields for 1 y and 3 to 5 y was 2,640 and 2,600 mg kg-l, respectively. A high correlation was observed between the contents of the labile pool in fraction A and total soil organic matter. The ratio of the pool in fraction A to total soil organic matter apparently remained constant through the input-output balance in the pool. The content of the labile pool in fraction B was the highest among the fields cultivated for 1 y after the slash and burn practice and it decreased in the course of the fallow period. The content of organic C was 548 mg kg-l in the fields cultivated for 1 y and 235 mg kg-l in the fallow fields for 8-15 y, respectively. There was a reverse relation between the contents of the pool in fraction B and microbial biomass. Therefore, the origin of the pool in fraction B was attributed to the microbial debris associated mainly with a decrease in the soil moisture content in the dry season. On the other hand, in the relatively intensive cultivation system, there was no significant difference in the contents of the labile pools both in fractions A and B among the land use stages, suggesting that the preservation mechanism of these pools, which was observed in the traditional cultivation system, did not operate well in the intensive system. In alternative farming systems in future, it will be essential to apply organic materials to soils to supply organic matter and to maintain the microbial biomass.

Effect of burning on soil organic matter content and N mineralization under shifting cultivation system of Karen people in Northern Thailand

“…Organic C and organic N contents in fraction A were significantly correlated with the T -C and T -N contents, respectively, among the soils at all sampling times in this study (r=0.629** and 0.724**). These results are consistent with those obtained previously (Tanaka et al 1998a). However, there was no significant correlation between the amount of hexose-C in fraction A and T-C.…”

“…On the other hand, the ratio of fraction A to total organic matter decreased in the rainy season (Table 2), suggesting that fraction A was more prone to decomposition during the rainy season among the organic materials in the soils. In the previous study (Tanaka et al 1998a), the contents of total organic matter and of fraction A changed significantly during the fallow period. Therefore, in the shifting cultivation system, the contents of total organic matter and of fraction A may be used to detect changes in the qualitative properties of soil organic matter if the chemical composition could be further analyzed.…”

“…Most of the soils in the village area were classified into Ustic Dystropepts in the USDA classification system (Soil Survey Staff 1992;Funakawa et al 1997a). Detailed information on our study site and the soils was given in the previous reports (Funakawa et al 1997a, b;Tanaka et al 1997Tanaka et al , 1998a.…”

N mineralization process of the surface soils under shifting cultivation in northern Thailand

“…At the end of the dry season, a cropping field was prepared by slash and burn, and crops were seeded at the onset of the rainy season. Detailed information about the region and soils was given in the previous reports (Funakawa et al 1997a, b;Tanaka et al 1997Tanaka et al , 1998.…”

“…In the previous report (Tanaka et al 1998), we described the labile pools of organic matter in the surface 0-5 cm soil layers at the end of the dry season. In this report, we constructed statistical models for the determination of the N mineralization patterns of the soils by the incubation method and analyzed the relationship between the labile pools of organic matter and N mineralization process depending on the land use stages under shifting cultivation.…”