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Graduate School of Agriculture, Kyoto University

Kosaki Takashi^{Kadono Atsunobu, Funakawa Shinya}, Kosaki Takashi, 2003

Sakyo-ku, Kyoto, Japan

Abstract

In this study, the factors controlling readily mineralizable carbon and nitrogen content were HF, LF, clay, LF N and LF C/N. For readily mineralizable nitrogen however, the factor LF C/N played negative role, whist positive role for readily mineralizable carbon. The contribution of LF to MinC was relatively high, but not so high for MinN. These facts implying that LF caused immobilization of nitrogen. Cropland soil retained little organic matter, and thus MinC and MinN in cropland sites were much lower than in forest and grassland sites.

Readily mineralizable organic carbon

20 g aliquots of fresh soil, adjusted to a moisture content of 60% water holding capacity, were incubated at 30C in sealed plastic bottles with 1M NaOH (Anderson, 1982). CO2 trapped in the alkali solution was measured by titration after 7, 35, 63 and 133 days. The amount of CO2 released from the soil during the 133 days incubation was assumed to represent the quantity of MinC. This experiment was conducted in duplicate.

Readily mineralizable organic nitrogen

Aliquots of fresh soil equivalent to 10 g in dry weight were weighed into glass bottles and adjusted to 60% water holding capacity (Tanaka et al., 1998). The bottles were sealed with aluminum foil and incubated at 30 C for 133 days. Ammonium and nitrate ions mineralized were extracted from the soil with 50 ml of 2 M KCl solution by shaking for 1 hour. These ions were separately determined by a steam distillation method. The amount of MinN was estimated by summation of ammonium and nitrate-N.

Physicochemical analysis

Soil pH was measured with a pH meter (Iwaki glass, pH /ion meter 225) using a soil to water ratio of 1:5, and shaking for 1 hour.

Contents of light fraction (LF) and heavy fraction (HF) were determined as follows: 10 g aliquots of air-dried soil were dispersed in sodium iodide solution (1.6 g cm^-3) and then centrifuged at 3000 rpm (Strickland and Sollins, 1987). Material in the supernatant was considered to be LF (mostly partially decomposed plant residues), whereas that in the sediment was HF (more fully-decomposed residues and mineral material). Carbon and nitrogen contents in LF (LF C and LF N) were measured by dry combustion with an NC analyzer (Sumika, NC-800-13N). Carbon and nitrogen contents in HF (HF C and HF N) were determined by subtraction of LF C and N from total C and N.

Statistical analysis

Principal component analysis (PCA) was performed on the variables of soil pH, sand, silt and clay content, carbon and nitrogen content of LF and HF, C/N ratio of LF and HF, and percent of LF in total weight and total carbon and nitrogen. The factors controlling MinC and MinN were then determined by linear regression with the stepwise method, using the extracted factors. Statistical analysis was performed with SYSTAT 8.0 (SPSS Inc. 1998).

Results and Discussion

Amounts of MinC and MinN under different land use

The relationship between MinC and MinN for each land use were plotted in Fig.1.

The average values (±SD) of MinC and MinN in cropland sites were 966±543 mgC kg^-1 and 76±34 mgN kg^-1, while those in forests were 3367±1995 mgC kg^-1 and 167±96 mgN kg^-1, and in grasslands were 2334±1284 mgC kg^-1 and 169±84 mgN kg^-1. Therefore, the amounts of MinC and MinN in cropland sites were significantly lower than other two sites. Though MinC in forest sites were higher than in grasslands, MinN in these two sites were not different.

Soil properties

The average values of each soil property and their coefficients of variance (CV) are shown in Table 1.

Higher amounts of SOM were accumulated in forest sites than in grassland and cropland sites. Light fraction C (LF C) and N (LF N) in forest sites were about 5 times larger than in cropland. Janzen et al. (1992) reported that LF (< 1.7 g cm^-3) accounted for 0.2-2.4% of soil weight, 2-17% of organic carbon, and 1-12% of nitrogen in a study of Canadian crop rotations. Sollins et al. (1984) showed that LF (< 1.6 g cm^-3) in forest sites accounted for 0.8-14% of soil weight, 5-42% of C and 3-40% of N. In this study, LF in cropland sites accounted for 0.03-3.5% of soil weight, 1.1-20% of C, and 0.4-17% of N, whilst in forest these values were 0.2-18%, 4.5-63% and 2.1-61%, respectively. Since the concentrations of carbon and nitrogen in the LF were not very different between forests and cropland, these differences are due to a higher LF content in forest soils.

Factors controlling the amount of readily mineralizable organic matter

Soil properties were summarized into 6 factors by PCA, and correlation coefficients between factor scores and each soil property are shown in Table 2. Judging from the correlation between factors and soil properties, these factors were named “LF”, “clay”, “LF C/N”, “HF”, “LF N”,and “HF C/N”.

Linear regression analysis was conducted for MinC and MinN using the 6 factors. The following two equations were obtained:

MinC (mgC/kg) = 2425 + 833 (HF factor) + 831 (LF factor) +576 (clay

factor) +349 (LF N factor) +206 (LF C/N factor) (R² = 0.53)

MinN (mgN/kg) = 138 + 46 (HF factor) + 33 (LF factor) +29 (clay factor)

+27 (LF N factor) -12 (LF C/N factor) (R² = 0.61)

The correlation between LF and mineralization of C and N has been widely studied (Janzen et al., 1992; Hassink, 1995; Barrios et al., 1996). According to the coefficients in the equation above, LF and HF contributed similarly to MinC, in spite of the fact that HF C was about 7.4 times larger than LF C. This probably reflects the relatively labile nature of LF C, which is more easily decomposed than HF C. There are several reports that LF is enriched in carbohydrates relative to both the whole soil and HF (Oades, 1972; Whitehead et al., 1975; Murayama et al., 1979; Dalal and Henry, 1988).

The contribution of LF to MinN was apparently lower than that of HF however. And LF C/N factor played a negative role in MinN, while positive role in MinC. Using an anaerobic incubation technique, Boone (1994) showed that LF N was less available than HF N. Later Wharlen et al. (2000) reported that addition of LF to soil decreased net N mineralization due to N immobilization. This could explain the relatively small contribution of LF to MinN, compared to the case of MinC, i.e. LF is both a sink for MinN (through immobilization) as well as a source.

According to its coefficient in the regression equation, the clay factor played a positive role in both MinC and MinN. Generally, clay change the microenvironment of microorganisms (Zeck et al., 1997). Microorganisms are able to convert substrate C more efficiently to biomass in the presence of clays (Martin and Heider, 1986).

In this study, the average amount of MinC was higher in forest sites than in grassland, while no difference in MinN. According to the equations, only the factor “LF C/N” contribute oppositely to the amount of readily mineralizable organic matter. The clay content and LF N in these sites were no different and the amount of LF and HF were higher in forest sites (Table 2). It would be concluded that the lower value of LF C/N in grassland might cause the lower amount of MinC and the higher amount of MinN.

organic mineralized substance agricultural soil

Conclusion

In this study, the factors controlling readily mineralizable carbon and nitrogen content were HF, LF, clay, LF N and LF C/N. For readily mineralizable nitrogen however, the factor LF C/N played negative role, whilst positive role for readily mineralizable carbon. The contribution of LF to MinC was relatively high, but not so high for MinN. These facts implying that LF caused immobilization of nitrogen. Cropland soil retained little organic matter, and thus MinC and MinN in cropland sites were much lower than in forest and grassland sites.

Fig 1 Readily mineralizable C and N distribution in land use

Table 1

Soil properties of total samples and average in each land use

LF C or N (%): C and N concentration in LF;

LFWS: LF content of the soil weight;

For each variables in land use, different letter indicates significant difference (p<0.05).

Table 2

The Peason's correlation matrix between factors and variables

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