Seasonal changes in soil properties caused by slash and burn agriculture practice in a humid temperate region of northeast Japan

ABSTRACT

We investigated the influence of slash and burn (SBA) practice on selected soil properties and its influence on the dynamics of mineral nutrients in a typical humid temperate climate region of northeast Japan. Soils were sampled from an SBA site and compared to an adjacent original forest site (control) located in Shonai area, Yamagata Prefecture, from August 2018 to August 2019 with monthly intervals for SBA and bi-monthly intervals for control, except the snow cover period. The results showed that SBA could not evidently affect soil pH, electric conductivity, available phosphorus, exchangeable cations (Ca²⁺, Mg²⁺ and K⁺), and effective cation exchange capacity. This was likely due to the low severity of fire in the SBA site. On the other hand, cultivation of turnip affected seasonal changes of soil NH₄⁺ and NO₃⁻ mainly attributed to crop absorption. Soil MBC and MBN were significantly increased after one month of burning due to available soil nutrients from plant biomass burning which stimulated microbial growth. There were no significant changes in SOC and TN between the clear-cut stage (before SBA), and after one year at the end of the study period. This was largely due to the short turnip cultivation period and the quick covering by succession plants. In conclusion, our results showed the SBA practice affected soil properties in this study was not relatively significant.

KEY WORDS:

• Biomass burning
• Japanese cedar
• slash and burn agriculture (SBA)
• soil organic matter
• soil properties

1. Introduction

Slash and burn agriculture (SBA), also known as swidden-fallow agriculture, involves intermittent felling and burning of dried forest vegetation for short-term subsistence cultivation, followed by prolonged periods of fallow in which the forest regenerates (Kleinman, Pimentel, and Bryant 1995; Cramb et al. 2009). SBA was an ancient and prevailing subsistence cultivation practice in the tropical and subtropical regions with an abundance of land and low population density. Now several SBA practices are still carried out in some tropical developing countries, such as in Papua New Guinea, Indonesia, Cameroon, and Thailand (Tanaka et al. 2001; Kikuta et al. 2018; Kukla et al. 2019; Sugihara et al. 2019). However, unlike in these regions, SBA practices in the temperate regions are rarely practiced. For example, historically distributed SBA practices in Japan as a means of subsistence in different mountainous regions for hundreds of years have vanished (Yokoyama et al. 2014). Even so, SBA is still practiced in Japan particularly in some areas to specifically preserve local cultural traditions and produce special regional products. For example, red turnip (Brassica rapa var. glabra) is produced by SBA practice in bordering area between Yamagata and Niigata Prefectures (Egashira 2006; Ohtsuka et al. 2007).

As a land-use and management change, SBA practices have been reported to affect significant positive changes in soil physicochemical and microbial properties, soil nutrient availability, and soil organic matter (SOM) (Funakawa et al. 1997; Tanaka et al. 2001; Kukla et al. 2019). However, these positive soil nutritional changes following biomass burning are short-lived as a result of, but not limited to, increased soil erosion and runoff, and nutrient leaching (Juo and Manu 1996). In addition, researchers have reported a decrease in soil organic carbon following SBA practices (Kotto-Same et al. 1997; Vashum and Jayakumar 2016). Principally, these conclusions are largely limited to the tropical and subtropical regions. Thus, the objective of this research was to understand the effects of SBA practice on main soil chemical and biological properties in the temperate region, Shonai Area, Yamagata Prefecture, northeast Japan.

2. Materials and methods

2.1. Site description

The hilly area of Fujisawa, Tsuruoka, near Yutagawa onsen, Shonai Area, Yamagata Prefecture, northeast Japan (38°40ʹ44 N; 139°46ʹ36 E) was chosen as the study site. The area is considered as a village-vicinity forest. Human settlement is surrounded by lowland rice paddies in the flat areas and SBA is practiced in the steep mountain foothills. The secondary forest planted on the slopes of the mountain foothills is dominated by Japanese cedar (Cryptomeria japonica D. Don.), and has been planted for over 30 years.

According to the Japan Meteorological Agency database for Tsuruoka Meteorological Observatory (http://www.data.jma.go.jp/obd/stats/etrn/index.php), the mean annual temperature and precipitation in the region were 12.9°C and 2,191 mm, respectively, for the 30 years average from 1991 to 2020. Annually, the study site is generally covered by thick snow for approximately 3 months from the end of December to early March (Figure S1).

2.2. Study sites designation

An approximately 20 m x 25 m rectangular forest field was selected for subsistence SBA practices to cultivate the traditional local vegetable turnip. In the summer of 2018, around the month of July, cedar trees in the selected area were felled. The big tree trunks were removed from the field while the branches and leaves were left to dry before burning. The branches and leaves biomass were an estimated 32.5 ton ha⁻¹ as modeled by Furukawa (2012). The biomass was burned in the field on 20 August 2018 for about 3 hours (08.30 am to 11.30 am). The turnip seeds were sown in the afternoon on the same day and the full turnips were harvested from October until November 2018. After the harvest, the field was covered by snow until March 2019. Then, the field was abandoned and covered by succession plants, mostly horseweed (Erigeron canadensis). An adjacent forest ― dominated by the Japanese cedar tree― was chosen as the control treatment (hereinafter forest).

2.3. Soil sampling

Soil samples for the SBA and forest sites were taken from the upper, middle, and lower sections of the slopes at the 0–5 cm soil depth in five replications. Each sampling location was marked with bamboo stakes for the subsequent sampling. For the SBA site, soil samples were taken on the 8^th August 2018 (before burning), 26^th September (after burning), and October 2018, and in 2019 on the 22^nd April, and 26^th June, July, and August. We could not take the soil sample immediately after the burning (on 20^th August 2018), since the farmer sowed the red turnip seeds few hours after the burning, we were prohibited to enter the field until one month after sowing to wait for the seedlings to grow well. For the forest site, soil samples were taken on 8^thAugust and 26^th October 2018, respectively, and on 22^nd April, and 26^th June and August in 2019. Soil sampling was discontinued from November 2018 to early April 2019 due to the intense snowfall in this area.

2.4. Soil chemical and biological analysis

Fresh soil samples collected from both sites of SBA and forest were sieved through a 2 mm grid to remove plant materials and small stones, homogenized and divided into two equal portions. From the first portions, soil pH (H₂O) and electrical conductivity (EC) were measured using a pH and EC meter (Horiba D-51, Kyoto, Japan), and the mineral N (NO₃⁻ and NH₄⁺) by colorimetric methods as detailed in JSSSPN (Japanese Society of Soil Science and Plant Nutrition) (1986) and Cheng et al. (2017). In addition, microbial biomass carbon and nitrogen (MBC and MBN) were determined using a fumigation experiment (Vance, Brookes, and Jenkinson 1987). The absorbance of NH₄⁺-N and NO₃^–_N was read by the Hitachi U-2900 Spectrophotometer (Hitachi High-Tech Science Corporation, Tokyo, Japan) at 655 nm and 540 nm, respectively. On the other hand, extracted MBC and MBN were measured using a TOC analyzer equipped with a TN detector (Shimadzu TOC-L CSH/CSN) and calculated using Equation (1)(1) $\mathrm{M}\mathrm{B}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{M}\mathrm{B}\mathrm{N}=\left(\mathrm{F}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{N}-\mathrm{U}\mathrm{n}\mathrm{f}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{N}\right)\times 2.2$(1) below;

(1) $\mathrm{M}\mathrm{B}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{M}\mathrm{B}\mathrm{N}=\left(\mathrm{F}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{N}-\mathrm{U}\mathrm{n}\mathrm{f}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{N}\right)\times 2.2$(1)

Subsequently, the second portions were air-dried in a greenhouse, sieved similar to the fresh samples and reduced to a fine powder to enable the measurements of available phosphorus (P) (Truog), cations exchange capacity (CEC) by semi-micro Schollenberger method, exchangeable cations (Ca²⁺, Mg²⁺, and K⁺), and the soil organic carbon (SOC) and total nitrogen (TN). Available P was quantified colorimetrically and read by Hitachi U-2900 Spectrophotometer (Hitachi High-Tech Science Corporation, Tokyo, Japan) at 880 nm. Each cation was measured using atomic absorption spectrophotometer (Z-5000, Hitachi, Tokyo, Japan) (JSSSPN (Japanese Society of Soil Science and Plant Nutrition) 1986; Cheng et al. 2017). The contents of SOC and TN were measured by the dry combustion method using an NC analyzer (SUMIGRAPH NC-220 F; Sumika Chemical Analysis Service Ltd, Tokyo Japan) from the soils sampled at the beginning (August 2018) and the end of this research in August 2019 only.

2.5. Statistical analysis

A one-way analysis of variance (ANOVA) was carried out to compare the means of the different sampling times. To compare mean differences at sampling times and between study sites, Bonferroni test was calculated at the 0.05 probability level. Correlation analysis was conducted using the average values of each parameter. All statistical analysis was performed in SPSS Statistics version 22.0 for Windows (IBM Corp., Armonk, NY, USA).

3. Results

3.1. Seasonal change of soil pH, EC, inorganic N, and available P

As shown in Figure 1(a,b), soil pH and EC values, respectively, at the SBA site ranged from 5.1 to 5.4 units, and 81.0 to 133.6 µS cm⁻¹, while those of the forest site ranged from 5.2 to 5.5 units, and 97.4–179.0 µS cm⁻¹. There were no significant differences of soil pH and EC between both sites of SBA and forest or among the different sampling months.

Figure 1. Seasonal changes in soil pH (H₂O) (a) and EC (b) in the top soil (0–5 cm depth). Bars indicate standard error (n = 5). The same lowercase indicated no significant differences for SBA site among different sampling periods.

At SBA site, compared to the clear-cut stage, concentrations of inorganic N (NH₄⁺-N and NO₃^–_N) decreased significantly after biomass burning in August 2018, until the thick snow cover period. The concentration of NH₄⁺-N decreased from 23.0 to 18.0 µg N g⁻¹ in October 2018 (Figure 2(a)), while that of NO₃^–_N decreased from 31.0 to 12.0 µg N g⁻¹ in October 2018 (Figure 2(b)). The concentrations of inorganic N from April 2019 to August 2019 were relatively low compared to inorganic N concentrations in August 2018 to October 2018. The concentrations of inorganic N from April 2019 to August 2019 were ranged from 2.0 to 9.0 µg N g⁻¹ for NH₄⁺-N, and from 0.1 to 2.0 µg N g⁻¹ for NO₃^–_N (Figure 2(a,b)).

Figure 2. Seasonal changes in soil NH₄⁺-N (a), NO₃^–N (b) and Available P (c) in the top soil (0–5 cm depth). Bars indicate standard error (n = 5). The lowercases indicated significant differences for SBA site among different sampling periods. The asterisk (*) indicates significant differences between SBA and forest sites.

Soil available P values for both study sites were relatively stable throughout the study period with those of SBA ranging from 15.0 to 29.0 µg P₂O₅ g⁻¹, and from 15.0 to 18.0 µg P₂O₅ g⁻¹ for the forest site (Figure 2(c)).

3.2. Seasonal change of MBC and MBN, and MBC/MBN ratio

As shown in Figure 3(a,b), MBC and MBN at SBA site were increased significantly after one month of biomass burning (September 2018), compared to the clear-cut stage in August 2018. The MBC and MBN values at the clear-cut stage were 162.0 µg C g⁻¹ and 25.0 µg N g⁻¹, respectively, while those in September 2018 were 1681.0 µg C g⁻¹ and 59.0 µg N g⁻¹, respectively. After September 2018, MBC and MBN values numerically decreased in the subsequent months, the range were 159.7 to 560.9 µg C g⁻¹ and 18.9 to 40.2 µg N g⁻¹, respectively (Figure 3(a,b)). The ratio of MBC to MBN ranged from 3.7 to 19.1 at the SBA site and 7.4 to 19.7 at the forest site, respectively (Figure 3(c)).

Figure 3. Seasonal changes in soil MBC (a), MBN (b) and MBC/MBN (c) in the top soil (0–5 cm depth). Bars indicate standard error (n = 5). The lowercases indicated significant differences for SBA site among different sampling periods. The asterisk (*) indicates significant differences between SBA and forest sites.

3.3. Seasonal change of exchangeable cations and CEC

The levels of exchangeable calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺), and the CEC for the two study sites are shown in Figure 4(a–d), respectively. Briefly, at the SBA site, the levels of exchangeable cations ranging from 13.0 to 18.0 Ca²⁺, 7.0 to 9.0 cmol kg⁻¹ for Mg²⁺, and 6.0 to 8.0 cmol kg⁻¹ for K⁺, and the effective CEC from 39.0 to 54.0 cmol kg⁻¹, were relatively stable throughout the study period. However, compared to the SBA site, intermittently higher levels of cations and CEC values were recorded in the forest site during the study period.

Figure 4. Seasonal changes in soil exchangeable Ca²⁺ (a), Mg²⁺ (b) K⁺ (c), and CEC (d) in the top soil (0–5 cm depth). Bars indicate standard error (n = 5). The lowercases indicated significant differences for SBA site among different sampling periods. The asterisk (*) indicates significant differences between SBA and forest sites.

3.4. Effect of slash and burn agriculture practice on SOC and TN

The changes in SOC and TN contents for the two study sites are presented in Table 1. At the beginning of the study (August 2018) the SOC and TN contents were 6.7% and 0.50% in the SBA site, while those were 9.2% and 0.70% in the forest site, respectively. At the end of the study (August 2019), the SOC and TN contents were 6.9% and 0.50% in the SBA site, while those were 7.8% and 0.60% in the forest site, respectively. There was no significant difference between beginning of the study in August 2018 and end of the study in August 2019 for both SOC and TN contents in each SBA and forest sites.

Table 1. Changes in soil organic carbon (SOC), total nitrogen (TN) and C/N ratio in top soil (0–5 cm) between SBA and forest sites before and after one year SBA

Year (Month)	Site	SOC		TN		C/N
		(%)
2018 (August)	SBA	6.7 ± 0.59	b	0.50 ± 0.05	a	12.7 ± 1.0	a
	Forest	9.2 ± 0.59	a	0.70 ± 0.04	a	14.0 ± 0.3	a
2019 (August)	SBA	6.9 ± 0.86	b	0.50 ± 0.05	a	13.8 ± 0.4	a
	Forest	7.8 ± 0.38	ab	0.60 ± 0.03	a	14.1 ± 0.3	a

Data are shown as mean ± standard errors, and the values within a column followed by different letters differ significantly among the study sites during the study period (T-test [P < 0.05, n = 5]).

4. Discussion

4.1. SBA effect on the seasonal change of soil chemical and biological properties

In this study, compared to the clear-cut stage in August 2018, no significant differences in soil pH, EC, available P, exchangeable cations, and effective CEC were observed in September 2018 (one month later after biomass burning). These results were contrary to several studies conducted in different tropical regions (Nye and Greenland 1964; Sanchez, Villachica, and Bandy 1983; Kyuma, Tulaphitak, and Pairintra 1985). These differences were largely attributed to: (1) the soil temperature was low during the biomass burning process, (2) the soil samples in our study were taken from the SBA site without ash depositions, and (3) the turnip cultivation period was short (2–3 months in autumn season) and aboveground and roots biomass productions of turnip were low as we saw in the field.

Although we did not measure the soil temperature during the burning process in this study, Ohtsuka et al. (2007) and Seidel et al. (2017) reported that the maximum temperature of soil during the burning process was around 71°C to 78°C at the surface soil in SBA sites in Niigata and Yamagata Prefectures, Japan. The low temperature during burning was affected by high soil water content (SWC) in this area, Ohtsuka et al. (2007) reported high soil moisture (30.0%) during burning day. In this study, SWC was 39.9% before burning and 43.3% after one month of burning (in September) (Figure S2). The temperature was significantly lower compared to those reported in the tropic by other researchers, which temperature of the surface soil were reported around 100–500°C (Giardina, Sanford, and Døckersmith 2000; Tanaka et al. 2001).

During the turnip growing period (August to October 2018), the decrease in inorganic N concentrations at the SBA site was mainly attributed to turnip absorption, though we did not measure the turnip biomass and yield in this study. These results were in agreement with previous observations by Ohtsuka et al. (2007) in Murakami, Niigata Prefecture.

Soil MBC and MBN increased significantly by 937.7% and 145.8%, respectively, one month after biomass burning (September) compared to the clear-cut stage (August 2018). Biomass burning has been reported to significantly increase readily available soil nutrients through ash deposition, consequently stimulating microbial growth (Kara and Bolat 2009). The ratio of soil MBC/MBN is an important indicator of changes in the microbial community. In this study, compared to the clear-cut stage, biomass burning significantly increased the ratio of MBC/MBN by 137.5% after one month (Figure 3(c)). The increase of MBC/MBN ratio was implied that fungi proportion was increased, while bacteria proportion was decreased in the soils on SBA site after the biomass burning (Zhou and Wang 2015). Fungi has been reported to survive as thermotolerant spores. For example, the fungi Neosartorya fischeri survives fire in the form of spores and becomes dominant in the post-fire environment because its ascospores are thermotolerant and their germination is stimulated by thermic stress (Mataix-Solera et al. 2009).

4.2. SBA effect on SOC and TN

In this study, compared to the beginning of the experiment, SBA practices did not significantly change the SOC and TN contents (Table 1). The ranges of SOC and TN contents in our study were similar to those reported by Ohtsuka et al. (2007), at 6.5% to 10.0% for SOC and 0.4% to 0.6% for TN, in one SBA site with similar tree species to our study site. These observations were attributed to the insignificant change in SOM as a result of the short duration of SBA practice. Kukla et al. (2019) reported on the influence of duration on SOM loss in SBA practices vis-à-vis the immediate effect after biomass burning. Additionally, Fujisaki et al. (2017) reported a significant decrease in SOC stocks in an SBA site after five years with no significant decreases after 18 months. Furthermore, at our SBA site, minimal to no-tillage practices were carried out during the turnip cultivation. Minimal soil disturbances may contribute to minor effects on SOM. Beare et al. (1994) reported higher soil aggregates stability affecting SOM protection loss from rapid mineralization in zero tilled soils compared to conventionally tilled sites. At SBA site, the succession plants grew rapidly right after the abandonment of the field, and at the end of the experiment (August 2019), the site was already fully covered by the vegetation. Rapid growth and coverage of succession plants at SBA site could also be another reason affecting minor SOM loss. A similar observation was reported by Kukla et al. (2019) in an SBA practice in Papua New Guinea, where the field was mostly covered by plants. The presence of the plant roots could protect the disaggregation of soil and thus significantly reduce the loss of organic matter through soil erosion. Forest as control in this study numerically decrease in SOC and TN even though is not significant, the reason was hard to explain to the lack of data and evidence. Further study is needed to understand this case.

5. Conclusion

The results in our study indicated that SBA could not obviously affect soil pH, EC, available P, exchangeable cations (Ca²⁺, Mg²⁺ and K⁺) and CEC. This was attributed to the low severity fire in this study site. Additionally, cultivation of turnip affected seasonal changes of soil NH₄⁺ and NO₃⁻ mainly due to crop absorption. Soil MBC and MBN were significantly increased after one month of burning due to available soil nutrients from plant biomass burning which stimulated microbial growth. There were no significant changes in SOC and TN between before SBA and after one year at the end of the experiment due to the short turnip cultivation period and quickly covering by succession plants. Since our investigation was one case study in a typical humid temperate climate zone with heavy snow in winter season, more investigations should be carried out in future to confirm long-term effects of SBA on soil properties, especially for the SOM change which contributes to climate change.

Acknowledgments

First author, Putu Oki Bimantara, is thankful to the Okazaki Kaheita International Scholarship Foundation for supporting his master degree studies in Japan. We are grateful to Mr. Masatoshi Goto, the SBA farmer and the students in the Plant Nutrition and Soil Science Lab of the Faculty of Agriculture, Yamagata University for helping with the soil sampling.

Disclosure statement

No potential conflict of interest was reported by the author.

Supplementary material

Supplemental data for this article can be accessed here

Additional information

Funding

This research was funded partly by the Sumitomo Foundation with ID 173275 and the Heiwa Nakajima Foundation.