2.1. Site Description and Plant Cultivation
The experiment was conducted at the lysimeter plots of the Akita Prefectural Agricultural Experiment Station (39°35′ Ν, 140°12′ Ε), Akita, Japan for six years (June 2008 to May 2014). Three lysimeter plots (15 m2
in area and 2 m in depth for each plot) were filled with gray lowland soil (Eutric Fluvisols; Food and Agriculture Organization/UNESCO) with subsurface drainage at 60 cm depth in each plot. Soil fertilities differed among the plots due to preceding compost application to forage rice (Oryza sativa
L. cv. Bekoaoba) cultivation for four consecutive years (2004–2007) before this study. During the cultivation, 3.0 kg m−2
(as fresh matter) of immature or mature compost made of livestock manure (mixture of poultry/swine/cattle = 2:3:7, C/N ratios: 18.2–24.3 and 10.0–16.9, respectively) was applied to the plots (i.e., immature compost or mature compost plots, respectively) each year. In the control plot, forage rice was cultivated without compost application. Chemical fertilizers were applied to all plots equally. All treatments were conducted with one replication (lysimeter). More detailed information is provided in related papers [17
]. The chemical properties of studied soils (0–10 cm) in each plot are provided in detail by Takakai et al. [17
]. Briefly, the soil pH and cation exchange capacity (CEC) ranged from 5.6 to 6.0 and 21.8 to 23.9 cmolC
, respectively. The total nitrogen contents of the immature and mature compost plots (2.03 and 2.14 g kg−1
, respectively) were higher than that of the control plot (1.67 g kg−1
) at the beginning of the experiment. The total carbon contents of the immature and mature compost plots (25.0 and 26.4 g kg−1
, respectively) were also higher than that of the control plot (18.0 g kg−1
) (Table S1
The mean annual temperature and precipitation recorded by the automated meteorological data acquisition system (AMeDAS) at a location 5 km from the experimental field are 10.9 °C and 1775 mm, respectively (Table S2
). In this region, snowfall is generally observed from December to March.
Soybean (upland) and staple rice (paddy) was cultivated for the three consecutive years (2008–2010 and 2011–2013), respectively. Plant cultivation was conducted based on the guidelines of Akita Prefecture [19
]. All plots did not have applications of any organic materials throughout the study period. All agricultural practices (i.e., chemical fertilizers and agrochemicals) were applied to all plots equally. Detailed information about plant cultivation was described in our previous paper [18
Soybean (Glycine max (L.) Merr. cv. Ryuho) was cultivated with applying chemical fertilizer at the rate of 0 or 2 g N m−2 (ammonium sulfate) as a basal fertilizer to all plots in 2008 or 2009 and 2010, respectively. In all years, top-dressing of fertilizer was not conducted. Soybean seeds were stripe-sown (10.9 plants m−2) in early June. Ridging was conducted once at the end of June or July. Plant residue after harvesting (early October) was subsequently scattered to each plot. After the harvesting, ridges were leveled with a hoe.
Thereafter, staple rice cultivar (Oryza sativa L. cv. Yumeobako or Akitakomachi for 2011 or 2012–2013, respectively) was cultivated for three years. In the middle of May, plowing and basal fertilizer application (chemical fertilizer with the rate of 0 or 6 g N m−2 (ammonium sulfate) was applied to all plots in 2011 or 2012 and 2013, respectively), with puddling conducted. In late May, transplanting was carried out at a density of 20.8 hills m−2. In late July, a total of 3 or 2 g N m−2 of chemical fertilizer (2011 or 2012–2013, respectively) was top-dressed. During the flooding period before mid-season drainage, flooding water depth was kept around 3–5 cm by irrigation and surface drainage. Mid-season drainage was conducted from late June to the middle of July according to plant growth and field moisture condition. Intermittent drainage was carried out during the end of mid-season drainage and final drainage in late August. Harvest was conducted in late September. Rice straw after harvesting was subsequently scattered to each plot and was left until plowing in the next spring.
For the first year of soybean and rice cultivation (2008 and 2011), to avoid over-luxuriant growth of crops due to increased soil nitrogen supply caused by paddy–upland rotation, basal fertilizers were not applied based on the guidelines of Akita Prefecture [19
]. The plant nitrogen accumulations in 2008 (soybean) and 2011 (rice) were similar to or higher than those in the corresponding other two years despite no application of basal fertilizer [18
2.2. CH4 and N2O Fluxes
O fluxes were measured using a closed-chamber method based on the method described in Takakai et al. (for soybean upland [17
] and rice paddy [18
For soybean uplands, cylindrical stainless-steel chambers (18.5–21.0 cm in diameter and 25 cm in height) were used for the measurements. Two stainless-steel bases equipped with a groove for sealing by water were installed into the soil between the rows and on the rows of each plot. In total, the gas flux measurement was conducted for each plot with four replicates. After ridging, the difference in height of measurement points between “inter-rows” and “on the rows” was approximately 20 cm. During the snow period, gas fluxes from the snow surface were measured by inserting the chamber into snow directly. Measurements were conducted almost once a week during the growing period (June–October) and one to three times per month during the fallow season (December–May, including the snow period). The frequency of measurement increased during one month after fertilization, plowing and sowing. Gas samples were taken at 0 and 20 min after the chamber was closed. Soil temperature at a depth of 5 cm and volumetric soil water content at a depth of 0–6 cm was measured simultaneously with each gas flux measurement by thermometer and amplitude domain reflectometry (ADR, ML2 Theta Probe Delta-Y Devices, Cambridge, UK), respectively. The volumetric soil water content was converted into a value of water filled pore space (WFPS) by soil porosity measured using soil core samples.
For rice paddies, rectangular transparent acryl chambers (30 × 60 × 50 or 100 cm in length × width × height) were used for the measurements during the rice growing period (end of May to late September). The flux measurement was conducted with three replicates per plot. Gas samples were taken at 1, 11 and 21 min after the chamber was closed. During the fallow period, gas fluxes from soil surface were measured in the same manner with soybean upland. Measurements were conducted almost once a week during the growing period and one to three times per month during the fallow season. Soil redox potential (Eh) at a depth of 5 cm was measured using platinum-tipped electrodes and a portable Eh meter (PRN-41, Fujiwara Scientific Company Co. Ltd., Tokyo, Japan) simultaneously with each measurement of gas fluxes. After transplanting rice, three electrodes were inserted into the soil at a depth of 5 cm per plot and kept in place throughout the rice growing period. Soil temperature was also measured. To avoid any disturbance during the flux measurements, all operations were performed from a boardwalk.
The CH4 and N2O concentrations were analyzed using a gas chromatograph (GC-14B, Shimadzu, Kyoto, Japan) equipped with a flame ionization detector and an electron capture detector, respectively. CH4 and N2O fluxes were calculated using a linear regression method. Annual emissions of CH4 and N2O were calculated by integrating the daily fluxes by linear interpolation.
2.3. Soil Carbon Storage and Decrease Rate
Changes in soil carbon storage were calculated by using the soil samples obtained by Takakai et al. [18
]. Briefly, bulk soil samples were taken at three different depths (0–10, 10–20, and 20–30 cm), with three replicates conducted at each plot before the start of this study (November 2007), after 3 years of soybean cultivation (May 2011) and after 3 years of rice cultivation (April 2014). The total carbon content of air-dried, sieved (2-mm mesh) and finely ground samples were measured using an N/C analyzer (NC-900 and NC-22F, Sumika Chemical Analysis Service, Ltd., Osaka, Japan). Soil carbon storage (0–30 cm) was calculated based on soil mass [3
], using the value of bulk density in May 2008 (before plowing) as described in Takakai et al. [18
In this study, assuming that all carbon losses from soil contributed to CO2 and CH4 emissions, annual CO2 emissions from soils were calculated by subtracting annual CH4 emissions from the annual rate of carbon loss.