Kanta Kuramochi

Effects of agricultural land-use change and forest fire on N2O emission from tropical peatlands, Central Kalimantan, Indonesia

Abstract Nitrous oxide (N2O) fluxes from tropical peatland soils were measured at a grassland, three croplands, a natural forest, a burned forest and a regenerated forest in Central Kalimantan, Indonesia. Only croplands received fertilization (665–1278 kg N ha−1 year−1). Mean annual N2O emissions from croplands were 21–131 kg N ha−1 year−1 in 2002–2003 and 52–259 kg N ha−1 year−1 in 2003–2004, and were significantly higher than the emissions from other comparable sites. Cropland N2O emissions were among the highest values reported from cultivated tropical, temperate and boreal organic soils. Mean annual N2O emissions were 7.1 (2002–2003) and 23 (2003–2004) kg N ha−1 year−1 from grassland, and were significantly higher than in natural, regenerated and burned forests (0.62, 0.40 and 0.97 kg N ha−1 year−1 in 2002–2003 and 4.4, 4.0 and 1.5 kg N ha−1 year−1 in 2003–2004, respectively). Annual N2O emissions did not differ significantly between forests in 2002–2003, but were significantly lower in burned forest in 2003–2004. Annual N2O emission was significantly correlated between years. Regression analysis revealed that annual N2O emissions in 2003–2004 were 1.9-fold the corresponding 2002–2003 value (annual precipitation of 2339 and 1994 mm, respectively). N2O fluxes were higher during the rainy season than during the dry season at all sites except the regenerated forest. N2O fluxes in cropland and grassland were significantly lower when the water-filled pore space (WFPS) was less than 60–70%, and increased with an increase in soil NO3–N concentration when WFPS exceeded this threshold. Thus, changes in soil moisture were important in controlling seasonal changes in N2O emission. Our results suggest that changing land use from forestry to agriculture will increase N2O production. The effect of forest fires on N2O emission from these soils was not clear.

Hydrological process controls on nitrogen export during storm events in an agricultural watershed

Abstract The dynamic characteristics of nitrogen (N) and suspended solids (SS) were investigated in stream water during four storm events in 2003 in the Shibetsu watershed, eastern Hokkaido, Japan. Analysis showed that total nitrogen (TN), nitrate-N (NO- 3-N), dissolved organic nitrogen (DON), particulate nitrogen (PN) and SS concentrations all peaked sharply during the rising limb of the discharge hydrograph, but peaks in PN and SS were more significant than the peak in dissolved N. Particulate N and SS consistently displayed clockwise hysteresis with higher concentrations during rising flows, whereas NO- 3-N and DON showed different patterns among the storms depending on the antecedent soil moisture. An M (V) curve, defined as the nutrient mass distribution versus the volume of discharge, showed that a “first flush” of PN, NO- 3-N, DON and SS was observed; however, the distribution of nutrient loads in the discharge was different. Particulate N and SS had a shorter flushing characteristic time constant (t 1/e, defined as the time interval required for a decline in nutrient concentrations in discharge water to e−1 [37%] of their initial concentrations), but contributed 80% of fluxes during the first 50% of the discharge, whereas the longer flush time (t 1/e) of NO- 3-N and DON with slowly decreased concentrations led to half loads during the recession of the discharge. These data indicate that flush mechanisms might be distinguished between particulate nutrients and dissolved N. Analysis showed that the concentrations of PN and SS derived from soil erosion were related to surface run-off. In contrast, NO- 3-N originated from the near-surface soil layer associated with the rising shallow groundwater table and mainly flushed with subsurface run-off. The different flushing mechanisms implied that different watershed best management practices should be undertaken for effectively mitigating water quality degradation.

Nitrogen budget and relationships with riverine nitrogen exports of a dairy cattle farming catchment in eastern Hokkaido, Japan

Abstract Dairy farming regions are important contributors of nitrogen (N) to surface waters. We evaluated the N budget and relationships to riverine N exports within the Shibetsu River catchment (SRC) of a dairy farming area in eastern Hokkaido, Japan. Five drainage basins with variable land-cover proportions within the SRC were also evaluated individually. We quantified the net N input (NNI) to the catchment from the difference between the input (atmospheric deposition, chemical fertilizers, N fixation by crops and imported food and feed) and the output (exported food and feed, ΔS liv and ΔS hu, which are the differences between input and output in livestock and human biomass, respectively) using statistical and measured data. Volatilized ammonia (NH3) was assumed to be recycled within the catchment. The riverine export of N was quantified. Agricultural N was a dominant source of N to the SRC. Imported feed was the largest input (38.1 kg N ha−1 year−1), accounting for 44% of the total inputs, followed by chemical fertilizers (32.4 kg N ha−1 year−1) and N fixation by crops (13.4 kg N ha−1 year−1). The exported food and feed was 24.7 kg N ha−1 year−1 and the ΔS liv and ΔS hu values were 7.6 and 0.0 kg N ha−1 year−1, respectively. As a result, the NNI amounted to 54.6 kg N ha−1 year−1. The riverine export of total N from the five drainage basins correlated well with the NNI, accounting for 27% of the NNI. The fate of the missing NNI that was not measured as riverine export could possibly have been denitrified and/or retained within the SRC. A change in the estimate of the deposition rate of volatilized NH3 from 100 to 0% redeposited would have decreased the NNI by 37%, although we believe that most NH3 was likely to have been redeposited. The present study demonstrated that our focus should be on controlling agricultural N to reduce the impact of environmental pollution as well as on evaluating denitrification, N stocks in soil and the fate of NH3 volatilization in the SRC.

Nitrous oxide emission derived from soil organic matter decomposition from tropical agricultural peat soil in central Kalimantan, Indonesia

Our previous research showed large amounts of nitrous oxide (N2O) emission (>200 kg N ha−1 year−1) from agricultural peat soil. In this study, we investigated the factors influencing relatively large N2O fluxes and the source of nitrogen (N) substrate for N2O in a tropical peatland in central Kalimantan, Indonesia. Using a static chamber method, N2O and carbon dioxide (CO2) fluxes were measured in three conventionally cultivated croplands (conventional), an unplanted and unfertilized bare treatment (bare) in each cropland, and unfertilized grassland over a three-year period. Based on the difference in N2O emission from two treatments, contribution of the N source for N2O was calculated. Nitrous oxide concentrations at five depths (5–80 cm) were also measured for calculating net N2O production in soil. Annual N fertilizer application rates in the croplands ranged from 472 to 1607 kg N ha−1 year−1. There were no significant differences in between N2O fluxes in the two treatments at each site. Annual N2O emission in conventional and bare treatments varied from 10.9 to 698 and 6.55 to 858 kg N ha−1 year−1, respectively. However, there was also no significant difference between annual N2O emissions in the two treatments at each site. This suggests most of the emitted N2O was derived from the decomposition of peat. There were significant positive correlations between N2O and CO2 fluxes in bare treatment in two croplands where N2O flux was higher than at another cropland. Nitrous oxide concentration distribution in soil measured in the conventional treatment showed that N2O was mainly produced in the surface soil down to 15 cm in the soil. The logarithmic value of the ratio of N2O flux and nitrate concentration was positively correlated with water filled pore space (WEPS). These results suggest that large N2O emission in agricultural tropical peatland was caused by denitrification with high decomposition of peat. In addition, N2O was mainly produced by denitrification at high range of WFPS in surface soil.

Modeling the Water Balance Processes for Understanding the Components of River Discharge in a Non-conservative Watershed

The study was conducted in the Shibetsu watershed, eastern Hokkaido, Japan, to examine the possibility of using the Soil and Water Assessment Tool (SWAT) model in a non-conservative watershed (the surface watersheds are lying on a discontinuous impervious horizon) with external contribution (EXT). After confirming the capability of model simulation, the EXT was estimated to understand the components of river discharge. The EXT is difficult to measure directly and simulate by SWAT due to its subsurface circulation. In this study, the EXT was roughly estimated from the water balance equation using measured data. The average daily flux of EXT (1.38 mm d-1) was assumed as a point-source discharge in SWAT. The simulation of daily streamflow during the calibration and validation periods produced satisfactory results, with R2 values of 0.65 and 0.66, respectively. In addition, the simulated daily baseflow, monthly streamflow, surface runoff, and evapotranspiration (ET) all showed good agreement with the corresponding observations. Our simulation suggested that the EXT assigned as the assumed discharge in SWAT can help us to reasonably simulate the streamflow in the Shibetsu watershed. The EXT was then investigated indirectly by considering the difference between the observed streamflow and simulated streamflow using calibrated SWAT without adding the assumed EXT. The result indicated that the EXT was an important water source in the Shibetsu watershed, accounting for 47% of streamflow during the study period.

Influence of Agricultural Activity on Nitrogen Budget in Chinese and Japanese Watersheds

Abstract To analyze the effect of agricultural activity on nitrogen (N) budget at the watershed scale, a comparative study was conducted at two Japanese watersheds, the Shibetsu River watershed (SRW) and Upper-Naka River watershed (UNRW), and one Chinese watershed, the Jurong Reservoir watershed (JRW). The total area and the proportion of agricultural area (in parentheses) of the watersheds were 685 (51%), 1 299 (21%), and 46 km2 (55%) for SRW, UNRW, and JRW, respectively. The main agricultural land use in SRW was forage grassland, while paddy fields occupied the highest proportion of cropland in UNRW (11% of total area) and JRW (31% of total area). The farmland surplus N was 61, 48, and 205 kg N ha−1 year−1 for SRW, UNRW, and JRW, respectively. The total input and output for the whole watershed were 89 and 76, 83 and 61, and 353 and 176 kg N ha−1 year−1 for SRW, UNRW, and JRW, respectively. The proportion of discharged N to net anthropogenic N input was 31%, 37%, and 1.7% for SRW, UNRW, and JRW, respectively. The two watersheds in Japan showed similar proportions of discharged N to those of previous reports, while the watershed in China (JRW) showed a totally different characteristic compared to previous studies. The high N input in JRW did not increase the amount of discharged N at the outlet of the watershed due to high proportions of paddy fields and water bodies, which was an underestimated N sink at the landscape scale.

Characteristics of nutrient load in a stream flowing through a livestock farm during spring snowmelt

Abstract We investigated the stream water quality during the snowmelt period in a livestock farm, located in Shizunai, southern Hokkaido, Japan. The water quality was very different between the early stage of the snowmelt period (March 15–21) and the later stage (March 22–April 5) in 2001. The load of nutrients (NH4 +-N, T-P, K+) was large along with the increase of the flow rate and nutrient concentrations during the early stage of the snowmelt period. The molar ratios of Si / T-N and Si / T-P of the stream water, as an index of eutrophication, were also under the threshold values (2.7, 64.3, respectively) during the early stage of the snowmelt period. In addition, the relationship between the Si and nutrient concentrations (NH4 +-N, T-P, K+) showed a significantly negative correlation (R 2 = 0.65), which indicated that the amount of nutrient load might be derived from surface runoff. Consequently both the quantity and quality of the stream water during snowmelt may exert an adverse effect on coastal waters, leading to eutrophication. Furthermore, since the main effluent source appeared to be surface runoff at the early stage of the snowmelt period, attention should be paid to land management before and during the snow covering period.

Simulation of stream nitrate-nitrogen export using the Soil and Water Assessment Tool model in a dairy farming watershed with an external water source

Quantitative assessment of stream nitrate-nitrogen (NO3-N) export in a watershed with an external underground water source is complex because the underground flux for both the water discharge and NO3-N cannot be directly measured. A modeling approach, using the Soil and Water Assessment Tool (SWAT), was tested to simulate the NO3-N export in the 672 km2 (417 mi2) Shibetsu Watershed with external water source (EXT) in eastern Hokkaido, Japan. The EXT and its NO3-N loading contents were added as the assumed point source discharge and loading in the SWAT model. The results yielded high Nash-Suttcliffe coefficient (Ens) values for daily streamflow (over 0.5) and monthly NO3-N loading (over 0.7) for the simulation during the calibration and validation periods. The simulated seasonal patterns of discharge and NO3-N loading were also well matched with the measured data during 2004 to 2008. However, the spatial patterns of NO3-N concentrations showed a poor r2 correlation (r2 = 0.34) with the measured data in 2004. These results showed that, although the model had limitations for the simulation of spatial patterns, the method of adding EXT and NO3-N loading as assumed points in SWAT was reasonable for the assessment of the stream NO3-N export. Therefore, the calibrated model was repeated without adding EXT and NO3-N loading as assumed points. Approximately 66% of the NO3-N export was from the watershed itself, and the NO3-N export was high in the grasslands. These results indicated that most of the NO3-N export was related to dairy farming in the Shibetsu Watershed, and the best management practices for controlling nonpoint source pollution should focus on manure applications on grasslands in the future. Meanwhile, the EXT (47% of streamflow) contributed to 34% of the annual NO3-N loading, which may cause the overestimation of the stream NO3-N export when using only the measured data. Therefore, the EXT should not be ignored when estimating the annual NO3-N loading.