**3. Cultivation managements for the mitigation of N loss from paddy-upland rotation fields with soybean cultivation**

As mentioned above, soybean productivity in Japan is low and needs to be improved in the future. As soybean yields increase, a corresponding increase in N loss from the field is expected [24]. Hence, measures to improve the N budget to maintain the soil N fertility become more important. To improve the N budget in the field, N inputs need to be increased.

To increase N2 fixation, which is a major N input in soybean cultivation fields, control of groundwater level in converted paddy fields has been reported to be effective [26, 27]. Deep placement application of slow-release fertilizers [3, 28–35] has been proposed as a fertilization method that does not inhibit N2 fixation in soybean. Nitrogen supply by application of organic matter is also effective in improving the N budget. Organic matter not only supplies nutrients but also affects the physical, chemical, and biological properties of the soil to promote soybean growth. The application of livestock manure compost (LMC) has been reported to increase soybean production by improving N availability and N2 fixation in soybean nodules [11, 36–39]. Green manure cultivation of N-fixing legumes is also effective in improving the N budget. In a converted paddy field, cultivation of the leguminous green manure hairy vetch (HV) before soybean cultivation has been shown to promote soybean growth by increasing N supply to the crop and improving soil physical properties [11, 40–42]. A standard application rate of 2 kg m−2 of LMC [20] is expected to supply about 20 g N m−2 of N to the field. Nitrogen supply by HV cultivated before soybean cultivation ranges from 10 to 20 g N m−2 [43, 44]. These N supplies are sufficient to compensate for the N losses during soybean cultivation described above (−11.9 g N m−2 y −1, [19]). However, increased N supply to the field may alter other N flows, such as inhibition

### *Nitrogen Budget in a Paddy-Upland Rotation Field with Soybean Cultivation DOI: http://dx.doi.org/10.5772/intechopen.103023*

of N2 fixation in nodules and increased leaching and N2O emission. Therefore, there is a need to quantitatively evaluate the effect of organic matter application on the N budget in soybean cultivation in converted paddy fields.

The preliminary results of an experiment for different types of organic matter (HV and LMC; **Figure 5**) application conducted in lysimeter plots at the Center of Field Education and Research, Faculty of Bioresource Sciences, Akita Prefectural University are reported. The results of this study are for a single year, the first year of conversion from paddy. Three lysimeter plots were filled with soil collected from a rice paddy field on gley lowland soil (Fluvic Gleysols in WRB), one of the major paddy soils in this region (soil texture: heavy clay). The three plots were designated as a control plot with no organic matter application except for crop residue, HV, and LMC plots. In the HV plot, HV was sown and cultivated after paddy rice cultivation in the autumn of the previous year, cultivated until before soybean sowing in early June, and then plowed into the soil. In the LMC plot, 2 kg m−2 of cow dung-based LMC was applied and incorporated into the soil before soybean sowing. Soybean (cv. Ryuho) was cultivated from early June to early October. No chemical N fertilizer was applied to all plots. Soybean cultivation and N budget measurements were conducted basically as in Section 2 [18, 19].

The effect of organic matter application on the N flows and budgets in a converted soybean field is shown in **Figure 6**. The soybean yields in the HV and LMC plots (358 and 343 g m−2, respectively) were higher than that in the control plot (314 g m−2) (data not shown). Although soybean in the HV plots grew more vigorously than in the other two plots, damage to harvested grain by insects was significant, and the difference in grain yield excluding damaged grains among the plots was small. The total grain yield including damaged grains, which was used in the calculation of N budget, in the control, HV, and LMC plots was 414, 550, and 443 g m−2, respectively.

The N inputs from HV and LMC application were 18.0 and 25.1 g N m−2, which were higher than the respective N input from soybean N2 fixation. Symbiotic N2 fixation by soybean nodules was lower in the HV plot (7.9 g N m−2) and higher in the LMC plot (12.1 g N m−2) compared to the control plot (10.2 g N m−2). The percentages of N accumulation derived from N2 fixation for the control, HV, and LMC plots were 30, 19, and 34%, respectively. Hairy vetch has a low C/N ratio of around 10 [43, 44], indicating rapid mineralization of its N after incorporating into the soil. Therefore, inorganic N could inhibit soybean nodule growth and N2 fixation activity [45]. On the other hand, LMC may not inhibit N2 fixation because of its slow decomposition

**Figure 5.** *Hairy vetch (left) and livestock manure compost (right).*

### **Figure 6.**

*Effect of organic matter application on the nitrogen (N) flows and budgets in a converted soybean field. Positive and negative values indicated N input and output, respectively. The N budget was calculated by subtracting N output from input. HV, hairy vetch; LMC, livestock manure compost; N2, dinitrogen; N2O, nitrous oxide.*

(mineralization). The lower amount and percentages of N accumulation derived from N2 fixation than the values shown in Section 2 (**Figure 4**, [19]) may be due to the fact that the study site was located in a reclaimed land and soil N fertility was high [46]. Therefore, soil N uptake via soybean roots could be high and dependence on symbiotic N2 fixation could be low.

The major components of N output were harvested grain and leaching (83–87% and 12–13%, respectively). The N output by harvested grain in the HV plot (29.7 g N m−2) was higher than that of the control and LMC plots (23.0 and 24.1 g N m−2, respectively) because of the higher total grain yield. The increase in leached N by organic matter application (1.5 and 0.2 g N m−2 y −1 for the HV and LMC plots, respectively) was much smaller than that of the amount of applied N. The leaching may have been low because the texture of studied soil is heavy clay and its drainage is poor.

The N budget in the control plot without organic matter application was negative (−13.7 g N m−2 y −1 ; **Figure 6**), and was similar to the value reported previously (−11.9 g N m−2 y −1 ; **Figure 4**) [19]. The N loss during soybean cultivation was mitigated by HV application, and N accumulation occurred by LMC application (−3.5 and +11.8 g N m−2 y −1, respectively). The application of HV increased N inputs, but only mitigated the N loss because it suppressed symbiotic N2 fixation and increased N output by harvested grain and leaching. The application of LMC did not suppress symbiotic N2 fixation, but rather promoted it. It was shown that different types of applied organic matter had different effects on the N budgets in converted paddy fields.

Application of organic matter during soybean cultivation in paddy-upland rotation fields can improve the N budgets. For effective use of hairy vetch, it may be necessary to consider management practice to avoid inhibition of N2 fixation and to reduce the environmental load such as leaching and N2O emission. Unlike chemical fertilizers, the N supplied to plants by organic matter such as LMC continues for

*Nitrogen Budget in a Paddy-Upland Rotation Field with Soybean Cultivation DOI: http://dx.doi.org/10.5772/intechopen.103023*

several years [47]. Nitrogen derived from organic matter applied during upland crop (soybean) cultivation is expected to affect the N budget during subsequent paddy rice cultivation. In the future, it will be necessary to evaluate the effect of organic matter application on the N budget in the entire paddy-upland rotation system, including paddy rice cultivation. Furthermore, accumulation of soil N due to continuous application of organic matter [6, 48–50] is considered to affect the field N budget. Therefore, it is necessary to evaluate the effects of organic matter application on the N dynamics and budget in the field and on crops on a long-term basis.
