**2. Detailed nitrogen budget measurement in a paddy-upland rotation field with soybean cultivation**

Detailed N budget in a paddy-upland rotation field was evaluated for 6 years (3 years for upland soybean, then 3 years for flooded paddy rice) in a lysimeter plot at the Akita Prefecture Agricultural Experiment Station, located in the Tohoku region, northern Japan [18, 19]. The lysimeter was filled with soil collected from a rice paddy field on gray lowland soil, which is a major paddy soil in this region (soil texture: clay loam). No organic matter other than soybean and paddy rice residues was applied, and the crop was grown according to the guidelines for soybean and paddy rice cultivation in Akita Prefecture [20, 21]. Soybean (cv. Ryuho) was cultivated from early June to early October. Paddy rice (cv. Yumeobako or Akitakomachi) was cultivated from late May to mid-September. The major N flows of inputs (fertilizer, bulk N deposition, irrigation, and symbiotic N2 fixation in soybean) and outputs (harvested grain, leaching, surface drainage, and N2O emission) were measured (**Figure 3**). Symbiotic N2 fixation in soybean was measured using the relative ureide method [22, 23]. Other N flows were estimated from literature values. The N budget was calculated by the difference between the total input N flow and the total output N flow. Positive and negative values indicate net N accumulation and loss in the field, respectively. Nitrogen budgets were measured for 3 years in soybean and rice cultivation, respectively, and averaged to give annual values.

The average yields of soybean and rice were 341 and 519 g m−2, respectively. The yield of soybean was much higher than the average of Akita Prefecture (about 140 g m−2) [20], whereas the yield of paddy rice was lower than the target value of Akita Prefecture (570 g m−2) [21]. It could be due to severe damage by insects in the third year (422 g m−2) [19].

Among the N inputs during soybean cultivation, symbiotic N2 fixation by nodule accounted for the majority of the inputs, about over 80% (**Figure 4**). The percentages of N accumulation derived from N2 fixation for 3 years ranged from 60 to 69% [19]. On the other hand, the N input from fertilizer was about 6% of the total.

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

### **Figure 3.**

*Outline of major nitrogen (N) flows in soybean upland field and rice paddy field. NH3, ammonia volatilization; N2, dinitrogen; N2O, nitrous oxide. N2 emission via denitrification in upland was not considered in this study. Modified from [19].*

### **Figure 4.**

*Comparison of the nitrogen (N) flows and budgets in soybean and rice cultivated fields. Positive and negative values indicated N input and output, respectively. The N budget was calculated by subtracting N output from input. NH3, ammonia; N2, dinitrogen; N2O, nitrous oxide. Modified from [19].*

This is due to the fact that in soybean cultivation in converted paddy fields, the amount of N fertilizer applied is set as low as 0–2 g N m−2 to prevent over-luxuriant growth [20]. The major components of N output were harvested grain and leaching (74 and 25%, respectively). During the soybean cultivation under upland conditions, the annual N budget was negative (−11.9 g N m−2 y −1), indicating net N loss from the field.

A review of N balance in soybean cultivation reported that the mean value of partial N balance, which is calculated from the difference between the input due to N2 fixation and the output due to harvest, is close to neutral (−0.4 g N m−2 growing season−1) [24]. However, the value is likely to be negative if a detailed N balance is obtained, taking into account N losses such as leaching, as in this study. Similar to the present study, the N budget including N flow due to water movement such as leaching in a converted paddy field with soybean cultivation in Shiga, central Japan, was negative (−5.4 to −4.0 g N m−2 growing season−1) [25].

During the paddy rice cultivation, the major input N flow was fertilizer application (63%), whereas the major output N flows were harvested grain and leaching (49 and 29%, respectively; **Figure 4**). Although less than soybean cultivation, the N budget during paddy rice cultivation was also negative (−2.3 g N m−2 y −1), indicating N loss from the field. The N loss during the paddy rice cultivation could be due to the limitation of N fertilization to paddy fields converted from upland fields. In Akita Prefecture, to avoid over-luxuriant growth and lodging due to the increased N uptake, it is recommended that basal N fertilization decreases by 100% and by 50–70% in the first and second years after conversion, respectively [21].

In paddy-upland rotation fields including soybean cultivation, the field N budget in both upland soybean and paddy rice is negative, and soil N fertility is likely to decrease due to repeated rotation [6]. It will be essential to take measures to improve the N budget in paddy-upland rotation fields to maintain soil N fertility and crop productivity.
