Nitrogen Budget in a Paddy-Upland Rotation Field with Soybean Cultivation

*Fumiaki Takakai, Takemi Kikuchi, Tomomi Sato, Masato Takeda, Saki Kanamaru, Yasuhiro Aono, Shinpei Nakagawa, Kentaro Yasuda, Takashi Sato and Yoshihiro Kaneta*

### **Abstract**

To reduce the over-production of rice, the paddy-upland rotation system, which alternates every few years between paddy rice cultivation and upland crop cultivation in drained (converted) paddy fields, is now commonly practiced in Japan. Recently, depletion of available soil nitrogen (N) and a subsequent decline in soybean yield in converted upland fields with repeated rotation have been reported in northern Japan. To evaluate the N budget in the paddy-upland rotation field with soybean and rice, a 6-year lysimeter experiment was conducted. In the rotation system, a considerable loss of N occurred in both the upland soybean and paddy rice cultivation periods (−11.9 and − 2.3 g N m−2 y −1, respectively). To mitigate the N loss in the rotation system, N supply from organic matter application is required. The effects of applying different types of organic matter (leguminous green manure, hairy vetch, and livestock manure compost) on the N budget in soybean cultivated fields were investigated. Compared to the N loss in the control plot without organic matter application, the N loss was mitigated in the hairy vetch plot, and N accumulation occurred in the livestock manure compost plot (−13.7, −3.5, and +11.8 g N m−2 y −1, respectively).

**Keywords:** flooded paddy rice, hairy vetch, livestock manure compost, nitrogen budget, organic matter application, paddy-upland rotation, upland soybean

### **1. Introduction**

In Japan, rice production has been restricted for more than 40 years due to declining rice consumption. The area planted to paddy rice during summer, which was more than 3 million ha in the 1960s, has continued to decline since 1970, reaching 1.58 million ha in 2019 (**Figure 1**) [1, 2]. As a countermeasure, crop rotation in shifting the cultivation of paddy fields to crops other than staple food rice (crop rotation) has been implemented in earnest since 1970. As of 2019, 18% of the total paddy area was planted with crops other than paddy rice in the summer (**Figure 1**).

**Figure 1.** *Trends in crop cultivation in paddy fields during the summer season in Japan. Source: [2].*

One of the systems of crop rotation is "paddy-upland rotation," in which paddy fields are planted with rice and upland crops alternately for one to several years. Although there are no statistics on how much of the total area is under the paddyupland rotation, the rotations have become a major cultivation system for crop rotation. In Japan, a three-crop in two-year rotation system: paddy rice in summer, followed by wheat or barley from autumn to next early summer, and then soybean cultivation in summer has been conducted. On the other hand, in northern Japan, where it is relatively cold, a rotation system with annual cropping of paddy rice and upland crops such as soybean has also been conducted.

As mentioned above, soybean is an important rotational crop cultivated in paddy fields in Japan. Of the total area under soybean cultivation in Japan, about 80% is planted in paddy fields, and 90% in the Tohoku region of northern Japan, a major paddy field area (**Table 1**) [2]. The area of soybean cultivated in paddy fields is 115,900 ha, or 29% of the total area of cultivation with crops other than paddy rice shown in **Figure 1** (403,000 ha).

While the average yield of the world's major soybean-producing countries is approaching 3 Mg ha−1, the yield in Japan has remained low at 1.55–1.65 Mg ha−1 [3, 4]. Shimada [4] pointed out that there are many factors contributing to the low soybean yield in Japan, but one of the main factors is the inhibition of N2 fixation due to wet damage and drought stress in paddy-upland rotation fields.

Soybean assimilates N from atmospheric N2 by symbiotic N2 fixation in root nodules [3]. The contribution of atmospheric N2 to the N accumulation of soybean is highly variable and depends largely on the surrounding environment such as oxygen and moisture. Yoneyama et al. [5] reported that the average percentage of soybean N accumulation derived from N2 fixation in Japan was 50%. Ohyama et al. [3] reported that the percentages of soybean N accumulation derived from N2 fixation in rotated paddy fields in Niigata, Japan ranged from 59 to 75%, whereas soybean plants require a large amount of N compared to other crops because of the large protein


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

### **Table 1.**

*Soybean cultivation area by agricultural region in Japan (2019). Source [2].*

accumulation in their seeds (about 35–40%). In order to meet this high N requirement, N derived from N2 fixation in root nodules alone is not sufficient; soybean should also absorb significant amounts of N from the soil. Then, most of the accumulated N in soybean could be removed from the field as harvested grain. Therefore, there is a possibility that N output from the soybean cultivated field exceeds the N input to the field, and thus the N loss could occur. Therefore, the N budget of a converted paddy field with soybean cultivation could be negative, indicating N loss from the field.

Recently, depletion of available soil N followed by a decline in soybean yield in a repeated paddy-upland rotation field has been reported in northern Japan [6]. Nishida et al. [7, 8] reported a decrease in available soil N with an increase in upland frequency (i.e., the number of years in soybean cultivation per total cultivation years) in fields with paddy-upland rotation in Akita, Tohoku region, northern Japan (**Figure 2**). This indicates that soybean cultivation reduces the soil N fertility of paddy-upland rotation fields. They also reported that when the upland frequency exceeded 60%, the amount of available soil N was less than the minimum value of suitable concentrations of available soil N in the paddy field (80 mg kg−1) [9]. And that the concentration of available soil N could be increased by repeated cattle manure compost application at the rate of 2–3 kg m−2. A similar trend of declining available soil N has been reported in various parts of Japan in recent years [10–13].

Takahashi et al. [14] reported that soil N fertility of a converted paddy field could be a major controlling factor for soybean yield when moisture injury is not severe. In maintaining soil N fertility in paddy-upland rotation fields with soybean cultivation, it is necessary to manage N during soybean cultivation based on the N budget. In general, the N budget in a rice paddy field is considered to be neutral [15] or slightly positive (accumulation) [16], suggesting that the N loss from the rotated paddy field occurs mainly during soybean cultivation. However, the N budget in rice paddy fields could vary widely depending on field management practices [17]. Therefore, in the paddy-upland rotation fields, the N budget including that during paddy rice cultivation should be evaluated and measures to improve the budget should be considered.

#### **Figure 2.**

*Relationship between upland frequency and available soil nitrogen (N). \*\*\*P < 0.001, \*\*P < 0.01. CMC, cattle manure compost. Broken lines indicate the minimum (80 mg kg−1) and maximum (200 mg kg−1) values of the suitable range of available soil N in paddy fields [9]. Modified from [8].*
