Seeding and Harvesting Times and Climate Conditions Are Important for Improving Nitrogen and Fiber Contents of Green Manure Sunn Hemp
1
NARO Kyushu Okinawa Agricultural Research Center, Suya 2421, Koshi, Kumamoto 861-1192, Japan
2
Department of Agriculture, Tokai University, Toroku 9-1-1, Higashi-ku, Kumamoto 862-8652, Japan
3
Range Cattle Research and Education Center, University of Florida, Ona, FL 33865, USA
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(9), 7103; https://doi.org/10.3390/su15097103
Submission received: 15 March 2023
/
Revised: 10 April 2023
/
Accepted: 20 April 2023
/
Published: 24 April 2023
Abstract
:The efficient use of green manure enhances sustainable nitrogen (N) cycling in agroecosystems. Appropriately utilizing the differences in N and fiber of green manure between cultivars or harvesting times strengthens the N use efficiency for subsequent crops. Therefore, it is necessary to clarify these differences. We clarified differences in nitrogen and fibers from four sunn hemp (Crotalaria juncea L.) commercial cultivars at two harvesting times after spring and summer seeding for three years. Only a few significant differences were found for any components between the four commercial cultivars. Although sunn hemp produced almost no flowers after the May seeding, the earliest flower opened at 50 days after seeding (DAS) and then significantly increased by 60 DAS (p < 0.05), after the July seeding. Sunn hemp DMY at 60 DAS had significantly larger values than those at 50 DAS all three years after the May and July seedings (p < 0.01). Significant differences between 50 DAS and 60 DAS were also observed for more than two years for the sunn hemp total carbon (TC), total nitrogen (TN), C/N ratio (CN), nitrate N (NO3-N), acid detergent fiber (ADF), and neutral detergent fiber (NDF) (p < 0.05). The sunn hemp N and ADF yields were significantly greater at 50 DAS than at 60 DAS in 2019 and 2020 (p < 0.05). These results suggest that the seeding and harvest time are important for decomposition estimations, although the differences between cultivars with equivalent flowering characteristics were not worth considering. Additionally, significant differences between years were found for all measurements except for NDF after the July seeding. This result suggests that the yearly effect of the difference in weather conditions, which causes differences in the yield, nitrogen, and ADF contents, is also an important factor for decomposition.
1. Introduction
The overuse of chemical fertilizers causes environmental losses, such as high N2O emissions [1] and nitrate leaching, leading to global warming and soil acidification [2]. For better nitrogen (N) cycling with lower N pollution and greenhouse gas emissions, many researchers have attempted to improve the N fertilizer use efficiency [3,4]. On the other hand, green manure was used as a substitute for inorganic fertilizers to reduce chemical inputs in agriculture [5]. In particular, using biological nitrogen-fixing legume cover crops contributes to N sustainability as part of a sustainable agricultural system [6]. Green manure plays a role in long-term sustainable land management and climate change mitigation, with lower N2O emissions per unit of N input of leguminous green manure than chemical fertilizer [7]. Sunn hemp (Crotalaria juncea L.), a rapidly growing shrubby legume of the Crotalaria spp., is used worldwide as a green manure for increasing the N fertility of soil and nematode suppression. Although sunn hemp is a tropical legume, it is used in the tropics, subtropics, and temperate zones as an annual summer legume [8,9].
Decomposition and nutrient release patterns must be achieved to improve the synchrony of nutrient release from green manure with crop nutrient demands [10]. This improvement of the synchrony is a key point of N sustainability. Decomposition and N release essentially depend on the chemical composition of decomposed plants [11]. A low C/N ratio favors the initial rapid decomposition of the cover crops [12]. Baitsaid et al. [13] described that for slower-growing crops such as perennials, or if the same area is to be used for cropping over successive years, the benefits from the greater biomass and total N content added to the soil could be obtained from the standard practice of mowing when the sunn hemp is 70–120 days old, despite the slower N mineralization rate. The decomposition rates in the soil also differ due to the concentrations of fiber contents, such as lignin [14]. Like other green manure crops, the sunn hemp NDF and ADF contents increase with age [15]. The lower ADF and NDF contents in younger sunn hemp raised the rate of tissue breakdown compared with 42-, 77-, and 112-day-old mowing and the incorporation of sunn hemp into the soil [13]. Therefore, the N and fiber characteristics of sunn hemp change depending on age, which also affects its decomposition in soil.
For use as a green manure, sunn hemp is plowed or disced standing before it reaches the full-bloom stage and becomes too fibrous for effective use [16]. Sunn hemp has the characteristic of blooming in a short day [17]. Cultivars differ between their time to flowering (83 to more than 174 days from seeding), herbage accumulation, and N in Florida [18]. Kaneko et al. [19] also reported the differences between cultivars for the time to flowering in temperate zones. However, the differences between sunn hemp cultivars regarding N and fiber characteristic changes before and after flowering were not clarified.
Therefore, it is hypothesized that the differences in N and fiber between cultivars or harvesting times must be understood before using sunn hemp efficiently to contribute to sustainable agroecosystems in a temperate zone. The prediction of N mineralization from organic materials, including green manure, has already been launched in Japan [20,21]. However, no model has been developed that reflects the N and fiber components that accompany the growth of crops for green manure. The improvement of the model by incorporating differences in N and fiber between cultivars or harvesting times will advance the N cycle with green manure into a sustainable agricultural system. The objectives of this study were to clarify the importance of cultivar and harvesting time selection for the estimation of N decomposition based on measurements of N and fiber components.
2. Materials and Methods
2.1. Study Site
This study was conducted at Kyushu Okinawa agricultural research center, NARO, Kumamoto, Japan (32°53′ N, 130°44′ E, 78 m a.s.l.), from May 2018 through September 2020. As the average temperature and precipitation values during the experimental period, we used the values from weather observation equipment installed in the center. Monthly temperature and precipitation data are shown in Figure 1. The soil, which was heavy clay (Hydric Pachic Melanudands by Soil Taxonomy [22]), was categorized as Andosol using the Soil Classification System of Japan [23]. The soil properties at a 0–15 cm of depth were measured before this examination, as follows: the mean soil pH (in water) was 5.5; the Truog (0.002N-H2 SO4) extractable p was 2.0 mg/100 gDS; the pH 7.0 N-CH3COONH4 extractable K, Mg, and Ca were, respectively, 15.6, 9.9, and 125 mg/100 gDS; the 1N-KCL extractable NH4-N and NO3-N were 0.8 and 0.6 mg/100 gDS, respectively.
2.2. Treatments and Experiment Design
The field was divided into halves in the east and west. The May seeding was conducted on the east side. The July seeding was conducted on the west side. The treatments were a factorial combination of four sunn hemp commercial cultivars and two harvesting times in a randomized complete block design with three replicates. We used three commercial cultivars in Japan, ‘Nekobukiller (Takii and Co., Ltd., Kyoto, Japan)’, ‘Nemakorori (Snow brand Seed Co., Ltd., Sapporo, Japan’, and ‘Kobutorisou (Sakata Seed Co., Yokohama, Japan)’, and a commercial cultivar, ‘Ubon (Tropical seeds, LLC, Miami, USA)’, from the USA. The plots were 3 × 4 m2 (3 × 10 m2 only in 2020). For a basal fertilizer, a compound fertilizer (N: P2O5: K2O, 16%:16%:16%) was applied at rates equivalent to 30 g N/m2 immediately before sowing the seeds. As the seeding rate of sunn hemp, 60 kg/ha of commercial seed was adopted for the recommended rate of seed manufacture for this region. A seed bed was produced by rotary tilling and with a Cambridge roller. The seed was broadcasted and then suppressed using a roller. The seeding dates were 22 May and 20 July in 2018, 24 May and 25 July in 2019, and 25 May and 21 July in 2020.
2.3. Harvests and Measurements
The harvests were obtained around 50 and 60 days after seeding (DAS). Actually, 50 DAS was set as the first flowering after May seeding in the first year. Then, 60 DAS was set as the blooming stage. The 1 × 1 m2 quadrats were harvested at 5 cm in height from ground level for each plot at each harvest. All harvests were separated into sunn hemp and weeds (including dead material). Then, the separated sunn hemp plant length, stem diameter at 5 cm from the edge of the cut, and presence of opened flowers were measured at ten plants for each plot before drying. After drying at 70 °C for over 3 days, the dried samples were weighed to ascertain their biomass.
Dried and milled samples were used to ascertain the concentrations of total nitrogen (TN), total carbon (TC), nitrate nitrogen (NO3–N), acid detergent fiber (ADF), and neutral detergent fiber (NDF). The concentrations of TN and TC were measured using a macro coder (JM1000CN; J-science Co. Ltd., Tokyo, Japan). Furthermore, we analyzed NO3–N as the available nitrogen using an alkaline reduction diazotization method with a spectroflame photometer (SFP-3; Fujihira Industry Co., Ltd., Tokyo, Japan). ADF and NDF as green manure decomposition rate control parameters were measured using the official AOAC method [24].
2.4. Statistical Analysis
The data were analyzed using an ANOVA with the cultivars, harvesting times, their interactions, and year as fixed effects for three years. If the differences were significant (p < 0.05) based on the ANOVA, the differences between cultivars were analyzed using Tukey’s HSD test. The response variables were the plant length, stem diameter, open flower, sunn hemp DMY, TN, TC, CN, NO3–N, ADF, NDF, N yield, ADF yield, and weed mass.
3. Results
3.1. Plant Length, Stem Diameter, and Flower Opening (Table 1)
The plant length and stem diameter were investigated as growth parameters, and the flower opening was investigated as a growth stage parameter. After the May and July seedings, no significant difference was found between cultivars for the plant length, stem diameter, or flowering, except for the stem diameter for 60 DAS after the July seeding of 2020 (p < 0.05). The plant length and stem diameter after the May and July seedings for 60 DAS showed significantly larger values than those for 50 DAS all three years (p < 0.01), except after the July seeding of 2018. The plant length and stem diameter after the July seeding had larger values than those after the May seeding for all cultivars and 50 and 60 DAS, except for the stem diameter at 60 DAS in 2020. It bloomed slightly in 2018 but not in 2019 and 2020 at 50 DAS and 60 DAS for all cultivars after the May seeding. After the July seeding, the earliest flowering occurred for all cultivars by 50 DAS, except after the July seeding of 2020. The flower opening rate was significantly higher for 60 DAS than 50 DAS. Significant differences between years were found for all plant lengths, stem diameters, and flower opening measurements (p < 0.05).
Table 1.
Sunn hemp plant length, stem diameter, and flower opening values at two harvests with two seeding times.
Table 1.
Sunn hemp plant length, stem diameter, and flower opening values at two harvests with two seeding times.
| Year, Cultivars and Items | May Seeding | July Seeding | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant Length (cm) | Stem Diameter (cm) | Opened Flower (n/10 plant) | Plant Length (cm) | Stem Diameter (cm) | Opened Flower (n/10 plant) | |||||||
| 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | |
| 2018 | ||||||||||||
| Nekobukiller | 107.4 | 141.6 | 5.2 | 6.2 | 0.0 | 1.0 | 150.8 | 185.0 | 7.7 | 7.1 | 1.3 | 5.0 |
| Nemakorori | 97.9 | 148.5 | 4.8 | 5.9 | 0.0 | 0.3 | 146.6 | 176.0 | 6.9 | 6.7 | 1.7 | 4.7 |
| Kobutorisou | 114.8 | 137.2 | 5.2 | 5.7 | 0.7 | 1.0 | 157.3 | 181.0 | 7.5 | 7.4 | 2.0 | 6.0 |
| Ubon | 101.3 | 158.8 | 4.7 | 7.0 | 0.0 | 0.0 | 134.0 | 180.9 | 6.3 | 6.7 | 1.3 | 3.7 |
| p value Cultivar | 0.64 | 0.59 | 0.71 | 0.46 | 0.052 | 0.16 | 0.33 | 0.94 | 0.32 | 0.86 | 0.91 | 0.65 |
| Harvest | <0.01 | <0.01 | 0.04 | <0.01 | 0.76 | <0.01 | ||||||
| CA × HB | 0.37 | 0.33 | 0.32 | 0.66 | 0.87 | 0.86 | ||||||
| 2019 | ||||||||||||
| Nekobukiller | 94.6 | 131.2 | 5.0 | 6.5 | 0.0 | 0.0 | 121.3 | 168.6 | 6.5 | 7.9 | 0.0 | 5.3 |
| Nemakorori | 100.4 | 133.2 | 5.5 | 6.3 | 0.0 | 0.0 | 116.4 | 163.8 | 6.2 | 7.7 | 0.0 | 3.7 |
| Kobutorisou | 101.5 | 137.9 | 5.3 | 6.5 | 0.0 | 0.0 | 137.2 | 163.4 | 7.1 | 7.5 | 0.3 | 5.3 |
| Ubon | 91.9 | 129.3 | 5.2 | 6.4 | 0.0 | 0.0 | 127.9 | 154.4 | 6.5 | 7.0 | 0.5 | 5.7 |
| p value Cultivar | 0.44 | 0.14 | 0.62 | 0.87 | - | - | 0.20 | 0.75 | 0.24 | 0.39 | 0.47 | 0.46 |
| Harvest | <0.01 | <0.01 | - | <0.01 | <0.01 | <0.01 | ||||||
| C × H | 0.92 | 0.56 | - | 0.38 | 0.27 | 0.63 | ||||||
| 2020 | ||||||||||||
| Nekobukiller | 59.1 | 93.0 | 4.0 | 5.2 | 0.0 | 0.0 | 102.1 | 141.7 | 5.4 | 6.8a | 0.0 | 0.0 |
| Nemakorori | 63.2 | 121.3 | 3.2 | 5.7 | 0.0 | 0.0 | 98.7 | 143.6 | 4.2 | 5.2b | 0.0 | 4.0 |
| Kobutorisou | 68.1 | 76.0 | 3.5 | 4.5 | 0.0 | 0.0 | 98.2 | 141.3 | 4.0 | 5.4ab | 0.0 | 4.3 |
| Ubon | 55.6 | 118.3 | 3.2 | 5.4 | 0.0 | 0.0 | 101.7 | 134.7 | 4.3 | 5.3ab | 0.0 | 4.3 |
| p value Cultivar | 0.73 | 0.42 | 0.24 | 0.74 | - | - | 0.99 | 0.70 | 0.20 | 0.03 | - | 0.053 |
| Harvest | <0.01 | <0.01 | - | <0.01 | <0.01 | <0.01 | ||||||
| C × H | 0.37 | 0.40 | - | 0.89 | 0.84 | 0.03 | ||||||
| Three years | ||||||||||||
| p value Year | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | ||||||
| Year × H | 0.67 | 0.83 | 0.04 | 0.89 | 0.03 | 0.14 | ||||||
Means followed by different lowercase letters in the columns differ based on Tukey’s HSD test (p < 0.05). AC: cultivars; BH: harvest (50 DAS vs. 60 DAS).
3.2. Sunn Hemp Dry Matter Yield (DMY) and Weed Mass (Table 2)
The DMY was investigated as a biomass parameter incorporated as green manure. After the May and July seedings for the sunn hemp DMY, no significant difference was found between cultivars, except after the July seeding of 2020 (p < 0.05). There was no significant difference in weed mass between cultivars. After both the May and July seedings, the sunn hemp DMY values at 60 DAS were significantly larger than those at 50 DAS for all three years (p < 0.01). For weed mass, no significant difference was found between harvesting times (50 vs. 60 DAS) after either the May or July seeding. Significant differences between years were found for the sunn hemp DMY and weed mass measurement (p < 0.05).
Table 2.
Sunn hemp yield and weed mass values at two harvests with two seeding times.
| Year, Cultivars and Items | May Seeding | July Seeding | ||||||
|---|---|---|---|---|---|---|---|---|
| Sunn Hemp | Weed | Sunn Hemp | Weed | |||||
| (gDM/m2) | (gDM/m2) | (gDM/m2) | (gDM/m2) | |||||
| 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | |
| 2018 | ||||||||
| Nekobukiller | 221.3 | 341.0 | 77.4 | 214.7 | 453.5 | 625.1 | 7.5 | 8.3 |
| Nemakorori | 255.4 | 562.9 | 88.0 | 85.0 | 399.0 | 606.3 | 10.1 | 4.5 |
| Kobutorisou | 326.9 | 359.5 | 79.4 | 244.6 | 444.2 | 668.7 | 2.6 | 1.4 |
| Ubon | 228.3 | 497.6 | 73.4 | 66.5 | 394.2 | 584.1 | 4.9 | 3.5 |
| p value Cultivar | 0.76 | 0.37 | 0.99 | 0.24 | 0.88 | 0.78 | 0.54 | 0.36 |
| Harvest | <0.01 | 0.07 | <0.01 | 0.42 | ||||
| CA × HB | 0.39 | 0.28 | 0.98 | 0.79 | ||||
| 2019 | ||||||||
| Nekobukiller | 252.3 | 391.0 | 9.4 | 4.7 | 179.4 | 445.5 | 58.5 | 26.9 |
| Nemakorori | 326.9 | 405.5 | 4.2 | 5.3 | 181.3 | 307.5 | 13.8 | 94.2 |
| Kobutorisou | 261.1 | 459.5 | 11.7 | 5.3 | 230.7 | 366.0 | 47.9 | 43.1 |
| Ubon | 253.9 | 402.9 | 3.0 | 3.7 | 244.8 | 365.0 | 11.9 | 40.3 |
| p value Cultivar | 0.30 | 0.46 | 0.49 | 0.94 | 0.51 | 0.57 | 0.56 | 0.28 |
| Harvest | <0.01 | 0.36 | <0.01 | 0.35 | ||||
| C × H | 0.31 | 0.64 | 0.51 | 0.18 | ||||
| 2020 | ||||||||
| Nekobukiller | 153.5 | 227.6 | 5.9 | 10.5 | 232.0 | 469.2a | 71.1 | 84.5 |
| Nemakorori | 121.9 | 314.1 | 3.3 | 4.4 | 164.8 | 358.2ab | 117.0 | 179.6 |
| Kobutorisou | 131.0 | 117.0 | 2.2 | 10.8 | 202.9 | 341.0ab | 91.4 | 142.3 |
| Ubon | 85.4 | 247.1 | 7.1 | 11.1 | 176.9 | 256.7b | 120.8 | 161.6 |
| p value Cultivar | 0.68 | 0.20 | 0.68 | 0.59 | 0.66 | 0.04 | 0.53 | 0.27 |
| Harvest | <0.01 | 0.08 | <0.01 | 0.07 | ||||
| C × H | 0.20 | 0.76 | 0.30 | 0.89 | ||||
| Three years | ||||||||
| p value Year | <0.01 | <0.01 | <0.01 | <0.01 | ||||
| Year × H | 0.44 | <0.05 | 0.53 | 0.19 | ||||
Means followed by different lowercase letters in the columns differ based on Tukey’s HSD test (p< 0.05). AC: cultivars; BH: harvest times (50 DAS vs. 60 DAS).
3.3. Sunn Hemp Nitrogen, Carbon, and Fiber Contents (Table 3 and Table 4)
The carbon, nitrogen, and fiber components were investigated as parameters related to cycling and decomposition. Significant differences were found between cultivars for the sunn hemp TC, TN, CN ratio, NO3-N, ADF, and NDF contents (p < 0.05). However, these differences were not observed for more than two years.
All variables were significantly greater at 50 DAS than at 60 DAS for over two years (p < 0.05). However, no significant differences were found for TN, CN, and NO3-N after the May seeding of 2018; for NO3-N after the May and July seedings of 2019; and for ADF after the July seeding of 2020. Significant differences between years were found for the sunn hemp TC, TN, CN, NO3-N, and ADF measurements (p < 0.05).
Table 3.
Sunn hemp nitrogen, carbon, and fiber contents at two harvests of the May seeding.
| Year, Cultivars and Items | TC(%DM) | TN(%DM) | CN Ratio | NO3-N(%DM) | ADF(%DM) | NDF(%DM) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | |
| 2018 | ||||||||||||
| Nekobukiller | 44.6 | 45.7 | 2.4 | 2.2 | 19.0 | 21.0 | 0.09 | 0.05 | 41.5 | 49.8 | 49.7 | 58.5 |
| Nemakorori | 44.6 | 45.4 | 2.3 | 2.3 | 19.3 | 20.2 | 0.08 | 0.09 | 41.0 | 51.4 | 50.2 | 59.2 |
| Kobutorisou | 45.0 | 45.9 | 2.3 | 2.4 | 20.1 | 19.0 | 0.08 | 0.06 | 44.2 | 49.5 | 52.5 | 56.4 |
| Ubon | 44.6 | 45.5 | 2.5 | 2.2 | 18.0 | 21.1 | 0.10 | 0.08 | 42.1 | 51.5 | 49.3 | 59.6 |
| p value Cultivar | 0.53 | 0.19 | 0.65 | 0.60 | 0.59 | 0.69 | 0.98 | 0.35 | 0.79 | 0.37 | 0.90 | 0.36 |
| Harves | <0.01 | 0.34 | 0.17 | 0.38 | <0.01 | <0.01 | ||||||
| CA × HB | 0.86 | 0.35 | 0.39 | 0.82 | 0.55 | 0.61 | ||||||
| 2019 | ||||||||||||
| Nekobukiller | 43.9 | 44.4 | 2.8 | 2.2 | 15.8 | 20.5 | 0.22 | 0.23 | 43.1 | 51.6 | 48.2ab | 59.2 |
| Nemakorori | 43.7 | 44.8 | 2.7 | 2.3 | 16.0 | 19.8 | 0.26 | 0.25 | 44.2 | 50.7 | 49.7ab | 58.3 |
| Kobutorisou | 44.0 | 44.5 | 2.6 | 2.2 | 17.1 | 20.5 | 0.22 | 0.24 | 45.9 | 50.7 | 51.9a | 57.5 |
| Ubon | 43.6 | 44.3 | 2.9 | 2.3 | 15.2 | 19.2 | 0.23 | 0.22 | 41.5 | 49.7 | 46.8b | 56.8 |
| p value Cultivar | 0.80 | 0.70 | 0.21 | 0.84 | 0.19 | 0.83 | 0.16 | 0.73 | 0.10 | 0.80 | 0.02 | 0.72 |
| Harvest | <0.01 | <0.01 | <0.01 | 0.87 | <0.01 | <0.01 | ||||||
| C × H | 0.79 | 0.80 | 0.89 | 0.67 | 0.41 | 0.20 | ||||||
| 2020 | ||||||||||||
| Nekobukiller | 44.6 | 45.3c | 3.1 | 2.9 | 14.2 | 16.0 | 0.08 | 0.07 | 37.5 | 41.9 | 46.2ab | 49.1 |
| Nemakorori | 45.2 | 45.9b | 3.1 | 3.0 | 14.6 | 15.4 | 0.09 | 0.05 | 39.1 | 41.6 | 49.5a | 49.3 |
| Kobutorisou | 45.0 | 45.8b | 3.4 | 3.1 | 13.5 | 15.2 | 0.11 | 0.04 | 36.2 | 38.1 | 47.3ab | 48.0 |
| Ubon | 44.8 | 46.3a | 3.3 | 2.8 | 13.8 | 16.8 | 0.06 | 0.04 | 32.1 | 43.7 | 41.9b | 52.4 |
| p value Cultivar | 0.12 | <0.01 | 0.30 | 0.83 | 0.27 | 0.78 | 0.19 | 0.15 | 0.21 | 0.68 | <0.05 | 0.69 |
| Harvest | <0.01 | 0.04 | 0.01 | <0.01 | 0.02 | 0.04 | ||||||
| C × H | 0.02 | 0.76 | 0.67 | 0.12 | 0.31 | 0.10 | ||||||
| Three years | ||||||||||||
| p value Year | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | ||||||
| Year × H | 0.46 | 0.01 | 0.02 | 0.14 | 0.27 | 0.02 | ||||||
Means followed by different lowercase letters in the columns differ based on Tukey’s HSD test (p < 0.05). AC: cultivars; BH: harvest times (50 DAS vs. 60 DAS).
Table 4.
Sunn hemp nitrogen, carbon, and fiber contents at two harvests of the July seeding.
| Year, Cultivars and Items | TC (%DM) | TN (%DM) | CN Ratio | NO3-N (%DM) | ADF (%DM) | NDF (%DM) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | |
| 2018 | ||||||||||||
| Nekobukiller | 44.8 | 45.8 | 2.9 | 2.4a | 15.8 | 19.2b | 0.17 | 0.13 | 46.6 | 50.4b | 53.8 | 58.3b |
| Nemakorori | 44.8 | 46.0 | 2.8 | 2.2b | 16.5 | 21.3b | 0.16 | 0.12 | 47.1 | 52.7ab | 54.2 | 61.0ab |
| Kobutorisou | 45.1 | 46.4 | 3.1 | 1.9c | 14.8 | 24.4a | 0.16 | 0.13 | 45.3 | 55.0a | 49.7 | 64.8a |
| Ubon | 44.7 | 45.8 | 3.0 | 2.3ab | 14.9 | 20.4b | 0.16 | 0.14 | 44.9 | 52.0b | 51.3 | 60.1b |
| p value Cultivar | 0.45 | 0.27 | 0.68 | <0.01 | 0.63 | <0.01 | 0.85 | 0.59 | 0.67 | <0.01 | 0.39 | <0.01 |
| Harvest | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | <0.01 | ||||||
| CA × HB | 0.95 | 0.11 | <0.01 | 0.65 | 0.09 | 0.02 | ||||||
| 2019 | ||||||||||||
| Nekobukiller | 45.8 | 46.3a | 2.5 | 2.3 | 18.5 | 20.5 | 0.06 | 0.07 | 47.0 | 48.8 | 54.1 | 58.0 |
| Nemakorori | 44.8 | 46.3a | 2.4 | 2.0 | 18.8 | 24.6 | 0.08 | 0.04 | 46.2 | 51.1 | 54.5 | 60.4 |
| Kobutorisou | 45.6 | 46.2ab | 2.4 | 2.1 | 18.8 | 22.5 | 0.08 | 0.06 | 48.3 | 50.5 | 56.1 | 59.5 |
| Ubon | 45.5 | 46.0b | 2.5 | 2.3 | 18.1 | 20.2 | 0.08 | 0.06 | 46.6 | 49.3 | 54.9 | 57.4 |
| p value Cultivar | 0.34 | 0.03 | 0.95 | 0.62 | 0.98 | 0.58 | 0.69 | 0.49 | 0.82 | 0.79 | 0.47 | 0.75 |
| Harvest | <0.01 | 0.04 | 0.03 | 0.06 | 0.04 | <0.01 | ||||||
| C × H | 0.21 | 0.92 | 0.77 | 0.38 | 0.81 | 0.79 | ||||||
| 2020 | ||||||||||||
| Nekobukiller | 44.0b | 45.0b | 2.8 | 2.5 | 15.5 | 17.8 | 0.15a | 0.08a | 45.6 | 46.5 | 53.0 | 54.4 |
| Nemakorori | 44.4ab | 45.5a | 2.8 | 2.3 | 16.0 | 20.0 | 0.10ab | 0.03b | 46.9 | 51.2 | 55.0 | 60.1 |
| Kobutorisou | 44.7a | 45.7a | 2.9 | 2.6 | 15.6 | 17.7 | 0.10ab | 0.03b | 46.1 | 47.2 | 53.3 | 56.8 |
| Ubon | 44.5ab | 45.5a | 2.8 | 2.5 | 15.8 | 18.4 | 0.09b | 0.03b | 45.8 | 48.2 | 53.3 | 57.2 |
| p value Cultivar | 0.02 | <0.01 | 0.99 | 0.68 | 0.98 | 0.64 | <0.05 | <0.01 | 0.91 | 0.23 | 0.88 | 0.28 |
| Harvest | <0.01 | <0.01 | <0.01 | <0.01 | 0.052 | 0.02 | ||||||
| C × H | 0.92 | 0.92 | 0.82 | 0.94 | 0.65 | 0.80 | ||||||
| Three years | ||||||||||||
| p value Year | <0.01 | <0.01 | <0.01 | <0.01 | 0.02 | 0.22 | ||||||
| Year × H | 0.21 | 0.01 | 0.08 | <0.01 | <0.01 | <0.01 | ||||||
Means followed by different lowercase letters in the columns differ by Tukey’s HSD test (p < 0.05). AC: cultivars; BH: harvest times (50 DAS vs. 60 DAS).
3.4. Sunn Hemp Nitrogen Yield and ADF Yield (Table 5)
Nitrogen and ADF yields were investigated as a of N and fiber mass parameter incorporated as green manure. After the May and July seedings for sunn hemp N and ADF yields, no significant difference was found between cultivars except N yield after the July seeding of 2020 (p < 0.05). Sunn hemp N and ADF yields were significantly greater at 50 DAS than at 60 DAS (p < 0.05) except for the N yield after the May and July seedings of 2018 and ADF yield after the May seeding of 2018. Significant differences between years were found for sunn hemp N and ADF yield measurement (p < 0.05).
Table 5.
Sunn hemp nitorgen and ADF yields at two harvests with two seeding times.
| Year, Cultivars and Items | May Seeding | July Seeding | ||||||
|---|---|---|---|---|---|---|---|---|
| Nitrogen Yield | ADFA Yield | Nitrogen Yield | ADF Yield | |||||
| (gN/m2) | (gADF/m2) | (gN/m2) | (gADF/m2) | |||||
| 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | 50 DAS | 60 DAS | |
| 2018 | ||||||||
| Nekobukiller | 5.3 | 7.6 | 91.8 | 169.7 | 13.0 | 15.0 | 211.2 | 314.8 |
| Nemakorori | 5.9 | 12.7 | 104.7 | 289.4 | 11.0 | 13.1 | 188.1 | 319.7 |
| Kobutorisou | 7.4 | 8.7 | 144.5 | 178.1 | 13.6 | 12.7 | 201.4 | 367.7 |
| Ubon | 5.7 | 10.8 | 96.0 | 256.4 | 11.8 | 13.1 | 177.2 | 303.9 |
| p value Cultivar | 0.84 | 0.53 | 0.74 | 0.30 | 0.55 | 0.57 | 0.90 | 0.55 |
| Harvest | 0.63 | 0.45 | 0.24 | <0.01 | ||||
| CB × HC | 0.59 | 0.32 | 0.56 | 0.81 | ||||
| 2019 | ||||||||
| Nekobukiller | 7.0 | 8.6 | 108.7 | 201.7 | 4.5 | 10.2 | 84.4 | 217.5 |
| Nemakorori | 8.9 | 9.2 | 144.5 | 205.6 | 4.4 | 6.1 | 83.7 | 157.2 |
| Kobutorisou | 6.7 | 10.0 | 119.7 | 232.9 | 5.6 | 7.6 | 111.5 | 185.0 |
| Ubon | 7.3 | 9.3 | 105.3 | 200.4 | 6.2 | 8.5 | 114.0 | 179.8 |
| p value Cultivar | 0.26 | 0.63 | 0.27 | 0.55 | 0.49 | 0.16 | 0.54 | 0.74 |
| Harvest | <0.01 | <0.01 | <0.01 | <0.01 | ||||
| C × H | 0.30 | 0.45 | 0.17 | 0.71 | ||||
| 2020 | ||||||||
| Nekobukiller | 4.8 | 6.6 | 57.6 | 95.5 | 6.6 | 11.9a | 105.9 | 218.2 |
| Nemakorori | 3.8 | 9.3 | 47.6 | 130.7 | 4.6 | 8.3b | 77.3 | 183.4 |
| Kobutorisou | 4.4 | 3.6 | 47.4 | 44.5 | 5.8 | 8.9ab | 93.5 | 160.9 |
| Ubon | 2.8 | 6.9 | 27.4 | 108.0 | 5.0 | 6.4b | 81.0 | 123.8 |
| p value Cultivar | 0.68 | 0.13 | 0.63 | 0.29 | 0.69 | <0.01 | 0.68 | 0.24 |
| Harvest | 0.02 | <0.01 | <0.01 | <0.01 | ||||
| C × H | 0.13 | 0.27 | 0.26 | 0.50 | ||||
| Three years | ||||||||
| p value Year | <0.01 | <0.01 | <0.01 | <0.01 | ||||
| Year × H | 0.35 | 0.14 | 0.26 | 0.20 | ||||
AADF: Acid detergent fiber. BC: Cultivars CH: Harvest times (50 DAS vs. 60 DAS).
4. Discussion
After the July seeding of 2020, the sunn hemp growth was suppressed by drought. It was suggested that this suppression differed depending on the cultivars, causing the growth parameter differences between cultivars. After the May seeding of 2020, a sunn hemp growth delay occurred because of low air temperatures and heavy rain after 35 DAS. Similarly, after the July seeding of 2019, a sunn hemp growth delay occurred because of low air temperatures with heavy rain after 29 DAS. It was suggested that these growth delays provide the differences between cultivars for the carbon, nitrogen, and fiber contents.
A short-day characteristic sunn hemp cultivar planted in late April in north-central Florida (subtropics) produced very few flowers and no seed pods until late in the growing season [25]. From our results, the four commercial cultivars examined for this study showed no differences in flower opening time (Table 1). All were regarded as short-day-sensitive because they bloomed during 50–60 DAS after the July seeding in the temperate zone. This information was referred to from our earlier report [19], but that finding reconfirmed that the flowering was delayed when sown in May. Furthermore, no difference between cultivars in sunn hemp growth parameters (plant length, stem diameter) (Table 1) or DMY (Table 2) was reproducible between the three years, which indicated that the growth characteristics were also similar among cultivars, although the biomass was low in some seasons because of the weather conditions.
Our results showed a decline in TN and increase in ADF and NDF, TC, and the C/N ratio from 50 DAS to 60 DAS after the May and July seedings (Table 3 and Table 4). These findings demonstrated our hypothesis that there are differences in nitrogen and fiber before and after flowering. The whole plant TN concentration depends on the leaf/stem ratio in some legumes due to the concentrations of TN in leaves and stems differing [26]. Mansoer et al. [27] and Lepcha and Naumann [28] also reported differences in sunn hemp TN concentrations between leaves and stems. In addition, Mansoer et al. [27] reported a significant TN decrease from 3 to 6 weeks after planting and a slight TN decrease from 6 to 9 weeks after planting in both leaves and stems. Lepcha et al. [15] reported a decrease in N and increases in ADF and NDF with increasing DAS from 35 to 55. Therefore, there were changes in our results because the proportion of the stem increased by the flower stem scape elongation and the thicker stem after flowering (Table 1). On the other hand, another part of our hypothesis was that there are differences in N and fiber between cultivars, which was denied. The sunn hemp N contents decreased with greater maturity in short-day-insensitive varieties, but these contents did not decline with flowering in short-day-sensitive varieties [18]. In regions where varieties with different flowering characteristics are available, additional studies must be conducted to elucidate differences between varieties having different flowering characteristics.
If the NO3-N, the available N in the soil, is not absorbed by subsequent crops, it will be leached. In plants, NO3-N is a form of stored excess N. At the flowering stage, when the stage changes from vegetation to reproduction, the N use increases with increased reproductive activity (including flower stalk, bud, flower, and pod formation activities), even though the absorption does not increase rapidly. Regarding our results, from the differences in flowering situations between harvesting times (50 vs. 60 DAS) (Table 1), the transition to the reproductive stage progressed more after the July seeding than after the May seeding, even on the same days from seeding. This finding suggests that changes in the N budget along with the progression of flowering led to a decrease in the nitrate nitrogen concentration after the July seeding. In order to prevent a loss attributable to nitrate leaching from the soil after sunn hemp incorporation into the soil, incorporation after flowering with low NO3-N contents is recommended. In addition, our C/N ratio results showed an increase from 50 DAS to 60 DAS. A low C/N ratio enhances the initial rapid decomposition of the cover crops [12]. Our results also showed a significant increase in nitrogen yield after only 10 days of growth extension (Table 5). Therefore, it would be better to delay the incorporation of sunn hemp with an increase in the total amount of nitrogen for long-growing crops with low nitrate leaching.. This increase in the total amount of input nitrogen based on biological nitrogen fixation contributes to N sustainability strengthening in the agroecosystem through chemical fertilizer reduction.
Our result demonstrated higher ADF and NDF contents at 50 DAS than 60 DAS (Table 3 and Table 4). Moreover, the stems were thicker when sown in July, and higher ADF and NDF values in 50 DAS were found after July seeding than after May seeding. Mansoer et al. [27] demonstrated the shrubby characteristics of sunn hemp, showing that the ADF (>60%DM) and NDF (>70%DM) in the stems are higher than the ADF (<30%DM) and NDF (<40%DM) in the leaves at 9 weeks after planting. Additionally, they reported that the proportion of stems in the DMY had increased from around half at 6 weeks to over two-thirds at 9 weeks after planting. Lepcha and Naumann [28] reconfirmed these differences in ADF and NDF between the leaves and stems. They pointed out that the fiber concentration variation because of the cumulative degree days is associated with the proportions of structural tissues, e.g., xylem and sclerenchyma. From this shrubby growth, it can be inferred that the enlargement of the stems is mainly attributable to thick cell wall development. Our results can be explained based on the consideration that the enlargement of the stems is mainly attributable to cell wall development. As Baitsaid et al. [13] reported, our results for the ADF and NDF indicated that younger sunn hemp is expected to decompose faster, which suggests that the seeding and harvest times are important for the decomposition rate. However, the differences in decomposition among the four cultivars with equivalent flowering characteristics were not worth consideration.
Additionally, significant differences between years were found for all measurements except for the NDF after the July seeding (Table 1, Table 2, Table 3, Table 4 and Table 5). This suggests that the yearly effect, namely the difference in weather conditions, causes differences in the yield, N, and fiber composition. The differences between years included drought and heavy rain. In the future, when extreme weather is expected to increase, the yearly effect will also be a significant factor for decomposition. Although there has been an attempt to determine a key parameter related to the half-life degradation time of legumes used as green manure by directly using patterns identified by association rule networks [29], the seeding time, harvest time, and year (weather) were not listed as parameters. The seeding time, harvest time, and weather were considered important factors for the nitrogen mineralization prediction model in green manure.
5. Conclusions
No reproducible difference was found for any components of the four commercial cultivars with equivalent flowering characteristics. On the other hand, a decrease in N content and increases in biomass and fiber, along with increased growth periods after the May and July seedings, were demonstrated. Additionally, significant differences between years were found for all measurements. Based on these results, improving the nitrogen mineralization prediction model in green manure by adding seeding and harvest times and weather conditions would enhance the sustainable N cycling in agroecosystems.
Author Contributions
Conceptualization, M.K., I.H., and J.M.B.V.; methodology, M.K. and J.M.B.V.; investigation, M.K., N.K, I.H., and M.M.; data curation, M.K.; writing—original draft preparation, M.K.; writing—review and editing, N.K., M.M., and J.M.B.V.; project administration, M.K.; funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by JSPS KAKENHI Grant Number JP19K06008.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Acknowledgments
We thank S. Ishimatsu and S. Ogata for the field management and measurements during the study.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Average temperature and precipitation during the experiment period.
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Kaneko, M.; Kato, N.; Hattori, I.; Matsuoka, M.; Vendramini, J.M.B. Seeding and Harvesting Times and Climate Conditions Are Important for Improving Nitrogen and Fiber Contents of Green Manure Sunn Hemp. Sustainability 2023, 15, 7103. https://doi.org/10.3390/su15097103
AMA Style
Kaneko M, Kato N, Hattori I, Matsuoka M, Vendramini JMB. Seeding and Harvesting Times and Climate Conditions Are Important for Improving Nitrogen and Fiber Contents of Green Manure Sunn Hemp. Sustainability. 2023; 15(9):7103. https://doi.org/10.3390/su15097103
Chicago/Turabian StyleKaneko, Makoto, Naoki Kato, Ikuo Hattori, Makoto Matsuoka, and Joao M. B. Vendramini. 2023. "Seeding and Harvesting Times and Climate Conditions Are Important for Improving Nitrogen and Fiber Contents of Green Manure Sunn Hemp" Sustainability 15, no. 9: 7103. https://doi.org/10.3390/su15097103
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