Prediction of nitrogen uptake by sugar beet (Beta vulgaris L.) by scoring organic matter and nitrogen management (N-score), in Hokkaido, Japan

Abstract

To improve the prediction of nitrogen (N) uptake by sugar beet for the effective N fertilization, field experiments were carried out in 55 arable fields under various N management regimes throughout Hokkaido, Japan, from 2003 to 2006. The fields comprised nine soil types. Firstly, N uptake by sugar beet at different soil depths (0–20, 20–40, 40–60, 60–80, 80–100 cm) was examined using ¹⁵N-labeled fertilizer in Andosol fields in 2005. Secondary, soil nitrate N at a depth of 0 to 60 cm in the fields was determined to analyze the relationship between soil nitrate N and N uptake by sugar beet. Thirdly, the effects of soil type, the quantity of organic materials and fertilizers applied, and the history of land use on N uptake by sugar beet were investigated. Although sugar beet took up nitrate N mainly at the soil depth of 0 to 60 cm, N uptake at the depth below 60 cm was not negligible. Moreover, soil nitrate content at the depth of 0 to 60 cm could not evaluate N released by decomposition of applied organic matter during the cropping season. Therefore, the evaluation of soil nitrate content at the depth of 0 to 60 cm was not practical for predicting N uptake by sugar beet. However, sugar beet N uptake could be predicted by the sum of N-score and applied fertilizer N amount using a linear regression analysis (N-score is the estimated value of N available to the next crop from the applied organic matter, the incorporated crop residue and the other field practices). This predictability may indicate that sugar beet effectively takes up N released from the applied N sources, including fertilizer and organic matters, partly because of the plant's deep root elongation, long cropping period and harvest in autumn when the decomposition of organic matter has passed its peak. As a result of this study, it was concluded that this methodology (N-score) could be useful for farmers to decide the most effective N fertilizer management for sugar beet.

Keywords:

• nitrogen management
• nitrogen uptake
• N-score
• organic matter application
• sugar beet.

Introduction

Although nitrogen (N) is an essential nutrient for sugar beet, excessive N supply causes several unfavorable results such as lower sugar purity and yield, nitrate pollution of groundwater, and increase in fertilizer cost. Sugar yield is closely associated with N uptake by sugar beet. Nishimune (1984) showed that in the range of N uptake up to 150 kg ha⁻¹, sugar yield increases linearly as N uptake increases. In the range between 150 and 250 kg ha⁻¹, it continues to increase linearly as N uptake increases but its increasing rate is lower than that in the range below 150 kg ha⁻¹. After sugar yield reaches a peak at 250 kg ha⁻¹, it goes into a decline and decreases as N uptake increases. Thus, there exists an optimum N uptake at which the sugar yield in sugar beet is maximized. A metareview of reports from the United Kingdom, Germany, the Netherlands, and Japan (Hokkaido) by Konno (2001) indicated that the optimum N uptake by sugar beet for sugar production is universally between 200 and 240 kg ha⁻¹, irrespective of climatic and soil conditions.

However, it has been difficult to adjust precisely N uptake by sugar beet to 200–240 kg ha⁻¹, not only because of the large variability in soil N fertility (soil N supply) among fields (Goodlass et al. 1996; Konno 2001; Sato et al. 2008; Fueki et al. 2010), but also because the responses of yield and N uptake to N application rate are quite different, depending on field conditions such as soil type and organic matter application (Hasegawa and Nomura 1973; Giles et al. 1975; Imura and Hayasaka 1982; Igarashi and Nakamura 1983; Nishimune 1984; Shinsenji et al. 1984; Okumura et al. 1989). Hence, considerable efforts have been exerted to develop an effective soil test for evaluating N fertility. In Hokkaido, which is a cool temperate area that has 400–800 mm of precipitation surplus (annual precipitation – annual evapotranspiration; Miki et al. 2000), the performance of various tests such as incubated N (Shinsenji et al. 1984; Konno 2001), autoclaved N (AC-N; Igarashi and Nakamura 1983; Takada and Dempo 1988; Konno 2001), and autoclaving extracted mineral N (AC-mineral N; Konno 2001) have been studied. Currently, both the AC-N and AC-mineral N methods have been employed for formally recommended soil diagnosis in Hokkaido, but only the AC-mineral N method is employed in the eastern-coastal Abashiri district (Department of Agriculture, Hokkaido Government 2002). The procedure to decide N fertilizer amount for sugar beet by AC-N (or AC-mineral N) is as follows (Department of Agriculture, Hokkaido Government 2002): (1) identifying the recommended N application rate, which is suitable for the field conditions in terms of the region and the soil type, (2) adjusting this rate according to the soil test result (AC-N or AC-mineral N), (3) subtracting the N supply from organic matters if it is expected. All the information necessary for this procedure is compiled in Hokkaido Fertilizer Recommendations 2002, which is further explained later (Department of Agriculture, Hokkaido Government 2002). However, this procedure has been underutilized because (1) the combined system (AC-N and AC-mineral N) is complicated for users and thus, only the AC-N method is actually employed throughout Hokkaido, and (2) the evaluation of soil N fertility by AC-N for the topsoil (0–20 or 0–30 cm) might not reflect the accumulated N in the subsoil.

Recently, Sato et al. (2008) have found that use of the soil nitrate N test (at a depth of 0 to 60 cm) in the spring is quite effective for both understanding soil N supply and helping farmers to determine the amount of N fertilizer to be applied for winter wheat (Triticum aestivum L.), to which no organic matter is applied in many cases. This technique may be a precise and effective methodology for sugar beet, because sugar beet is also a deep-rooted crop as winter wheat (Suzuki and Shiga 2004). Giles et al. (1975) and Bilbao et al. (2004) reported that soil nitrate N test was a useful method for accessing N fertilizer rate in sugar beet production, if no organic matter was applied prior to planting sugar beet.

However, sugar beet is usually planted after winter wheat in Hokkaido, and organic matters are applied soon after the harvest of winter wheat (mainly in August, middle of summer). Therefore soil N test, which can evaluate not only soil mineral N at planting time (spring) but also N released from applied organic matters after planting, may be more effective than the soil nitrate N test for sugar beet, as Konno (2001) suggested.

Recently, Fueki et al. (2010) reported that soil mineral N (0–100 cm depth) could be accurately predicted by scoring the type of the applied organic matter and the field management conventionally practiced (“N-score”). The N-score is calculated from the amount of applied organic matter including incorporated crop residue and green manure, the history of their application, and conversion from paddy rice fields to upland fields within the last two years (Fueki et al. 2010). It is based on the numerous experimental results, most of which are available on the Internet (Hokkaido Department of Agriculture 2010) and are summarized in the advisory guidebook for soil and plant nutrient management, entitled Hokkaido Fertilizer Recommendations 2002 (Department of Agriculture, Hokkaido Government 2002). Because the N-score takes account of N released from applied organic matters, it can be used for establishing a new soil test for N management of sugar beet.

Therefore, our objectives in this study are: (1) to clarify how deep sugar beet roots can take up soil nitrate N, in order to understand the necessary depth of soil for measurement of soil nitrate N test, (2) to analyze the relationship between N uptake by sugar beet and soil mineral N (soil nitrate N, at a depth of 0 to 60 cm), with taking organic matter application into account, and (3) to analyze the relationship between N uptake and N-score. To fulfill objectives 1–3, we conducted experiments in 55 typical arable fields in Hokkaido, Japan, from 2003 to 2006.

Materials and Methods

Determination of soil depth until which the root took up nitrate nitrogen

In accordance with the procedure described by Sato et al. (2008), the rate of N recovery by sugar beet was determined in different soil layers. This experiment was carried out in a conventionally cultivated sugar beet field at the Tokachi Agricultural Experiment Station. The sugar beet variety used was Etopirika, which was planted on April 27, 2005. The row width, plant distance and N fertilizer amount were 60 cm, 23 cm and 171 kgN ha⁻¹, respectively. The soil type of the field was Andosol (Low-humic Andosol), found in Memuro, Hokkaido. Five plots (length, 115 cm; width, 120 cm; see Fig. 1) were set (no replication) in the sugar beet field. In each plot, four holes were made with same depth using a soil auger (diameter: 2.2 cm) on May 11, 2005. The depths of the holes were 10, 30, 50, 70, and 90 cm, and were different among the plots. Then, 40.4 g (5 gN m⁻²) of ¹⁵N-labeled N (powdered calcium nitrate, 5.35 atom (%); Shoko Co., Ltd, Tokyo, Japan) was placed at the bottom of each hole. The ¹⁵N-labeled N was used to determine the N recovery rate at soil layers at 0–20, 20–40, 40–60, 60–80, and 80–100 cm depths. At harvest time (October 20, 2005), 10 plants in each plot were sampled and oven-dried, and the oven-dried plant samples (leaves and roots) were used to determine ¹⁵N recovery rate by the combustion method [Elemental Analyzer-Isotope Ratio Mass Spectrometer (EA-IRMS) system]. We calculated the ¹⁵N atom% of each sample by subtracting the natural proportion of ¹⁵N (0.365%) from the determined value. Nitrogen recovery rate was calculated using the equation below. Nitrogen uptake of leaves or roots was determined as described later (see section entitled “Measurement of nitrogen uptake by sugar beet”).

where

15Nuptake-leaf	=	N uptake of leaves from applied 15N-labeled N (gN m−2)
15Nuptake-root	=	N uptake of roots from applied 15N-labeled N (gN m−2)
Nuptake-leaf	=	N uptake of leaves (gN m−2)
Nuptake-root	=	N uptake of roots (gN m−2)
15Natom-leaf	=	15N atom % of leaf
15Natom-root	=	15N atom % of root
0.365	=	15N atom % of natural proportion
5.35	=	15N atom % of applied 15N-labeled N
5	=	amount of 15N-labeled N applied (gN m−2)

Figure 1 Placement of nitrogen-15 (¹⁵N)-labeled fertilizer and the areas where sugar beet was sampled. The 10 sugar beet plants surrounded by a broken line were sampled at harvest time, October 20, 2005. @, Sugar beet plant; •, hole where ¹⁵N-labeled nitrogen was applied.

Experimental fields

Fifty-five fields with various soil types and locations throughout Hokkaido were randomly selected (Table 1). Hokkaido is located between latitude 41.3° and 45.5° north and longitude 137.5° and 145.8° east; the annual average temperature ranges from 4°C to 10°C, the annual average precipitation is between 600 and 2000 mm, and the annual evapotranspiration is between 540 mm and 770 mm (Hokunoukai 1987). In this study, the agricultural areas/districts in Hokkaido were assigned to five categories: Abashiri, Hokkaido-central, Tokachi-central, Tokachi-coastal, and Tokachi-mountainous, in accordance with categories defined in Hokkaido Fertilizer Recommendations 2002 (Department of Agriculture, Hokkaido Government 2002).

Table 1 Field description

Site ID	Year	Area/district	City/town/village	Soil type	Nitrogen treatment (number of plots)	Nitrogen fertilizer amount† (kg N ha^−1)
1	2003	Tokachi-central	Makubetsu	Andosol	4	160–220
2	2003	Tokachi-central	Sarabetsu	Andosol	4	108–200
3	2003	Tokachi-central	Sarabetsu	Andosol	4	80–200
4	2004	Abashiri	Abashiri	Brown Forest soil	2	144–168
5	2004	Abashiri	Abashiri	Brown Forest soil	2	168–192
6	2004	Abashiri	Abashiri	Sand-dune Regosol	2	168–216
7	2004	Abashiri	Bihoro	Andosol	2	168–208
8	2004	Abashiri	Bihoro	Brown Forest soil	2	149–183
9	2004	Abashiri	Higashimokoto	Gray Lowland soil	2	156–204
10	2004	Abashiri	Saroma	Gray Upland soil	4	162–222
11	2004	Abashiri	Shari	Andosol	1	220
12	2004	Hokkaido-central	Chitose	Volcanogenous Regosol	4	66–126
13	2004	Hokkaido-central	Naganuma	Gray Lowland soil	3	154–216
14	2004	Tokachi-central	Honbetsu	Andosol	4	112–192
15	2004	Tokachi-central	Memuro	Andosol	1	200
16	2004	Tokachi-central	Obihiro	Wet Andosol	2	176–198
17	2004	Tokachi-central	Sarabetsu	Brown Forest soil	4	140–200
18	2004	Tokachi-central	Sarabetsu	Wet Andosol	1	200
19	2004	Tokachi-central	Sarabetsu	Andosol	1	200
20	2004	Tokachi-central	Sarabetsu	Andosol	3	107–180
21	2004	Tokachi-central	Shihoro	Wet Andosol	2	150–180
22	2004	Tokachi-coastal	Churui	Andosol	2	160–200
23	2004	Tokachi-coastal	Taiki	Brown Lowland soil	4	160–220
24	2004	Tokachi-mountainous	Shikaoi	Wet Andosol	4	0–160
25	2005	Abashiri	Abashiri	Brown Forest soil	2	144–168
26	2005	Abashiri	Abashiri	Brown Forest soil	2	132–192
27	2005	Abashiri	Higashimokoto	Andosol	3	110–176
28	2005	Abashiri	Saroma	Gray Upland soil	5	48–192
29	2005	Hokkaido-central	Chitose	Volcanogenous Regosol	4	48–135
30	2005	Hokkaido-central	Naganuma	Gray Lowland soil	3	88–176
31	2005	Tokachi-central	Honbetsu	Andosol	5	57–228
32	2005	Tokachi-central	Memuro	Andosol	9	48–335
33	2005	Tokachi-central	Sarabetsu	Brown Forest soil	4	100–160
34	2005	Tokachi-central	Sarabetsu	Andosol	4	100–200
35	2005	Tokachi-coastal	Churui	Andosol	2	140–200
36	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	184–244
37	2005	Tokachi-coastal	Toyokoro	Andosol	2	206–286
38	2005	Tokachi-coastal	Toyokoro	Andosol	2	180–220
39	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	200–263
40	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	209–233
41	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	176–198
42	2005	Tokachi-coastal	Toyokoro	Andosol	2	183–223
43	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	200–265
44	2005	Tokachi-coastal	Toyokoro	Peat soil	2	180–238
45	2005	Tokachi-coastal	Toyokoro	Gray Lowland soil	2	272–318
46	2005	Tokachi-mountainous	Shikaoi	Wet Andosol	2	0–27
47	2006	Abashiri	Abashiri	Brown Forest soil	5	42–231
48	2006	Abashiri	Abashiri	Brown Forest soil	2	144–168
49	2006	Abashiri	Saroma	Gray Upland soil	5	48–264
50	2006	Tokachi-central	Honbetsu	Andosol	6	45–225
51	2006	Tokachi-central	Memuro	Andosol	11	49–392
52	2006	Tokachi-coastal	Toyokoro	Gray Lowland soil	7	54–324
53	2006	Tokachi-mountainous	Shikaoi	Wet Andosol	2	0–21
54	2006	Tokachi-mountainous	Shimizu	Wet Andosol	2	80–192
55	2006	Tokachi-mountainous	Shimizu	Wet Andosol	2	80–160
†Nitrogen (N) fertilizer amount applied in the cropping season. Some were applied just before transplanting, others were broadcasted until late May.

On the basis of Classification of Cultivated Soils in Japan, Third Approximation (Cultivated Soil Classification Committee 1995), the soil types in the fields were classified as (1) Andosol (Low-humic Andosol and Haplic Andosol), (2) Brown Forest soil (Aquic Brown Forest soil and Humic Brown Forest soil), (3) Brown Lowland soil (Haplic Brown Lowland soil), (4) Gray Lowland soil (Humic Gray Lowland soil, Gleyed Gray Lowland soil and Haplic Gray Lowland soil), (5) Gray Upland soil (Haplic Gray Upland soil), (6) Peat soil (Low-moor Peat soil), (7) Sand-dune Regosol (Haplic Regosol), (8) Volcanogenous Regosol (Haplic Volcanogenous Regosol), and (9) Wet Andosol (Haplic Wet Andosol).

The chemical properties of topsoil in all the fields were as follows: soil pH (glass-electrode method, dried soil/water = 1:2.5), 5.9 ± 0.3 [mean ± standard deviation (SD)] with a range of 5.5 to 6.5, which is within the appropriate range of soil diagnosis criteria (Department of Agriculture, Hokkaido Government 2002); soil organic matter content determined by the Tyurin method, 32 ± 22 g kg⁻¹ (mean ± SD) with a range of 2 to 110 g kg⁻¹; and soil N content determined by the Kjeldahl method, 2.8 ± 1.5 g kg⁻¹ (mean ± SD) with a range of 0.7 to 8.9 g kg⁻¹.

Field experiment

In each field, sugar beet was planted from the middle of April to early May by using Hokkaido's conventional cultivation method (paper-pot transplanting). There were 1–11 plots in which different amounts of N fertilizer were applied (the range was 0–392 kgN ha⁻¹) in each experimental field (Table 1). Every plot consisted of three replicates. Agrochemical spraying and weed control were performed following each farmer's conventional method.

Soil sampling and measurement of nitrate nitrogen

In each field, immediately after snowmelt (before fertilization, early-to-mid April), soil samples for measuring nitrate N were taken using a soil auger (diameter 2.2 cm) at depths of 0–20, 20–40, and 40–60 cm. Four replicates were taken at each depth and mixed in the same plastic bag. Soil samples in the closed plastic bags were stored at 4°C. Within a week after sampling, each fresh soil sample was weighed and sieved through a 5 mm mesh and mixed thoroughly. Twenty grams of each fresh sample was then placed into a polyethylene bottle and shaken with 100 mL of 1 mol L⁻¹ potassium chloride (KCl) solution for 1 h. After shaking, the suspension was percolated, and the nitrate N content of the percolated solution was measured using an autoanalyzer (AACS-II, BRAN+LUEBBE). Soil nitrate N data were expressed in kg ha⁻¹ using bulk density, which was calculated from the volume (length and diameter of soil core) and weight of each fresh soil sample.

Measurement of nitrogen uptake by sugar beet

At harvest time (October), sugar beet plant samples were taken from each plot (4–5 m² area per plot). The plant samples were divided into leaves and roots. Then, their N content was measured using the Kjeldahl method.

Farm survey of organic matter application and other field management data

We collected information on the previous crops, the recent history of organic matter application, and other field management items (e.g. the amount of N fertilizer applied at the sowing of green manure crops before planting sugar beet, or the history after conversion to upland fields from paddy fields) from every farmer who cultivated the experimental fields, in accordance with the methods described by Fueki et al. (2010).

Scoring organic matter application and other field management by N-score

The N-scores were calculated from the data shown in Table 2, according to Fueki et al. (2010). In this study, the N-scores for cattle slurry, pig manure, and poultry manure were also defined on the basis of the Hokkaido Fertilizer Recommendations 2002 (Department of Agriculture, Hokkaido Government 2002) (Table 3).

Table 2 Field description data

Site ID	Previous crop	Organic matters and fertilizers applied after harvest of previous crop and history of land use
1	Winter wheat	Nothing applied.
2	Winter wheat	Last fall: cattle manure† 40 Mg ha^−1, spring: poultry manure 4 Mg ha^−1.
3	Winter wheat	Last fall: urea 200 kg ha^−1(92 kg N ha^−1) for green manure + cattle manure† 60 Mg ha^−1.
4	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 40 Mg ha^−1.
5	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 60 Mg ha^−1.
6	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 50 Mg ha^−1.
7	Sugar beet	Last fall: cattle manure† 50 Mg ha^−1(Beet-top was applied last fall).
8	Potato	Last fall: cattle manure† 30 Mg ha^−1.
9	Sugar beet	Nothing applied (Beet-top was applied last fall).
10	Winter wheat	Last fall: cattle manure† 40 Mg ha^−1.
11	Winter wheat	Spring: poultry manure 2 Mg ha^−1.
12	Winter wheat	Spring: poultry manure 50 Mg ha^−1.
13	Paddy rice	Nothing applied, but this had been paddy field until the last year. From the investigated year, this was converted to upland field.
14	Winter wheat	Last fall: urea 400 kg ha^−1 (184 kg N ha^−1) for green manure.
15	Winter wheat	Last fall: cattle manure† 30 Mg ha^−1.
16	Winter wheat	Spring: pig manure 1.5 Mg ha^−1.
17	Winter wheat	Last fall: cattle manure† 40 Mg ha^−1, spring: cattle manure† 40 Mg ha^−1.
18	Winter wheat	Spring: poultry manure 5 Mg ha^−1.
19	Winter wheat	Last fall: urea 200 kg ha^−1(92 kg N ha^−1) for green manure + cattle manure† 60 Mg ha^−1.
20	Winter wheat	Last fall: cattle manure† 30 Mg ha^−1 + calcium cyanamide 200 kg ha^−1 (40 kg N ha^−1), spring: poultry manure 10 Mg ha^−1.
21	Potato	Last fall: cattle manure† 50 Mg ha^−1.
22	Winter wheat	Last fall: cattle manure† 30 Mg ha^−1 + cattle urine 10 Mg ha^−1.
23	Winter wheat	Last fall: cattle manure† 30 Mg ha^−1 (every year over for a decade).
24	Winter wheat	Last fall: pig slurry 150 Mg ha^−1 + cattle manure† 50 Mg ha^−1(every year over for a decade).
25	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 40 Mg ha^−1.
26	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 40 Mg ha^−1.
27	Sugar beet	Nothing applied (Beet-top was applied last fall).
28	Winter wheat	Last fall: cattle manure† 40 Mg ha^−1.
29	Winter wheat	Spring: poultry manure 63 Mg ha^−1.
30	Sugar beet	Nothing applied, but this had been paddy field until two years ago. From the last year, this was converted to upland field (Beet-top was applied last fall).
31	Winter wheat	Last fall: urea 400 kg ha^−1 (184 kg N ha^−1) for green manure.
32	Winter wheat	Nothing applied.
33	Winter wheat	Last fall: cattle manure† 30 Mg ha^−1, spring: cattle manure† 30 Mg ha^−1.
34	Winter wheat	Last fall: urea 200 kg ha^−1 (92 kg N ha^−1) for green manure + cattle manure† 60 Mg ha^−1.
35	Winter wheat	Last fall: cattle urine 20 Mg ha^−1 + cattle manure† 40 Mg ha^−1(every year over for a decade).
36	Winter wheat	Nothing applied.
37	Winter wheat	Last fall: cattle manure† 20 Mg ha^−1.
38	Winter wheat	Last fall: cattle manure† 20 Mg ha^−1.
39	Winter wheat	Last fall: ammonium sulfate 400 kg ha^−1 (84 kg N ha^−1) for green manure.
40	Winter wheat	Last fall: urea 200 kg ha^−1 (92 kg N ha^−1) for green manure + cattle manure† 30 Mg ha^−1.
41	Winter wheat	Last fall: cattle manure† 20 Mg ha^−1.
42	Winter wheat	Last fall: urea 150 kg ha^−1 (69 kg N ha^−1) for green manure + cattle manure† 30 Mg ha^−1.
43	Winter wheat	Last fall: urea 250 kg ha^−1 (115 kg N ha^−1) for green manure.
44	Winter wheat	Last fall: cattle manure† 10 Mg ha^−1.
45	Winter wheat	Last fall: cattle manure† 10 Mg ha^−1.
46	Winter wheat	Last fall: cattle manure† 50 Mg ha^−1 + pig slurry 50 Mg ha^−1, spring: pig manure 40 Mg ha^−1.
47	Winter wheat	Last fall: cattle manure† 60 Mg ha^−1.
48	Winter wheat	Last fall: cattle manure† 60 Mg ha^−1.
49	Winter wheat	Last fall: cattle manure† 60 Mg ha^−1.
50	Winter wheat	Last fall: urea 400 kg ha^−1 (184 kg N ha^−1) for green manure.
51	Winter wheat	Nothing applied.
52	Winter wheat	Last fall: calcium cyanamide 200 kg ha^−1 (40 kg N ha^−1).
53	Winter wheat	Last fall: cattle manure† 50 Mg ha^−1 + pig slurry 50 Mg ha^−1.
54	Winter wheat	Last fall: ammonium sulfate 200 kg ha^−1 (42 kg N ha^−1) for green manure + cattle manure† 20 Mg ha^−1.
55	Winter wheat	Last fall: cattle slurry 80 Mg ha^−1 + cattle urine 100 Mg ha^−1.
†In this study, cattle manure means the cattle excrement without being composted, the composted cattle manure (several years) and the cattle manure mixed with bark.

Table 3 N-scoring of organic matter application and field practices

Organic matter application/field practice	N-score
Cattle manure†, single year application	1.0 kg N Mg^−1 of manure
Cattle manure†, more than 10 years consecutive application	3.0 kg N Mg^−1 of manure
Cattle slurry	1.3 kg N Mg^−1 of slurry
Pig slurry	1.3 kg N Mg^−1 of slurry
Cattle urine	2.5 kg N Mg^−1 of urine
Pig manure	3.7 kg N Mg^−1 of manure
Poultry manure	13 kg N Mg^−1 of manure
Nitrogen fertilizer amount at sowing green manure	100% of the nitrogen amount (kg N ha^−1)
Incorporating sugar beet leaf	40 kg N ha^−1
Converting to upland field from paddy rice field within 2 years	10 kg N ha^−1
†In this study, cattle manure means the cattle excrement without being composted, the composted cattle manure (several years) and the cattle manure mixed with bark.

Results and Discussion

Nitrogen recovery rate at different soil depths

Table 4 shows the N recovery rate at each soil depth. The N recovery rates at 0–60 cm were 66–74%, which were similar to those reported in previous studies for sugar beet (68–75% in Igarashi and Nakamura 1983; 63–86% in Nishimune 1984; 70–85% in Konno 2001) and those at 0–100 cm (36–74%) were not different from the results for other crops (Nishimune 1984; 62–78% for potato, 64–89% for corn, 36–56% for winter wheat). The N recovery rates at 60–100 cm were 36–48%, which were lower than those of the upper layers (0–60 cm). Nishimune (1984) reported that if the difference in N recovery rate was more than 5%, the difference was significant. Thus, it was assumed in this study that the difference larger than 10% was significant.

Table 4 Nitrogen (N) uptake as determined from ¹⁵N measurement and N recovery rate of ¹⁵N applied at each depth

^15N-applied depth (cm)	^15N atom (%)		N uptake from applied ^15N-labeled N (g m^−2)			N recovery rate (%)
	Leaf	Root	Leaf	Root	Total
0–20	1.16	1.03	2.4	1.3	3.7	74.0
20–40	1.18	1.15	1.8	1.5	3.3	66.0
40–60	1.10	1.03	2.4	1.0	3.4	68.0
60–80	1.01	0.90	1.4	1.0	2.4	48.0
80–100	0.83	0.74	1.1	0.7	1.8	36.0

Compared with a similar study of the winter wheat's N recovery rate of 28.4% at a depth of 80 to 100 cm (Sato et al. 2008), the recovery rates of 36 to 48% observed at 60–100 cm in this study would be higher and thus, N uptake by sugar beet from those layers could not be negligible. This corresponded to the fact that sugar beet roots can elongate up to 90–150 cm, unless anaerobic subsoil is present (Fueki and Takeuchi 2010). From these results, we can understand that sugar beet takes up nitrate N mainly at the soil depth of 0 to 60 cm, but N uptake at the depth below 60 cm is not negligible.

Relationship between nitrogen uptake and soil nitrate nitrogen (0–60 cm)

The relationship between N uptake and N supply (N fertilizer amount + soil nitrate N at 0–60 cm) was analyzed. The relationship was quite complex, and it was necessary to separate the data by area/district. The most representative and comprehensive case is that of Tokachi-central (Fig. 2). The correlations differed depending on whether or not organic matter was applied. In the case of no organic matter was applied, the correlation coefficient was high (r = 0.88, n = 24, p < 0.01). This result corresponded to those of Giles et al. (1975) and Bilbao et al. (2004). They also conducted field experiments where no organic matter was applied and indicated that the assessment of soil nitrate N at a depth of 0 to 60 cm or 0 to 30 cm, respectively, was useful for predicting N uptake.

Figure 2 Representative data demonstrating the relationship between nitrogen (N) uptake by sugar beet and N supply (defined as N fertilizer amount + soil nitrate N at 0–60 cm depths) in the Tokachi-central area. ○, No organic matter applied; ×, organic matter applied. Regression on ○, y = 0.61x +  68, n = 24, r = 0.88, **(p < 0.01). Regression on ×, y = 0.24x + 250, n = 45, r = 0.48, **(p < 0.01).

However, in fields where organic matters were applied, the correlation coefficient was lower (r = 0.48, n = 45, p < 0.01). This fact suggested that the soil nitrate N test at planting time (0–60 cm) cannot be used to predict N uptake by sugar beet because it could not evaluate N released by decomposition of applied organic matter during the cropping season. Sugar beet may be more susceptible to the effects of organic matter application before planting than the other crops owing to longer cropping season in Hokkaido, more than half year (from April–May to October–November). Moreover, N uptake by sugar beet at the depth below 60 cm is not negligible (Table 4), as stated earlier. These facts explain the reason why soil nitrate N at 0–60 cm failed to predict N uptake by sugar beet if organic matters were applied.

From these results, it was concluded that the soil nitrate N test at planting time (0–60 cm) is not a practical index for predicting N uptake by sugar beet in Hokkaido, where organic matters are applied in most cases.

Relationship between nitrogen uptake and N-score

Figure 3 shows the relationship between N uptake by sugar beet and N-score + N fertilizer amount. They are closely related to each other (y = 0.68x + 100, n = 140), and the correlation coefficient (r = 0.75, p < 0.001) is similar to that found through analysis using AC-mineral N at 0–50 cm (r = 0.77, p < 0.001, Konno 2001). As shown in Fig. 3, we eliminated the data points in the fields where “Excess organic matter (n = 18)” or growth inhibition (n = 13) were found. “Excess organic matter” was set when the “N-score is over 220 or more than 30 Mg ha⁻¹ of manure is applied consecutively”, on the basis of the following findings. Central Agricultural Experiment Station et al. (2005) reported that nitrate N concentration of groundwater could be kept less than 10 mg L⁻¹ when N uptake by sugar beet is less than 250 kg ha⁻¹. Because N uptake by sugar beet reaches 250 kg ha⁻¹ at N-score of 220 according to the relationship shown in Fig. 3, N leaching could occur even without additional fertilization (and additional fertilization surely raises the risk of groundwater pollution) on conditions that N-score is over 220. Nakatsu et al. (2000) found that if more than 30 Mg ha⁻¹ manure is applied consecutively, the nitrate N concentration of groundwater could be over 10 mg L⁻¹.

Figure 3 Relationship between nitrogen (N) uptake by sugar beet and N-score + fertilizer N amount. ×, Excess organic matter (N-score is over 220, and/or more than 30 Mg ha⁻¹ manure applied consecutively), n = 18; •, confounding problems (diseased sugar beet or the topsoil is too hard), n = 13; ○, the majority of cases except the above cases, y = 0.68x + 100, n = 140, r = 0.75, *** (p < 0.001).

In the cases of excess organic matter (cross points: ×, in Fig. 3), N uptake did not increase as N-score + N fertilizer increased in most cases (13 of the total 18 cross points). This suggests that if N supply from organic matter is in excess (Fig. 3), sugar beet cannot take up N in proportion to the N supply, because the upper limit of N uptake by sugar beet is thought to be 300–400 kg ha⁻¹ (Konno 2001). Alternatively, another speculation is related to the eight points plotted at an N-score + N fertilizer of over 700 kg ha⁻¹. These eight points were taken in the sites 12 and 29, where a large amount of poultry manure was applied (Table 2). In this study, the N-score of poultry manure was set to be 13 kg N Mg⁻¹ of poultry manure, on the basis of the Hokkaido Fertilizer Recommendations 2002 (Department of Agriculture, Hokkaido Government 2002). However, as Kitta et al. (2002) described, the N contents of poultry manure under commercial usage ranged widely (total N: 24–70 kg N Mg⁻¹, inorganic N: 0.8–7.6 kg N Mg⁻¹, uric acid N: 0–34 kg N Mg⁻¹) because of the N loss caused by the decomposition of uric acid and ammonia volatilization. This wide variability suggests that further study is necessary to improve the accuracy of the N-score of poultry manure. Growth inhibition included (1) brown spot disease and (2) hard soil [the penetrability was 26 mm (1640 kPa) by the Yamanaka-type penetrometer]. Generally, root elongation could be inhibited when soil penetrability is more than 20–22 mm (616–837 kPa) (Nakatsu et al. 2004).

As stated earlier, we found that N uptake by sugar beet could be predicted using the N-score + fertilizer N amount, without taking the variability of original soil fertility into account. This indicates that sugar beet efficiently takes up N released from the applied N sources, including organic matter and fertilizer. This simple and interesting result could be explained by the following facts: (1) sugar beet can take up N from deeper soil layer (until 100 cm) (Table 4), (2) sugar beet has a long cropping period and is harvested in autumn when the decomposition of organic matter in soil has passed its peak (Konno 2001; Suzuki and Shiga 2004), (3) most arable lands in Hokkaido are made up of “mature fields” because several decades have passed since land reclamation (Sakuma 1987) and N fertility is affected mainly by farmers’ soil management practices such as organic matter application, rather than the initial soil fertility. The fact that the intercept of the regression equation (Fig. 3) was 100 (kg ha⁻¹) also suggests the natural soil fertility has become high, (4) Hokkaido Fertilizer Recommendations 2002 (Department of Agriculture, Hokkaido Government 2002), from which N-score was defined, is quite reliable in its quantitative evaluation of N supply from organic matter to the sugar beet. This study opened the possibility that farmers in Hokkaido can calculate by themselves the suitable amount of N fertilizer for sugar beet from (1) optimum N uptake by sugar beet (200–240 kg ha⁻¹, Konno 2001), (2) N-score in their field determined from Table 4, and (3) the regression formula in Fig. 3.

Nevertheless, it remains unclear whether or not N-score can predict N uptake by sugar beet in areas that are wetter/drier than Hokkaido. The applicability of the N-score for use in other regions will require further study. The development of an evaluation method such as N-score is certainly worthwhile as long as a reliable agricultural guidebook such as the United Kingdom's Fertilizer Recommendations (Ministry of Agriculture, Fisheries and Food 2000), is available.

Conclusions

Although sugar beet took up nitrate N mainly at the soil depth of 0 to 60 cm, N uptake at the depth below 60 cm was not negligible. Moreover, soil nitrate content at the depth of 0 to 60 cm could not evaluate N released by decomposition of applied organic matter during the cropping season. Therefore, the evaluation of soil nitrate content at the depth of 0 to 60 cm was not practical for predicting N uptake by sugar beet. However, the N-score, as defined on the basis of the regional experimental results related to organic matter application (Department of Agriculture, Hokkaido Government 2002), was quite easy to interpret and predict the N uptake of the sugar beet accurately. This methodology could be available for deciding the most effective N fertilizer management for sugar beet.

Acknowledgments

The authors wish to thank the Hokkaido Sugar Beet Association for funding this work and would like to express their gratitude to Dr Hiroyuki Shiga for his profound suggestions. They are very grateful to advisers at the agricultural extension centers and sugar beet companies in Hokkaido who helped with the experiments and investigation. The authors would also like to thank the owners of the fields used for the investigation, for readily allowing the authors to conduct the field experiments and investigations.