In Korea, tidal lands have been utilized by the demand for food requirement. However, reclaimed tidal land has poor physical and chemical properties, and thus crop growth is inhibited by unfavorable soil conditions (Lee et al., 2003; Sohn et al., 2009). Although rice production system is recognized one of the most efficient practices in reclaimed tidal land, the interest in cultivation of upland crops has been increased over the past decade because it has more economic benefits compared to rice cultivation (RRI, 2007). Due to low soil organic matter and plantavailable nutrient contents in reclaimed tidal lands, upland crop productivity is relatively low compared to paddy rice at its early stages (Yang et al., 2008). Nutrient replenishment and enhanced quality of reclaimed tidal lands can be achieved by the addition of fertilizers and/or organic matters. However, the use of chemical fertilizer is restricted to reduce non-point source water pollution by excess nutrients.

The inclusion of organic matter has long-term benefits in enhancing soil quality due to its impact on the physical, chemical, and biological properties of the soil. It is well documented that the cultivation of green manure crops (GMCs) can improve soil fertility (Blanco-Canqui et al., 2011; Ruffo et al., 2004; Wang et al., 2005). Green manure crops are especially effective for use in soils with poor fertility as in reclaimed tidal lands. The addition of GMCs to reclaimed tidal land can also ameliorate resalinization by increasing infiltration to the water table. For instance, Li and Zhu (2002) found that GMCs improved soil physical and chemical properties through increment of organic matter contents and microbial activities.

Indeed, the increase in soil organic matter may improve soil physical properties. Blanco-Canqui et al. (2011) reported that summer GMCs reduced soil bulk density and increased wet aggregate stability and infiltration rate. Soil bulk density also influences the infiltration rate, which is especially important to manage the salt damage in reclaimed tidal lands, where need salt leaching through vadose zone. The impact of GMCs on soil fertility and crop production depends on the management practices including soil management, cropping systems, type of GMCs, growing season, and weather variations.

Several types of GMCs have been investigated for their ability to survive in high-salt soil as well as productive plant biomass (Lee et al., 2003; Lee et al., 2007). It may provide potential for direct and indirect salt degradation and modification of the soil environment with increased organic matter. Therefore, the objectives of this study were to 1) evaluate the effects of GMCs on soil chemical and physical properties, 2) provide management options aimed at maximizing the supplemental nutrients to subsequent crops in reclaimed tidal lands.

MATERIALS AND METHODS

Study Field Characteristics

Field experiments were conducted at Kwang-whal district in Saemangeum reclaimed tidal land from 2007 to 2011. Experimental site (35°50'N, 126°42'E) was established in April 2006 and it was desalinized by fresh water to keep soil salinity level for optimum crop production. The average annual precipitation at this location was 1,250 mm, of which, on average, 54% falls between June and August. The average annual temperature was 13°C and usually ranged from 26°C in August to -1°C in January. The dominant soil series is Mangyeong (coarse silty, mixed, nonacid, mesic, Fluaguentic Endoaguepts) that is commonly found in Saemangeum area. The physical and chemical properties of experimental site are shown in Table 1.

Cultivation and Application of Green Manure Crop

Three kinds of summer and winter crops were selected according to their high yield potential in this reclaimed tidal land (Lee et al., 2003; Lee et al., 2007). For summer GMCs, sesbania (Sesbania sesban), wild millet (Echinochloa crus-galli), and sorghum-sudangrass hybrid (Sorghum bicolor) were grown from June to October at each growing season. Oat (Avena sativa), rye (Secale cereals L.), and barley (Hordeum vulgare) were used as winter GMCs from October to May or June. The cultivation of summer crops followed by winter cereal crops is a common practice in this area. Thus, nine combinations of summer and winter GMCs were main treatments, which were planted in a 2.4 × 10 m experimental plot in a completely randomized block design with three replications at each growing season.

Summer crops were drilled with 40 kg/ha seeding rate in early June at each growing season. Winter crops were broadcasted in late October each year with the recommended seeding rate by RDA (2005). Each crop was fertilized and managed according to the recommended practices for maximizing crop production (Table 2). All aboveground biomass of GMCs were terminated by flail mowing and amended into the plots. Then, all residues were incorporated with the top 15 cm surface soil using a spader. Termination times were selected to maximize biomass yield and then allow field preparation for planting the subsequent crop, which were within five day difference among each growing season.

Analysis of Soil and Plant

Soil samples were randomly collected from surface (0- 15 cm) and sub-surface (15-30 cm) depth by a hand probe at each growing season. Core samples (vol. 100 cm3) were collected from each soil depth and measured bulk density, porosity, and field volumetric water content. Soil physical and chemical analysis in this study were conducted based on the standard method by NIAST (2000). Soil samples were air-dried at room temperature and ground to pass a 2 mm sieve and analyzed for chemical properties. Soil pH and EC were determined on a 1 : 5 soil : water suspension using an Orion pH/EC meter. Total carbon (C) and total nitrogen (N) contents were determined by dry combustion method (Seo et al., 2004) with a CN Sauto analyzer (Vario Max CNS Elementar, Germany). Soil organic matter (OM) was calculated from C content. Available phosphate (P₂O₅) was analyzed by Lancaster method. Exchangeable cations (K, Ca, Mg and Na) were extracted by 1N-NH₄OAC (adjusted pH 7.0) and quantified by inductively coupled plasma emission spectroscopy (Varian Inc., USA).

Aboveground and belowground plant samples were collected from sub plot (1 m²) right before they were terminated by flail mowing. Plant samples were oven dried at 70°C for 72 hour before they were ground to pass through a 0.5 mm sieve for chemical analysis. The content of P₂O₅ and K₂O in the green manure crops were analyzed by vanadate method and ICP method, respectively, after wet digestion. C and N contents were also determined by dry combustion method.

Statistical Analysis

Experimental data was statistically analyzed using analysis of variance (ANOVA) and general linear model (GLM) procedures with the SAS statistical program (SAS institute, 1988). Least significant differece (LSD) at 95% confidence level was used for a mean comparison when Ftest was statistically significant for main effects with the ANOVA.

RESULTS AND DISCUSSION

Climate Condition

The mean air temperature and monthly precipitation during the experiment period and long-term average (1981-2010) are shown in Fig. 1. Average monthly air temperatures typically were between 15 and 26°C for summer GMC cropping periods and ranged from -1#x00B0;℃ in January to 17°C in late May for winter GMCs. The mean monthly temperature in July 2008 was 1.5°C higher than in other growing seasons and long-term average. In 2011, the mean monthly temperature in January was 4.5°C below normal. Above normal precipitation was recorded in 2007 for the months of August and September, in 2009 for July, and in 2010 for August. In 2007, the precipitation in September was approximately four-fold greater than the long-term average and twenty-fold more than in 2008. In 2007, the precipitation between June and October was approximately two-fold greater than the long-term average, and the amount of precipitation during the same period in 2008 was 35% less than the long-term average. The periods of October through May for winter GMCs were relatively dry and were received similar amounts of precipitation each growing season.

Green Manure Crop Productivity

There were significant differences in the dry weight biomass yield among GMCs during growing seasons (Table 3). Biomass yield for summer GMCs were greater in the first year (2007 - 2008) of the study, with the differences likely due to year to year weather variation. Despite some fluctuation of monthly temperature between growing seasons, GMCs had similar growing degree days at 30 days after planting (average 590, 4°C base) and termination time (2,550) between each growing year (data not shown). These results indicate that GMCs tended to depend more on timely precipitation. Year to year and monthly fluctuations in precipitation during the growing season are reflected in the yield performance of GMCs. During the growing season, it was warmer and wetter in 2007 than the other growing seasons, which contributed to more rapid crop establishment and greater growth.

The biomass yield for winter GMCs was also greater in the first year of the study, but less biomass yield was produced in 2010 - 2011 compared to the rest. The less biomass yield can be attributed to unfavorable weather condition, for example, seed damages from winter cold in January (Fig. 1). Winter GMCs with different seeding rate produced the similar biomass yield, showing that it is unlikely that the different seeding rate was large enough to impact biomass yield.

Among GMCs, sorghum-sudangrass hybrid (SS hybrid) produced the greatest amounts of biomass across growing seasons, while the rest production ranged 9.7-10.7 ton/ha (Table 3). The result suggests that SS hybrid can return at least 40% more biomass compared to the other GMCs. Because of high biomass input, SS hybrid is expected to provide an effective to supply significant amounts of decomposable materials.

Biomass yield of GMCs obtained in this study is comparable with other results. For instance, SS hybrid and sesbania produced wide range of biomass yields (2-22 and 2-16 ton/ha, respectively) from three reclaimed tidal lands (Sohn et al., 2009), indicating that it was influenced by soil salinity variation in reclaimed tidal lands. On the other hand, Shin et al. (2005) produced barley of 4.5 ton/ha and rye of 2.6 ton/ha on silty loam reclaimed tidal lands. It is probably yield reduction due to the high-salt concentration in study soils, although barley and rye are recognized as one of the most adapted crops in high-salt soils.

Changes in soil chemical properties

Green manure crops have beneficial effect on major soil nutrients including N, P, and K, which can be released from the residues of GMCs by decomposition procedures. The nutrient contents of GMCs are shown in Table 4. The total N contents in GMCs ranged from 84 in wild millet to 275 kg/ha in sesbania. Even though the total amount of sesbania biomass was similar to other crops except SS hybrid, the accumulation of total N by sesbania was greater than that accumulated by other crops. It is probably due to the ability of legume crop to produce N through the fixation of atmospheric N. This trend was observed in P₂O₅ contents, which ranged between 33 and 76 kg/ha. Sesbania and SS hybrid accumulated similar amounts of P₂O₅, which were greater than those accumulated by the other crops. SS hybrid accumulated higher amounts of K₂O than the other GMCs, while wild millet accumulated less amounts of K₂O. Among GMCs, sesbania and SS hybrid were expected to provide significant amounts of nutrients into the soils.

Aboveground forage crops are usually used as feed stock and belowground biomass constitute an important source for soil organic carbon and soil fertility. One of several management practices proposed to increase soil organic matter is to expand the area of crops such as perennial forages that increase the annual crop residue carbon inputs to soils. The ratio of root to shoot biomass had widely ranged from 0.04 in barley to 0.21 in SS hybrid (Table 4). These findings suggest that belowground biomass contributed 4 to 21% of aboveground biomass in GMCs. If assumed that aboveground biomass was removed for feed stock, SS hybrid would be the most contribute to biomass accumulation in soils. The proportion of aboveground to belowground biomass can vary since belowground biomass can vary more than aboveground biomass due to variation in belowground growth in the soil profile due to the heterogeneity in belowground. There was no significant difference on root/shoot ratio between summer and winter GMCs. Although the beneficial effects of winter GMCs may be diminished in this area with lower precipitation and temperature, winter GMCs can also supplement nutrients to subsequent crops.

The aboveground and belowground total C and N contents varied on the types of GMCs (Table 5). The N content in sesbania was greatest and wild millet had the lowest content, whereas the content of C had similar between GMCs, ranging from 39.2 to 43.2%. The C/N ratio ranged from 21 to 35 for aboveground biomass and from 32 to 53 for belowground biomass. The C/N ratio of biomass has frequently been used as a tool for predicting the rate of decomposition and N mineralization. In this study, the C/N ratio of aboveground biomass in wild millet and rye had more than 30, indicating that net mineralization was unlikely to occur upon their decomposition and induce N deficiency in subsequent crops. The C/N ratio of belowground biomass was greater than in aboveground and C/N ratio was greater than 30 regardless of GMC types. These data indicate that biomass residues from GMCs have high potential for N immobilization in the soil and thus management practices need to facilitate the decomposition rate by decreasing C/N ratio.

In this study, the different harvesting time for feed stock was influenced on C/N ratio (data not shown). For instance, C content in rye was decreased with one month earlier harvesting and N content was increased from 1.37 to 1.42%, consequently the ratio of C/N was decreased in both aboveground and belowground part. The greater N availability from soil N can also decrease C/N ratio by allowing for greater N uptake by GMCs (Ruffo et al., 2004). Green manure crops improve soil organic matter content and reduce N leaching, which typically have lower N concentration or higher C/N ratio. C/N ratio also can be influenced by culture management.

Ranells and Wagger (1996) observed that as N concentration in rye increased from monoculture to biculture with legume crop of hairy vetch, the C/N ratio decreased from 42 to 16. As a result, more N was released from hairy vetch and rye biculture residue in the soil than from rye residue. Therefore, biculture of forage and legume can reduce the C/N ratio of forage and increase the potential for soil N mineralization, which can facilitate their rapid decomposition to increase nutrient availability for the subsequent crop.

Cultivation and application of GMCs significantly influenced on soil pH, organic matter, nutrient contents (Table 6). Reclaimed tidal land soils are susceptible to the changes in soil chemical properties as well as crop production and its feed value. The experimental soil contains a high content of carbonate (CO₃), which causes the soil to be high in pH, which was gradually decreased into 5.6. It is probably due to the continuous use of physiologically acidic fertilizer used in this study, suggesting that proper fertilizer selection should be followed in reclaimed tidal lands. Organic matter content was increased from 1.3 to 6.0 g/kg by 1.2 g/kg per year with GMCs, but it is far less than the optimum ranges (20 - 30 g/kg) for upland soils.

Cultivation and application of GMCs also continuously increased total N and available phosphate content, while exchangeable K and Ca contents were fluctuated. Especially, Ca content was relatively lower than optimum range and thus lime application is recommended in this reclaimed tidal soil, which can also improve soil pH.

Changes in Soil Physical Properties

Green manure crops may contribute to crop growth through not only nutrient cycling but also improvement in soil physical properties including field water content, bulk density, and aeration. The soil in reclaimed tidal land has poor infiltration and hydraulic conductivity (Ryu, 2010). In this study, bulk density in surface and sub-surface was 1.44 and 1.48 Mg/m³, respectively. Green manure crops significantly decreased bulk density at surface soil in four year treatment (Fig. 2(a)). This result suggests that an increase in soil organic matter may reduce soil bulk density. Yang et al. (2008) reported that GMCs caused decreasing bulk density and increasing porosity in coarse textured reclaimed tidal lands.

By the way, there were no changes in bulk density (ranging from 1.21 to 1.24 Mg/m³) after second-year treatment of GMCs, which was in the optimum ranges for the common upland crops (Fig. 2(b)). This data indicate that bulk density is maximized with two year cultivation of GMC, and further cultivation was not improved soil bulk density. The improvement of soil properties may concomitantly help to growth and development of subsequent crops.

Effect of GMC Combination on the Change in Soil Properties

Table 7 shows the effect of GMC combination on change in soil organic matter and nitrogen content, and bulk density in the 4th year experiment. SS hybrid treatment as summer GMC had the greatest content in soil organic matter (6.4 g/kg) due to its highest biomass yield which was incorporated into soil, but there was no consistent trend among winter GMCs. Although nitrogen application for sesbania was smaller than other GMCs (Table 2), total N content in soil was same or greater. This result was probably due to legume crop’s nitrogen fixing capacity, indicating that sesbania would be useful for low input cultivation on reclaimed tidal land. Soil bulk density was also lowest on SS hybrid treatment. It is probably due to its high biomass and C/N ratio. Among winter GMCs, rye cultivation showed the lowest bulk density as affected by its higher C/ N ratio (Table 5). The combination of SS hybrid and rye showed the lowest soil bulk density (1.04 Mg/m³) among combinations of summer and winter GMCs.

Green manure crops may have contributed to water capture by reducing runoff losses and increasing water infiltration. However, less field water content under green manure crops was observed compared to the non-crop plots in this study (data not shown). The result in this case was contrast with the other researches that showed greater field water content under cover crops due to the reduced evaporation and maintained soil water content (Blanco- Canqui et al., 2011; Wang et al., 2005). It was unlikely that GMCs can readily replenish the water consumed during the growth. The contrast results are probably due to the different impacts of GMCs and unfavorable soil conditions. Green manure crops used water and reduced soil water storage due to evapotranspiration, although GMC biomass left on the soil surface and conserved soil water by reducing evaporation.

CONCLUSIONS

Results from this study showed that GMCs can return significant amounts of residues into the reclaimed tidal lands, although the amount of residue input had a high fluctuation by growing conditions. Summer GMCs, especially SS hybrid, produced large amounts of residues in a short period of 12 weeks. Green manure crops significantly improved soil organic matter content and bulk density of surface soil, although management practices to facilitate nutrient cycling and residue decomposition should be considered. Further research is needed to determine the concentration of readily available or mineralizable nutrients under GMCs for a better understanding of net release or nutrient contribution to subsequent crops.

적 요

새로 조성된 간척농지의 토양개량을 위하여 새만금간척지에 서 2007년부터 2011년까지 4년 동안 녹비작물 (여름철 : 세스 바니아, 제주재래피, 수수×수단그라스, 겨울철 : 귀리, 호밀, 보 리) 재배 및 시용이 토양이화학성에 미치는 영향을 조사하여 얻은 결과는 다음과 같다.

이상의 결과로 미루어 볼 때 간척지에서의 녹비작물 재배 및 시용은 토양의 비옥도 요인을 일부 개선시킴으로써 간척농 지 개량에 기여할 수 있을 것으로 기대된다. 앞으로 신간척지 에서의 유기물과 관련된 다각적인 연구가 필요할 것으로 생각 된다.