1. Introduction

Land transformation of rangelands to croplands and their subsequent abandonment is a significant ecosystem change that causes land degradation in dryland. In Mongolia, cropping was initiated in the 1950s under the Soviet regime; however, crop abandonment has increased since 1990 because of the shift from socialism to a market economy caused by dissolution of the Soviet Union.

Dominant species, such as Stipa krylovii, of a typical perennial grass are not found in abandoned croplands even decades after abandonment. In addition, changes are observed in soil physicochemical properties, decreasing organic nutrients, particle size distribution, and hydraulic properties. The biotic and abiotic legacy of crop abandonment results in poor adaptation of the regional species pool and lead to areas remaining in a degraded state for decades. Since grazing has been practiced for many years in Mongolia, the regional species pool is completely adapted to grazing disturbance. Therefore, it is important to clarify the altered physicochemical properties that are acceptable and not acceptable for plant species in Mongolian rangelands.

Many studies have reported that plant growth is highly associated with precipitation and soil water stress; however, studies on unsaturated hydraulic properties in semi-arid regions are few. In particular, the hydraulic properties of the soil profile under different climatic conditions are required to clarify how degraded the conditions are in abandoned croplands.

In addition, drought, due to sparse rainfall, and salinity (nutrient), due to excess fertilizer use, are two serious plant stressors in the semi-arid ecosystems of abandoned croplands in Mongolia. However, previous evaluations of drought and salinity stresses for plants have been based on individual static treatments, such as continuous wet or dry conditions, even though drought and salinity stresses vary continuously in situ. Hence, information on stress tolerance under various drought and salinity stresses is required for typical species in the Mongolian rangelands. An understanding of how these drought and salinity stresses affect altered soil hydraulic properties under different climatic conditions is also required.

Therefore, the objective of this study was to evaluate the effects of crop abandonment on vegetation and soil properties, and to provide vital knowledge on species adaptation for soil properties (Chapter 2), in particular soil hydraulic properties (Chapter 3), with stress tolerance (Chapter 4) under altered soil hydrological processes following crop abandonment (Chapter 5) in semi-arid Mongolian rangelands.

Study sites were croplands that had been abandoned for 18 years (CA 18), 9 years (CA9), and 2 years (CA2), and a grazed semi-natural grassland (SNG) with a grazing gradient (10, 50, 100, 200, and 500 m from the center of a livestock holding) provided representative sites of a grazing-disturbance regime of semi-arid grasslands in Mongolia. Each study site was a moderately grazed grassland situated in the buffer zone of Hustai National Park, where the livestock number is strictly controlled.

2. Response of plant functional traits to crop abandonment in rangelands

Functional traits are a consequence of historical selective pressures. I therefore used the change in composition of functional traits along a grazing gradient as representation of vegetation and soil variation due to adaptation to livestock grazing. I then examined croplands abandoned at different times to investigate the differences in the composition of functional traits and soil properties from those of rangelands.

Moderate grazing (100, 200, and 500 m, and SNG) and the late successional trajectory of abandoned croplands were represented by an increase in zoochorous species and fine sand content. S. krylovii was the most frequently found zoochorous species. Rhizome-forming species were common on intensively grazed rangeland (10 and 50 m) and during the early successional trajectory of abandoned croplands (CA9). The main rhizomatous species was Leymus chinensis. S. krylovii and L. chinensis were considered important species during the late and early successional trajectories of abandoned croplands, respectively. The CA2 site, where there was a higher content of coarse sand, was characterized by an absence of zoochorous and rhizomatous perennial species. The plant functional traits, which differed from those of the rangelands, were apparent in the abandoned croplands, where a difference in particle size distribution influences soil hydraulic properties.

3. Effects of changes of soil hydraulic properties on plant transpiration following crop abandonment in semi-arid Mongolian grasslands

To investigate changes in the hydraulic properties of a soil profile under different climatic conditions following crop abandonment, I measured the unsaturated hydraulic properties of undisturbed core soil samples from different depths (i.e., 0-5, 10-15, and 30-35 cm) at abandoned croplands (CA2, CA9, and CA18) and the SNG site. The hydrological properties of soil profiles were simulated with soil hydraulic properties at the study site under drought and non-drought conditions. I selected data from non-drought (1999) and drought (2004) summers as a typical climate range. The amounts of precipitation during the period considered were 158 and 81 mm, respectively.

Although the difference in volumetric water content was not large, there were differences in the pressure head at the soil surface among sites in drought and non-drought years. The present study suggests that the pressure head at the soil surface indicated the soil hydraulic condition more clearly than volumetric water content. The pressure heads at the soil surface at the CA9 and CA18 sites were lower than those of the CA2 site because of the high fine sand content. In drought conditions, the soil pressure head at CA18 was lower than that at CA9 for subsurface soil. This suggests that a change in soil hydraulic properties under a successional trajectory of abandoned croplands readily occurs in drought years. In addition, this change may contribute to preventing the growth of plants such as annual species, which rely on soil water in the surface soil for growth.

4. Comparing the drought and salinity tolerances of two dominant perennial grasses in Mongolia

Pot experiments were conducted in a greenhouse to compare the tolerance of two dominant perennial grasses to drought and salinity stresses in continuously changing drought and salinity conditions. The tolerances are evaluated in terms of parameter values of a root water uptake model.

The drought and salinity tolerances of these species appeared to depend on the degree of matric and osmotic head. L. chinensis had a higher tolerance to relatively low drought and salinity stresses than S. krylovii. On the other hand, S. krylovii had a higher tolerance to relatively high drought stress than L. chinensis. For high salinity stress, the measured osmotic head in the pot experiment was extremely extended, so the osmotic head was estimated from the electrical conductivity of field data. I therefore simulated only the effect of drought stress tolerance with soil hydraulic properties on cumulative transpiration in Chapter 5.

5. Effects of invasion of perennial grasses on soil hydraulic properties in abandoned croplands

A numerical simulation was developed to clarify the effects of drought tolerances of L. chinensis and S. krylovii on transpiration and soil hydraulic properties at different times after fields had been abandoned under drought and non-drought climatic conditions. The effect of drought tolerance of each species was evaluated by cumulative transpiration.

The cumulative transpiration of L. chinensis was higher than that of S. krylovii at each site in non-drought years with a relatively high pressure head. On the other hand, the cumulative transpiration of S. krylovii was higher than that of L. chinensis at each site in drought years with a relatively low pressure head. In addition, S. krylovii seemed to make conditions less favorable for other plants in terms of the pressure head; that is, a low pressure head was created as S. krylovii grew. This may indicate the competitive superiority of S. krylovii in dry Mongolian rangelands. In addition, drought would create an opportunity for the replacement of L chinensis by S. krylovii in abandoned croplands. This implies that substantial annual variability in climatic conditions may contribute to plant species replacement in Mongolian rangelands.

6. Conclusion

The present study finding that the typical dry conditions of the soil surface and climate can play an important role in the successional trajectory of abandoned croplands. Crop abandonment seemed to lead to a high-pressure head condition at the surface of the soil, particularly immediately after crop abandonment, which decreases the advantage of drought tolerance of typical perennial grass species. The results of the present study imply that hydraulic conditions, which promote infiltration to the subsurface soil where typical perennial grass species may have an advantage of root water uptake, are a cause for the successional trajectory in rangelands.

20世紀以降、社会経済的背景の変化により、耕作放棄地面積の増加が指摘されている。特に、草原域においては、最も開墾されやすい植生帯であるとともに、耕作放棄された場合、土地荒廃が引き起こされることが指摘されている。そこで、本研究では、国土の97%が草原であり、草原を利用した遊牧が長期にわたり営まれてきたモンゴル国の耕作放棄地を対象地とした。モンゴル国では、1950年代以降、旧ソ連体制の下、大規模な開墾が行われ、1990年代の旧ソ連体制崩壊とともにその多くが放棄された。この様な耕作放棄地では、家畜の嗜好性の低い一年生草本の優占や土壌構造の変化に伴う土壌水分環境の変化が指摘されている。このような耕作放棄地を対象に、一年生草本の刈り取り試験や多年生草本の播種試験が行われてきたが、耕作放棄地の土壌環境や多年生草本の生育に必要な環境が不明であることや、干ばつの影響により、期待した成果を得られていない。そこで、本研究では、土壌荒廃や半乾燥気候を踏まえた詳細な現状分析を行い、放牧草原における耕作放棄地の管理への提言を行うことを目的とした。

本研究では、放棄年代別のサイト(放棄後2、9、18年)と対照地である、耕作履歴のない放牧草原と放牧強度別の植生・土壌環境を把握するために、家畜の冬営地を設定した。

まず、耕作とその放棄が土壌環境や植生及ぼす影響について、植物の種特性と土壌理化学性を典型的な放牧草原と比較することで評価した。種特性は、生態系に与えられてきた、長期的なストレスである、気象条件や土地利用によって形成されるといわれており、本研究では、土地荒廃の程度を把握するため、種特性を用いた。その結果、耕作放棄初期では、放牧地において典型的な種特性が確認されない地点が放棄年代別に存在した。耕作放棄初期の典型的な種特性が確認されない地点では、土壌炭素・窒素量が少なく、リン酸量や粗砂が他の地点よりも多い傾向があった。一方、耕作放棄後9-18年経過したものの、放牧草原の典型的な種特性が確認されない地点では、細砂の含有率が典型的な放牧草原よりも少ないことが明らかになった。また、種特性の出現頻度を用いた統計解析から、耕作放棄後重要な種特性は、放棄後9年の耕作放棄地では、根茎繁殖を行う種であり、放棄後18年程度経過した耕作放棄地では、動物媒介の種子散布形態をとる種であることが明らかになった。これらの種特性は、前者がLeymus chinensis、後者がStipa kryloviiに代表されることが明らかになった。

次に、耕作放棄地の種特性において、典型的な放牧草原と異なる種特性が確認されたところでは、粒径組成が異なることが示されたことから、耕作放棄後の土壌断面における水移動特性の変化について調べた。この際に、半乾燥地特有の、不規則に発生する干ばつの影響を考慮して、干ばつ年とそうでない年との土壌水分移動特性について調べた。その結果、耕作放棄直後では、粗砂の含有率が高いため、透水係数が大きく、土壌水のサクションが低く維持されているのに対し、放棄後9-18年経過した耕作放棄地では、多年生草本の生育により、土壌地表面の安定に伴い、細砂が捕捉され、細砂の堆積に伴い、透水係数が小さくなり、土壌水のサクションが高い傾向であることが示唆されました。また、この傾向は干ばつ年に促進される可能性が示唆された。

ここまで、耕作放棄地においては、放棄年数の経過によって、土壌水分移動特性が異なることが分かったが、耕作放棄地では、同時に耕作時の施肥の影響により、土壌の富栄養化が進行し、それによる植生回復の遅れが指摘されている。そこで、種特性の解析から示された、放棄初期に重要種である、L. chinensisと放棄9-18年目において重要種であるS. kryloviiの耐乾性・耐栄養塩類性についてポット試験により検証した。その結果、蒸散量半減させる程度の乾燥・栄養塩ストレスに対しては、L. chinensisの方に耐性があり、非常に高い乾燥ストレスについては、S. kryloviiの方に耐性があることが示唆された。

次に、放棄年数に伴う土壌水分環境の変化が気象条件によって異なり、それに伴い重要主が変化し、重要種間にも耐乾性に違いがあることが示された。よって、放棄年数の経過や気象条件に伴い変化する土壌水分環境に対して、L. chinensisとS. kryloviiがどのようにストレス耐性を発揮しているか、成長期間の積算蒸散量から評価した。その結果、L. chinensisとS. kryloviiの耐乾性の違いにより成長量の差は放棄年別の土壌水分移動特性の違いよりも降水量の違いに顕著に現れ、L. chinensisからS. kryloviiへの置き換わりは干ばつ年において促進される可能性が示唆された。

以上から、放棄初期は土壌水分環境が典型草原と大きく異なり、土壌水分環境と気象条件との相互作用といいた、日生物的な要因が典型的な植物群落における優占種への置き換わりに影響していると考えられた。よって、モンゴルの半乾燥地放牧草原に典型的な乾燥した条件や、半乾燥地域特有の不規則に発生する干ばつといった環境が重要であり、放棄初期には、典型的な土壌水分環境の創出も含めた修復技術が必要であることが示唆された。また、放棄後10年程度経過し、多年生草本の生育が確認されるようになった際には、干ばつといった半乾燥地特有の気象条件を踏まえた修復技術が必要であると考えられた。

以上要するに、本研究は、乾燥地の耕作放棄地における土地荒廃に対して、土壌化学、土壌物理学に加え、生態学、植物生理学の視点を取り込んだ評価手法によって、乾燥地の耕作放棄地に今後期待される管理に関する重要な知見を提供した。得られた知見は、学術的な価値を有するだけでなく、応用的側面でも有用な結論を導き出しており、審査委員一同、博士(農学)の学位を与えるに十分値する論文であると判断した。