| Soil degradation can be defined as a decline of soil productivity, quality and sustainability which caused by natural and, particularly, anthropogenic factors leading the loss of physical, chemical and microbial processes in the past, current and the future. As a kernel process of land deterioration, soil degradation impacts global climate through damaging resources pool and productivity of ecosystems, emitting greenhouse gases, unbalancing water and energy, resulting in unstable global politic and society. Soil restoration, therefore, become to one of the most demanded projects to agricultural and environmental development also a hot topic of ecological and pedological research programs in the 21st century. Soil restoration has been initiated in some developed countries for some time with the principal purpose of improving sol fertility for agricultural productivity but less for improving natural ecosystem's restoring. Traditionally, programs for restoring damaged ecosystems emphasized on soil modifications suit desire species, rather than using inherent resources (adapted species and their matching relationship with soils) of the degraded ecosystems that can positively improve the soil. Therefore, soil restoration has been reported mostly as a description of so called 'the ecological effect' of revegetation. Soil degradation in Xerothermic regions in the world brings more risks to the ecosystems and agriculture because it is commonly indicated a dry soil condition, serious erosion and climatic extremes. Thus, increasing importance has been attached to soil ecosystems to address the constraints, to understand the process of soil degradation in this region in recent ten years but rare research program concerned about the soil restoration. This provides plenty of opportunities to this study.Xerothermic valley regions in South Western China is characterized by hot and seasonal dry regional climate with reasonable rainfall but extremely high evaporation and soil and water loss caused by erosion due to the loss of vegetation and developed hilly and mountainous terrain. It has suffered and still at the risk of land degradation and subsequent soil degradation which is characterized by extremely low vegetative cover, hot and severe drought in the whole dry season and scorching spells in the rain season, erosion prone, low nutrition and unavailable water supply capacity to plants, making it a most revegetation difficult region in China. Although relevant ecology-based programs on soil degradation in these regions have just at the beginning and rarely on soil restoration, the clear facts have being indicated that soil degradation in these areas is largely attributed to water erosion caused by deforestation, overgrazing and over-exploitation of wild-lands. Moreover, ecology-based programs on soil and/or land degradation and their rehabilitation or reclamation in these areas have been decades behind that focus on revegetation practices mainly through afforestation and rely mainly on exoticspecies rather than native genes of grasses which is the most dominant species of local vegetation. This brings series technical disadvantages to maintain the establishment because of low sustainabiliry and high costs of the mono-specie artificial community. In addition, there has been a long time absence of theoretical supports of all of the restoration approaches, particularly, the understanding of soil condition that is the critical and essential base of vegetation and their functioning to natural ecosystems. This study is an initiation of soil restoration that emphasize on addressing effects of restoring native dominant grasses on two type of soil ecosystems, Xerothermic soil and Vertisol, and the mechanisms of few functioning processes, with the following objectives: i) identifying the role and functions of native herbage in the restoration of degraded soil ecosystems in xerothermic valley regions by understanding their major coenogical characteristics, revealing the effects of restoring native dominant grasses through closed conservation on soil water regimes, soil fertility, soil microbes and the dynamics of vegetation alteration, and, understanding mechanisms of the principal self - repairing processes; ii) presenting a new approach and concept for a sustainable revegetation on degraded ecosystems in xerothermic valley regions.This research emphasize mostly on the hilly and mountainous zone with the altitude of 1000-1350m that has been long recognized as the most difficult in revegetation in Yuanmou Longchuan River valley, a preventative of Xerothermic valley regions, Yunnan province. Field survey, long-term located sites method, quantitative and qualitative analysis, pot experiment and field trials were employed to assemble data. Field survey covers most parts of soil degraded areas through the investigation at 21 sampling plots. Long-term located experimentations, pot experiments and field trials were conducted at the Institute for Ecological agriculture in tropics, YAAS, which is sited in this valley region. The long-term located experimentations are based on the very simple concept of closing plant conservation in 1992, and the data mostly covers a term from 1992 - 2004. Two adjacent free grazing wild-lands were taken as control sites for the two degraded soil types respectively. Excel and STATISTICA soft wares were used in data and correlation analysis. Several results and major conclusions obtained are as follows:1. COENOGICAL CHARACTERISTICS OF HERBACEOUS PLANTS OF DEGRADED SOILS1.1 Vegetation structure on degraded soilsThere are 96 plant species, of which 7 are arbores, 14 are bushes and 65 are herbs on degraded Xerothermic soils and Vertisols, posing a vegetation landscape of open herbosa grassland and / or sparse patch grassland, even bare land, indicating the regressive succession. Herbs aboveground biomass is accounted for 59.53%~ 100.00% of the total of the community, largely contributing to the materialcycling of the system. Herbaceous species take the responsibility for material cycling ,play an important role in maintaining the structure, functions and capabilities of degraded soil-plants systems against to adverse environment.1.2 Species composition and biomass of native herbageThere are 62 herb species from 15 plots of degraded Xerothermic soils grouped into 14 families, of which 25 are Gramineaes, 12 are Papilionaceaes, 7 are Compositaes and 14 belong to rest 11 families. However, there 23 herbs from 6 plots of degraded Vertisols classified to 6 families, of which 9 are Gramineaes, 8 are Papilionaceaes, 2 are Euphorbiaceae and 4 are the rest families. Graminaceous species contributes? 1.19% ~ 100.00% and 62.85% ~ 95.31% in Xerothermic soils and Vertisols, respectively. It can be concluded that Graminaceous plants dominated the herbage and the primary productivity of degraded soil systems in this region. The significant difference of relative biomass was not found between those two soil systems indicating a catholicity of above conclusion.1.3 The dominant herbaceous plants on degraded soilsFor the majority of dominant species, composition differed statistically between sampled areas even within the same type of soil's plots resulting in the diverse community patterns with particular dominants adapted to specific soils and environment. The differences largely associated with parent materials, interferences, severity of degradation and the erodibility. With the 100.00% presence, Heteropogon contortus (H. c) has the >60% SDR value in more than 53.33% of 15 plots showing the highest dominance on degraded Xerothermic soils. Bothriocola pertusa (B. p.) and /or Dichanthium annulatum (D. a) present on all Vertisols and Xerothermic soils with pH>7.6, and B. p. is more dominant in open areas. This implies that soil pH may come up to one of the important determinants of dominant species on degraded soils.No significance of grasses' aboveground biomass correlates to soil organic matter, total nitrogen, total phosphor and soil bulk density (n=21, r<0.1524,p<0.5); but extremely significant positive correlations were found to plant coverage (n=21, r=0.5907, p<0.005) and average height (n=21, r=0.6146, p<0.003), high positive correlation was found to species number (n=21, r=0.3605, p<0.1).2. ECOLOGICAL EFFECTS OF SOIL RESTORATION BY RESTORING NATIVE HERBAGE Continues 10 years observations and experimentations at five located sites produced followingmajor results:2.1 Impacts of res tor ing herbs on soil water condition26 Months water content data from the restored two types of soil ecosystems in the 0~180mprofile from 2002 to 2004 showed that significant alterations of soil water condition occurred during theten years restoration. Firstly, infiltration increased. With comparison of the control plots (free grazing), the infiltration depth increased notably in Xerothermic soil restoring Heteropogon contortus community (H. c) with an increment of 100m, but less significant in Vertisol restoring Bothriocola pertusa (B. p ), and Dichanthium annulatum (D. a) with the 20cm and 40cm deeper infiltration, respectively. Secondly, water-storage capacity of above three varied dramatically between restored plots and their controls, also between those tree restored sites, with the increments, respectively, of 63 mm, 36mm, and 102mm of the annual average value, 60 mm, 45mm and 107 mm of the extremes in dry seasons and 85mm, 20mm and 92mm of the extremes in wet seasons. The average volumetric water content of 0-180cm in rain season, periods did not differ prominently with the respective increment of 4.0%, 5.1% and 4.5%; 1.5%, 1.3%, 1.9% and; 5.4%, 5.4% and 5.3%.However, remarkable differences were recorded in the dry seasons in the two degraded soil types. In Xerothermic soils, there is a persistent-low-moisture-extreme period in both restored and grazing sites, but the water content showed 6.0% higher in the former than the later. Moreover, the extremes only occurred at 20cm layer in the restored site and persisted 20 days, 10 days less than that in the grazing site. The fact was much more serious in the grazing site with 20 days persistence through out the whole profile and surpassed 70 days within 60cm. Data from the grazing Vertisol sites showed that low extremes was 1.8% and 6.45% less than that of the soil under B. p and D. a lasting trough the whole dry season and the beginning of rain season at 20 ~ 80cm layer, whereas the two restored sites presented the same extremes merely in 20 ~ 40cm lasting 70 ~ 80 days. The water content at 20 ~ 60cm increased 16.5% and 11.8%, respectively, further indicating the alteration.It is worth to notice that the water shortage remains in the dry season, although the soil moisture condition improved by the restoration, intrinsically due to the unchangeable climate pattern.2.2 Impacts of herbaceous communities on soil fertilitySoil fertility data of 1992, 1999 and 2002 indicated that grasses restoring has resulted in increases in the top 0 ~ 20cm of all analyzed factors, namely, soil organic mater (SOM), total nitrogen (TN), available nitrogen (AN) and available potassium (AK), but declines in pH; available phosphorus remained trace though out ten years in all sites. Grasses Restoration created substantial increases of SOM from 5.18g/kgto10.01g/kg in the Xerothermic soil under H. c, and 5.0g/kgto14.1g/kgand 5.1g/kg to11.8g/kg in the Vertisols under B. p and D. a, respectively, whereas slightly decline in the 0-10cm layer in the two controls. All other factors increased statistically along SOM, but with less increment except AK.The presence of herbaceous plants contributed to the restoration of soil fertility. Positivesignificance was found between SOM and all factors of herbs communities, extremely high with aboveground biomass (AB) at p < 0.01, litter (LIT) and plants individuals (IN) at p < 0.05. Similar trends of TN were found except less significant with litter but preferentially with species richness (SR) at p < 0.05. A positively correlated to AB, LIT and the height of the communities at p < 0.5; CEC and AK showed the similar significance with IN, community cover (CC) and root biomass (RB) at p < 0.5; and pH had the extreme negative correlation to AB at < 0.01 and LIT at p < 0.1. In conclusion, a steady supply of organic materials by restored herb community through liter and roots, sorts of secretions and the activities of soil organism benefited from the plants are contributed to the fertility restoring, particularly, to the SOM, leading the co-improving of soil structure and other fertility factors.2.3 Effects of native herbaceous plants on principal soil microbesThe differences of total amount of soil microbes (TAM) were recorded in the top 0-10cm soil between the restoring sites and controls. The highest difference of total amount of three major microbes was noted in mid-rain-season (MR) and the end rain season (ER) in Xerothermic soils under H. c which is 1.78 and 2.56 times of the control, whereas in the ER in Vertisols under B. p and D.a with the multiples of 3.08 and 2.09, respectively. Highest levels of TAM were recorded in rain season, while the lowest were noted in mid dry season (MD) at all five sites. It was positively proved that water is the determinants of the activities of soil microbes in this area, even sufficient food are available.Higher levels of three major groups of microbes were recorded in the three restoring sites but merely occurred in 0 ~10cm topsoil. Bacteria was the most active group (50 % ~ 97%) principally in MR at the H. c Xerothermic soil sites (1.33 times of the control) and mainly at ER in B. p. (3.20 times of control) and D. a (1.21 times of control) Vertisols. Fungi and Actinomyces were more active in the restoring soils any seasons except the MR, particularly, in the ED which was 1.18 ~ 2.48 times of the controls.TAM and the amount of BC showed the extremely significant positive correlations to AB, IN, SR and RB of plant communities, and the significance to LIT and ground coverage; the number of Fungi showed extremely high correlations to the SR and RB. It can be thereby concluded that steady inputs of organic materials by the herbaceous communities through litter and root maintain the supply of organic carbon and nitrogen to the soil resulting the increase of soil microbes. Furthermore, species richening may lead to the establishment and propagation of rhizophytes, which are crucial to soil particles aggregation..2.4 Succession dynamic of plant communities in degraded soil ecosystemsNative species including bushes have been establishing along with the restoring. Species number inH .c. Xerothermic soil, B. c. Vertisol and D. c. Vertisol increased from 10, 7, and 16 in 1992 to 17, 15 and 31, respectively, in 2002,. Gramineaes sustained the highest dominance followed by Papilionaceaes during the succession. Perennial tufted grasses, perennial forbs, annuals and biennials gradually increased; Semi-shrub & sub-shrub species also augmented. Inter-invasions of grasses occurred between adjacent communities of the two types of soil. No any arbor and prominent bushes species invasion were recorded. However, Dodanaea augustifola in H. c. Xerothermic soil increased dramatically of Dodanaea augustifola from 0.00826 plant/m2 in 1992 to 0.273 plant /m2 in 2004; and Sophora davidi and Dodanaea augustifola in D. c. Vertisol increased sharply both from 0.00275 plant/m2 to 0.674 plant/m2 and 0.662 plant/m2, respectively, in the same period, posing a succession tendency to semi-savanna plant landscape that has been identified as the disclimax community in this Xerothermic valley region.2.5 Integrated assessment of the effects of herbaceous plants restoration on degraded soil Results of graphical analysis by using relative values of 14 indices of soil fertility, water condition, microbes and plant community revealed that restoring native herbaceous plants benefited most to B. p. Vertisol followed in sequence by H. c. Xerothermic soil and D. c. Vertisol. The most significant influences of restoring herbage on degraded soils are the establishment of ground cover with increment of 7.4~8.4 compared with control; the dramatic improvement on material productivity producing the increases of 15.86~22.84times aboveground biomass and 58.1 ~91.3 litter, which may positively contribute to soil organic matter accumulation. The notable improvement of soil water condition was recorded of the decreases of the duration of the low water content extremes which were could not be illustrated directly by the graphical analysis.3. MECHANISM OF NATIVE HERBS REPAIRING DEGRADED SOIL3.1 Strategies of population growth of dominant grasses to improve and develop the herbage communitiesResults of long-term observations and simulated trials showed that the high fertility with over 2340 seeds/m2; synaptosperm seeds transmission and the unique process buried into the soil by the spiral awns contributed wholly to the H. c. to colonize for new habitats and develop the population. Abundant seeds give presume the high population potential. Sharp and bristle-inserted seeds, not only could prevent themselves from damaging but also easily being transmitted. Moreover, the way of synaptosperm seeds spread pattern could reduce the loss with runoff. Most critically, the spiral awn plays the important role in seed burial in the field. 5~12 cm long awns twisted spirally together when matured and produced elasticity when they were slightly wetted burying 4.76%~7.69% seeds into thefields through the movement.B. c. presents the great performance on Vertisols by tufted growth of both culms and roots to avoid damaging by the swelling. In addition, the tuft produces many stolons (runners) that readily take root and expand crown to 190cm far from the original bunch. Another result showed that B. c. had the self-nutrition capability when there were slope erosion occurred by intercepting sediment and litter into tufted clums and making fertile "soil matrix", which has much higher nutrients content than the inter-tuft-soil. It played a role of providing refuge and nutrition to its propagulums.Less reliable results were found in this research about the mechanisms that D. c. develope the population. However, it is clear that D. c. poses a wonderful performance on degraded soils with high pH values (> 7.6 in this research) and showed very high dominance. So does B. c. The results therefore suggest that B. c. and D. c. may remain mechanisms of adaptability to high pH soils to be revealed in the future.3.2 HydrologicalJunctions of native herbaceous plants and their litterObservation results of 11 erosion events on Xerithermic soil under H. c. restoration and free-grazing in 1998 showed that restoring 6 years of H. c. dominated herbs could reduced 57% (gradient is about 5) runoff compared with the control. An indoor experiment indicated that the maximum water holding rate of native herbs' litter was 286.67%~304.72% while the capacity was 1.22~3.03kg/m2, equal to 0.12~0.3mm rainfall. The results of an field pots trial showed, after wetted by the water equal to 57mm rainfall, which is the mean value of the last raining month in the region, Xerothermic soil with litter mulching could kept the water more than 64 days while the bare soil only last 24 days; after wetted 64 days, the water content was 1.56 times of the initial moisture in B.p litter mulched Vertisol and 1.69 times in D. c. litter mulched ones.3.3 Relationship between the soil organic accumulation and the dynamics of litter-fall and litter decomposition of dominated species on degraded soilsA nine months monitoring result indicated that H. c, B. p. and D. c. retained 90.0 %~92.3 % of standing litter to the mid rain season in next year so that more organic materials accumulated due to less decomposition in the dry season. And, results from an incubated decomposition showed that the residues of H. a, B. p. and D. c. litter after 24 months decomposing in the corresponding degraded soils were 61.24%, 70.45% and 60.8%, respectively, posing low decomposition rates so that contributing organic matter to the soil. Further results from a test in early rain season in 2002 illustrated that the plants communities dominated by H. c, B. p. and D. c. produced 11.78g/1000cm\ 8.373g/l 000cm3. 4.165 g/1000cm more dead root biomass than open grazed plants providing reliable organic matter to thedegraded soil. All these results suggested that the loss of soil organic matter in the Xerothermic valley regions mainly due to the loss of ground biomass caused by overgrazing, inappropriate cutting rather than the high decomposition rates during the hot-wet rain season.3.4 Correlation of soil water stable aggregates to native herbage rootsIn layer 0~20cm, the significant correlation were found of the water stable aggregates in different degraded soils involved at l~2mm, >5mm and >0.25mm particle fractions and the aggregation state, aggregation ratio to the root biomass, the length of root and soil organic matter. Notable changes of soil structure conditions were produced by herbage restoring as follows:After 10 years of restoring herbage the soil micro-aggregates reduced in 0~20cm soil layer at the particle fraction of 0.25~0.05mm but increased significantly at >0.25mm in H.c. Xerothermic soils with the multiples 5.0(0~10cm soil layer)and 4.3( 10~20cm soil layer)of open grazing soils. Similar increase occurred in B. p. Vertisol and D. c. Vertisol with lower increments. Also, the percentage of aggregates 0.25~0.05mm and 0.05~~0.01mm prominently increased compared with the free grazing site in the restoring Vertisols while the percentage at O.001 declined. The aggregation state, degree of aggregation in H. c. Xerothermic soil were 4.0 times, 4.4 times (layer 0~10cm) and 3.5times, 3.9 times (layer 10~20cm), respectively, of that in control soil. Same multiples were 2.5, 3.7 and 1.3, 1.97 in B. p. Vertisol while were 2.2 > 2.9 and 1.33, 1.17 in B. p. Vertisol. The dispersion ratio of//, c. Xerothermic soil were 40% and 20% less than the control in layer 0~10cm and layer 10~20 cm respectively. The difference were close in two types of Vertisols and 6% and 10% lower than the control soil.Following major conclusions were drawn from above results:(0 Vegetation on Xerothermic soils and Vertisols in Yuanmou Longchuang River valley are posing a notable regressive succession. Species richness, bushy species decrease dramatically with the strengthen of degradation causes, such as erosion, grazing intensity etc., resulting a physiognomy of herbage dominant open dwarfing grasses, patching grasses, even bare lands.(2) Heteropogon contortus, Bothriocola pertusa and / or Dichanthium annulatum are the most potential and promising species for revegetation on degraded Xerothermic soils and Vertisols respectively, because of their high population growth through unique mechanism.(3) Accumulation of soil organic matter is attributed to the declined loss of biomass and ground organic mater, to the lag of formation of ground-litter, to the reduced microbes' activity in the dry season. The low decomposition rates of Heteropogon contortus on Xerothermic soils and Bothriocola pertusa and Dichanthium annulatum contributes to the soil organic mater as well.
|
PDF Full Text Request