A tentative discussion about plant-soil relationships on spatial heterogeneity by means of geostatistical approach.
Nature is simply too complex to be understood even when looking at a single fundamental question. (Dec 16, 2009. My Weblog)
“Changes of Soil Matrix and Vegetation Dynamic of Artemisia ordosica Krasch“
(Thesis Research, 2004 – 2007, Nankai University)
HOW WAS THIS TOPIC BROUGHT FORWARD?
During a graduate fieldtrip in grassland conservation in semi-arid region of Northwestern (NW) China, We found a significant increase of soil compaction where settled by Artemisia ordosica, a constructive plant species of sandyland. Later chronosequence studies on plant community implied a significant variation of soil properties through successional time. Then two questions came to me: How about the variation of soil properties in plant successional series? How about A. ordosica plant communities respond to soil heterogeneity in this area?
In other words, how is soil changed by plants, and how do plants respond to such changes?
WHY IS THIS RESEARCH?
- Why Artemisia ordosica: A. ordosica is the important constructive plant in sandy land, and distributes wildly over arid/semi-arid region (120, 000 km2) of NW China.
- Why this plant-soil relationship: The study of the vegetation process of A. ordosica and development of aeolian sandy soil would help to approach the stability mechanism of A. ordosica community, and provide theoretical bases for vegetation restoration in semi-arid/arid region where degradation of vegetation and desertification are notorious.
WHAT IS THIS RESEARCH ABOUT?
1. What are the changes in communities? 
1.1 What is the change of plant?
1.1.1 The successions of A. ordosica community
In study area (Fig 1):
A. ordosica community on semi-fixed sandy land (OS) →A. ordosica community on fixed sandy land (OF) →A. frigida community on fixed sandy land (FF) → Oxytropis aciphylla on sandy terrace (AT)
What is the main driving force of succession of A. ordosica?
1.2 What is the change of soil?
1.2.1 Development of aeolian sandy soil in successional community series
In the study area: shifting aeolian sandy soil → semi-fixed aeolian sandy soil → fixed aeolian sandy soil
1.2.2 What are the changes of soil properties?
Along with the successions of A. ordosica community from the early to the late stage,
- Soil Organic Matter (SOM) contents and Total Nitrogen contents increased significantly in the top soil, especially within 0-5cm depth of soil.
- CaCO3 was accumulated gradually and reached the peak value in 40cm depth in the late successional stage. (Fig. 2). And
- The number of Bacteria increased significantly within 0-20cm depth of soil. (Tab. 1)
How do the soil properties affect the plant successions?
1.2.3 What is the change of soil respiration?
Along with the successions of A. ordosica community from the early to the late stage,
- Soil Respiration Rate declined in 20-40 cm depth of soil. (Tab. 2)
So, all the above results indicated,
- With the accumulated of CaCO3, the caliche is formed in 20-40 cm depth of soil in the middle or late successional stages.
- The formation of caliche inhibited the activities of roots by undermining soil water condition in this depth in the middle or late successional stages.
- There was a same tendency between the increment of SOM content and accumulation of CaCO3.
Is the formation of pedogenic CaCO3 dominated by a biological process?
2. How do plants affect soil?
2.1 Is the formation of CaCO3 dominated by a biological process?
If the accumulation of CaCO3 is associated with SOM strongly, that is, dominated by the biological process, there should be a positive relationship between their contents and a coincidence in spatial distribution with each other.
- A significantly logarithmic relationship between SOM contents and CaCO3 was found by regression analysis. (Fig. 3) (Tab. 3)
- The coincidence in spatial distribution was found by geostatistics analysis on spatial heterogeneities, which was indicated by a significant positive power regression between Ds of CaCO3 content and of SOM content. (Fig. 4)
So, the formation of CaCO3 in A. ordosica community was dominated by a biological process. The SOM derived from the decomposition of litter fall of A. ordosica was an important source to the accumulation of CaCO3 in the study area.
How does SOM contribute to the formation of CaCO3 in A. ordosica community?
2.2 How about accumulation of CaCO3 derived from biological process?
The content of the annual accumulation of CaCO3 by biological process can be calculated by determination of annual precipitation of Ca2+ derived from decomposition of litter fall, and annual production of litter fall in plant community.
The calculation of decomposition rate follows an exponential decay model:
- Annual production of litter fall in study area (Tab. 4)
- Average contents of Ca in different organs of A. ordosica (Fig. 5)
3. How does the soil affect plants?
The influences of the change of soil properties on the dynamic of plant population/community in different successional stages can be interpreted by their relationships in spatial heterogeneities.
Spatial autocorrelation is calculated by semivariogram:
Scale problem and zoning problem
Criteria of the experimental design and divisions
- Essential characteristics of population/community should be shown within the sample plot
- The area of each block divided from sample plot should be enough to represent a certain microhabitat type
- The number of quadrate within every block should be equal, and N(h)>30, 50
- Divisions of sample plots of OS (early stage) and OF (middle stage) are showed in Fig. 6 and Fig. 7.
3.1 How do soil properties affect plant dynamic in early succession stage?
- Distribution pattern: A. ordosica population follows clumped distribution. (Tab. 5) (Tab. 6)
- Spatial Heterogeneity: D (or sill) of plant density and biomass varied with D (or sill) of soil moisture identically, which indicated that the spatial heterogeneity of plant density and biomass were correlated with that of soil moisture. In particular, a significant positive correlation between Ds (or sills) of biomass and soil moisture was found, indicating that soil moisture affected the spatial heterogeneity of biomass strongly in study area. (Fig. 8) (Fig. 9)
So, In early successional stage,
- The spatial pattern of soil moisture played a decisive role in affecting the distribution pattern of A. ordosica population.
How does the soil affect plants after A. frigida has appeared in the middle succession stage?
3.2 How do soil properties affect plant dynamic in middle successional stage?
3.2.1 Spatial distribution
- Distribution Pattern: Both A. ordosica and A. frigida population followed clumped distribution. (Tab. 7) (Tab. 8)
The spatial distributions of the density of A. frigida and of the soil moisture showed a similarity (Fig. 10 A, C), while the spatial distribution of density of A. ordosica and of SOM showed a similarity (Fig. 10 B, D).
Are there any mathematical bases of these relationships?
3.2.2 Spatial heterogeneity
- there was a significant positive correlation between sill of density of A. ordosica and of SOM, while no significant correlation between that of A. frigida and of SOM;
- There was a significant positive linear correlation between sill of density of A. frigida and soil moisture, while no significant correlation between that of A. ordosica and of SOM (Tab. 9) (Fig. 11)
So, in the middle succession stage,
- Soil moisture exerted great influence on A. frigida.
- SOM content was mainly contributed by A. ordosica. And
- more individuals of A. frigida distributed in quadrates of lower soil moisture, which indicated A. frigida has more competitive advantage than A. ordosica in drier microhabitat.
How about next stage?
WHAT ARE THE MAIN CONCLUSIONS?
- Along with the succession of A. ordosica community, the accumulation of CaCO3 was driven by a biological process which associated with the decomposition of SOM.
- Along with the succession of A. ordosica community, the settlement of A. ordosica enriched SOM content, and then promoted the formation of caliche in soil. Meanwhile, the formation of caliche caused the deterioration of soil water condition under this layer, thus facilitated the settlement of shallow rooted plant, such as A. frigida.
- The high spatial heterogeneity of soil moisture is a prerequisite to the stability of A. ordosica population in early succession stage. And the change in spatial heterogeneity of soil properties would dominate the changes of spatial distribution and the succession of A. ordosica community in this area.
WHAT IS NEW?
- This study can be considered as a tentative discussion towards “how to applying environmental spatial heterogeneity to explain the heterogeneity of vegetation”. In this study,
- We managed to use the combination of geostatistical and statistical methods to test the relationships between the changes of soil properties and the dynamic of vegetation in the A. ordosica community.
- We tried to analyze the relationships between plant and soil properties in spatial heterogeneities along the A. ordosica successional stages, and thus we discuss the stability mechanism of A. ordosica community by means of comparing the variations of spatial heterogeneities.
WHAT ARE THE DEFICIENCIES/FUTURE WORKS?
- The evaluation of Ca2+ precipitation has not been completed.
- We tried to analyze plant-soil interactions in the early and middle successional stages, but the interaction in the late succession stage has not been concerned.
- Similar researches should be carried out in larger scales to verify and extend current studies, and a combined means of GIS technology, remote sensing and fieldworks could be considered.
ANY NEW TOPICS GENERATED FROM THIS RESEARCH?
- This research also brings forth two topics to me as follows:
- Considering A. ordosica geographic populations distributed in NW China are meta-population, how about the origin and evolution of A. ordosica in NW China?
- Considering severe water stress would be caused by the accumulation of CaCO3 in soil, how about the physiological adaptability or phenotypic plasticity of A. ordosica in this area?
WHAT HAVE I LEARNT FROM THIS RESEARCH?
- Laboratory/Fieldwork skills
- Teamwork; Skepticism; Self-study; Independent thinking