| Other Abstract | As the important carbon and nitrogen stock, soil has the important status in the global
carbon and nitrogen cycle, which affect directly the greenhouse gas emissions of terrestrial
ecosystem and global climate change. Correct Estimating the soil organic carbon (SOC) and
total nitrogen (TN) level and its change，is the important content of studying global climate
change, is also the hot fields of international geological and ecological community in recent
years. The Loess Plateau is the main ecological regions. Along with the implementation of
returning farmland to forest and grass, it is one of the important contents of environment
effect evaluation that studying the distribution characteristics, stock estimation and future
development trend of SOC and TN. In this paper, Zhi Fanggou is taken as the study area,
where belongs to typical small catchment in the Loess Plateau. The area is studied to explore
its spatial variation, distribution characteristics and influencing factors of soil organic carbon
content (SOCC) and total nitrogen content (TNC) under the condition of different land use
types, by using GPS to even the stationing of the whole catchment, combined with the
traditional statistics, geostatistics and GIS. Through study, it is revealed that the
Spatial-Temporal Variability of SOC and TN in the catchment during 20 years. And,
catchment-scale SOC is simulated and predicted with Century model, which is one dynamic
model of SOC. The main research results as follows:
1. The average SOCC and TNC in depth of 0-100cm are 3.08 g·kg -1 , 0.32 g·kg -1
respectively. SOCC and TNC decreases with the increase of soil depth, the relationship of
which can be expressed by power function. Every layer’s SOCC and TN belong to medium
intensity variation, and conform to logarithmic normal distribution, which can be fitted with
different theoretical model. SOCC has medium spatial correlation above 40 cm, affected by
random and structural factors together. And it has strong spatial correlation under 40 cm,
mainly affected by structural factors. TNC has strong spatial correlation in the two layers of 0-10cm, 40-60cm, mainly affected by structural factors. And it has medium spatial
correlation in the remained 4 layers, affected by random and structural factors together.
These spatial distribution differences about the catchment have a close relationship with
factors such as the terrain, land use and human activity. The situation of SOC spatial
distribution can be directly reflected by ordinary Kriging interpolation.
2. In depth of 0-100cm, the content and density of SOC and TN are as follows: shrub >
forest > grassland > farmland, and the soil bulk density is in the order: farmland > grassland >
forest > shrub. The soil bulk density of farmland in each layer is significantly higher than
that of other land use type. The depth is not obvious effect on soil bulk density. The
relationship between the soil bulk density and SOCC can be represented by a power function.
Under different land use types, there are extremely significant linear correlation (p < 0.01)
between SOC and TN. From big to small, The C/N is in the order: forest > shrub > farmland >
grassland. C/N of grassland, forest and farmland decreases with the increase of soil depth,
but that of shrub is not obvious. SOCC of 8° -15° slope is significantly higher than that of
other slope (p > 0.05), while < 5° slope significantly lower than other slope. TNC of 15°-25°
slope is significantly lower than that of other slope, and the difference between other slopes
is not significant. SOCC and TNC in different aspects are shown as: cloudy slope > sunny
slope > half sunny slope > half cloudy slope. It is showed the similar trends that SOCC and
TNC vary with the change of altitude: with the increase of altitude, first increased, and then
decreased. In vertical direction, the average level is higher for the profile distribution of
SOCC and TNC above 60 cm, and the trend reduced with the increase of the depth is
obvious. The average SOCC and TNC below 60 cm are low and stable.
3. In 0-100cm depth of Zhi Fanggou catchment, the average soil organic carbon density
(SOCD) and total nitrogen density (TND) are 2.63 kgC·m -2 and 0.267 kgC·m -2 respectively,
and soil organic carbon stock (SOCS) and total nitrogen stock (TNS) are 21.62 ×10 6 kgC,
2.20×10 6 kgN respectively. SOCD has strong spatial correlation, affected by structural
factors. And TND has medium spatial correlation, affected by random and structural factors
together. SOCD and TND are fitted with semivariance function, whose curve tendency
accords with the index and spherical model respectively. They show the ladder-like and
annular pattern in the spatial distribution. The different land use type、landforms and soil
texture is important influence factor to distribution characteristics of SOCD and TND. It
can be expressed by power function that the relationship between soil depth and SOCS and
TNS. The soil area account about 65.58% and 59.75% of the total area, whose SOCD and
TND are less than average value. These show that SOCC and TNC of most area is low in the
catchment, and soil total SOCS and TNS are small. 4. In recent two decades, SOCC and TNC of 0-20 cm are increased 2.37 g·kg -1 and 0.12
g·kg -1 respectively. SOCS and TNS compared to 1992 increase by 78% and 30%
respectively. Two phase data belong to medium variability and conform to logarithmic
normal distribution. In 2010, the variation coefficient and spatial correlation degree of SOC
and TN are higher than ones in 1992. Range data shows that the spatial distribution
uniformity is abated in the study area, small scale variation is enhanced, and the overall
distribution is more complex during the past 20 years. Spatial distribution show that the high
value area from the catchment periphery of higher terrain to low-lying area of relatively flat
terrain. It is the main reason for spatial-temporal variation that the land use change and
human activity caused by returning farmland to forest and grass.
5. In this paper, the Century model is first used in the Loess Plateau, with which the
simulation and prediction in catchment scale are made. The results show that the Century
model can be applied to simulate the SOC dynamic changes of the catchment in the Loess
Plateau. In the balance status of Century model running, the total SOC pool in the study area
is 1.792 kgC·m -2 , and the active, slow and passive SOC pool are respectively 3%, 68% and
29% of the total. After the balance was destroyed, the total SOC has the change trend of
“increasing-reduction-increasing”, and final more than the original natural state of
equilibrium. the change of each component SOC pool has a close relationship with the land
use type and agricultural management mode. Simulated with Century model, in 2110, SOCD
and SOCS in depth of 0-20 cm are 1.86kgC/m 2 , 15.4×10 6 kgC respectively. Under varied
land use types, SOC increase in different degree, which shows obvious soil "carbon sink"
effect. In the study area, land use and landform will affect the spatial distribution of SOC for
a long time.
Key Words：The Loess Plateau; Soil organic C,N; Environmental variables；GIS；
Spatial-temporal variability; Century model |
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