The Status and Evaluation of M ain Soil Nutrients in Xilin Gol Grassland

Jiguang SHI

Xilinhot National Climate Observing Station，Xil in hot026000，China

Soil is considered the＂skin of the earth＂with interfaces between the lithosphere，hydrosphere，atmosphere and biosphere.Soil consists of a solid phase（minerals＆organic matter）as well as a porous phase that holds gases and water.Accordingly，soils are often treated as a three-state system.Soil is the foundation of farming and animal husbandry，and the abundance of soil nutrient is not only related to the growth of plants，farming and animal husbandry product quality and the sustainable development of ecological environment［1］.

In recent years，due to the irrational use of grassland resources and the grassland ecosystem imbalance，64%of the grassland in Xilin Gol has degraded，the soil nutrients have been reduced and there has been a serious decline in the quality of the grassland［2-3］.Currently，the soil quality studies in China are mostly the sporadic studies of soil fertility and degradation，and there is a shortage of long-term monitoring and positioning researches，so that the soil quality evaluation data are short and the study area is not systematic［4］.

Gong Zitong［5］and Liu Duosen［6］conducted a study of how to distinguish the soil types.Su Qingrui［7］carried out the evaluation of black soil fertility.He Jieping et al［8］evaluated the soil nutrient in the Xilin River Basin.Zhao Na et al［9］conducted a study of the relationship between elm density and soil nutrient.However，the studies on long-term monitoring and positioning of grassland soil nutrient in Xilin Gol have not been reported.

Therefore，using SPSS 11.5 and Microsoft Excel 2003，we carry out comprehensive analysis and evaluation of the grassland soil nutrient in Xilin Gol，and reveal the characteristics and status of soil nutrient distribution，in order to provide a theoretical basis for promoting the stability of grassland ecosystem and improving the grassland productivity.

1 Materials and methods

1.1 Overview of the study areaXilin Gol（111°59′-120°00′E，42°32′-46°41′N）is one of 12 prefecture level divisions of Inner Mongolia，with a total area of 203000 km2.The seat is Xilinhot，while the area is 202580 km2.The league's economy is based on mining and agriculture.Xilin Gol borders Mongolia to the north，Chifeng，Tongliao and Hinggan League to the east，Ulanqab to thewestand Hebei to the south.Xilin Gol is divided into two county-level cities，one county and nine banners.The elevation is839-1 469m，and it featuresa semi-arid temperate continental climate.The average annual temperature is 2.5℃，the rainfall is 273 mm，and the sunshine hours are 3024.7 h.

There are three kinds of grassland in Xilin Gol League：the meadow grassland in the east，with the constructive species of Leymus chinensis，Stipa baicalensis and so on；typical grassland in the central part，with the constructive species of S.kryloyii，and Leymus chinensis；desertified grassland in the western part，with the constructive species of semi-shrub and so on.

The companion species of grassland in Xilin Gol include Cleistogenes squarrosa，Anemarrhena asphodeloides，Scorzonera subacauli，and Artemisia frigida.The soil is mainly based on Aeolian sandy soil，and there are also chestnut soil，calcic brown soil and meadow soil.

1.2 Data sources and methodsData are from the nutrient content of soil organicmatter，total nitrogen，total phosphorus and total potassium in Xilin Gol grassland，determined by the analysis laboratory of Xilinhot National Climate Observatory during the pe-riod 2006-2011.The samples for experimentare from 13 banners and counties and municipal ecological monitoring field in Xilin Gol，and are the soil samples obtained in early October each year.

S-shaped distribution point method is used to for sampling.The collection depth is30 cm，and 6 points are selected.Every 3 points are mixed into a soil sample，and 1 kg of soil sample is collected in each point.Then the plastic bag is used to evenly mix the soil samples in the points，and the quarteringmethod is used to keep 1 kg ofmixed soil sample.The samples are sealed in the plastic bag and brought to the laboratory for natural air drying.After the impurities are removed，the samples are sifted and kept for determination.

The determination of soil organic matter，total nitrogen，total phosphorus and total potassium is based on the soil agricultural chemical conventional analysis method for content analysis［10-12］.The grading standards for nutrient content are based on soil nutrient grading standards of the second national soil survey［13-14］（Table 1）.Soil variation is based on the study results of Yi Liang et al［15］and Hou Lin et al［16］.When the coefficient of variation（CV）≤10%，it is weak variation；when the coefficient of variation（CV）＞10%，it is moderate variation；when the coefficient of variation（CV）＞100%，it is intense variation.

Table1 Soilnutrientgradingstandardsofthesecondnationalsoilsurvey Unit：g／kg

1.3 Data analysisBased on the principal component analysis principle，the comprehensive evaluation of soil nutrient is carried out using SPSS 11.5 and Microsoft Excel 2003.

2 Results and analysis

2.1 The overall distribution of soil total nutrient content

Through the statistical analysis of data（Table 1，2），in the Xilin Gol grassland soil，the organic matter content is atmoderate level（Level 3）；total nitrogen content is atmoderate level（Level3）；total phosphorus content is very scarce（Level5）；total potassium content is rich（Level 2）.The organic matter，total nitrogen and total phosphorus are moderately variable；total potassium is weakly variable.The coefficients of variation are 39%，35%，25%and 9%，respectively.

In the meadow grassland soil，the organic matter content is rich（Level 2）；the total nitrogen content is moderate（Level3）；the total phosphorus content is very scarce（Level 5）；the total potassium content is rich（Level 2）.

In the typical grassland soil，the organic matter content is moderate（Level 3）；total nitrogen content is moderate（Level 3）；total phosphorus content is scarce（Level4）；total potassium content is rich（Level2）.

In the desertified grassland soil，the organic matter content is scarce（Level4）；total nitrogen content is very scarce（Level5）；total phosphorus content is very scarce（Level5）；total potassium content is rich（Level 2）.

2.2 Geographical distribution of soil total nutrient content

The regions with rich organic matter content（Level2）include West Ujimqin Banner，Plain and Bordered White Banner；the regions with moderate organic matter content（Level 3）include Plain Blue Banner，Wulagai Management Area，East Ujimqin Banner，Xilinhot City，Taibus Banner and Bordered Yellow Banner；the regions with scarce organic matter content（Level 4）include Sonid Left Banner，Abag Banner，Sonid Right Banner and Duolun County；the region with very scarce organic matter content（Level 5）is Erenhot City.

The region with rich total nitrogen content（Level 2）is Xilinhot City；the regionswithmoderate totalnitrogen content（Level 3）include West Ujimqin Banner，Plain Blue Banner，Plain and Bordered White Banner，East Ujimqin Banner，Taibus Banner，Wulagai Management Area；the regions with scarce total nitrogen content（Level 4）include Bordered Yellow Banner，Duolun County，Sonid Left Banner；the regions with very scarce total nitrogen content（Level5）include Abag Banner，Sonid Right Banner；the region with extremely scarce totalnitrogen content（Level 6）is Erenhot City.

The region withmoderate total phosphorus content（Level3）is Xilinhot City；the region with scarce total phosphorus content（Level 4）is West Ujimqin Banner；the regions with very scarce total phosphorus content（Level 5）include Plain and Bordered White Banner，Plain Blue Banner，Bordered Yellow Banner，Taibus Banner，East Ujimqin Banner，Abag Banner，Sonid Left Banner，Wulagai Management Area，Duolun County，Sonid Right Banner，and Erenhot City.

The regions with very rich total potassium content（Level 1）include Abag Banner，Sonid Left Banner and Wulagai Management Area；the regions with rich total potassium content（Level 2）include Erenhot City，East Ujimqin Banner，West Ujimqin Banner，Plain Blue Banner，Duolun County，Taibus Banner，Sonid Right Banner，Plain and Bordered White Banner，Xilinhot City，and Bordered Yellow Banner（Fig.1）.

2.3 Comprehensive evaluation of soil nutrientsUsing the principal component analysis in the multivariate statistical methods［17-18］can quantify the weight of various indicators，and avoid the subjective arbitrariness，thereby improving the objectivity and accuracy of the results.Through the preselection of principal com-ponent variables，we select two principal component variables for principal component analysis.Table3 shows that organic matter，total nitrogen and total phosphoru shave a high load on the first principal component，indicating that the first principal component basically reflects the information on the three indicators（organic matter，total nitrogen and total phosphorus）；total potassium hasa high load on the second principal component，indicating that the second principal component reflects the information on total potassium.

Table2 Theme as uredmeanofsoilnutrientinthebanners，countiesandcities　Unit：g／kg

Fig.1 Leveldistributionmapoforganic matter，totalnitrogen，totalphosphorusandtotalpotassiuminXilinGolGrassland

Table3 Indicatorselectionandtheprincipalcomponentloadingmatrix

The variance contribution rate of principal components is 66.24%and 21.34%，respectively，and the cumulative contribution rate reaches 87.58%，representing 87.58%of the information on the samples for experiment and 12.42%of information loss. Using the principal component analysis，we quantify the selected soil indicators.The higher the overall quantification score，the higher the integrated nutrient.The results show that the integrated soil nutrient is the highest in the Xil in hot natural grassland and the lowest in Erenhot City.In terms of the integrated soil nutrient，the grassland types are sequenced in descending order as follows：meadow grassland＞typical grassland＞desertified grassland（Table 4）.

Table4 Rankingofprincipalcomponent，compositescoreandsoilnutrient

3 Conclusions and discussions

The overall Xilin Gol grassland soil fertility is at the moderate level，and the basic trend of change is decreasing from southeast to northwest，which is consistent with the trend of change in the precipitation of the region［19］，and in line with the actual situation of grassland types.It indicates that the soil fertility is affected by the vegetation type factors.When there is change in the vegetation community，there is also change in the accumulation and distribution of soil nutrients.The soil nutrients are very sensitive to changes in vegetation，and there is a significant positive correlation between different vegetation level characteristics and soil nutrient characteristics［20-22］.

At the same time，we also derive the results inconsistent with this conclusion.The ranking of soil fertility of the same grassland type is not in line with the ranking of pasture nutrients by Shi Jiguang［23］.This may be caused by the unsynchronized sampling of soil and grass，uneven distribution of grass types，and the sampling limitations.The soil fertility of different types of grassland is not necessarily correlated with the nutrientsof pasture.Due to the difference in the population structure of different grassland types，it is necessary to further study whether there is universality.

As for the content of main soil nutrients，only the total phosphorus is very scarce in the Xilin Gol grassland soil；organic matter，total nitrogen and total potassium are all above average.As for the variability，total potassium is weakly variable，but others are all moderately variable.In terms of the integrated soil nutrient，the grassland types are sequenced in descending order as follows：meadow grassland＞typical grassland＞desertified grassland.

This study result reveals the current situation of Xilin Gol grassland soil nutrient，provides a theoretical basis for the comprehensive understanding of the grassland soil nutrient status，and guides the rational use of grassland resources.

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