Differences of Soil Nutrient Contents under Different Land Use Patterns in Niyang River Basin

Heping MA Zhu DONG

Abstract    [Objectives]  This study was conducted to investigate the variation characteristics of soil nutrient contents under different land use patterns in the middle and upper reaches of the Niyang River in Tibet.

[Methods] With the cultivated land， grass land and forest land in the middle and upper reaches of the Niyang River as the research objects， 216 soil samples were collected in layers （0-10， 10-20， 20-30 cm） by the standard sampling method， and the soil total nitrogen， alkali-hydrolyzable nitrogen， total potassium， available potassium， total phosphorus， available phosphorus and organic matter were determined.

[Results]  The contents of soil organic matter， total nitrogen and alkali-hydrolyzable nitrogen in the middle and upper reaches of Niyang River in Tibet ranked as forest land>grass land>cultivated land， and there were significant differences between the three （ P <0.05）. The total potassium contents of grass land and forest land were not much different， and the contents of available potassium exhibited an order of forest land> grass land>cultivated land. The distribution of soil total nutrients and available nutrients under different land use patterns showed a certain degree of surface aggregation. The contents of total nutrients and available nutrients in the soil at the 0-10 cm depth were significantly higher than those at 10-20 and 20-30 cm， and there were significant differences between the three （ P <0.05）. The average content of alkali-hydrolyzable nitrogen was the largest in forest soil and the smallest in cultivated land under the three land use patterns in the study area. The average content of soil available phosphorus in cultivated land  （19.47 mg/kg）  was significantly higher than those in grass land  （5.73 mg/kg）  and forest land  （5.19 mg/kg）.  The above results indicate that long-term vegetation restoration can improve the soil quality of the middle and upper reaches of the Niyang River.

[Conclusions] The results of this study research provide basic support for improving the soil effect in the area.

Key words   Soil nutrient; Land use type; Soil quality; Available nutrient; Tibet

Soil nutrient contents are the material basis and important indexes to measure soil fertility.  Soil nutrients are the main sources of plant nutrition and the material basis of soil fertility， as well as the basic attribute and essential characteristic of soil  [1-2] . The contents of organic matter and inorganic salts of nitrogen， phosphorus and potassium in soil are important factors to determine soil fertility and yield and quality of agricultural products  [3] . Soil nutrient contents are affected by many factors， and spatial variation is ubiquitous and complex. Therefore， the spatial distribution of soil nutrients is of great significance for guidance， scientific fertilization， precision agriculture， and field crop management  [4] .

Land use is an important reflection form of human agricultural activities， which directly affects soil nutrient status  [5-6] . Land use is closely related to soil nutrients. It is an important factor affecting the changes of soil organic carbon， total nitrogen， total phosphorus and other nutrients. The distribution of soil properties is different under different land use patterns  [7] . Therefore， it is of great significance to study the characteristics of soil nutrients under different land use patterns， maintain the ecological stability of vegetation， improve the effective use of soil nutrients， and improve soil texture and sustainable soil utilization. Based on this， in this study， with middle and upper reaches of the Niyang River in Tibet as the study area， the soil total nitrogen， alkali-hydrolyzable nitrogen， total phosphorus， total potassium， available phosphorus and available potassium under different land use patterns were determined， and the characteristics of soil nutrient contents under different land use patterns were analyzed， aiming to provide a scientific basis for soil nutrient management， soil fertility cultivation and land optimization and rational fertilization in the Niyang River Basin of Tibet.

Materials and Methods 

General situation of the sampling area

The experiment was carried out in the upper and middle reaches of the Niyang River in Nyingchi area （92°45′-93°53′ E，29°47′-29°54′ N）. The average altitude of this area is about  3 590 m;  the annual average temperature is 7-16 ℃， and the annual accumulated temperature above 10 ℃ is 2 272 ℃; the frost-free period is about 180 d; and the annual average precipitation is about 600-800 mm. This area has a plateau temperate monsoon semi-humid and semi-arid climate， which is warm， and it has a long frost-free period， abundant heat and abundant rainfall. However， the annual rainfall is unevenly distributed， and the dry and wet seasons are obvious. The precipitation from June to September accounts for more than 80% of the whole year. The soil in the area is mainly sandy loam  [8] .

Soil sample collection

GPS positioning and sampling were performed between Baiba Town and Songduo Township in the middle and upper reaches of the Niyang River on June 20， 2020. The layout of the plots and eight sampling points were determined. The distribution of  sampling  points included the main land use pattern （grass land， forest land and cultivated land） in the study area. Considering the characteristics of incomplete soil development and thin soil layers in the study area， multiple sampling points were set up in each land use pattern， and three replicates were set for each sampling point. The 0-10， 10-20 and 20-30 cm soil layers were sampled， and a total of 216 soil samples were obtained. The collected soil samples were transported back to the laboratory for treatment and used to determine soil properties. Meanwhile， the soil layer thickness， land use pattern， vegetation type， slope and other information of each sample point were investigated in detail  [9] . The  information of each sampling point in this study is shown in Table 1.

Determination of soil physical and chemical properties

Soil nutrients were determined according to the method of Lu Rukun  [10] . The soil pH value was determined by the potentiometric method with a soil-water ratio of 1∶ 5. The soil organic matter content was determined by the potassium dichromate oxidation-heating method. The total nitrogen content was determined by the semi-micro Kjeldahl method. The alkali-hydrolyzable nitrogen content was determined by the alkali-hydrolysis diffusion method. Total phosphorus and total potassium were determined by sulfuric acid-perchloric acid melting-molybdenum-antimony colorimetric method. The rapidly available phosphorus content was determined by sodium bicarbonate leaching-molybdenum-antimony colorimetric method. The rapidly available potassium content was determined by  amine  acetate extraction-flame photometry.

Data analysis

Excel 10 and SPSS 20 were used for data analysis， and one-way ANOVA was used for multiple comparisons （ P <0.05） to analyze the contents of relevant indices between different land use  patterns.

Results and Analysis 

General situation of soil nutrients in the study area

Table 2 shows the soil nutrient contents of the 0-30 cm soil layer under different land use patterns in the middle and upper reaches of the Niyang River in Tibet. It can be seen from Table 2 that， in terms of soil organic matter， total nitrogen， and alkali-hydrolyzable nitrogen contents， forest land was the highest， while cultivated land was the smallest. The contents of total phosphorus and available phosphorus were the highest in cultivated land， and the lowest in forest land. The total potassium content of grass land was not much different from that of forest land， but it was higher than that of cultivated land. For the content of available potassium， forest land showed the highest content， while the content in cultivated land was the smallest.

It can be seen from Table 2 that the variation coefficients， standard deviations and amplitudes of variation of soil nutrient indexes under the three land use patterns ranked as available potassium>available phosphorus>total phosphorus>total nitrogen>total potassium> alkali-hydrolyzale nitrogen>organic matter. In this study area， the soil nutrient indices were all with moderate spatial variability， which is consistent with the research results of Wang  et al.   [9] .

Organic matter content of the soil

Organic matter is the foundation of soil fertility. The changes of soil organic matter in the three land use patterns are shown in Fig. 1. It can be seen from Fig. 1 that under different land use patterns， soil organic matter showed an order of forest land>grass land>cultivated land， and there were significant differences between the three （ P <0.05）. The distribution of organic matter at different soil depths showed a certain degree of surface aggregation. The content of organic matter at the 0-10 cm depth was significantly higher than those of other soil layers， and there were significant differences between the three （ P <0.05）.

Total nutrient contents of the soil

Nitrogen is one of the important nutrients for crop growth. Soil nitrogen plays a very important role in soil fertility， and soil total nitrogen content is an important index to measure soil nitrogen supply  [11-12] . Fig. 2 shows the variations of soil total nitrogen contents under different land use patterns. It can be seen from Fig. 2 that under the same land use pattern at different soil depths， as far as cultivated land was concerned， the soil total nitrogen content was not significantly different between 0-10 and 20-30 cm   （ P <0.05） ， while the 0-10 and 20-30 cm depths were significant  different from the 10-20 cm depth （ P <0.05）. For grass land， the soil total nitrogen content was significantly different between  0-10  and 10-20 cm， and between 0-10 and 20-30 cm  （ P <0.05） ， while no significant difference was found between  10-20  and 20-30 cm （ P >0.05）. For forest land， the soil total nitrogen content was significantly different between 0-10， 10-20 and 20-30 cm （ P <0.05）.

Different lowercase letters indicate significant differences between different depths of the same land use type， and different uppercase letters indicate significant differences between  different land use pattern at the same depth （ P <0.05）. The same below.

Agricultural Biotechnology 2022

In the analysis of different land use patterns at the same depth， for the total nitrogen content of the soil in the depth of  0-10 cm，  there was no significant difference between cultivated  land and grass land （ P >0.05）， but they were significantly different  from forest land （ P <0.05）. In the depth of 10-20 cm， there was no significant difference between grass land and forest land  （ P <0.05） ， but there were significant differences between the two and cultivated land （ P <0.05）. For the 20-30 cm depth， there were significant differences between cultivated land， grass land and forest land （ P <0.05）. In general， the total nitrogen contents of different land use patterns were significantly different， which was manifested as forest land>grass land>cultivated land.

The total phosphorus contents of the soil under different land use patterns are shown in Fig. 3. It can be seen from Fig. 3 that the soil total phosphorus was significantly different between different soil depths in the same land use pattern （ P <0.05）， and the total potassium content of cultivated land was significantly higher than those of grass land and forest land. There were significant differences between different land use patterns at the same depth. Specifically， at the 0-10 and 10-20 cm， the differences between grass land and forest land were not significant （ P >0.05）， while cultivated land was significantly different from grass land and forest land （ P <0.05）. At 20-30 cm， there was no significant difference between cultivated land and grass land （ P >0.05）， while cultivated land and grass land were significantly different from forest land （ P <0.05）.

The total potassium contents of the soil under different land use patterns are shown in Fig. 4. It can be seen from Fig. 4 that in the same land use pattern at different depths， the soil total potassium content was significantly different between 0-10 and 10-20 cm， and between 0-10 and 20-30 cm （ P <0.05）， while no significant difference was observed between 10-20 and 10-20 cm （ P >0.05）. At the same soil depth，  i.e. ， at 0-10， 10-20 and 20-30 cm， there were no significant differences in soil potassium content between grass land and forest land （ P >0.05）， while significant differences were observed between cultivated land and  grass land， and between cultivated land and forest land （ P <0.05）.  

Available nutrient contents of the soil

It can also be seen from Table 1 that the average value of soil alkali-hydrolyzable nitrogen content was the highest in forest land， followed by grass land， and the smallest in cultivated land in the study area， and the value of forest land was significantly larger than those of cultivated land and grass land. It is consistent with the findings of Yu  et al.   [13] . The average soil available phosphorus content in cultivated land （19.47 mg/kg） was significantly higher  than those in grass land （5.73 mg/kg） and forest land （5.19 mg/kg），  while the values of grassland and forest land were relatively close. In terms of coefficient of variation， the order was forest land  （79.75）  > cultivated land （46.47） > grassland （40.14）， and the values of grass land and forest land were relatively close.  The average values of soil available potassium content exhibited an order of forest land>grass land>cultivated land， which is contrary to the research results of Xiao  et al.   [14] . Their research showed that changes in land use pattern had a significant impact on the nutrients of the two kinds of bedrock-developed soils， and after the conversion of forest land to cultivated land， the available potassium of limestone soil would increase. In this study area， after the conversion of forest land to cultivated land， the available potassium in the soil would decrease， which is related to the fertilization habits of local farmers and soil texture in the region.

Discussion

In this study， soil organic matter， total nitrogen， total potassium， total phosphorus， available phosphorus and available potassium that are closely related to crop growth were selected as crop evaluation indexes， and the variation characteristics of the above indices at different depths of the same land use pattern and under different land use patterns at the same depth were analyzed. This study showed that for cultivated land， the average value of soil organic matter was 35.05 g/kg; the average value of total nitrogen was 2.09 g/kg; the average value of alkali-hydrolyzable nitrogen was 54.59 mg/kg; the average value of total phosphorus was 0.58 g/kg; the average value of total potassium was 17.99 g/kg; the average value of available phosphorus was 19.47 mg/kg; and the average value of available potassium was 161.02 mg/kg. In grass land， the average value of organic matter was 39.14 g/kg; the average value of total nitrogen was 2.34 g/kg; the average value of alkali-hydrolyzable nitrogen was 120.81 mg/kg; the average value of total phosphorus was 0.54 g/kg; the average value of total potassium was 29.71 g/kg; the average value of available phosphorus was 5.73 mg/kg; and the average value of available potassium was 230.33 mg/kg. In terms of forest land， the average value of organic matter was 49.43 g/kg; the average value of total nitrogen was 2.49 g/kg; the average value of alkali-hydrolyzable nitrogen was  177.52 mg/kg; the average value of total phosphorus was 0.48 g/kg;  the average value of total potassium was 29.04 g/kg; the average value of available phosphorus was 5.19 mg/kg， and the average value of available potassium was 249.59 mg/kg.

It could be seen from the above study that the soil organic matter content in study area was higher than that of other provinces in China， whether it was cultivated land， grass land or forest land. The reason is that the average altitude is above 3 150 m in the middle and upper reaches of the Niyang River， and even higher for the higher upper reaches， where the highest altitude can reach  5 100 m.  In such high-altitude area， the soil temperature is low for a long time， and the low temperature inhibits the activity of microorganisms， so it is difficult for the litter on the ground to be decomposed by microorganisms into nutrients that can be absorbed and utilized by plants in a short time. As a result， the organic matter content in the soil is getting higher and higher after long-term accumulation. However， cultivated land is more affected by humans than grass land and forest land. The soil is often tilled， and a large number of plant residues in the loose soil are easily decomposed， so the content of organic matter in the soil is lower than that of other two land use patterns. The litter content of forest land is higher than those of cultivated land and grass land， and the long-term accumulation makes the organic matter content in the soil of forest land higher， which is consistent with the research results of Ma  et al.   [15] . Another reason is that the study area is a semi-agricultural and semi-pastoral area， where people raise a large number of cattle， sheep and Tibetan pigs， and they apply a large amount of livestock excrement to the cultivated land every year. Under low temperature conditions， the manure is difficult to degrade in a short time， which increases the organic matter content of the soil on the other hand.

The essential nutrients for plants are mainly obtained from the soil. With different land use methods， vegetation patterns and the intensity of human disturbance （such as farming management） are also different， which directly affect the input and output of soil nutrients， which in turn leads to differences in the distribution of  total  and available nutrients at different soil depths  [16] . In this study， the contents of total nitrogen， alkali-hydrolyzable nitrogen， total potassium and available potassium in the soil of forest land were significantly higher than those in cultivated land and grass land （mainly for grazing）， that is， forest land had the highest soil nutrient contents. Considering that the soil in this study area is not fully developed and the soil layer is thin， the sampling depth of the soil samples in this study was only 0-30 cm. Since the sampling soil layers were shallow， there were certain limitations in terms of the effects of nutrient contents at different soil depths. Generally speaking， the farmland with a higher degree of tillage maturation has richer soil nutrient contents， but with the increase of the tillage years and the increase of the multiple cropping index， the partial application of nitrogen fertilizer and nitrogen and phosphorus fertilizer has promoted the increase of crop yields， but also aggravated the consumption of soil potassium， resulting in generally low soil potassium content  [17] . However， in this study， the content of  available  potassium in the soil was relatively abundant. It is analyzed that the variation of available potassium may be affected by non-human natural factors （topography， climate， parent material，  etc. ）  [18] . Moreover， it may be related to the difference in agricultural farming methods in the area. The local people are very extensive in cultivating crops， and a large amount of farmyard manure is mainly applied in the farmland， while the application amount of potassium fertilizer is small， so the distribution law of available potassium content is relatively little affected by human factors.

Conclusions

The contents of soil organic matter， total nitrogen and alkali-hydrolyzable nitrogen in the middle and upper reaches of Niyang River in Tibet ranked as forest land > grass land > cultivated land. The soil organic matter content was the highest in forest land， and the lowest in farm land， and there were significant differences between the three （ P <0.05）. The contents of total phosphorus and available phosphorus were the highest in cultivated land， and the lowest in forest land. The total potassium content of grass land was not much different from that of forest land，  and the contents of available potassium under the different land use patterns exhibited an order of forest land>grass land>cultivated land.

Under different land use patterns at different soil depths， the distribution of total soil nutrients and available nutrients showed a certain degree of surface aggregation. The soil total nutrients and available nutrients at the depth of 0-10 cm were significantly higher than those at 10-20 and 20-30 cm， and there were significant differences between the three （ P <0.05）. The average values of alkali-hydrolyzable nitrogen content in the soil under the three land use patterns in the study area ranked as forest land>grass land>cultivated land. The average value of soil available phosphorus content in cultivated land （19.47 mg/kg） was significantly higher than those in grass land （5.73 mg/kg） and forest land （5.19 mg/kg）.

References

[1]  XIAO SZ， HE JL， ZENG C，  et al.  Nutrient content of soil developed from limestone and dolomite in karst areas of Southwest China[J]. Southwest China Journal of Agricultural Sciences， 2020， 33（6）： 1247-1252. （in Chinese）.

[2] HU M， HAN C， YANG XY，  et al.  Soil nutrient content and distribution characteristics in Tongguan County[J]. Journal of Henan Agricultural Sciences， 2016， 45（1）： 61-64. （in Chinese）.

[3] ZHANG ZY， LI JY， ZULPIYA MAMAT，  et al.  Spatial heterogeneity of soil nutrients and salinization risk assessment of a small-scale farmland in Ebinur Basin in northwest China[J]. Acta Ecologica Sinica， 2017， 37（3）： 819-828. （in Chinese）.

[4] CHEN Q， YANG JS， YAO RJ，  et al.  Spatial variability of soil nutrients in cultivated land in typical county of Hetao Plain[J]. Chinese Agricultural Science Bulletin， 2020， 36（10）： 102-108. （in Chinese）.

[5] FU B， LIU S， LU Y，  et al.  Comparing the soil quality changes of different land uses determined by two quantitative methods[J]. Journal of Environmental Sciences， 2003， 15（2）： 167-172.

[6] Yu HY， Li TX， Zhang XZ. Nutrient budget and soil nutrient status in greenhouse system[J]. Agricultural Sciences in China， 2010， 9（6）： 871-879.

[7] SHAN ZJ， YU Y， YIN Z，  et al.  Soil nutrient characteristics in different land use of Mengzi gabin basin[J]. Journal of Central South University of Forestry & Technology， 2019， 39（7）： 85-91. （in Chinese）.

[8] ZHANG C， LIU B， ZHANG WX，  et al.  Study on soil physical and chemical properties of wetlands in Linzhi Reaches of Niyang-Yaluzangbu River Valley in Tibet[J]. Journal of Anhui Agricultural Sciences， 2011， 39（31）： 19119-19121. （in Chinese）.

[9] WANG XM， CHAI ZP， YANG XF. Analysis on soil nutrients difference under different land use patterns in desert oasis region[J]. Agricultural Research In The Arid Areas， 2017， 35 （1）： 91-96. （in Chinese）.

[10]  LU RK. Soil agrochemical analysis methods[M]. Beijing： China agriculture Science and Technique Press， 1999. （in Chinese）.

[11] ZHU YQ， CUI YX， LI WD，  et al.  Analysis of soil pH， organic matter， nitrogen and phosphate characteristics under different land use types in Taige Canal Valley[J]. Journal of Ecology and Rural Environment， 2020， 36 （2）： 171-178. （in Chinese）.

[12] QIN Y. Effects of different land use methods on soil nutrients and vegetation in Maqu alpine grassland[J]. Journal of Ecology and Rural Environment， 2020， 36（2）： 171-178. （in Chinese）.

[13] YU YJ， LI YJ， LI QC，  et al.  Effect of different land use types on soil nutrient contents under grassland ，woodland and cultivated land in the farming-pastoral ecotone in the Siziwang Banner of Inner Mongolia[J]. Inner Mongolia Agricultural Science and Technology， 2019， 47（2）： 66-72. （in Chinese）.

[14] XIAO SZ， HE JH， ZENG C，  et al.  Nutrient content of soil developed from limestone and dolomite in karst areas of Southwest China[J]. Southwest China Journal of Agricultural Sciences， 2020， 33（6）： 1247-1252. （in Chinese）.

[15] MA HP， GUO QQ， LIU HM，  et al.  Soil organic carbon pool at the western side of the sygera mountains， southeast Tibet， China[J]. Acta Ecologica Sinica， 2013， 33（10）： 3122-3128. （in Chinese）.

[16] PAN B， DUAN LX， ZHANG F，  et al.  Responsive sensitivity of nutrients in red soil profile to land use change[J]. Chinese Journal of Ecology， 2018， 37（9）： 2707-2716. （in Chinese）.

[17] ZHANG H， ZHAO XM， OUYANG ZC，  et al.  Effects of different farmland use types on soil nutrients in Jiangxi Province[J]. Research of Soil and Water Conservation， 2018， 25（6）： 53-60. （in Chinese）.

[18] MUHTAR AMAT， BUMARYAM KIREM， WU ZX. Spatial distribution characteristics of plough layer soil nutrients in Zhongying Town， Hefeng County[J]. Journal of Kashgar University 2019， 40（3）： 54-61. （in Chinese）.

 Editor： Yingzhi GUANG   Proofreader： Xinxiu ZHU