De-Ling Yang，Li-Hai Wang，Wen-Shu Lin，Shu-E Ji

(1.College of Engineering and Technology，Northeast Forestry University，Harbin 150040，China;2.College of Automobile and Traffic Engineering，Heilongjiang Institute of Technology，Harbin 150050，China)

1 Introduction

With the implementation of Natural Forest Protection Project，timber production in forestry regions in China has reduced significantly.The cutting area becomes more scatter and selective cutting is used commonly.Therefore，large skidding machines are not suitable for the current production status and small skidding machines are needed in practice［1］.A small four-wheeled tractor is a kind of skidding machine with high efficiency and multi-purpose which aims to the plantation and secondary stand with small individual volume and less skidding operation.

Skidding operation is an important part in forest harvesting.The process of tractor skidding operation can cause varying degrees of soil compaction，which changes the physical and chemical properties of soil，limits the microbial activity，and affects the soil respiration and the normal growth of trees［2］.Therefore，the research of the small four-wheel tractor skidding on soil respiration can provide references for the analysis of the impact of interference on soil respiration，as well as the ecological harvesting theory of plantation.

2 Materials and Methods

2.1 Materials

The study site is in the No.1 subcompartment of No.58 compartment in Daxi Forest farm，which belongs to Jilin Forest Industry Group.The study area is 10.9 hm2，with elevation of 670 m and southwest orient.The average slope is between 6°and 8°.The soil is dark brown forest soil with average soil depth of 20 cm.The soil is wet.The vegetation type is Equisetum with medium coverage，and the type of bushes is Acer ukurunduense with dense coverage.This study site is a mixed coniferous forest plantation with major tree species of Larix gmelinii， Pinus koraiensis， Fraxinus mandshurica Rupr，T.tuan SzyszyL，and others.The average forest age is 48 y;the average diameter at breast height is 24 cm，and the average height is 18 m.The canopy density is 0.9，and timber volume is 244 m3/hm2.

2.2 Methods

2.2.1 Measurement of soil compaction

The soil compaction of sample points at depth of 5 cm，10 cm，and 15 cm were measured by the TJSD-750-II compaction measurement in Sept.5-7，2010.The distribution of sample points is shown in Fig.1.

2.2.2 Measurement of soil respiration

The forest roads within the study site were rebuilt in 2009.The soil respiration was firstly measured in Sept 5-7，2009，prior to skidding operation on the rebuilt skidding trails with testing equipment of the Li-8100 automatic soil carbon flux measurement system.The soil respiration of the skidding trails was measured again one year later after skidding operation.The distribution of sample points is also shown in Fig.1.Z1－Z4 are the sample points of soil respiration on the main skidding road;five 10cm-long PVC pipes were placed on each sample point，and 2 － 3 cm of them were above the ground;C1－C3 are sub skidding roads，and the settings of the sample points are the same as thoseon themain skiddingroad.The measurement of soil temperature and soil moisture at 10cm below the ground was conducted by using the temperature and humidity sensor equipped with the Li-8100;CK are the control points.A total of 47 sets of data were collected，including 20，15，and 12 sets of data for the main skidding roads，sub skidding roads，and control plots，respectively.

Fig.1 Distribution of sample points for measuring soil respiration

3 Results and Analysis

3.1 Compactness of Soil

3.1.1 Compactness of soil on the main skidding-road

The measure results of four different distribution points on the main skidding-road are shown in Fig.2.The pressure decreased from Z1 to Z4 in the range of soil depth of 5 cm，10 cm，and 20 cm.From the distribution of Z1 to Z4 in Fig.1，Z1 is located at the exit of the skidding road，therefore the number of skidding machinery passes was highest;Z4 was the least，and Z2，Z3 were in the middle.This shows that the more times the skidding machine rolls，the larger the value of soil compaction pressure is.From Z1 to Z4，pressure values at different soil depths gradually decreased，but the decreasing degree varied from each other.The minimum pressure difference between the values was shown at 20 cm soil depth in the main logging-road，which shows that the change of soil compaction will tend to steady state and eventually get a stable value as the soil depth increased.

Except for Z1 point，the compaction of soil increaseswith soildepth. During skidding， the skidding machines can not only have impacts on soil compacting but also drag and turn the surface soil which causes strong interference to soil.

3.1.2 Soil compaction of sub skidding-roads

Fig.3 shows the soil compaction of sub skidding roads.The skidding volumes on the three sub skidding roads were different.There is no significant variation of soil compaction at different soil depth among the three sub skidding roads.The average value ofsoil compaction on C1，C2，and C3 were 862.4 kPa，1170.4 kPa and 1377.79 kPa，respectively.It can be seen that C1 is the point of minimum soil tightness and C2 follows and C3 is the largest.

3.1.3 Comparison of soil compaction in different skidding-roads

The average values of soil compaction at different depths of corresponding points，on the main skiddingroad and sub skidding roads were calculated separately. These results were:1181.18 kPa，1136.86 kPa，and 978.13 kPa.Soil compactions of main skidding road and sub skidding roads are larger than that of the comparison points，and main skiddingroad are larger than that of sub skidding roads.

Fig.2 Values of soil compaction on the main logging road

Fig.3 Values of soil compaction on the sub logging roads

3.2 Soil Respiration in Skidding-Road

3.2.1 Soil respiration in main skidding-road

Z1 is the exit of the main skidding-road and it suffers the most serious mechanical rolling.Degree of mechanical rolling on the main skidding road decreased from Z1 to Z4(as shown in Table 1).Based on the value of soil respiration in 2010，soil respiration gradually reduced with the decrease of rolling degree from Z1 to Z4.Compared with the values in 2009，the rate of soil respiration of the main skidding road reduced significantly after skidding operation and Z1 showed themaximum reduction.This isbecause compaction from skidding machine after logging on soil alters water content and heat of soil，damages the physical structure of soil surface，reduces soil porosity，and obstructs soil respiration.All of these factors lead to reduction of soil respiration rate.The research from He et al.(2010)also showed that compaction changes the physical structure of soil surface，leading to the reduction of release rate of CO2［3］.

Table 1 Values of soil respiration on the main skidding road

3.2.2 Soil respiration in sub skidding roads

The values of soil respiration before and after skidding were compared.The results showed that soil respiration rates reduced in all three sub skidding roads and C3 showed the most obvious variation(as shown in Table 2).Through the values of soil respiration in 2010 from C1 to C3，C1 had the maximum value of soil respiration，followed by C2 and C3.Some evidences are shown above:C1 has the minimum soil compaction;C2 is the second one，and C3 is the largest one.All above results show that the larger soil compaction has，the lower soil respiration value is.

Table 2 Values of soil respiration on the sub skidding roads

3.2.3 Comparisons of soil resiration in different skidding-road

Taking the average of soil respiration value in various depth on the main skidding-road，sub skidding roads and comparison points，the results are 1.43 μmol/(m2·s)，1.59 μmol/(m2·s)，and 2.11 μmol/(m2· s)respectively.Combined with the results of soil compaction，the greater the degree of soil compaction is，the smaller soil respiration is.

3.3 Effect of Soil Temperature and Humidity to Soil Respiration

Temperature and humidity of soil are the key factors which make contribution to soil respiration［4－6］.Temperature also affects soil micro biotic activity and respiratory enzyme activity of plants root［7］.Soil micro biotic and the plants root system need soil humidity.

Let y represent soil respiration rate(μmol/(m2·s));x1represents surface temperature(℃);x2representsthe relative humidity RH (%);x3represents temperature(℃)of 10cm underground，and x4representhumidity(g/cm3)of10 cm underground［8］.According to the measurement data，regression analysis were conducted between soil respiration rate and four factors including surface temperature，relative humidity，soil temperature and moisture［9］.We used 17，12，and 9 sets of data to establish the regression models for the main skidding roads， sub skidding roads， and control plots，respectively，and the rest were used to test the corresponding models.The results indicated that there is a good correlation between soil respiration rate and these fourfactors.So the multi-factorregression equationsofsoilrespiration are established and presented in Table 3.The variations of soil respiration on the main skidding road，sub skidding roads and the soil respiration rate that can be explained by soil surface temperature and relative humidity，temperature and humidity of 10 cm underground are 22.0%，68.8%and 77.6% ，respectively［10］.Therefore，the fourfactors are the main factors affecting soil respiration of sub skiddingroads and comparison points，while the soil respiration in the main skidding road is less sensitive to the changes of the four factors.This is because the main skidding road is suffered much more skidding operation which leads to large soil disturbance and instability of soil respiration.It also indicates that the skidding volume has direct impact on soil respiration.The average relative error is 10%，7%，and 6%for the regression models of the main skidding roads，sub skidding roads，and control plots，respectively.It is noted that the sample size is small，and the precision of the regression model is rather low.Therefore，more measured data is needed in order to improve the models.

Table 3 Multiple regression of soil respiration model

4 Conclusions

Skidding operation is an importantpartin harvesting.The study of skidding road compaction and soil respiration can provide scientific references to the establishment of ecological theory of harvesting.This study shows that the more times of skidding machine passes，the more compacted soil compaction is.The pressure difference is minimum under 20 cm depth of soil surface at the main skidding road.Increasing the soil depth，soil compaction will be stabilized and eventually stable. During skidding， the skidding machines can not only have impacts on soil compacting but also drag and turn the surface soil upside down which causes strong interference to soil.

From the comparisons of main skidding road，sub skidding roads and vice control point of soil respiration，the higher the degree of soil compaction is， the lower the soil respiration value is. After compaction， the soildensity increases， and the porosity decreases，and soil aeration and permeability also become worse than before［11］.The variation of soil physical properties directly affects the soil temperature and humidity.Temperature and humidity are the main factors to soil breathing.Based on the temperature，humidity and soil model，some conclusion can be conducted that the greater the compaction of skidding road is，the lower the sensitivity of soil respiration to relative factors is.

This paper has studied the relationship between soil compaction and soil respiration of skidding roads.Future research includes the establishment of soil compaction or skidding frequency vs.amount of soil respiration model.

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