Abstract
The change of soil organic carbon and its influencing factors after afforestation in sandy land should be taken into account. Here, the factors would be revealed which would influence the SOC dynamics to a depth of 100âcm during the development of Mongolian pine plantations in Horqin sandy land, northeast China. The chronosequence method was used to quantify the change of SOC in vertical distribution and influencing factors following conversion grassland to Pinus sylvestris var. mongolica forest in semi-arid sandy land, northeast China. Then the traditional statistical approaches were used to assessed the influence of the identified factors. Stand age played a major role in SOC dynamics. It took 38 years for SOC in 0â10âcm layer to recover to its initial level after afforestation, and 46 years for 10â20âcm layer. SOC accumulation increased with the age of Mongolian pine plantation. Over-mature forest fully embodied the advantage of SOC accumulation. In addition, the changes of SOC in 0â10âcm layer were also affected by TN, TP, TK and soil moisture, and those below 10âcm soil layers were related to the effects of TN, TP, TK, BD and CS.
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Introduction
Land use change is the main factor that drives changes in soil organic carbon (SOC) stocks and the global carbon cycle1. SOC in the form of soil organic matter plays an important role in the formation and conservation of soil structure, soil nutrient cycling and soil biodiversity. SOC is a natural resource for the sustainable development of human society and a key foundation for sustainable forestry development2.
Studies on SOC are usually conducted near the soil surface (0â20âcm or 0â30âcm) and deep (below 20âcm or 30âcm) layers. Plant residues are the most important source of SOC. The SOC near the soil surface is mainly derived from aboveground litter and fine roots distributed near the soil surface layer, whereas deep SOC is mainly derived from root exfoliation and exudates3. The distribution of SOC is largely heterogeneous because the soil properties4, site characteristics5,6, plant species1,7, environment4,5,8,9,10,11,12, land use7,11,13,14,15 and management16,17, the dynamic carbon processes differ and the responses to external environmental changes are distinct between the surface and deep soil. However, results regarding the effect of time since conversion on SOC stock are inconsistent17,18,19,20. In small areas with the same climate and soil type, the factors that affect SOC include land use and management, plant species, site characteristics (slope position, aspect), intrinsic soil properties (e.g., BD, soil texture), stand age18,19,21, and stand density22,23,24. For example, in plains in Australia with rainfall less than 500âmm, different land uses (grass or planting) have no significant impacts on SOC12, but changing land management practices has been demonstrated to affect deep SOC17 and SOC stratification16.
After the conversion of grassland to forest land, the root system of the forest is able to regulate the redistribution of deep soil water25 and provide C9. Furthermore, the active plant roots can promote the formation of soil aggregates and make the most of the root-derived C that was occluded in soil aggregates26, thereby promoting the accumulation of soil organic matter in degraded grasslands3. There are a large of sandy grassland in northern China27. To increase the primary productivity of the land, the land use mode (grassland into agricultural or forest land) has been changed. Pine trees have high yields and low afforestation costs28, and introduced pine plantations are promoted for their presumed capacity to provide a net sink of atmospheric C29. Accordingly, Pinus sylvestris var. mongolica was first introduced to Horqin Sandy Land in 1955 and achieved good benefits30. Although some scholars have studied the effects of the pine plantations on SOC during growth, the maximum stand age that has been studied was 30 years old, and research has been limited to the 0â10âcm surface layer31, and Chen et al.32 also studied only 0â30âcm soil layer and certain stand age, not the whole growth process. Guo et al.33 concluded that carbon stocks were reduced by approximately 20% when plantations were less than 40 years old. Meta-estimates calculated for the periods <30âyrs and >30âyrs since afforestation revealed a shift from initial loss to later gain of SOC34. Chapela et al.29 demonstrated that during the 12 years after pine trees were planted on grassland, SOC remained almost unchanged in the 0â10âcm soil layer but decreased by 30% and 44% in the 10â20 and 20â30âcm soil layers, respectively. Therefore, estimates of the changes of SOC during the whole growth process may be inaccurate when only stand ages less than 30 years and the 0â10âcm or 0â30âcm soil layer are considered. The earliest introduced Pinus sylvestris var. mongolica individuals in Horqin sandy land have been growing for 56âyrs and have entered a stage of slow growth. How has SOC changed over these 56âyrs, and what are the relationships of SOC with other soil properties? How do Pinus sylvestris var. mongolica forests affect SOC?
The hypothesis that the drivers of SOC vary with stand age after afforestation with the exotic species Pinus sylvestris var. mongolica in sandy land, namely that the factors influencing the amount of SOC vary with stand age and also differ from those influencing the SOC of topsoil and deep soil layer. The method of stepwise regression analysis which was assessed the importance of various possible explanatory factors on the change of SOC in a depth of 100âcm was be used to test this hypothesis. The data of SOC and other soil properties were derived from 20 sites, laboratory testing and calculation from Horqin sandy land, northeast China.
Material and Methods
Study area
The research site is established in the southern part of the Horqin Sandy Land in Zhanggutai region of Liaoning Province, Northeast China (42°39â²-42°43â²N, 122°23â²-122°33â²E) with an average altitude of 225âm (Fig. 1).The area is a typical semi-arid sandy land with the annual average temperature of 4.6â6.3â°C and the average annual precipitation of 500âmm. The soil is eolian sandy type developed from sandy parent material through wind action. The annual average wind speed is 4.5âm·sâ1, and winds can blow gusts of sand and dirt in spring. The area is a typical combination of grassland and agricultural land, representative plants include Potentilla anserina, Cleistogenes chinensis, Armeniaca sibirica, Lespedeza daurica, Salix gordejevii and Agriophyllum squarrosum30. Before afforestation, this area was a sandy grassland.
Site selection
The survey was conducted in the above study area (Table 1). A paired forest land-grassland method was adopted, assuming that the SOC of the adjacent grassland represented the SOC at the initial stage of afforestation. Accordingly, the selection of the sample sites was based on the following criteria: (i) the forests should be planted on native grasslands and have soil properties similar to those in the adjacent grassland; (ii) plots should be on well-drained soils with no anoxic conditions influencing the decomposition rate of organic matter; (iii) sampled stands should, to the greatest extent possible, include young, middle-aged, mature and over-mature ages; and (iv) there should be 4â6 repetitions per stand age. In total, 20 suitable stands were identified and sampled and the site should have small slope and relatively flat slope. All the sampled stands were still in their first rotation after conversion from grassland. The growth stage was classified into four categories: young forest, with 13âyrs of afforestation, middle-aged forest, with 25âyrs; mature forest, with 44âyrs; and over-mature forest, with 56âyrs. There was a control grassland in each stand of forest age. After reviewing the afforestation archives, it was found that 2-year-old bare root seedlings of Pinus sylvestris var. mongolica were used during the original afforestation. Before afforestation, the herbs were removed with a diameter of approximately 20âcm. There was limited disturbance to the soil using this method. The soil in the study area was free of carbonate reactions (Xie 1982, Unpublished data). It means a lack of Ca2+ and Mg2+.
Soil sampling took place in May 2017 and 2018. At each plot of 20âÃâ20âm2, soil from forest land and adjacent grassland were sampled. For each the forest land, a survey was conducted to locate an average tree in the stand and a soil profile with a length, width and depth of 1âm each was excavated. The distance of the soil profile from the average tree was approximately 1âm, then, the soil was divided into 6 layers at fixed depths (0â10, 10â20, 20â40, 40â60, 60â80, 80â100âcm). To determine soil properties (moisture content, BD, TN, TP, TK, soil texture, pH), each layer was sampled natural soils with a cutting ring with an inner diameter of 70âmm and a height of 52âmm, 3 repetitions. For each the control grassland, the soil profile was dug at the centre of each plot and the same sampling method as forest land was performed. In addition, 5 sampling points were randomly selected in each plot (forest land and grassland), and each sampling point was stratified-sampled at fixed depths using a soil drill with a length of 1âm. To reduce the number of samples, 6 samples from the same layer were mixed into one sample. All samples were taken back to the laboratory to determine the soil property values.
Soil preparation
The sealed cutting ring and natural soils were weighed and dried at 105â°C until constant weight. After the samples cooled, the dry natural soil and cutting ring were weighed for BD and soil moisture determination35. The other soil samples were air-dried for at least one week. The coarser roots and stones (>2âmm) were removed and weighed. Because the stone (>2âmm) contents in the samples were zero, each sample was sieved through a 0.25âmm sieve. After sieving, the samples were analysed for SOC, total nitrogen (TN), total phosphorus (TP), total potassium (TK), and pH.
Laboratory analyses
The soil aggregates were crushed and sieved using a 2-mm sieve. The soil texture was measured by an HORIBAâs LA-300 Laser Diffraction Particle Size Distribution Analyser (Japan). The sieved soil samples were evenly divided into three parts (approximately 25âg each). Each part was ground in a grinding dish until the soil particles were separated; then, the samples were sieved through 0.5-mm, 0.25-mm, and 0.1-mm sieves and to obtain three soil particle sizes (0.5â0.25âmm, 0.25â0.1âmm, <0.1âmm), which were categorized as mass 1, mass 2, and mass 3. Three replicates were collected for each sample. The sizes of soil particles >0.1âmm were determined by the sieving method, and those <0.1âmm were measured using an LA-300 HORIBA laser particle size analyser. Three analyses were carried out and averaged to determine the particle size distribution. Because there were fewer clays and silts in the soil, the summed percentages of clay (<0.002âmm) and silt (0.002â0.05âmm) were used to represent the fine soil percentage (FS), and the percentage of sand (0.05â0.5âmm) was used to represent the coarse soil percentage (CS) at each site.
The SOC concentration was determined by the potassium dichromate (K2Cr2O7) volumetric-external heating method, the TN content was determined by the semi-micro-Kelvin method, the TP content was determined by the NaOH melt-molybdenum anti-colorimetric method, the TK was determined by the NaOH melt-flame photometric method, the pH was measured in a suspension of 10âg soil and 25âmL 0.01âmol·Lâ1 CaCl2 using a glass calomel electrode GK2401. The determination and calculation of SOC, TN, TP, TK and pH were referred to literature35.
Calculations
The change in each factor was estimated as:
where \(\Delta {X}_{ij}\) is the variation of factor i, Xi is the measured value of factor i in forest land, and Xj is the measured value of factor i in adjacent grassland.
The relative change rate of each factor was estimated as:
where Pi is the relative change rate of factor i.
Statistical analyses
The relative change rate of SOC was used as the dependent variable, and the other influencing factors were used as the independent variables for stepwise regression analysis. The various influencing factors that exhibited collinearity were excluded, and the relative change rate of SOC was obtained with the following regression equation:
where PSOC is the relative change rate of SOC, β0 is a constant, and βi is the coefficient estimates of affecting factor i.
The SOC was estimated as
where SOCj is the measured value of SOC in adjacent grassland.
The positive and negative effects of SOC variation and its influencing factors have the same positive and negative effects of SOC as their corresponding factors, but the coefficient estimates are different.
To detect the effects of soil properties and stand age on SOC changes. Stepwise regression analysis was carried out in which the SOC was taken as dependent variable while other all factors were independent variables. Before the stepwise regression analysis, the linear models was used to test whether the soil properties were interacted. If there were linear correlations between two soil properties, one of them would be selected for analyse, which was done to eliminate the influence of collinearity on the results. Finally, the factor that significantly affected the SOC was found by stepwise regression analysis, and the stepwise regression analysis was done for each factor with significant influence as a dependent variable to identify the main factors affecting these factors, all analyses were performed by SPSS 20.0 developed by IBM.
Results
Change of SOC with stand age and soil depth
After the conversion of grassland to Pinus sylvestris var. mongolica forest, stand age was the main driving factor of the SOC in the 0â40 and 80â100âcm layers (Table 2). The analysis of SOC in Pinus sylvestris var. mongolica forest of different growth stages revealed that at 13âyrs, the amount of SOC was increased in the 10â100âcm layer but significantly decreased in the 0â10âcm layer. The magnitude of SOC change was significantly higher in the 40â60âcm soil layer than that in the 80â100âcm layer (Fig. 2a). At 25âyrs, the decrease of SOC in 0â20âcm layer resulted in lower amount of SOC in 0â100âcm layer than that in control grassland. However, the SOC was increased in the 40â60, 80â100âcm soil layer, and almost unchanged in the 20â40, 60â80âcm layer (Fig. 2b). At 44âyrs, the SOC in the 0â10âcm layer recovered to the initial grassland level and increased slightly, and the level in the 10â20âcm soil layer remained lower than the initial level, while others were increased. The vertical distribution indicated that the magnitude of SOC change was increased with depth (Fig. 2c). By 56âyrs, the magnitude of SOC change in 0â100âcm layer was significantly higher than that in the initial grassland, especially in the 0â10âcm layer.
On average, the data from the grassland converted to the Pinus sylvestris var. mongolica forests showed that the SOC contents in the 0â100âcm layers increased by 1.17, 1.29, and 7.58âg·kgâ1 at 13, 44, and 56âyrs, respectively, and decreased by 0.37âg·kgâ1 at 25âyrs. At the same time, the data analysis showed that the SOC changes in the surface and deep soil of the forest land were inconsistent, and the SOC changed faster in the topsoil (Fig. 3a), while that in the deep soil changed relatively slowly (Fig. 3câf). soil organic C content in the <10âcm layers generally decreased by 0.047% per year relative to the initial SOC content during 13 years of afforestation, followed by a increase (up to 0.087% per year) in the rate decline from 13 to 25 years, eventually recovered to the initial level in 44-year-old forests. During the 12âyrs from 44â56âyrs, SOC was in a rapid accumulation phase and increased by 0.089% per year in the 0â10âcm layer. SOC of 10â20âcm layer was increased before 13âyrs, but decreased from 25âyrs, still slightly decreased at 44âyrs, and significantly increased at 56âyrs. The SOC was increased from 13 to 56âyrs and did not reach equilibrium at soil depths >30âcm. Surprisingly, compared with mature forests, SOC changes in all soil layers increased significantly in the over-mature forests. Starting from the middle-aged forests, the SOC increased with stand age, and the SOC in 0â10âcm layer increased faster than that in deeper layers (Fig. 2). Stepwise regression analysis showed that the magnitude of SOC was a significant positive correlation with stand age in the 0â10, 10-20, and 80-100âcm soil layers (Pâ<â0.001, 0.002, <0.001, respectively). Based on the estimated value of coefficients, stand age played the most important role in SOC increase (Table 2).
The effects of soil properties on SOC
Soil moisture was positively correlated with the SOC in only the 0â10âcm soil layer (Pâ=â0.044), it was showed that rainfall had a significant effect on SOC content in 0â10âcm soil layer, and SOC increased with the increase of rainfall, in the other soil layers, soil moisture was not correlated with the SOC. There was no significant correlation between SOC and BD in the other soil layer except for 20â40âcm soil layer (Pâ=â0.003), The content of SOC in this layer (20â40âcm layer) increases with the increase of BD. The effects of soil texture (CS) were mainly found in 10â20, 40â60 and 80â100âcm soil layers. The CS was significantly negatively correlated with the SOC (Pâ=â0.008, 0.012, 0.036, respectively). The coarser the soil texture, the more the SOC increased with the reduction in CS after conversion to the Pinus sylvestris var. mongolica plantation. Therefore, the sandy soil with high content of coarse grains is very helpful to improve SOC reserves. pH value was not correlated with the SOC in any soil layers.
The effects of soil nutrient on the SOC were also different. SOC was significantly negatively correlated with TK, TP, TN in the 0â10âcm soil layer, significantly negatively correlated with TP in 10â20, 80â100âcm soil layer, and significantly negatively correlated with TN in 10â20, 60â80âcm soil layer. However, TK was significantly positively correlated with SOC in the 20â40âcm soil layer.
Discussion
SOC change with soil depth and stand age
Following the afforestation in grassland, soil physicochemical properties differed with soil depth36, and soil depth significantly affected SOC37. Our results were similar to those observed for surface (0â20âcm) soil. The SOC content of the surface soil decreased initially, and then increased continuously until it returned to the initial state after afforestation at 44âyrs (Fig. 2aâc), while the SOC in deep layer (below 20âcm) has been increasing since afforestation. Similar to the present study29,34,38, remarkable changes in SOC at different soil depths due to afforestation were previously demonstrated for various soil properties. However, most studies had shown that the SOC in the initial stage was decreased after afforestation1,33,39. The age at which SOC decreased to its lowest value was consistent with Hu et al.31, but the age at which SOC recovered to initial levels was inconsistent with the findings of Hu et al.31. Our results were more reliable than those of the previous study because our plots included the whole growth stage of Pinus sylvestris var. mongolica forests.
After grassland afforestation, within the soil 10âcm from the surface during the first 13âyrs of planting, there was little above-ground input of C due to the small forest biomass and low rate of litter fall36. In addition, at this stage, the mass of fine roots (<1âmm) under various forests was only one-fifth lower than that under adjacent pasture sites40. Tree roots were sources of organic matter that were less important than grass roots because much of the tree root system lives for many years. The annual turnover of organic matter from dying tree roots was, therefore, smaller than that from grass roots33. However, SOC from residues of the preceding pasture phase would continue to decompose during this time. Therefore, the initial decrease in soil C was due to the loss of C through decomposition, which outweighs the gains in C from litter production40,41,42. Soil disturbances involved in the establishment of plantations could result in decomposition of SOM and carbon losses occurring at different rates in different parts of the soil profile. For example, the initial decrease in soil C following forest establishment it was often attributed to site preparation43. The partial land preparation of removing surface vegetation no bigger than 20âcm in diameter was regarded as a low-level disturbance39. This disturbance still increased the surface temperature of the afforestation site and resulted in accelerated decomposition of the residual organic matter during the initial stage of afforestation. On the other hand, because the surface cover was removed after soil preparation, the soil C stocks decreased due to accelerating soil erosion (wind or water erosion)44,45,46. C leaching could play a role in the reduction in soil C stocks in these areas. However, this impact was not evident from the analysis of the data on soil sampling depth. Finally, the growth of woody plants resulted in a decrease in SOC. The decrease in SOC at 25âyrs in the 10â20âcm soil layer in this study may be due to the growth of woody plants, despite the fact that woody plants produced a greater amount of more recalcitrant material45.
The afforestation method used in this study did not disturb the deep soil. The SOC change in the deep soil was mainly dominated by the growth of deep-rooted plants such as Pinus sylvestris var. mongolica after afforestation9.
Due to the growth of plants, the root exudates produced by its root system continuously deposited and decomposed during the growth process in 0â20 and 80â100âcm soil layer (Table 2) and provided a consistent source of top and deep SOC3. In addition, accompanied by rainfall and soil water movement, soil soluble organic carbon migrated from litter to the surface soil and deep soil47,48; although the proportion was small, it contributed to the SOC increase in deep soil. For the whole soil layer within 1âm, the SOC increased in forest of all ages except 25âyrs, in which SOC decreased. Guo et al.33 reported that grassland afforestation reduced the soil C stock by 10% rather than increasing it, which may have reflected that fact that deep SOC was not considered.
Afforestation in grasslands had a significant positive effect on SOC changes in the studied region. The Pinus sylvestris var. mongolica forest floors may have enhanced the positive effects of afforestation on SOC (Unfortunately, there were no related data in this paper). The research showed that the stand age was the main driving factors of SOC changes in the 0â100âcm layer (Fig. 3), especially in 0â20 and 80â100âcm soil layers (Table 2), and many soil properties were also directly or indirectly affected by stand age (Table 3), such as W, TP, TN, TK, CS. The regression analysis showed that stand age was the most important factor that affected the TN and TP in the 0â20âcm soil layer, and W decreased with the increase in stand age in the 0â10âcm soil layer (Table 3). Many studies have also shown that stand age was one of the main drivers of SOC change3,33,34,39,49,50,51,52,53.
The SOC increased more rapidly with stand age increased, indicating that grassland afforestation could significantly increase SOC. Zhao et al.21 showed that the total carbon density of the soil layer in sparse elm forests and poplar plantations in the Hunshandake Sandy Land increased with stand age, and the results of global forest and stand age studies also showed that C increased with stand age54. However, the SOC changes with time were limited after afforestation in Northern Europe34. Walter et al.19 studied 18 woodlands at ages between 8 and 35âyrs in Central Europe. Rowe et al.20 studied 12 poplar or willow plots for 4â23âyrs in the UK. Both studies found no significant change in the SOC stocks with stand age. Similarly, Mao et al.18 did not find significant changes in SOC in a time series study of poplar plantations. This discrepancy may be related to the initially lower SOC content. At initial stages, Pinus sylvestris var. mongolica plantations had little impact on the 0â20âcm soil. Site preparation and the legacy of the former grasslands (e.g., soil microbial activity, soil physical properties, availability of soil nutrients) probably played major roles34. As forests developed, the input of C from litterfall increased and stabilized many years after afforestation due to canopy closure55, whereas SOC losses were generally observed following the afforestation in grasslands33,39,56. After the establishment of a plantation, a transition phase was often reported to occur41. The characteristics of a transition phase were the loss of SOC followed by a recovery phase. Meta-estimates calculated for the periods <30âyrs and >30âyrs since afforestation revealed a shift from initial loss to subsequent gain in SOC34.
Although the lengths of the transition and recovery phases were determined by factors such as the soil characteristics, species, and climate, for the 1âm deep soil layers, stand age was the most important driving factor affecting SOC change after the conversion of sandy grassland to Pinus sylvestris var. mongolica forest. In the 0â10âcm soil layer, the initial stage (<38âyrs) was decreased (Fig. 3a), and the SOC level in the grassland was reached in approximately 38âyrs. The established correlation equation showed that the SOC increased in the 10â20âcm soil layer for the first 16âyrs, decreased to the lowest level by 32âyrs, and then began to increase and reached initial level approximately by 46âyrs (Fig. 3b). The SOC change increased with stand age in the >20âcm soil layers (Fig. 3câf). compared with grassland, SOC in 56-year-old plantations increased significantly. The magnitude of SOC change in each layer from 0â100âcm had not reached equilibrium, and there kept a large increase potential (Fig. 2). Compared with mature forests, the over-mature forests had stronger carbon sequestration ability (Fig. 2). Therefore, the pattern of conversion of sandy grassland to Pinus sylvestris var. mongolica was worth promoting in the mid-temperate sandy grasslands of China.
Effects of soil properties on SOC
Soil properties significantly affected SOC. In general, in arid and semi-arid regions, a decrease in pH was associated with an increase in SOC57, as was also found in Chen et al.32, Giddens et al.58, Mueller et al.59 and Fang et al.60. Low pH values were often associated with low soil microbial activity, and low microbial activity reduced the mineralization of organic matter. Afforestation reduces soil Ca2+ and Mg2+ levels, resulting in increasing H+ levels37,57,61,62,63. However, our findings showed that SOC in any soil layers was not significantly correlated with pH value after planting Pinus sylvestris var. mongolica forests, as was also found in Liu et al.13. The viewpoint that high soil pH had a consistently negative effect on SOC storage32 was not suitable for sandy soil.
The SOC was only significantly positively correlated with soil moisture in the 0â10âcm layer (Pâ=â0.044) in this study (Table 2), Deng et al.37 found that SOC was significantly positively correlated with soil moisture in China, indicating that soil moisture played an important role in soil C sequestration of forest ecosystems because high soil moisture contributed to a high net primary productivity and a high SOC accumulation32,37. Regression analysis also showed that TK (0â10, 20â40âcm soil layers) and CS (10â20, 40â60, 80â100âcm soil layers) which affect SOC change were significantly correlated with soil moisture. However, after planting Pinus sylvestris var. mongolica, soil moisture decreased with the increase of stand age (Table 3). Low soil moisture would affect SOC accumulation. Therefore, appropriate supplementation of soil moisture after afforestation would increase SOC accumulation.
Previous studies had shown that SOC content had a significant impact on BD64,65, BD changes are primarily driven by SOC content and soil texture49. After grassland afforestation, the variation of BD in 0â10âcm soil layer was greater (coefficient of variation was 78%). The BD variation in 20â60âcm soil layers decreased first and then increased with the increase of stand age of Pinus sylvestris var. mongolica, and showed a decreasing trend in 60â100âcm soil layers66. We found that BD was only positively correlated with SOC in 20â40âcm soil layer, and our results indicated that changes in BD could also be attributed to soil nutrient changes (Table 3).
After afforestation, the soil of Pinus sylvestris var. mongolica plantation is still dominated by sand grains (about 95%). The content of CS in different soil layers tends to decrease, with a significant change in 0â10âcm layer. With regard to soil textural class, the negative impact of afforestation on SOC was found in the 10â20, 40â60 and 80â100âcm layers; that was, the SOC decreased with the increase in CS in these three soil layers. This result meant that the SOC increased with the increase in FS. Jobbágy et al.1 showed that the clay content determines the SOC content of deep soil. The general pattern that had been previously observed was high SOC accumulation in FS39,53, and our study observed this phenomenon in the 10â20, 40â60 and 80â100âcm soil layers. However, in northern Europe, the CS was the most prone to SOC increases following afforestation, whereas FS negatively affected SOC sequestration34. There effects of soil texture on SOC stocks with land use change continue to be debated, as other studies had not detected any effect of texture67,68.
The content of TN and TP were lower in 13âyrs forest land than in control grassland, and then gradually accumulated with the increase of stand age, and they were restored to grassland level in 44âyrs and 25âyrs, respectively. While the content of TK restored to grassland level in the 25âyrs, and then decreased with the increase of stand age. The application of fertilizer was a common practice during plantation establishment, and many studies had found that it increased C accumulation on plantations by increasing both above- and below-ground growth and litter production39,69,70. Soil phosphorus was a major nutrient that controlled plant growth and development in forest ecosystems; it had been shown that soil phosphorus decreased concurrently with increasing SOC after afforestation (e.g., with conifers) of former grassland71 and that SOC was significantly and positively correlated with soil TP37. The results of our study at the surface were inconsistent with this result (Table 2). Luo et al.72 reported that N dynamics were a key variable that regulated long-term terrestrial C sequestration. Furthermore, C-N interactions were important when determining whether C sinks in land ecosystems could be sustainable over long periods73,74. It was likely that N became progressively more limiting as C accumulated in an ecosystem with elevated carbon dioxide (CO2) if the amount of N in the ecosystem did not change72. If additional C inputs stimulated capital gains of N by biological fixation and atmospheric deposition, increasing N uptake of available soil N or decreasing N loss, then the progressive development of N limitation would not occur37,73. Accordingly, there was a significant correlation between SOC storage and TN content in soil35. An increase in N could promote the fixation of soil C75. This phenomenon had been observed during the early cultivation of artificial pine forests76. Due to the influences of afforestation species and growth phase, organic carbon and total N content sometimes showed a significant negative correlation23. In this study, the analysis revealed that the negative correlation between SOC and TN in Pinus sylvestris var. mongolica forests was significant in the 0â10, 10â20, 60â80âcm soil layer, the other soil layers were not correlated, supporting the view that the C-N relationship was complex52. In most cases, the N, P, and K contents were significantly positively correlated with SOC77. According to this relationship, some scholars had attempted to determine the change in SOC stock by evaluating soil fertility, including N, P, and K contents, after land use changes77. However, we found that there was a significant negative correlation between TN, TP, TK and SOC in 0â10âcm soil layers after afforestation (Table 2), indicating that afforestation caused an increase in SOC while the TN, TP, TK content in the soil gradually decreased. TN, TP and TK returned to soil by litter decomposition could not meet the growth needs of trees. Zhao et al.21 found that the P returned to the soil by litter was insufficient to maintain forest demand in the area, too. The results indicated that using soil fertility changes to infer SOC changes in sandy land could be arguable. In the future, the accumulation mechanism of soil organic carbon in over-mature stage of Pinus sylvestris var. mongolica forest will be studied.
Conclusion
After establishment of Pinus sylvestris var. mongolica plantation on sandy grassland, the SOC accumulation increased with the age of Pinus sylvestris var. mongolica plantation during the whole growth process. The SOC of near the soil surface underwent the stages of decline, recovery and increase during the growth of the stand. In the process of SOC change in the topsoil layer, the SOC was negatively correlated with TN, TP, TK and CS, and positively correlated with soil moisture. Therefore, the establishment of Pinus sylvestris var. mongolica plantation on sandy grassland increased SOC content, especially, the over-mature forest had a significant role in promoting the increase of SOC. Clear cutting of Pinus sylvestris var. mongolica over-mature stand would lead to a significant decline in SOC. We should pay enough attention to the management of over-mature forest.
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Acknowledgements
Thanks for the National Natural Science Foundation (Grant NO. 31570709) of China for its financial support.
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Zeyong LEI wrote the main manuscript text and translation of manuscript; Dongwei YU mainly carried out indoor experiments, test data processing, compiled tables and plotted figures and translation of manuscript. Yansong Zhang and Deliang YU carried out field sampling, indoor experiments and translation of manuscript; Fengyan Zhou carried out indoor experiments; Yanping Zhou carried out indoor experiments; Yangang HAN carried out indoor experiments. All authors reviewed the manuscript.
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Lei, Z., Yu, D., Zhou, F. et al. Changes in soil organic carbon and its influencing factors in the growth of Pinus sylvestris var. mongolica plantation in Horqin Sandy Land, Northeast China. Sci Rep 9, 16453 (2019). https://doi.org/10.1038/s41598-019-52945-5
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DOI: https://doi.org/10.1038/s41598-019-52945-5
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