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Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China
Journal of Integrative Agriculture
March 2012
2012, 11(3): 512-520
RESEARCH ARTICLE
Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China WANG Yang, LIU Jing-shuang, WANG Jin-da and SUN Chong-yu Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, P.R.China
Abstract The carbon (C), nitrogen (N) and phosphorus (P) variations of a temperate wetland soil under continuous cultivation for 40 yr were determined and evaluated in the Sanjiang Plain, Northeast China. The results showed that the soil organic carbon (SOC) and total nitrogen (TN) contents in each soil layer decreased sharply after cultivation for 2-3 yr, and exhibited minor differences after cultivation for 11 yr, which showed an exponential decline curve with the increase of cultivation years. The reduction rates of carbon and nitrogen reserves were 14.79% and 28.53% yr-1 at the initial reclamation stages of 2-3 yr and then decreased to 2.02-3.08% yr-1 and 1.98-2.93% yr-1 after cultivation for 20 yr, respectively. Soil total phosphorus (TP) reserves decreased within cultivation for 5 yr, and then gradually restored to the initial level after cultivation for 17 yr. Both SOC and TN could be restored slightly when the farmland was left fallow for 8 yr after reclamation for 11 yr, whereas TP had no significant difference. These results demonstrated that wetland cultivation was one of the most important factors influencing on the nutrient fate and reserves in soil, which could lead to the rapid nutrient release and slow restoration through abandon cultivation, therefore protective cultivation techniques preventing nutrients from loss should be immediately established after wetland reclamation. Key words: wetland reclamation, Sanjiang Plain, organic carbon, total nitrogen, total phosphorus
INTRODUCTION Wetland, regarded as a nutrient pool of carbon, nitrogen and phosphorus in environment, is an important field for the biogeochemical processes of nutrient elements. The carbon reserves in all types of wetlands account for about 10-15% of the total carbon store in the terrestrial ecosystem (Tarnocai and Stolbovoy 2006). Due to the anaerobic condition and higher organic matter, soil carbon, nitrogen and phosphorus cycle in wetland exhibit a special environment behavior and take an important function in nutrient reserves, which are different from those in other ecosystems. It has Received 30 December, 2010
been proposed that about 68% of the total global area of wetlands, as well as presumably a similar percentage of the carbon reserves, has been lost since the Industrial Revolution due to the environmental changes and human activities, such as peat harvesting and converting wetland to the cropland, forestry, and urban areas (Neher et al. 2003; Zhang et al. 2008). Carbon and nitrogen mineralization processes are of great importance in maintaining the soil quality and fertility as well as the substantial basis for plant growth (Carter 1996). The key questions of wetland reclamation are that the nutrient exhaustion and the soil quality degradation, which can result in the decline of soil function. With cultivation time, the soil hydrothermal
Accepted 18 March, 2011
Correspondence LIU Jing-shuang, Tel: +86-431-85542232, Fax: +86-431-85542298, E-mail: [email protected]
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Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China
conditions and physicochemical properties would be greatly changed, and the nutrients would migrate and move outside of the soil system (Bowman et al. 1999; Song et al. 2004). Some studies have found that the transformation of the wetland to the cropland could lead to the organic matter decomposition and the nutrient decrease in soil (Zhang et al. 2007). In addition, the wetland reclamation might be an important reason for greenhouse gas elevation (Gregorich et al. 2000; Johnson et al. 2005). The Sanjiang Plain (43°49´55´´-48°27´40´´N, 129°11´20´´-135°05´26´´E) is a river basin illuviated by Heilongjiang River, Songhua River and Usuri River, located in Northeast China. It covers an area of 1.09×105 km2, which was mostly dominated by marsh wetlands before the 1900s, and presently is the second largest marsh in China (Liu and Ma 2000). The area had experienced an intensive reclamation over the past 50 yr with the population growth and migration. Most of the virgin marshland has been converted, resulting in the increase of cultivated land from about 8.2×105 ha in 1949 to 5.24×106 ha at present (Liu and Ma 2000; Liu et al. 2004). The soil nutrients migrate between different ecosystems intensively, caused by the wetland reclamation and the vegetation destruction (Saggar et al. 2001; Templer et al. 2005). The abilities of a soil to supply nutrients, store water, resist physical degradation, and produce crops within a sustainable managed framework are strongly affected by the quality and quantity of the organic matter that it contains (Ress et al. 2000). The predominant degradative processes, for example, soil erosion, depletion of soil organic matter, and a decline in soil structure, are accelerated (Lal 2002). Considerable attention has been focused on the restoration of soil structure, organic matter, and soil water-holding capacity. However, there is little information on the influences of the continuous cultivation on nutrient changes in the Sanjiang Plain after wetland reclamation. The present study was conducted to analyze and evaluate the carbon, nitrogen and phosphorus variations in soil of natural wetland, cropland at Honghe Farm, Sanjiang Plain. It is expected that the results of this study will provide insight into the relationships between nutrient reserves and cultivation time, and provide theoretical guidance to reduce the cultivation impact on nutrient losses in soil.
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RESULTS Influences of wetland reclamation on soil organic carbon As shown in Fig. 1, SOC contents in different layers were all exhibited a decrease trend in the wetland and all cultivated farmland. SOC contents in 0-15 cm layer of wetland were much higher and decreased sharply with the rates of 48.63 and 81.42% after reclamation for 2 and 5 yr, respectively, and then had no significant changes within 20-40 yr. SOC contents in 15-30 cm layer of wetland were 9.02 g kg-1 and also sharply decreased within reclamation for 5 yr, and then were stable between 2.73 and 3.83 g kg-1 within cultivation for 40 yr. SOC contents in 30-45 cm layer of wetland were only 4.97 g kg-1 and decreased by 26.49% after reclamation for 2 yr, and then fluctuated between 1.16 and 1.53 g kg-1 after cultivation for 5 yr.
Fig. 1 Influences of wetland reclamation on the SOC in different soil layers.
The relationships between SOC content (x) in different layers and reclamation year (y) were all fitted with the exponential decline curves and the equations were as follows, respectively: the plough layer, y=25.54 exp(-x/1.86)+4.05, R2=0.976; the transition layer, y=6.48 exp(-x/2.86)+2.86, R2=0.937; and the plough sole, y=3.87 exp(-x/2.38)+1.24, R2=0.884. The results indicated that the SOC decrease rates gradually reduced with the soil depth increase.
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Influences of wetland reclamation on total nitrogen Regularities for TN contents in different soil layers after reclamation were shown in Fig. 2. TN decreased from the top soil to the 45 cm depth in wetland and all cultivated farmland. TN contents in the plough layer, transition layer and plough sole decreased by 64.29, 56.27 and 69.13% after cultivation for 2 yr, respectively, and then decreased slightly after cultivation for 8 yr. The reduction rates of nitrogen in all soil layers fluctuated between 0.55-2.51% yr-1, which had a tendency of keeping a constant value after cultivation for about 20 yr.
crease trend from the top soil to the 45 cm depth in wetland and all cultivated farmland, and decreased by 19.51-39.72%, 21.82-38.99% and 37.74-38.67% in the plough layer, transition layer and plough sole within cultivation for 3-5 yr, respectively. TP in the plough layer and transition layer began to increase after cultivation for 5 yr and reached to the initial levels within 18 yr except for the plough sole which restored a little slowly. TP in the plough layer, transition layer and plough sole had a continuous increase trend with the cultivation time, which was different from those of the organic carbon and nitrogen.
Fig. 3 Influences of wetland reclamation on the TP in different soil layers. Fig. 2 Influences of wetland reclamation on the TN in different soil layers.
The relationships between TN content (x) in different soil layers and reclamation year (y) were all fitted with exponential decline curves and the equations were as follows, respectively: the plough layer, y=8.66 exp (-x/1.46)+2.06, R2=0.931; the transition layer, y=4.05 exp(-x/1.84)+1.50, R 2=0.890; and the plough sole, y=1.69 exp(-x/0.989)+0.466, R2=0.915. The results indicated that the TN decrease rates gradually reduced with the soil depth increase.
Influences of wetland reclamation on total phosphorus Influences of wetland reclamation on TP in different soil layers were shown in Fig. 3. TP exhibited a de-
Variations of C/N and C/P Influences of cultivation on ratios of C/N and C/P in different soil layers were shown in Fig. 4. C/N in different farmland soil layers significantly increased at the initial reclamation stages and then gradually declined with the increase of cultivation time. The values of C/N in different soil layers were plough sole (30-45 cm)>plough layer (0-15 cm)>transition layer (15-30 cm) within reclamation for 10 yr, whereas, almost the same in the plough layer and transition layer, and higher in the plough sole afterwards. The orders of C/P in wetland and farmland soil were the same as follows: plough layer (0-15 cm)>transition layer (15-30 cm)> plough sole (30-45 cm). The values of C/P in different farmland soil layers gradually declined with the increase of cultivation years and were almost the same after cultivation for 17 yr.
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Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China
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Table 1 The soil bulk density at different cultivation time Bulk density (g cm -3)
Reclamation years (yr) 0 2 5 8 12 17 23 35 40 11(8) 1) 1)
Fig. 4 Influences of cultivation on ratios of C/N and C/P in different soil layers.
Variations of soil nutrient reserves after wetland reclamation The wetland soil is supersaturated or saturated by water all the time before reclamation and resulting in organic matter accumulation. The soil nutrient transformation mostly concentrated in the layers from the topsoil to the 45 cm depth because of the cultivation perturbation, which were selected to estimate the nutrient reserves after reclamation. The nutrient reserves could be estimated through the soil bulk density (Table 1) and nutrient content in different layers, and the calculation formula is as follows: (1) Lsi=dvi×Ci×hi/100 Where, Lsi, the nutrient reserves of the soil profile in the ith layer (g m-2), Ci, the nutrient content in the ith layer (mg kg-1), dvi, the soil bulk density of the ith layer (g cm-3), and hi, the soil depth of the ith layer (cm). The soil nutrient reserves in the unit area (Ls) are the summation of the nutrient reserves in the layer from j to n, and the calculation formula is as follows: (2) Where, Ls, the summation of the nutrient reserves (g m-2). The carbon, nitrogen and phosphorus reserves of the unit area in wetland and farmland were calculated according to formulae (1) and (2) and shown in Table 2.
0-15
15-30
30-45
0.426±0.380 0.635±0.045 0.917±0.061 0.984±0.069 1.042±0.064 1.057±0.059 1.061±0.061 1.141±0.077 1.195±0.072 0.689±0.047
0.782±0.064 0.896±0.066 1.084±0.058 1.147±0.064 1.246±0.076 1.272±0.079 1.297±0.089 1.313±0.084 1.362±0.088 1.082±0.061
1.481±0.090 1.498±0.094 1.513±0.097 1.522±0.091 1.549±0.087 1.665±0.107 1.721±0.096 1.768±0.116 1.776±0.112 1.536±0.095
11(8) presents abandoning cultivation for 8 yr after cultivation for 11 yr. The same as below.
Carbon and nitrogen reserves in wetland soil above 45 cm were 5.316 kg m-2 and 2 301.53 g m-2, respectively. The reduction rates of carbon and nitrogen reserves were about 14.79 and 28.53% yr-1 averagely at the initial reclamation stages within 2 yr, and then gradually decreased and kept stability at 2.02-3.08% yr -1 and 1.98-2.93% yr-1 after cultivation for 20 yr. Phosphorus reserves reduction rates were about 8.17 % yr-1 at the initial reclamation stages, and then decreased slowly with the increase of cultivation years. Moreover, phosphorus reserves in soil could be recovered to the initial level after cultivation for 20 yr, which indicated that the reclamation influenced the soil phosphorus slightly. The organic carbon, nitrogen and phosphorus in the soil fallow for 8 yr after reclamation for 11 yr were restored 1.80, 0.084 and 0.43% yr-1, respectively, as shown in Table 2. The restoration rates of organic carbon, nitrogen and phosphorus in different soil layers were 0-15 cm>30-45 cm>15-30 cm, 30-45 cm>0-15 cm>15-30 cm and 0-15 cm>15-30 cm>30-45 cm, respectively.
DISCUSSION Effects of environmental conditions The site condition of the wetland soil would be completely changed after reclamation, especially in the aspects of water condition, aeration, tillage, plant type and the added nutrients, which result in the organic matter decomposition rapidly (Yang and Wander 1999; Zhao et al. 2008). When the site condition in a relatively stable level again, the new dynamic balance of the soil nutrient reserves would be established. The
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Table 2 Carbon, nitrogen and phosphorus reserves in soil at different cultivation time and the annual variation rates (AVRs) Organic carbon
Total nitrogen
Total phosphorus
Reclamation years (yr) Reserves (kg m ) -2
0 2 5 8 11 17 23 35 40 11(8)
5.32±0.34 3.74±0.26 2.43±0.18 1.54±0.15 1.55±0.16 1.74±0.19 1.55±0.14 1.55±0.12 1.54±0.11 1.80±0.21
AVRs (% yr ) -1
0 -14.79 -10.84 -8.87 -6.44 -3.96 -3.08 -2.02 -1.78 +1.80
wetland soil was water saturated in long period and the organic matter slowly decomposed and gradually accumulated because of the anaerobic conditions (Compton and Boone 2000). Once reclamation, the soil physicochemical conditions would be changed greatly and the organic matter decomposed even quickly, especially in the plough layer, which was the main reason for carbon and nitrogen decrease (Milkha and Sukhdev 2005). The study suggested that cultivation could enhance the processes of organic carbon mineralization, and subsequently organic nitrogen. The above study indicated that SOC decrease rates gradually declined from the plough layer to the plough sole and a little variation of SOC in all layers was found after reclamation for 8 yr. Similarly, previous studies had shown that cultivation of wetland resulted in the organic matter reduction which was initially rapid, and total soil nitrogen levels was also decreased (Fayez 2006). The crop assimilation and harvesting were other reasons for the carbon and nitrogen loss. The aboveground biomass of the farmland crop was about 520-660 g m-2, which decreased about 64.71-69.25% compared with the initial wetland aboveground biomass, and even was removed out of the soil-crop system. The underground biomass of the farmland crop was about 130-360 g m-2, which decreased 58.62-85.06% compared with that of the wetland (Zhang et al. 2007), indicating that the plant carbon and nitrogen accumulation effect was reduced. Total phosphorus presented an accumulative status in soil and accumulated more quickly in plough layer than that of in transition layer and plough sole. Soil organic phosphorus constitutes about 50% of total soil TP (Lopez-Gutierrez et al. 2004), and is derived from microbes, plants or animals, and may be recycled into
Reserves (g m ) -2
2 301.53±118.06 988.12±72.32 842.06±54.65 715.95±45.53 784.95±42.16 807.96±51.31 750.75±40.27 700.82±38.52 696.46±47.05 790.24±52.26
AVRs (% yr ) -1
0 -28.53 -12.68 -8.61 -5.99 -3.82 -2.93 -1.99 -1.74 +0.084
Reserves (g m-2) 314.62±19.25 263.24±16.17 255.45±14.27 269.44±21.23 290.80±22.41 302.02±24.82 380.07±27.39 377.03±21.04 381.27±19.83 300.85±17.58
AVRs (% yr-1) 0 -8.16 -3.76 -1.80 -0.69 -0.24 +0.90 +0.57 +0.53 +0.43
the soil microbial biomass or stabilize in the soil matrix. The basal organic phosphorus mineralization rates in non-irradiated samples were 1.4-2.5 mg P kg-1 yr-1 (Fritz et al. 2004), which was approximately equivalent to, or higher than water soluble phosphorus. But phosphorus deficiency is a major constraint to the crop production and most of the inorganic phosphorus added in the form of chemical fertilizers is rapidly sorbed by or precipitated with (i.e., fixed with) Al and Fe components and thereby becomes less available to crops. In the Sanjiang Plain, plenty of underground water, containing large amount of Fe, was pumped up to satisfy the demand of irrigation (Zou et al. 2008). The phosphorus entered in the soil would combine with Fe and become unavailable. Some researchers suggest that phosphorus efficiency can be achieved by the enhancement of biologically mediation in organic phosphorus mineralization with large P-fixing capacities (Bunemann et al. 2004). Therefore regular utilization of phosphorus fertilizer might be the main reason for the phosphorus increase after reclamation for several years. In this study, we found that the soil carbon and nitrogen declined rapidly at the initial reclamation stages, and reduced by 0.576 and 0.292 kg m-2 yr-1 within reclamation for 20 yr, which accounted for about 76.14 and 91.18% within reclamation for 40 yr. Estimation of carbon and nitrogen reserves within different land management and cropping system is an important element in the design of land use styles that would protect or sequester carbon and nitrogen. Apezteguia et al. (2009) estimated that there would be 44-45% loss of the organic matter when a temperate scrubland converted to the cropland. Conversely, in these soils the adoption of sustainable practices like maize-soybean rotation under no tillage can produce a capture of 100.51
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Kg C ha -1 yr -1 and would make soils act as sinks of carbon. Wetland reclamation accelerated the circulation of carbon and nitrogen, whereas partly lead to the increase of CO2, CH4 and N2O emission to the air. In fact, the results of wetland reclamation in some extent would be one of the important reasons for global warming (Kirschbaum 2000). How to prevent soil carbon and nitrogen reduction after wetland reclamation is an essential issue in the field of agro-environment and climate change (Li et al. 2006). The SOC and the ratios of C/N and C/P are important indices of the soil quality, which affected the soil organic matter decomposition and the balance between carbon and nitrogen in microorganisms (Bruun et al. 2005). Microorganism activities increased when C/N dropped off, and with the adequate nitrogen the microorganisms would demand more carbon maintaining their activities, which could accelerate the organic matter mineralization and cause the farmland become the sources of carbon (Guo and Gifford 2002; Dignac et al. 2002; Meng et al. 2005; Ren et al. 2006). The utilization of nitrogen and phosphorus fertilizer alone might further aggravate the loss of soil nutrient reserves due to the imbalance of C/N and C/P (Chatigny et al. 1999). Suitable quantity of organic and inorganic fertilizers should be used at the initial reclamation stages to ensure the slightly decrease of nutrient level.
Wagner 1998). There are many protective cultivation techniques for the Sanjiang Plain (Wang et al. 2007). Usually, ridge cultivation combined with subsoiling between rows should be taken, which build ridges 15-20 cm high and 25-30 cm wide and subsoiling 30-40 cm depth at 60-120 cm intervals. When it comes to the area initially reclaimed, reduction tillage by skipping tillage operations is necessary, while the main tillage operation between two crops will be skipped, and the appropriately rotation would be maize-coarse cerealssoybean. Some researchers concern on the land utilization type or management suggested that about 6070% lost carbon would be recovered and made the farmland to be the carbon sinks through the application of conservation tillage techniques (Lal 2002; Apezteguia 2009). Composted stalks also should be applied to farmland mulching soil in order to keep moisture from evaporating and directly increase the organic matters (Shepherd et al. 2001; Koch and Stockfisch 2006). In addition, converting the irrational reclamation land to the wetland could guarantee the normal evolution of the existing wetland and achieve the healthy development between the regional ecological environment and agriculture production in the Sanjiang Plain (Liu and Ma 2002).
The nutrients restoration
Soil nutrients are the prerequisites for the sustainability of agriculture production and the stability of wetland evolution. The reclamation caused 70.8 and 69.6% loss of the soil carbon and nitrogen reserves after 40 yr, whereas 19.8% increase of the soil phosphorus because of blindfolded fertilizer utilization and lack of rational management. Although abandoning cultivation could partly restore the carbon and nitrogen reserves, whereas we should maintain the moderate reclamation and keep the appropriate scale between the farmland and wetland in order to satisfy with food needs and keep the region ecological safety as well together. It’s necessary to establish rational farming systems and take sound conservation tillage techniques for preventing soil nutrients from losing after wetland reclamation, which could keep the nutrient balance in the soil-crop system with high-yielding production in the long run and alleviate the global climate changes in some extent.
The reasons for the different changes between nitrogen and phosphorus might be that the organic nitrogen was mineralized more quickly in the top layers or adsorbed infirmly and easily to be leached to the down layers when compared with phosphorus. If the vegetation gradually recovered, the mineralization rates of organic matter would be decreased, which caused the nitrogen and phosphorus accumulation through the soil structural improvement and moisture conservation. The slow nutrient restoration after abandoning cultivation meant that the nutrients could be consumed easily and more difficult to be restored (Knops and Tilman 2000). Both ecological restoration measures which return the farmland to grassland or wetland and the conservation tillage techniques should be taken to prevent the reduction of soil carbon and nitrogen (Buyanovsky and
CONCLUSION
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MATERIALS AND METHODS Site descriptions The study site is located at the Honghe Farm in Sanjiang Plain, northeastern Heilongjiang Province, China (47°35´N, 133°31´E), and based on the research platform of the Sanjiang Mire Wetland Ecosystem Experimental Station of the Chinese Academy of Sciences. The average elevation of the study area is about 55.4-57.9 m with a gradient of 1/5 000. The mean lowest, highest, and annual mean air temperatures are -18-21°C, 21-22°C and 1.6-1.9°C, respectively. The freezing period is up to 5 months and the nadir is 190 cm below ground (Zhao 1999). The average annual precipitation is 565-600 mm, of which more than 60% takes place from June to August, and the average annual evaporation is about 542-580 mm. The natural wetland, dominated by Deyeuxia angustifolia, Carex lasiocarpa and Carex pseudocuraica communities, is a representative among the Sanjiang Plain wetlands. The farmlands have been reclaimed since the 1950s in succession and most of the farmlands cultivate soybean or paddy.
Sample schemes The soybean farmlands, originally covered by typical wetland plants and cultivated for 2, 5, 8, 11, 17, 23, 35, and 40 yr, were selected as research objects. The natural wetlands, dominated by Deyeuxia angustifolia communities, were selected as references. The sample sites were distributed in the Honghe Farmland and shown in Fig. 5. Soybeans were planted continuously each year in May and harvested in September or October, and no fertilizer was added at the initial reclamation stages (about 5-8 yr). Three layers of the soil were sampled: plough layer (0-15 cm), transition layer (15-30 cm) and plough sole (30-45 cm). The natural wetland soil was also sampled as the same layers mentioned above. All samples were collected in autumn before freezing entirely. Three sampling sites were set in different farmland and wetland, and 3-5 samples of the same soil layer for one type in the same site were mixed together.
Fig. 5 The Sanjiang Plain region, Northeast China and sampling sites (rectangular area) in the Honghe Farm.
determined by UV-2500 Spectrophotometer (Manufactured by Shimadzu, Japan), using ascorbic acid molybdenum blue method (Murphy and Riley 1962).
Statistical analysis Statistical analysis and plots were carried out using SPSS software package for Windows and ORIGINPRO 7.5. In all analyses where P<0.05, the factor was tested and the relationship was considered significantly.
Acknowledgements We gratefully acknowledge Profs. Zhang Xuelin and Wang Guoping for their constructive comments on the manuscript and Drs. Yang Jisong, Sun Zhigao and Zhou Wangming for their valuable help with the sampling, Zhang Yuxia, Zhao Haiyang and Lou Bokun for samples analyses (Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences). The research was financially supported by the National Natural Science Foundation of China (41071056) and the Discovery Research Project of Chinese Academy of Sciences (KZCX2-YW-309).
Sample analysis References The soil bulk density and soil water content were also measured for 0-15 cm, 15-30 cm and 30-45 cm depths by using cutting rings of 5.3 cm in diameter. Soil samples were air-dried at ambient temperature, crushed, and sieved through a 0.25 mm sieve to remove the coarse fraction (gravel and roots). Soil organic carbon (SOC) and total nitrogen (TN) were measured with a FLASHEA 1112 CHNS analyzer (Manufactured by CE Elantech, Inc., USA). The total phosphorus in the digests extracted from the soil was
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Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China
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2012, 11(3): 512-520
RESEARCH ARTICLE
Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China WANG Yang, LIU Jing-shuang, WANG Jin-da and SUN Chong-yu Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, P.R.China
Abstract The carbon (C), nitrogen (N) and phosphorus (P) variations of a temperate wetland soil under continuous cultivation for 40 yr were determined and evaluated in the Sanjiang Plain, Northeast China. The results showed that the soil organic carbon (SOC) and total nitrogen (TN) contents in each soil layer decreased sharply after cultivation for 2-3 yr, and exhibited minor differences after cultivation for 11 yr, which showed an exponential decline curve with the increase of cultivation years. The reduction rates of carbon and nitrogen reserves were 14.79% and 28.53% yr-1 at the initial reclamation stages of 2-3 yr and then decreased to 2.02-3.08% yr-1 and 1.98-2.93% yr-1 after cultivation for 20 yr, respectively. Soil total phosphorus (TP) reserves decreased within cultivation for 5 yr, and then gradually restored to the initial level after cultivation for 17 yr. Both SOC and TN could be restored slightly when the farmland was left fallow for 8 yr after reclamation for 11 yr, whereas TP had no significant difference. These results demonstrated that wetland cultivation was one of the most important factors influencing on the nutrient fate and reserves in soil, which could lead to the rapid nutrient release and slow restoration through abandon cultivation, therefore protective cultivation techniques preventing nutrients from loss should be immediately established after wetland reclamation. Key words: wetland reclamation, Sanjiang Plain, organic carbon, total nitrogen, total phosphorus
INTRODUCTION Wetland, regarded as a nutrient pool of carbon, nitrogen and phosphorus in environment, is an important field for the biogeochemical processes of nutrient elements. The carbon reserves in all types of wetlands account for about 10-15% of the total carbon store in the terrestrial ecosystem (Tarnocai and Stolbovoy 2006). Due to the anaerobic condition and higher organic matter, soil carbon, nitrogen and phosphorus cycle in wetland exhibit a special environment behavior and take an important function in nutrient reserves, which are different from those in other ecosystems. It has Received 30 December, 2010
been proposed that about 68% of the total global area of wetlands, as well as presumably a similar percentage of the carbon reserves, has been lost since the Industrial Revolution due to the environmental changes and human activities, such as peat harvesting and converting wetland to the cropland, forestry, and urban areas (Neher et al. 2003; Zhang et al. 2008). Carbon and nitrogen mineralization processes are of great importance in maintaining the soil quality and fertility as well as the substantial basis for plant growth (Carter 1996). The key questions of wetland reclamation are that the nutrient exhaustion and the soil quality degradation, which can result in the decline of soil function. With cultivation time, the soil hydrothermal
Accepted 18 March, 2011
Correspondence LIU Jing-shuang, Tel: +86-431-85542232, Fax: +86-431-85542298, E-mail: [email protected]
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Effects of Wetland Reclamation on Soil Nutrient Losses and Reserves in Sanjiang Plain, Northeast China
conditions and physicochemical properties would be greatly changed, and the nutrients would migrate and move outside of the soil system (Bowman et al. 1999; Song et al. 2004). Some studies have found that the transformation of the wetland to the cropland could lead to the organic matter decomposition and the nutrient decrease in soil (Zhang et al. 2007). In addition, the wetland reclamation might be an important reason for greenhouse gas elevation (Gregorich et al. 2000; Johnson et al. 2005). The Sanjiang Plain (43°49´55´´-48°27´40´´N, 129°11´20´´-135°05´26´´E) is a river basin illuviated by Heilongjiang River, Songhua River and Usuri River, located in Northeast China. It covers an area of 1.09×105 km2, which was mostly dominated by marsh wetlands before the 1900s, and presently is the second largest marsh in China (Liu and Ma 2000). The area had experienced an intensive reclamation over the past 50 yr with the population growth and migration. Most of the virgin marshland has been converted, resulting in the increase of cultivated land from about 8.2×105 ha in 1949 to 5.24×106 ha at present (Liu and Ma 2000; Liu et al. 2004). The soil nutrients migrate between different ecosystems intensively, caused by the wetland reclamation and the vegetation destruction (Saggar et al. 2001; Templer et al. 2005). The abilities of a soil to supply nutrients, store water, resist physical degradation, and produce crops within a sustainable managed framework are strongly affected by the quality and quantity of the organic matter that it contains (Ress et al. 2000). The predominant degradative processes, for example, soil erosion, depletion of soil organic matter, and a decline in soil structure, are accelerated (Lal 2002). Considerable attention has been focused on the restoration of soil structure, organic matter, and soil water-holding capacity. However, there is little information on the influences of the continuous cultivation on nutrient changes in the Sanjiang Plain after wetland reclamation. The present study was conducted to analyze and evaluate the carbon, nitrogen and phosphorus variations in soil of natural wetland, cropland at Honghe Farm, Sanjiang Plain. It is expected that the results of this study will provide insight into the relationships between nutrient reserves and cultivation time, and provide theoretical guidance to reduce the cultivation impact on nutrient losses in soil.
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RESULTS Influences of wetland reclamation on soil organic carbon As shown in Fig. 1, SOC contents in different layers were all exhibited a decrease trend in the wetland and all cultivated farmland. SOC contents in 0-15 cm layer of wetland were much higher and decreased sharply with the rates of 48.63 and 81.42% after reclamation for 2 and 5 yr, respectively, and then had no significant changes within 20-40 yr. SOC contents in 15-30 cm layer of wetland were 9.02 g kg-1 and also sharply decreased within reclamation for 5 yr, and then were stable between 2.73 and 3.83 g kg-1 within cultivation for 40 yr. SOC contents in 30-45 cm layer of wetland were only 4.97 g kg-1 and decreased by 26.49% after reclamation for 2 yr, and then fluctuated between 1.16 and 1.53 g kg-1 after cultivation for 5 yr.
Fig. 1 Influences of wetland reclamation on the SOC in different soil layers.
The relationships between SOC content (x) in different layers and reclamation year (y) were all fitted with the exponential decline curves and the equations were as follows, respectively: the plough layer, y=25.54 exp(-x/1.86)+4.05, R2=0.976; the transition layer, y=6.48 exp(-x/2.86)+2.86, R2=0.937; and the plough sole, y=3.87 exp(-x/2.38)+1.24, R2=0.884. The results indicated that the SOC decrease rates gradually reduced with the soil depth increase.
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Influences of wetland reclamation on total nitrogen Regularities for TN contents in different soil layers after reclamation were shown in Fig. 2. TN decreased from the top soil to the 45 cm depth in wetland and all cultivated farmland. TN contents in the plough layer, transition layer and plough sole decreased by 64.29, 56.27 and 69.13% after cultivation for 2 yr, respectively, and then decreased slightly after cultivation for 8 yr. The reduction rates of nitrogen in all soil layers fluctuated between 0.55-2.51% yr-1, which had a tendency of keeping a constant value after cultivation for about 20 yr.
crease trend from the top soil to the 45 cm depth in wetland and all cultivated farmland, and decreased by 19.51-39.72%, 21.82-38.99% and 37.74-38.67% in the plough layer, transition layer and plough sole within cultivation for 3-5 yr, respectively. TP in the plough layer and transition layer began to increase after cultivation for 5 yr and reached to the initial levels within 18 yr except for the plough sole which restored a little slowly. TP in the plough layer, transition layer and plough sole had a continuous increase trend with the cultivation time, which was different from those of the organic carbon and nitrogen.
Fig. 3 Influences of wetland reclamation on the TP in different soil layers. Fig. 2 Influences of wetland reclamation on the TN in different soil layers.
The relationships between TN content (x) in different soil layers and reclamation year (y) were all fitted with exponential decline curves and the equations were as follows, respectively: the plough layer, y=8.66 exp (-x/1.46)+2.06, R2=0.931; the transition layer, y=4.05 exp(-x/1.84)+1.50, R 2=0.890; and the plough sole, y=1.69 exp(-x/0.989)+0.466, R2=0.915. The results indicated that the TN decrease rates gradually reduced with the soil depth increase.
Influences of wetland reclamation on total phosphorus Influences of wetland reclamation on TP in different soil layers were shown in Fig. 3. TP exhibited a de-
Variations of C/N and C/P Influences of cultivation on ratios of C/N and C/P in different soil layers were shown in Fig. 4. C/N in different farmland soil layers significantly increased at the initial reclamation stages and then gradually declined with the increase of cultivation time. The values of C/N in different soil layers were plough sole (30-45 cm)>plough layer (0-15 cm)>transition layer (15-30 cm) within reclamation for 10 yr, whereas, almost the same in the plough layer and transition layer, and higher in the plough sole afterwards. The orders of C/P in wetland and farmland soil were the same as follows: plough layer (0-15 cm)>transition layer (15-30 cm)> plough sole (30-45 cm). The values of C/P in different farmland soil layers gradually declined with the increase of cultivation years and were almost the same after cultivation for 17 yr.
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Table 1 The soil bulk density at different cultivation time Bulk density (g cm -3)
Reclamation years (yr) 0 2 5 8 12 17 23 35 40 11(8) 1) 1)
Fig. 4 Influences of cultivation on ratios of C/N and C/P in different soil layers.
Variations of soil nutrient reserves after wetland reclamation The wetland soil is supersaturated or saturated by water all the time before reclamation and resulting in organic matter accumulation. The soil nutrient transformation mostly concentrated in the layers from the topsoil to the 45 cm depth because of the cultivation perturbation, which were selected to estimate the nutrient reserves after reclamation. The nutrient reserves could be estimated through the soil bulk density (Table 1) and nutrient content in different layers, and the calculation formula is as follows: (1) Lsi=dvi×Ci×hi/100 Where, Lsi, the nutrient reserves of the soil profile in the ith layer (g m-2), Ci, the nutrient content in the ith layer (mg kg-1), dvi, the soil bulk density of the ith layer (g cm-3), and hi, the soil depth of the ith layer (cm). The soil nutrient reserves in the unit area (Ls) are the summation of the nutrient reserves in the layer from j to n, and the calculation formula is as follows: (2) Where, Ls, the summation of the nutrient reserves (g m-2). The carbon, nitrogen and phosphorus reserves of the unit area in wetland and farmland were calculated according to formulae (1) and (2) and shown in Table 2.
0-15
15-30
30-45
0.426±0.380 0.635±0.045 0.917±0.061 0.984±0.069 1.042±0.064 1.057±0.059 1.061±0.061 1.141±0.077 1.195±0.072 0.689±0.047
0.782±0.064 0.896±0.066 1.084±0.058 1.147±0.064 1.246±0.076 1.272±0.079 1.297±0.089 1.313±0.084 1.362±0.088 1.082±0.061
1.481±0.090 1.498±0.094 1.513±0.097 1.522±0.091 1.549±0.087 1.665±0.107 1.721±0.096 1.768±0.116 1.776±0.112 1.536±0.095
11(8) presents abandoning cultivation for 8 yr after cultivation for 11 yr. The same as below.
Carbon and nitrogen reserves in wetland soil above 45 cm were 5.316 kg m-2 and 2 301.53 g m-2, respectively. The reduction rates of carbon and nitrogen reserves were about 14.79 and 28.53% yr-1 averagely at the initial reclamation stages within 2 yr, and then gradually decreased and kept stability at 2.02-3.08% yr -1 and 1.98-2.93% yr-1 after cultivation for 20 yr. Phosphorus reserves reduction rates were about 8.17 % yr-1 at the initial reclamation stages, and then decreased slowly with the increase of cultivation years. Moreover, phosphorus reserves in soil could be recovered to the initial level after cultivation for 20 yr, which indicated that the reclamation influenced the soil phosphorus slightly. The organic carbon, nitrogen and phosphorus in the soil fallow for 8 yr after reclamation for 11 yr were restored 1.80, 0.084 and 0.43% yr-1, respectively, as shown in Table 2. The restoration rates of organic carbon, nitrogen and phosphorus in different soil layers were 0-15 cm>30-45 cm>15-30 cm, 30-45 cm>0-15 cm>15-30 cm and 0-15 cm>15-30 cm>30-45 cm, respectively.
DISCUSSION Effects of environmental conditions The site condition of the wetland soil would be completely changed after reclamation, especially in the aspects of water condition, aeration, tillage, plant type and the added nutrients, which result in the organic matter decomposition rapidly (Yang and Wander 1999; Zhao et al. 2008). When the site condition in a relatively stable level again, the new dynamic balance of the soil nutrient reserves would be established. The
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Table 2 Carbon, nitrogen and phosphorus reserves in soil at different cultivation time and the annual variation rates (AVRs) Organic carbon
Total nitrogen
Total phosphorus
Reclamation years (yr) Reserves (kg m ) -2
0 2 5 8 11 17 23 35 40 11(8)
5.32±0.34 3.74±0.26 2.43±0.18 1.54±0.15 1.55±0.16 1.74±0.19 1.55±0.14 1.55±0.12 1.54±0.11 1.80±0.21
AVRs (% yr ) -1
0 -14.79 -10.84 -8.87 -6.44 -3.96 -3.08 -2.02 -1.78 +1.80
wetland soil was water saturated in long period and the organic matter slowly decomposed and gradually accumulated because of the anaerobic conditions (Compton and Boone 2000). Once reclamation, the soil physicochemical conditions would be changed greatly and the organic matter decomposed even quickly, especially in the plough layer, which was the main reason for carbon and nitrogen decrease (Milkha and Sukhdev 2005). The study suggested that cultivation could enhance the processes of organic carbon mineralization, and subsequently organic nitrogen. The above study indicated that SOC decrease rates gradually declined from the plough layer to the plough sole and a little variation of SOC in all layers was found after reclamation for 8 yr. Similarly, previous studies had shown that cultivation of wetland resulted in the organic matter reduction which was initially rapid, and total soil nitrogen levels was also decreased (Fayez 2006). The crop assimilation and harvesting were other reasons for the carbon and nitrogen loss. The aboveground biomass of the farmland crop was about 520-660 g m-2, which decreased about 64.71-69.25% compared with the initial wetland aboveground biomass, and even was removed out of the soil-crop system. The underground biomass of the farmland crop was about 130-360 g m-2, which decreased 58.62-85.06% compared with that of the wetland (Zhang et al. 2007), indicating that the plant carbon and nitrogen accumulation effect was reduced. Total phosphorus presented an accumulative status in soil and accumulated more quickly in plough layer than that of in transition layer and plough sole. Soil organic phosphorus constitutes about 50% of total soil TP (Lopez-Gutierrez et al. 2004), and is derived from microbes, plants or animals, and may be recycled into
Reserves (g m ) -2
2 301.53±118.06 988.12±72.32 842.06±54.65 715.95±45.53 784.95±42.16 807.96±51.31 750.75±40.27 700.82±38.52 696.46±47.05 790.24±52.26
AVRs (% yr ) -1
0 -28.53 -12.68 -8.61 -5.99 -3.82 -2.93 -1.99 -1.74 +0.084
Reserves (g m-2) 314.62±19.25 263.24±16.17 255.45±14.27 269.44±21.23 290.80±22.41 302.02±24.82 380.07±27.39 377.03±21.04 381.27±19.83 300.85±17.58
AVRs (% yr-1) 0 -8.16 -3.76 -1.80 -0.69 -0.24 +0.90 +0.57 +0.53 +0.43
the soil microbial biomass or stabilize in the soil matrix. The basal organic phosphorus mineralization rates in non-irradiated samples were 1.4-2.5 mg P kg-1 yr-1 (Fritz et al. 2004), which was approximately equivalent to, or higher than water soluble phosphorus. But phosphorus deficiency is a major constraint to the crop production and most of the inorganic phosphorus added in the form of chemical fertilizers is rapidly sorbed by or precipitated with (i.e., fixed with) Al and Fe components and thereby becomes less available to crops. In the Sanjiang Plain, plenty of underground water, containing large amount of Fe, was pumped up to satisfy the demand of irrigation (Zou et al. 2008). The phosphorus entered in the soil would combine with Fe and become unavailable. Some researchers suggest that phosphorus efficiency can be achieved by the enhancement of biologically mediation in organic phosphorus mineralization with large P-fixing capacities (Bunemann et al. 2004). Therefore regular utilization of phosphorus fertilizer might be the main reason for the phosphorus increase after reclamation for several years. In this study, we found that the soil carbon and nitrogen declined rapidly at the initial reclamation stages, and reduced by 0.576 and 0.292 kg m-2 yr-1 within reclamation for 20 yr, which accounted for about 76.14 and 91.18% within reclamation for 40 yr. Estimation of carbon and nitrogen reserves within different land management and cropping system is an important element in the design of land use styles that would protect or sequester carbon and nitrogen. Apezteguia et al. (2009) estimated that there would be 44-45% loss of the organic matter when a temperate scrubland converted to the cropland. Conversely, in these soils the adoption of sustainable practices like maize-soybean rotation under no tillage can produce a capture of 100.51
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Kg C ha -1 yr -1 and would make soils act as sinks of carbon. Wetland reclamation accelerated the circulation of carbon and nitrogen, whereas partly lead to the increase of CO2, CH4 and N2O emission to the air. In fact, the results of wetland reclamation in some extent would be one of the important reasons for global warming (Kirschbaum 2000). How to prevent soil carbon and nitrogen reduction after wetland reclamation is an essential issue in the field of agro-environment and climate change (Li et al. 2006). The SOC and the ratios of C/N and C/P are important indices of the soil quality, which affected the soil organic matter decomposition and the balance between carbon and nitrogen in microorganisms (Bruun et al. 2005). Microorganism activities increased when C/N dropped off, and with the adequate nitrogen the microorganisms would demand more carbon maintaining their activities, which could accelerate the organic matter mineralization and cause the farmland become the sources of carbon (Guo and Gifford 2002; Dignac et al. 2002; Meng et al. 2005; Ren et al. 2006). The utilization of nitrogen and phosphorus fertilizer alone might further aggravate the loss of soil nutrient reserves due to the imbalance of C/N and C/P (Chatigny et al. 1999). Suitable quantity of organic and inorganic fertilizers should be used at the initial reclamation stages to ensure the slightly decrease of nutrient level.
Wagner 1998). There are many protective cultivation techniques for the Sanjiang Plain (Wang et al. 2007). Usually, ridge cultivation combined with subsoiling between rows should be taken, which build ridges 15-20 cm high and 25-30 cm wide and subsoiling 30-40 cm depth at 60-120 cm intervals. When it comes to the area initially reclaimed, reduction tillage by skipping tillage operations is necessary, while the main tillage operation between two crops will be skipped, and the appropriately rotation would be maize-coarse cerealssoybean. Some researchers concern on the land utilization type or management suggested that about 6070% lost carbon would be recovered and made the farmland to be the carbon sinks through the application of conservation tillage techniques (Lal 2002; Apezteguia 2009). Composted stalks also should be applied to farmland mulching soil in order to keep moisture from evaporating and directly increase the organic matters (Shepherd et al. 2001; Koch and Stockfisch 2006). In addition, converting the irrational reclamation land to the wetland could guarantee the normal evolution of the existing wetland and achieve the healthy development between the regional ecological environment and agriculture production in the Sanjiang Plain (Liu and Ma 2002).
The nutrients restoration
Soil nutrients are the prerequisites for the sustainability of agriculture production and the stability of wetland evolution. The reclamation caused 70.8 and 69.6% loss of the soil carbon and nitrogen reserves after 40 yr, whereas 19.8% increase of the soil phosphorus because of blindfolded fertilizer utilization and lack of rational management. Although abandoning cultivation could partly restore the carbon and nitrogen reserves, whereas we should maintain the moderate reclamation and keep the appropriate scale between the farmland and wetland in order to satisfy with food needs and keep the region ecological safety as well together. It’s necessary to establish rational farming systems and take sound conservation tillage techniques for preventing soil nutrients from losing after wetland reclamation, which could keep the nutrient balance in the soil-crop system with high-yielding production in the long run and alleviate the global climate changes in some extent.
The reasons for the different changes between nitrogen and phosphorus might be that the organic nitrogen was mineralized more quickly in the top layers or adsorbed infirmly and easily to be leached to the down layers when compared with phosphorus. If the vegetation gradually recovered, the mineralization rates of organic matter would be decreased, which caused the nitrogen and phosphorus accumulation through the soil structural improvement and moisture conservation. The slow nutrient restoration after abandoning cultivation meant that the nutrients could be consumed easily and more difficult to be restored (Knops and Tilman 2000). Both ecological restoration measures which return the farmland to grassland or wetland and the conservation tillage techniques should be taken to prevent the reduction of soil carbon and nitrogen (Buyanovsky and
CONCLUSION
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MATERIALS AND METHODS Site descriptions The study site is located at the Honghe Farm in Sanjiang Plain, northeastern Heilongjiang Province, China (47°35´N, 133°31´E), and based on the research platform of the Sanjiang Mire Wetland Ecosystem Experimental Station of the Chinese Academy of Sciences. The average elevation of the study area is about 55.4-57.9 m with a gradient of 1/5 000. The mean lowest, highest, and annual mean air temperatures are -18-21°C, 21-22°C and 1.6-1.9°C, respectively. The freezing period is up to 5 months and the nadir is 190 cm below ground (Zhao 1999). The average annual precipitation is 565-600 mm, of which more than 60% takes place from June to August, and the average annual evaporation is about 542-580 mm. The natural wetland, dominated by Deyeuxia angustifolia, Carex lasiocarpa and Carex pseudocuraica communities, is a representative among the Sanjiang Plain wetlands. The farmlands have been reclaimed since the 1950s in succession and most of the farmlands cultivate soybean or paddy.
Sample schemes The soybean farmlands, originally covered by typical wetland plants and cultivated for 2, 5, 8, 11, 17, 23, 35, and 40 yr, were selected as research objects. The natural wetlands, dominated by Deyeuxia angustifolia communities, were selected as references. The sample sites were distributed in the Honghe Farmland and shown in Fig. 5. Soybeans were planted continuously each year in May and harvested in September or October, and no fertilizer was added at the initial reclamation stages (about 5-8 yr). Three layers of the soil were sampled: plough layer (0-15 cm), transition layer (15-30 cm) and plough sole (30-45 cm). The natural wetland soil was also sampled as the same layers mentioned above. All samples were collected in autumn before freezing entirely. Three sampling sites were set in different farmland and wetland, and 3-5 samples of the same soil layer for one type in the same site were mixed together.
Fig. 5 The Sanjiang Plain region, Northeast China and sampling sites (rectangular area) in the Honghe Farm.
determined by UV-2500 Spectrophotometer (Manufactured by Shimadzu, Japan), using ascorbic acid molybdenum blue method (Murphy and Riley 1962).
Statistical analysis Statistical analysis and plots were carried out using SPSS software package for Windows and ORIGINPRO 7.5. In all analyses where P<0.05, the factor was tested and the relationship was considered significantly.
Acknowledgements We gratefully acknowledge Profs. Zhang Xuelin and Wang Guoping for their constructive comments on the manuscript and Drs. Yang Jisong, Sun Zhigao and Zhou Wangming for their valuable help with the sampling, Zhang Yuxia, Zhao Haiyang and Lou Bokun for samples analyses (Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences). The research was financially supported by the National Natural Science Foundation of China (41071056) and the Discovery Research Project of Chinese Academy of Sciences (KZCX2-YW-309).
Sample analysis References The soil bulk density and soil water content were also measured for 0-15 cm, 15-30 cm and 30-45 cm depths by using cutting rings of 5.3 cm in diameter. Soil samples were air-dried at ambient temperature, crushed, and sieved through a 0.25 mm sieve to remove the coarse fraction (gravel and roots). Soil organic carbon (SOC) and total nitrogen (TN) were measured with a FLASHEA 1112 CHNS analyzer (Manufactured by CE Elantech, Inc., USA). The total phosphorus in the digests extracted from the soil was
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