Response of soil C:N:P stoichiometry, organic carbon stock, and release to wetland grasslandification in Mu Us Desert
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Wetlands in Mu Us Desert have severely been threatened by grasslandification over the past decades. Therefore, we studied the impacts of grasslandification on soil carbon (C):nitrogen (N):phosphorus (P) stoichiometry, soil organic carbon (SOC) stock, and release in wetland-grassland transitional zone in Mu Us Desert.
Materials and methods
From wetland to grassland, the transition zone was divided into five different successional stages according to plant communities and soil water conditions. At every stage, soil physical and chemical properties were determined and C:N:P ratios were calculated. SOC stock and soil respirations were also determined to assess soil carbon storage and release.
Results and discussion
After grasslandification, SOC contents of top soils (0–10 cm) decreased from 100.2 to 31.79 g kg−1 in June and from 103.7 to 32.5 g kg−1 in October; total nitrogen (TN) contents of top soils (0–10 cm) decreased from 3.65 to 1.85 g kg−1 in June and from 6.43 to 3.36 g kg−1 in October; and total phosphorus (TP) contents of top soils (0–10 cm) decreased from 179.4 to 117.4 mg kg−1 in June and from 368.6 to 227.8 mg kg−1 in October. From stages Typha angustifolia wetland (TAW) to Phalaris arundinacea L. (PAL), in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 19.0 to 11.8 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October. SOC stock decreased and soil respiration increased with grasslandification. The decrease of SOC, TN, and TP contents was attributed to the reduction of aboveground biomass and mineralization of SOM, and the decrease of soil C:N, C:P, and N:P ratios was mainly attributed to the faster decreasing speeds of SOC than TN and TP. The reduction of aboveground biomass and increased SOC release led by enhanced soil respiration were the main reasons of SOC stock decrease.
Grasslandification led to lowers levels of SOC, TN, TP, and soil C:N, C:P, and N:P ratios. Grasslandification also led to higher SOC loss, and increased soil respiration was the main reason. Since it is difficult to restore grassland to original wetland, efficient practices should be conducted to reduce water drainage from wetland to prevent grasslandification.
KeywordsCarbon stock and release C:N:P stoichiometry Grasslandification Nutrient status Wetland
Wetlands such as rivers, lakes, coastlines, and bogs constitute important ecosystems which perform many vital functions. Wetlands play a critical role in regulating climate, improving ecological environment and maintaining biodiversity (Beuel et at. 2016). Wetlands also serve as a major stock of carbon and can significantly affect global warming through the emission of greenhouse gases like methane and carbon dioxide (Nyssen et al. 2008; Ceddia et al. 2015; Wang et al. 2016). However, health of the global ecosystems and survival of human beings has been threatened by the loss and degradation of the wetlands. These severe consequences are mainly attributed to anthropogenic activities such as drainage and agricultural reclamations (Mitsch 2005; Hu et al. 2017).
Grasslandification implies conversion of a wetland into a grassland as a result of drainage or climatic change (Shang et al. 2013). Since the process of grasslandification is usually caused by reduction in water levels, it will be accompanied by the reduce of soil water content (SWC). Water conditions play a key role in maintainance and accumulation of soil organic carbon (SOC) which is strongly driven by hydrological restoration and sediment deposition (Ballantine and Schneider 2009; Chen et al. 2017). Another key process that occurs as a result of grasslandification is vegetation succession. There have been reports with opposing conclusions regarding the effect of grasslandification on the levels of SOC, nitrogen (N), and phosphorus (P) in the soil (Dunne et al. 2010; Shang et al. 2013; Wang et al. 2014; Liu et al. 2017a, b). Also, the studies elucidating the effects of the use of different land type on SOC, soil N, and P contents, gave little emphasis on the process of grasslandification (Qu et al. 2014; Wang et al. 2014; Zhao et al. 2015; Liu et al. 2017a, b). Although both grasslandification and farmland cultivation in a wetland are soil-drying processes, soil characteristics in a grassland and a farmland can be quite different (Wang et al. 2014; Liu et al. 2017a, b). These observations indicate the need for further research on the changes in SOC, soil N, and P contents as a result of grasslandification.
In this study, we applied the C:N:P stoichiometry approach to improve our understanding of C, N, and P status and cycling in wetland systems. Ecological stoichiometry is the study of element ratios in organisms and environments like soils and marine water (Cleveland and Liptzin 2007). Redfield found that C:N:P ratios of phytoplanktons in ocean and marine water were consistent at approximately 106:16:1 (Redfield 1960). This observation paved way for numerous applications of C:N:P stoichiometry in the ecological research of soils, plants, and microorganisms (Reich and Oleksyn 2004; Li et al. 2012; Fan et al. 2015; Schreeg et al. 2016). The average soil C:N:P ratio was reported to be 186:13:1 globally (Cleveland and Liptzin 2007) and 134:9:1 in China (Tian et al. 2010). It was also reported by Xu and Post (2013) that the best estimate of soil C:N:P ratio was 287:17:1. Since C:N ratios in soil are affected by atmosphere, soil nutrient conditions, tree species, and humus types, there are various reports debating the constrained/unconstrained nature of C:N:P ratio in soils (Tian et al. 2010; Cools et al. 2014; Fan et al. 2015). It was reported that SOM C:N and C:P ratios were lower than biomass and higher than litter (Nicolas et al. 2013; Zechmeister-Boltenstern et al. 2016), and it is reasonable that soil C:N:P ratios can be affected by litter composition. It was also reported that SOM decomposition process can be enhanced by higher C inputs when there were higher aboveground biomass and soil C:N:P ratios can hence be affected (Craine et al. 2007; Blagodatskaya and Kuzyakov 2008).
The process of wetland grasslandification is accompanied by changes of water condition, plant species, and aboveground biomass and how soil C:N:P ratios will response to these changes. Shang et al. (2013) reported that wetland grasslandification in the Tibetan Plateau resulted in decreased ratios of soil N:P and C:P, indicating a faster depletion in C as compared with N and P reserves. Although it was found that soil C:P ratios and N:P ratios in wetland were higher than grassland in the Ili River region (Liu et al. 2017a, b), opposing observations were reported in the Fujian Province (Wang et al. 2014). Literature research reveals documentation of numerous studies based on the response of the ratios of soil C:N:P to the use of different land types (Shang et al. 2013; Wang et al. 2014; Qu et al. 2014; Gao et al. 2014; Zeng et al. 2016; Zhao et al. 2015; Liu et al. 2017a, b); however, there were limited reports on the changes in ratios of soil C:N:P in response to grasslandification. There is no record on the response of soil C:N:P stoichiometry to wetland grasslandification process in the Mu Us Desert.
Mu Us Desert is a sand land located in northwestern China, at the junction of the north Shaanxi province and the south Inner Mongolia autonomous region. It is the transitional zone between the monsoon and the non-monsoon regions and thus, has very fragile ecosystems. Wetlands like rivers, lakes, and bogs in this region play critical roles in maintaining the health of natural environment. However, these wetlands are severely threatened. Climate change and human activities like farming and draining have caused the shrinkage of the water bodies in the wetland. The stock of SOC in wetland can have great effects on soil quality and nutrient status while releasing the reserves of SOC in the form of CH4 and CO2 gases would promote global warming (Lal 2004; Setia et al. 2010). Besides, soil N and P status are critical for the growth of wetland plants in order to maintain a better and more stable ecological environment. Grasslandification has been on a rise for agricultural development. It is urgent to know the effects of grasslandification on soil carbon, nitrogen, and phosphorus status and C:N:P stoichiometry in wetlands of the Mu Us Desert.
This study investigated the following: (1) What are the effects of grasslandification on N and P status and C:N:P stoichiometry? (2) What are the effects of grasslandification on carbon stock and release in wetland soil of the Mu Us Desert? (3) What are the major influencing physicochemical factors in the grasslandification process? The study predicted: (1) SOC, total nitrogen (TN), and total phosphorus (TP) contents and soil C:P ratios will decrease; (2) SOC stock will decrease and SOC release will increase when soil transforms from water-submerged condition to air-exposed condition; (3) soil water condition, soil water content, and aboveground biomass will be major influencing factors in grasslandification process.
2 Materials and methods
2.1 Study site and vegetation investigation
Mu Us Desert is one of the four deserts in China and is located at 107° 20′–111° 30′ E and 37° 27.5′–39° 22.5′ N. It covers the lands of southern part of Ordos City in Inner Mongolia, northern part of Yulin City in Shaanxi province, and northeast part of Yanchi City in Ningxia autonomous region. It covers a total area of 42,200 km2. The annual temperature of this sand land is 6.0 to 8.5 °C while the temperature in January is − 9.5 to 12 °C. The annual precipitation is 250 to 440 mm and confined mainly to the months of July to September. This contributes approximately 60 to 75% of the total annual. The east part is a dry grassland of chestnut soil and has an annual precipitation of 400 to 440 mm while the west part is a semi-desert land of brown soil. Mu Us Desert is located at the junction of several natural regions, and the vegetations and soils have been characterized by transitional properties (Li and Xiao 2007; Wang et al. 2010).
Typha angustifolia wetland (TAW) is a natural or virgin wetland. T. angustifolia is a tall hydrophyte which is higher than 2 m. The water depth of this zone is 70–120 cm. This wetland has been used to represent the shallow water area (water depth < 120 cm) of this lake wetland.
Phragmite australis wetland (PAW) is characterized by higher P. australis plants, no water submerging, and comparatively wetter soils. This is the first dry land area and the first stage of transformation from a natural wetland.
Phragmite australis dry land (PAD) is characterized by smaller and shorter P. australis plants and drier soil as compared with PAW. This is the third stage of the wetland grasslandification process.
Phragmite australis and Phalaris arundinacea L.-mixed grassland (PA+PAL) is characterized by two dominant species, P. australis and P. arundinacea L. P. australis plants in this stage are much smaller and shorter than PAD and PAW.
Phalaris arundinacea L. dry land (PAL) is characterized by the dominant species P. arundinacea L. and dry soils with more sand content. This is the last stage of grasslandification.
Characteristics of vegetation during different successional stages in June
Biomass (g m−2)
193.6 ± 22.2
55.7 ± 8.3
93.6 ± 10.5
1305.3 ± 86.3
131.2 ± 15.3
72.3 ± 10.5
76.1 ± 8.2
1016.1 ± 77.5
79.4 ± 12.3
58.2 ± 7.6
77.5 ± 9.4
876.2 ± 54.3
65.8 ± 8.6
51.1 ± 6.8
74.1 ± 8.3
681.1 ± 45.6
56.3 ± 5.7
41.2 ± 5.6
66.2 ± 7.5
515.3 ± 40.6
Characteristics of vegetation during different successional stages in October
Biomass (g m−2)
201.5 ± 15.3
281.4 ± 22.3
93.1 ± 5.2
1658.1 ± 192.8
108.6 ± 10.4
334.1 ± 27.8
98.5 ± 4.7
1219.2 ± 138.5
100.1 ± 9.9
179.2 ± 20.9
92.1 ± 3.2
940.6 ± 100.6
95.6 ± 4.5
257.9 ± 30.1
85.3 ± 4.9
950.7 ± 115.8
90.1 ± 5.8
164.1 ± 20.5
80.4 ± 5.5
931.2 ± 120.5
2.2 Field sampling and laboratory analysis
The collection of soil samples was conducted in June 2016 and October 2016. Three plots (50 × 50 m) were selected for each stage of grasslandification, three subplots (10 × 10 m) were arranged in each plot and five quadrats (1·0 × 1·0 m) were randomly chosen in each plot. Soil samples were collected with a soil-drilling sampler (9 cm i.d.) corer at intervals of 0–10, 10–20, and 20–30 cm at each quadrat (Liu et al. 2017a, b). Five soil samples which were collected at the same depth in each subplot were mixed to form one sample. Nine soil samples at the same depth were collected at each stage, and thus, a total of 27 soil samples were collected in each stage. The final total number of soil samples were 135 (5 stages × 9 subplots × 3 depths). After collection, soil samples were transported to the laboratory with ice bags in thermal containers. Plant root tissues and leaves were removed before the soil samples were kept under laboratory conditions. When taken to the laboratory, half of wet soil samples were stored at 4 °C and the other half were air dried and sieved (< 2 mm) for the analysis of chemical and physical properties.
Soil bulk density (BD) was obtained using the cutting ring method (Liu et al. 2017a, b). Soil electrical conductivity and pH were measured using portable conductivity meter (HI993310, HANNA, Italy) and pH meter (HQ11d, HACH, USA), respectively. Soil particle size (clay, silt, and sand) was measured using a laser particle analyzer (LS-609, OMEC, China).
The TN content in soil was determined by semi-micro Kjeldahl method, and TP content was determined by perchloric acid digestion method followed by molybdate colorimetry method using UV-2550 spectrophotometer (Shimadzu, Japan). Nitrate nitrogen content and ammonium nitrogen content were determined by continuous flow analyzer (AutoAnalyzer 3, SEAL, Germany). Total organic carbon content was determined by potassium dichromate oxidation method after digestion with concentrated sulfuric acid (Wang et al. 2015a).
Soil respiration was measured by Li-8100A Soil Flux System under field conditions. Among the soil respiration measurement plots, a PVC collar (11 cm in diameter and 8 cm in height) was inserted into the soil to a depth of 5 cm at the center of each plot for measuring soil CO2 efflux. Living plants inside the PVC collars were clipped at the soil surface at least once a day before measurements to eliminate the effects of aboveground biomass respiration. A 24-h determination of soil respiration was done, and we found that soil respiration from 9:30 to 10:30 am is near to the average value of the whole day. Soil respiration of studied sites was determined from 6 to 11 June 11 and from 2 to 7 October. The average values for every site were then calculated.
2.3 Statistic analysis
The differences in the soil variables with different successional stages and depths (stages × depths) were conducted by a two-way ANOVA with SPSS 20.0. Pearson correlation analysis was conducted with R language 3.51, using the package “corrplot.” Type II standard major axis (SMA) was performed by SPSS 20.0 after a logarithmic conversion. The stoichiometric relationship was analyzed with the model: logy = a + blogx, which was similar to the model: y = axb. When the slope was approximately 1, the relationship was described as isometric, which means x and y are in a linear relationship (Cleveland and Liptzin 2007).
Discriminant functional analysis (DFA) was used to separate the soils of different stages from each other based on their physical and chemical characteristics. Principal component analysis (PCA) was used here to determine the relationships between physical characteristics and chemical characteristics in the form of vectors. DFA and PCA were performed together using R language, with the package “factoextra.” The contributions of every component and factor in PCA was analyzed using R language, with the package “FactoMineR.” Redundancy analysis (RDA) was used to compare physical properties and chemical properties of the soil. RDA between C:N:P stoichiometry and soil respiration, SOC stock, and other chemical properties were also conducted using R language.
SOCD (kg m−2) is the reserve of soil SOC at a depth of 0–30 cm; i represents the depth of the soil; Ci is the soil SOC content at the depth of i; ρb is the bulk density of soil at layer i; and di is the thickness of soil at layer i.
3.1 Soil physical and chemical properties in different stages of grasslandification
From stages TAW to PAL, in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 732.4 to 382.9 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October (Figs. 4 and 5). C:P ratios in the top soils decreased steadily from the TAW stage to the PAL stage in June and October while soil C:N ratios and soil N:P ratios of top soils (0–10 cm) showed no continuous changing trend from the TAW stage to the PAL stage in June and October (Figs. 4 and 5). Soil C:P ratios and N:P ratios were higher in top soils (0–10 cm) than in subsoils (10–20 cm) in June and October while soil C:N ratios had no such regularity (Figs. 4 and 5).
Results of ANOVA on effects of depths and different successional stages on soil physical and chemical properties in June and October are presented in Tables S3 and S4 (ESM). Evidently, both in June and October, soil depths and different successional stages had significant effects on SWC, BD, SOC, TN, nitrate nitrogen, ammonium nitrogen and TP contents, and soil C:P and N:P ratios (Tables S3 and S4 (ESM)).
3.2 Correlation between soil physical and chemical properties
3.3 Type II-standardized major axis analysis for soil carbon, nitrogen, and phosphorus
Evaluation of the comparative changing speed between soil carbon, nitrogen, and phosphorus was performed using the SMA analysis. The model was logy = a + blogx or y = axb.
Standardized major axis analysis of the levels of SOC, TN, and TP in June
Standardized major axis analysis of the levels of SOC, TN, and TP in October
3.4 Soil carbon stock, respirations, and their relationship with soil physicochemical properties
For results in October, it was observed that the soil respirations increased from stages TAW to PA+PAL and reached the highest value at the mixed grassland stage (PA+PAL). Average soil respiration rates in stages TAW, PAW, PAD, PA+PAL, and PAL were 0.14, 1.93, 3.76, 6.55, and 3.16 μmol m−2 s−1. SOC stock in TAW stage was much higher than other stages. From PAW to PAL, SOC stock decreased with a much slower speed. SOC stocks in stages TAW, PAW, PAD, PA+PAL, and PAL were 23.8, 10.1, 8.51, 8.29, and 6.27 kg m−2.
Correlations between soil respiration, SOC stock, and soil physicochemical properties in June and October
3.5 RDA and PCA for soils from different stages
PCA for soils from different successional stages based on content of soil water, pH, bulk density, electrical conductivity, silt, clay, and sand in June and October are presented in Figs. S1 and S4 (ESM). In the PCA result, the first two components explained 85.0% of the variances in June (Fig. S1 (ESM)) and 80.1% of the variances in October. Among all the physical properties, maximum contributions were made by soil water, silt, clay, and sand contents while pH and bulk density had little effect leading to the variations among different soils in June and October (Figs. S1 and S4 (ESM)). Soils of natural wetland (TAW) had significant differences as compared with the other land types generally because they have high soil water contents and silt and clay contents. Similarly, lower soil water, silt, clay, and sand contents and higher bulk density of the soils of grassland (PAL) also resulted in differences as compared with other land types. Soils of stages PAW and PAD and mixed grassland (PA+PAL) shared similar characteristics (Figs. S2 and S5 (ESM)).
PCA for soils from different successional stages based on soil SOC, TN, TP, nitrate nitrogen, ammonium nitrogen contents, C:N ratios, C:P ratios, and N:P ratios are presented in Figs. S1 and S4 (ESM). In the PCA result, the first two components explained 76.8% of the variances in June (Figs. S1 and S4 (ESM)) and 74.9% of the variances in October. Among all the chemical properties, maximum contributions were made by SOC, TN, TP, and soil N:P and C:P ratios while ammonium nitrogen, nitrate nitrogen, and C:N ratio had little effects on the variances (Figs. S1 and S4 (ESM)). Soils in the natural wetland (TAW) had significant differences as compared with the other stages because of higher soil SOC, TN, TP, N:P ratios, and C:P ratios. Soils in grassland (PAL) exhibited significant differences as compared with the other four land types due to lower levels of SOC, TN, TP, N:P ratios, C:P ratios, and C:N ratios (Figs. S3 and S6 (ESM)).
4.1 Changes of soil carbon, nitrogen, and phosphorus contents along the different stages of grasslandification
Experimental evidence from this study site revealed that grasslandification led to lower levels of SOC, TN, and TP and a faster reduction in C was observed as compared with N and P. These results were consistent with the research on wetland grasslandification conducted in the Tibetan Plateau (Shang et al. 2013). In this study, we attributed the depletion in soil SOC, TN, and TP to the reduction in SWC and the aboveground biomass (Table 1). Another study reported water to be a key factor in maintaining levels of C and other nutrients like N and P (Yang et al. 2010) and reduction in levels of SWC to less than 30%, decreased the levels of soil C and N by more than 50% (Shang et al. 2013). Other studies also reported severe SOC loss led by the drainage of a wetland (Zedler and Kercher 2005; Iost et al. 2007; Zhang et al. 2008). Good vegetation also significantly contributed in maintaining SOC in natural ecosystems and litters were an important supply for SOC and other nutrients like N and P (Setia and Marschner 2013; Wang et al. 2015a, b). Other studies also reported that the reduction of aboveground biomass led to lower levels of C, N, and P inputs to soils, especially for C and N (Cui et al. 2012; Gao et al. 2014; Lou et al. 2015). In this study, 99% of the total nitrogen in the soil was organic nitrogen (Fig. 3). This observation was consistent with the studies conducted in the Minjiang estuary wetland which implied that for wetland and other land use types which are cultivated from wetland, organic nitrogen has always been the major component of the total nitrogen present in soil (> 99%) (Wang et al. 2014, 2015b). In the studies performed in Ili River, a higher proportion of soil inorganic nitrogen (10–15%) was recorded in the TN composition. The dominant ratio of organic nitrogen implies little mineralization of the organic nitrogen. Other studies also reported the small fraction of soil inorganic nitrogen compared with TN (Ouyang et al. 2013; Chen et al. 2015). Thus, the depletion in soil nitrogen content was attributed to the reduction in organic nitrogen inputs led by lower aboveground biomass.
Contrary to some previous studies, lower levels of TP were recorded in the natural wetland (TAW) than the grassland (PAL) (Wang et al. 2014; Liu et al. 2017a, b). Human activities like fertilization in grassland can lead to higher TP contents in grassland than wetland (Bennett 2001; Simpson et al. 2014), and wetland-grassland ecotone in this study was not interfered by human acitivities. TP in soils were composed of organic P (Po), which were mostly in forms of phytate and inorganic P (Pi) which were in forms of Fe/Al–P in acid soils (Yang et al. 2012) and were in forms of Ca–P in neutral or alkaline soils (Maranguit et al. 2017). In some places, Po can surpass half of total phosphorus content (Berry et al. 2009), and it was found that Po were closely related to SOM. It was reported that the rapid mineralization of organic matter in Histosols drained for agricultural use released an estimated 80 kg P ha−1 year−1 (Brady and Weil 2008), and we attributed the decrease of TP contents to the mineralization of SOM and decrease of Po contents. In this study, preliminary analysis revealed that Pi was mainly present in the form of Ca–P (Latati et al. 2014; Maranguit et al. 2017). This indicated minimum impact of the reduction in aboveground biomass and SOM on the composition of phosphorus. Also, the reducing speeds of phosphorus were the slowest compared with carbon and nitrogen.
4.2 Effects of grasslandification on soil C:N:P stoichiometry
Soil C:N ratios were in a range of 16.9–33.6 in June and 11.8–19.0 in October while the average soil C:N ratio is 14.3 globally (Cleveland and Liptzin 2007) and 14.4 in China (Tian et al. 2010). In this study, grasslandification led to lower soil C:N ratios compared with virgin wetland (TAW) and the reduction of soil C:N ratios were attributed to faster decreasing speeds of SOC than TN (Tables 3 and 4). However, some other studies reported that soil C:N ratios were less affected when a wetland was transformed to a grassland (Qu et al. 2014; Wang et al. 2014, 2015a). To determine the factors related to soil C:N ratio, Cools et al. (2014) did a systematic analysis of the influential factors and it was found that tree species followed by soil types and humus types were the major factors involved. It was reported that microbes had lower C:N ratios while litters had higher C:N ratios and SOM C:N ratios were in the range between microbes and litters (Mooshammer et al. 2014; Nicolas et al. 2013) and soil C:N ratios were variable and can be affected by litter composition and other factors like N addition and warming (Zechmeister-Boltenstern et al. 2016).
The ratios of soil N:P and C:P ratios decreased progressively with the grasslandification process which were due to the faster decreasing speeds of SOC and TN than TP as discussed above (phosphorus was mainly from mineral and was less affected) (Tables 3 and 4). This result was consistent with the study conducted by Shang et al. (2013) which also reported lower levels of C:P and N:P ratios in grasslands as compared with the natural wetlands. However, there are other studies which have reported opposing evidences on the same ratios (Wang et al. 2014; Liu et al. 2017a, b). In our study, soil C:P ratios in top soils (0–10 cm) were in a range of 580.5–1519.2 in June and 320.6–732.4 in October; soil N:P ratios in top soils (0–10 cm) were in a range of 28.2–46.9 in June and 31.1–35.2 in October. The average soil C:P ratio is 186.0 globally and 136.0 in China; the average N:P ratio is 13.0 globally and 9.30 in China (Cleveland and Liptzin 2007; Tian et al. 2010). Besides, in the Minjiang estuary, wetland soil C:P ratios were less than 240 and N:P ratios were less than 9 (Wang et al. 2014, 2015a). In Yellow River delta wetland, soil C:P ratios were less than 75 and soil N:P ratios were approximately 2 (Qu et al. 2014). Higher C:P and N:P ratios in this study site were attributed to higher SOC contents and TN contents and lower TP contents compared with soils in the Minjiang estuary (Wang et al. 2014, 2015b). It has already been noted that soil N:P ratios decrease during the process of grasslandification, since soil P suffers minimal impact from aerobic conditions and litter input. Considering that macro-aquatic plants had much more P demand as compared with P. arundinacea L. in the grassland (Palmborg et al. 2014), the situation of P limitation was alleviated after grasslandification.
4.3 Effects of grasslandification on soil organic carbon stock and release
SOC stock was observed to be higher in the TAW stage with little to no difference in the other stages of grasslandification (Fig. 8). Major carbon loss, which occurred during soil transformation from the natural wetland to P. australis wetland was driven by the sharply increased soil respiration (Fig. 9). Transformation of soils from anaerobic to aerobic conditions enhanced soil respiration in microorganisms and faster loss of SOC (Setia and Marschner 2013; Wang et al. 2014, 2015a). Though there was little CO2 released from the natural wetland, it is known that soils under anaerobic condition can still release carbon in the form of CH4 (Chen et al. 2015). It was reported that the release rate of CH4 from submerged soils was 13.77 μmol m−2 h−1 (Chen et al. 2015), and an average CO2 flux in P. australis wetland (PAW) in this study was 195.6 μmol m−2 h−1 which was much higher than the carbon release of CH4. Yang et al. (2013) reported that CO2 flux was 17.6 times of CH4 flux indicating the dominant role of CO2 release in the process of carbon loss. SOC stock were positively correlated with soil C:P ratios and SWC (p < 0.01) in June and October (Table 5). In this study, soils in the TAW stage had higher SWC and SOC contents and lower BD while soils in the PAL stage had lower SWC and SOC contents and higher BD. Higher SOC stock in the TAW stage was attributed to higher SOC contents. However, it was reported that SOC stock (0–30 cm) was significantly negatively correlated with SWC in an alpine wetland in the Qinghai-Tibet Plateau (Bai et al. 2010). In the study of Qinghai-Tibet Plateau, higher SOC contents occurred in peat soils with lower water contents than higher water contents (this was quite different with this study), and this led to negative correlation between SOC stock and SWC (Bai et al. 2010).
For the four land stages of grasslandification, soil respiration rates were highest in mixed grassland (PA+PAL) and lowest in P. arundinacea grassland (PAL) (Fig. 5). Soil respiration rates in the PAW and PAD stages were approximately the same. Previous studies reported close relationships between electrical conductivity (salinity), soil N:P ratios, and soil respiration (Setia and Marschner 2013; Wang et al. 2014, 2015a). However, there was no correlation observed between soil respiration and electrical conductivity in this study. It was reported that soil respiration was mainly driven by the productivity and increase in plant diversity (Dias et al. 2010). Besides this, it was also reported that soils respirations were closely related to tree species and litter quality (Vesterdal et al. 2012). In this study, we attribute the lower respiration rates in grassland (PAL) to the lower litter inputs and the higher respiration rates in mixed grassland (PA+PAL) to the higher diversity of plant species.
Grasslandification led to reduced levels of SOC and soil TN and TP, which was mainly attributed to decreased aboveground biomass and mineralization of SOM. Soil C:N ratios, C:P ratios, and N:P ratios decreased after grasslandification because of faster decreasing speeds of SOC than TN and TP and faster decreasing speeds of TN than TP. However, we insisted the alleviation of P limitation after grasslandification because of lower C:N and N:P ratios and lower P demands of plants in grassland. Grasslandification led to lower SOC stock and higher SOC release led by enhanced soil respiration. Most of SOC was released during the transformation of natural wetland (TAW) to PAW. This implies that it is critically important to maintain water submerging and anaerobic status for the soil to hold carbon. Plant productivity and diversity had limited effects on soil respiration as compared with changing of the soil water condition. We recommended efficient practices to be conducted by the local government to reduce water drainage from wetlands to prevent grasslandification.
Acknowledgements This study was financially supported by Western Light Program of CAS (XBZG2011015) and State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau Foundation (A314021402-1605).
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