Soil coarseness is the main process decreasing soil organic matter and threatening the productivity of sandy grasslands. Previous studies demonstrated negative effect of soil coarseness on soil carbon storage, but less is known about how soil base cations (exchangeable Ca, Mg, K, and Na) and available micronutrients (available Fe, Mn, Cu, and Zn) response to soil coarseness. In a semi-arid grassland of Northern China, a field experiment was initiated in 2011 to mimic the effect of soil coarseness on soil base cations and available micronutrients by mixing soil with different mass proportions of sand: 0 % coarse elements (C0), 10 % (C10), 30 % (C30), 50 % (C50), and 70 % (C70). Soil coarseness significantly increased soil pH in three soil depths of 0–10, 10–20 and 20–40 cm with the highest pH values detected in C50 and C70 treatments. Soil fine particles (smaller than 0.25 mm) significantly decreased with the degree of soil coarseness. Exchangeable Ca and Mg concentrations significantly decreased with soil coarseness degree by up to 29.8 % (in C70) and 47.5 % (in C70), respectively, across three soil depths. Soil available Fe, Mn, and Cu significantly decreased with soil coarseness degree by 62.5, 45.4, and 44.4 %, respectively. As affected by soil coarseness, the increase of soil pH, decrease of soil fine particles (including clay), and decline in soil organic matter were the main driving factors for the decrease of exchangeable base cations (except K) and available micronutrients (except Zn) through soil profile. Developed under soil coarseness, the loss and redistribution of base cations and available micronutrients along soil depths might pose a threat to ecosystem productivity of this sandy grassland.
Dryland ecosystems, accounting for 41 % of the total land area of the
world, are prone to desertification which would result in soil coarseness
(Cerdà et al., 2014; T. Wang et al., 2015). Dryland ecosystems represent
25 % of land surface area in Latin America, with 75 % of them having
desertification problems (Torres et al., 2015). Desertified land area has
been reported to reach 45.6 million km
In China, most of the grasslands have undergone degradation and desertification with 50 % distributed in the agro-pastoral transition zone of Northern China (T. Wang et al., 2015; Yan and Cai, 2015). Continuous grazing and intense cultivation can reduce vegetation cover and litter accumulation, which exposes the ground surface to wind erosion in the erosion-prone sandy lands (Su et al., 2005). Desertification and wind erosion processes in fragile arid and semi-arid rangelands have contributed to increased soil coarseness (Yan and Cai, 2015). Together with reduction of plant cover, increased soil coarseness contributes to a loss of agricultural productivity, environmental deterioration, and associated social and economic disruptions (Vieira et al., 2015; Xie et al., 2015). In this case, it is urgent to combat desertification and study the causes, processes, consequences, and mechanisms of soil coarseness (Xu et al., 2012; Weinzierl et al., 2016).
Soil base cations are not only essential nutrient cations for both plants and soil microbes but also serve as one of the main mechanisms of soil acid buffering capacity (Lu et al., 2014) and are a good indicator of soil fertility (Zhang et al., 2013). Micronutrient availabilities essentially affect terrestrial net primary production, plant quality, and consequently food and forage supply worldwide (Cheng et al., 2010; Marques et al., 2015). Current research about desertification and soil coarseness mainly focuses on their effects on degradation of forest and grasslands due to logging and overgrazing (Conte et al., 1999; Cao et al., 2008), C and N depletion in soils and plant components (Zhou et al., 2008; Bisaro et al., 2014), soil compaction and erosion risk (Allington and Valone, 2010), and soil physical properties of particle size distributions (Su et al., 2004; Huang et al., 2007). However, less is known about the changes in soil base cations and availabilities of micronutrient during dryland desertification and soil coarseness.
Soil coarseness is suggested to cause a decrease of soil silt and clay contents (Zhou et al., 2008), decrease of soil C and nutrient (such as N and P) concentrations (Xie et al., 2015), and losses in species diversity and productivity (Zhao et al., 2006; Huang et al., 2007). As biogeochemical cyclings of base cations and micronutrients are largely controlled by soil organic matter (SOM) (complexation and chelation; Sharma et al., 2004) and properties of soil mineral (reversible sorption and desorption processes; Jobbágy and Jackson, 2004), decrease of SOM and soil fine particles would potentially decrease soil base cations and micronutrient availability. Also, the changes in SOM along soil depth could shape the vertical distribution of base cations and available micronutrients (Sharma et al., 2004).
The Horqin Sandy Land, or Horqin sandy grassland, is an important part of
Inner Mongolian grassland and one of the main sandy areas in Northern China,
covering approximately 43 000 km
Location of the experimental site.
The study was conducted at the Desertified Grassland Restoration Research
Station maintained by the Institute of Sand Fixation and Utilization, Liaoning
Academy of Agricultural Sciences. The study site (42
In May 2011, a complete randomized design was applied to the site. Within a
24 m
Mean and range of soil chemical characteristics for 0–10 cm soil in different soil coarseness degrees from 0 % sand addition (C0) to 70 % (C70).
In October 2015 (i.e., after 2 years of plant community settled), a composite soil sample was taken from three randomly selected locations within each plot from three soil layers of 0–10, 10–20, and 20–40 cm. Fresh soil samples were sieved through 2 mm screen and visible plant roots were taken out. After transportation to laboratory, the soils were air-dried and a subsample of the soil was ground for C and N analysis.
Soil pH was determined in a
Soil base cations were determined using the CH
Available Fe, Mn, Cu, and Zn were extracted by diethylenetriaminepentaacetic
acid (DTPA) according to the method of Lindsay and Norvell (1978). Briefly, 10 g
of soil samples was mixed with 20 mL 0.005 M DTPA
The normality of data was tested using the Kolmogorov-Smirnov test, and
homogeneity of variances using Leven's test. Effects of soil coarseness on
soil pH, fine particles, base cations, and available micronutrients were
determined by one-way ANOVA. Multiple comparisons with Duncan design were
performed to determined difference in soil parameters among soil coarseness
degrees. Pearson correlation analysis was used to examine the relationship
among soil parameters. Multivariate linear regression analyses (stepwise
removal) were conducted to determine variables that made significant
contributions to variance of soil base cations and available micronutrients.
All statistical analyses were performed in SPSS 16.0 (SPSS, Inc., Chicago,
IL, USA) and statistical significance was accepted at
Results (
Soil pH values for three soil depths
Soil coarseness significantly increased soil pH by up to 8.8 % across three soil depths (Fig. 2a; Table 2). For both 0–10 and 10–20 cm soils, the highest soil pH was detected in C70 (7.3 and 7.4, respectively) and C50 (7.2 and 7.3, respectively) soils, which were followed by C30 and C10 soils (Fig. 2a). Significant and positive overall effect of soil depth was detected on soil pH (Fig. 2a; Table 2). For both C0 and C50 treatments, soil pH of 10–20 and 20–40 cm was significantly higher as compared to that of 0–10 cm soil (Fig. 2a). Soil pH in 10–20 cm of C10 and C30 was significantly higher than that in 0–10 cm of C10 and C30, respectively (Fig. 2a). Significant interactive effect of soil coarseness and soil depth was found on soil pH (Table 2).
Proportions of soil fine particles (< 0.25 mm) were determined in 0–10 cm soil. Soil fine particles significantly decreased with soil coarseness degree by 6.3 % (for treatment of C10), 17.7 % (C30), 34.1 % (C50), and 55.6 % (C70) as compared to C0 (Fig. 2b). The lowest proportion of soil fine particles was detected in C70 (39.1 %), followed by C50 (58.1 %; Fig. 2b). The proportion of clay particles significantly decreased under soil coarseness (Fig. S1).
Soil base cations of exchangeable Ca
Across three soil depths, soil coarseness significantly decreased both exchangeable Ca and Mg concentrations by up to 29.8 and 47.5 %, respectively, as compared to C0 (Fig. 3a, b). Both exchangeable Ca and Mg concentrations were the lowest in the C70 and followed by C50 as compared to C0 in all soil depths (Fig. 3a, b). Soil depth significantly decreased soil-exchangeable Mg, while showed no effect on exchangeable Ca (Table 2). Both soil coarseness and soil depth had no impact on soil-exchangeable K (Fig. 3c). At 0–10 cm, C50 and C70 significantly decreased soil-exchangeable Na by 22.3 and 24.2 %, respectively, as compared to C0 (Fig. 3d). Soil-exchangeable Na did not change with soil depth (Fig. 3d, Table 2).
Soil available Fe significantly decreased with soil coarseness degree by as much as 17.1 % in C10, 22.0 % in C30, 36.6 % in C50, and 62.5 % in C70 across three soil depths (Fig. 4a). Soil coarseness significantly decreased Soil available Mn for 0–10 cm (by up to 17.3 % in C70) and 10–20 cm (by up to 45.4 % in C70) soils (Fig. 4b). Both Soil available Fe and Mn significantly decreased with soil depth (Fig. 4a, b; Table 2). Significant negative soil coarseness effect was detected on Soil available Cu by 14.7–44.4 % as compared to C0 in 0–10 cm soil (Fig. 4c). For both C30 and C50 treatments, Soil available Cu concentration of 10–20 cm soil was significantly higher than that in 0–10 and 20–40 cm soils. Soil available Zn concentration was not affected by soil coarseness but it decreased with soil depth (Fig. 4d; Table 2).
Soil available micronutrients of available Fe
All regression analysis were conducted for 0–10 cm soil as the data of fine particles (< 0.25 mm) were only available for 0–10 cm soil. At 0–10 cm soil, soil pH significantly and negatively correlated with exchangeable Ca, Mg, and Na and with available Fe, Mn, and Cu (Table 3). Soil fine particles (< 0.25 mm) significantly and positively correlated with exchangeable Ca, exchangeable Mg, exchangeable Na, available Fe, available Mn, and available Cu (Table 3). The SOC significantly and positively correlated with exchangeable Ca, Mg, Na, available Fe, Mn, and Cu (Table 3).
According to multiple regression models, change of soil fine particles explained 65.5, 75.7, 31.4, and 24.0 % of variations in exchangeable Ca, Mg, Na, and available Mn (Table 3). Soil pH explained 75.7 % of variation in available Fe (Table 2). The SOC explained 59.3 % of variation in available Cu (Table 2).
Regression statistics relating soil base cations (exchangeable Ca, Mg, K, and Na) and available micronutrients (Fe, Mn, Cu, and Zn) to soil pH, soil fine particles (< 0.25 mm), and soil organic carbon (SOC).
Values are
Significant decrease in exchangeable Ca and Mg concentrations in three soil
depths and exchangeable Na in 0–10 cm soil as affected by soil coarseness
partially supported our first hypothesis. The decrease of exchangeable Ca,
Mg, and Na might be due to increase of soil pH under soil coarseness as
suggested by the significant and negative correlation between soil pH and
exchangeable Ca, Mg, and Na (Table 3). Indeed, with the increase of soil pH,
soil base cations (such as Ca
Soil fine particles (< 0.25 mm), especially clay inside these fine particles, were suggested to provide additional binding surfaces for exchangeable base cations and available micronutrients (Beldin et al., 2007). Confirmed by the positive correlations of soil fine particles with both base cations (exchangeable Ca, Mg, and Na) and available micronutrients (Fe, Mn, and Cu), the decrease of soil fine particles and clay content (Fig. S1) might also contribute to lower base cations and available micronutrients under soil coarseness. Consistent with our findings, previous studies also suggested that the decrease of soil fine particles and increase of soil coarseness resulted in a loss of SOM as well as a reduction in the nutrient storage (Lopez, 1998; Zhao et al., 2006; Zhou et al., 2008).
As the essential role of SOM in retaining base cations and micronutrients by its functional groups (Oorts et al., 2003), significantly lower soil base cations and micronutrients would possibly be due to lower C (the largest component of SOM) concentration in coarsen soils. This can be further enhanced by the significant positive correlation of soil C with both base cations and micronutrients (Table 3). Consistently, Vittori Antisari et al. (2013) reported that humified organic compounds in soil could retain base cations and decrease their leaching from soils. As compared to higher soil coarseness degree, higher soil microbial activities (unpublished data) under conditions of lower soil coarseness degree could promote humification or microbial processing of the SOM (R. Wang et al., 2015), potentially increasing the availability of functional groups to complex with the base cations and micronutrients. Additionally, higher net primary production and plant nutrient demands would induce the activation of base cation and micronutrients from the soils under lower soil coarseness degree (Burke et al., 1999). Due to the fact of reduction in ecosystem productivity under cation deficiencies (Lawrence et al., 1995; Cheng et al., 2010), the loss of base cations and available micronutrients as developed under soil coarseness could constrain both plant growth and pasture productivity of this nutrient-poor sandy ecosystem.
The hypothesized decrease of exchangeable base cations and available micronutrients with soil depth was partially supported as only exchangeable Mg (Fig. 3b) and available Fe (Fig. 4a), Mn (Fig. 4b), and Zn (Fig. 4d) decreased with soil depth. Vertical distribution of soil nutrients can be influenced by two opposite processes: leaching and biological cycling (such as plant absorption; Truggill, 1988). Being a ubiquitous process in ecosystems, plant absorption of nutrients can transport soil elements aboveground and return the litterfall to soil surface (Stark, 1994). Especially in this sandy land or desertified grassland, plants tend to accumulate SOM or nutrients to form “island of fertility” (Cao et al., 2008). In these sandy soils, leaching is also an essential process in shaping the vertical distribution of soil nutrients (Truggill, 1988). As leaching moves nutrients downward while biological cycling moves them upward (Jobbágy and Jackson, 2001), the unchanged Ca, K, and Na concentrations might be the combining effects of leaching and biological cycling. This area experiences freeze–thaw cycles for at least 4–5 months per year (Alamusa et al., 2014). Freeze–thaw cycles might promote the leaching of exchangeable Ca, K, and Na from surface to subsoil, resulting in the unchanged base cations along soil profile. Our results are in contrast with previous studies, suggesting that ecosystem were more capable to retain K than other base cations (Nowak et al., 1991; Jobbágy and Jackson, 2001). In this case, it is obvious that many environmental factors, like soil types and plant community composition, can be drivers for the vertical distribution of base cations and micronutrients (Burke et al., 1999; Van der Ploeg et al., 2012). The leaching of base cations to subsoils might enhance mineral weathering process and pedogenesis by forming kaolinite in topsoils as rapid removing of water-soluble elements (such as exchangeable Ca and Na; Chadwick and Chorover, 2001). Stronger effect of plant absorption than leaching might contribute to the shallower distribution of exchangeable Mg (Fig. 3b) and available Fe (Fig. 4a), Mn (Fig. 4b), and Zn (Fig. 4d). The dominant role of plant cycling in determining the vertical distribution of Mg, Fe, Mn, and Zn might illustrate that these elements were scarcer and more limiting nutrients for plant growth in this semi-arid sandy ecosystem (Jobbágy and Jackson, 2001).
The results showed that grassland soil coarseness decreased soil base cations of exchangeable Ca, Mg, and Na as well as available micronutrients of Fe, Mn, and Cu. The loss of SOM, decrease of soil fine particles, and increase of soil pH were the main driving factors for the decrease of base cations and micronutrient availability as affected by soil coarseness. Unchanged concentrations of exchangeable Ca, K, and Na along the soil depth might result from the balance between plant cycling and leaching effects. The dominant role of plant cycling over leaching shaped the shallower distribution of exchangeable Mg as well as available Fe, Mn, and Zn. The reduction and re-distribution of soil base cations and available micronutrients would potentially influence soil fertility and plant productivity in this desertified grassland ecosystem.
Zhengwen Wang, Guoqing Yu, and Xingguo Han designed the experiments; Linyou Lü and Yan Zhao carried them out. Heyong Liu and Jinfei Yin helped with the laboratory analysis. Linyou Lü and Ruzhen Wang prepared the manuscript with contributions from all authors. Yong Jiang helped to revise the manuscript. Jiangtao Xiao created the figure of our experimental location.
This work was financially supported by the National Natural Science Foundation of China (41371251). Edited by: A. Jordán