Biologia 64/3: 424—427, 2009 Section Botany DOI: 10.2478/s11756-009-0112-9
Soil microbial parameters and stability of soil aggregate fractions under different grassland communities on the Loess Plateau, China Shao-shan An1,2, Axel Mentler 2, Veronica Acosta-Martínez3 & Winfried E. H. Blum2* 1
Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Northwest A&F University, Yangling, P.R. China, 712100 2 Institute of Soil Science, Department of Forest and Soil Sciences, University of Natural Resources and Applied Life Sciences, Vienna, Peter-Jordan Straße 82, A-1190 Vienna, Austria; e-mail:
[email protected] 3 USDA-ARS, Cropping Systems Research Laboratory, Wind Erosion and Water Conservation Research Unit, 3810, 4th St., Lubbock TX. 79415, USA
Abstract: Over-grazing and large-scale monocultures on the Loess plateau in China have caused serious soil erosion by water and wind. Grassland revegetation has been reported as one of the most effective counter measures. Therefore, we investigated soil aggregation, aggregate stability and soil microbial activities as key parameters for soil remediation through grassland revegetation. The results showed that soil microbial biomass carbon (Cmic) and microbial biomass nitrogen (Nmic) increased under revegetated grass communities compared to cropland and overgrazed pastures and were higher in surface layers (0–10 cm) than in the subsurface (10–20 cm). Although there are variations between the four investigated grassland communities, their values were 10 to 50 times higher in comparison to the cropland and overgrazed pastures, similar to the increase in soil enzyme activities, such as β-glucosidase and β-glucosaminidase. Soil aggregate stability (SAS) showed clear differences between the different land uses with two main soil aggregate fractions measured by ultra sound: < 63 µm and 100–250 µm, with approximately 70% and 10% of the total soil volume respectively. We also found positive correlations between SAS and soil microbial parameters, such as Cmic, Nmic, and soil enzyme activities. From this, we concluded that revegetation of eroded soils by grasses accelerates soil rehabilitation. Key words: enzyme activities; grassland revegetation; soil microbial biomass C(Cmic); soil microbial biomass N(Nmic); soil aggregate fractions and stability; Loess Plateau of China
Introduction Over-grazing and large scale monocultures have caused soil and ecosystem degradation on the Loess Plateau in China, an area of about 9,6 Mha in the upper and middle catchment of the Yellow River (Fu et al. 2000). The soil of this region has been called the “most highly erodible soil on earth” (Laflen 2000). Recently, several measures have been taken by engineering and biological approaches to restore the degraded ecosystems (Wang 2002). Revegetation has been reported as the most effective way to combat soil degradation on the Loess Plateau (Hou et al. 2002) and to restore the ecology of disturbed soil systems (Montalvo et al. 1997). The misuse of the soils had a negative impact on soil quality, especially on soil microbiology. Therefore, soil organic matter was considered as a key factor which promotes the formation of larger aggregates and aggregate stability and influences the percentage of water stable aggregates (WSA), which are an indicator for the resilience of soil against degradation (Udawatta et al. * Corresponding author
c 2009 Institute of Botany, Slovak Academy of Sciences
2008). Especially macroaggregates (diameter >250 µm) are important for soil porosity, microbial habitats, and the increase of soil organic matter (Christensen 2001; Carter 2004). Soil microbial biomass carbon (Cmic), microbial biomass nitrogen (Nmic) and enzyme activities (EAs) are key parameters used in soil quality monitoring. Enzyme activities can also be used to evaluate rapid soil changes due to management practices and for understanding soil sensitivity to environmental stress. Aggregate stability is used as an indicator of soil structure (Six et al. 2000). The stability of the aggregates and the pore system influences the storage of water and air, biological activity, and the growth of crops (Zhang & Miller 1996). Maintaining high soil aggregate stability is essential for preserving soil productivity, minimizing soil degradation. Arshad & Cohen (1992) proposed aggregate stability as an indicator of soil quality. Hortensius & Welling (1996) included this parameter in the international standardization of soil quality measurement.
Soil microbial parameters and stability of soil aggregate fractions In 1999 the Chinese government launched the national revegetation project “Grain for Green” in Northwest China, to answer the questions: (1) What are the effects of revegetation on soil quality? and (2) How long and in which direction should revegetation be undertaken in order to obtain adequate results? For this, an improved knowledge of the links between the biochemical soil characteristics and soil aggregate formation and stability is required (Abiven et al. 2007). Therefore, we investigated the effects of revegetation by different grassland communities on soil quality parameters such as soil aggregate formation and stability, microbial biomass, and enzyme activities. Material and methods Study sites and sampling protocol A permanent grassland in the Ningxia province was selected for this study, located at 106◦ 24 ∼ 106◦ 28 longitude and 36◦ 13 ∼ 36◦ 19 latitude at the Yunwu Observatory for Vegetation Protection and Eco-environment, which is the only remaining grass region of the Loess Plateau. The total area comprises about 1000 ha and was protected by fences since 1982. We studied 4 stable grass communities on 2 sites, with a natural succession: Stipa gradiss (St.G), Stipa bungana Trin Ledeb (S.B), Artemisia sacrorum Ledeb. (A.S), and Thymus mongolicus Ronnm (T.M.), and in contrast Hierochloe ordorata, characteristic for overgrazed grasslands (O.G.) and a traditional cropland (Cr.) near the protected grassland as control. The study area is characterized by distinct wet and dry seasons with heavy seasonal rainfall and periodic local flooding as well as droughts. The average annual rainfall at the experimental site is 400 mm (1941–2000), with a rainy season from July to September. The mean annual temperature is about 7 ◦C. Most of the area is located at an altitude of 1,800–2,040 m and is dissected by steep erosion gullies. The soil type is a Haplic-Ustic Cambisol according to Keys to Chinese Soil Taxonomy (3rd edition, 2001). Soil pH ranges from 7.95 to 8.10, soil clay (< 0.002 mm) from 173.2–291.9 g kg−1 , silt (0.2–0.002 mm) from 172.7 to 266.6 g kg−1 , sand (> 0.2 mm) from 462.4 to 578.8 g kg−1 , and inorganic carbon (C inorg) from 5.07 to 15.58 g kg−1 . Soil samples were taken from 0–10 cm and 10–20 cm depths in March 2008 for the measurement of pH, total organic carbon (Ct), total nitrogen (Nt), Cinorg, soil microbial parameters and soil aggregate stability. An area of 60 × 60 m was selected for each site and within this area three 20 × 20 m plots were chosen for sampling. Seven core samples were taken from each plot and mixed to a bulk sample of about 1 kg. The fresh samples were transported to the laboratory in cooling boxes. Soil samples for aggregate stability measurements were taken by 200 cm3 cylinders. Each soil sample was sieved (2 mm) and air dried. Methods Soil microbial biomass C (Cmic), microbial biomass N (Nmic) were determined by the fumigation-extraction method using 15–g field-moist soil sample (< 2 mm) and 0.5 M K2 SO4 as the extractant (Vance et al. 1987; Brookes et al. 1985; Zhou & Li 1998). The N mic was determined colorimetrically. The microbial biomass C was calculated using a kEC factor of 0.45 (Wu et al. 1990) and N mic using a kEN factor of 0.54 (Vance et al. 1987). All analyses were performed in triplicate on fresh samples.
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The activity of β-glucosidase was measured on air-dried subsamples as described by Tabatabai (1994). The activity of β-glucosaminidase was determined similarly by the method of Parham & Deng (2000). Results for all enzyme activities are reported as the product (p-nitrophenol = PN) released after 1 h incubation. Each sample was measured in duplicate plus one control. Soil aggregate stability (SAS) was determined by wet sieving according to ON L1072 (2004). Ultrasonic soil aggregate stability (USAS): The degree of aggregate stability was measured through the specific ultrasonic energy absorbed by a soil-water-mixture. Soil aggregate distribution was measured by Ultrasonic dispersion (Mayer et al. 2002; Mentler et al. 2004). The mass fractions in the ultrasonic experiments were determined by wet sieving immediately after the treatments. The aggregates were analysed with standard sieves and classified in different aggregate fractions: macro aggregates (1000–630 µm, 630–250 µm) and micro aggregates (250–63 µm). The determination of total organic carbon (Ct) content and total nitrogen (Nt ) content was carried out according to Austrian Standard ON L 1080-99 (1999) and ON L 1082-99 (1999) with an elementary analyzer using dry combustion technique and gas chromatography. One-way ANOVA followed by the Duncan test (P < 0.05) was used to compare the sites representing different land use and plant communities. An exponential regression model was fitted to describe the relationships between the variables. All statistical analyses were carried out with Excel 5.0 and SPSS 15.0.
Results Ct content under grassland is shown in Table 1. The highest content was about 32 g kg−1 under the Stipa gradiss community (0–10 cm). There is no significant difference among 3 stable permanent grass communities in the 0–10 cm layer except for Stipa gradiss. The Ct content in the subsurface layer under overgrazed grassland and cropland was considerably lower, with 14 g kg−1 and 16 g kg−1 respectively. The Ct content between surface and subsurface layer under grassland was not very different which indicated that the grass cover increased soil organic carbon content until greater depths. Compared to Ct, Nt did not change very much between natural grass communities and overgrazed grassland (Table 1). But there were significant differences between surface layer and subsurface layer. Tables 1 and 2 show that soil Cmic and Nmic have obviously increased under grass communities compared to cropland and overgrazed land. Moreover, in the revegetated area, soil Cmic and Nmic were higher in the surface layer (0–10 cm) and in the subsurface layer (10– 20 cm). Although Cmic and Nmic are different in the 4 grassland communities, they were all 10 to 50 times higher than in cropland and overgrazed land. Both soil enzyme activities, β-glucosidase and βglucosaminidase were higher in 0–10 cm than at 10– 20 cm soil depth, which is consistent with our previous results (Tables 1 and 2). The enzyme activities of the cropland were much lower than under grassland.
S.-S. An et al.
426 Table 1. Soil Ct, Nt and microbiological properties under different plant communities (0–10 cm). Plant communities
Ct (g kg−1 )
Nt (g kg−1 )
Cmic (mg kg−1 )
Nmic (mg kg−1 )
β-glucosidase (mg PN kg−1 h−1 )
β-glucosaminidase (mg PN kg−1 h−1 )
32.67 28.52 28.97 28.79 20.29 17.36
2.42 2.92 2.33 2.11 2.14 1.93
312.22 324.67 1086.89 1242.07 916.44 1580.67
323.71 234.86 232.36 299.86 46.53 52.65
321.63 187.30 314.60 201.83 159.02 86.55
46.84 40.37 47.45 53.27 24.83 17.66
St.G: S.B.: A.S.: T.M.: O.G.: Cr.
St.G: Stipa gradiss; S.B.: Stipa bungana Trin Ledeb; A.S.: Artemisia sacrorum Ledeb.; T.M.: Thymus mongolicus Ronnm.; O.G.: overgrazed grassland; Cr.: Traditional cropland.
Table 2. Soil Ct and Nt and microbial properties under different plant communities (10–20 cm). Plant communities
Ct (g kg−1 )
Nt (g kg−1 )
Cmic (mg kg−1 )
Nmic (mg kg−1 )
β-glucosidase (mg PN kg−1 h−1 )
β-glucosaminidase (mg PN kg−1 h−1 )
29.28 27.28 23.71 23.84 14.83 16.23
2.13 2.01 0.93 0.90 1.33 1.22
1090.15 754.22 830.74 985.63 262.59 207.63
176.65 166.50 196.43 211.27 41.02 43.21
165.54 170.19 120.66 129.02 58.47 82.41
39.09 34.87 41.06 38.20 18.97 17.84
St.G S.B. A.S. T.M. O.G. Cr.
St.G: Stipa gradiss; S.B.: Stipa bungana Trin Ledeb; A.S.: Artemisia sacrorum Ledeb.; T.M.: Thymus mongolicus Ronnm.; O.G.: overgrazed grassland; Cr.: Traditional cropland.
(A)
(B)
(C) >1000
>1000
Soil aggregate stability(%)
100
0-10cm
c d
10-20cm
80
d
c
b
60
d c
c
c
b aa
80
1000-630
80
1000-630
70
630-250
70
630-250
60
250-100
60
250-100
50
100-63
50
100-63
40
S.B > T.M. > O.G. > Cr.) and at 10–20 cm (St. G. ≈ S.B > A.S. ≈ T.M. > Cr. > O.G). Similar and significant differences were observed for β-glucosaminidase activity at 0–10 cm (T.M. > A.S. > St. G. > S.B > O.G. > Cr.) and at 10–20 cm (A.S. > T.M. ≈ St. G. > S.B > O.G. ≈ Cr.). Soil aggregate stability under different grassland communities, cropland and overgrazed pasture was also found different (Fig. 1A). Only 40% of the soil aggregates under cropland were stable, both in 0–10 cm and 10–20 cm. In overgrazed pasture land, SAS was about 68% in the surface layer (0–10 cm) and 50% in the subsurface layer (10–20 cm). In comparison, the soil under revegetated grassland showed higher aggregate stability, with about 80% in the surface layer and over 75% in the subsurface layer. Compared to the grassland communities, soil aggregate stability decreased 50% un-
der cropland and 20–30% under overgrazed pasture. Therefore, soil aggregate stability was not as sensitive as the soil microbiological parameters Cmic and Nmic. The distribution of soil aggregate fractions under different plant communities were shown in Fig. 1B and Fig. 1C for 0–10 cm and 10–20 cm respectively. The main two fractions were < 63 µm and 250–100 µm, with approximately 70% and 10%, respectively. There were clear differences between cropland soil and grassland soil. Soil aggregate stability is positively correlated with Cmic, Nmic, β-glucosidase and β-glucosaminidase, with the coefficients R2 0.7753, 0.6756, 0.6003 and 0.8483 (n = 12) respectively. The positive and significant relationship between SAS and soil microbiological properties shows changes in soil properties through revegetation. With the increase of soil Cmic, Nmic and enzyme activities, soil aggregate stability also increased.
Soil microbial parameters and stability of soil aggregate fractions Discussion Soil microbial parameters can be used as indicators for changes in soil quality and the success of revegetation of disturbed soil ecosystems. The biochemical and microbiological parameters measured in this study allowed for distinguishing the soils according to differences in microbial population size and activity due to revegetation. In the present study, soil Cmic and Nmic also showed positive relationships with soil organic matter, and soil enzyme activities, which changed with different land use and plant communities. According to previous studies in this area (An et al. 2009), the natural grassland succession on the sites began after the stop of overgrazing. The overgrazed and degraded Thymus mongolicus communities gradually developed to a stable Stipa bungana community (Zou & Guan 1997). After almost thirty years fencing, total plant biomass and height increased visibly, together with Ct, Cmic and Nmic. Among the most important factors that affect aggregate structure and stability of semiarid soils are the level of soil organic matter and the microbiological activity (Diaz et al. 1994). Soil structure influences the growth and activities of soil organisms, which in return affect positively the structure (Angers & Caron 1998). Like the soil microbial properties, soil aggregate stability was higher under grassland compared to cropland and overgrazed land, thus stabilizing soils against erosion. Our results clearly show that revegetation by grasses is a useful approach for improving soil structure and microbial properties in order to control the risks of erosion on the Loess Plateau of China. Acknowledgements This study was sponsored by the National Natural Sciences Foundation of China (40701095) and the West Light Foundation of The Chinese Academy of Science.
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