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Differential responses of litter decomposition to increased soil nutrients and water between two contrasting grassland plant species of Inner Mongolia, China
Applied Soil Ecology 34 (2006) 266–275 www.elsevier.com/locate/apsoil
Differential responses of litter decomposition to increased soil nutrients and water between two contrasting grassland plant species of Inner Mongolia, China Ping Liu a,b, Jianhui Huang a, Xingguo Han a, Osbert J. Sun a,*, Zhiyong Zhou a,b a
Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received 1 August 2005; received in revised form 22 December 2005; accepted 28 December 2005
Abstract Soil chemistry and physical conditions are the key factors controlling litter decomposition. We studied the effects of increased soil nitrogen (N), phosphorus (P), and water on the decomposition rates and associated nutrient dynamics of two dominant grassland plant species (i.e. Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal. and Stipa krylovii Roshev.) with contrasting life forms and tissue chemistry in a typical steppe of Inner Mongolia, China. The treatments included addition of urea at the rates equivalent to 0, 8, 16, and 32 gN/m2, and additions of mixed urea and triple superphosphate at the rates equivalent to 0, 8 gN/4 gP, 16 gN/8 gP, and 32 gN/16 gP/m2, with and without water added. We found marked differences between the two species in the rates, as well as in the responses to water and addition rates of N and P, of litter decomposition. Additions of N alone or in mixture with P stimulated the rate of litter decomposition in both species. Adding water significantly increased the values of decay constant, k, in A. bidentatum, but not in S. krylovii. N and P concentrations in litters of A. bidentatum and S. krylovii all increased corresponding to increases in the rates of N or mixed N and P additions. Our results clearly indicate that the decomposition of high quality litter is more likely to be limited by soil moisture regimes, while that of low quality litter is more sensitive to nutrient availability. Our findings suggest that plant species with different litter qualities should be taken into consideration when we are to model the carbon cycle and nutrient dynamics in grassland ecosystems and that A. bidentatum is expected to contribute more than S. krylovii to the carbon cycle and nutrient dynamics of the semi-arid grasslands of Inner Mongolia. # 2006 Elsevier B.V. All rights reserved. Keywords: Litter decomposition; Grassland ecosystems; Allium bidentatum; Stipa krylovii
1. Introduction Decomposition of plant litter plays an important role in nutrient cycling and carbon (C) fluxes of the terrestrial ecosystems (Swift et al., 1979; Berg and McClaugherty, 1989; Sun et al., 2004). The rate and process of litter decomposition greatly influence soil * Corresponding author. Tel.: +86 10 62836510. E-mail addresses: [email protected], [email protected] (O.J. Sun). 0929-1393/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2005.12.009
development and the availability of nitrogen (N), phosphorus (P) and other nutrients to plants and soil microorganisms (Huang et al., 1998; Liu et al., 2000). Understanding the key factors and processes that control the rate of litter decomposition under different environmental conditions and in different habitats is therefore fundamental to quantitative analysis of C and nutrient cycling of terrestrial ecosystems. Decomposition is primarily driven by microbial activities and can be best predicted by environmental factors such as temperature and precipitation, as well as
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litter quality (Meentemeyer, 1978; Berg et al., 1993; Aerts, 1997; Taylor et al., 1989; Wang et al., 2000; Moretto et al., 2001), but soil chemistry and physical conditions can also influence the rate of litter decomposition (Gijsman et al., 1997; Seneviratne et al., 1999). As decomposer microbes require nutrients from either litter material or surrounding soils to maintain their life activities (Ocio et al., 1991; Sinsabaugh et al., 1993), soil nutrient availability has long been suggested as one of the controlling factors affecting the rate of litter decomposition (Swift et al., 1979). However, results of the studies concerning the effects of increased soil N and P on the rate of litter decomposition and nutrient dynamics to date have been controversial. For example, some studies (Hunt et al., 1988; Fenn, 1991; Ostertag and Hobbie, 1999) found that increased N and P could stimulate litter decomposition, while others found no (i.e. Prescott et al., 1999; Dukes and Field, 2000) or depressing effects (So¨derstro¨m et al., 1983; Magill and Aber, 1998). Berg et al. (1982) and Berg and Matzner (1997) found a positive response to N in the initial decomposition phase but a negative response in the later stages. Kwabiah et al. (1999) suggested that responses of plant litter decomposition to soil nutrients were determined by litter quality. Such inconsistency in the relationships between litter decomposition and soil nutrients, therefore, calls for continued investigations of the subject. Water availability can influence the rates of litter decomposition and nutrient release through its effects on the activities of the decomposer communities (Orchard and Cook, 1983; Berg, 1986; Clein and Schimel, 1994). Water supply in the form of rainfall can also affect decomposition by facilitating leaching and breakdown of surface litter (Swift et al., 1979). Moreover, water availability may affect litter decomposition indirectly by altering the litter quality in terms of lignin and nutrient concentrations of plants (Pastor and Post, 1988; Austin and Vitousek, 2000). Global change will inevitably alter nutrient cycling processes in terrestrial ecosystems along with changes in C and water fluxes. The accelerated rates of N mineralization likely to occur with global climate warming (Rustad et al., 2001) and increased N deposition due to anthropogenic activities (Bobbink et al., 1998) could result in N enrichment under many land use and land cover types, including grassland ecosystems. The global precipitation pattern is anticipated to change under the climate change scenarios, especially in arid and semi-arid areas where summer precipitation is expected to increase (Melillo, 1990). All
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these changes would potentially result in changes in litter decomposition that could affect C and nutrient cycles, and affect how we simulate the C cycling process in the context of global change. However, there is a lack of regionally based, species-specific information on the rates of litter decomposition for most of the terrestrial ecosystems in China. The increased intensity of land use on the grasslands of northern China has caused severe land degradation since the late 1970s, resulting in decreased productivity and ecosystem stability. Much of the area is now in various stages of degradation, where soil N, P and water are considered to be the principal limiting factors to net primary productivity. The recycling of nutrients through plant litter decomposition is an essential mechanism to maintain productivity (Tiessen et al., 1994). To examine how increased soil nutrient availability, predominantly N and P, and water would affect the litter decomposition rates and nutrient dynamics of two dominant grassland plant species with contrasting life forms and tissue chemistry (i.e. Allium bidentatum and Stipa krylovii), we conducted two field experiments with varying addition rates of N, P, and water. Our objectives were to determine: (1) how increased soil N would affect the decomposition rates and nutrient dynamics of A. bidentatum and S. krylovii, and (2) how simultaneous increases in soil N and P, with or without increased water availability, would affect the rates of decomposition of the two species. Our underlying hypotheses were that species with contrasting life forms and tissue chemistry would differ significantly in the rates of litter decomposition, and that increased soil nutrient and water would stimulate litter decomposition in the degraded semi-arid grassland ecosystems. 2. Materials and methods 2.1. Study site This study was conducted at a field site of the Duolun Restoration Ecology Experimentation and Demonstration Station in Inner Mongolia, China (latitude 428020 N; longitude 1168160 E; altitude 1344 m a.s.l.). The mean annual, minimum, and maximum air temperatures for the area are 1.6, 18.3, and 18.7 8C. The mean annual precipitation is 385 mm, occurring mainly from July to September (accounting for 67% of the total), with Penman evaporative potential of 1748 mm. Soils of the region are commonly referred to as chestnut type (Calcic Kastanozems) in the Chinese classification, and are classified as Calcic–orthic Aridisol in the US soil taxonomy classification system (Yuan et al., 2005).
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The area has suffered from severe degradation since 1979 as a result of overgrazing. The primary vegetation is of the typical steppe, with perennial herb A. bidentatum and bunch grass species S. krylovii being among the dominant species of the plant community.
the time of initial deployment to determine the ratio between air-dried mass and oven-dried mass. This ratio was used to convert the initial air-dried mass of the litter to oven-dried mass. 2.4. Chemical analysis
2.2. Experimental design and treatments We conducted two decomposition experiments on a fenced site from July and October 2004, both lasting for 100 days. The first experiment contained four levels of N addition rates (0, 8, 16, and 32 gN/m2; designated as N0, N8, N16, and N32, respectively), and the second experiment contained four addition rates of mixed N and P (0, 8 gN/4 gP, 16 gN/8 gP, and 32 gN/16 gP/m2; designated as N0P0, N8P4, N16P8, and N32P16, respectively) with and without increased water availability. Both of the experiments were deployed based on a randomized block design with four blocks and each plot size of 4 m 4 m. N was added as granular urea, and P in the form of granulated triple superphosphate, which were applied 5 days before the litterbags were deployed. In treatments with increased water availability, water was applied on 15 July, 10 August, 29 August, and 22 September, following major rain events in the previous week. For each water application, 15 mm of water was added, simulating an average increase of rainfall by 30% of the corresponding period. 2.3. Litter decomposition In early July 2004, litters of A. bidentatum (predominantly leaves) and S. krylovii (predominantly culms) were collected from the study area, including freshly fallen and senesced tissues attached to the plants. After being air-dried to constant mass, they were clipped into fragments of 10 cm in length, and placed into the 10 cm 15 cm polyethylene litterbags (mesh size 1 mm2) that each contained 3 g of air-dried litter material. Each treatment plot contained nine litterbags of the same species. A total of 864 litterbags were prepared and deployed onto the treatment plots on 9 July 2004. The remaining litter was retrieved 35, 70, and 100 days after initial litterbag deployment. For each sampling time, three litterbags for each species from each treatment plot were collected. In the laboratory, extraneous matter such as other plant materials, rocks and small animals were handpicked from the litterbags. The retrieved litters were then oven-dried at 70 8C for 48 h, to determine the remaining dry mass. Five samples for each litter type were oven-dried at 70 8C for 48 h at
After determination of the dry mass, litters of the same plant species from the three within-plot replications were pooled for chemical analysis. Total C, N and P concentrations were determined for the final samples from the two litter decomposition experiments. Total C was measured by the standard method of wetcombustion, total N by semi-micro Kjeldahl method and total P by molybdenum blue colorimetric method (Bao, 1999). Five litter samples for each plant species were also analyzed for total C, N and P to determine the initial litter chemistry. 2.5. Data analysis The value of decay constant, k, was determined by fitting the following exponential function (Olson, 1963): xt ¼ ekt x0 where, xt is the remaining litter mass after a given time period t, x0 is the initial litter mass. The remaining litter nutrients were calculated by multiplying the sample mass by the respective nutrient concentration. Tests of significance of the remaining mass were performed by analysis of variance (ANOVA; three-way without water treatment and four-way with water treatment). Data analyses were performed using procedures of SPSS (v.11.0). The least significant difference (LSD) was used for comparisons of means with confidence level of P < 0.05. 3. Results 3.1. Initial litter chemistry Total C contents in the initial litters were similar between A. bidentatum and S. krylovii (Table 1). Allium bidentatum had twice the N and P concentrations, and half the C:N and C:P ratios, of S. krylovii (Table 1). 3.2. Species difference in litter decomposition Allium bidentatum litter decomposed faster than S. krylovii in both experiments (Fig. 1). After 100 days of decomposition, the remaining litter mass was 70% of
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Table 1 Initial litter chemistry of Allium bidentatum and Stipa krylovii in the degraded typical steppe of Inner Mongolia, China Species
C (g/kg)
N (g/kg)
P (g/kg)
C:N ratio
C:P ratio
Allium bidentatum Stipa krylovii
481.9 6.0 486.3 7.2
5.0 0.3 2.8 0.2
0.42 0.02 0.23 0.01
97 6 174 15
1273 65 2720 101
Values are means S.E. (n = 5).
the initial in A. bidentatum, but >80% in S. krylovii. The patterns of litter decomposition over the duration of the experiment were well described by the exponential model, xt/x0 = ekt (values of r2 ranged between 0.89 and 0.99, and P < 0.05; Table 2). The values of decay
constant, k, of A. bidentatum were about twice of S. krylovii across all the treatments (Table 2). 3.3. Effects of nutrient addition and water on litter decomposition In the experiment I, addition of N alone to the substrate soils only slightly affected the rate of litter decomposition in both A. bidentatum and S. krylovii as shown by three-way ANOVA and changes in the k values, with two species showing the similar patterns of responses (Tables 2 and 3). Four-way ANOVA showed that the litter mass remaining was significantly (P < 0.05) affected by retrieval time, plant species, water treatment, addition rates of mixed N and P, and some of the interactions in the experiment II (Table 4). Increased rates of N and P Table 2 Decay constant (k; y1, r2 range 0.89–0.99, P < 0.05) for litters of Allium bidentatum and Stipa krylovii as affected by additions of N alone (experiment I) or in mixture with P with or without adding water (experiment II) in the degraded typical steppe of Inner Mongolia, China. Treatment
Fig. 1. Effects of N addition or concurrent additions of N and P with or without water added on litter decomposition of Allium bidentatum (open symbols) and Stipa krylovii (closed symbols). Vertical bars represent standard errors (n = 4). *P < 0.05; **P < 0.01; ***P < 0.001.
k Allium bidentatum
Stipa krylovii
Experiment I N0 N4 N8 N16
1.20 0.07 1.24 0.10 1.30 0.10 1.33 0.08
a a a a
0.53 0.07 0.58 0.07 0.63 0.10 0.67 0.06
b ab ab a
Experiment II No water added N0P0 N8P4 N16P8 N32P16
1.20 0.07 1.23 0.11 1.37 0.05 1.48 0.13
c bc ab a
0.63 0.06 0.69 0.02 0.77 0.07 0.85 0.10
c bc ab a
Water added N0P0 N8P4 N16P8 N32P16
1.37 0.07 1.41 0.15 1.48 0.09 1.56 0.09
b ab ab a
0.63 0.01 0.71 0.02 0.79 0.04 0.87 0.05
d c b a
The values of k are means S.E. (n = 4), which were derived based on exponential decay model, xt/x0 = ekt. Data in the same column followed by the same letter are not significantly different (P < 0.05).
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Table 3 F and P (shown in parentheses) values from three-way ANOVA for mass remaining (M, % of initial mass) and two-way ANOVA for residual N and P (% of initial mass) during decomposition of litter materials in soils with N addition (experiment I), with retrieval time, plant species and N addition (fertilizer) as main effects Source of variation
d.f.
F M
Time (T) Species (S) Fertilizer (F) TS TF SF TSF
2 1 3 2 6 3 6
79.3 286.5 2.4 10.5 0.9 1.4 0.9
(<0.001) (<0.001) (0.073) (<0.001) (0.513) (0.241) (0.504)
additions significantly (P < 0.05) accelerated the rate of litter decomposition in both A. bidentatum and S. krylovii; the values of k increased with increasing rates of N and P additions in both species regardless water treatment (Table 2). Adding water significantly (P = 0.001) increased the k value in A. bidentatum, but not in S. krylovii (Table 2). The decomposition rates of S. krylovii litter were more responsive to changes in the rates of mixed N and P additions, as significant treatment effects of N and P additions were shown for the remaining mass litter of the species on the Day 70 and 100 after the commencement of litter decomposition experiment (Fig. 1), and the changes in the values of k with increasing rates of N and P additions were greater in S. krylovii than in A. bidentatum (Table 2). In both species, the k value responded more to the concurrent changes in N and P (Experiment II) than changes in N alone (Experiment I; Table 2).
N
P
14.3 (<0.001) 34.1 (<0.001)
65.2 (<0.001) 7.7 (0.001)
3.5 (0.030)
0.01 (0.999)
3.4. Litter nutrient dynamics At the end of the 100-day study, the N and P concentrations in litters of A. bidentatum and S. krylovii all increased with increases in the addition rates of N in the experiment I (Fig. 2), and the addition rates of mixed N and P in the experiment II (Fig. 3). Adding water decreased the N and P concentrations in A. bidentatum and S. krylovii in the experiment II (Fig. 3). Two-way and three-way ANOVA showed that the residual amounts of litter N and P were significantly (P < 0.01) affected by plant species and addition rates of N in the experiment I (Table 3), and by plant species, addition rates of mixed N and P, and water treatment in the experiment II (Table 4). The absolute amounts of litter N increased in nearly all treatments after decomposing for 100 days, in both
Table 4 F and P (shown in parentheses) values from four-way ANOVA for mass remaining (M, % of initial mass) and three-way ANOVA for residual N and P (% of initial mass) during decomposition of litter materials in soils with mixed N and P additions with or without water added (experiment II), with retrieval time, plant species, water and N and P additions (fertilizer) as main effects Source of variation
d.f.
F M
Time (T) Species (S) Water (W) Fertilizer (F) TS TW SW TSW TF SF TSF WF TWF SWF TSWF
2 1 1 3 2 2 1 2 6 3 6 3 6 3 6
266.1 930.3 4.0 23.2 28.2 1.1 5.9 2.6 1.1 0.6 1.7 2.9 0.6 2.7 0.4
(<0.001) (<0.001) (0.047) (<0.001) (<0.001) (0.343) (0.016) (0.075) (0.374) (0.586) (0.115) (0.039) (0.740) (0.047) (0.879)
N
P
44.5 (<0.001) 10.8 (0.002) 116.5 (<0.001)
120.9 (<0.001) 20.9 (<0.001) 59.7 (<0.001)
0.1 (0.789)
0.5 (0.464)
6.6 (0.001)
0.3 (0.825)
0.2 (0.908)
0.8 (0.496)
0.2 (0.926)
0.2 (0.889)
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decomposition, whereas in S. krylovii, the absolute amounts of litter P increased (Table 5). Similar to the patterns of litter N and P concentrations, the absolute amounts of N and P in the remaining litter from the two experiments all increased with increasing addition rates of N alone or mixed N and P (Table 5). Adding water significantly reduced the absolute amounts of litter N and P in A. bidentatum (P = 0.004 and <0.001, respectively) and the absolute amounts of litter P in S. krylovii (P = 0.025; Table 5). 4. Discussion
Fig. 2. Effects of N addition on litter N and P concentrations of Allium bidentatum and Stipa krylovii after decomposing for 100 days in the field. Vertical bars represent standard errors (n = 4).
A. bidentatum and S. krylovii (Table 5). The changes in the absolute amounts of litter P, however, differed between the two species: in A. bidentatum, the absolute amounts of litter P decreased slightly after 100-day
On a smaller spatial scale, litter quality is considered as the most important factor influencing decomposition rate (Melillo et al., 1982; Aerts and De Caluwe, 1997). High quality litters are often characterized by higher N concentrations and lower C:N ratios, and can decompose faster in comparison with low quality litters (Sanchez, 2001). In this study, the litters of the perennial herb A. bidentatum had twice the N concentration, and about half of the C:N ratio, of the bunch grass species S. krylovii. The marked differences in the rate of litter decomposition between these two species can, therefore, attribute to differences in litter quality. We also found that increasing the rates of N additions resulted in slight increase in the rates of litter decomposition in the two grass species. This is in general agreement with many other studies showing
Table 5 Residual amounts of N and P (% of initial mass) in Allium bidentatum and Stipa krylovii after decomposing for 100 days as affected by additions of N alone (experiment I) or in mixture with P with or without adding water (experiment II) Treatment
Allium bidentatum N
Stipa krylovii P
N
P
Experiment I N0 N8 N16 N32
103.1 4.0 105.3 3.8 111.7 5.3 119.6 4.0
c c b a
86.2 2.5 91.0 4.5 95.3 2.6 98.8 2.6
c bc ab a
101.4 3.2 111.2 4.3 120.2 9.3 134.5 5.7
c b b a
102.3 4.2 106.3 7.6 111.0 7.6 114.6 8.5
b ab ab a
Experiment II No water added N0P0 N8P4 N16P8 N32P16
102.3 2.0 109.2 4.5 116.0 5.2 122.3 3.4
d c b a
86.7 3.0 96.4 3.9 105.1 5.0 113.3 3.7
d c b a
101.8 3.2 114.4 5.9 125.7 4.2 137.3 3.3
d c b a
100.7 4.1 110.8 6.0 117.3 8.5 122.5 5.6
c b ab a
Water added N0P0 N8P4 N16P8 N32P16
96.4 3.1 104.8 4.2 112.1 3.7 120.3 2.2
d c b a
83.0 4.1 92.4 3.8 97.6 4.7 102.6 4.4
c b ab a
97.4 5.0 112.2 6.4 122.2 7.3 133.5 5.9
d c b a
96.7 5.4 107.2 4.4 112.3 4.5 116.4 4.5
c b ab a
Values greater than 100 indicate nutrient immobilization, and less than 100 indicate net mineralization. Data in the same column followed by the same letter are not significantly different (P < 0.05). Values are means S.E. (n = 4).
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Fig. 3. Effects of mixed N and P additions with or without water added on litter N and P concentrations of Allium bidentatum and Stipa krylovii after decomposing for 100 days in the field. Vertical bars represent standard errors (n = 4).
small or no responses of litter decomposition to increasing soil N input (Pastor et al., 1987; Theodorou and Bowen, 1990; Downs et al., 1996). One of the possible explanations could be that other nutrients, such as phosphorus, were more limiting for decomposer microbes as found by others (Ostertag and Hobbie, 1999; Cleveland et al., 2002). In our second experiment with concurrent changes in N and P, the values of decay constant varied more markedly than in the first experiment. The above results suggest synergistic effects of N and P on litter decomposition rates in these two species that differ in litter quality. Apart from the marked difference in the rates of litter decomposition between the two species, Allium bidentatum and S. krylovii also differed in the responses of litter decomposition to increased soil water and concurrent changes in soil N and P. The litter decomposition of A. bidentatum was found to be more responsive to soil water, whereas S. krylovii responded more to concurrent changes in the soil N and P. The stronger effect of soil water on the litter decomposition of A. bidentatum than S. krylovii could be mainly due to the differences in litter quality as reflected by both litter N concentration and C:N ratio. The differences in
physical structures between the two litters could also partly attribute to the different responses of litter decomposition to soil water. Allium bidentatum is a perennial herb with more fragile structures, while S. krylovii is a bunch grass species with tissues that are more fibrous. Many studies have demonstrated that nutrient additions can affect litter nutrient dynamics without directly affecting the rate of decomposition (Hunt et al., 1988; O’Connell, 1994; Downs et al., 1996). Such mode of nutrient immobilization decouples nutrient mineralization from decomposition temporally, which may have implications to nutrient retention in degraded ecosystems. Organic N and P are much less mobile than inorganic N and P and both plants and microbes can make use of organic P by producing extracellular phosphatases to mineralize common organic P compounds into labile phosphates (McGill and Cole, 1981). Incorporation of nutrients into decomposing litter provides an important nutrient-retention mechanism for the system (McGroddy et al., 2004). In this study, we demonstrated marked differences in the rates, as well as in the responses to changes in soil nutrients and water, of litter decomposition between
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two dominant grassland plant species that differ in life forms and tissue chemistry. Our results clearly indicate that the decomposition of high quality litter is more likely to be limited by soil moisture regimes, while that of low quality litter is more sensitive to nutrient availability. Our study area, Duolun County of Inner Mongolia, belongs to the agro-pastoral ecotone of the Inner Mongolia Plateau. Here S. krylovii is the main construction species of the typical steppes, with A. bidentatum as one of the predominant companion species. In this agro-pastoral ecotone, sheep and cattle grazing was once a major land use practice. Most of the local grasslands were subjected to heavy grazing from early 1980s to 2000, resulting in severe land degradation. Beginning in 2001, a national policy was put into effect to ban grazing from the whole Duolun County in an effort to reverse the land degradation problem. Instead the grasses are now mowed for forage production and officially free of grazing. Existing literature on grassland vegetation dynamics of northern China has mostly concerned with Leymus chinensis and S. grandis steppes (e.g. Wang et al., 1996a, 1996b, 2000a, 2000b; Liu et al., 2002). Although specific information on the succession dynamics of S. krylovii steppe during degradation and restoration is lacking, the patterns should be similar to the S. grandis steppe. In general, long-term overgrazing reduces the status of S. krylovii into a sub-dominant species, while A. bidentatum becomes a minor or rare species. Under moderate grazing, the production of A. bidentatum and S. krylovii decreases in a similar proportion as other plants; Allium bidentatum would be more exposed and hence likely to be more susceptible to grazing than other plants in degraded grasslands because of its better palatability to livestock. When protected from grazing, the two species can both quickly restore their dominance in the plant communities; but the recovery is faster for A. bidentatum than Stipa spp. (Wang et al., 1999). In a separate study, we found that in grassland communities subjected to mowing, the maximum biomass of A. bidentatum was 50% greater than S. krylovii and ranked second in all plant species 5 years after exclusion of grazing (unpublished data). The much greater increases of A. bidentatum than S. krylovii during the recovering of the degraded grassland ecosystems emphasized greater importance of A. bidentatum to the carbon cycle and nutrient dynamics of the plant communities because of its faster decomposition rate and greater sensitivity to soil water. In conclusion, our findings suggest that plant species with different litter qualities should be taken into
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consideration when we are to model the carbon cycle and nutrient dynamics in grassland ecosystems and that A. bidentatum is expected to contribute more than S. krylovii to the carbon cycle and nutrient dynamics of the semi-arid grasslands of Inner Mongolia. Acknowledgements This research was supported by the National Natural Science Foundation of China (30330150), and by a startup fund to O.J. Sun from Institute of Botany of the Chinese Academy of Sciences. We thank the Duolun Restoration Ecological Experimentation and Demonstration Station for access permission to the study site and technical assistance, and Q.S. Chen for helps with field sampling and data analysis. References Aerts, R., 1997. Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship. Oikos 79, 439–449. Aerts, R., De Caluwe, H., 1997. Nutritional and plant-mediated controls on leaf litter decomposition of Carex species. Ecology 78, 244–260. Austin, A.T., Vitousek, P.M., 2000. Precipitation, decomposition and litter decomposability of Metrosideros polymorpha in native forests on Hawaii. J. Ecol. 88, 129–138. Bao, S.H.D., 1999. Agriculture Soil Chemical Analysis. Science Press, Beijing, China, pp. 263–271. Berg, B., 1986. Nutrient release from litter and humus in coniferous forest soils—a mini review. Scand. J. For. Res. 1, 359–369. Berg, B., Berg, M.P., Bottner, P., Box, E., Breymeyer, A., Calvo de Anta, R., Couteaux, M., Escudero, A., Gallardo, A., Kratz, W., Madeira, M., Ma¨lko¨nen, E., McClaugherty, C., Meentemeyer, W., Mun˜oz, F., Piussi, P., Remacle, J., Virzo de Santo, A., 1993. Litter mass loss rates in pine forest of Europe and Eastern United States: some relationship with climate and litter quality. Biogeochemistry 20, 127–159. Berg, B., Matzner, E., 1997. Effect of N deposition on decomposition of plant litter and soil organic matter in forest systems. Environ. Rev. 5, 1–25. Berg, B., McClaugherty, C., 1989. Nitrogen and phosphorus release from decomposing litter in relation to the disappearance of lignin. Can. J. Bot. 67, 1148–1156. Berg, B., Wessen, B., Ekbohm, G., 1982. Nitrogen level and decomposition in Scots pine needle litter. Oikos 38, 291–296. Bobbink, R.M., Hornung, M., Roelofs, J.G.M., 1998. The effects of air-borne pollutants on species diversity in natural and seminatural European vegetation. J. Ecol. 86, 717–738. Clein, J.S., Schimel, J.P., 1994. Reduction in microbial activity in birch litter due to repeated drying and rewetting events. Soil Biol. Biochem. 26, 403–406. Cleveland, C.C., Townsend, A.R., Schmidt, S.K., 2002. Phosphorus limitation of microbial processes in moist tropical forests: evidence from short-term laboratory incubations and field studies. Ecosystems 5, 680–691.
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Differential responses of litter decomposition to increased soil nutrients and water between two contrasting grassland plant species of Inner Mongolia, China Ping Liu a,b, Jianhui Huang a, Xingguo Han a, Osbert J. Sun a,*, Zhiyong Zhou a,b a
Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received 1 August 2005; received in revised form 22 December 2005; accepted 28 December 2005
Abstract Soil chemistry and physical conditions are the key factors controlling litter decomposition. We studied the effects of increased soil nitrogen (N), phosphorus (P), and water on the decomposition rates and associated nutrient dynamics of two dominant grassland plant species (i.e. Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal. and Stipa krylovii Roshev.) with contrasting life forms and tissue chemistry in a typical steppe of Inner Mongolia, China. The treatments included addition of urea at the rates equivalent to 0, 8, 16, and 32 gN/m2, and additions of mixed urea and triple superphosphate at the rates equivalent to 0, 8 gN/4 gP, 16 gN/8 gP, and 32 gN/16 gP/m2, with and without water added. We found marked differences between the two species in the rates, as well as in the responses to water and addition rates of N and P, of litter decomposition. Additions of N alone or in mixture with P stimulated the rate of litter decomposition in both species. Adding water significantly increased the values of decay constant, k, in A. bidentatum, but not in S. krylovii. N and P concentrations in litters of A. bidentatum and S. krylovii all increased corresponding to increases in the rates of N or mixed N and P additions. Our results clearly indicate that the decomposition of high quality litter is more likely to be limited by soil moisture regimes, while that of low quality litter is more sensitive to nutrient availability. Our findings suggest that plant species with different litter qualities should be taken into consideration when we are to model the carbon cycle and nutrient dynamics in grassland ecosystems and that A. bidentatum is expected to contribute more than S. krylovii to the carbon cycle and nutrient dynamics of the semi-arid grasslands of Inner Mongolia. # 2006 Elsevier B.V. All rights reserved. Keywords: Litter decomposition; Grassland ecosystems; Allium bidentatum; Stipa krylovii
1. Introduction Decomposition of plant litter plays an important role in nutrient cycling and carbon (C) fluxes of the terrestrial ecosystems (Swift et al., 1979; Berg and McClaugherty, 1989; Sun et al., 2004). The rate and process of litter decomposition greatly influence soil * Corresponding author. Tel.: +86 10 62836510. E-mail addresses: [email protected], [email protected] (O.J. Sun). 0929-1393/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2005.12.009
development and the availability of nitrogen (N), phosphorus (P) and other nutrients to plants and soil microorganisms (Huang et al., 1998; Liu et al., 2000). Understanding the key factors and processes that control the rate of litter decomposition under different environmental conditions and in different habitats is therefore fundamental to quantitative analysis of C and nutrient cycling of terrestrial ecosystems. Decomposition is primarily driven by microbial activities and can be best predicted by environmental factors such as temperature and precipitation, as well as
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litter quality (Meentemeyer, 1978; Berg et al., 1993; Aerts, 1997; Taylor et al., 1989; Wang et al., 2000; Moretto et al., 2001), but soil chemistry and physical conditions can also influence the rate of litter decomposition (Gijsman et al., 1997; Seneviratne et al., 1999). As decomposer microbes require nutrients from either litter material or surrounding soils to maintain their life activities (Ocio et al., 1991; Sinsabaugh et al., 1993), soil nutrient availability has long been suggested as one of the controlling factors affecting the rate of litter decomposition (Swift et al., 1979). However, results of the studies concerning the effects of increased soil N and P on the rate of litter decomposition and nutrient dynamics to date have been controversial. For example, some studies (Hunt et al., 1988; Fenn, 1991; Ostertag and Hobbie, 1999) found that increased N and P could stimulate litter decomposition, while others found no (i.e. Prescott et al., 1999; Dukes and Field, 2000) or depressing effects (So¨derstro¨m et al., 1983; Magill and Aber, 1998). Berg et al. (1982) and Berg and Matzner (1997) found a positive response to N in the initial decomposition phase but a negative response in the later stages. Kwabiah et al. (1999) suggested that responses of plant litter decomposition to soil nutrients were determined by litter quality. Such inconsistency in the relationships between litter decomposition and soil nutrients, therefore, calls for continued investigations of the subject. Water availability can influence the rates of litter decomposition and nutrient release through its effects on the activities of the decomposer communities (Orchard and Cook, 1983; Berg, 1986; Clein and Schimel, 1994). Water supply in the form of rainfall can also affect decomposition by facilitating leaching and breakdown of surface litter (Swift et al., 1979). Moreover, water availability may affect litter decomposition indirectly by altering the litter quality in terms of lignin and nutrient concentrations of plants (Pastor and Post, 1988; Austin and Vitousek, 2000). Global change will inevitably alter nutrient cycling processes in terrestrial ecosystems along with changes in C and water fluxes. The accelerated rates of N mineralization likely to occur with global climate warming (Rustad et al., 2001) and increased N deposition due to anthropogenic activities (Bobbink et al., 1998) could result in N enrichment under many land use and land cover types, including grassland ecosystems. The global precipitation pattern is anticipated to change under the climate change scenarios, especially in arid and semi-arid areas where summer precipitation is expected to increase (Melillo, 1990). All
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these changes would potentially result in changes in litter decomposition that could affect C and nutrient cycles, and affect how we simulate the C cycling process in the context of global change. However, there is a lack of regionally based, species-specific information on the rates of litter decomposition for most of the terrestrial ecosystems in China. The increased intensity of land use on the grasslands of northern China has caused severe land degradation since the late 1970s, resulting in decreased productivity and ecosystem stability. Much of the area is now in various stages of degradation, where soil N, P and water are considered to be the principal limiting factors to net primary productivity. The recycling of nutrients through plant litter decomposition is an essential mechanism to maintain productivity (Tiessen et al., 1994). To examine how increased soil nutrient availability, predominantly N and P, and water would affect the litter decomposition rates and nutrient dynamics of two dominant grassland plant species with contrasting life forms and tissue chemistry (i.e. Allium bidentatum and Stipa krylovii), we conducted two field experiments with varying addition rates of N, P, and water. Our objectives were to determine: (1) how increased soil N would affect the decomposition rates and nutrient dynamics of A. bidentatum and S. krylovii, and (2) how simultaneous increases in soil N and P, with or without increased water availability, would affect the rates of decomposition of the two species. Our underlying hypotheses were that species with contrasting life forms and tissue chemistry would differ significantly in the rates of litter decomposition, and that increased soil nutrient and water would stimulate litter decomposition in the degraded semi-arid grassland ecosystems. 2. Materials and methods 2.1. Study site This study was conducted at a field site of the Duolun Restoration Ecology Experimentation and Demonstration Station in Inner Mongolia, China (latitude 428020 N; longitude 1168160 E; altitude 1344 m a.s.l.). The mean annual, minimum, and maximum air temperatures for the area are 1.6, 18.3, and 18.7 8C. The mean annual precipitation is 385 mm, occurring mainly from July to September (accounting for 67% of the total), with Penman evaporative potential of 1748 mm. Soils of the region are commonly referred to as chestnut type (Calcic Kastanozems) in the Chinese classification, and are classified as Calcic–orthic Aridisol in the US soil taxonomy classification system (Yuan et al., 2005).
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The area has suffered from severe degradation since 1979 as a result of overgrazing. The primary vegetation is of the typical steppe, with perennial herb A. bidentatum and bunch grass species S. krylovii being among the dominant species of the plant community.
the time of initial deployment to determine the ratio between air-dried mass and oven-dried mass. This ratio was used to convert the initial air-dried mass of the litter to oven-dried mass. 2.4. Chemical analysis
2.2. Experimental design and treatments We conducted two decomposition experiments on a fenced site from July and October 2004, both lasting for 100 days. The first experiment contained four levels of N addition rates (0, 8, 16, and 32 gN/m2; designated as N0, N8, N16, and N32, respectively), and the second experiment contained four addition rates of mixed N and P (0, 8 gN/4 gP, 16 gN/8 gP, and 32 gN/16 gP/m2; designated as N0P0, N8P4, N16P8, and N32P16, respectively) with and without increased water availability. Both of the experiments were deployed based on a randomized block design with four blocks and each plot size of 4 m 4 m. N was added as granular urea, and P in the form of granulated triple superphosphate, which were applied 5 days before the litterbags were deployed. In treatments with increased water availability, water was applied on 15 July, 10 August, 29 August, and 22 September, following major rain events in the previous week. For each water application, 15 mm of water was added, simulating an average increase of rainfall by 30% of the corresponding period. 2.3. Litter decomposition In early July 2004, litters of A. bidentatum (predominantly leaves) and S. krylovii (predominantly culms) were collected from the study area, including freshly fallen and senesced tissues attached to the plants. After being air-dried to constant mass, they were clipped into fragments of 10 cm in length, and placed into the 10 cm 15 cm polyethylene litterbags (mesh size 1 mm2) that each contained 3 g of air-dried litter material. Each treatment plot contained nine litterbags of the same species. A total of 864 litterbags were prepared and deployed onto the treatment plots on 9 July 2004. The remaining litter was retrieved 35, 70, and 100 days after initial litterbag deployment. For each sampling time, three litterbags for each species from each treatment plot were collected. In the laboratory, extraneous matter such as other plant materials, rocks and small animals were handpicked from the litterbags. The retrieved litters were then oven-dried at 70 8C for 48 h, to determine the remaining dry mass. Five samples for each litter type were oven-dried at 70 8C for 48 h at
After determination of the dry mass, litters of the same plant species from the three within-plot replications were pooled for chemical analysis. Total C, N and P concentrations were determined for the final samples from the two litter decomposition experiments. Total C was measured by the standard method of wetcombustion, total N by semi-micro Kjeldahl method and total P by molybdenum blue colorimetric method (Bao, 1999). Five litter samples for each plant species were also analyzed for total C, N and P to determine the initial litter chemistry. 2.5. Data analysis The value of decay constant, k, was determined by fitting the following exponential function (Olson, 1963): xt ¼ ekt x0 where, xt is the remaining litter mass after a given time period t, x0 is the initial litter mass. The remaining litter nutrients were calculated by multiplying the sample mass by the respective nutrient concentration. Tests of significance of the remaining mass were performed by analysis of variance (ANOVA; three-way without water treatment and four-way with water treatment). Data analyses were performed using procedures of SPSS (v.11.0). The least significant difference (LSD) was used for comparisons of means with confidence level of P < 0.05. 3. Results 3.1. Initial litter chemistry Total C contents in the initial litters were similar between A. bidentatum and S. krylovii (Table 1). Allium bidentatum had twice the N and P concentrations, and half the C:N and C:P ratios, of S. krylovii (Table 1). 3.2. Species difference in litter decomposition Allium bidentatum litter decomposed faster than S. krylovii in both experiments (Fig. 1). After 100 days of decomposition, the remaining litter mass was 70% of
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Table 1 Initial litter chemistry of Allium bidentatum and Stipa krylovii in the degraded typical steppe of Inner Mongolia, China Species
C (g/kg)
N (g/kg)
P (g/kg)
C:N ratio
C:P ratio
Allium bidentatum Stipa krylovii
481.9 6.0 486.3 7.2
5.0 0.3 2.8 0.2
0.42 0.02 0.23 0.01
97 6 174 15
1273 65 2720 101
Values are means S.E. (n = 5).
the initial in A. bidentatum, but >80% in S. krylovii. The patterns of litter decomposition over the duration of the experiment were well described by the exponential model, xt/x0 = ekt (values of r2 ranged between 0.89 and 0.99, and P < 0.05; Table 2). The values of decay
constant, k, of A. bidentatum were about twice of S. krylovii across all the treatments (Table 2). 3.3. Effects of nutrient addition and water on litter decomposition In the experiment I, addition of N alone to the substrate soils only slightly affected the rate of litter decomposition in both A. bidentatum and S. krylovii as shown by three-way ANOVA and changes in the k values, with two species showing the similar patterns of responses (Tables 2 and 3). Four-way ANOVA showed that the litter mass remaining was significantly (P < 0.05) affected by retrieval time, plant species, water treatment, addition rates of mixed N and P, and some of the interactions in the experiment II (Table 4). Increased rates of N and P Table 2 Decay constant (k; y1, r2 range 0.89–0.99, P < 0.05) for litters of Allium bidentatum and Stipa krylovii as affected by additions of N alone (experiment I) or in mixture with P with or without adding water (experiment II) in the degraded typical steppe of Inner Mongolia, China. Treatment
Fig. 1. Effects of N addition or concurrent additions of N and P with or without water added on litter decomposition of Allium bidentatum (open symbols) and Stipa krylovii (closed symbols). Vertical bars represent standard errors (n = 4). *P < 0.05; **P < 0.01; ***P < 0.001.
k Allium bidentatum
Stipa krylovii
Experiment I N0 N4 N8 N16
1.20 0.07 1.24 0.10 1.30 0.10 1.33 0.08
a a a a
0.53 0.07 0.58 0.07 0.63 0.10 0.67 0.06
b ab ab a
Experiment II No water added N0P0 N8P4 N16P8 N32P16
1.20 0.07 1.23 0.11 1.37 0.05 1.48 0.13
c bc ab a
0.63 0.06 0.69 0.02 0.77 0.07 0.85 0.10
c bc ab a
Water added N0P0 N8P4 N16P8 N32P16
1.37 0.07 1.41 0.15 1.48 0.09 1.56 0.09
b ab ab a
0.63 0.01 0.71 0.02 0.79 0.04 0.87 0.05
d c b a
The values of k are means S.E. (n = 4), which were derived based on exponential decay model, xt/x0 = ekt. Data in the same column followed by the same letter are not significantly different (P < 0.05).
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Table 3 F and P (shown in parentheses) values from three-way ANOVA for mass remaining (M, % of initial mass) and two-way ANOVA for residual N and P (% of initial mass) during decomposition of litter materials in soils with N addition (experiment I), with retrieval time, plant species and N addition (fertilizer) as main effects Source of variation
d.f.
F M
Time (T) Species (S) Fertilizer (F) TS TF SF TSF
2 1 3 2 6 3 6
79.3 286.5 2.4 10.5 0.9 1.4 0.9
(<0.001) (<0.001) (0.073) (<0.001) (0.513) (0.241) (0.504)
additions significantly (P < 0.05) accelerated the rate of litter decomposition in both A. bidentatum and S. krylovii; the values of k increased with increasing rates of N and P additions in both species regardless water treatment (Table 2). Adding water significantly (P = 0.001) increased the k value in A. bidentatum, but not in S. krylovii (Table 2). The decomposition rates of S. krylovii litter were more responsive to changes in the rates of mixed N and P additions, as significant treatment effects of N and P additions were shown for the remaining mass litter of the species on the Day 70 and 100 after the commencement of litter decomposition experiment (Fig. 1), and the changes in the values of k with increasing rates of N and P additions were greater in S. krylovii than in A. bidentatum (Table 2). In both species, the k value responded more to the concurrent changes in N and P (Experiment II) than changes in N alone (Experiment I; Table 2).
N
P
14.3 (<0.001) 34.1 (<0.001)
65.2 (<0.001) 7.7 (0.001)
3.5 (0.030)
0.01 (0.999)
3.4. Litter nutrient dynamics At the end of the 100-day study, the N and P concentrations in litters of A. bidentatum and S. krylovii all increased with increases in the addition rates of N in the experiment I (Fig. 2), and the addition rates of mixed N and P in the experiment II (Fig. 3). Adding water decreased the N and P concentrations in A. bidentatum and S. krylovii in the experiment II (Fig. 3). Two-way and three-way ANOVA showed that the residual amounts of litter N and P were significantly (P < 0.01) affected by plant species and addition rates of N in the experiment I (Table 3), and by plant species, addition rates of mixed N and P, and water treatment in the experiment II (Table 4). The absolute amounts of litter N increased in nearly all treatments after decomposing for 100 days, in both
Table 4 F and P (shown in parentheses) values from four-way ANOVA for mass remaining (M, % of initial mass) and three-way ANOVA for residual N and P (% of initial mass) during decomposition of litter materials in soils with mixed N and P additions with or without water added (experiment II), with retrieval time, plant species, water and N and P additions (fertilizer) as main effects Source of variation
d.f.
F M
Time (T) Species (S) Water (W) Fertilizer (F) TS TW SW TSW TF SF TSF WF TWF SWF TSWF
2 1 1 3 2 2 1 2 6 3 6 3 6 3 6
266.1 930.3 4.0 23.2 28.2 1.1 5.9 2.6 1.1 0.6 1.7 2.9 0.6 2.7 0.4
(<0.001) (<0.001) (0.047) (<0.001) (<0.001) (0.343) (0.016) (0.075) (0.374) (0.586) (0.115) (0.039) (0.740) (0.047) (0.879)
N
P
44.5 (<0.001) 10.8 (0.002) 116.5 (<0.001)
120.9 (<0.001) 20.9 (<0.001) 59.7 (<0.001)
0.1 (0.789)
0.5 (0.464)
6.6 (0.001)
0.3 (0.825)
0.2 (0.908)
0.8 (0.496)
0.2 (0.926)
0.2 (0.889)
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decomposition, whereas in S. krylovii, the absolute amounts of litter P increased (Table 5). Similar to the patterns of litter N and P concentrations, the absolute amounts of N and P in the remaining litter from the two experiments all increased with increasing addition rates of N alone or mixed N and P (Table 5). Adding water significantly reduced the absolute amounts of litter N and P in A. bidentatum (P = 0.004 and <0.001, respectively) and the absolute amounts of litter P in S. krylovii (P = 0.025; Table 5). 4. Discussion
Fig. 2. Effects of N addition on litter N and P concentrations of Allium bidentatum and Stipa krylovii after decomposing for 100 days in the field. Vertical bars represent standard errors (n = 4).
A. bidentatum and S. krylovii (Table 5). The changes in the absolute amounts of litter P, however, differed between the two species: in A. bidentatum, the absolute amounts of litter P decreased slightly after 100-day
On a smaller spatial scale, litter quality is considered as the most important factor influencing decomposition rate (Melillo et al., 1982; Aerts and De Caluwe, 1997). High quality litters are often characterized by higher N concentrations and lower C:N ratios, and can decompose faster in comparison with low quality litters (Sanchez, 2001). In this study, the litters of the perennial herb A. bidentatum had twice the N concentration, and about half of the C:N ratio, of the bunch grass species S. krylovii. The marked differences in the rate of litter decomposition between these two species can, therefore, attribute to differences in litter quality. We also found that increasing the rates of N additions resulted in slight increase in the rates of litter decomposition in the two grass species. This is in general agreement with many other studies showing
Table 5 Residual amounts of N and P (% of initial mass) in Allium bidentatum and Stipa krylovii after decomposing for 100 days as affected by additions of N alone (experiment I) or in mixture with P with or without adding water (experiment II) Treatment
Allium bidentatum N
Stipa krylovii P
N
P
Experiment I N0 N8 N16 N32
103.1 4.0 105.3 3.8 111.7 5.3 119.6 4.0
c c b a
86.2 2.5 91.0 4.5 95.3 2.6 98.8 2.6
c bc ab a
101.4 3.2 111.2 4.3 120.2 9.3 134.5 5.7
c b b a
102.3 4.2 106.3 7.6 111.0 7.6 114.6 8.5
b ab ab a
Experiment II No water added N0P0 N8P4 N16P8 N32P16
102.3 2.0 109.2 4.5 116.0 5.2 122.3 3.4
d c b a
86.7 3.0 96.4 3.9 105.1 5.0 113.3 3.7
d c b a
101.8 3.2 114.4 5.9 125.7 4.2 137.3 3.3
d c b a
100.7 4.1 110.8 6.0 117.3 8.5 122.5 5.6
c b ab a
Water added N0P0 N8P4 N16P8 N32P16
96.4 3.1 104.8 4.2 112.1 3.7 120.3 2.2
d c b a
83.0 4.1 92.4 3.8 97.6 4.7 102.6 4.4
c b ab a
97.4 5.0 112.2 6.4 122.2 7.3 133.5 5.9
d c b a
96.7 5.4 107.2 4.4 112.3 4.5 116.4 4.5
c b ab a
Values greater than 100 indicate nutrient immobilization, and less than 100 indicate net mineralization. Data in the same column followed by the same letter are not significantly different (P < 0.05). Values are means S.E. (n = 4).
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Fig. 3. Effects of mixed N and P additions with or without water added on litter N and P concentrations of Allium bidentatum and Stipa krylovii after decomposing for 100 days in the field. Vertical bars represent standard errors (n = 4).
small or no responses of litter decomposition to increasing soil N input (Pastor et al., 1987; Theodorou and Bowen, 1990; Downs et al., 1996). One of the possible explanations could be that other nutrients, such as phosphorus, were more limiting for decomposer microbes as found by others (Ostertag and Hobbie, 1999; Cleveland et al., 2002). In our second experiment with concurrent changes in N and P, the values of decay constant varied more markedly than in the first experiment. The above results suggest synergistic effects of N and P on litter decomposition rates in these two species that differ in litter quality. Apart from the marked difference in the rates of litter decomposition between the two species, Allium bidentatum and S. krylovii also differed in the responses of litter decomposition to increased soil water and concurrent changes in soil N and P. The litter decomposition of A. bidentatum was found to be more responsive to soil water, whereas S. krylovii responded more to concurrent changes in the soil N and P. The stronger effect of soil water on the litter decomposition of A. bidentatum than S. krylovii could be mainly due to the differences in litter quality as reflected by both litter N concentration and C:N ratio. The differences in
physical structures between the two litters could also partly attribute to the different responses of litter decomposition to soil water. Allium bidentatum is a perennial herb with more fragile structures, while S. krylovii is a bunch grass species with tissues that are more fibrous. Many studies have demonstrated that nutrient additions can affect litter nutrient dynamics without directly affecting the rate of decomposition (Hunt et al., 1988; O’Connell, 1994; Downs et al., 1996). Such mode of nutrient immobilization decouples nutrient mineralization from decomposition temporally, which may have implications to nutrient retention in degraded ecosystems. Organic N and P are much less mobile than inorganic N and P and both plants and microbes can make use of organic P by producing extracellular phosphatases to mineralize common organic P compounds into labile phosphates (McGill and Cole, 1981). Incorporation of nutrients into decomposing litter provides an important nutrient-retention mechanism for the system (McGroddy et al., 2004). In this study, we demonstrated marked differences in the rates, as well as in the responses to changes in soil nutrients and water, of litter decomposition between
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two dominant grassland plant species that differ in life forms and tissue chemistry. Our results clearly indicate that the decomposition of high quality litter is more likely to be limited by soil moisture regimes, while that of low quality litter is more sensitive to nutrient availability. Our study area, Duolun County of Inner Mongolia, belongs to the agro-pastoral ecotone of the Inner Mongolia Plateau. Here S. krylovii is the main construction species of the typical steppes, with A. bidentatum as one of the predominant companion species. In this agro-pastoral ecotone, sheep and cattle grazing was once a major land use practice. Most of the local grasslands were subjected to heavy grazing from early 1980s to 2000, resulting in severe land degradation. Beginning in 2001, a national policy was put into effect to ban grazing from the whole Duolun County in an effort to reverse the land degradation problem. Instead the grasses are now mowed for forage production and officially free of grazing. Existing literature on grassland vegetation dynamics of northern China has mostly concerned with Leymus chinensis and S. grandis steppes (e.g. Wang et al., 1996a, 1996b, 2000a, 2000b; Liu et al., 2002). Although specific information on the succession dynamics of S. krylovii steppe during degradation and restoration is lacking, the patterns should be similar to the S. grandis steppe. In general, long-term overgrazing reduces the status of S. krylovii into a sub-dominant species, while A. bidentatum becomes a minor or rare species. Under moderate grazing, the production of A. bidentatum and S. krylovii decreases in a similar proportion as other plants; Allium bidentatum would be more exposed and hence likely to be more susceptible to grazing than other plants in degraded grasslands because of its better palatability to livestock. When protected from grazing, the two species can both quickly restore their dominance in the plant communities; but the recovery is faster for A. bidentatum than Stipa spp. (Wang et al., 1999). In a separate study, we found that in grassland communities subjected to mowing, the maximum biomass of A. bidentatum was 50% greater than S. krylovii and ranked second in all plant species 5 years after exclusion of grazing (unpublished data). The much greater increases of A. bidentatum than S. krylovii during the recovering of the degraded grassland ecosystems emphasized greater importance of A. bidentatum to the carbon cycle and nutrient dynamics of the plant communities because of its faster decomposition rate and greater sensitivity to soil water. In conclusion, our findings suggest that plant species with different litter qualities should be taken into
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