Taogetao Baoyin

Land-use impact on soil carbon and nitrogen sequestration in typical steppe ecosystems, Inner Mongolia

To explore the optimal land-use for soil carbon (C) sequestration in Inner Mongolian grasslands, we investigated C and nitrogen (N) storage in soil and soil fractions in 8 floristically and topographically similar sites which subjected to different land-use types (free-grazing, grazing exclusion, mowing, winter grazing, and reclamation). Compared with free-grazing grasslands, C and N storage in the 0-50 cm layer increased by 18.3% (15.5 Mg C ha−1) and 9.3% (0.8 Mg N ha−1) after 10-yr of grazing exclusion, respectively, and 21.9% (18.5 Mg C ha−1) and 11.5% (0.9 Mg N ha−1) after 30-yr grazing exclusion, respectively. Similarly, soil C and N storage increased by 15.3% (12.9 Mg C ha−1) and 10.2% (0.8 Mg N ha−1) after 10-yr mowing, respectively, and 19.2% (16.2 Mg C ha−1) and 7.1% (0.6 Mg N ha−1) after 26-yr mowing, respectively. In contrast, soil C and N storage declined by 10.6% (9.0 Mg C ha−1) and 11.4% (0.9 Mg N ha−1) after 49-yr reclamation, respectively. Moreover, increases in C and N storage mainly occurred in sand and silt fractions in the 0-10 cm soil layer with grazing exclusion and mowing. Our findings provided evidence that Inner Mongolian grasslands have the capacity to sequester C and N in soil with improved management practices, which were in the order: grazing exclusion > mowing > winter grazing > reclamation.

The responses of soil respiration to nitrogen addition in a temperate grassland in northern China.

Anthropogenic activities have increased nitrogen (N) inputs to grassland ecosystems. Knowledge of the impact of soil N availability on soil respiration (RS) is critical to understand soil carbon balances and their responses to global climate change. A 2-year field experiment was conducted to evaluate the response of RS to soil mineral N in a temperate grassland in northern China. RS, abiotic and biotic factors, and N mineralization were measured in the grassland, at rates of N addition ranging from 0 to 25gNm(-2)yr(-1). Annual and dormant-season RS ranged from 241.34 to 283.64g C m(-2) and from 61.34 to 83.84g C m(-2) respectively. High N application significantly increased RS, possibly due to increased root biomass and increased microbial biomass. High N treatment significantly increased soil NO3-N and inorganic N content compared with the control. The ratio of NO3-N to NH4-N and the N mineralization rate were significantly positively correlated with RS, but NH4-N was not correlated or negatively correlated with RS during the growing season. The temperature sensitivity of RS (Q10) was not significantly affected by N levels, and ranged from 1.90 to 2.20, but decreased marginally significantly at high N. RS outside the growing season is an important component of annual RS, accounting for 25.0 to 29.6% of the total. High N application indirectly stimulated RS by increasing soil NO3-N and net nitrification, thereby eliminating soil N limitations, promoting ecosystem productivity, and increasing soil CO2 efflux. Our results show the importance of distinguishing between NO3-N and NH4-N, as their impact on soil CO2 efflux differed.

Effects of mowing regimes and climate variability on hay production of Leymus chinensis (Trin.) Tzvelev grassland in northern China

Leymus chinensis (Trin.) Tzvelev grassland is the most widely distributed native steppe vegetation type suitable for haymaking in the semiarid pastoral regions of north-east Asia. The long-term effects of four mowing regimes (mowing once a year, twice a year, once every 2 years and twice every 3 years), and climatic variability on the hay production from L. chinensis grassland were investigated using permanent plots over 27 years. The results showed that (i) the overall cumulative annual herbage production over the 27 years was the highest under ‘mowing once a year’, and ‘mowing twice a year’ did not produce any significant change in the amount of harvested herbage; (ii) mowing induced a reduction in herbage production mostly in the first couple of years, and the reduction was larger under frequent than less frequent mowing. The annual herbage production in harvest years was higher under less frequent mowing (once every 2 years, or twice every 3 years) than that under annual mowing, but the higher herbage production in harvest years under less frequent mowing could not compensate for the herbage not made into hay in the years without harvest; (iii) annual herbage production generally increased with annual precipitation, but the response of annual herbage production to precipitation was best described by quadratic (instead of linear or logarithmic) equations, which suggested a decline in annual herbage production under the highest annual precipitation in the region, and the relationships stood under different mowing regimes; and (iv) an analysis of hay yield and quality, and costs of the mowing operation, showed that the haymaking from L. chinensis grassland was financially viable as long as the herbage production was not extremely low under drought. It is concluded that the best practice for haymaking from L. chinensis grassland should be based on the rule of ‘mowing once a year’ in high-production years and grazing in low-production years; and a light grazing early in the season may increase hay quality mowed in the autumn if the grassland grows well early in the season in high-production years.

Long-term effects of mowing on plasticity and allometry of Leymus chinensis in a temperate semi-arid grassland, China

Mowing is an important land management practice for natural semi-arid regions. A growing body of empirical evidence shows that different mowing regimes affect the functioning of grassland ecosystems. However, the responses of plant functional traits to long-term mowing and their allometric scaling under long-term mowing are poorly understood. For a better understanding of the effects of mowing on grassland ecosystems, we analyzed the allometric traits of leaves and stems of Leymus chinensis (Trin.) Tzvel., a dominant grass species in eastern Eurasian temperate grassland, at different mowing intensities (no clipping, clipping once every two years, once a year and twice a year). Experiments were conducted on plots established over a decade ago in a typical steppe of Xilinhot, Inner Mongolia, China. Results showed that most of the functional traits of L. chinensis decreased with the increased mowing intensity. The responses of leaves and stems to long-term mowing were asymmetric, in which leaf traits were more stable than stem traits. Also significant allometric relationships were found among most of the plant functional traits under the four mowing treatments. Sensitive traits of L. chinensis (e.g. leaf length and stem length) were primary indicators associated with aboveground biomass decline under high mowing intensity. In conclusion, the allometric growth of different functional traits of L. chinensis varies with different long-term mowing practices, which is likely to be a strategy used by the plant to adapt to the mowing disturbances.

Phytolith-occluded organic carbon as a mechanism for long-term carbon sequestration in a typical steppe: The predominant role of belowground productivity

Phytolith-occluded organic carbon (phytOC) has recently been demonstrated to be an important terrestrial carbon (C) fraction resistant to decomposition and thus has potential for long-term C sequestration. Existing studies show that plant leaves and sheath normally have high phytOC concentration, thus most of phytOC studies are limited to the aboveground plant parts. Grassland communities comprise herbaceous species, especially grasses and sedges which have relatively high concentrations of phytoliths, but the phytOC production from grassland, especially from its belowground part, is unknown. Here we determined the phytOC concentration in different parts of major plant species in a typical steppe grassland on the Mongolian Plateau, and estimated the phytolith C sequestration potential. We found that the phytOC concentration of major steppe species was significantly (p<0.05) higher in belowground (0.67gkg-1) than aboveground biomass (0.20gkg-1) and that the belowground net primary productivity (BNPP) was 8-15 times the aboveground net primary productivity (ANPP). Consequently, the phytOC stock in belowground biomass (12.50kgha-1) was about 40 times of that in aboveground biomass (0.31kgha-1), and phytOC production flux from BNPP (8.1-15.8kgha-1yr-1) was 25-51 times of that from ANPP. Our results indicate that BNPP plays a dominant role in the biogeochemical silica cycle and associated phytOC production in grassland ecosystems, and suggests that potential phytolith C sequestration of grasslands may be at least one order of magnitude greater than the previous estimation based on ANPP only. Our results emphasize the need for more research on phytolith and phytOC distribution and flux in both above and below ground plant parts for quantifying the phytolith C sequestration.

Recovery succession drives the convergence, and grazing versus fencing drives the divergence of plant and soil N/P stoichiometry in a semiarid steppe of Inner Mongolia

Background and aimsUnderstanding the plant and soil elemental stoichiometry during grassland dynamics is important for developing measures to enhance the restoration of degraded grassland. A number of restoration practices have been applied in the degraded typical steppe grassland in Inner Mongolia, either for research purpose or as actual restoration projects. However, the effects of different restoration measures on soil and plant N/P stoichiometry remain unclear.MethodsHere we explored the effects of three restoration measures (i.e., natural recovery, NR; shallow ploughing, SP; and harrowing HA) on the N, P stoichiometry of plant and soil in a typical steppe of Inner Mongolia, by comparing plant and soil N, P content and N/P ratio among the grasslands restored through NA, SP and HA, and that under sustained animal grazing (GR).ResultsLong-term restoration increased aboveground plant biomass, litter accumulation and changed soil and plant N/P ratio. Soil N and P contents in restored grassland (NR, SP or HA) were higher than those under grazing (GR); the restored grasslands shared a common slope of N-P linear regression, which was significantly greater than that of grazing grassland. Plant N content and N/P ratio decreased firstly and then increased during the restoration of degraded grassland.ConclusionSoil N limitation is greater than soil P limitation in typical steppe of Inner Mongolia. Soil N limitation is smaller in naturally recovered grassland and grazing grassland than in the restored grassland following shallow ploughing and harrowing. The restoration succession over 26-years after the exclusion of animal grazing have changed the N-P coupling relation in grassland soil, with a common N-P relation converged under grassland that are treated with different restoration measures.

Nutrient Characteristics in Relation to Plant Size of a Perennial Grass Under Grazing Exclusion in Degraded Grassland

Identifying the linkages between nutrient properties and plant size is important for reducing uncertainty in understanding the mechanisms of plant phenotypic plasticity. Although the positive effects of grazing exclusion on plant morphological plasticity has been well documented, surprisingly little is known about the relationship of nutrient strategies with plant shoot size after long-term grazing exclusion. We experimentally investigated the impacts of grazing exclusion over time (0, 9, 15, and 35 years) on the relationships of nutrient traits (nutrient concentration, allocation, and stoichiometry) of with morphological plasticity in Leymus chinensis, which is a dominant species in grasslands of Inner Mongolia, China. Our results showed that there was a significantly negative correlation between the degrees of plasticity and stability of various morphological traits. Increases in plant size by 126.41, 164.17, and 247.47% were observed with the increase of grazing exclusion time of 9, 15, and 35 years, respectively. Plant size was negatively correlated with nitrogen (N) and phosphorus (P) concentrations, but was positively correlated with carbon (C) concentration. Biomass partitioning and leaf to stem ratios of nutrient concentrations contributed more than 95% of the changes in N, P, and C allocation in L. chinensis leaves and stems induced by grazing exclusions. Nine years’ grazing exclusion rapidly changed the nutrient concentrations (averaged by -34.84%), leaf to stem nutrient allocations (averaged by -86.75%), and ecological stoichiometry (averaged by +46.54%) compared to free-grazing, whereas there was no significant trend of these nutrient traits across the 9, 15, and 35 years’ grazing exclusion in L. chinensis individuals. Our findings suggest that with the increase of the duration of the grazing exclusion, time effects on plant performances gradually weakened both in plant morphological plasticity and nutrient properties. There is a significant negative effect between plant sizes and nutrient traits under long-term grazing exclusion.

Plant sizes mediate mowing-induced changes in nutrient stoichiometry and allocation of a perennial grass in semi-arid grassland

Abstract While mowing‐induced changes in plant traits and their effects on ecosystem functioning in semi‐arid grassland are well studied, the relations between plant size and nutrient strategies are largely unknown. Mowing may drive the shifts of plant nutrient limitation and allocation. Here, we evaluated the changes in nutrient stoichiometry and allocation with variations in sizes of Leymus chinensis, the dominant plant species in Inner Mongolia grassland, to various mowing frequencies in a 17‐yr controlled experiment. Affected by mowing, the concentrations of nitrogen (N), phosphorus (P), and carbon (C) in leaves and stems were significantly increased, negatively correlating with plant sizes. Moreover, we found significant trade‐offs between the concentrations and accumulation of N, P, and C in plant tissues. The N:P ratios of L. chinensis aboveground biomass, linearly correlating with plant size, significantly decreased with increased mowing frequencies. The ratios of C:N and C:P of L. chinensis individuals were positively correlated with plant size, showing an exponential pattern. With increased mowing frequencies, L. chinensis size was correlated with the allocation ratios of leaves to stems of N, P, and C by the tendencies of negative parabola, positive, and negative linear. The results of structure equation modeling showed that the N, P, and C allocations were co‐regulated by biomass allocation and nutrient concentration ratios of leaves to stems. In summary, we found a significant decoupling effect between plant traits and nutrient strategies along mowing frequencies. Our results reveal a mechanism for how long‐term mowing‐induced changes in concentrations, accumulations, ecological stoichiometry, and allocations of key elements are mediated by the variations in plant sizes of perennial rhizome grass.