The Mongolian Steppe is one of the largest remaining grassland ecosystems. Recent studies have reported widespread decline of vegetation across the steppe and about 70 percent of this ecosystem is now considered degraded. Among the scientific community there has been an active debate about whether the observed degradation is related to climate, or overgrazing, or both. Here, we employ a new atmospheric correction and cloud screening algorithm (MAIAC) to investigate trends in satellite observed vegetation phenology. We relate these trends to changes in climate and domestic animal populations. A series of harmonic functions is fitted to MODIS observed phenological curves to quantify seasonal and inter-annual changes in vegetation. Our results show a widespread decline (of about 12 percent on average) in MODIS observed NDVI across the country but particularly in the transition zone between grassland and the Gobi desert, where recent decline was as much as 40 percent below the 2002 mean NDVI. While we found considerable regional differences in the causes of landscape degradation, about 80 percent of the decline in NDVI could be attributed to increase in livestock. Changes in precipitation were able to explain about 30 percent of degradation across the country as a whole but up to 50 percent in areas with denser vegetation cover (p0.05). Temperature changes, while significant, played only a minor role (r20.10, p0.05). Our results suggest that the cumulative effect of overgrazing is a primary contributor to the degradation of the Mongolian steppe and is at least partially responsible for desertification reported in previous studies.

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Satellite observed widespread decline in Mongolian grasslands largely due to overgrazing

Soil respiration and human effects on global grasslands.

We analyzed 17 months (August 2005 to December 2006) of continuous measurements of soil CO 2 efflux or soil respiration (Rs) in an 18-year-old west-coast temperate Douglas-fir stand that experienced somewhat greater than normal summertime water deficit. For soil water content at the 4 cm depth (0) > 0.11 m 3 m -3 (corresponding to a soil water matric potential of -2 MPa), Rs was positively correlated to soil temperature at the 2 cm depth (Ts). Below this value of 0, however, Rs was largely decoupled from Ts, and evapotranspiration, ecosystem respiration and gross primary productivity (GPP) began to decrease, dropping to about half of their maximum values when 0 reached 0.07 m 3 m -3 . Soil water deficit substantially reduced R s sensitivity to temperature resulting in a Q 10 significantly <2. The absolute temperature sensitivity of Rs (i.e. dRs/dTs) increased with 0 up to 0.15 m 3 m -3 , above which it slowly declined. The value of dRs/dTs was nearly 0 for θ < 0.08 m 3 m -3 , thereby confirming that Rs was largely unaffected by temperature under soil water stress conditions. Despite the possible effects of seasonality of photosynthesis, root activity and litterfall on Rs, the observed decrease in its temperature sensitivity at low 0 was consistent with the reduction in substrate availability due to a decrease in (a) microbial mobility, and diffusion of substrates and extracellular enzymes, and (b) the fraction of substrate that can react at high T s , which is associated with low 0. We found that an exponential (van't Hoff type) model with Q 10 and R 10 dependent on only 0 explained 92% of the variance in half-hourly values of Rs, including the period with soil water stress conditions. We hypothesize that relating Q 10 and R 10 to 0 not only accounted for the effects of Ts on Rs and its temperature sensitivity but also accounted for the seasonality of biotic (photosynthesis, root activity, and litterfall) and abiotic (soil moisture and temperature) controls and their interactions.

Effect of soil water stress on soil respiration and its temperature sensitivity in an 18-year-old temperate Douglas-fir stand

Climate change scenarios predict increases in temperature, changes in precipitation patterns, and longer drought periods in most semi-arid regions of the world. Ecosystems in these regions are prone to land degradation, which may be aggravated by climate change. Soil respiration is one of the main processes responsible for organic carbon losses from arid and semi-arid ecosystems. We measured soil respiration over one year in two steppe ecosystems having different degrees of land degradation under three ground-covers: with vegetation, bare soil, and an intermediate situation between plants and bare soil. The largest differences in soil respiration rates between the sites were observed in spring, coinciding with the highest level of plant activity. The degraded site had drier and hotter soils with less soil water availability and a longer drought period. As a result, vegetation on the degraded site did not respond to spring rainfall events. Soil respiration showed a strong seasonal variability, with average annual rates of 1.1 and 0.8 μmol CO2 m−2 s−1 in the natural and degraded sites, respectively. We did not observe significant differences in soil respiration rates associated with ground-cover i.e., the temporal variation was much larger than the spatial variation. At both sites, soil moisture was the controlling driver of soil respiration for most of the year, when temperatures were above 20 °C and constrained the response to temperature for the few months when the temperature was below 20 °C. An empirical model based on soil temperature and soil moisture explained 90% and 72% of the seasonal variability of soil respiration on the natural and degraded sites, respectively. For the first time, this study suggests that land degradation may alter the carbon balance of these ecosystems through changes in the temporal dynamics of soil respiration and plant productivity, which have important negative consequences for ecosystem functioning and sustainability.

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Impact of land degradation on soil respiration in a steppe (Stipa tenacissima L.) semi-arid ecosystem in the SE of Spain

Progress in combating aeolian desertification (land degradation resulting from wind erosion) has been achieved in an agro-pastoral ecotone of northern China since the mid-1980s. This paper reviews three common measures used to combat and control aeolian desertification in such regions. In addition, it introduces a case study on the recovery of a degraded semi-arid ecosystem to provide regional lessons and support theoretical and practical approaches to desertification prevention and reversal on a global scale. On the basis of the analysis and evaluation of three kinds of typical measures and one regional scale case, this study shows that human-caused aeolian desertified land can be rehabilitated. Although the technologies and management of combating aeolian desertification in an agro-pastoral ecotone of northern China still need further improvement through more experimentation and practical application in the future, the experience gained to date contains important lessons for the recovery of degraded land on a global scale. Copyright © 2013 John Wiley & Sons, Ltd.

Combating Aeolian Desertification in Northern China

Effects of temperature and soil water-content on soil respiration of grazed and ungrazed Leymus chinensis steppes, Inner Mongolia

Based on the enclosed chamber method, soil respiration measurements of Leymus chinensis populations with four planting densities (30, 60, 90 and 120 plants/0.25 m 2 ) and blank control were made from July 31 to November 24, 2003. In terms of soil respiration rates of L. chinensis populations with four planting densities and their corresponding root biomass, linear regressive equations between soil respiration rates and dry root weights were obtained at different observation times. Thus, soil respiration rates attributed to soil microbial activity could be estimated by extrapolating the regressive equations to zero root biomass. The soil microbial respiration rates of L. chinensis populations during the growing season ranged from 52.08 to 256.35 mg CO2 m K2 h K1 . Soil microbial respiration rates in blank control plots were also observed directly, ranging from 65.00 to 267.40 mg CO2 m K2 h K1 . The difference of soil microbial respiration rates between the inferred and the observed methods ranged from K26.09 to 9.35 mg CO2 m K2 h K1 . Some assumptions associated with these two approaches were not completely valid, which might result in this discrepancy. However, these two methods’ application could provide new insights into separating root respiration from soil microbial respiration. The root respiration rates of L. chinensis populations with four planting densities could be estimated based on measured soil respiration rates, soil microbial respiration rates and corresponding mean dry root weight, and the highest values appeared at the early stage, then dropped off rapidly and tended to be constant after September 10. The mean proportions of soil respiration rates of L. chinensis populations attributable to the inferred and the observed root respiration rates were 36.8% (ranging from 9.7 to 52.9%) and 30.0% (ranging from 5.8 to 41.2%), respectively. Although root respiration rates of L. chinensis populations declined rapidly, the proportion of root respiration to soil respiration still increased gradually with the increase of root biomass. q 2005 Elsevier Ltd. All rights reserved.

Partitioning root and microbial contributions to soil respiration in Leymus chinensis populations

Soil respiration, canopy temperature, soil moisture, above and belowground biomass were observed in 2001, 2002, 2004 and 2005 at fenced and grazed typical Leymus chinensis steppes in Inner Mongolia. Based on soil respiration data obtained by the enclosed chamber method, diurnal and seasonal dynamics of soil respiration and their controlling factors were analyzed. The effects of grazing on diurnal and seasonal soil respirations were not significant. The diurnal patterns of soil respiration could be expressed as a one-humped curve and the lowest and highest values appearing from 1:00 to 3:00 and from 11:00 to 14:00, respectively. Canopy temperature had a strong influence on the diurnal variation of soil respiration. The rates of soil respiration rose to a seasonal maximum from the middle of June to the end of July and then gradually decreased. Soil moisture explained about 71.3% and 58.3% of the seasonal variation in soil respiration at fenced and grazed plots, respectively, and canopy temperature only 33.9% and 39.7%. Soil respiration rate, above and belowground biomass and soil moisture were significantly increased at the fenced plots compared to the grazed plots (P < 0.05), but the difference was not significant in canopy temperature. The mean soil respiration rates were 247.85 and 108.31 mgCO2 m−2 h−1 during the whole experiment at fenced and grazed plots, respectively. Soil respiration rate was enhanced significantly at the fenced plots, which might attribute to the increasing soil moisture and biomass. The response of soil respiration rate to grazing varied among different sites and might be related to local soil moisture status.

Effects Of Grazing On Soil Respiration Of Leymus Chinensis Steppe

Accurately partitioning soil respiration into autotrophic and heterotrophic components is important for understanding how ecosystem carbon budgets will respond to climate change. Usually, heterotrophic respiration can be estimated by a linear relationship between soil respiration and root biomass. In this study, however, we found that an exponential relationship was more appropriate than a linear relationship for relating soil respiration to root biomass in a temperate desert steppe in Inner Mongolia, China.

Partitioning soil respiration in a temperate desert steppe in Inner Mongolia using exponential regression method

Based on two-year soil respiration observing data at fenced and grazing typical Leymus chinensis steppe, Nei Monggol by the enclosed chamber method, soil respiration and its relationships with various affecting factors were analyzed. The major results include: (1) The diurnal and seasonal dynamics of soil respiration are similar at fenced and grazing typical Leymus chinensis steppe, but soil respiration rate is larger at the fenced plots than that at the grazing plots, and their values were 219.18 mg x (m2 x h)(-1) and 111.27 mg x (m2 x h)(-1), respectively. The reason of soil respiration rate increasing in the fenced plots might be the increases of biomass in the belowground and soil water content. (2) Among the influencing factors on soil respiration, grazing decreases the effects of soil water content and relative humidity and increases the effect of photosynthetic rate, and has no significant effects on other factors. (3) At the fenced plots, the correlation of soil respiration rate with soil water content is largest, followed by photosynthetic rate, air temperature, relative humidity, air CO2 concentration, intercellular CO2 concentration, stomatal conductance and the lowest being photosynthetically active radiation. Soil water content and air temperature are the main environmental factors, and photosynthetic rate is the critical biotic factor. The affecting factors on soil respiration are the same at the fenced and grazing plots, however the importance of those influencing factors is changed quantitatively.

Fengyu Wang

Papers

Effects of temperature and soil water-content on soil respiration of grazed and ungrazed Leymus chinensis steppes, Inner Mongolia

Partitioning root and microbial contributions to soil respiration in Leymus chinensis populations

Effects Of Grazing On Soil Respiration Of Leymus Chinensis Steppe

Partitioning soil respiration in a temperate desert steppe in Inner Mongolia using exponential regression method

[Soil respiration and its influencing factors at grazing and fenced typical Leymus chinensis Steppe, Nei Monggol].