Grasslands In Semi-arid Regions Research Articles

Effect of Grass Components on the Temporal Response of Surface Soil Water to Precipitation and Air Temperature in the Loess Plateau

The Loess Plateau of China is facing increasingly serious challenges related to water shortage issues despite the successful vegetation restoration. Surface soil water (0–40 cm) is crucial to water transfer and discharge in the soil-plant-atmosphere continuum, which is controlled by precipitation (P), air temperature (Ta), and plant components. This study selected one typical grass species and set different treatments of natural condition (NC), removing litter (CR), and only roots (OR), to analyze the responses of soil water storage (W) and its change (∆W) to P and Ta at different time scales (including daily, weekly and monthly scales). The results showed that P had significantly positive effects on W and ∆W, while Ta negatively influenced W and ∆W. Meanwhile, Ta also controlled the significance of P affecting W. P contributed more (53–63%) to ∆W variation at the daily scale, but Ta showed more effect (73–80%) at the weekly scale, and both of them contributed more than 77% at the monthly scale. P and Ta existed a threshold phenomenon for increasing soil water, where P thresholds were higher than 0.93, 4.98, and 28.17 mm and Ta threshold were lower than 28.69, 28.89, and 29.96°C at the daily, weekly and monthly scales, respectively. Moreover, grass components also affected the responses of ∆W to P and Ta, and the OR treatment displayed the lowest sensitivity and had the lowest loss rate of soil water. The total infiltration, total loss amount, and rainwater retention rate exhibited: NC > CR > OR. Overall, grass canopy and litter improved soil water condition, and the roots most strongly protected soil water after infiltration. This study can provide a better understanding of surface soil water responding to climate change under drought stress with water shortage implications, and may strength the management of grassland in semi-arid region.

Impacts of nitrogen enrichment on vegetation growth dynamics are regulated by grassland degradation status

AbstractPlant phenological responses under global climate change is a hot topic in current ecological research. However, it also has comprehensive interactions with other anthropogenic environmental factors including nitrogen (N) enrichment owing to atmospheric deposition and fertilization, and grassland degradation, which attracts less attention. Continuous photographic techniques enabled us to monitor the plant cover dynamics and extract the phenophases (start, end, length and the peak cover position of growing season) in a field experimental platform with six N treatment levels on five grasslands along a degradation gradient indicated by a human disturbance index (HDI). The results showed that N addition significantly promoted seasonal plant covers of degraded grasslands, while mature grassland responding less sensitively. Moreover, increasing rates during spring green‐up and decreasing rates during autumn senescence were promoted by N addition, while those of grasslands with lower degradation levels showed non‐linear saturation patterns. Phenophases at the community level were generally insensitive to N addition, with the most degraded grassland (totally degraded grassland, TD) and mature grassland (MG) responding significantly. The length of growing season of TD was shortened by 8 days and that of MG lengthened by 15 days on average from N addition. The responses of extracted phenophases were regulated more by the proportions of three groups of degradation indicators (annuals, moderate grazing degradation indicator species, and climax species) than those of grasses and forbs. Our findings highlight the relationship between community phenology and grassland degradation, which can benefit the assessment and restoration of degraded grasslands in semiarid regions.

Restoration of degraded grasslands, but not invasion by Prosopis juliflora, avoids trade-offs between climate change mitigation and other ecosystem services

Grassland degradation and the concomitant loss of soil organic carbon is widespread in tropical arid and semi-arid regions of the world. Afforestation of degraded grassland, sometimes by using invasive alien trees, has been put forward as a legitimate climate change mitigation strategy. However, even in cases where tree encroachment of degraded grasslands leads to increased soil organic carbon, it may come at a high cost since the restoration of grassland-characteristic biodiversity and ecosystem services will be blocked. We assessed how invasion by Prosopis juliflora and restoration of degraded grasslands in a semi-arid region in Baringo, Kenya affected soil organic carbon, biodiversity and fodder availability. Thirty years of grassland restoration replenished soil organic carbon to 1 m depth at a rate of 1.4% per year and restored herbaceous biomass to levels of pristine grasslands, while plant biodiversity remained low. Invasion of degraded grasslands by P. juliflora increased soil organic carbon primarily in the upper 30 cm and suppressed herbaceous vegetation. We argue that, in contrast to encroachment by invasive alien trees, restoration of grasslands in tropical semi-arid regions can both serve as a measure for climate change mitigation and help restore key ecosystem services important for pastoralists and agro-pastoralist communities.

Responses of soil organic carbon and nitrogen to land-use changes in a semiarid region of northwest China

Currently, we lack a clear understanding of how climate influences the responses of soil organic carbon (OC) and nitrogen (N) to land-use changes in semiarid regions. Thus, to clarify the mechanisms involved, we measured the soil OC and total N (TN) concentrations and stocks in the 0–100 cm soil layer in adjacent grassland, farmland, and woodland at five sites on the Loess Plateau in China, which varied in terms of the mean annual precipitation (MAP) and mean annual temperature (MAT). The farmland or woodland was converted from grassland at each site. Conversion of grassland to farmland significantly reduced the soil OC and TN, with greater decreases in sites with relatively lower MAP and MAT (–0.60 Mg OC ha−1 yr−1 and –0.22 Mg TN ha−1 yr−1, respectively) than sites with relatively higher MAP and MAT (–0.35 Mg OC ha−1 yr−1 and –0.04 Mg TN ha−1 yr−1). Conversion of grassland to woodland increased OC and TN at sites with higher MAP and MAT, but decreased them at sites with lower MAP and MAT. OC and TN increased to a depth of 100 cm with MAP and MAT after this land-use change. OC and TN decreased at relatively drier and colder sites, but increased at relatively wetter and warmer sites. These findings provide insights into the effects of land-use changes on the soil OC and TN under different climate conditions, thereby facilitating the sustainable management of grasslands in semiarid regions.

Environmental control on water vapour and energy exchanges over grasslands in semiarid region of Brazil

A micrometeorological experiment was conducted over grasslands in a semi-arid region of north-eastern Brazil (São João, Pernambuco) from January to December 2011, using the Bowen ratio energy balance method, to improve the current understanding of energy partitioning and water vapour exchange over this ecosystem in this region. The objectives of the present study were to quantify the seasonal and diurnal variations in energy and water vapour exchanges over grasslands and understand the biotic and abiotic factors controlling the energy partitioning of this ecosystem. In the dry period, the low stored soil water limited the grass production and leaf area index, and as a consequence of these conditions, most of the annual net radiation (58%) was consumed in sensible heat flux. During the course of the study the evaporative fraction was linearly related to the leaf area index. The total annual evapotranspiration and its daily maximum were 543.8 mm and 3.14 mm d-1. The seasonal and diurnal variations in energy partitioning and evapotranspiration were controlled by soil water availability and leaf area index.

Investigation of Rainfall-Runoff Processes and Soil Moisture Dynamics in Grassland Plots under Simulated Rainfall Conditions

The characteristics of rainfall-runoff are important aspects of hydrological processes. In this study, rainfall-runoff processes and soil moisture dynamics at different soil depths and slope positions of grassland with two different row spacings (5 cm and 10 cm, respectively, referred to as R5 and R10) were analyzed, by means of a solution of rainfall simulation experiments. Bare land was also considered as a comparison. The results showed that the mechanism of runoff generation was mainly excess infiltration overland flow. The surface runoff amount of R5 plot was greater than that of R10, while the interflow amount of R10 was larger than that of R5 plot, although the differences of the subsurface runoff processes between plots R5 and R10 were little. The effects of rainfall intensity on the surface runoff were significant, but not obvious on the interflow and recession curve, which can be described as a simple exponential equation, with a fitting degree of up to 0.854–0.996. The response of soil moisture to rainfall and evapotranspiration was mainly in the 0–20 cm layer, and the response at the 40 cm layer to rainfall was slower and generally occurred after the rainfall stopped. The upper slope generally responded fastest to rainfall, and the foot of the slope was the slowest. The results presented here could provide insights into understanding the surface and subsurface runoff processes and soil moisture dynamics for grasslands in semi-arid regions.

半干旱地区不同土地利用方式下土壤团聚体的微生物量属性

团聚体是土壤基本结构单元,是土壤质量和肥力的基础,研究了解不同土地利用方式下,不同团聚体的生物学属性,有助于深入了解土壤肥力本质,以及不同土壤肥力差异的根本原因.采集半干旱地区林地、旱地、灌溉耕地和草地4种土壤利用类型的表层土壤,用干筛法获得6级团聚体,测定微生物量碳(MBC)、三磷酸腺苷(ATP)含量、基础呼吸速率(BR)和基质诱导呼吸量(SIR)等指标.结果表明:土壤微生物量ATP浓度在7.79-9.26 μmol ATP/g C_mic,灌溉耕地最高、林地最低;微生物代谢熵(<em>q</em>CO₂)和SIR分别为0.35-0.69 μL CO₂ g^-1C_mic d^-1和88.7-132.2 μg CO₂ g^-1 d^-1,灌溉耕地最高、旱地最低.土壤团聚体的微生物量ATP浓度在6.54-9.32 μmol/g C_mic之间,平均值为8.22 μmol/g C_mic;<em>q</em>CO₂和SIR分别为0.29-0.71 μL/g C_mic和64-120 μg CO₂/g土,大团聚体含量总体上高于小团聚体,可能暗示不同团聚体土壤微生物群落活性及生理状态存在差异.土壤ATP含量、SIR和<em>q</em>CO₂与微生物量碳呈显著相关性 (<em>r</em>=0.929、0.865、0.885, <em>P</em> < 0.01),SIR和BR之间也有存在较高的相关性 (<em>r</em>=0.828、<em>P</em> < 0.01),说明尽管各级团聚体微生物学属性的绝对值存在很大的差异,但相互关系并未随团聚体大小而变化.;Soil aggregates are the basic units of soil structure and play an important role in soil quality and fertility. It is thus meaningful to investigate the natures and differences in biochemical properties of different soil aggregates. Moreover, it may be benefit to understand the essence of soil fertility and fundamental reason for the difference in fertility of different soils. In this study, 4 surface soils were sampled from woodland, none-irrigated cropland, irrigated cropland, and grassland in semi-arid region of Inner Mongolia. Six classes of soil aggregates with different diameters were collected with dry sieving method. The measured parameters included soil basic properties such as clay, organic matter, total nitrogen, K₂SO₄ extracted carbon, and soil microbial biomass C (MBC), ATP, basal respiration and substrate-induced respiration. The results showed that the irrigated soil had highest clay and organic matter contents, about 32.2 g/kg and 29.93 g/kg respectively, but lowest values in grassland soil, only 12.2 g/kg and 12.48 g/kg. The irrigated soil contained microbial biomass C of 336.03 mg/kg, but the woodland soil and none-irrigated cropland soil had about 207.53 mg/kg and 216.11 respectively. The ATP content in soils ranged 0.0016 - 0.0031 μmol/g soil, higher in the irrigated soil, lower in the woodland soil. The soil microbial biomass ATP concentration was the highest in the irrigated soil, about 9.26 μmol ATP/g C_mic, but lowest in the woodland soil, only 7.79 μmol ATP/g C_mic. The soil basal respiration and substrate-induced respiration were the highest in the irrigated soil, 10.31 μg CO₂-C g^-1soil d^-1 and 132.2 μg CO₂ g^-1 d^-1 respectively, but very low in the no-irrigated soil, only 3.33 μg CO₂-C g^-1soil d^-1 and 88.66 μg CO₂ g^-1 d^-1. The microbial biomass metabolic quotient (<em>q</em>CO₂) was high in the irrigated soil, about 0.69μg CO₂-C g^-1soil d^-1 but very low in the none-irrigated cropland soil, about 0.35 μL CO₂-C g^-1soil d^-1. The result showed that the microbial activity in the irrigated soil was higher than other soils, which might be linked with the quantity and quality of organic C input, water soluble carbon content and water content. Different soil aggregates had different natures of microbial biochemical properties. The basal respiration, <em>q</em>CO₂ and substrate-induced respiration in different soil aggregates were 0.29-0.71 μL/g C_mic, 0.29-0.71 μL/μg C_mic and 64-120 μg CO₂/g soil, respectively. The ATP content in soil aggregates changed among 0.83-3.02 nmol/g soil. The soil microbial biomass ATP concentration in soil aggregates were fallen within the range of 6.54-9.32 μmol/g C_mic. In general, the larger the soil aggregates, the higher values of those biochemical parameters were measured, which may imply the difference in microbial community and physiological activity among the soil aggregates. It may also indicate that microorganisms in larger aggregates (> 0.25 mm) have higher carbon utilization efficiency than those of smaller aggregates (< 0.25 mm). Soil microbial biomass ATP concentration, soil basal respiration and substrate-induced respiration were significantly and positively correlated with the soil microbial biomass C(<em>r</em>=0.929, 0.865, 0.885, <em>P</em> < 0.01). There was also a significant correlation between basal respiration and substrate-induced respiration (<em>r</em>=0.828, <em>P</em> < 0.01). It is evident that the relationships among the soil microbial parameters does not change with soil aggregates though dramatically different in the absolute values.

Effects of adding water on seasonal variation of soil nitrogen availability under sandy grasslands in semi-arid region

Water is usally thought of a limiting factor for the restoration of semi-arid ecosystem. In the growing season of 2006, a study was conducted to determine the effects of modeling precipitation on seasonal patterns in concentrations of soil-available nitrogen and to describe the seasonal patterns in soil nitrogen availability and seasonal variation in the rates of net nitrogen mineralization of topsoil at Daqinggou ecological station in Keerqin sand lands, Inner Mongolia Autonomous Region, China. Manipulation of water (80 mm) was designed to be added to experiment plots of sandy grasslands in dry season. Water addition (W) treatment and control (CK) treatment were separately taken in six replications and randomly assigned in 12 plots (4 m×4 m for each) with 2-m buffers betweens. Results showed that the content of soil inorganic nitrogen and net nitrogen mineralization rate were not affected by adding water in sandy grassland of Keerqin sand lands. Net nitrogen mineralization rates ranged from 0.5 μg·μg−1·month−1 to 4 μg−1g−1·month−1. The highest values of soil inorganic nitrogen and net nitrogen mineralization occurred on October 15 in control plots. The seasonal changes of soil inorganic nitrogen contents exhibited “V” shape pattern that was related to seasonal patterns of soil ammonium-N (ascending trend) and nitrate-N transformation (descending trend).

Simulation of CO2 and sensible/latent heat fluxes exchange between land surface and atmosphere over cropland and grassland in semi-arid region, China

A comparison between simulated land surface fluxes and observed eddy covariance (EC) measurements was conducted to validate Integrated Biosphere Simulator (IBIS) at Tongyu field observation station (44°25′N, 122°52′E) in Jilin Province, China. Results showed that the IBIS model could reproduce net ecosystem CO2 exchange (NEE), sensible and latent heat fluxes reasonably, as indicated by correlation coefficients exceeding the significant level of 0.05. It was also evident that the NEE and sensible heat fluxes were characterized by diurnal and seasonal variation both in the grassland and the cropland ecosystems, while the latent heat fluxes correlated with evapotranspiration, only took on the diurnal variation during the growing season. Moreover, both sensible heat fluxes and the latent heat fluxes were larger in the cropland ecosystem than that in the degraded grassland ecosystem. This different characteristic was possibly correlated with vegetation growing situation in the two kinds of ecosystems. A close agreement between observation and simulation on NEE, sensible heat fluxes and latent heat flux was obtained both in the degraded grassland and the cropland ecosystems. In addition, the annual NEE in the model was overestimated by 23.21% at the grassland and 27.43% at the cropland, sensible heat flux with corresponding 9.90% and 11.98%, respectively, and the annual latent heat flux was underestimated by 4.63% and 3.48%, respectively.