Jimin Cheng

Effects of black locust (Robinia pseudoacacia) on soil properties in the loessial gully region of the Loess Plateau, China

Black locust (Robinia pseudoacacia) has been widely planted in the Loess Plateau for soil and water conservation. The effects of black locust on soil properties has significant role in land use and ecosystem management. However, this beneficial effect has been little studied in the Loess Plateau. The soil properties below black locust and native grass growing in Nanxiaohe and Wangdonggou watersheds, located in the loessial gully region of the Loess Plateau, were studied for changes in soil properties after establishment of black locust. The black locust significantly increased soil cation exchange capacity, organic carbon, total nitrogen, nitrate, and carbon:nitrogen and carbon:phosphorus (P) ratios, as well as some enzymes like alkaline phosphatase and invertase in 0–20xa0cm or 0–80xa0cm depths of soil compared to the native grassland in Nanxiaohe and Wangdonggou watersheds. However, the effects on ammonium, total P, and extractable P and potassium were not consistent in both watersheds. There were more obvious differences in soil properties between black locust land and grassland for Nanxiaohe watershed than for Wangdonggou watershed, suggesting that the effects of black locust on most soil properties increase with black locust age. The results indicate that black locust has potential to improve soil properties in the loessial gully region of the Loess Plateau and the improvements were greater in long-term than middle-term black locust stands.

Soil organic carbon losses due to land use change in a semiarid grassland

Background and AimsKnowledge about the effect of land use change on soil organic carbon (OC) in semiarid grassland is essential for understanding C cycles and for forecasting ecosystem C sequestration. Our objectives were (1) to study the effect of land use change on aggregate size distribution, aggregate-associated OC concentrations, and aggregate-associated stocks in a semiarid grassland area and (2) to relate changes in the aggregate fractions to changes in total soil OC.MethodsCropland and shrubland plots were established in a semiarid grassland area in 1982. We collected soil samples from adjacent grassland, cropland, and shrubland plots 27xa0years later and measured OC concentrations in the macroaggregate (>0.25xa0mm), microaggregate (0.25–0.053xa0mm) and silt+clay (<0.053xa0mm) fractions.ResultsTotal soil OC concentrations and stocks decreased significantly after the grassland was converted to cropland or shrubland. Soil microbial biomass C, root biomass, and root C also declined. The proportion of soil in the macroaggregate fraction decreased after conversion to cropland or shrubland. Decreases in macroaggregate-associated OC stocks accounted for more than half of the OC losses that occurred when grassland was converted to cropland. The decreases in macroaggregate-associated OC stocks were due to declines in both macroaggregation and macroaggregate-associated OC concentrations after conversion to cropland. In contrast, decreases in microaggregate-associated OC stocks accounted for more than half of the OC losses when grassland was converted to shrubland. The declines in microaggregate-associated OC stocks were primarily due to a decrease in microaggregate-associated OC concentrations after conversion to shrubland.ConclusionsLand use changed caused significant decreases in soil OC stocks. Conversion to cropland soil resulted in large decreases in macroaggregate-associated OC stocks whereas conversion to shrubland resulted in large decreases in microaggregate-associated OC stocks. Any changes in land use in semiarid grasslands could cause the grassland soil to become a source of atmospheric CO2; therefore extreme caution should be taken to avoid this hazard.

Ecosystem carbon and nitrogen accumulation after grazing exclusion in semiarid grassland.

The grazing exclusion in degraded grassland has been extensively used to prevent the loss of grassland resources and to improve grassland services. The effects of grazing exclusion on C and N balance, however, have not been well addressed but are essential for assessing grassland C sinks, the sustainable use of grassland resources and the support of grassland services. To understand the response of ecosystem C and N to grazing exclusion in semiarid grassland, we determined the C and N in litter, aboveground biomass, roots and soils from ungrazed grassland fenced at different times in northwest China. Our results showed that the aboveground biomass, root biomass and plant litter were 70–92%, 56–151% and 59–141% higher, respectively, in grazer excluded grassland than in grazed grassland. Grazing exclusion significantly increased C and N stored in plant biomass and litter and increased the concentrations and stocks of C and N in soils. Grazing exclusion thus significantly increased the C and N stored in grassland ecosystems. The increase in C and N stored in soil contributed to more than 95% and 97% of the increases in ecosystem C and N storage. The highest C and N stocks in ecosystems were observed in 17-year grazer excluded grassland. The results from this study indicate that grazing exclusion has the potential to increase C and N storage in degraded semiarid grassland and that the recovery of ecosystem C and N was mainly due to the accumulation of C and N in soils.

Effects of land-use change on soil organic carbon and nitrogen in density fractions and soil δ13C and δ15N in semiarid grasslands

AimsThe objectives of this study were to determine the response of soil organic carbon (SOC) and nitrogen (N) in different density fractions and the response of δ13C and δ15N signatures of soil to land-use change in a semiarid grassland.MethodsWe measured soil δ13C and δ15N and the concentrations of SOC and N in whole soil and in the light and heavy fractions in adjacent tracts of native grassland, cropland, and shrubland in a semiarid region of China. The cropland and shrubland were established on existing grassland 27xa0years ago.ResultsThe conversion of grassland to cropland or to shrubland significantly decreased SOC and N concentrations and stocks in both whole soil and density fractions in the 0–80xa0cm soil layer. The decreases in SOC and N stocks associated with the heavy fraction accounted for >90xa0% of the total decrease in the 0–80xa0cm soil layer after grassland cultivation or afforestation. The conversion of grassland to cropland or to shrubland significantly enriched soil δ13C but depleted δ15N.ConclusionsOur results suggested that the losses of SOC and N in whole soil after land-use changes in this semiarid grassland were primarily due to losses of SOC and N associated with the heavy fraction.

Response of soil CO2 efflux to precipitation manipulation in a semiarid grassland.

Soil CO2 efflux (SCE) is an important component of ecosystem CO2 exchange and is largely temperature and moisture dependent, providing feedback between C cycling and the climate system. We used a precipitation manipulation experiment to examine the effects of precipitation treatment on SCE and its dependences on soil temperature and moisture in a semiarid grassland. Precipitation manipulation included ambient precipitation, decreased precipitation (-43%), or increased precipitation (+17%). The SCE was measured from July 2013 to December 2014, and CO2 emission during the experimental period was assessed. The response curves of SCE to soil temperature and moisture were analyzed to determine whether the dependence of SCE on soil temperature or moisture varied with precipitation manipulation. The SCE significantly varied seasonally but was not affected by precipitation treatments regardless of season. Increasing precipitation resulted in an upward shift of SCE-temperature response curves and rightward shift of SCE-moisture response curves, while decreasing precipitation resulted in opposite shifts of such response curves. These shifts in the SCE response curves suggested that increasing precipitation strengthened the dependence of SCE on temperature or moisture, and decreasing precipitation weakened such dependences. Such shifts affected the predictions in soil CO2 emissions for different precipitation treatments. When considering such shifts, decreasing or increasing precipitation resulted in 43 or 75% less change, respectively, in CO2 emission compared with changes in emissions predicted without considering such shifts. Furthermore, the effects of shifts in SCE response curves on CO2 emission prediction were greater during the growing than the non-growing season.

Distribution of organic carbon and nitrogen in soil aggregates of aspen (Populus simonii Carr.) woodlands in the semi-arid Loess Plateau of China

The distribution and turnover of organic carbon (OC) and nitrogen (N) associated with aggregates in soils is critical for understanding the behaviour of C and N in soils. We collected soil samples from aspen (Populus simonii Carr.) woodland in the semi-arid Loess Plateau of China to investigate the distribution of aggregate-associated OC and N. The distribution of aggregates and aggregate-associated OC and N were measured, and OC and N stocks in each aggregate fraction were calculated. Across the sites and soil depths, microaggregates and the siltu2009+u2009clay fraction dominated the distribution of soil aggregates, which varied with site. Organic C and N accumulated mainly in the macro- and micro-aggregate fractions in loamy soils but in the siltu2009+u2009clay fractions in sandy soils. The OC and N stocks in the bulk soil of aspen woodland were determined by the OC and N stocks associated with siltu2009+u2009clay fraction. The results of this study indicate that soil texture may play an important role in assessing the distributions of soil OC and N in both bulk soils and aggregate size fractions in aspen woodland, especially in semi-arid regions. Furthermore, the establishment of aspen woodland would result in greater accumulation of OC and N in loam soils than in sandy soils.

Isotherms and kinetics of Si adsorption in soils

Abstract Many scientists consider Si to be a ‘quasi-essential’ element for plants. Farmers in a number of countries now apply Si-containing fertilizers to the soil in order to improve crop yield. The adsorption of Si onto soil particles is an important process affecting the availability of Si to plants, yet little is known about the fate of Si after it is added to the soil. The objective of this paper was to study the adsorption of Si in three soils at different initial Si concentrations, temperatures (293 and 303 K), and reaction times. Si-adsorption behavior varied significantly between soil types. Si adsorption and adsorption rate were highest in the Yellow Drab soil and lowest in the Lou soil. In all the three soils, the slopes of the adsorption isotherms remained constant as initial Si concentrations increased from 0 to 50 mg Si/L, but declined rapidly as initial Si concentrations increased from 50 to 200 mg Si/L. This suggests that these soils contained multiple Si-adsorption sites. Langmuir, Freundlich, and Temkin equations all described the adsorption of Si as a function of Si concentration reasonably well, however their goodness of fit varied according to soil type. The soils also showed significant differences in buffering capacities, supply parameters, and percent saturation. The reaction kinetics for Si adsorption in the Yellow Drab and Purple Paddy soils could be divided into two stages, an initial fast reaction (0~2 h) followed by a slow reaction (2~12 h). Si adsorption in the Lou soil was slow but steady throughout the reaction period. The correlation coefficients (r 2 ) for all the equations used in this study were significant at p<0.05 significant levels. Among the kinetic equations used in this study, the parabolic diffusion, bi-constants function, Langmuir, and Elovich equations were the best for describing the relationship between reaction time and the amount of Si adsorbed onto soil particles. An analysis of activation energy (E a ) suggested that the rate-limiting step for Si adsorption in the Yellow Drab soil and the Purple Paddy soil was a diffusion-controlled process, while Si adsorption in the Lou soil seemed to be a chemically controlled process. This study shows that there are significant differences in the adsorption of Si between soil types and highlights the importance of future studies to investigate the mechanisms for Si adsorption in soil.

Early-spring soil warming partially offsets the enhancement of alpine grassland aboveground productivity induced by warmer growing seasons on the Qinghai-Tibetan Plateau

AimsThe response of vegetation productivity to global warming is becoming a worldwide concern. While most reports on responses to warming trends are based on measured increases in air temperature, few studies have evaluated long-term variation in soil temperature and its impacts on vegetation productivity. Such impacts are especially important for high-latitude or high-altitude regions, where low temperature is recognized as the most critical limitation for plant growth.MethodsWe used Partial Least Squares regression to correlate long-term aboveground net primary productivity (ANPP) data of an alpine grassland on the Qinghai-Tibetan Plateau with daily air and soil temperatures during 1997–2011. We also analyzed temporal trends for air temperature and soil temperature at different depths.ResultsSoil temperatures have steadily increased at a rate of 0.4–0.9xa0°C per decade, whereas air temperatures showed no significant trend between 1997 and 2011. While temperature increases during the growing season (May–August) promoted aboveground productivity, warming before the growing season (March–April) had a negative effect on productivity. The negative effect was amplified in the soil layers, especially at 15xa0cm depth, where variation in aboveground productivity was dominated by early-spring soil warming, rather than by increasing temperature during the growing season.ConclusionsFuture warming, especially in winter and spring, may further reduce soil water availability in early spring, which may slow down or even reverse the increases in grassland aboveground productivity that have widely been reported on the Qinghai-Tibetan Plateau.

Effects of wildfire and topography on soil nutrients in a semiarid restored grassland

BackgroundWildfire and topography each have significant effects on soil biogeochemical cycles, but their interactive effects on soil nutrients remain largely unclear, hindering the precise prediction of the effects of fire on soil biogeochemical cycles in a larger spatial scale.MethodsWe examined soil nutrient contents from restored grass slopes that had suffered wildfires and adjacent restored grass slopes without any wildfire. Topographic factors included slope aspects (north and south slopes) and positions (upper, middle and lower slopes).ResultsFire significantly increased the contents of soil organic carbon (OC), total nitrogen (TN) (0–10xa0cm), ammonium (NH4+), and extractable phosphorous (EP), decreased the contents of nitrate (NO3−) and available potassium (AK), and had minimum influence on total phosphorus (TP) content. Slope aspect and position also affected soil nutrients, with higher contents in the north slope than the south slope and at the upper slope than the lower slope. The effects of fire on soil OC, TN, and NO3− were consistent across north and south slopes, but the effects on soil NH4+, TP, EP and AK varied with slope aspect. However, the effects of fire on soil nutrients were not influenced by slope position.ConclusionThese results indicate that slope aspect should be considered in predicting the response of soil biogeochemical cycles to fire.