Liping Qiu

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.

The Accumulation of Organic Carbon in Mineral Soils by Afforestation of Abandoned Farmland

The afforestation of abandoned farmland significantly influences soil organic carbon (OC). However, the dynamics between OC inputs after afforestation and the original OC are not well understood. To learn more about soil OC dynamics after afforestation of farmland, we measured the soil OC content in paired forest and farmland plots in Shaanxi Province, China. The forest plots had been established on farmland 18, 24, 48, 100, and 200 yr previously. The natural 13C abundance of soil organic matter was also analyzed to distinguish between crop- and forest-derived C in the afforested soils. We observed a nonlinear accumulation of total OC in the 0–80 cm depth of the mineral soil across time. Total soil OC accumulated more rapidly under forest stands aged 18 to 48 yr than under forest stands aged 100 or 200 yrs. The rate of OC accumulation was also greater in the 0–10 cm depth than in the 10–80 cm depth. Forest-derived OC in afforested soils also accumulated nonlinearly across time, with the greatest increase in the 0–20 cm depth. Forest-derived OC in afforest soils accounted for 52–86% of the total OC in the 0–10 cm depth, 36–61% of the total OC in the 10–20 cm depth, and 11–50% of the total OC in the 20–80 cm depth. Crop-derived OC concentrations in the 0–20 cm depth decreased slightly after afforestation, but there was no change in crop-derived OC concentrations in the 20–80 cm depth. The results of our study support the claim that afforestation of farmland can sequester atmospheric CO2 by increasing soil OC stocks. Changes in the OC stocks of mineral soils after afforestation appear to be influenced mainly by the input of forest-derived C rather than by the loss of original OC.

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 silt + clay 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 silt + clay 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 silt + clay 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.