Qiong Zhao

Effect of Land Cover Change on Soil Phosphorus Fractions in Southeastern Horqin Sandy Land, Northern China

In the past 50 years, large areas of the Horqin sandy land were afforested to prevent desertification. Although the afforestation policy appears successful, many people now doubt whether it is suitable to plant trees with high density on the poor soils in semiarid regions. Little is known about the impacts of afforestation on the sandy soil properties, although the evaluation of these impacts is fundamental to judge the rationality of afforestation policy. Soil phosphorus (P) fractions, acid phosphomonoesterase activities, and other soil chemical properties were compared among five adjoining typical ecosystems on poor sandy soils in southeastern Horqin sandy land. The ecosystems studied are natural elm savanna, degraded grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, Chinese pine (Pinus tabulaeformis) plantation, and mixed plantation of Mongolian pine and poplar (Populus simonii). The results showed that organic P dominated soil P (47%-65%) was the principal source of available P. The degradation of elm savanna to grassland significantly reduced soil pH and resulted in an overall reduction in soil fertility, although slightly increased labile inorganic P. Grassland afforestation had no significant influence on soil pH, organic carbon, and total N but significantly reduced total P. Impacts of grassland afforestation on soil P fractions depended on tree species. Natural elm savanna had higher soil P conserving ability than artificial plantations. Therefore, with the aim of developing a sustainable ecosystem, we suggested that vegetations with low nutrient demand (particularly P) and efficient nutrient cycling would be more suitable for ecosystem restoration in the semiarid region.

Soil Inorganic Nitrogen and Microbial Biomass Carbon and Nitrogen Under Pine Plantations in Zhanggutai Sandy Soil

The dynamics of soil inorganic nitrogen (NH4+-N and NO3--N) and microbial biomass carbon (C-mic) and nitrogen (N-mic) under 30-year-old fenced Pinus sylvestris L. var. mongolica, Litvin (SF), unfenced P. sylvestris L. var. mongolica Litvin (SUF), and unfenced Pinus densiflora Siebold et Zucc. (DUF) plantations in the Zhanggutai sandy soil of China were studied during Apr. to Oct. 2004 by the in situ closed-top core incubation method. All mentioned C and N indices in each stand type fluctuated over time. The ranges of inorganic N, C-mic, and N-mic contents in the three stand types were 0.7-2.6, 40.0-128.9, and 5.4-15.2 mu g g(-1), respectively. The average contents of soil NH4+-N and C-mic under the three 30-year-old pine plantations were not different. However, soil NO3--N and total inorganic N contents decreased in the order of SUF >= SF >= DUF, the N-mic content was in the order of SF = SUF > DUF, and the C-mic:N-mic ratio was3 in the order of SUF = DUF > SF. Seasonal variations were observed in soil inorganic N, microbial biomass, and plant growth. These seasonal variations had certain correlations with microbe and plant N use in the soil, and their competition for NH4+-N was mostly regulated by soil N availability. The influence of tree species on inorganic N and N-mic were mainly because of differences in litter quality. Lack of grazing decreased the C-mic:N-mic ratio owing to decreased carbon output and increased the ability of soil to supply N. The soil N supply under the P. sylvestris var. mongolica plantation was lower than under the P. densiflora plantation.

Responses of biomass to the addition of water, nitrogen and phosphorus in Keerqin sandy grassland, Inner Mongolia, China

The effects of water, nitrogen and phosphorus on productivity of sandy grassland were investigated with a fully factorial experiment to find out the main factors limiting natural restoration of grassland productivity in the southeastern Keerqin sandy land. In total, eight treatments were designed as water addition (W), nitrogen fertilizer addition (N), phosphorus fertilizer addition (P), water + nitrogen fertilizer addition (WN), water + phosphorus fertilizer addition (WP), nitrogen fertilizer + phosphorus fertilizer addition (NP), water + nitrogen fertilizer + phosphorus fertilizer addition (WNP) and control (CK). Each treatment was replicated six times and randomly assigned to 48 plots (4 m × 4 m) that were separated by a 2-m buffer. Results show that restoration of productivity is only limited by nitrogen factor for sandy grassland of Keerqin sandy land and not limited by water and phosphorus. Relative to CK plots, the biomass and the aboveground net primary productivity (ANPP) of all the plots added with nitrogen fertilizer were significantly enhanced (P < 0.05) in 2005 growing season. Grass root mass is dominant in underground biomass. The present study possibly underestimates net primary productivity of grassland in northern China, due to limitation of underground biomass measurements.

Aboveground biomass and nutrient allocation in an age-sequence of Larix olgensis plantations

Biomass and nutrient (N, P, K, Ca, Mg) stock in various aboveground tree components (stemwood, stembark, branches and leaves) were quantified in an age sequence of pure Larix olgensis plantations (20, 35, 53 and 69 years old) in Northeast China. The results show that the aboveground biomass allocation in various tree components was in the order of stemwood (62%–83%), branches (9%–21%), stembark (7%–11%) and leaves (1%–6%) for all stands. The proportion of stemwood biomass to total aboveground biomass increased whereas that of other tree components decreased consistently with stand age from 20 to 53 years old, but kept relatively constant with stand age from 53 and 69 years old. The nutrient allocation in various tree components generally followed the same pattern as the biomass allocation (i.e. stemwood > branches > stembark > leaves). The proportion of nutrient stock in leaves to total aboveground nutrient stock decreased consistently with increasing stand age, while that in stemwood increased with stand age from 20 to 53 years old but then decreased from 53 to 69 years old. The rate of nutrient removal for stands was estimated at different stand ages under different logging schemes, showing that the rate of nutrient removal would be unchanged when the rotation length was shortened to 20 years by the harvest of stem only, but greatly increased by the harvest of total aboveground biomass. The rate of nutrient removal would be a considerable reduction for all elements by debarking, especially for Ca.

Asymmetric effects of litter removal and litter addition on the structure and function of soil microbial communities in a managed pine forest

AimsVariation in tree litter inputs and understory vegetation caused by human disturbances and climate change in forest plantations can extend to alter forest stability and productivity over time. Here, we explore how tree litter inputs interact with understory plant management to influence belowground processes in a managed forest plantation.MethodsWe conducted a two-factor nested experimental manipulation of pine litter and understory vegetation in a nutrient-poor Pinus sylvestris var. mongolica plantation. Three levels of tree litter manipulation (ambient litter, litter removal and litter addition) were nested in two levels of understory manipulation (understory intact and understory removal). After two years of manipulation, mineral soils were analyzed for total and extractable C, N and P concentrations, N mineralization, enzyme activities, as well as the microbial community structure (as indicated by phospholipid fatty acids).ResultsLitter removal had little impact on C and nutrient cycling as well as microbial biomass and community structure in this low nutrient pine plantation; however, litter addition and the removal of the understory vegetation had large impacts on these processes. Litter addition elevated soil microbial biomass, acid phosphatase and β-1, 4-glucosidase activities, by a much greater degree when the understory vegetation was intact than when the understory was removed. Litter addition also reduced soil available P by 39% when the understory vegetation was intact, and reduced soil available P by 74% and NO3−–N by 45% when the understory was removed. Litter addition significantly reduced the ratio of Gram-positive to Gram-negative bacteria as well as the ratio between PLFA markers cy17:0 and 16:1ω7. Understory removal reduced the ratio of PLFA markers cy17:0 to 16:1ω7.ConclusionsOur study results show that, in this managed pine plantation, soil microbial community structure and function were more sensitive to an increase rather than to a decrease in pine litter inputs. Further, we found that the presence of understory vegetation can increase soil microbial biomass and alleviate the reduction in available N and P concentrations induced by pine litter addition. Thus, preservation of the understory vegetation is an effective way to maintain the functional stability of managed forests on nutrient-poor soils.

Responses of plant diversity and species composition to the cessation of fertilization in a sandy grassland

Nitrogen was the main limiting nutrient of net primary production in the southeastern Keerqin Sandy Lands, Northeast China. Species richness declined and biomass increased after five consecutive years of nitrogen fertilization of these sandy grasslands (2004–2008). After fertilization had been stopped for three years (2009–2011), we surveyed vegetation on previously fertilized plots to quantify changes in community composition. Respect species richness showed an increasing trend over time since the cessation of fertilization. Respect vegetation height and coverage showed decreasing trends over time since the cessation of fertilization. Species composition changed after fertilization ceased, the dominant species shifting from Cannabis sativa, Phragmites communis and Chenopodium acuminatum in 2008 to Cannabis sativa, Phragmites communis and Artemisia scoparia in 2011. Dominance of dominant species declined from 66.2% in 2008 to 57.5% in 2011. The importance value of annual plants in the earlier nitrogen addition plots was higher than in control plots, but the differences were not significant in 2011. The importance value of perennial plants differed significantly between treatments from 2009 to 2011. The reversion rate not only differed between community characteristics, but also between functional groups in the same community characteristic. Although the residual effect of nitrogen addition on vegetation was still observed three years after fertilization ceased, the vegetation showed signs of recovery.

Effects of freeze-thaw on soil nitrogen and phosphorus availability at the Keerqin Sandy Lands, China

A laboratory simulated freeze-thaw was conducted to determine the effects of freeze-thaw on soil nutrient availability in temperate semi-arid regions. Soil samples were collected from sandy soils (0–20 cm) of three typical ecosystems (grassland, Mongolian pine plantation and poplar plantation) in southeastern Keerqin Sandy Lands of China and subjected to freeze-thaw treatment (−12°C for 10 days, then 20°C for 10 days) or incubated at constant temperature (20°C for 20 days). Concentrations of the soil NO3−-N, NH4+-N, NaHCO3 extractable inorganic P (LPi) and microbial biomass P (MBP) were determined on three occasions: at the start of the incubation, immediate post-thawing and at the 10th day post-thawing. The results showed that soil net nitrification and N mineralization rates at three sites were negatively affected by freeze-thaw treatment, and decreased by 50%–85% as compared to the control, of which the greatest decline occurred in the soil collected from poplar plantation. In contrast, the concentration of soil NH4+-N, NaHCO3 extractable inorganic P (LPi) and microbial biomass P were insignificantly influenced by freeze-thaw except that LPi and NH4+-N showed a slight increase immediate post-thawing. The effects of freeze-thaw on soil N transformation were related to soil biological processes and the relatively constant available P was ascribed to severe soil aridity.

Altered leaf functional traits by nitrogen addition in a nutrient-poor pine plantation: A consequence of decreased phosphorus availability

This study aimed to determine how specific leaf area (SLA) and leaf dry matter content (LDMC) respond to N addition and understory vegetation removal in a 13-year-old Mongolian pine (Pinus sylvestris var. mongolica) plantation. Traits (SLA, LDMC, individual needle dry weight, N and P concentrations) of different-aged needles and their crown-average values were measured, and their relationships with soil N and P availability were examined. N addition and understory removal reduced soil Olsen-P by 15–91%. At the crown level, N addition significantly reduced foliar P concentration (by 19%) and SLA (by 8%), and elevated N concentration (by 31%), LDMC (by 10%) and individual leaf dry weight (by 14%); understory removal did not have a significant effect on all leaf traits. At the needle age level, traits of the previous year’s needles responded more strongly to N addition and understory removal than the traits of current-year needles, particularly SLA and N concentration. SLA and LDMC correlated more closely with soil Olsen-P than with soil inorganic N, and LDMC correlated more closely with soil Olsen-P than SLA did. These results indicate that aggravated P limitation resulting from N addition and understory removal could constrain Mongolian pine growth through their effects on the leaf traits.

Effects of Nitrogen Addition on Litter Decomposition and CO2 Release: Considering Changes in Litter Quantity.

This study aims to evaluate the impacts of changes in litter quantity under simulated N deposition on litter decomposition, CO2 release, and soil C loss potential in a larch plantation in Northeast China. We conducted a laboratory incubation experiment using soil and litter collected from control and N addition (100 kg ha−1 year−1 for 10 years) plots. Different quantities of litter (0, 1, 2 and 4 g) were placed on 150 g soils collected from the same plots and incubated in microcosms for 270 days. We found that increased litter input strongly stimulated litter decomposition rate and CO2 release in both control and N fertilization microcosms, though reduced soil microbial biomass C (MBC) and dissolved inorganic N (DIN) concentration. Carbon input (C loss from litter decomposition) and carbon output (the cumulative C loss due to respiration) elevated with increasing litter input in both control and N fertilization microcosms. However, soil C loss potentials (C output–C input) reduced by 62% in control microcosms and 111% in N fertilization microcosms when litter addition increased from 1 g to 4 g, respectively. Our results indicated that increased litter input had a potential to suppress soil organic C loss especially for N addition plots.

Effects of ultraviolet (UV) radiation and litter layer thickness on litter decomposition of two tree species in a semi-arid site of Northeast China

Forests and grasslands in arid and semi-arid regions receive high-intensity ultraviolet (UV) radiation year-round. However, how the UV radiation affects the litter decomposition on the forest floor remains unclear. Here, we conducted a field-based experiment in 2011 in the southeastern Horqin Sandy Land, Northeast China, to investigate the effects of UV radiation, litter layer thickness, and their interaction on the mass loss and chemical properties of decomposing litter from Xiaozhuan poplar (Populus × xiaozhuanica) and Mongolian pine (Pinus sylvestris var. mongolica) plantation trees. We found that UV radiation accelerated the decomposition rates of both the Xiaozhuan poplar litter and Mongolian pine litter. For both species, the thick-layered litter had a lower mass loss than the thin-layered litter. The interaction between UV radiation and litter layer thickness significantly affected the litter mass loss of both tree species. However, the effects of UV radiation on the chemical properties of decomposing litter differed between the two species, which may be attributed to the contrasting initial leaf litter chemical properties and morphology. UV radiation mostly had positive effects on the lignin concentration and lignin/N ratio of Xiaozhuan poplar litter, while it had negative effects on the N concentration of Mongolian pine litter. Moreover, litter layer thickness and its interaction with UV radiation showed mostly positive effects on the N concentration and lignin/N ratio of Xiaozhuan poplar litter and the ratios of C/N and lignin/N of Mongolian pine litter, and mostly negative effects on the C/N ratio of Xiaozhuan poplar litter and the N concentration of Mongolian pine litter. Together, these results reveal the important roles played by UV radiation and litter layer thickness in the process of litter decomposition in this semi-arid region, and highlight how changes in the litter layer thickness can exert strong influences on the photodegradation of litter in tree plantations.