Lili Jiang

The role of plant–soil feedbacks and land-use legacies in restoration of a temperate steppe in northern China

Plant–soil feedbacks affect plant performance and plant community dynamics; however, little is known about their role in ecological restoration. Here, we studied plant–soil feedbacks in restoration of steppe vegetation after agricultural disturbance in northern China. First, we analyzed abiotic and biotic soil properties under mono-dominant plant patches in an old-field restoration site and in a ‘target’ steppe site. Second, we tested plant–soil feedbacks by growing plant species from these two sites on soils from con- and heterospecific origin. Soil properties generally did not differ between the old-field site and steppe site, but there were significant differences among mono-dominant plant patches within the sites. While soil species origin (i.e., the plant species beneath which the soil was collected) affected biomass of individual plant species in the feedback experiment, species-level plant–soil feedbacks were ‘neutral’. Soil site origin (old-field, steppe) significantly affected biomass of old-field and steppe species. For example, old-field species had higher biomass in old-field soils than in steppe soils, indicating a positive land-use legacy. However, soil site origin effects depended on the plant species beneath which the soils were collected. The predictive value of abiotic and biotic soil properties in explaining plant biomass differed between and within groups of old-field and steppe species. We conclude that the occurrence of positive land-use legacies for old-field species may retard successional replacement of old-field species by steppe species. However, high levels of idiosyncrasy in responses of old-field and steppe plant species to con- and heterospecific soils indicate interspecific variation in the extent to which soil legacies and plant–soil feedbacks control successional species replacements in Chinese steppe ecosystems.

Plant species effects on soil carbon and nitrogen dynamics in a temperate steppe of northern China

We evaluated plant species effects on soil carbon (C) and nitrogen (N) dynamics in a steppe ecosystem of northern China. In two subsequent years, we measured soil properties in the top 10xa0cm of the soil under replicated mono-dominant plant patches in two sites that differed in land-use history: a cultivated site (old field) and an uncultivated site (steppe). Both in the cultivated site and the uncultivated site, we selected patches of three of the dominant plant species. Contrast analyses in ANOVA showed that soil organic carbon (SOC) and total N content (g per m2) was on average lower in the cultivated site than in the uncultivated site. On average, soil respiration was also lower in the cultivated site than in the uncultivated site. However, overall differences in soil C and N dynamics between the cultivated site and the uncultivated site (if existing) were generally small compared to the effects of individual plant species. Soil respiration differed among plant species in the cultivated site, but not in the uncultivated site. In contrast, SOC content, total N, and N mineralization rate differed among plant species in the uncultivated site, but not in the cultivated site. Mineralization and nitrification rates strongly varied among the dominant plant species, particularly in the uncultivated site. Variation in both C and N pools and fluxes could be best explained by a combination of plant biomass, litter, and soil microbial and micro-climatic parameters. Cultivation can directly affect soil C and N dynamics. However, importantly, our data suggest that indirect effects through changes in plant species composition are also important, and probably strongly interact with direct effects in affecting soil C and N dynamics after land-use change. Hence, evaluation of land-use history on soil C and N dynamics requires integral analyses of changes in plant community composition.

Responses of sequential and hierarchical phenological events to warming and cooling in alpine meadows

Organisms life cycles consist of hierarchical stages, from a single phenological stage (for example, flowering within a season), to vegetative and reproductive phases, to the total lifespan of the individual. Yet phenological events are typically studied in isolation, limiting our understanding of life history responses to climate change. Here, we reciprocally transfer plant communities along an elevation gradient to investigate plastic changes in the duration of sequential phenological events for six alpine species. We show that prolonged flowering leads to longer reproductive phases and activity periods when plants are moved to warmer locations. In contrast, shorter post-fruiting leaf and flowering stages led to shorter vegetative and reproductive phases, respectively, which resulted in shorter activity periods when plants were moved to cooler conditions. Therefore, phenological responses to warming and cooling do not simply mirror one another in the opposite direction, and low temperature may limit reproductive allocation in the alpine region.

Effects of flue gas desulfurization gypsum by-products on microbial biomass and community structure in alkaline–saline soils

PurposeFor an alkaline–saline region in Northwest China, we examined the responses of soil microbial communities to flue gas desulfurization gypsum by-products (FGDB), a new ameliorant for alkaline–saline soils. In 2009 and 2010, we collected soils from 0–20xa0cm and 20–40xa0cm depths along an experimental FGDB gradient (0, 0.74, 1.49, 2.25, and 3.00xa0kg FGDB m−2).Materials and methodsAs a measure of microbial community composition and biomass, we analyzed phospholipid fatty acids (PLFAs). We used real-time quantitative polymerase chain reaction (qPCR) to measure abundance of bacterial 16xa0S rRNA copy numbers. Additionally, physicochemical soil parameters were measured by common laboratory methods.Results and discussionMicrobial community composition differed along the FGDB gradient; however, the microbial parameters did not follow a linear response. We found that, in 2009, total PLFA concentrations, and concentrations of total bacterial and Gram-negative bacterial PLFAs were slightly higher at intermediate FGDB concentrations. In 2010, total PLFA concentrations, and concentrations of total bacterial, Gram-positive bacterial, Gram-negative bacterial, and fungal PLFAs as well as the fungal:bacterial PLFA ratio were highest at 1.49xa0kg FGDB m−2 and 3.00xa0kg FGDB m−2. PLFA concentrations often differed between 2009 and 2010; however, the patterns varied across the gradient and across microbial groups. For both years, PLFA concentrations were generally higher at 0–20xa0cm depth than at 20–40xa0cm depth. Similar results were obtained for the 16xa0S rRNA copy numbers of bacteria at 0–20xa0cm depth. FGDB addition resulted in an increase in soil Ca2+ and NO3−–N and a decrease in pH and electrical conductivity (EC). Shifts in PLFA-based microbial community composition and biomass could partly be explained by pH, soil organic carbon, total nitrogen (TN), soil moisture, EC, inorganic nitrogen, C/N, and Ca2+. Indirect effects via shifts in abiotic soil properties, therefore, seem to be an important pathway through which FGDB affect soil microbial communities.ConclusionsOur results demonstrate that addition of FGDB leads to significant changes in soil physicochemical and microbial parameters. As such, addition of FGDB can have large impacts on the functioning of soil ecosystems, such as carbon and nitrogen cycling processes.

Relatively stable response of fruiting stage to warming and cooling relative to other phenological events

The timing of the fruit-set stage (i.e., start and end of fruit set) is crucial in a plants life cycle, but its response to temperature change is still unclear. We investigated the timing of seven phenological events, including fruit-set dates during 3xa0yr for six alpine plants transplanted to warmer (approximately +3.5°C in soils) and cooler (approximately -3.5°C in soils) locations along an altitudinal gradient in the Tibetan area. We found that fruit-set dates remained relatively stable under both warming and cooling during the 3-yr transplant experiment. Three earlier phenological events (emergence of first leaf, first bud set, and first flowering) and two later phenological events (first leaf coloring and complete leaf coloring) were earlier by 4.8-8.2xa0d/°C and later by 3.2-7.1xa0d/°C in response to warming. Conversely, cooling delayed the three earlier events by 3.8-6.9xa0d/°C and advanced the two later events by 3.2-8.1xa0d/°C for all plant species. The timing of the first and/or last fruit-set dates, however, did not change significantly compared to earlier and later phenological events. Statistical analyses also showed that the dates of fruit set were not significantly correlated or had lower correlations with changes of soil temperature relative to the earlier and later phenological events. Alpine plants may thus acclimate to changes in temperature for their fruiting function by maintaining relatively stable timings of fruit set compared with other phenological events to maximize the success of seed maturation and dispersal in response to short-term warming or cooling.

Grazing modifies inorganic and organic nitrogen uptake by coexisting plant species in alpine grassland

To study how grazing affects the uptake of inorganic and organic N forms, three focal plant species (i.e., the graminoid species Kobresia pygmaea, which decreases with grazing, and the forbs Potentilla bifurca and Potentilla multifida, which increase with grazing) were selected in ungrazed and grazed plots in an alpine meadow on the Tibetan Plateau. Three times during the growing season (i.e., June, July, and September), these plots were injected with 15N-labeled NO3−-N, NH4+-N, or glycine-N, or with only water as a control. Two hours after 15N injection, exchangeable NH4+-N, glycine-N, and NO3−-N as well as plant and soil samples were collected and analyzed for 15N/14N and total N content. Our result showed that all three plant species took up glycine-N, but uptake of inorganic N was generally predominant. The graminoid K. pygmaea took up all three N forms equally in June but preferred NO3−-N in July (particularly under grazing) and exchangeable NH4+-N in September. The forbs P. bifurca and P. multifida preferentially took up exchangeable NH4+-N in July (particularly under grazing), while NO3−-N was the dominant form of N uptake in September. P. bifurca generally preferred exchangeable NH4+-N, but preference shifted toward NO3−-N under grazing in June. P. multifida preferred glycine-N in ungrazed plots and shifted its preference to NO3−-N under grazing in June. In conclusion, the three plant species showed niche partitioning for uptake of three forms of N across the season, which was modified by grazing. These findings indicate that plant N uptake patterns should be considered for better understanding the mechanisms of grazing effects on plant diversity and species coexistence.

Effects of Reed Straw, Zeolite, and Superphosphate Amendments on Ammonia and Greenhouse Gas Emissions from Stored Duck Manure

Stored poultry manure can be a significant source of ammonia (NH) and greenhouse gases (GHGs), including nitrous oxide (NO), methane (CH), and carbon dioxide (CO) emissions. Amendments can be used to modify physiochemical properties of manure, thus having the potential to reduce gas emissions. Here, we lab-tested the single and combined effects of addition of reed straw, zeolite, and superphosphate on gas emissions from stored duck manure. We showed that, over a period of 46 d, cumulative NH emissions were reduced by 61 to 70% with superphosphate additions, whereas cumulative NO emissions were increased by up to 23% compared with the control treatment. Reed straw addition reduced cumulative NH, NO, and CH emissions relative to the control by 12, 27, and 47%, respectively, and zeolite addition reduced cumulative NH and NO emissions by 36 and 20%, respectively. Total GHG emissions (as CO-equivalents) were reduced by up to 27% with the additions of reed straw and/or zeolite. Our results indicate that reed straw or zeolite can be recommended as amendments to reduce GHG emissions from duck manure; however, superphosphate is more effective in reducing NH emissions.

Effect of grazing on the abundance of functional genes associated with N cycling in three types of grassland in Inner Mongolia

PurposeThe aim of the study was to investigate the patterns of soil nitrogen (N)-cycling functional gene abundance along a precipitation gradient on the Mongolian Plateau, and the effects of grazing on the population size of microbial functional group under different precipitation regimes.Materials and methodsSoil samples were taken from grazing and non-grazing plots of meadow steppe, typical steppe, and desert steppe plots on the Mongolian Plateau for soil gravimetric moisture content, pH, and soil organic carbon (SOC), total N, and inorganic N (NH4+-N and NO3−-N) concentrations, and the abundance of functional genes associated with N2 fixation (nifH gene), nitrification (AOA and AOB genes), and denitrification (narG, nirS, nirK, and nosZ genes) was studied. The relationships between environmental variables, soil physicochemical properties, and functional microbial abundance were examined.Results and discussionSoil properties (soil moisture, pH, soil organic carbon, total nitrogen, NH4+-N, and NO3−-N content) and abundance of N-cycling groups all varied with precipitation. Compared with desert steppe, precipitation significantly decreased the abundance of nifH gene by 1 order of magnitude, but markedly increased the abundance of AOA and AOB genes by 1.32 to 4.72 times and denitrifying genes narG, nirS, nirK, and nosZ by 0.66 to 9.02 times in meadow steppe. Grazing significantly decreased the abundance of functional groups in desert steppe and typical steppe (pu2009<u20090.001), while there was no difference between grazing and non-grazing treatments in meadow steppe which had the highest precipitation level. Soil pH was the main factor affecting the abundance of nifH gene according to simple linear regression (R2u2009=u20090.934, pu2009<u20090.001), while moisture was positively related with population sizes of nitrifier and denitrifier groups, explaining 53.8–92.34xa0% of the variation in the abundance of AOA, narG, nirS, and nosZ genes in all three steppes.ConclusionsSoil pH was the major factor that significantly affected the gene abundance of nitrogen fixation process, and soil moisture was the dominant factor controlling the gene abundance of nitrification and denitrification process along the precipitation gradient. Grazing had no effect on the gene abundance of N-cycling process in meadow steppe but decreased it in desert and typical steppe. Our results suggest that grazing may not necessarily be associated with a reduction in microbial functional potentials when soil moisture was relatively good but will decrease the soil microbial functional potentials in a more arid environment in northern China grasslands.

Effects of grazing on the acquisition of nitrogen by plants and microorganisms in an alpine grassland on the Tibetan plateau

Background and aimsNitrogen (N) limitation leads to intense competition between plants and soil microorganisms for available N. However, it is unclear how grazing affects the acquisition of N by plants and microorganisms.MethodsWe conducted short-term 15N tracer experiments during the growing season (June, early growing season; July, mid-growing season; and September, late growing season) in an alpine grassland on the Tibetan Plateau to investigate the effects of grazing on the acquisition of NO3−-N, NH4+-N, and glycine-N by plants and soil microorganisms. Three dominant plant species (one graminoid, Kobresia pygmaea, and two forbs, Potentilla bifurca and Potentilla multifida) were selected for the study. As these species represented >90% of the vegetation, the plant recovery of 15N reflected competition at the plant community.ResultsGrazing decreased the recovery of 15N by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of 15N by plants and microorganisms and altered the partitioning of 15N between them. Significant interactions were found between grazing, season and the different forms of N. In the absence of grazing, plants acquired relatively more N than soil microorganisms for the three forms of N in July and September, whereas the microorganisms obtained relatively more 15N glycine in July and all three forms of N in September under grazing conditions. Under grazing, the plant root biomass played an important role in controlling plant–microbial N acquisition.ConclusionsGrazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial 15N recovery to a lesser extent than plant 15N recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems.

Soil bacterial community responses to warming and grazing in a Tibetan alpine meadow

Warming and grazing significantly affect the structure and function of an alpine meadow ecosystem. Yet, the responses of soil microbes to these disturbances are not well understood. Controlled asymmetrical warming (+1.2/1.7°C during daytime/nighttime) with grazing experiments were conducted to study microbial response to warming, grazing and their interactions. Significant interactive effects of warming and grazing were observed on soil bacterial α-diversity and composition. Warming only caused significant increase in bacterial α-diversity under no-grazing conditions. Grazing induced no substantial differences in bacterial α-diversity and composition irrespective of warming. Warming, regardless of grazing, caused a significant increase in soil bacterial community similarity across space, but grazing only induced significant increases under no-warming conditions. The positive effects of warming on bacterial α-diversity and grazing on community similarity were weakened by grazing and warming, respectively. Soil and plant variables explained well the variations in microbial communities, indicating that changes in soil and plant properties may primarily regulate soil microbial responses to warming in this alpine meadow. The results suggest that bacterial communities may become more similar across space in a future, warmed climate and moderate grazing may potentially offset, at least partially, the effects of global warming on the soil microbial diversity.