Songnen Grassland Research Articles

Application of soil phytoliths to the quantitative characterization of temperate grassland ecosystems: a case study in Northeast China

Although phytolith analysis is a promising tool for reconstructing the palaeovegetation in grassland ecosystems, no phytolith study has yet attempted to quantitatively reconstruct the corresponding plant community characteristics. Questions remain regarding whether or not soil phytoliths can quantitatively reflect various aspects of grassland physiognomy, and how well modern phytoliths represent the quantitative characteristics of grassland ecosystems. These are important scientific issues in the field of phytolith analysis, which require further research on the modern phytoliths of grassland ecosystems. In order to explore the potential of phytolith analysis in the temperate grassland region, we have for the first time established a modern soil phytolith reference dataset by exploring the relationships between soil phytoliths and plant species inventory data from 77 sites in the Songnen grassland in Northeast China. The results show that phytolith assemblages can clearly distinguish Leymus chinensis grassland, Stipa grassland and mesophytic herb grassland. In addition, we found that the Ic phytolith index could discriminate samples of L. chinensis grassland from the other samples, while the Iph phytolith index proved to be less reliable in the region. In addition, in terms of quantitative plant community characteristics, the soil phytoliths showed a good correspondence with plant community cover and species richness, but not with biomass and diversity index. The transfer functions established in this study can provide reliable estimates of community cover and species richness in the Songnen grassland. In addition, phytoliths are shown to have the potential to reconstruct the taxonomic abundance of the aboveground plant community. This study is a major advance in the application of phytolith analysis to the quantitative estimation of plant community characteristics, and it provides a modern phytolith reference for the quantitative reconstruction of the paleovegetation of grassland ecosystems.

Continuous aridification since the mid‐Holocene as the main cause of C3/C4 dynamics in the grasslands of northeastern China

AbstractEcological responses to past climate change as determined from palaeorecords offer insights into responses that may accompany future climate change. In arid and semi‐arid lands, the interactions between regional vegetation and climate change are not yet well understood, partly due to a lack of suitable palaeovegetation proxies that can provide accurate and continuous tracers for past vegetation dynamics. To gain a better understanding of long‐term vegetation dynamics, this study employs a multiproxy approach applied to sand‐palaeosol sediments of northeastern China's Songnen grasslands. Phytolith analyses and data on the stable carbon isotope composition (δ13C) of organic matter are used to reconstruct palaeovegetation composition, namely, the changing abundance of C3 and C4 species, whereas a geochemical weathering index (Fed/Fet ratios) tracks past East Asian summer monsoon (EASM) intensity. The phytolith assemblages and indices and δ13C of the soil indicate that C4 species' abundance has been increasing in the Songnen grasslands since the mid‐Holocene, although C3 vegetation is still dominant. Statistically significant negative correlations between the δ13C data and Fet/Fed ratios suggest that continuous weakening of the EASM since the mid‐Holocene may be responsible for the 13C‐enrichment of the sediments in the Songnen grasslands. Field vegetation surveys, modern topsoil phytoliths and δ13C calibration data indicate that the expansion of C4 species since the mid‐Holocene is mainly due to their ability to cope with aridity when growing season temperature is not undergoing a significant decrease. Future precipitation decreases in arid and semi‐arid lands should make C4 species more competitive in the grasslands of northeastern China.Highlights A continuous grassland landscape history is reconstructed from a sand‐palaeosol sequence via phytolith analysis. Multiple independent approaches were used to reconstruct past EASM intensity and palaeovegetation patterns. Phytolith and δ13C analyses indicate an increase in C4 species since the mid‐Holocene. Aridification drives the increase in C4 species within the grassland ecosystem.

Nitrogen addition increases sexual reproduction and improves seedling growth in the perennial rhizomatous grass Leymus chinensis

BackgroundThe Eurasian steppe is an important vegetation type characterized by cold, arid and nitrogen poor conditions. At the Eastern edge, including in the Songnen grassland, the vegetation is dominated by Leymus chinensis (henceforth L. chinensis) and is increasing threatened by elevated anthropogenic nitrogen deposition. L. chinensis is a perennial grass that mainly reproduces vegetatively and its sexual reproduction is limited. However, sexual reproduction plays an important role influencing colonization after large disturbances. To develop an understanding of how elevated nitrogen deposition changes the plant community structure and functioning we need a better understanding how sexual reproduction of L. chinensis changes with nitrogen enrichment. Here we report on a field experiment where we added 10 g N m− 2 yr− 1 and examined changes in seed traits, seed germination and early seedling growth.ResultsNitrogen addition increased seed production by 79%, contributing to this seed increases were a 28% increase in flowering plant density, a 40% increase in seed number per plant and a 11% increase in seed weight. Seed size increased with a 42% increase in large seeds and a 49% decrease in the smallest seed size category. Seed germination success improved by 10% for small seeds and 18% for large seeds. Combined, the increased in seed production and improved seed quality doubled the potential seed germination. Subsequent seedling above and below-ground biomass also significantly increased.ConclusionsAll aspects of L. chinensis sexual reproduction increased with nitrogen addition. Thus, L. chinensis competitive ability may increase when atmospheric nitrogen deposition increases, which may further reduce overall plant diversity in the low diversity Songnen grasslands.

Coupled influence of precipitation regimes and seedling emergence time on the reproductive strategy in Chloris virgata.

Precipitation regime and seedling emergence time both influence plant growth and reproduction. However, little attention has been given to the effects of these combined factors on the reproductive strategy of Chloris virgata, which is a vital species in Songnen grassland. Here, we simulated the changes in the precipitation regime and seedling emergence time to evaluate tiller traits and seed production. The results showed that tiller number behaved similarly among three precipitation regimes when sowed on 15 May (T1), while it increased significantly with precipitation regimes when sowed on 15 June (T2) and 15 July (T3). Tiller number decreased significantly with the seedling emergence time under the same water supply treatment. The proportional allocation of reproductive tiller number to total tiller number was significantly higher at T3 than at T1 and T2. Seed number remained similar under different precipitation regimes at T2 and T3, whereas it was significantly lower under low precipitation than under other water levels at T1. Seed number reached the maximum values at T2 under the same level of precipitation treatment. Seed size was significantly lower under low precipitation compared to other water supply treatments and the lowest values in seed size, about 0.5 mg, occurred at T2 under all the precipitation regimes. The lowest values in spike number were under low precipitation at all seedling emergence times. Seed yield exhibited similar trends with seed size under different precipitation regimes, while the greatest gains in these values were at T1 under all the precipitation regimes. Our findings showed that simulated precipitation regimes and seedling emergence time affected the reproductive strategy of C. virgata. Typical and high precipitation, as well as early seedling emergence, will improve the seed yield and seed quality in this species.

Effect of simulated warming on the functional traits of Leymus chinensis plant in Songnen grassland.

Leymus chinensis grassland in Northeast China provides a natural laboratory for the investigation of climate change. The response of L. chinensis to experimental warming can provide insight into its regeneration behaviour and the likely composition of future communities under warmer climate. We used MSR-2420 infrared radiators to elevate temperature and examined soil organic carbon and nitrogen and soil total phosphorus and determined the growth and physiology of L. chinensis in response to manipulations of ambient condition and warming. Results showed that compared with the control, L. chinensis subjected to warming treatment showed increased soil organic carbon and soil total nitrogen, but no significant difference was observed in soil total phosphorus. Climate warming increased shoot biomass, ecosystem respiration, and ecosystem water-use efficiency and reduced net ecosystem CO2 exchange and evapotranspiration. This result implies that warming could rapidly alter carbon fluxes. The effect of warming treatment significantly increased the contents of glucose and fructose and significantly inhibited sucrose synthesis. However, the TCA cycle was enhanced when citric and malic acid contents further accumulated. The results implied that L. chinensis probably enhanced its warming adaption mechanism mainly through increasing glycolysis consumption when it was exposed to elevated temperature. These results provide an understanding of the fundamental evidence explaining the primary metabolism of L. chinensis in response to warming and suggest the future impact of the terrestrial carbon-cycle feedback on global climate change.

Run Theory and Copula-Based Drought Risk Analysis for Songnen Grassland in Northeastern China

Droughts are among the more costly natural hazards, and drought risk analysis has become urgent for the proper planning and management of water resources in grassland ecosystems. We chose Songnen grassland as a case study, used a standardized precipitation evapotranspiration index (SPEI) to model drought characteristics, employed run theory to define the drought event, and chose copula functions to construct the joint distribution for drought variables. We applied two kinds of return periods to conduct a drought risk assessment. After evaluating and comparing several distribution functions, drought severity (DS) was best described by the generalized extreme value (GEV) distribution, whereas drought duration (DD) was best fitted by gamma distribution. The root mean square error (RMSE) and Akaike Information Criterion (AIC) goodness-of-fit measures to evaluate their performance, the best-performing copula is Frank copula to model the joint dependence structure for each drought variables. The results of the secondary return periods indicate that a higher risk of droughts occurs in Keshan county, Longjiang county, Qiqiha’er city, Taonan city, and Baicheng city. Furthermore, a relatively lower risk of drought was found in Bei’an city, Mingquan county, Qinggang county, and qian’an county, and also in the Changling county and Shuangliao city. According to the calculation of the secondary return periods, which considered all possible scenarios in our study, we found that the secondary return period may be the best indicator for evaluating grassland ecosystem drought risk management.

Warming and nitrogen deposition accelerate soil phosphorus cycling in a temperate meadow ecosystem

Phosphorus (P) is an essential element for living organisms and a major limiting factor in many ecosystems. In recent years, global warming and nitrogen (N) deposition have become increasingly serious, with significant effects on the P cycle in terrestrial ecosystems. A series of studies were carried out on the soil P cycle, but how climate change affects this remains unclear. Field experiments with warming and N addition were implemented since April 2007. Infrared radiators manipulated temperature, and aqueous ammonium nitrate (10 g m–2 year–1) was added to simulate N deposition. Compared with the control, N addition reduced soil total P; warming and N addition decreased soil available P; warming, N addition and warming plus N addition decreased microbial biomass P, but increased litter P; and warming and N addition increased phosphatase activity significantly. Correlation analysis showed that soil total P, available P, microbial biomass P and phosphatase activity were positively correlated with soil temperature and water content. Soil total P was positively correlated with microbial biomass P and phosphatase activity; and available P was positively correlated with microbial biomass P but negatively correlated with litter P. The results showed that warming and N deposition accelerated the soil P cycle by changing soil physical and chemical properties and soil biological activities (microbial and phosphatase activities). However, N addition reduced the capacity of microbes to fix P and reduced microbial biomass P, resulting in losses to the soil P pool, further aggravating P limitation in the Songnen Grassland ecosystem.

Characteristics of soil micro-and macro-arthropods and soil properties in Songnen Grassland of China during different periods of freeze–thaw season

ABSTRACTSoil properties have an important influence on soil fauna in the grassland ecosystem. However, the relationship between the structural characteristics of soil fauna and properties in the grassland ecosystem in freeze–thaw season remains unclear. Hence, the feature of soil arthropods and properties in Songnen Grassland of China were investigated in fall–winter alternating (T1), completely frozen (T2) and winter–spring alternating periods (T3) during freeze–thaw season in three years. Results showed slight differences in the community composition of soil animals with Oribatida, Prostigmata and Mesostigmata as co-dominant groups in all sampling periods. The total number of individuals of soil arthropod at low temperature was low. The pH value, electrical conductivity (EC), and moisture content had the same order of T3 > T2 > T1. The activities of invertase and urease increased with increasing soil temperature, whereas protease activity had no relationship with soil temperature, soil moisture, EC and soil organic matter (SOM), activities of protease and urease were principal factors affecting individual abundance of soil animals. The sequence of their effect degrees was moisture content > EC > SOM > protease activity > urease activity. The changes in the quantitative characteristics of soil animals were related to soil properties. Therefore, soil properties can affect the structural characteristics of soil arthropod in the Songnen Grassland of China in the freeze–thaw season.

Diversity and specificity of arbuscular mycorrhizal fungi in the rhizosphere of six plants in the Songnen grassland, China

ABSTRACTArbuscular mycorrhizal fungi (AMF) are widely distributed in grassland ecosystems. However, the diversity and specificity of AMF in the rhizosphere of grassland plants remain largely unexplored. The species and quantitative characteristics of AMF in the rhizosphere soil of 6 common plant species in the Songnen grassland in Northeast China were investigated by using wet sieving and morphological identification methods. The results showed that there were 24 AMF species belonging to 6 genera in the rhizosphere of the 6 plant species. Glomus claroideum and Glomus melanosporum were the common dominant AMF species for most plants. The subdominant species and companion species of the AMF community were different. The diversity index, spore density and infection rate of AMF were significantly different among the rhizosphere of the different plants (p < 0.05). Similarity coefficients of AMF composition varied greatly among plant species. Therefore, the AMF species showed a certain level of host specificity, and this result partially supported the singular hypothesis. Differences in plant species and soil microenvironment were the main reasons explaining the specificity of the AMF in the rhizospheres of grassland plant species.

Warming limits daytime but not nighttime activity of epigeic microarthropods in Songnen grasslands

Both climate warming and nitrogen addition have the potential to affect soil fauna. The effect of these changes on the diel variation of soil microarthropods motility remains unknown. In northeastern China, in temperate grasslands with limited moisture and nutrients, epigeic microarthropods were captured by pitfall traps and activity-density and species richness of epigeic Collembola and Oribatida were calculated after four years of simulated environmental change by warming and/or nitrogen addition both in daytime and nighttime. Warming with or without nitrogen addition reduced the total activity-density of epigeic Collembola. However, this effect was pronounced only in daytime, but not nighttime. Epigeic Oribatida was not impacted by warming treatment. Nitrogen addition had no effect on epigeic Oribatida and Collembola. Our results indicate that warming has a pronounced, adverse effect on epigeic Collembola in water-limited temperate grassland and that the effect was microarthropod taxon-specific. We conclude that warming could limit the activity-density of epigeic microarthropods' species that are not resistant to water deficiency and these alterations would not be alleviated by concurrent nitrogen deposition.

Metabolic response and correlations between ions and metabolites in Phragmites communis under long-term salinity toxicity

Phragmites communis has a long history in Songnen grassland of China and has a series of biological, ecological as well as genetic characteristics contributing to its adaptation to the specific local climatic and edaphic conditions. The aim of the present study was to investigate the ions balance and their relationship to metabolites in P. communis under three salinity stress conditions. Results showed that the contents of Na+, Cl−, and SO42− significantly increased in P. communis leaves, while K+, Mg2+, and Mn2+ decreased. Moreover, Na+ and Cl− had significant negative correlations with metabolites involved in the tricarboxylic acid cycle (TCA cycle), and significant positive correlations with glycolysis. The metabolite results showed that high contents of sugars and proline played important roles in developing salinity tolerance, indicating that glycolysis and proline biosynthesis were enhanced; however, this consumes large amounts of energy and likely caused the TCA cycle to be inhibited. The results suggested that P. communis might enhance its salinity tolerance mainly through increased glycolysis and energy consumption. In addition, restricting Na+ accumulation and increasing of Cl−, and rearrangement of metabolite production in P. communis tissues are possible causes of salinity tolerance. Therefore, salinity caused systems alterations in widespread metabolic networks involving TCA cycle, glycolysis and proline biosynthesis. These findings provided new insights for the P. communis metabolic adaptation to salinity and demonstrated the ions balance and metabolites in P. communis are possibly attributable to development of salinity tolerance.

Nitrogen addition interacted with salinity-alkalinity to modify plant diversity, microbial PLFAs and soil coupled elements: A 5-year experiment

Atmospheric nitrogen (N) deposition has greatly increased in the past 50 years due to fossil fuel combustion and the use of synthetic fertilizers. Because N is the limiting nutrient for plant growth in terrestrial ecosystems, atmospheric deposition of N has the potential to increase biomass production, reduce species richness, inhibit microbial growth, and result in soil acidification. However, few studies have evaluated whether and how N inputs affect plant species diversity and the interactions among soil elements in saline-alkaline grassland ecosystems. The Songnen grassland of China is characterized by heterogeneous patches of saline-alkaline soils. A 5-year nitrogen deposition experiment was conducted in two contrasting saline-alkaline communities. Results showed that increasing N input increased the plant biomass production and reduced plant species richness in the low saline-alkaline stress community, but salinity-alkalinity was able to alleviate the negative effects of nitrogen input on community diversity. Plant communities were more sensitive to N addition than microbial communities. N addition induced a shift from N to P as the limiting factor in temperate grassland ecosystems. In particular, salinity-alkalinity in the high stress community increased the conversion rate from N to P limitation. These results highlight the importance of the integrative study of the plant-soil-microbe system in ecosystems under global N enrichment. Our study emphasizes the need for future research to understand the mechanisms that control biogeochemical cycling and maintain ecosystem stability under the combined conditions of N addition and other stress factors, such as saline-alkalinity.

Phenotypic plasticity of four Chenopodiaceae species with contrasting saline-sodic tolerance in response to increased salinity-sodicity.

It is unknown whether phenotypic plasticity in fitness‐related traits is associated with salinity–sodicity tolerance. This study compared growth and allocation phenotypic plasticity in two species with low salinity–sodicity tolerance (Chenopodium acuminatum and C. stenophyllum) and two species with high salinity–sodicity tolerance (Suaeda glauca and S. salsa) in a pot experiment in the Songnen grassland, China. While the species with low tolerance had higher growth and allocation plasticity than the highly tolerant species, the highly tolerant species only adjusted their growth traits and maintained higher fitness (e.g., plant height and total biomass) in response to increased soil salinity–sodicity, with low biomass allocation plasticity. Most plasticity is “apparent” plasticity (ontogenetic change), and only a few traits, for example, plant height:stem diameter ratio and root:shoot biomass ratio, represent “real” plasticity (real change in response to the environment). Our results show that phenotypic plasticity was negatively correlated with saline–sodic tolerance and could be used as an index of species sensitivity to soil salinity–sodicity.

Response of soil nutrients and stoichiometric ratios to short-term land use conversions in a salt-affected region, northeastern China

Soil organic carbon (SOC), nitrogen (N), phosphorus (P) and potassium (K) are essential soil nutrients needed for plant growth and their stoichiometric ratios are important indicators of the elemental balance in soil ecosystem. However, changes in contents and stoichiometric ratios of SOC, N, P and K under different land uses in salt-affected region remain poorly understand. Soils from five land use treatments, including corn cropland (CL), alfalfa perennial forage (AF), monoculture Lyemus chinensis grassland (AG), monoculture Lyemus chinensis grassland for hay (AG + M) and successional regrowth grassland (RG) were used to assess the influences of land use conversion on the contents and stoichiometric ratios of SOC, N, P and K in a salt-affected region, northeastern China. Results showed that conversion of cropland to forage and grasslands increased by 40.42%, 17.66%, 15.71% and 11.5% for SOC, total N, total P and total K contents in 0–20 cm depth, while the effects of land use conversion on these soil nutrients in 20–50 cm depth were very limiting. Similar with these soil nutrients, the influences of land use conversion on soil stoichiometric ratios were higher in topsoil (0–20 cm) than that in the subsoil (20–50 cm), and re-vegetation increased most of the soil stoichiometric ratios due to the increase of soil nutrient contents. The average C:N, C:P and N:P ratios in the study site were 7.4, 21.3 and 3.0, respectively, in 0–20 cm depth, which were relatively lower than the average values in China and global. The C:K, N:K and P:K ratios were relatively consistent in Songnen grassland, which were 2.0, 0.3 and 0.1, respectively. These findings suggested that land-use type is a major factor that affect the contents and stoichiometric ratios of soil nutrients, and re-vegetation on the cropland improves the soil nutrients in northeastern China.

The clonal grass Leymus chinensis overcomes salt stress by over-compensatory growth of individual ramets

Soil salinisation and overgrazing are two important factors limiting plant growth in the Songnen Grassland, Northeast China. Leymus chinensis, a dominant rhizomatous grass, resists grazing and tolerates saline–alkali stress. However, its adaptive mechanisms to the dual effects of grazing and saline–alkali stress remain largely unknown. A two-factorial field experiment was conducted in two consecutive years in the natural L. chinensis community, combining the addition of mixed saline–alkali solution (NaCl:NaHCO3:Na2CO3 1:1:1, amount 559.13 g m–2 year–1) with clipping (removal of 60% of aboveground biomass, AGB). Saline–alkali addition significantly increased AGB and total biomass in the no clipping but not in the clipping treatment. Irrespective of clipping, ramet density was significantly decreased, and individual ramet biomass was significantly increased under salt stress. The significant increase in AGB was due to a high K+:Na+ ratio, high water-use efficiency, and an increase in leaf area index and net photosynthesis rate of individual ramets under salt–alkali stress. Clipping significantly decreased AGB and total biomass regardless of saline–alkali addition, possibly because of decreased sugar content of rhizomes. Saline–alkali and clipping had an interactive effect on AGB and total biomass of L. chinensis. The significant reduction in AGB and total biomass were mainly caused by reduced proline and water-soluble carbohydrate content under dual stress. A modified and simplified graphic model of the limiting resource model was proposed based on our results. Leymus chinensis can grow well under saline–alkali stress via ramet biomass compensation, in which the significant decrease in ramet density is compensated by the significant increase in individual ramet biomass. Ramet compensation and clonal integration were identified to be main mechanisms of herbivory and saline–alkali tolerance.

Phenology-Based Residual Trend Analysis of MODIS-NDVI Time Series for Assessing Human-Induced Land Degradation.

Land degradation is a widespread environmental issue and an important factor in limiting sustainability. In this study, we aimed to improve the accuracy of monitoring human-induced land degradation by using phenological signal detection and residual trend analysis (RESTREND). We proposed an improved model for assessing land degradation named phenology-based RESTREND (P-RESTREND). This method quantifies the influence of precipitation on normalized difference vegetation index (NDVI) variation by using the bivariate linear regression between NDVI and precipitation in pre-growing season and growing season. The performances of RESTREND and P-RESTREND for discriminating land degradation caused by climate and human activities were compared based on vegetation-precipitation relationship. The test area is in Western Songnen Plain, Northeast China. It is a typical region with a large area of degraded drylands. The MODIS 8-day composite reflectance product and daily precipitation data during 2000–2015 were used. Our results showed that P-RESTREND was more effective in distinguishing different drivers of land degradation than the RESTREND. Degraded areas in the Songnen grasslands can be effectively detected by P-RESTREND. Therefore, this modified model can be regarded as a practical method for assessing human-induced land degradation.

Tissue physiological metabolic adaptability in young and old leaves of reed (Phragmites communis) in Songnen grassland

Common reed (Phragmites communis) is widely distributed as the dominant plant species in the Songnen Plain of China. The aim of this study was to investigate different physiological adaptive mechanisms to salinity tolerance between young and old leaves. The profiles of 68 metabolites were measured and studied in reed leaves by gas chromatography-mass spectrometer. The nitrogen, carbon, and pigment contents showed stronger growth inhibition for older leaves with salinity stress. In young leaves, high K+ contents not only promoted cell growth, but also prevented influx of superfluous Na+ ions in cells; the Ca2+ accumulation in old leaves implied that Ca2+ triggered the SOS-Na+ exclusion system and reduced Na+ toxicity. Thus, the mechanism of enhanced tolerance differed between young and old leaves. The metabolite results indicated that the young and old leaves had different mechanisms of osmotic regulation; sugars/polyols and amino acids played important roles in developing salinity tolerance in young leaves but high contents of fatty acids were important for old leaves. These results implied dramatically enhanced sugars and amino acid synthesis but inhibited energy metabolism in young leaves. In contrast, fatty acid synthesis was enhanced in old leaves. The results extended our understanding of the differences in physiological metabolism in adaptive to the salt-alkalization of soil in Songnen grassland between young and old leaves of reeds.

Plant litter effects inversely on air and soil temperatures in Leymus chinensis grassland, Northeast China

Plant litter is an important component of grassland ecosystem, and has significantly reduced by human activities. These changes in natural systems cause variations in air and soil temperatures. This study described an experiment that different amounts of plant litter effect on air and soil temperatures in Songnen grassland, Northeast China. Plant litter reduction resulted in decreasing air temperature at 10 cm above surface and increasing soil temperature at 20 cm below soil surface in the growing season. The mean, maximum, and amplitude of air temperatures at 10 cm above soil surface of plots with plant litter were significantly higher than plots without litter. The mean, extreme, and amplitude of soil temperatures at 20 cm depth increased significantly as plant litter decreased in the growing season. Temperature differences between air and soil temperatures decreased significantly as plant litter decreased in the growing season. The results show that the soil and air temperatures respond differently to plant litter, and the changing temperature difference between air and soil temperatures has the potential to produce complex effects on grassland ecosystem.

Short-term land use conversions influence the profile distribution of soil salinity and sodicity in northeastern China

Soil salinity and sodicity are important factors leading to soil degradation. Understanding the changes in soil salinity and sodicity is important to maintain soil quality and health and for sustainable land use. The main objective of this study was to investigate the short-term influences of land uses on soil salinity and sodicity in northeastern China. The eight dominant soil soluble ions (sodium, potassium, calcium, magnesium, carbonate, bicarbonate, chloride and sulfate), pH, electrical conductivity (EC), total soluble salt content and sodium adsorption ratio (SAR) were determined at 0–50 cm depth under five land uses consisting of corn (Zea mays L.) cropland (Corn), alfalfa (Medicago sativa L.) forage land (Alfalfa), Leymus chinensis (Trin.) Tzvelev grassland (AG), Leymus chinensis (Trin.) Tzvelev grassland for mowing (AG + M) and restored grassland (RG). The results showed that sodium was the predominant cation in Songnen grassland, while the contents of carbonate, bicarbonate, chloride and sulfate had slight differences in the 0–50 cm depth. Compared with soil salinity, soil sodicity was a more serious problem due to the higher soil pH and SAR. EC and SAR were more sensitive to land use conversions due to the higher discrimination ability and value ranges by different land use treatments. Under AG + M, AG and RG treatment, soil EC (20.59%, 23.03% and 19.61%, respectively) and SAR (42.46%, 39.66% and 37.62%, respectively) were lower in the 0–10 cm depth, while soil EC (36.03%, 25.75% and 17.40%, respectively) and SAR (30.86%, 20.18% and 23.09%, respectively) in the 10–50 cm depth were higher than those under Corn. Soil EC and SAR under Alfalfa in the 0–50 cm depth were 4.69% and 1.97% lower than that under Corn, demonstrating that alfalfa planting was better than other land uses to improve soil salinity and sodicity in northeastern China. These results suggested that soil sodicity was a very serious problem in the Songnen grassland, and soil salinity and sodicity can be expected to gradually decrease with the conversion of cropland to grasslands in semi-arid agroecosystems.

Effect of Autumn Cutting Date on Regrowth, Turning Green, Yield and Quality of &amp;lt;i&amp;gt;Leymus chinensis&amp;lt;/i&amp;gt; Grassland in Songnen Plain, Northeast China

A field study was carried out to quantify the effects of different cutting time on Leymus chinensis population and community characteristic in Songnen grassland of northeast China from August, 2006 to August, 2007. The treatment interval was ten days with the remaining stubble height of 5 cm from 15th August to 4th October in 2006. Results showed that the L. chinensis made up about 90% of the dry matter of aboveground biomass, and there were about ten species in the L. chinensis community in the 1 m × 1 m plot. The L. chinensis and Kalimeris integrifolia regrew rapidly after autumn cutting in 2016. In the second year, the coefficients of the community similarity were high between the different cutting time treatments. The density, height, aboveground biomass, stem/leaf ratio, and crude protein of L. chinensis were not significant at the turning green stage and harvesting stage, the companion species of L. chinensis community contained higher crude protein, but the crude protein of the plant community were not significant. Thus, considering the weather factor for haymaking, the harvest dates should be concentrated from the middle ten days of August to the first ten days in September.