Effects Of Alfalfa Reseeding On The Productivity And Diversity Of Degraded Grassland

More than 90 % of grasslands in China are considered degraded with depletion of biological diversity and soil erosion caused by overstocking, climate change, mining and other types of development, is common throughout the past decade. Reseeding legumes into native grassland systems effectively replaces commercial N fertilization,however, lack of competitiveness and limited persistence and seed dispersal are obstacles to overcome in utilizing legumes for restoration of highly diverse and productive grasslands.Yellow-flowered alfalfa (Medicago falcata) (YFA) is indigenous to the Middle East and Central Asia and widely distributed in northeast of China. It has been proven that YFA is more winter-hardy, more drought tolerant, and grazing tolerant than other alfalfas due to its deep-set crown and fibrous root systems. Our research combined field and greenhouse experiments to address the following three contents:(1) The effect of alfalfa reseeding, phosphorus amendments, and mowing on grassland productivity, diversity and microbial community in northeast China(2) The effect of vegetation gaps on reseeded yellow-flowered alfalfa performance and production in native grasslands(3) The effects of AMF-rhizobium symbiotic association and resource availability on the productivity, and species composition of alfalfa reseeded grasslandThe first study was initiated to examine the effects of reseeding native yellow-flowered alfalfa on Meadow Steppe in Hulunbeir, Inner Mongolia, China. Reseeding was performed successfully using a no-till planter in summer 2013. After one year of YFA establishment, Phosphorus amendments, mowing, or the combination of P amendments and mowing were randomly assigned to reseeded or non-seeded (control)plots. Forage production, plant species community composition, soil nutrients and microbial community were assessed each fall, 2014,2015, and 2016. Reseeding alfalfa increased aboveground biomass by 15 (cultivated) to 34 % (native) compared to the non-seeded control plots. Plant productivity in the reseeded plots was also increased following the combination of P amendments and mowing as compared to corresponding non-seeded plots, and the increased forage yields were mostly attributed to the biomass of reseeded alfalfa. Species diversity declined in native alfalfa reseeded plots when amended with P, presumably from intensified aboveground light competition when the proportional aboveground biomass exceeded 30 % leguminous species. In reseeded plots, the proportional biomass of the native dominant grass, Leymus chinensis, which composed the majority of aboveground biomass in non-seeded plots, significantly decreased, and mowing further reduced the dominance of this grass species by increasing the proportion of high quality forages such as alfalfa and low canopy forbs.Across all treatments, mowing resulted in significantly increased species diversity.Temporal niche overlap among species declined in mowed plots. Increased light and nutrient resource availability alleviated competitive exclusion, facilitated niche complementarities in grasslands, and ultimately resulted in higher species diversity. In addition, soil total N, SOC, MBC, MBN and microbial PLFAs were greater in alfalfa reseeded plots than non-reseeded grassland. Overall, native alfalfa reseeding and appropriate P amendments combined with mowing could be a practical way to help rehabilitate degraded grasslands while supporting both productive and diverse plant communities.The second study aimed to examine the role of microclimates of various sized vegetation gaps on seedling performance and adult plant production of YFA. Vegetation gaps with or without adjacent roots exclusion were artificially constructed in the meadow steppe of Hulunber, Inner Mongolia, China. Seedling performance, plant production, the microclimate within vegetation gaps, and soil nutrients (plant-available N, P, and K, total N concentration) were assessed at the end of each growing season from 2013 to 2015. Our results indicate light availability, topsoil temperature, and air temperature directly above the soil surfaces of each gap were significantly increased as gap size increased, while topsoil moisture and relative humidity near the gap surface were decreased in larger gaps. Small gaps (diameter <10 cm) improved seedling emergence, survival, biomass, and root nodulation, as compared with seedling performance associated with the larger gaps, presumably in response to increased shade and moisture. Additionally, large gaps (> 20 or > 40 cm) were characterized by significantly lower plant-available P, total N concentrations, plant-available K, and soil pH due to diminished litter and root decomposition and greater nutrient volatilization.However, root exclusion treatments did not improve overall seedling performance,plant production, or soil properties, as compared to corresponding microsites without roots, regardless of within each gap size. Our results suggest that reseeding YFA into grasslands where disturbance, such as light grazing, has resulted in small gaps will be more effective than in highly degraded grasslands.Another greenhouse study was conducted to examine the effects of AMF, rhizobia and their potential interactions on the alfalfa reseeded grassland. The significant interactive effects of AMF and rhizobium were detected on aboveground biomass of big blue, belowground biomass of alfalfa and plant community and alfalfa root nodule number in polyculture. Alfalfa took advantage from grassland community, the productivity and nodulation of alfalfa were greater in polycultures than monocultures.In contrast to alfalfa, the productivities of big blue and ratibida decreased in polycultures, as compared to these in monocultures. Clipped significantly increased aboveground biomass of all three species, root colonization, and species coexistence,Ratibida was the only one to benefit from the N amendments, as indicated by the increased belowground biomass and root colonization. In addition, root colonization was positively correlated with each species biomass and alfalfa root nodulation in the microcosms with all clipped and N amendments treatments. Our results demonstrated the strong interactions between AMF and rhizobia on plant productivity and community structure, suggesting that the belowground community is critical for the success of grassland restoration.