Effects of Afforestation Restoration on Soil Potential N2O Emission and Denitrifying Bacteria After Farmland Abandonment in the Chinese Loess Plateau

Denitrification is a critical component of soil nitrogen (N) cycling, including its role in the production and loss of nitrous oxide (N2O) from the soil system. However, restoration effects on the contribution of denitrification to soil N2O emissions, the abundance and diversity of denitrifying bacteria, and relationships among N2O emissions, soil properties, and denitrifying bacterial community composition remains poorly known. This is particularly true for fragile semiarid ecosystems. In order to address this knowledge gap, we utilized 42-year chronosequence of Robinia pseudoacacia plantations in the Chinese hilly gullied Loess Plateau. Soil potential N2O emission rates were measured using anaerobic incubation experiments. Quantitative polymerase chain reaction (Q-PCR) and Illumina MiSeq high-throughput sequencing were used to reveal the abundance and community composition of denitrifying bacteria. In this study, the afforestation practices following farmland abandonment had a strong negative effect on soil potential N2O emission rates during the first 33 years. However, potential N2O emission rates steadily increased in 42 years of restoration, leading to enhanced potential risk of greenhouse gas emissions. Furthermore, active afforestation increased the abundance of denitrifying functional genes, and enhanced microbial biomass. Actinobacteria and Proteobacteria were the dominant denitrifying bacterial phyla in the 0 to 33-years old sites, while the 42-years sites were dominated by Planctomycetes and Actinobacteria, implying that the restoration performed at these sites promoted soil microbial succession. Finally, correlation analyses revealed that soil organic carbon concentrations had the strongest relationship with potential N2O emission rates, followed by the abundance of the nosZ functional gene, bulk density, and the abundance of Bradyrhizobium and Variovorax across restoration stages. Taken together, our data suggest above-ground restoration of plant communities results in microbial community succession, improved soil quality, and significantly altered N2O emissions

Erratum: Yu, W.; Jiao, J. Sustainability of Abandoned Slopes in the Hill and Gully Loess Plateau Region Considering Deep Soil Water. <i>Sustainability</i> 2018, <i>10</i>, 2287

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Sustainability of Abandoned Slopes in the Hill and Gully Loess Plateau Region Considering Deep Soil Water

Soil desiccation of the deep soil layer is considered one of the main limiting factors to achieving sustainable development of ecosystems in the hill and gully Loess Plateau region. In this study, slope croplands were selected as the control, and deep soil water was studied on abandoned slopes, including natural abandoned slopes, Robinia pseudoacacia plantations, and Caragana korshinskii plantations. Then, we explored deep soil water characteristics of different vegetation types and slope aspects and the variation tendencies of deep soil water at different recovery stages. The results showed that there were no significant differences in deep soil water content between sunny and shady slopes, and thus, slope aspect was not the key impact factor affecting deep soil water. Deep soil water content on R. pseudoacacia plantations and C. korshinskii plantations was lower than that on natural abandoned slopes; there were no significant differences in soil water content between the natural abandoned slopes and slope croplands. Soil desiccation did not exist on natural abandoned slopes; thus, natural vegetation restoration is an appropriate way to achieve a sustainable ecosystem with respect to deep soil water. In contrast, soil desiccation intensified until it was difficult for vegetation to obtain available water in the deep soil layer on the plantations; soil desiccation began to appear at the 11–20-year stage, and it became increasingly severe until the deep soil water was close to the wilting coefficient at the ≥30-year stage on R. pseudoacacia plantations. Deep soil water was rapidly consumed, and soil desiccation began to appear at the 1–10-year stage and then was close to the wilting coefficient in the later stages on C. korshinskii plantations. According to the results, the plantations needed to be managed in a timely manner to prevent or reduce soil desiccation

Seed removal on loess slopes in relation to runoff and sediment yield

Overland flow and sediment transport can carry away seeds at the soil surface and in the soil, cause a secondary seed dispersal event, lead to seed redistribution, and influence the spatial distribution of seedling renewal, which plays an important role in vegetation restoration and succession. The objectives of this study were to investigate the process of seed loss on loess slopes and its relationship to the yield of runoff and sediment, the effects of rainfall intensity, slope gradient and seed morphology on seed removal. Rainfall simulation experiments were carried out in 1 m(2) plots on 10 degrees, 15 degrees, 20 degrees and 25 degrees loess slopes for a 60-minute duration with intensities of 50 mm/h, 100 mm/h and 150 mm/h, respectively. A mixture of 75 seeds from 16 species on the Chinese hilly-gullied Loess Plateau had been placed in these plots, and we measured the number of seeds lost, the distance seeds displaced, the runoff rate and amount, and soil loss rate and amount. Results showed that the accumulated seed loss rates in the rainfall process were closely related to the corresponding sediment yield, and even more closely related to the runoff amount. The seed removal obviously increased with rainfall intensity but did not obviously change with slope gradient, and results varied among species. At 50 mm/h rainfall, there was almost no seed loss on the four slopes, but 30-45% of the seeds moved from their original position. However, 79.5% and 86.4% of the seeds were eroded at 100 mm/h and 150 mm/h, respectively. Of these, 46.9% and 20.4% of the seeds were displaced, and 32.6% and 66.0% of the seeds were lost. Total seed removal was also affected by the seed amount and position on slopes, species composition of the seeds, and slope length. It was suggested that seed removal during water erosion events can affect seed redistribution and, consequently, species composition and vegetation spatial distribution. (C) 2010 Elsevier B.V. All rights reserved

Influence of Afforestation on the Species Diversity of the Soil Seed Bank and Understory Vegetation in the Hill-Gullied Loess Plateau, China

The Chinese Loess Plateau region has long been suffering from serious soil erosion. Thus, large-scale afforestation has continued during the past decades in order to control soil erosion. Afforestation can dramatically alter nutrient cycles, affect soil-carbon storage, and change hydrology. However, it is unknown how afforestation influences species diversity of the soil seed bank and understory vegetation compared with spontaneous restoration of abandoned land. Forest land with trees planted 30 years ago, abandoned slope land restored spontaneously for 30 years, and the corresponding slopes with remnant natural vegetation were selected as sampling sites. The species richness both in the soil seed bank and vegetation was significantly higher on the afforested slope compared to the spontaneously restored abandoned land. The species similarity between the afforested slope and the remnant slope land was high both in the soil seed bank and standing vegetation compared to the abandoned land. The soil seed bank density varied from 1778 ± 187 to 3896 ± 221 seeds/m2, and more than half of it was constituted by annual and biennial species, with no significant difference among sampling habitats. However, the afforested slope had higher seed density of grass and shrub/subshrubs compared to the abandoned slope. The present study indicates that in the study region, characterized by serious soil erosion, afforestation can better facilitate vegetation succession compared to spontaneously restoration of abandoned slope land