Scientia Agricultura Sinica

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Responses of Community Species Diversity and Productivity to Nitrogen and Phosphorus Addition During Restoration of Degraded Grassland

WANG HongYi, CHANG JiFang, WANG ZhengWen

Scientia Agricultura Sinica 2020, 53 (13): 2604-2613. DOI: 10.3864/j.issn.0578-1752.2020.13.009

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【Objective】 Overgrazing and mowing of grassland in north China caused serious vegetation degradation and soil nutrient deficiency. The purpose of this study was to investigate the effects of nitrogen and phosphorus fertilizers on plant community diversity and productivity in the degraded grassland. 【Method】 A four-year nitrogen and phosphorus addition experiment was conducted with two kinds of fertilizers, including calcium superphosphate and ammonium nitrate, scheduled 2 nitrogen rates (0, and 10 g N·m^-2·a^-1) and 6 phosphorus rates (0, 2, 4, 6, 8 and 10 g P·m^-2·a^-1) with interaction between the both, a total of 12 treatments, and each treatment repeated 5 times. Fertilizers were applied when grassland was turning green in late May, and the community was investigated in the period of maximum biomass in the middle of August every year. In the process of research, the community productivity, species diversity and relative biomass of four functional groups were analyzed. 【Result】 From 2014 to 2017, it was found that the species richness decreased by year, and the average species of the control treatment (neither nitrogen nor phosphorus) was 20.2, 17.1, 14.7 and 15.2, respectively. Based on plant community α diversity index (richness and Simpson dominance index) , the study showed that nitrogen addition significantly decreased the species diversity of the community, but phosphorus addition and cumulative effects of phosphorus did not affect the species diversity of the community. There was a significant difference in diversity index among years, and the interaction between nitrogen and year had a significant effect on it. The aboveground net primary productivity (ANPP) of grassland community varied significantly among years, with the highest in 2014 and the lowest in 2015, the range was from 400 g·m ^-2 to 100 g·m^-2, and which in dry years was significantly lower than that in normal precipitation years. Phosphorus addition had little effect on ANPP, while nitrogen addition significantly increased ANPP, and the effect of nitrogen and phosphorus addition together was greater than that of phosphorus alone. In the normal precipitation growing season, nitrogen was the main limiting factor of Hulunber meadow steppe. Nitrogen addition significantly affected the relative biomass of functional groups, while phosphorus addition had little effect on it, and there was a significant difference in the relative biomass of each functional group between years. Nitrogen addition alone significantly increased the relative biomass of the perennial rhizomatous grass (PR), but reduced the relative biomass of the perennial fobs (PF) and perennial bunch grass (PB). Phosphorus addition alone increased relative biomass of PB and perennial leguminous (LE), while had no effect on relative biomass of PR and PB. Combined addition of nitrogen and phosphorus significantly increased the relative biomass of PR, but significantly decreased the relative biomass of the other functional groups. In addition, the nitrogen and phosphorus combined addition partly reduced the plant community stability. 【Conclusion】 The differentiation of different functional types’ responses caused by nitrogen and phosphorus addition led to the changes of the original hierarchical level between different functional groups, especially nitrogen addition could cause a rapid increase in PR relative biomass, and thus led to the plant community structure developing towards the direction of increasing dominance of PR, and decreasing species diversity and community stability.

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Effects of Organic Fertilizer on Soil Bacterial Community Diversity in Leymus chinensis Steppe

SHANG LiRong,WAN LiQiang,LI XiangLin

Scientia Agricultura Sinica 2020, 53 (13): 2614-2624. DOI: 10.3864/j.issn.0578-1752.2020.13.010

Abstract （279）

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【Objective】 In order to provide a practical and scientific basis for improvement, restoration and reasonable use of degraded grassland, the effects of different organic fertilizers on soil nutrient status and soil bacterial diversity were revealed in the natural Leymus chinensis steppe degraded grassland in Hulunbeier. 【Method】 The field experiments were designed by single factor randomized block design, and the seven treatments were as follows: the control (ck), vermicompost 15 t·hm^-2(a1), 30 t·hm^-2(a2), 45 t·hm^-2(a3), mushroom residues 15 t·hm^-2(b1), 30 t·hm^-2(b2), 45 t·hm^-2(b3). Combined with the physical and chemical properties of soil, this study used the high-throughput sequencing technology of Miseq to analyze the effects of different organic fertilizer treatments on the diversity of soil bacterial communities, and to explore the environmental drivers of bacterial community changes under different organic fertilizer treatments. 【Result】 The results showed that organic fertilization improved the soil nutrient status and shaped the distinct bacterial communities. Compared with ck, the AP content increased significantly by 37.27% under a3 treatment. AK content under b3 and a3 treatments increased significantly by 62.99% and 40.53% compared with the control, respectively. And aboveground biomass was significantly higher than other treatments under b3 treatment (244.11 g·m^-2). Moreover, vermincompost and mushroom residues fertilizers significantly increased the richness of the bacterial community. Compared with ck, the richness index increased significantly under a1 and b2 treatments. At the phylum level, a total of 31 taxa were obtained from 21 samples. Actinobacteria, Proteobacteria, Acidobacteria, Verrucomicrobia and Chloroflexi were the dominant groups, and relative abundances account for the bacterial community 85% or more. Actinomycetes were the most abundant under a2 treatment (36.79%). Proteobacteria was higher under b1 and b3 treatments (23.29% and 22.32%, resptectively). Acidobacteria was the highest under a1 treatment (20.69%). And LEfSe showed that more bacterial taxonomic groups were detected under b3 treatment (17 clades, 1 class, 1 order, 4 families and 11 genera). In addition, AN (P=0.001), AK (P=0.005), and SOM (P=0.006) had extremely significant effects on the composition of bacterial communities in the soil (P<0.01), while TK (P=0.014) had not. The composition of soil bacterial community had a significant effect (P<0.05). It showed that AN, AK, SOM and TK were the main driving factors of bacterial community. 【Conclusion】 Organic fertilizer changed the soil bacterial community structure in Leymus chinensis steppe. Our results indicated vermicompost and mushroom residues at 45 t·hm^-2 increased available nutrient content, but also enhanced the biodiversity of soil bacterial communities in the grasslands of Leymus chinensis, which contributed to the sustainable development of grassland agro-ecosystems.

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Effects of Nitrogen Application and Clipping Height on Vegetation Productivity and Plant Community Composition of Haying Meadow Steppe

WANG KaiLi,YANG HeLong,XIAO Hong,SUN Wei,RONG YuPing

Scientia Agricultura Sinica 2020, 53 (13): 2625-2636. DOI: 10.3864/j.issn.0578-1752.2020.13.011

Abstract （187）

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【Objective】 The aim of this study was to improve soil nutrients in haying steppe, to increase grassland productivity, and to maintain sustainable utilize of grassland. 【Method】 Five nitrogen application levels (0, 10, 20, 30, and 40 g N·m^-2·a^-1) and two mowing heights (4 and 8 cm) were set in the haying field of Hulunbuir Grassland Agro-ecosystem Experiment Station with spit-plot design, which was carried out in mid-June and mid-August from 2016 to 2019, respectively, and the responses of nitrogen application and clipping height to community and plant functional group’s species richness and aboveground biomass, crucial species’ important value, dominant species’ functional traits, and soil properties were investigated. 【Result】 The results showed that nitrogen application and mowing height had no significant effect on community and plant functional group’s species richness (P<0.05). Nitrogen application significantly increased the aboveground biomass of grasses and community by 69.2%-115.3% and 36.5%-84.8%, respectively (P<0.05), but there was no significant difference within 10-40 g·m^-2·a^-1. Lower cutting height significantly reduced the aboveground biomass of grasses by 18.3% (P<0.05). Nitrogen application significantly increased and reduced the important value of Leymus chinensis and Bromus inermis, respectively (P<0.05). Lower mowing height significantly reduced the important value of Leymus chinensis, which increased the value of Bromus inermis; nitrogen application increased the important value of Leymus chinensis, which reduced the important value of Bromus inermis(P<0.05). Lower cutting height significantly increased the important value of Potentilla bifurca and Potentilla acaulis and reduced the important value of Cleistogenes squarrosa (P<0.05). Nitrogen addition significantly increased the plant height, leaf area and shoot nitrogen content of Bromus inermis and Leymus chinensis (P<0.05), but there was no significant difference within 20-40 g·m^-2·a^-1. The soil pH and soil water content decreased significantly along with the increase of nitrogen application rate, while NH₄⁺-N, NO₃^--N and inorganic nitrogen (ION) in soil increased (P<0.05). The species richness of community, grasses and nongraminous forbs was positively correlated with soil water content, while the aboveground biomass of community and grasses was negatively correlated with soil water content (P<0.05). 【Conclusion】 Short-term nitrogen application and suitable clipping height were beneficial to improve vegetation productivity and maintain community composition, and the effects of nitrogen addition were strongly dependent on water availability. It was suggested that the suitable mowing height of grassland in Hulunber haying meadow steppe was 8 cm, and the comfortable nitrogen application rate was 10-20 g·m^-2·a^-1.

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Molecular Ecological Network Analyses Revealing the Effects of Nitrogen Application on Soil Microbial Community in the Degraded Grasslands

ZHU RuiFen,LIU JieLin,WANG JianLi,HAN WeiBo,SHEN ZhongBao,XIN XiaoPing

Scientia Agricultura Sinica 2020, 53 (13): 2637-2646. DOI: 10.3864/j.issn.0578-1752.2020.13.012

Abstract （369）

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【Objective】 Nitrogen input affects the sustainability of the global grassland ecosystem. Paying attention to the soil microbial community and its molecular ecological network can provide a theoretical basis for grassland degradation restoration. 【Method】 Taking Songnen degraded Leymus chinensis grassland as the research object, the molecular ecological network of soil microbial community was constructed by applying high-throughput sequencing and random matrix network construction with and without nitrogen treatment. To explore the effects of nitrogen management on the soil microbial community structure and network in degraded Leymus chinensis grassland, the key microbial changes in the microbial network structure under the condition of nitrogen addition was studied, and the interaction between microorganisms during the process were investigated, and the conditions for external nitrogen addition key points and regularity of soil bacterial dynamic change were analyzed. 【Result】 At the level of phylum classification, there were 22 bacterial phylum in nitrogen-applied grassland and 23 without nitrogen. The 7 phylum were the dominant phylum of the nitrogen-applied and non-nitrogen-applied grasslands. Among them, Proteobacteria was the phyla containing the largest number of OTUs, accounting for about 30.46% of the total sequence. The next largest genus was about 30.15% of the total sequence. The genus Gemmatimonadetes was the third genus containing OTUs, accounting for 8.14% of the total sequence. Actinomycete accounted for about the total 6.15% of the sequence, while Chloroflexi, Bacteroidetes and Nitrospirae accounted for 17.16% of the total sequence. The relative abundances of Proteobacteria, Actinomycota and Bacteroides in soil microorganisms in nitrogen-applied grassland were significantly higher than those in non-nitrogen applied grassland soil; The relative abundances of Proteobacteria, Actinomycete and Bacteroidetes were significantly higher than the soils of nitrogen-applied grassland (P<0.01), and no significant difference was found between the nitrogen application and non-nitrogen treatment of other bacteria. The forward connection ratio, the average path length, the average clustering coefficient, and the modularity of the characterizing network were all significantly lower than the nitrogen-free treatment (P<0.001). In the molecular ecological network of soil, there were 16 modular hubs without nitrogen treatment (Zi>2.5, Pi≤0.62), and there were 6 modular hubs under nitrogen treatment, all of which belong to Acidobacteria, Gemmatimonadetes and Actinomycete. Nitrogen application led to changes in soil microbial species relationships, which in turn changed the overall soil ecological network. 【Conclusion】 Nitrogen application reduced the complexity and tightness of soil network structure of degraded grassland, and reduced the relative abundance of Acidobacteria and Chloroflexi in degraded grassland, while which improved the relative abundance of Proteobacteria, Actinomycete and Gemmatimonadetes. The number of microbial key species (OTU) in soil decreased from 16 (no nitrogen application) to 6 and there was no overlapping OTU in both soils, indicating that nitrogen application regulated key species of its community network and thus changes its molecular ecological network.

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