Abstract
In the present paper, we investigated the effects of afforestation on nitrogen fractions and microbial communities. A total of 24 soil samples were collected from farmland (FL) and three afforested lands, namely Robinia pseudoacacia L (RP), Caragana korshinskii Kom (CK), and abandoned land (AL), which have been arable for the past 40 years. Quantitative PCR and Illumina sequencing of 16S rRNA genes were used to analyze soil bacterial abundance, diversity, and composition. Additionally, soil nitrogen (N) stocks and fractions were estimated. The results showed that soil N stock, N fractions, and bacterial abundance and diversity increased following afforestation. Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla of soil bacterial compositions. Overall, soil bacterial compositions generally changed from Actinobacteria (Acidobacteria)-dominant to Proteobacteria-dominant following afforestation. Soil N fractions, especially for dissolved organic nitrogen (DON), were significantly correlated with most bacterial groups and bacterial diversity, while potential competitive interactions between Proteobacteria (order Rhizobiales) and Cyanobacteria were suggested. In contrast, nitrate nitrogen (NO3−-N) influenced soil bacterial compositions less than other N fractions. Therefore, the present study demonstrated that bacterial diversity and specific species respond to farmland-to-forest conversion and hence have the potential to affect N dynamic processes in the Loess Plateau.
Highlights
In the present paper, we investigated the effects of afforestation on nitrogen fractions and microbial communities
Compared with FL, the ammonium nitrogen (AN), dissolved organic nitrogen (DON) and microbial biomass nitrogen (MBN) contents in RP40 were more than 83.22–204.21% higher, and similar trends were found in CK40 and AL40 (115.87–156.33% and 57.49–124.26%, respectively) (Fig. 1c,e,f); the NN content was higher in AL40
The beta diversity of the soil bacterial communities was highly variable with respect to afforestation (Adonis: R2 = 0.286, P = 0.001), the soil bacterial communities between replicates in RP40 were more similar to that in CK40 than AL40 and FL, and both aggregation degrees of the sample points in AL40 were far away from that in FL (Fig. 2)
Summary
We investigated the effects of afforestation on nitrogen fractions and microbial communities. With rising concern regarding climatic consequences, afforestation has often been proposed as an effective and cost-efficient mitigation response to climate change[1,2] This widespread shift in ecosystem have strong potential to alter key ecosystem processes, the resultant alterations can affect N cycles at the ecosystem and regional scale[3,4,5], further influence the forest N stock under the process of ecological restoration[6]. Elucidating the response of soil N stocks and fractions to changes in the ecosystem, especially for below-ground ecosystems such as soil bacterial community, are essential to revealing the biogeochemical cycling process. The shift in soil bacterial communities and the soil N fractions following afforestation and the interactions among these components may in turn provide feedback to N stock and plant productivity. Little research has applied high throughput sequencing technology to assess the soil microbial diversity following afforestation, and the possibility that soil N fractions to afforestation as well as their relationship to soil bacterial communities remain unclear
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