Fazhu Zhao

Responsiveness of soil nitrogen fractions and bacterial communities to afforestation in the Loess Hilly Region (LHR) of China.

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.

Population Structure and Spatial Pattern of Main Tree Species in Secondary Betula platyphylla Forest in Ziwuling Mountains, China

This study investigated a typical secondary Betula platyphylla forest in the Ziwuling Mountains, Loess Plateau, China. In the sample plot, the DBH (diameter at breast height) class structure of B. platyphylla was bimodal. Individuals with small and large DBH values were abundant. The DBH structures of Quercus wutaishanica and Pinus tabulaeformis were close to that of the logistic model, thus suggesting the increasing population of these species. B. platyphylla and Populus davidiana showed random spatial distributions at almost all scales. However, Q. wutaishanica and P. tabulaeformis were significantly clumped at small scales. B. platyphylla had a negative spatial relation with Q. wutaishanica at small spatial scales. P. tabulaeformis and Q. wutaishanica showed negative spatial correlations at small scales, but they had positive correlations at large scales. These results suggest that P. tabulaeformis and Q. wutaishanica shared habitat preferences at these scales. In the future, the secondary B. platyphylla forest in the Ziwuling Mountains in the Loess Plateau will probably change into a multi-species mixed forest (Quercus–Pinus mixed forest). Assisted restoration strategies must be employed to improve the regeneration dynamics of the forest in the long term.

Effect of Soil C, N and P Stoichiometry on Soil Organic C Fractions After Afforestation

Afforestation is recognized as an important driving force for soil organic C (SOC) dynamics and soil element cycling. To evaluate the relationships between soil C:N:P stoichiometry and SOC fractions, soil C:N:P stoichiometry distributions at 0–200 cm soil depths were analyzed and the contents of SOC fractions were evaluated in 9 typical land-use systems on the Loess Plateau of China. The contents of light fraction organic C, particulate organic C (> 53, 53–2 000, and > 2 000 µm), labile organic C, microbial biomass C, and dissolved organic C decreased with increasing soil depth and were higher in afforested soil than in slope cropland soil. Compared with the slope cropland, different vegetation types influenced soil C:N, C:P, and N:P ratios, especially when C:P and N:P ratios were significantly higher (P < 0.05). Moreover, SOC fractions at the 0–10 and 10–40 cm depths were particularly affected by soil C:P ratio, whereas those at the 40–100 and 100–200 cm soil depths were significantly affected (P < 0.05) by soil N:P ratio. These results indicate that changes in SOC fractions are largely driven by soil C:P and N:P ratios at different soil depths after afforestation.