Agricultural management methods, such as cultivation or fallowing, have led to significant changes in soil fertility and hence, crop yield. Such changes may have stemmed from changes in soil microbial communities and associated biogeochemical processes. This phenomenon is particularly true in organic-poor soil in the Loess Plateau of China. In this study, we examined three existing soil management regimes as part of a 10-year field experiment and evaluated their effects on fungal and bacterial community structures by performing high-throughput 454 pyrosequencing. These management regimes were (i) fertilized winter wheat (Triticum aestivum L.) (FW), (ii) continuous natural fallow with weeds but without crop grown (NF), and (iii) continuous bare fallow without weeds or crop grown (BF). After 10 years, soil organic carbon (SOC), microbial biomass carbon (MBC), and available potassium (K) concentrations were highest in NF. Soil N behaved differently, with BF obtaining the highest nitrate nitrogen (N). Meanwhile, slight differences in total N (TN) were observed among FW, NF, and BF. Available phosphorus (P) was highest and available K was lowest in FW. Microbial communities were dominated by Ascomycota (59.1% of fungal sequences), and Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria (75.7% of bacterial sequences) in FW, NF and BF at the phylum level. Soil management regimes did not affect the fungal and bacterial richness and diversity but significantly modified their community compositions. Compared with FW, the abundances of Ascomycota (fungi phylum) and Alternaria, Gibberella, and Emericella (fungi genus) were increased by NF, whereas the values of Chaetomium, Humicola, and Cryptococcus (fungi genus) were decreased by BF. The abundances of Verrucomicrobia (bacteria phylum), and Steroidobacter (bacteria genus) were increased by NF, and Bacteroides (bacteria genus) was increased by BF. Canonical correspondence analysis showed that SOC, available P, and TN might be the key factors in community formation. Therefore, the decadal absence of plants (BF) affected soil fertility by increased available K and nitrate N, whileas natural fallow (NF) affected soil fertility by increased SOC, available K, and MBC, and they all changed fungal and bacterial community compositions.

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2016): Effects of fallow or planting wheat (Triticum aestivum L.) and fertilizing P or fertilizing P and N practices on soil carbon and nitrogen in a low-organic-matter soil, Soil Science and Plant Nutrition, ABSTRACTSoil carbon (C) and nitrogen (N) are important for maintaining soil fertility, and they are considerably affected by soil use and management. In the present study, we conducted an 8-year field experiment on loessial dryland soil (Eum-Orthic Anthrosol, Food and Agriculture Organization of the United Nations (FAO)) in the southern Loess Plateau, China. We tested four soil management regimes-i.e., winter wheat (Triticum aestivum L.) cultivation with phosphorus (P) fertilization (WP), winter wheat cultivation with N and P fertilization (WNP), natural fallow (NF) and bare fallow (BF)-to evaluate their effects on soil C and N fractions. After 8 years, compared with the WNP treatment, the total soil organic nitrogen (SON) in the WP treatment decreased by 14.6% and 36.8%, and microbial biomass nitrogen (MBN) by 35.6% and 61.1%, at 0-20 and 20-40 cm soil depths, respectively. The soil heavy fraction nitrogen (HFN) and light fraction nitrogen (LFN) in the WP treatment also decreased by 36.6% and 39.4%, respectively. Furthermore, BF treatment decreased total soil organic carbon (SOC), heavy fraction carbon (HFC), LFN and MBN at both soil depths with average reductions of 43.4%. The NF treatment decreased light fraction carbon (LFC) by 17.0% at 0-20 cm soil depth, as well as MBN by 24.8% and 71.2%, and inorganic C by 29.1% and 23.8%, at 0-20 and 20-40 cm soil depths, respectively. There was no significant difference of microbial biomass C concentration among the WP, NF and BF treatments. These results confirmed that a lack of N fertilization decreased SON, BF reduced both SOC and SON, and NF decreased soil inorganic C. Therefore, the managements of a recommended rate of N fertilizer application and shortened time of bare fallow are critical for maintaining or increasing SON fraction sequestration, and natural fallow management is not a useful method for maintaining soil fertility in dryland in the Loess Plateau in China.

Although nitrate and ammonium transporter genes of wheat have been cloned, little is known about their expression characteristics. A greenhouse experiment was established to study temporal expression patterns over a growing season for four nitrate-transporter genes (TaNRT2.1, TaNRT2.2, TaNRT2.3 and TaNRT1.2) and two ammonium-transporter genes (TaAMT1.1 and TaAMT1.2) in wheat genotypes with different nitrogen (N)-uptake efficiencies. Genotypes that were N-efficient (XY107) and N-inefficient (XY6) were planted in soils that were N-limited (no N added) and N-adequate (added 0.3 g N kg–1 soil). Roots were sampled at tillering, jointing, heading and grain-filling stages, and the expression of the six genes was quantified using real-time, reverse transcription PCR (polymerase chain reaction). Results indicated that maintaining active N uptake during reproduction was the main strategy used by genotype XY107 to sustain its high N-uptake efficiencies in both N treatments. The expression of all NRT and AMT genes showed significant temporal dynamics, and generally matched the pattern of in-season N uptake of wheat plants. Several NRT or AMT genes (especially TaNRT2.1) showed greater expression at reproduction in the N-efficient genotype, XY107, than in the N-inefficient genotype, XY6, suggesting that nitrate and ammonium transporters play important roles in determining the genotypic variation of N uptake in wheat.

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