Abstract: It is not clear how long-term natural restoration affects soil aggregate distribution and C∶N∶P ecological stoichiometric characteristics in red soil. Soil samples (0-15 cm depth) were taken from two treatments, conventional farming (CF) and natural restoration (NR), in a 27-year field experiment in Hunan Red Soil Experimental Station. The aim of this study is to investigate the response of soil aggregate distribution and the contents and stoichiometric ratios of soil organic C (SOC), total N (TN) and total P (TP) to long-term natural restoration. Compared with CF, (1) The total soil waterstable macroaggregates (>0.25 mm) in NR significantly increased, and the proportion of microaggregates (0.053-0.25 mm) and mineral particles(<0.053 mm) significantly decreased; (2) The contents of SOC and TN in bulk soil of NR significantly increased. While the SOC content significantly increased in each aggregate size class, TN content in macroaggregates (>2 and 0.25-2 mm) and microaggregates (0.053-0.25 mm) significantly increased. In contrast, TP content significantly decreased in bulk soil and each aggregatesize fraction. The 0.25-2 mm aggregate size class contributed most of nutrients in the bulk soil. (3) The ratios of C∶N, C∶P and N∶P in bulk soil of NR significantly increased. The C∶N ratio of the four aggregates remained constant, while C∶P and N∶P showed greater variability and significantly increased in each aggregate size class. In conclusion, long-term natural restoration enhances soil aggregation effect, which results in more concentrated C, N and P in macroaggregates. While natural restoration can improve soil structure and aggregate quality, attention should be paid to the exacerbated phosphorus limitation in future.

Key words: long-term fertilization, returning straw to field, lignin, accumulation characteristics, soil enzyme activity.