Abstract: 【Objective】Paddy soils in subtropical China derived from Quaternary red clay are generally deficient in available nutrients, and fertilizers have long been identified as dominant contributors to increase of crop production. Therefore, a long-term experiment was set up in a paddy field that used to be a wasteland earlier to study the distribution of nutrients and enzyme activities in water stable aggregates under long term application of fertilizers, and the findings will provide a sound basis for rational application of fertilizers to ensure sustainable crop production. 【Method】Soil samples were collected from a 24-year long-term field experiment, which was established in 1990 in the Yingtan Red Soil Ecological Experiment Station. The experiment included four treatments: CK (without fertilization), SM (straw application plus manure), NSM (straw application plus manure and nitrogen fertilizer), NPKSM (straw application plus manure, nitrogen, phosphorus and potassium fertilizers). Undisturbed bulk soils were separated into five aggregate-size classes (＞2 mm, 1-2 mm, 0.25-1 mm, 0.053-0.25 mm and ＜0.053 mm) by wet sieving. Three soil enzymes, invertase, urease, and phosphatase, in water-stable aggregates (WSA) and total nitrogen (TN), available phosphorus (AP), and soil organic C (SOC) were determined. 【Result】Application of fertilizers to infertile paddy soil significantly increased the percentage of macro aggregates (larger than 0.25 mm), decreased the percentage of micro aggregates (smaller than 0.25 mm), and increased the mean weight diameter (MWD) of WSA, which could improve soil structure. The combined application of straw, manure and inorganic fertilizer significantly increased the soil enzyme activities in each size fraction. NSM treatment had the largest impact on invertase and urease activities. NPKSM treatment had the most significant impact on acid phosphatase activity. Compared with the control, NSM treatment increased invertase activity in five size fractions by 20.3%-396.2%; urease increased by 58.6%-372.1%. In NPKSM treatment, acid phosphatase activity in five size fractions increased by 48.9%-94.5%. Compared with the control, SOC in each size fraction of NSM treatment increased by 31.6%-65.1%. Total N increased by 19.8%-51.9%. In NPKSM treatment, available phosphorus content in each of the size fractions increased by 7.4-10 times. Aggregated boosted trees (ABT) analysis showed that the relative influence of SOC on invertase was the largest, accounting for 40.6% of the variation; the composition of soil particle had the largest relative influence on the activity of urease, accounting for 44.9% of the variation. Soil AP had the largest contribution of 41% to the variation in the activity of acid phosphatase. Nonmetric multidimensional scaling (NMDS) analysis indicated that soil aggregates within NPKSM treatment differed from those in CK, SM and NSM treatments. However, SM and NSM treatments showed similar effect on soil fertility. 【Conclusion】The combined application of straw, manure, and inorganic fertilizer significantly increased the mean weight diameter (MWD) of WSA, the contents of SOC, TN, AP, and soil enzyme activities, which improved the soil structure and biological functions.

Key words: paddy soil, straw return, water-stable aggregates, soil nutrients, soil enzymes