Objectives Taihu Lake region produces large amounts of aquatic plants in both water body and intensive aquiculture system every year. To assimilate these aquatic plants, compost practice of aquatic plants is one of major measures. This study is to effectively promote the utilization of water purified plant residues by evaluating the effects of aquatic plant compost in rice–wheat system in the Taihu Lake region.

Methods A field experiment was conducted for successive four years from 2012 to 2016 in the Taihu Lake region. The experiment was composed of 7 treatments: CK (no fertilization), urea-N, and 5 levels of compost N, which were 1.0, 1.5, 2.0, 2.5 and 3.0 times of the urea-N rate and recorded as M_1.0, M_1.5, M_2.0, M_2.5 and M_3.0, respectively. The urea-N application rates for rice and wheat were N 270和225 kg/hm², respectively. The yield, N, P and K uptake of crops, and the soil nutrient contents and soil pH were investigated.

Results Compared to urea-N, both the M_1.0 and M_1.5 treatments demonstrated similar yields, M_2.0, M_2.5 and M_3.0 demonstrated yield increases of 15.4%–28.6% in the first year. Application of compost had no significant effects on rice yield in the following years of the trial. Wheat showed greater yield response to the compost treatments than rice did. Wheat yields were improved with the increase of compost application rate, and there was a significant difference among the composting treatments in the first year. M_2.5 and M_3.0 showed similar yields, and were both significantly higher than that of urea-N treatment in the following years of the trial. Compared to urea-N treatment, both rice and wheat in compost treatments had lower nitrogen uptake, but higher phosphorus and potassium uptake. After 8 growth seasons, the total N contents in soil of M_2.0, M_2.5 and M_3.0 treatments were increased by 48.7%–89.9%; the organic C contents in soil of M_1.5–M_3.0 were increased by 42.1%–104.2%; the soil available phosphorus contents were significantly increased by 4.5–17.9 times and soil readily available potassium contents were significantly increased by 3.4–11.2 times in compost treatments. Compared to CK, soil pH became lower in urea-N treatment, whereas was increased by 0.66–1.31 units in compost treatments, which was increased with the increase of compost application rate. Statistical analyses showed a significant positive correlation between crop yield and soil carbon, nitrogen and potassium contents.

Conclusions Aquatic plant compost shows priorities in keeping rice and wheat yields and soil properties, so its reasonable use will not only effectively remove the aquatic plant wastes in Taihu Lake region, but also it is a good organic nutrient source for agricultural production. Comprehensively considering yields and soil nutrient balance, the reasonable rate of compost should be 1.5 times of the equivalent nitrogen amount of urea N.