Effects of rice straw returning and nitrogen regulation on the growth of Chinese milk vetch (Astragalus sinicus L.) and soil properties

Objectives  The growth of Chinese milk vetch (Astragalus sinicus L., CMV) and soil properties under different application rates of rice straw and nitrogen (N) fertilizer were studied to provide basis for optimizing high-yield cultivation of CMV and solving the problem of resource utilization of rice straw, and to realize the reduction of N fertilizer application rate and increase rice yield in CMV-rice rotation system.

Methods  The pot experiment had 12 treatments, including two factors, i.e. straw and N fertilizer. The three straw rates were 0 (RS0), 3000 (RS1), 6000 (RS2) kg/hm², respectively; and four N fertilizer rates were 0 (N0), 45 (N45), 90 (N90), 135 (N135) kg/hm², respectively. Fresh biomass and the N, P and K accumulations of CMV shoots were measured. The soil basic physicochemical properties and contents of soil microbial biomass carbon, dissolved organic carbon, the activities of six soil enzymes involved in soil C, N and P cycling (β-glucosidase; β-cellobiosidase; β-xylosidase; N-acetyl-glucosaminidase; L-leucine aminopeptidase; phosphatase) were analyzed. Then the correlations between CMV growth and soil properties were further studied.

Results  Both application of straw and N fertilizer significantly increased the fresh biomass and NPK accumulations of CMV. Compared with N0RS0 treatment, the combined application significantly increased CMV fresh biomass by 39.2%–323.8%. When N levels were N45, N90 and N135, CMV fresh biomass and N, P, K accumulations were all the highest in RS2 treatments. The shoot N seasonal recovery efficiency of CMV in treatment N45RS1 was the highest, which had no significant difference with those of treatments of N90 and N135 levels. The shoot N seasonal recovery efficiency of CMV in treatment N45RS1 was significantly higher than those in the others. Results of partial least squares path model showed that compared with straw application, N fertilizer had stronger positive effects on fresh biomass and nutrient uptake of CMV. Application of straw and N fertilizer significantly increased soil enzyme activities. The activity of soil L-leucine aminopeptidase increased at first and then decreased with the increased amount of straw at all the four N levels. The other five soil enzyme activities were strongest at RS2 treatments when N levels were N0, N45 and N135, while that in RS1 treatment was strongest at N90 level. Results of aggregated boosted trees analysis showed that soil readily available K content and N-acetyl-glucosaminidase activity were the most important indicators, and respectively contributed 52.6% and 30.0% to the fresh biomass of CMV.

Conclusions  Appropriate application of rice straw and N fertilizer significantly increases fresh biomass of CMV, NPK accumulation in shoot and soil enzyme activities, and promoted nutrient uptake of CMV, which provides suitable environment for the growth of CMV. Straw addition can increase NPK accumulations in shoot of CMV and contents of soil available nutrients, N application also has a significant positive effect on NPK accumulation in shoot of CMV. In conclusion, according to the yield and nutrient absorption of CMV in different treatments, and together with the purpose of straw resource utilization, 6000 kg/hm² straw combined with N 45–90 kg/hm² fertilizer is the suitable ratio of the combined utilization of rice straw and N fertilizer. The specific N application rate could be adjusted according to the local soil fertility.