Effects of returning Chinese milk vetch and rice straw to the field on the nitrification potential of paddy soil

Objectives Returning green manure and rice straw to the field are important methods of improving paddy soilfertility. Nitrification is a key process in the N cycle, but its regulation by the co-utilization of green manure and rice straw is unclear.

Methods A total of eight treatments were set up in a pot experiment, including: No soil amendment as control (CK), the addition of milk Vetch (MV), rice straw (RS), milk vetch + rice straw (MV+RS), chemical N fertilizer (N), milk vetch + chemical N fertilizer (MV+N), rice straw + chemical N fertilizer (RS+N), and milk vetch + rice straw + chemical N fertilizer (MV+RS+N). The pot experiment was conducted to study the effects of co-incorporation of Chinese milk vetch (Astragalus sinicus L.) and rice straw (RS) on soil properties and nitrification under N or no N fertilization. The effects of specific bacterial inhibitors (kanamycin and spectinomycin) were used to distinguish the relative contribution of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to nitrification.

Results MV+RS+N increased the total nitrogen (TN) and soil organic matter (SOM) but decreased the pH of the alkaline paddy soil. At the tillering stage of early rice, MV+RS+N increased ammonium nitrogen and nitrate-nitrogen in the soil (P < 0.05). Before transplanting the early rice, MV+RS increased the soil nitrification potential (NP), while MV reduced NP compared to the control (P < 0.05). Moreover, N increased NP (P < 0.05) . At the jointing stage of early rice and the harvest stage of late rice, RS and MV+RS reduced NP. Before transplanting early rice, MV+RS reduced the recovered nitrification potential (RNP), the contribution of AOA to RNP (RNP_AOA), the contribution of AOB to RNP (RNP_AOB), and reduced the proportion of AOB in RNP. At the tillering stage of early rice, compared with the CK and N, MV+N increased RNP_AOB. At the jointing stage of early rice, MV+RS+N increased RNP and RNP_AOB compared with MV+RS. At the harvest stage of early rice, MV, RS, and MV+RS all reduced the RNP of paddy soil. There was a positive correlation (P < 0.05) between NP and NO₃⁻-N content at the tillering stage, NP and soil pH and NO₃⁻-N, and a negative correlation (P < 0.05) between soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN). There was a positive correlation (P<0.05) between RNP_AOA and soil pH and ammonium nitrogen.

Conclusions The co-incorporation of Chinese milk vetch and rice straw reduced soil pH, increased soil nitrification potential in the early stage of rice growth, and reduced soil nitrification potential later. Nitrification potential positively correlates with soil pH and nitrate N. RNP_AOB was dominant in the early rice’s tillering and jointing stage. The co-utilization of milk vetch and rice straw reduced RNP_AOB compared with CK. RNP_AOA was dominant in the harvest stage of early and late rice. The co-utilization of milk vetch and rice straw increased RNP_AOA compared with the single return of the former or latter to the field.