Continuous Co-incorporation of Chinese Milk Vetch and Rice Straw with Reduced Chemical Fertilizer Maintains Rice Yield as a Consequence of Increased Carbon and Nitrogen Storage in Soil Aggregates

Despite increasing application of Chinese milk vetch (MV) and rice straw (RS), their combined effects on soil aggregate structure and nutrient storage are poorly understood. A field experiment was conducted to evaluate the effects of co-incorporation of MV and RS on rice yield and soil fertility. Five different fertilization regimes were adopted over a 3-year period. Chemical nitrogen (N) fertilizer was applied at a conventional rate (100%N) and a 40% reduced rate alone (60%N) or combined with MV (60%NMV), RS (60%NRS), and MV plus RS (60%NMVRS). Compared with the 100%N treatment, there were no distinct yield losses in the 60%NMV and 60%NMVRS treatments. Incorporation of organic amendments altered the composition of soil aggregates, as indicated by the increase of large (> 2 mm) and small (0.25-2 mm) macroaggregates and decrease of silt and clay (< 0.053 mm). The organic carbon and nitrogen storage were respectively decreased by 9.2% and 13.9% in the 60%N treatment, and this effect was diminished in the 60%NMVRS treatment, rather than the 60%NMV or 60%NRS treatment. The carbon and nitrogen storage in soil macroaggregates increased in response to organic amendments, and their peak values were observed in the 60%NMVRS treatment. Random forest analysis identified carbon and nitrogen storage in soil macroaggregates as the major contributors to rice yield. Continuous co-incorporation of Chinese milk vetch and rice straw maintained rice productivity with reduced nitrogen application as a consequence of improved soil aggregation (> 0.25 mm), increased carbon and nitrogen storage in macroaggregates, and enhanced nutrient availability.