Subsoil Tillage and Organic Fertilization Benefit Rice Root Growth and Yield by Ameliorating Soil Compaction and Fertility

Purpose The aim of this study was to clarify the effects of different tillage and organic fertilization practices on rice root growth, grain yield, and soil quality under rice–wheat rotation. Methods A three-year field experiment (2017–2020) was carried out under different practices of tillage (i.e., rotary tillage, RT; subsoil tillage, ST) with organic fertilization (i.e., rice/wheat straw, organic manure, straw plus manure). The control plots were rotary tilled and applied with chemical fertilizers only. There were three replications for each treatment. Results All the organic fertilization treatments led to variable increase (3.3%–12.7%) in the grain yield of rice compared with chemical fertilization only. ST caused an average yield increase of 3.1%–4.3% compared with RT under the same fertilization treatment. When ST plots received organic fertilization, the root length density of rice significantly increased from the booting stage compared with that in RT plots. The largest increase in root length density occurred in the subsoil (15–30 cm), which contributed to shoot N uptake. ST with organic fertilization also reduced subsoil compaction, without affecting topsoil compaction (0–15 cm). In the topsoil, organic matter, total nitrogen, available phosphorus, and available potassium contents were higher under RT than under ST, whereas the opposite trend was observed in the subsoil. Partial least squares path modeling revealed that tillage mainly reduced soil compaction and organic fertilization principally increased soil nutrients, thereby promoting root growth of rice. The influence of soil nutrients and compaction on root length density of rice grew stronger in the subsoil relative to the topsoil. Conclusions The co-application of crop straw and organic manure enhanced soil nutrient storage capacity, and combined ST alleviated subsoil compaction, promoted downward root growth, and facilitated plant nutrient uptake, which contributed to the improvement of grain yield in rice.