Straw mulching is more important than no tillage in yield improvement on the Chinese Loess Plateau

Conservation agriculture (CA) represents an efficient agro-ecosystems management for sustained crop production under the circumstances of global warming. However, the relative importance of three principles of CA has not been comprehensively assessed. Consequently, it has caused a controversy of whether no tillage (NT) is beneficial for crop yield improvement compared to conventional tillage, ranging from significant decrease to significant or insignificant increase. It remains unclear whether such a controversy is caused by statistical methodology or different acknowledge of the CA principles. This study aimed to clarify the relative importance of various tillage and straw mulching practices in crop yield, evapotraspiration (ET), and water use efficiency (WUE). Published data from experiments reporting the effects of CA on changes of crop yield and water use on the Chinese Loess Plateau were collected. A database of 49 sets of paired experiments was compiled, with tillage practices including conventional tillage without straw mulching (CT), conventional tillage with straw mulching (CTs), no tillage without straw mulching (NT), no tillage with straw mulching (NTs), reduced tillage without straw mulching (RT), reduced tillage with straw mulching (RTs), and subsoiling with straw mulching (Ss). Variable-controlling approach was used to divide these practices into reasonable groups. Paired t-tests were further conducted to systematically evaluate the relative importance of tillage reduction and straw mulching. The results showed the sole application of straw mulching as well as the combination of straw mulching and tillage reduction both resulted in a significant increase of crop yield. Tillage reduction itself including NT and RT caused a significant crop yield decline. The sole application of tillage reduction or straw mulching has neutral or negative impacts, whereas the combination of tillage reduction and straw mulching has neutral to positive on ET changes. WUE changes were similar to that of crop yield. The results illustrated that the increased crop yield reported on the Loess Plateau area is mainly due to the application of straw mulching, rather than the adoption of NT. The positive influences of CA practices on crop yield and water use tend to be higher under dry climate conditions compared to humid climate conditions. In addition, the adoption of paired t-test significantly improves the evaluation of the effects of CA compared to one way-ANOVA (LSD). Overall, the results help clarify the higher importance of straw mulching over tillage reduction and therefore challenge the centrality of NT in crop yield and WUE improvement.