Effects of subsoiling tillage on N/P ratios in roots, stems, leaves, and aboveground biomass in maize

Nitrogen and phosphorus in Plant organs have been two of the most important indicators of crop growth. The N/P stoichiometry in various organs has generally been selected to represent the balance of nutrients during plant growth. Alternatively, subsoiling tillage has widely been used as a better farming in the semi-arid area of Loess Plateau. Although subsoiling tillage can boost the crop yield and aboveground biomass, it remains unclear whether subsoiling tillage impacts crop N/P, or whether crop organ N/P can explain the mechanism of aboveground biomass growth. The goal of this research was to examine the mechanism of aboveground biomass production from the aspect of plant N/P. A field split plot experiment was carried out in the Loess Plateau, China, from 2016 to 2018, in order to investigate the effects of subsoiling tillage and fertilization on aboveground biomass, the N/P of roots, stems, and leaves of maize, particularly on the relationship between N/P stoichiometry and aboveground biomass. Furthermore, eight treatments were set, where tillage practices as main factors included subsoiling tillage (T1), rotary tillage (T2), plow tillage (T3), and no-tillage (T4), whereas, as secondary factors included two measures of fertilization: N0 (basic fertilizer 200 kg N/hm2) and N1 (basic fertilizer 200 kg N/hm2 + jointing stage fertilizer 100 kg N/hm2). The specific analysis was also associated with the split-plot variance, structural equation modeling, and a linear mixed-effect model. The results revealed that: 1) T1 significantly enhanced the aboveground biomass (P<0.05), where increased by 9.56 %, and 9.29% in 2016, while 4.67%, and 5.94% in 2018, compared with T3 and T4. 2) Similarly, both T1 and T2 considerably reduced the N/P of roots and leaves (P<0.05). T1 presented the greatest drop, with the N/P of roots and leaves of 19.90 and 17.74, respectively. Nevertheless, there was no significant effect of integrated fertilization and tillage practices on N/P of roots, stems, and leaves. 3) Effect of tillage practices on the root, and leaf N/P indirectly transferred 39% of the variation in aboveground biomass, according to structural equation modeling, with a total effect value of 0.24. Specifically, tillage practices indirectly affected aboveground biomass via the N/P of roots and leaves, with the effect values of 0.10 and 0.14, respectively. But there was no direct effect of tillage on the N/P of roots, stems, and leaves. Correspondingly, the nitrogen and phosphorus content of roots and leaves were two key indirect determinants to regulate the aboveground biomass. In addition, the random slope model better characterized the link between root, leaf N/P, and biomass, where the explanatory degree increased by 425% and 133%, respectively, indicating over fixed effects. More importantly, aboveground biomass was inversely linked with the N/P of roots and leaves, but there was no correlation with the N/P of stems, according to the linear mixed effect model. Consequently, T1 can be expected to boost the aboveground biomass via the lower N/P in maize leaves and roots, while the higher nitrogen and phosphorus nutrition balance. The findings can serve as a sound reference for the fertilization and the promotion of deep tillage, while greatly contribute to understanding the effects of tillage and fertilization on maize production, particularly nitrogen and phosphorus balance in farmland ecosystems.