Abstract：[Background] Root system plays key role in soil and water conservation, while coupling effect of planting density and soil nutrient on the root growth of plant remains unclear. Pinus massoniana is a dominant tree species in Guizhou province, and thus selected as study material in this study. This work aims to clarify 2 questions:1) the response mechanism of P. massoniana seedlings roots to planting density and soil nutrients; and 2) the mechanism or effect of planting density-nutrient coupling among roots of P. massoniana seedlings.[Methods] The pot experiment was carried out in this study, 6 planting densities (1-6 Trees/pot) and 6 soil types (A0-A5) were set to explore the growth of P. massoniana roots, A0-A5 consisted of different ratio of layer A (leaching layer) and layer B (deposition layer), and their nutrients were in order:A0 < A1 < A2 < A3 < A4 < A5. Root morphology indexes, including root length, root surface area, root volume, and root biomass, were analyzed by Expression 10000XL 1.0 and Win RHIZOC Pro 2004b, and the data were processed using Microsoft Excel 2013 and SPSS 20.0 software.[Results] Root surface area, root length, root volume and root biomass increased with the increase of soil nutrient, and were the maximum at planting density of 1 tree/pot. The total root length in the planting density 1 tree/pot was higher than that in planting density 6 trees/pot 30.7% (A0), 64.5% (A1), 51.2% (A2), 67.3% (A3), 54.7% (A4) and 72.2% (A5). The total root volume in planting density 1 tree/pot was higher than that in 6 trees/pot by 57.2% (A0), 55.3% (A1) than, 74.1% (A2), 47.2% (A3), 51.2% (A4) and 65.2% (A5). The competition intensity of roots in different soil nutrients showed as middle nutrient > high nutrient > low nutrient, and each nutrient had the highest competition index at planting density 6 trees/pot by 0.502 (A0), 0.641 (A1), 0.737 (A2), 0.623 (A3), 0.753 (A4) and 0.629 (A5). The correlation coefficient between total root length and soil nutrients was the largest (R2=0.945). Planting density had a greater impact on root growth than soil nutrients in the coupling effect of soil nutrient and planting density. There was a binary functional relationship between the two and the root system. The soil nutrient was positively correlated with root growth, and the positive correlation coefficient ranged in 0.044 and 1.436. The planting density was negatively correlated with root growth, and the negative correlation coefficient was in (-34.757) and (-1.901).[Conclusions] Under medium and medium planting density conditions, medium soil nutrient is more suitable for root growth of P. massoniana. There are other interactions between the roots of adjacent plants, which are hypothesized as inhibitory effects, the results of plant roots inhibiting the growth of adjacent roots. Increased soil nutrients weaken competition between roots, and increased planting density increases competition among roots. The binary density function can be used to explain the planting density and nutrient conditions suitable for root growth of P. massoniana.
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2019, Vol. 17 
