The Effect of Balanced N, P and K Fertilization on Fine Root Traits and Soil Properties in Sapindus mukorossi

To uncover the relationship between fine root traits and soil properties of trees under different fertilization treatments is important for understanding the nutrient acquisition strategies of woody oilseed plants under scientific fertilization management. An experiment of randomized complete block design was conducted in a Sapindus mukoross plantation using combinations of nitrogen (N), phosphorus (P) and potassium (K) fertilizers at different levels (Control: 0 kg·ha−2 of N, P and K; level 1: 300 kg·ha−2 of N, 250 kg·ha−2 of P, and 200 kg·ha−2 of K; level 2: 600 kg·ha−2 of N, 500 kg·ha−2 of P, and 400 kg·ha−2 of K; and level 3: 900 kg·ha−2 of N, 750 kg·ha−2 of P, and 600 kg·ha−2 of K). The response of fine root traits and soil properties of Sapindus mukoross to the application of different nitrogen (N), phosphorus (P) and potassium (K) combinations was investigated by measuring fine root traits and soil properties indices. The results showed that: (1) The soil surface is the main location of fine roots. In the 0–20 cm soil layer, the FRB, RLD, FSRL, and RAD of Sapindus mukoross were significantly increased under the fertilization treatment compared with the control. (2) In the 0–20 cm soil layer, the SOC, TN, TP, TK, AP, AK and AN of Sapindus mukoross soils were higher than those of the control group under different fertilization treatments, except for the pH. (3) Correlation analysis showed that FRB and RLD were significantly and positively correlated with SOC, AN and AP, while TP and AP were negatively correlated with FSRL and positively correlated with RAD. RDA showed that AP content in soil was the main factor which influenced the fine root traits of Sapindus mukoross. The results of the study revealed the response strategies of fine root traits and soil properties of Sapindus mukoross to N, P and K fertilizers, provided theoretical support for the scientific cultivation of woody oilseed species, and suggested scientific and rational fertilization and management strategies for Sapindus mukoross plantation forests, a species widely planted in the southern region of China.