Objectives We studied the effects of lignin-based humic acid liquid fertilizer on soil nutrient contents, soil enzyme activities, soil microorganism abundances, and banana seedling growth.

Methods  The tested soil was lateritic red soil, and the tested banana seedling was at 5-true-leaves stage. There were 6 treatments in the pot experiment, which were no fertilizer (CK), conventional liquid fertilizer (CF), humic acid liquid fertilizer (HF), LHF with ligin 10 g/L (LHF₁₀), 30 g/L (LHF₃₀), and 50 g/L (LHF₅₀), respectively. All kind of fertilizers had the same rates of nitrogen, phosphorus and potassium. The plants and soil were sampled when seedlings have been cultivated for three months. And then the indexes of plant growth and nutrient accumulation, soil physical and chemical properties, soil enzyme activities and soil microbial quantity were measured.

Results 1) Compared with CK, all fertilizer treatments significantly increased the contents of soil total N, NH₄⁺-N and NO₃^--N, available P and available K, but the total N, NH₄⁺-N and NO₃^--N content kept decreased with the increasing of lignin in LHF. Compared with CF, LHF₁₀ treatment did not significantly affect soil NH₄⁺-N, total N, available P and available K, significantly increased organic matter, while LHF₃₀ treatment significantly decreased available K and organic matter content. LHF₅₀ treatment increased available P, decreased NO₃^--N, total N and available K, and the decreases were higher than LHF₃₀ did. 2) Compared with CF, all the LHF treatments decreased urease activity, with the lowest (P<0.05) decrease in LHF₁₀. HF, LHF₁₀, and LHF₃₀ treatment significantly increased nitrate reductase activity but LHF₅₀ decreased it. LHF₃₀ significantly increased sucrase activity, whereas other LHF treatments and HF treatments did not. All the LHF treatments increased acid phosphatase activity. 3) The abundance of soil bacteria, fungi, and actinomycetes were in order of CK<CF<LHF (P<0.05). Among the four LHF treatments, bacteria abundance was in order of LHF₃₀>LHF₁₀>LHF₅₀>HF (P<0.05)_. Fungi abundance was highest in LHF₅₀ treatment, while the other three treatments did not significantly differ. The actinomycetes abundance was in order of LHF₃₀>LHF₃₀≈LHF₅₀>HF (P<0.05). 4) Compared with CF, HF treatment significantly increased root fresh weight; LHF₁₀, LHF₃₀, and LHF₅₀ increased the fresh and dry weight of both plant shoot and root. HF treatment had no effect on shoot and root NPK uptake, but LHF treatments had effects on shoot and root NPK uptake.

Conclusions Addition amount of lignin changed the effect of humic acid liquid fertilizer on soil and banana growth and nutrient uptake. Adding 10 g/L and 30 g/L of lignin in the liquid fertilizer improved soil mineral and total N content, but did not improve available P and K and organic matter content. It also increased sucrase, acid phosphatase and nitrate reductase activities, but obviously reduce urease activity. So lignin-based humic acid liquid fertilizer could markedly increase the abundance of bacteria and actinomycetes, and promote the growth and nutrient uptake of banana seedlings. Overall, the 10 g/L of lignin-based humic acid liquid fertilizer showed the most satisfactory effect in soil fertility and plant growth.