Biogas digestate as a potential nitrogen source enhances soil fertility, rice nitrogen metabolism and yield

Purpose

Substituting chemical fertilizers with biogas digestate can mitigate the negative impacts of fertilizers on soil quality to promote the recycling of livestock manure. This study aims to evaluate the effects of chicken manure biogas digestate as a replacement for chemical fertilizers in rice cultivation, assessing soil nutrients dynamics, bacterial communities, nitrogen (N) metabolism enzyme activity, and rice growth.

Methods

In this study, six treatments were established: no fertilizer (CK), conventional fertilization (CF), and varying proportions of biogas digestate and other N substitution fertilizers (25 %, 50 %, 75 %, and 100 %TP). A three-year field experiment was conducted to evaluate soil nutrient levels, bacterial community abundance, N functional gene expression, rice growth conditions, and N metabolism enzyme activities using standard physical and chemical methods, high-throughput sequencing, and enzyme activity assays.

Results

Our experimental results showed that biogas digestate application promoted rice growth, while the higher biogas digestate doses increased the leaf chlorophyll SPAD values and delayed rice maturity. Compared to the CF treatment, the 75 % TP treatment significantly enhanced rice yield and economic returns. Biogas digestate increased the activities of nitrate reductase (NTR), glutamate synthase (GTS), glutamine synthetase (GLS), glutamic oxalacetic transaminase (GOT), and glutamic pyruvic transaminase (GPT) in rice leaves and grains, leading to the higher amino acid and soluble protein contents in the grains. Compared to the CK treatment, biogas digestate application increased soil pH, soil organic matter, alkaline N, available phosphorus, and available potassium by 22.14 %, 37.04 %, 39.38 %, and 39.40 %, respectively. It also enhanced the levels of nitrate N, ammonium N, and soluble organic N in soil. Furthermore, biogas digestate application significantly (p < 0.05) improved soil bacterial community diversity and stability, as well as the abundance of N metabolism functional genes.

Conclusions

Our findings suggest that substituting 75 % of chemical fertilizers with biogas digestate can serve as an optimal application rate for rice cultivation. This application in paddy field does not only reduce chemical fertilizer usage but also enhance soil nutrient content and microbial diversity. It further promotes rice growth and N metabolism enzyme activity, benefiting the increase of both rice yield and quality.