Effects of biogas slurry application rates on soil physical and chemical properties and carbon storage of bamboo forest

Abstract: In order to solve the problem of resource utilization of biogas slurry and the reduction of chemical fertilizers, in this study, we worked on the effects of combined application of different biogas slurry dosages and chemical fertilizers (all treatments receiving the same amount of NPK application) on soil physical and chemical properties and carbon sink enhancement of bamboo forest. The result showed that the solo application of chemical fertilizers (NPK treatment) or combined application of biogas slurry and chemical fertilizers (N1P1K1Z1-total nitrogen in biogas slurry replaced 25% of the total application amount of chemical fertilizer, N2P2K2Z2-total nitrogen in biogas slurry replaced50% of the total application amount of chemical fertilizer, N3P3K3Z3-total nitrogen in biogas slurry replaced 75% of the total application amount of chemical fertilizer) had no obvious effects on the soil pH values and the contents of soil organic matter, total nitrogen, total phosphorus and total potassium (P>0.05), while the combined application of biogas slurry and chemical fertilizers (especially N3P3K3Z3 treatment -high dosage biogas slurry) significantly increased the contents of soil alkali hydrolyzable nitrogen, available phosphorus, and available potassium (P<0.05). Compared to the CK treatment, solo application of chemical fertilizers slightly decreased the proportion of actinomycetes (P>0.05), while the combined application of biogas slurry and chemical fertilizers significantly decreased the proportion of actinomycetes (P<0.05), and significantly increased the proportions of bacterial and fungi, respectively (P<0.05). Relative to the CK treatment, NPK treatment significantly increased the ratio of bacteria to fungi by 24.51% (P<0.05), while the combined application of biogas slurry and chemical fertilizers decreased the ratio of bacteria to fungi to different degrees with no significant difference (P>0.05). Solo application of chemical fertilizers had no significant effects on the contents of soil microbial biomass carbon and nitrogen of bamboo forest (P>0.05), but the combined application of biogas slurry and chemical fertilizers significantly increased the contents of soil microbial biomass carbon and nitrogen by 19.3%-62.0% and 52.7%-147.3%, respectively (P<0.05). Compared to the CK treatment, solo chemical fertilizers or combined application of biogas slurry and chemical fertilizers significantly decreased the ratio of soil microbial biomass carbon to microbial biomass nitrogen (P<0.05). The solo application of chemical fertilizers had no significant effect on invertase activity (P>0.05), but the combined application of biogas slurry and chemical fertilizers significantly increased the invertase activity by 46.2%-138.5% (P<0.05). The solo application of chemical fertilizers or combined application of biogas slurry and chemical fertilizers significantly improved urease activity by 7 to 14.5 times compared to the CK (P<0.05). Application of fertilizers significantly increased the acid phosphatase activity in the NPK and N1P1K1Z1 treatments compared to the CK, respectively (P<0.05), but no significant difference was found among CK, N2P2K2Z2 and N3P3K3Z3 treatments. There were no significant (P>0.05) difference among the CK, NPK, N1P1K1Z1 and N2P2K2Z2 treatments of catalase activity. In summary, the combined application of biogas slurry and chemical fertilizers could significantly increase soil available nutrient contents, the proportions of bacterial and fungi, soil microbial biomass carbon and nitrogen, and the activity of invertase, urease and catalase. In all fertilization treatments, the increase of above indicators under N3P3K3Z3 treatment was the most obvious. In addition, all fertilization treatments significantly increased the mean diameter at breast height of one-year bamboo, two-year bamboo and bamboo stand by 38.9%-61.7%, 11.8%-18.8%, and 13.1%-24.4%, respectively (P<0.05). In addition, all fertilization treatments significantly increased carbon storage of one-year bamboo and bamboo stand by 41.5%-179.0% and 26.3%-39.0%, respectively (P<0.05). For all fertilization treatments, the increase of mean diameter at breast height and carbon storage of one-year bamboo under N3P3K3Z3 treatment was the most obvious. It could be seen that the combined application of biogas slurry and chemical fertilizers could increase soil fertility, promote the growth of bamboo (especially newly grown bamboo) and increase carbon sequestration of bamboo.