Effects of the combined biogas slurry with chemical fertilizer on soil fertility and Asparagus quality in field

Soil fertility degradation can be attributed to a large amount of chemical fertilizer in the long term. Among them, the quality of asparagus shoots can depend mainly on the soil fertility in the process of asparagus planting. This study aims to investigate the effects of biogas slurry application with the chemical fertilizer on the soil fertility in the asparagus field. The quality of asparagus shoots was also evaluated in the same application of nitrogen, phosphorus, and potassium nutrients. Six treatments were set, including no fertilization (CK), conventional fertilization (NPK), the total nitrogen of biogas slurry replaced 25%, 50%, 75%, and 100% of the total nitrogen application amount of chemical fertilizer (25%NP1K1Z1, 50% NP2K2Z2, 75% NP3K3Z3, and 100% NP4K4Z4). Some influencing factors were selected, including the amount of biogas slurry application on the soil's chemical properties, enzyme activities, microbial biomass, and heavy metals in the asparagus field. The results showed that the soil organic matter contents increased by 32.7%-41.5% in the 50% NP2K2Z2 treatment, compared with the NPK (P<0.05). But there was no significant effect on the contents of soil total nitrogen, total phosphorus, and potassium (P>0.05). At the same time, the 75% NP3K3Z3 and 100% NP4K4Z4 treatments were significantly improved the contents of soil available potassium by 48.6% and 48.8%, while also significantly improved pH value (P<0.05), but there was no significant influence on the contents of soil alkali hydrolyzed nitrogen and available phosphorus (P>0.05). Comparison In comparison with the CK, the 50% NP2K2Z2 treatment was significantly reduced the activity of soil acid phosphatase (P<0.05), but there was no significant effect on the soil urease and catalase activities. By contrast, the rest treatments were enhanced the activity of soil sucrose enzyme. Compared with the NPK, the ratio of carbon and nitrogen of soil microbial biomass was significantly reduced by 31.4%-38.6% in the 50% NP2K2Z2 treatment (P<0.05). But, there was the a significant increase in the carbon and nitrogen contents of soil microbial biomass by 20.2%-61.2%, and 28.8%-162.0%, respectively (P<0.05) in the rest treatments. All treatments presented the little effect on the contents of Cu, Cr, Cd, Ni, Pb, and Zn in the topsoil of the asparagus field (P>0.05). In addition, the 50% NP2K2Z2, and 75% NP3K3Z3 treatments were improved the Zn contents and soluble protein contents in the tender stem of asparagus, compared with the NPK (P<0.05). Thus, the soil organic matter and available nutrients were improved for the soil fertility in the asparagus field. As such, the soluble protein and zinc contents were enhanced the quality of the tender stem of asparagus. Among them, the optimal treatment was achieved in the 75% NP3K3Z3 with the amount of total nitrogen of biogas slurry replaced by more than 75% of the total nitrogen application amount of chemical fertilizer.