4.1 Changes in soil physicochemical properties
Our results demonstrated that the amended treatments greatly affected soil physicochemical properties. We believed that the change of each soil property was likely to be affected by a single amendment or a combination of amendments that added in our experiment. Differences in soil pH changes after addition of amendment were likely due to variability in amounts of N addition. In general, biochar can quickly release alkaline earth metals to increase soil pH (Clough et al ., 2013). In our study, the application of biochar alone did contribute to an increase in soil pH, but no significant increase has been observed. However, we found that soil pH was significantly lower after N addition (P < 0.001)(especially in N3 level) than in CK treatment over the incubation period. This confirmed that the higher content of N addition may be an effective method for pH neutralization in alkaline soils in a certain range, as also observed in other recent studies (Panet al ., 2020; Wang et al ., 2020). The addition of N fertilizers may lead to decreases in soil pH due to the oxidation and nitrification of ammonia (Geisseler and Scow, 2014).
We observed a significant increase in SOC contents in all amended treatments than CK treatment over the incubation time. The increased SOC was likely due to the release of high OC contents from biochar into the soil. That biochar increases SOC contents has already been demonstrated in many studies (Agegnehu et al ., 2016; Forján et al ., 2017). In addition, we found that the addition of microbial agents, especially at high amounts, also lead to a higher increase in SOC contents, indicating that the addition of microbial agents to the mine soil in this study improved SOC contents; this may be attributed to the addition of microbial agents can produce a variety of enzymes (i.e. catalase, peroxidase, urease) by their life activities to promote the synthesis of soil organic matter (Song et al ., 2007). TN also had significant increases in amended treatments, especially in N3 level (highest in C-N3). This result indicated that the increase of TN may originated from high contents of N addition to mine soil. Subsequently, we subtracted TN contents introduced by N fertilizer and biochar, and TN contents was still higher in amended treatments; we attributed this to nitrogen retention in biochar. Phytoavailability of N is pivotal for soil quality, and low N contents often represent an immediate limitation to plant growth (Christensen and Schjønning, 2004). In our study, a significant increase in AN contents in all amended treatments than CK treatment similar to TN, and the increase was most obvious at the N3 level (especially in C-M2-N3 treatment). The first reason may be the higher contents of N fertilizer addition and N contents contained in the biochar itself. Another possible reason could be that higher microbial activity due to the addition of M contributed to N mineralization and increased AN content (Ahirwal and Maiti, 2018).
AP and AK also showed a significant increase in all amended treatments, also exhibited higher contents both at the N2 level (ordered by C-M2-N2, C-M1-N2, C-N2 treatments), showing AP and AK increased with increasing M contents in N2 level. Studies have shown that the addition of biochar can increase AP contents in soils because biochar itself contains large amounts of P with higher effectiveness (Agegnehu et al ., 2016; Rafael et al ., 2020). Meanwhile, we found AP and AK contents increased with increasing M contents in N2 level. A possible explanation for this is that the higher contents of M added into the soil can reduce the fixation of P, K and improve the availability of soil P and K; also, the number of soil microorganisms after the higher contents of M addition increased significantly, which promoted the mineralization of P and the conversion of organophosphorus to AP due to the release of P affected by bacteria (i.e. Bacillus subtilis) (Hu et al ., 2009). More importantly, AP and AK exhibited higher contents in N2 levels, which may be an appropriate amount of N input for N sources for microorganisms, promoting microbial activity, and subsequently promoting the activation and decomposition of insoluble substances in the soil. Above all, relatively higher contents of N and M addition based on biochar in the short-term can improve soil quality by neutralizing soil pH and increasing soil nutrient content.