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.