Accumulation of Soil Selenium Caused by Long-Term Foliar Spraying Reduced Soil Microbial Diversity and Enhanced Soil Denitrification in Rice Roots

Sustainable agricultural production
must be aligned with environmental
and biodiversity goals. Foliar selenium (Se) application can effectively
increase the rice Se content; however, its long-term effects on soil
microbial diversity and functional changes remain poorly understood.
Therefore, we conducted a field experiment (foliar Se for 3 years),
a simulation experiment (simulated foliar Se for 5 and 10 years),
and a meta-analysis (effects of exogenous Se application on the soil
microbial α-diversity) to evaluate the effects of foliar Se
application on rice production and soil microbial communities and
functions. Foliar Se application for 3 years effectively increased
the grain Se content, increased soil Se and organic carbon levels,
and altered root energy secretion metabolites with significant increases
in glutamic acid and glutamine. Soil Se application did not affect
the organic carbon content. Continuous foliar Se application for 3
years reduced the stability of the microbial community, with root
exudate metabolites exerting stronger effects than Se. The meta-analysis
indicated that high-concentration Se (≥0.4 mg·kg–1) significantly reduced the Shannon index of microbial communities.
Treatment with Na2SeO3 (for 10 years) increased
the soil Se content by 132.67% while decreasing microbial α-diversity
findings that align with the meta-analysis results. However, treatment
with the compound fertilizer containing Na2SeO3 (for 10 years) increased the soil Se content by 187.43% yet significantly
enhancing microbial α-diversity. This discrepancy may be attributed
to the oxides formed by iron and manganese ions in the compound fertilizer,
which reduce the bioavailability of Se in the soil. Both field and
simulation experiments confirmed that the exogenous Se application
accelerated the transformation of soil organic phosphorus to an inorganic
state soil available phosphorus content, which increased by 52.41%
and 5.09%–72.10%, respectively. Denitrification in the soil
nitrogen cycle was strengthened, and the increased abundance of norC
and nosZ enhanced the possibility of N2O emissions. These
results indicate that long-term foliar Se application increases the
soil Se content, reduces soil microbial diversity, and strengthens
microbial denitrification, which is detrimental to sustainable agricultural
production.