Soil organic matter and ammonium affect potassium retention in soil
microaggregates under long--term fertilization
Abstract
The adsorption and fixation of potassium in agricultural soils are
important as they influence K availability for crops. Soil organic
matter (SOM) and ammonium (NH4+) exist in soils and play indispensable
roles in soil fertility and crop yield; however, the effects of SOM and
NH4+ on K retention in soil aggregate remains unclear. This study aimed
to evaluate the effects of SOM and NH4+ on K adsorption and fixation in
soil microaggregates (<0.25 mm). Soil microaggregates were
extracted from three long-term fertilization treatments under rice-wheat
rotations: no fertilizer (CK), fertilized with inorganic NPK (NPK), and
inorganic NPK fertilizers combined with straw return (NPKS). Long-term
fertilization, particularly the application of inorganic NPK combined
with straw return, significantly improved the SOM content in
microaggregates. Both NPK and NPKS treatments increased K adsorption but
decreased K fixation, and SOM oxidation of microaggregates reduced K
adsorption but increased K fixation in all treatments, indicating the
positive and inhibitory effects of SOM on K adsorption and fixation,
respectively. NH4+ significantly inhibited K adsorption and fixation,
and this inhibitory effect was more significant in microaggregates with
a higher SOM content. Although NH4+ reduced the positive effect of SOM
on K adsorption, it enhanced the inhibitory effect of SOM on K fixation.
Conclusionally, long-term fertilization increases K adsorption but
reduces K fixation by improving SOM content, where NH4+ enhances SOM
inhibited K retention in soil microaggregates, which is considered to
improve K availability in soils amended with K fertilizers. Keywords:
soil organic matter, NH4+, K, adsorption, fixation