Larger Soil Water-Stable Aggregate May Exert a Negative Effect on Nutrient Availability: Results from Red Soil (Ultisol), in South China

Soil aggregates are the basic units of soil, which regulate soil carbon cycling and nutrient availability through the protective effect of soil aggregates on soil organic matter. It is still uncertain whether larger aggregates are more conducive to soil nutrient availability in red soil. This study explored the regulation of soil aggregates on soil nutrient availability by studying the distribution of soil aggregates, nutrient concentrations, nutrient availability and organo-mineral bonds in soil aggregates in a low-productivity Chinese fir forest, in south China. We sampled the 0-10 cm soil with nine repeated plots and analyzed the soil aggregate structure, total nutrients, available nutrients and organo-mineral bonds of soil aggregates. The results showed that the contribution of >2 mm soil aggregates to soil nutrients was highest, because the mass of >2 mm soil aggregates accounted for about 50% of the total mass of aggregates and was much higher than that of other aggregates. The availability (available nutrient/total nutrient) of nitrogen, phosphorus and potassium increased with decreases in soil aggregate size, indicating that soil aggregates with a larger particle size were more averse to nutrient availability. Strong organo-mineral bonds accounted for more than 80% of the total organo-mineral bonds in the soil aggregates of each size, and the proportion of weak organo-mineral bonds in the soil aggregate increased with decreases in the soil aggregate size. There was a significant negative correlation between the size of soil aggregates and the proportion of weak organo-mineral bonds in soil aggregates. The availability of carbon, nitrogen, phosphorus and potassium in soil aggregates was positively correlated with the proportion of weak organo-mineral bonds. These results suggest that Fe/Al oxides may play an important role in regulating nutrient availability, especially in red soil. A higher proportion of strong organo-mineral bonds in larger soil aggregates may exert a stronger negative effect on the accessibility of microorganisms to organic matter and result in a lower nutrient availability. In conclusion, this study shows that larger-sized soil macroaggregates may exert a negative effect on nutrient availability, owing to a higher proportion of strong binding bonds, which can better prevent microorganisms from mineralizing organic matter into effective nutrients in red soil.