Effect of mulching on labile soil organic matter pools, microbial community functional diversity and nitrogen transformations in two hardwood plantations of subtropical Australia

The aims of this study were to: (1) assess the effects of mulching on labile soil organic matter (SOM) pools (water-soluble organic matter, WSOM; hot water extractable organic matter, HWEOM; microbial biomass, MB; acid hydrolysable organic matter, AHOM), microbial community functional diversity and soil nitrogen (N) transformations and (2) quantify the relationships among labile SOM pools, soil N transformations and microbial community functional diversity. Soil (0–10 cm) was collected from two hardwood plantations in subtropical Australia to which plant residues (1.57 kg m−1, C/N ratio of 80) had previously been applied as mulch for 12 months. Both in southeast Queensland, Australia, one plantation was located in dry area (Pechey, average annual rainfall 851 mm) and the other at Proston, drier by comparison (average annual rainfall 613 mm). At both locations, soil WSOM-carbon (C), HWEOM-C, MB-C, and AHOM-C were significantly higher in the mulch treatment than in the non-mulch treatment. The soil microbial community functional diversity (Shannon's diversity index, H′) measured by BIOLOG GN2 micro-plates was significantly greater in the mulch treatment than in the non-mulch treatment. The mulch treatment had significantly higher soil gross N mineralization and gross N immobilization rates than the non-mulch treatment. However, there was no significant difference in soil gross nitrification between the two treatments. Stepwise multiple regressions revealed that HWEOM-C had significant correlation with soil microbial functional diversity and explained more of the variation in microbial functional diversity than any other labile SOM pools. The MB-N explained more of the variations in gross N transformations than other labile SOM pools.