Effect of mulch on nitrogen cycling in a hardwood plantation of subtropical Australia

We investigated the effect of mulch on N cycling in a hardwood plantation of subtropical Australia by examining monthly in situ net N mineralization for 12 months and natural N abundance (δN) of soil and plants. Monthly net N mineralization ranged from –23.0 μg g soil (net immobilization) to 52.6 μg g soil across 12 sampling months and across both mulch and nonmulch treatments. There was a significant effect of sampling month on net N mineralization; however, the effect of mulch on net N mineralization was not statistically significant. On average, net N mineralization was higher in soils collected in summer than other seasons. When monthly net N mineralization was partitioned into net ammonification and net nitrification, mulch treatment significantly decreased net nitrification, but did not affect net ammonification. The 0-10, 10-20, 20-30, 30-40, and 40-50 cm soil samples were collected for δN analysis 20 months after the mulch treatment was applied. The results showed that soil δN increased with depth and mulch treatment significantly decreased soil δN (0-10 cm) by 0.8‰. We explored the relationships between the annual averaged in situ net N mineralization rates and natural N abundance (δN) of 0-10 cm soils. Significant relationship was found between net nitrification and δN of 0-10 cm soils (p < 0.05), but no significant relationships were shown between δN of 0-10 cm soils and in situ net N mineralization or net ammonification rates. The foliage of 12-month old plantation was sampled from upper and outer canopy position pointing to north and analysed for δN. Surprisingly, mulch treatment increased foliar δN of trees by 0.7 ‰ compared to the non-mulch treatment. This may be due to isotopically light, soil NO3 (compared to soil NH4) constituting a more important N source for plants in the non-mulched plots relative in the mulched plots since we did find higher NO3 concentration in the non-mulched soils than in the mulched soils.